3D Electronic Sensors for Bio‐Interfaced Electronics and Soft Robotics

Multi‐dimensional electronic devices for sensing applications have received immense attention in the fields of bio‐interface electronics and soft robotics to achieve structural manifolds in recent years. The structural diversities of sensors are tuned based on the environments in which they encounter multipurpose and complex form factors of electronics. 3D electronics are one of the emerging sensor platforms characterized by structural adaptability and compatibility on free target surfaces, resulting in unprecedented improvement of spatial resolution. Herein, the recent progress made in the fabrication and usage of 3D electronic sensors that are classified into two groups based on the methodological concepts used for fabrication in bio‐interfaced electronics and soft robotics applications, is reviewed. The latest advances in the design and development of 3D printable and 2D to 3D shape‐deformable electronic sensors are reviewed with a focus on printable materials and deformation mechanisms, respectively. 3D printing techniques enable the facile fabrication of arbitrary and seamless electronics without the use of conventional microfabrication technologies. Deformable electronics cover their self‐tune architectures and functions depending on mechanical deformation based on buckling, origami, and kirigami fabrication strategies. Finally, the printed and deformable 3D sensors show their potential for use in next‐generation sensing platforms.


Introduction
[3][4][5] The emergence of internet of things technology has helped improve the application prospects of future electronics in DOI: 10.1002/adsr.202300013 the fields of medicine, robotics, and the fashion industry.In the upcoming era of convergence electronic devices, electronics can be customized and used to realize functionalized device form factors that can be integrated into complex and arbitrary spaces, including wearable healthcare materials, implantable medical devices, and soft robots.
[13] Electronic sensors applicable in the fields of soft robotics and those used for the advancement of biomedical fields should be compatible with joints in the target body to obtain highly accurate signals and information by which we can understand the health conditions of patients. [14,15]Although conventional 2D electronics developed following the vacuum-based microfabrication processes significantly contribute to improving the application prospects and integrity of the electronic systems, these can only utilize the information on flat surfaces.Therefore, it is essential to understand the spatial characteristics and performances of the electronic sensors to achieve effective utilization in the case of advanced applications with multi-dimensional architecture form factors.
Figure 1 presents the conceptual illustration of the application areas and the methodological classification of 3D electronic sensors.Diversification in the 3D workspaces related to the applications of electronic sensors has increased the need for changes in the architectures and form factors of the materials.Structural modification in electronic sensing systems has enabled applicable manifolds and extendability to advanced fields requiring real-time monitoring.Structural changes also help in developing unique spatial features, including sensitive shapeadaptiveness and high-resolution sensitivity on the diverse target substrate. [16,17]In recent years, electronic sensors have attracted immense attention as they can be used to realize the spatial resolution of sensing windows in the field of sensing applications.These materials can be used to advance the field of soft robotics, human-machine interfaces, and personalized medical devices.3D structures can introduce enhanced functionalities and novel characteristics that address the problems attributable to the spatiotemporal limitations of electronic sensors.
Figure 2 summarizes the key characteristics of the 2D and 3D electronic sensors.The 3D microstructures of functional materials can be tuned to control the spatial extendibility of sensing and the sensitivity of sensors by increasing the effective surface area of the materials. [18]The arbitrary architectures of electronic sensors also can improve the number of omnidirectional sensing points and widen the effective spatial regions. [19]It is noteworthy that 3D architectures improve the compatibility of the devices with the working environment and maintain the conformational features at the interfaces formed between the devices and organs, tissues, and skin. [20]It can be inferred that 3D structures can be potentially used to achieve high spatial resolution and tune freeform structures that can be applied in the field of sensing.
3D electronics can be defined following methodological approaches for developing freeform electronic devices.We classify and summarize two methodological strategies for building 3D structures of electronic sensors: 1) 3D printing technology and 2) the out-of-plane deformation method.3D printing is a key technology that enables the construction of stereoscopic architectures on arbitrary substrates for device-level implementation using resin-based composite materials.The 3D printingbased fabrication can be used to develop processable branches to develop a freeform of pre-designed architecture without executing microfabrication processes.These cannot be achieved using other 3D fabrication techniques, such as photolithography and vacuum deposition.The 3D printing technologies have been successfully used to achieve isolated and seamless 3D fabrication performance, which helps tune the electrical and mechanical properties of printed devices by controlling material combinations and processes in a composite system.This helps in the fabrication of functional freeform architectures.Nevertheless, 3D printing technologies have shown a critical drawback in XY-plane spatial resolution on the printed surface because the sensing efficiency of the electronic sensor is mainly dependent on the surface area on which performance is reactive with external stimuli.Out-of-plane deformable structuring methods for the transformation of 2D systems to 3D models have been widely used to fabricate 3D electronic devices using various scale driving forces.The two-step sequential fabrication and the on-demand 3D structuring method driven by appropriate driving forces post the fabrication of 2D electronics have been used to successfully achieve the nanometer-scale spatial resolution of 3D architectures of various functional materials.The 2D fabrication technologies can be exploited to achieve the on-demand surface spatial resolution of the as-formed structures.The out-of-plane deformation technique is a complementary technique that can help address the limitation of the 3D printing technology.The out-ofplane deformation strategy, which includes the trial for the freeform 3D structuring of 2D functional electronics, can be used for the development of 2.5D electronic structures.The application prospects of the method are limited by the fact that arbitrary 3D objects cannot be generated using the technique.Nevertheless, the complementary relationship of the two representative fabrication strategies reveals that the out-of-plane deformation strategy can be used for 3D object structuring for fabricating materials with advanced 3D functionalities, such as reversibility, scalability, and high responsiveness.The properties are based on the skeletal configuration of the materials.
Here, we review the materials, fabrication methods, and application prospects based on methodological strategies for the design and construction of the latest-reported 3D electronic sen-sors.The 3D sensing devices are classified based on printable and shape-deformable fabrication techniques and organized with their performance and stimuli which sensors transduce into an electrical signal.The progress made in the fields of 3D-printed multifunctional electronic sensors designed for use in fields such as soft robotics and bio-interfaced electronics is also presented.We also summarize the performance and application prospects of transformable electronic sensors and the fabrication strategies involving out-of-plane deformation techniques used to fabricate these materials.Finally, the advances in next-generation applications of the printed and shape-deformable electronic sensors characterized by the presence of sensor-integrated 3D structures are discussed.

3D Printed Electronics
3D printing is a promising technique that can be applied for fabricating free-standing and arbitrarily structured devices without using microfabrication procedures.][26][27] Printing technologies could be successfully used for the direct printing of stimuli-responsive sensing devices such as mechanical, thermal, optical, and radio frequency (RF) sensors even on omnidirectional surface geometries.Furthermore, the currently utilized 3D printing methods developed electronic sensors characterized by the form factors that can help employ artificial organs with biomimicking functions. [28,29]This section reviews printable electronic materials, their components, and their application prospects in various fields.We have also presented the use of various types of 3D-printed multi-dimensional sensors that can be used for smart monitoring and to develop bio-interfaced electronics and soft robots.

Printable Electronic Materials and Components
Various combinations of 3D printable ink developed using electrically functional fillers and polymer matrices can be used to develop customized composite systems that are stimuli-sensitive and can be used for signal conversion.3D printable inks have also been used to develop electronic components in the absence of substrates.[32][33][34] Electrically functional composites have been designed using viscoelastic resins and dispersed functional fillers.The intrinsic properties of printed composites depend on the inserted filler materials.These composite systems can be efficiently developed following a solidification process.A viscoelastic composite system is formed from fluidic filaments while conducting the additive bottom-up fabrication method in a liquid state during solidification to pile up the supporting basis continuously.The types of printable materials that can be developed following the 3D printing method depend on the solidifying mechanisms associated with the developing process.40][41][42][43][44][45][46] These methods can realize the seamless flows of functional composite materials following the solvent-casting or curing process in the presence of matrix resin.The filaments continuously form functional composite traces, which are polymer matrices containing nanofillers such as nanoparticles and nanowires, carbon-related materials, and organic semiconducting materials.Various types of functional fillers, including electrically conductive, semiconductive, and environmentally sensitive elements such as piezoelectric or photovoltaic materials, improve the electrical properties of composite systems fabricated following the extrusion and photoninducing printing methods.

Conductive Materials
Conductive filler materials form electron conduction pathways in a polymer matrix.Volumetric changes in filler materials control the electrical conductivity of printed composite systems beyond the percolation threshold point.In addition, the filler content can affect the mechanical properties of the printed traces.3D printable conductive materials, including metal nanoparticles, [24,30] carbon flakes (such as graphene and CNTs), [25,32,33] and liquid metals (EGaIn), [27] can be used to build pre-designed 3D structures from intrinsically conductive units and systemic architectures with polymer matrices.
Figure 3a presents electronic circuits, including passive electronic components, printed on various substrates in the form of freestanding 3D structures using silver composite ink. [24]RF transmission coils and circuit lines were extruded by using the DIW method, and the nozzle diameters were controlled. [24]In active RF electronic circuits, discrete transistors and wireless transmitters form GHz self-sustained and synchronized oscillators. [24] continuous extrusion step and a one-step printing process is followed for the device design and fabrication with seamless and arbitrary constructions. [24]Liquid solders or interconnects are absent in a 3D-printed electronic component with freeform geometries, irrespective of the substrate used. [24]The silver nanoparticle-based ink, immersed in a poly(acrylic acid) aqueous solution, was extruded via the nozzle, aligned with a focused infrared (IR) laser, and instantaneously solidified in the programmed position (laser-assisted DIW). [30]Free-surface structures of metal hemispherical spirals and conductive silver wires were constructed to demonstrate a method of fabricating printed 3D metal architectures and verify the ability of the method to print high-resolution, functional metal electrodes and complex structures that could be transformed into customized electronics, MEMs, and biomedical devices. [30]Subwavelength plasmonic and conductive-patterned 3D structures were printed following the simultaneous two-photon-initiated photoreduction of gold precursors in the presence of a polymer resin. [31]Several functional structures, including optically active planar chiral and plasmonic nanostructures, have been introduced into the system. [31]Ultrafine and geometric patterns for conductive paths can be generated in complex structures following the MPP process.Xiong et al. developed well-aligned multi-walled carbon nanotube (MWCNT)-acrylate-based 3D microstructures. [25]The maximum spatial resolution was recorded to be 100 nm (line width), and the material was fabricated following the two-photon polymerization (TPP) lithography method. [25]The method could be used to successfully generate ultrafine line widths and highresolution patterns and develop true 3D architectures based on micro-scale electronics. [25]Figure 3b shows the structural manifolds using a printable composite of MWCNTs fabricated by the TTP technique and characterized by the presence of 3D functional structures. [25]These architectures can be used for developing micro-scale 3D electronic components including capacitor and resistor arrays.The image also presents the structural features of MTA-based composite polymers, characterized by woodpile structure, containing 0.2 wt% of MWCNTs. [25]The systems impart the MWNT composites with good mechanical characteristics and help improve the electrical conductivity of the materials. [25]Guo et al. printed a 3D liquid sensor composed Figure 3. 3D printable electronic inks and their applications.a) 3D-printed passive devices fabricated following the direct ink writing (DIW) method using viscoelastic silver ink and a nozzle (10 μm).A 1.8 GHz radio frequency (RF) wireless transmitter was formed on a glass substrate.Reproduced with permission. [24]Copyright 2017, Wiley-VCH.b) 3D-printed conductive MWNT-thiol-acrylate (MTA) composite resins patterned using the two-photon polymerization (TPP) lithography technique for the fabrication of a series of microelectronic devices.Reproduced with permission. [25]Copyright 2016, Wiley-VCH.c) 3D-printed microelectronics, including parallel-plate capacitors, solenoid-type inductors, and meandering-shape resistors deposited using the dielectric ultraviolet (UV)-curable acrylic plastic (matrix block) and the liquid metal paste consisting of a silver suspension (filler).Reproduced with permission. [26]Copyright 2015, Springer Nature.d) 3D multilayered soft artificial skin electronic sensor made of EGaIn (liquid metal suspension).Microchannels were formed on flexible substrates as strain and pressure sensors.Reproduced with permission. [27]Copyright 2012, IEEE.e) Multi-photon lithography (MPL)-based fabrication of hybrid microelectrodes composed of UV curable acrylate and PEDOT:PSS as a conductive organic semiconductor for the fabrication of bioelectronics and biosensors.Reproduced with permission. [34]Copyright 2022, Wiley-VCH.f) Schematic representation of the ZnO UV photo-detecting semiconductor (top) and scanning electron microscopy (SEM) image recorded for the 3D ZnO microdevices (bottom).Reproduced with permission. [35]Copyright 2022, Wiley-VCH. of a freeform, helical polylactide (PLA)/MWCNT nanocomposite system. [32]The nanocomposite inks loaded with 5 wt% MWCNT determined the process-related viscosity of the inks and controlled their flow behavior.In addition, the electrical properties of the fabricated helix sensor exhibited a typical concentrationdependent percolation behavior. [32]The extent of percolation increased with an increase in the contents of MWCNT. [32]The extruded composite filament could be used to successfully fabricate a 3D free-floating helix of diameter 150 μm.The helices could be exploited to develop self-supporting structures. [32]The excellent sensitivity and high degrees of sensing responses achieved under conditions of liquid trapping in freestanding 3D helical sensors could be attributed to the structural features of the sensors. [32]arbon-based nanostructures have been widely developed into composite systems for the fabrication of printed free-standing electronic electrodes. [33]Emon et al. developed a direct printing system for the development of multi-materials that can be used for continuous and seamless printing. [40]Soft pressure sensors are directly fabricated following novel manufacturing techniques that impart design flexibility and help in customization.The methods can secure the product development using a wide range of materials. [40]They highlighted the 3D printing process, which enables sensors to be printed on substrates with complex and arbitrary geometry. [40]igure 3c presents the process of development of passive microelectronics in the presence of a photopolymer polymer matrix using a combination of 3D printing and metal paste filling methods. [26]Two commercial polyacrylates were deposited alternatively from dual nozzles for extruding molding structures as building blocks and hollow channels as sacrificial materials to fill metal inks. [26]These prints form multiple components of meander-shaped resistors, solenoid inductors, parallel plate capacitors, and LC circuits that are embedded in 3D structures. [26]he printed LC tanks were demonstrated to be passive and wireless resonant sensors, and these could be used for monitoring applications. [26]Liquid metal composites were printed onto stretchable substrates using the DIW method to fabricate multilayered conductive microchannels and liquid conductors. [26]Park et al. described the design and fabrication strategy of a highly compliant skin sensor for soft actuator technology. [27]Figure 3d presents the extrusion-printed conductive EGaIn ink that can be used to develop strain-sensitive and durable stretchable strain sensors composed of silicone rubber composites. [27]These 3D sensors function under structural operation modes. [27]

Semiconductive Materials
Semiconductive nanomaterials have also been used to fabricate extremely small 3D microstructure electronics for use in flexible electronic circuits.Multi-photon lithography (MPL)compatible polymer composites were used to construct 3D organic semiconductor microstructures using poly(ethylene glycol) diacrylate as a photosensitive resin and poly (3,4ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) as a doped organic semiconductor. [34]Figure 3e shows MPL-based 3D printed devices, including microresistors, microcapacitors, and μPCBs, which can be used for developing highly transparent microelectronics in the presence of a glucose sensing platform. [34]Liu et al. developed customizable metal-bound composite photo resins for the fabrication of 3D metal-oxide semiconductors based on the TPP printing method. [35]Figure 3f presents the schematic representation of the UV photodetector and the 3D architecture of the ZnO-based microdevices fabricated using ITO electrodes. [35]They prepared metal-bound photo resins using various metal-organic frameworks (imidazolate frameworks, ZIFs) and monomers (acrylates, epoxy resins, and water-soluble monomers) as the base resins. [35]The photoinitiator, photosensitizer, and tetraacrylate monomers were used to fabricate Co 3 O 4based complex 3D nanoarchitecture characterized by high spatial resolution and good surface properties. [35]The properties of the fabricated semiconductors could be attributed to the use of the photon-induced MPA method. [35]1.3.Printing Methods 3D electronic architectures for printable materials are developed following bottom-up fabrication methods.[20][21][22][23][24][25][26][27][30][31][32][33][34][38][39][40][41][42][47][48][49][50][51][52][53][54][55][56][57] These systems seamlessly form freeform 3D structures while maintaining solid states through a continuous extrusion and solidification process.The printing techniques followed for the development of polymer composites in printed electronics can be divided into two categories (solvent casting and photocuring) based on the associated solidification mechanisms.Extrusionbased printing methods use polymer resins, such as colloidal slurry inks, which are solidified by casting a liquid solvent under ambient conditions.Viscoelastic composites mix the metallic powder in polymer binders or organic solvents to realize adequate viscosity and impart the required functional properties. Photoinduced polymerizati methods utilize photocurable polymer matrices and photoinitiating agents with functional fillers.Therefore, properties of matrix resins such as molecular weight determine the processibility of 3D printing methods.Wei et al. designed a novel 3D printing method to fabricate freestanding electrodes with tunable structures and mechanical stretchability.[33] They obtained serpentine electrodes made of polydimethylsiloxane (PDMS)/CNT nanocomposites that were characterized by high stretchability (>315%) and excellent electrical stability (5% change at 100% strain).[33] The solvent-cast 3D printing approach can be followed to control the viscosity of the composite ink.This facilitates the fabrication of self-supporting architectures and 3D arbitrary structures for the development of printable electronic devices.[33] Thermal crosslinking methods are one of the solidification processes used for the fabrication of 3D-printed electronic sensors.[23,27,46,49,53,54,[58][59][60][61][62] Park et al. fabricated an artificial skin sensor using embedded microchannels of EGaIn liquid metals in a silicone-based matrix.[27] They utilized thermo-crosslinked polymers as the matrices or combined the materials with filler materials to form localized and high-resolution architectures of 3D-printed pressure sensors exhibiting electrical functionalities and sensing ability.[27] The printed materials developed in recent years for the fabrication of 3D electronic sensors can be divided into two categories based on single-and multi-material printing methods.3D-printed electronic sensors have been developed with printable materials, and their application prospects and novel functionalities have focused on the fields of bio-interfaced electronics and soft robotics.Multi-material printing enables multiple components and functions to integrate into a single device which can impart embedded characteristics of sensing capability to the printed electronic system.High-performance and novel types of form factors such as integrated circuit-level devices and optoelectronics, which consist of diverse multi-components can be fabricated using this method.[16,17,19,20,23,36,40,48,52,55,57,58,61,63,64] These types of printed sensors could be fabricated using various fabrication techniques such as extrusion-based, thermal-crosslinking, and photon-induced printing techniques.[19,20,23,36,40,64] Printed sensors fabricated using single components and monolithic materials primarily form line-type conductors and supporting structures that act as the electrical interconnection and body of sensors.[22,24,25,[27][28][29][31][32][33][37][38][39][42][43][44][45][46][47][49][50][51]53,54,56,59,60,62,[65][66][67][68][69][70][71][72] These materials have been fabricated using a single-material composite system following photon-induced methods such as SLA, DLP, and MPP.[21,25,34,37,41] 3D-printed electronic sensors are rapidly developing and can be used in the fields of biomedical electronics and soft robotics.The continuous development of nextgeneration sensing devices necessitates the use of advanced form factors of electronic sensors fabricated using multi-component and multi-functional units for the effective development of the desired architectures and the generation of target functionalities for the development of printed sensors.

3D-Printed Electronic Sensors
Sensors transduce external stimuli into electrical signals to recognize target species by analyzing signal intensities.The shapes of the sensors govern the sensing mode and sensing sensitivity of an electronic system.Structural modification in 3D electronic sensor could help in the development of the desired physical shapes of printed devices to secure the improvement of sensing performance of the sensor and impart the shape-adaptive property of the target geometry.3D printing techniques has been employed to control the architectures of the printed objects and customize the functionalities of the electronics. [17,30,48]In this section, we review the various types of electronic sensors and their applications classified based on external stimulus which they transduce into electrical signals.

Mechanical Sensors
3D-printed electronic sensors that respond to mechanical stimuli (such as pressure and strain) have been reported as the types of sensors with multiple sensing modes and tunable properties.Inorganic and organic materials with piezoelectric properties have attracted immense attention as they exhibit the ability to convert compressive and tensile forces into electrical power for the development of energy devices.
The structural dimensions of piezoelectric devices are a critical factor for maximizing the energy conversion efficiency of the systems.To date, the modification and modeling of piezoelectric devices using 3D printing techniques have focused on fabricating multi-dimensional devices of different shapes.Lead zirconate titanate (PZT), [21] polyvinylidene fluoride (PVDF), [41] and barium titanate (BaTiO 3 ) as piezoelectric materials [42] can be developed into multi-dimensional microstructures using printing technologies.BaTiO 3 nanoparticles were mixed with PVDF chains following the solvent-evaporation-assisted 3D printing method to induce an increase in the electroactive properties of the -phase fraction of the PVDF matrix. [42]The rheology of the BaTiO3-PVDF composite was controlled to fabricate selfsupporting structures with satisfactory mechanical robustness and high dielectric and piezoelectric properties. [42]3D-printed piezoelectric metamaterials with complex micro-architectures were composed of PZTs with functionalized particle surfaces and photosensitive monomers. [47]The design of a stacked piezoelectric infrastructure was analyzed to reveal the property of electrical-mechanical coupling anisotropy and understand the orientation effects in 3D piezoelectric responses associated with five operation node units. [47]It was observed that the properties of piezoelectric composites could be exploited to realize the inverse design of arbitrary piezoelectric tensors using the 3D printing method. [47]igure 4a presents the schematic representation of tactile sensors developed using silver composite ink. [48]The material was developed in three functional layers designed to fabricate multifunctional 3D-printed electronic sensors. [48]The contents of conductive silver filler in each layer were different, and the modes differed based on the types of physical stimuli received.The sensing characteristics of sensors can be tuned by tuning the 3D-printed layers and contents of extruded composite inks. [48]These sensors can be used to detect protean changes in resistance with changes in mechanical strain, pressure, and tactile motions of the hands.][50][51] Figure 4b presents the 3D-printed graphene aerogels that exhibit high pressure-sensing reversibility with high physical compressibility and electrical conductivity. [39]These were developed from geometrically porous and periodic sensors, and the degree of the response of porous structure was an order of magnitude higher than the degree of the response generated for bulk graphene materials. [39]The adaptability of the 3D printing techniques could enable facile and intimate fabrication process for the tunable device architectures and structural functionalities.Nanogenerators (NGs) which can be integrated into electronic systems, primarily utilize external mechanical forces to generate and convert the piezoelectric current. [52]The geometry in this harvesting system should be tuned and controlled to achieve high sensitivity.The triboelectric property is one of the desired characteristics for flexible and wearable devices that are used to develop highly customized and functionalized sensing devices, specifically in mobile and ubiquitous electronic systems. [52]Elastic and recoverable triboelectric nanogenerators (TENGs) with sophisticated 3D architectures have been fabricated to effectively harvest biomechanical energy and monitor self-powered physiological signals. [53]The high adaptability and controllability of the 3D printing technology have made it possible to tune the shapes and morphologies of 3DP-TENG sensors. [53]Insole and ring-shaped self-powered sensors used to harvest biomedical energy were studied, and it was revealed that 3D microscopic architectures played an important role in determining the sensing performance of materials. [53]The architectures also controlled the output voltage of the counterparts fabricated following the traditional molding method. [53]igure 4. 3D-printed stimulus-responsive electronic sensors integrated with functional and structural materials.a) 3D tactile sensor consisting of four printed components (isolating, electrode, sensing, and supporting layers). [48]The system was fabricated by using a mixed composite suspension composed of conductive silver nanoparticles (AgNPs) and silver nanowires (AgNWs) in base elastomer resin.Reproduced with permission. [48]Copyright 2017, Wiley VCH.b) Ultra-flexible 3D triboelectric nanogenerator (TENG) system fabricated following the hybrid 3D printing method.The diamond structure was exploited to generate an effective contact area for charging.Energy from human motions was harvested (usually less than 3 Hz).Reproduced with permission. [39]Copyright 2018, Elsevier.c) 3D-printed and pixelated bulk-heterojunction photodetectors and arrays characterized by high EQE in the UV-vis region.The systems were printed on a spherical surface using EGaIn liquid metal inks as the cathode and P3HT:PCBM as the photoactive layer.Reproduced with permission. [55]Copyright 2018, Wiley-VCH.d) Printed flexible wearable patch with a 3D sweat channel for continuous and noninvasive detection of the levels of multiple electrolytes present on the skin.Reproduced with permission. [58]Copyright 2021, Wiley-VCH.e) Schematic diagram of cylindrically printed flexible glucose sensor underlying the subcutaneous tissue.The system was based on an enzyme electrode composed of glucose oxidase (GOx), silver, and graphene ink.Reproduced with permission. [63]Copyright 2018, Royal Society of Chemistry.

Temperature Sensors
Heat can be collected and utilized for the detection of thermal information generated from bio-interfacial region to serve as temperature sensor.Huang et.al developed a flexible thermoelectric electronic module in a 3D-printed and integrated sensing device for the qualitative determination of -fetoprotein. [22]he decomposition of the substrate into the photothermal product was accelerated by the presence of the GOx-conjugated sec-ondary antibody in the immunoassay, resulting in a significant increase in the thermoelectric voltage generated during the process of biomarker detection. [22]The environment-adaptive and portable thermoelectric devices were fabricated following the processing approach of 3D printing. [22]Liu et al. demonstrated an extrusion-based 3D printing technology that utilized the DIW and solvent replacement methods to fabricate a structurespecialized MXene-bonded polyurethane (PU)/polyvinyl alcohol hydrogel. [54]3D-printed Ti 3 C 2 T x (MXene) hybrid hydrogel was fabricated into multimodal sensors that exhibited hightemperature sensitivity. [54]3D printing of a DIW-fabricated transition metal carbide-bonded hydrogel sensor exhibiting excellent strain-and temperature-sensing performance was also realized. [54]

Optical Sensors
Photodetectors developed using semiconductive polymer-based 3D-printed electronics exhibited photon-responsive external quantum efficiency. [23]The arbitrary 3D macrostructures showed emitted locally at a high resolution, and this could be attributed to the presence of closed structures capable of efficient omnidirectional sensing of light.These devices also could be developed without additional bulky optical components or using cleanrooms or conventional microfabrication techniques. [23]igure 4c presents the fully 3D-printed photodetectors containing interconnected photodetector arrays that are characterized by high sensitivity and can be used for the development of advanced image-sensing platforms. [55]The results demonstrate that it is possible to realize the multifunctional integration of multiple semiconducting devices that are 3D-printed on a single unit. [55]Architectures of 3D image sensors, directly printed on both planar and hemispherical surfaces, can be used to realize a wide field of view for optically-coupled photodetectors and LEDs. [55]3D-printed, miniaturized, complementary metal oxide semiconductor image sensors were developed by mimicking the natural vision of predators. [56]The eagle eye image-sensing system combines directly printed multi-lens objectives and four printed doublet lenses characterized by different focal lengths. [56]MOS image sensors allow the progress of fast design iterations, and these can be used to develop a plethora of miniaturized multi-aperture imaging systems as the four lenses are printed in a single step, and further assembly or alignment processes need not be conducted. [56]

Electromagnetic Sensors
One of the most prominent challenges associated with the development of wireless sensor network design is the generation of good power efficiency.3D-printed compact packages that can be used to enclose RF electronics were built to develop complex 3D origami structures for high-frequency applications.Kimionis et al. developed a complex 3D RF energy harvester with orthogonally positioned on-side antennas following the process of additive manufacturing fabrication. [57]On-package antennas for RF signal reception were printed as a planar structure with "smart" shape-memory hinges that allow origami folding to develop 3D structures post heating. [57]The planar structure could be folded into a 3D cube using a heat-activating shape-memory hinge. [57]he potential of utilizing the inkjet 3D printing method for RF applications opens new pathways to realize the reconfiguration of systems. [57]

Biosensors
Biological systems are characterized by sophisticated and multidimensional external structures of membranes in both motor and sensory modules.Electronic sensors for biomedical applications must be in physical contact with the conformal interface geometry of the organisms to detect and process vital signals generated from target organs with complex architectures.3D printing methods have imparted structural manifolds to the electronic systems applicable for biomedical electronic applications.Printing technologies such as photo-curable method and inkjet printing have been developed to address the problems attributable to the geometrical complexities of biological systems. [58,63,65]he biosensors were designed as conformable skin-like electronic devices to monitor the levels of sweat electrolytes, glucose, pH, and bioelectricity.Kim et al. developed wearable bioelectronic patch-type sensors for the in situ monitoring of sweat electrolytes present on the skin. [58]Figure 4d presents the process of fabrication of a multiplex and customized flexible all-inclusive integrated wearable sensor, which consists of 3D-printed ion sensors, electrodes, and soft substrates. [58]The sensor can continuously and noninvasively measure the levels of multiple electrolytes in sweat. [58]A flexible enzyme-responsive sensor with a nanostructural 3D working electrode (WE) was printed for continuous glucose monitoring. [58]Figure 4e shows the schematic representations of the cylindrical sensors fabricated following the rotated inkjet printing method, which enables the direct patterning of 3D microstructures on a curved polymer substrate. [63]he use of a 3D nanostructure consisting of graphene and platinum-nanoparticle-containing composites enhanced the sensitivity of the system and enabled hypoglycemia detection under the skin. [63]An integrated microneedle device for biosensing was developed to monitor subcutaneous glucose levels in normal and diabetic mice continuously. [65]The results revealed that microfabrication processes and electrochemical plating steps could be followed to detect glucose effectively. [65]The DLP printing method was also used to develop needle-like sensors. [65]

Stimuli-Responsive Sensors
Thermoactivated polymers can be printed as switching and actuating modules in electronic devices.Zarek et al. developed complex shape memory structures using a viscous polymer, SLA printer, and a customized heated resin bath. [59]Shape memory thermosets and polycaprolactone (semicrystalline polymer) were used to build blocks of shape memory objects that deformed with an increase in the surrounding temperature (Figure 5a). [59]The printed shape memory devices exhibited thermal switching and electrical actuation properties, indicating that these could be potentially used to fabricate soft robotics, medical devices, and wearable electronics. [59]Shape memory polymers can be used to develop systems of various shapes, such as mesh-type textiles and stacked filaments based on target applications. [43,44]Wei et al. designed an ambient-printable conductive polymer nanocomposite (CPN), including the Ag@CNFs hybrid system, to fabricate a printable conductive filler hybrid composite for electrical sensing and robot actuation. [45]Figure 5b revealed that the nanofiberbased CPNs could be directly printed to develop various geometries in the absence of additional support layers. [45]The printed structures could be used as strain sensors and EMI shielding scaffolds with good electroactive performance. [45]A smart gripper manufactured using CPN was triggered by an external voltage for target actuation. [45]igure 5. 3D electronic sensors for shape-memory and microfluidic applications.a) 3D-printed shape-memory temperature sensor fabricated following the stereolithography (SLA) technique using polycaprolactone (PCL) nanoparticles with methacrylate macromonomers.Reproduced with permission. [59]opyright 2016, Wiley-VCH.b) Electroactive smart gripper fabricated using hybrid silver-carbon nanofiber (Ag@CNFs) ink for electrical components, strain sensors, and electromagnetic interface (EMI) shielding scaffolds characterized by electrical-actuated shape-recovery.Reproduced with permission. [45]Copyright 2019, American Chemical Society.c) 3D-printed microfluidic device with integrated biosensors for the online analysis of tissue glucose and lactate levels during dialysate.Reproduced with permission. [66]Copyright 2015, American Chemical Society.

Microfluidics
Microfluidic devices use physiological analytes in different biofluids, such as interstitial fluid, sweat, tears, blood, and saliva, to monitor health conditions.These devices function by detecting sensor probes in microchannels in which target liquids flow continuously.3D-printed microfluidic devices with customized, integrated, and embedded microstructures have been reported as biosensors that detect electrochemical analytes.Gowers et al. reported the first example of a 3D-printed microfluidic device that was combined with integrated portable biosensors. [66]hese systems were directly utilized for clinical microdialysis in humans. [66]Figure 5c revealed that the microfluidic biosensing platform was fabricated using two 3D printers as tools for biofluid channels and encapsulation layers to detect glucose and lactate concentrations in sweat. [66]It is possible to readily integrate the system with commercially available microdialysis probes as the system is characterized by the dimensional controllability of the printed biosensor platform. [66]The embedded 3D-printed biosensor was fabricated using the 3D-printed flow cell system. [67]Disposable electrodes and 3D-printed flow cells continuously monitor hepatic oval cells by recognizing biomarkers on the cancer cell surface. [67]These function as promising diagnostic tools for detecting a wide range of markers in biofluids. [67]igure 6.Smart sensing system fabricated using 3D-printed electronics.a) 3D-printed smart cap for the rapid detection of the quality of liquid food items featuring wireless readout systems embedded with an LC sensor.Cross-sectional view of a smart cap fabricated following the liquid metal filling process.Reproduced with permission. [26]Copyright 2015, Springer Nature.b) Multilayer schematic of a 3D-printed smart glove integrated with integrated circuit (IC) chips and solid-state components consisting of liquid-state circuits (resistors, capacitors, and antennas) and liquid metal interconnects.Reproduced with permission. [16]Copyright 2016, Wiley-VCH.c) 3D fibers printed using Ag/PEDOT:PSS and PVDF-TrFE nanofiber-based floating inks for the fabrication of non-contact, wearable, and portable respiratory moisture sensors.Reproduced with permission. [68]Copyright 2020, AAAs.d) Large area monitoring system for the fabrication of H 2 S, temperature, and humidity sensors fabricated following the process of inkjet printing using silver-organo-complex (AOC) ink.Reproduced with permission. [64]Copyright 2017, Wiley-VCH.

Advanced Applications of 3D-Printed Sensors
Continuous progress has been made in the field of 3D-printed electronic sensors, and this has resulted in the advancement of new sensing platforms including the devices used to develop soft robotics and bio-interfacial sensing devices for real-time monitoring applications.The future development of 3D-printed electronic sensors may involve the shape-adaptive integration to the nature and connection with rehabilitation robotics and biological systems.In this section, we review and summarize the recent studies in the 3D-printed sensors used for real-time monitoring, soft robotics, and bio-interfaced electronics.

Smart Monitoring Sensors
Figure 6a presents the proposed "smart cap" wireless readout circuits, designed and fabricated using 3D-printed microelectronics and circuits using a combination of UV curable resins and liquid metal filling pastes. [26]A 3D-printed device embedded with an LC tank as a passive wireless sensor was constructed for the rapid detection of the quality of liquid food items under conditions of real-time monitoring. [26]Figure 6b presents the schematic representation of a printable smart glove used to develop sensing units integrated with a liquid metal-based heater and temperature sensor. [16]The multilayers in a 3D-printed glove are composed of stacks of printed substrates, conductive liquids, and other solid-state components.The gloves could be used to realize the on-demand development of customized objects and the personalization of programmable circuits.Thus, the systems could be potentially used in point-of-care settings to aid physical assistants and as therapeutic aids. [16]Wang et al. introduced a new method to develop on-and -off substrate fiber arrays using a onestep approach involving inflight fiber printing (iFP). [68]Figure 6c shows that the polyethylene oxide-sheathed fibers, containing inorganic silver and organic PEDOT:PSS, with diameters in the range of 1-3 μm, were continuously extruded to form fiber arrays.The method involves integrating conducting fibers with the fiberto-circuit connection. [68]The electrical and mechanical properties of the architectures were improved by fabricating an in-plane array of iFP-based conductive fibers and assembling these fibers into 3D structures. [68]The moisture sensor developed from a suspended organic fiber array exhibits superior responsiveness to water molecules over conventional film-based sensing devices that are characterized by small unit volumes and high surfaceto-volume ratios of fiber arrays. [68]Farooqui et al. developed a disposable wireless sensor node for real-time large-area environmental monitoring. [64]Figure 6d shows that the 3D-printed wireless sensor node was fabricated following the inkjet printing process. [64]It was integrated with fully packaged electronic sensors that could sense humidity, temperature, and H 2 S gas simultaneously.The cube-shaped functional sensor packages consisted of multiple 3D antennae for orientation communication and flood monitoring.

Soft Robotics
Sadegui et al. proposed a new type of robot (inspired by plant roots) that exhibits the ability to build its own body, enabling it to move in a granular medium developed following a layerby-layer deposition method. [69]Figure 7a exhibits that each root in the robotic plants is composed of a tubular body, a growing head that is a customized-3D printing system, and a sensor-based tip that controls the behavior of the robot. [69]The deposition of PLA filaments results in the development of a curved architecture that provides the proximity of root grapes. [69]This rootlike 3D soft robot presents interesting characteristics.It can be used for data transmission from a sensor-like root tip that helps monitor soil and pass oxygen, food, and drugs to targets. [69]She et al. fabricated an integrated soft robotic hand with fingers designed from shape memory alloy (SMA) strips. [60]The fingers were movable, and the shape recovery of SMA sensors helped in responsive shape feedback.Figure 7b presents the schematic representation of the Ni-Cr resistance wire, actuator, and sensor embedded into a 3D-printed robot body. [60]The designed robotic fingers can lift weights and recover their original shape before actuation based on the thermal response. [60]Figure 7c presents the somatosensitive soft robotic actuator fabricated following the multi-material 3D printing method. [61]Elastomers as matrices and conductive ionogels as strain-sensitive materials were printed into soft robotic grippers containing embedded haptic feedback sensors and curvature sensors to develop the desired bioinspired sensing platforms and realize actuation. [61]Ge et al. introduced the process of multi-material 3D printing to realize the fabrication of highly sophisticated and hybrid 3D structures developed from stretchable hydrogels covalently bonded with various types of UV-curable polymers. [70]They reported that the hydrogel-polymer hybrid 3D structures fabricated by the DLPbased multi-material 3D printing method could be used in the field of biomedicine. [70]The multi-material 3D printing of cardiovascular stents presented a new way to fabricate multifunctional soft devices and machines with drug-releasing properties.Soft pneumatic actuators with hydrogel also provided a strain-sensing platform. [70]

Bio-Interfaced Sensors
Mannoor et al. designed artificial cyborg ears composed of cellseeded alginate hydrogel as a bionic matrix and silver nanoparticles as a coil antenna for sound transmission. [28]Figure 8a shows the 3D-printed bionic ears, which can detect electromagnetic signals in the gigahertz RF range and receive transmitted RF signals in complementary ears on the opposite side. [28]The 3D printing of bionic ears (in both form and function) validates the fact that the tissue engineering process can be used to develop biointerfaced electronic systems. [28]The use of the bionic ears revealed that it was possible to realize bio-electronic conjunction without following conventional microfabrication processes. [28]omez et al. fabricated PVDF-based prosthetic ears following an additive manufacturing process that uses typical pressure and temperature sensors. [29]The 3D-printed PVDF-based ear is reliable for use under different conditions of pressure (0-16.3kPa) and temperature (0-90 °C). [29]Linear and inversely proportional properties were observed (with respect to frequency) when the external stimuli were pressure and temperature, respectively. [29]sulin et al. reported the development of 3D-printed heat tissues embedded in electronics fabricated using a cardiac cellcontaining extracellular matrix (hydrogel) combined with conductive and dielectric bio-inks. [46]Engineered heart tissues and electronics function simultaneously to record the extracellular potentials of cardiac cells and provide electrical stimulation to continue pacing (Figure 8b). [46]Lind et al. reported the automated design and fabrication method associated with cardiac microphysiological devices (Figure 8c). [62]The use of a combination of the digital manufacturing approach, multi-material printing method, and six functional inks based on piezoresistivity, high conductance, and biocompatible soft materials allows the fabrication of soft strain gauge sensors containing the self-assembly microstructures of laminar cardiac tissues. [62]They confirmed that contractile stresses of tissues could be read out from the electronic displays in the multi-material, printed sensors, and these sensors could respond to distributed drugs. [62]Glennon et al. developed a wearable sweat-monitoring platform embedded with 3D-printed electrochemical sensors and a commercial microfluidic chip designed to allow direct skin contact for sweat sampling. [71]Figure 8d shows that the 3D-printed sweat sensors associated with on-body prototype electronics can be used for the real-time sensing of electrolytes in sweat. [71]The data can be transmitted wirelessly through the incorporated Bluetooth circuits for data visualization and storage. [71]Laszczak et al. designed a 3D-printed capacitive sensor to quantify the pressure and shear stress applied to prosthetic devices.The device could potentially help in knee rehabilitation. [72]Figure 8e presents the miniaturized capacitive sensors loaded on the interface of the stump and prosthetic socket. [72]The sensors transduced the mechanical deformation on the film electrode to capacitive force in the sensor frame and effectively distinguished the two processes for mechanical movements. [72]This could help in classifying patient movement during the rehabilitation process. [72]

Summary
Previously reported 3D-printed electronic sensors were characterized by the structural manifolds enabling to impart arbitrary Applications of sensing and actuation using 3D-printed soft robotics.a) Sensorized tip-integrated robot with several growing roots composed of a magnetic encoder and a motor for the exploration of soil.Reproduced with permission. [69]Copyright 2017, Mary Ann Liebert, Inc. b) 3D-printed soft robotic fingers fully embedded with actuators and sensors for artificial perception.Reproduced with permission. [60]Copyright 2015, The American Society of Mechanical Engineers.c) Soft robotic gripper system for biomimetic somatosensitive actuators (SSAs) fabricated following an embedded 3D printing method.Reproduced with permission. [61]Copyright 2018, Wiley-VCH.
freeform structures to the printable and functionalized composite materials.The sensors present omnidirectional sensing abilities and shape-adaptive properties and can be used for the development of various types of electronic components and devices.Printable materials that present electrical and stimuliresponsive functions formed free-standing 3D architectures of electronic sensors that advance and develop sensing components in diverse polymer matrices.These devices can be fabricated using freeform electronic devices fabricated following extruded and photo-induced 3D printing methods.3D-printed sensors could generate novel sensing platforms with varying structural and electrical properties.The spatial characteristics of the 3D an additive manufacturing process.Reproduced with permission. [28]Copyright 2013, American Chemical Society.b) Heart patches fabricated using conductive and dielectric bio-inks and cardiac cell-containing ECM hydrogels to record extracellular potentials.Reproduced with permission. [46]Copyright 2021, Wiley VCH.c) Cardiac microphysiological device fabricated using six functional inks for the fabrication of piezoresistive, high-conductance, and biocompatible soft materials that can be used for data acquisition and long-term functional studies.Reproduced with permission. [62]Copyright 2017, Springer Nature.d) 3D-printed capacitive sensor to measure pressure and shear stress to study pressure and shear loading properties on a stumpsocket interface composed of an elastomeric material (TangoBlack).Reproduced with permission. [71]Copyright 2015, Elsevier.e) Platforms designed to wirelessly harvest and remotely analyze sweat on the skin surface under conditions of real-time monitoring.Reproduced with permission. [72]Copyright 2016, Wiley VCH.Reproduced with permission. [74]Copyright 2013, Elsevier.Spontaneously foldable silicon sheets formed under internal capillary force (middle), and foldable 2D nanomaterials formed under van der Waals force (right).Reproduced with permission. [75,76]Copyright 2019, AAAS.Copyright 2020, Wiley-VCH.b) FEA results and SEM images of mesostructures deformed under conditions of the pre-strain releasing process associated with the bottom polymer substrate (left). [77]SEM image of the bidirectional (middle) and multidirectional (right) buckling-based mesostructure.Reproduced with permission. [73,77]Copyright 2008, The National Academy of Sciences; Copyright 2016, Wiley-VCH.c) Origami-inspired structure formed following sequential folding (left) or buckling (right) processes.Reproduced with permission. [74,76,77]Copyright 2013, Elsevier BV.Copyright 2020, Wiley-VCH.Copyright 2016, Wiley-VCH.d) Kirigami-inspired structure formed following the sequential buckling of pre-designed and cut patterns.Reproduced with permission. [77,78]Copyright 2016, Wiley-VCH.Copyright 2017, Elsevier.
architecture of the sensors make them suitable for use in the field of soft robotics and biomedicine fields.The functions of the printable materials, including functional filler and polymer matrix, cannot be observed in the materials developed following conventional 2D microfabrication methods.3D-printed bio-interfaced sensors could be used to develop artificial organs with electrical functions that mimic living organs and support healthcare monitoring functions.The increase in the geometrical freedom of fabricating electronics will potentially result in the widening of the application prospects of future 3D electronic sensors that will be used to develop next-generation sensing technology.

Design Strategies for Three-Dimensionalized Electronics
3D electronics, transformed from 2D planar structures following the out-of-plane deformation method, take advantage of the 2D fabrication technologies and 3D functionalities.The 2D planar fabrication method results in 1) reproducibility based on reliable fabrication techniques, 2) scalability from nano-to milli-scale dimensions with high resolution, and 3) programmability under conditions of sophisticated sequence-controlled structuring skills in the case of 3D electronics.The on-demand modified 3D structure helps achieve 1) enhanced sensitivity, 2) sensing range controllability, and 3) unprecedented sensing ability based on the highly complex configuration and the high degree of freedom associated with directional movement.Pre-designed 2D materials and the selection of precise 3D structuring strategies help realize the target features.
2D to 3D structural transformation can be achieved by selecting the appropriate driving force based on structural dimension.][93][94][95] The operating forces depend on the dimensions of the device.101] Millimeter-scale N-isopropylacrylamide (NIPAM) and acrylamide (PAAM) bigel strip bent under conditions of varying acetone concentrations or temperatures. [74]Different degrees of swelling in different gel layers induced responsive stress, resulting in the formation of trap-like closed tongs. [74]ertices or sides of the milli-scale silicon sheets assembled at the center when the volume of the water droplet decreased under the influence of capillary force. [76,84]The edge of the nanoscale graphene sheet moved in a certain direction during folding. [75]he van der Waals force induced the movement under the action of the tip of the scanning tunneling microscope, forming a bilayered graphene structure. [75]echanically induced stress induces scale-free buckling (from the micrometer to the meter scale), as shown in Figure 9b. [73,77]uckling utilizes the driving force in the in-plane direction of 2D materials.The force is different from the folding-associated forces acting along the out-of-plane direction.The release of a certain amount of strain from a pre-stretched elastomer induces the generation of in-plane compressive stress that results in the protrusion of the unattached part of the 2D materials along the out-of-plane direction with low bending stiffness. [77]Buckling shapes can be realized in any scale by effectively controlling the pre-strain of the substrate. [77]The unattached part of SU8 or silicon sheets popped up from the surface of the elastomer. [73,77]his resulted in the formation of wave-like microstructures and starfish-like meter-scale structures under conditions of compressive stress induced by the uni-, bi-, and omni-axially released elastomers. [73,77]onstruction of complex 3D structures requires the use of strategically pre-designed 2D materials and 3D structuring methodologies.Representative examples include origami-and kirigami-inspired structures.Origami involves the development of 3D objects from sheets of paper.The paper sheets are folded to form various structures.Complex sequential folding and buckling steps are executed to transform various 2D materials.Kirigami is associated with cutting 2D materials formed from origami-based structures.This results in the formation of a highly complex structure.The number and location of the folding hinge or buckling sites, the shape of 2D materials, and the force exerting orientation are the design-determining factors in the case of origami. [74,76,77,102]The types of patterns and the number and location of the cuts made in the 2D materials determine the design of the structures formed via kirigami. [77,78]igure 9c presents examples of origami structures that control the number and location of hinges and buckling sites. [74,76,77,103]ruciform low-density polyethylene (LDPE) sheets with five carbon nanotube-integrated pNIPAM hydrogel hinges were used to form a millimeter-scale cube under conditions of responsive stress generated following the absorption of thermal energy at the hinges. [74]This induced the shrinkage of hydrogels. [74]he 2D dodecahedron template consisted of nickel panels and lead hinges that were hierarchically self-assembled at the micrometer-scale. [76,103]The formation of the dodecahedron was driven by the shrinkage of the hinge associated with the melting of lead. [76,103]SU8 sheets with thin hinges located periodically popped out of the plane in the cases of certain micrometer-scale buckling patterns. [77]It is feasible to design polyhedral buckling structures with certain shape-programmed configurations.Car-and soccer-ball-shaped objects and simple angular wave-like structures could be designed following the method. [77]Figure 9d presents examples of kirigami-assisted structures that control the cutting designs of 2D materials. [77,78]ertain pre-programmed structuring and cut patterns in plastic films, and polyimide sheets could be used to form complex buckling configurations, the dimensions of which ranged from the micrometer-to the millimeter-scale. [77,78]The substrate-releasing process mentioned above could be followed to generate these structures. [77,78]he details of the two basic 3D structuring strategies (folding and buckling) and the details of the two complex 3D structuring schemes (origami and kirigami) have been discussed based on the driving forces associated with different dimensions and sizes of the systems in the following sections.

Foldable 3D Structures
The folding strategy is widely used for transforming 2D structures into 3D units.Folding involves the localized bending of a 2D structure that consists of a bendable hinge and a relatively rigid plane. [104]The external force acting on the rigid plane and the internal force at the hinge transforms the 2D materials into 3D structures. [104]The challenge lies in matching the appropriate driving forces with the scales of the transforming structures.[93][94][95] Figure 10a presents an example of a nanometer-scale reversible graphene nanoisland folded under the action of van der Waals forces generated at the nano-controlled tip of a scanning tunneling microscope. [75]The pico-Newton scale force attracted the edge of the graphene units, and the tip dragged the graphene edge along a predetermined direction for the on-demand creation of 1D carbon intramolecular junctions in a fold. [75]Repeatable folding of the graphene nanoislands in arbitrary directions could be realized without causing structural defects. [75]Reconfiguration and tuning of 1D tubular carbon structures characterized by controllable chirality and electronic properties contributed to the development of graphene-based quantum electronic devices. [75]igure 10b presents an example where micrometer-scale folding of an etched hinge-integrated SiN membrane was realized under the action of capillary forces generated in a microscale water droplet. [105]The foldable structure consisted of a wing-shaped SiN membrane on a silicon wafer and a liquid-flowing microfluidic channel integrated into the central region of the wafer. [105]he channel connected the water pump and the surface of the SiN membrane. [105]The nano-Newton-scale capillary force generated following the processes of water injection and water suction (achieved via the microfluidic channels) pulled the SiN side Figure 10.Self-foldable 3D structures formed under the action of driving forces of various scales.a) Illustration (left) and STM image (right) of the foldable graphene nanoisland bent under the action of van der Waals force exerted by the tip of the scanning tunneling microscope along the programmed direction (nanometer scale).Reproduced with permission. [75]Copyright 2019, AAAS.b) Schematic representation of the microfluidic channel-integrated silicon-silicon nitride microstructure present on the silicon wafer used for inducing capillary force during the processes of fluid injection and suction (left). [105]Optical images of the chronological sequences associated with the folding process of a five-faced cubic structure (right).Reproduced with permission. [105]Copyright 2014, The American Institute of Physics.c) Schematic representation of the self-folding mechanism involving the selective expansion of volume in a differently crosslinked polymer layer under conditions of solvent exchange (left). [106]Optical microscopic images of flowershaped solvent-responsive MoS 2 -SU8 (right) (micrometer scale).Reproduced with permission. [106]Copyright 2019, American Chemical Society.
walls. [105]The bending of the silicon nitride hinges resulted in the formation of a programmed cubic microstructure. [105]Ten cycles of folding and unfolding were conducted, and the results revealed the stability of the platform. [105]It could be inferred that the system could be efficiently used as a sensing platform.Such foldable structures, controlled by the properties of the liquid droplets in an on-demand manner, can be potentially used in the field of 3D sensing and actuating engineering.
Figure 10c presents the process of micrometer-scale folding of a SU8 bilayer and a crosslinked SU8 layer. [106]The folding process was conducted under the action of responsive stress, which was induced by the differences in the swelling ratios of the SU8 layers. [106]The higher the degree of crosslinking, the higher the degree of swelling of the SU8 layer. [106]Swelling triggered folding toward the less crosslinked and less swelled SU8 layer. [106]The SU8 layer was characterized by a vertical gradient of crosslinking density, and it folded toward the less crosslinked part following the injection of water. [106]The reversible selffoldable structure that exhibits the property of solvent-induced swelling and deswelling did not require the action of other actuating energies. [106]A MoS 2 -based photodetector was developed on a flower-shaped foldable structure to verify the spatially resolved 3D photodetecting capability of the system under conditions of enhanced light-matter interaction. [106]The results revealed that the systems could be potentially used to fabricate wearable, portable, and energy-harvesting devices.Yoon et al.Various pop-up structures formed under conditions of buckling in the presence of dimension-and shape-controlled 2D precursors.a) Schematic illustration of the buckling process conducted under the action of external mechanical compressive stress in the presence of designed 2D precursors and elastomeric substrate.Reproduced with permission. [98]Copyright 2021, Elsevier.b) FEA results and SEM images of buckled mesostructures consisting of metal (Au) and polymeric (SU8) bilayer.The partial thickness ratio and length ratio of the 2D precursor were controlled.Reproduced with permission. [77]Copyright 2016, Wiley-VCH.c) SEM and optical microscopic images of 3D starfish-like mesostructures reveal the presence of polyimide sheets that span the submicron to meter range.Reproduced with permission. [78]Copyright 2017, Elsevier.d) Design of 2D precursors, red bonding site, FEA results, and SEM images of complex pop-up structures formed by controlling the numbers and locations of the bonding sites (left) and the overall shape (right) of the 2D precursor.Reproduced with permission. [99,108]Copyright 2015, The National Academy of Sciences.Copyright 2015, AAAS.e) SEM images of the complex 3D mesostructures constructed under conditions of omnidirectional buckling.Reproduced with permission. [108]Copyright 2015, AAAS.
reported a similar micrometer-scale foldable structure of pyramidal configurations. [107]The system exploited the swelling properties of the gradually crosslinked pNIPAM-AAc system. [107]The high-density crosslinking area was more responsive to hydrogen ions and thermal energy than the low-density crosslinked areas, realizing the low degree of swelling at low pH and hightemperature conditions. [107]The successful execution of stimuliinduced stress-responsive folding can be attributed to the different responsive swelling properties of the polymer materials.

Buckling-Based Pop-Up 3D Structures
Buckling is a representative 3D structuring strategy that involves out-of-plane deformation.Figure 11a presents the general buckling mechanism observed when compressive stress functions as the external mechanical driving force. [98]Stress is generated following the release of the pre-stretched elastomer substrate. [98]his results in the selective pop-up of non-bonding sites in 2D materials, generating 3D structures. [98]The buckling strategy can be used to form different shapes and dimensions during the formation of 3D structures from 2D structures.The 3D structures can be readily fabricated under the action of mechanical stress by modifying the design of 2D materials.The aspect ratio, overall dimension, thickness ratio of the hinges, and compressive stress-inducing pre-strain of the elastomer can be tuneable factors. [77,78,99,108]an et al. developed 3D mesostructures of various designs at the micro-scale by controlling the degree of pre-stretching of the substrate.Stretching resulted in an increase in the length of the substrate by 1.6-2.6 times (compared to the initial length).The aspect ratio of the 2D precursor increased from 0.05 to 0.27, and the thickness ratio of the hinge increased from one-sixth to one. [98]Figure 11b presents the results obtained by studying various representative final buckling configurations formed by controlling the overall aspect ratio and hinge thickness of Au metaldeposited SU8 sheets and the pre-strain of Dragon Skin. [77]The similarity in the borderline shape observed by studying finite element analysis (FEA) results and SEM images indicated that mesostructures could be formed by controlling various design factors. [77]This indicated that programmable buckling designs could be realized. [77]an et al. reported that it was possible to realize the scalable fabrication of buckling structures of all dimensions under the action of compressive stress. [78]Several 2D materials were developed, and this validated that submicrometer-to-meter-scale structures could be developed under conditions of the same tensile ratio of the pre-stretched substrate. [78]Figure 11c presents an example of a scalable buckling feature studied by analyzing the SEM and optical microscopic images recorded for starfish-like 3D mesostructures consisting of polyimide films. [78]op-up buckling structures that are significantly more complex than wrinkled structures can be developed by controlling various design factors, such as the number and location of bonding sites and the complex shape of the 2D precursors. [99,108]This reveals that the fabrication of on-demand design structures is feasible.Zhang et al. conducted computational and experimental studies on 3D buckled mesostructures of varying shapes. [99]he structures were generated by controlling the bonding siterelated and shape design-related factors. [99,108]It was observed that an increase in the number of bonding sites induced the formation of a large number of 3D wrinkling configurations, and a decrease in the distance between the bonding sites resulted in the outward-spreading protrusion of the structures. [99,108]Various 3D structures can be constructed by controlling the borderline shape of the 2D materials. [99]Figure 11d presents representative examples of mesostructures of varying shapes consisting of SiNM/SU8 sheets. [99,108]The structures were analyzed using the FEA and SEM techniques, and the structural features were tuned by controlling the position of the bonding site and the borderline shape of the 2D precursors. [99,108]The results suggested that the 3D design strategies could be used for material development using theoretically verified programmable materials.Xu et al. developed a one-step further complex mesostructures of varying shapes by periodically assembling different arrays. [108]hey also studied the effect of pre-strain associated with the elastomer on the final shape of the array using the FEA method. [108]igure 11e presents the representative SEM images recorded for the mesostructures of varying shapes. [108]The arrays consisted of monocrystalline silicon sheets, and tents, double-floor helices, a mixed array of tents and tables, and closed-loop filamentary serpentines were designed. [108]The results indicated that the system could be used for the fabrication of various 3D material-based electronic, optoelectronic, and electromagnetic devices.

Origami-and Kirigami-Inspired 3D Structures
[111] The degree of complexity of these structures is higher than that of the structures formed following folding and buckling techniques.Research works on origami-and kirigami-based structures focus on two directions: 1) The enhancement of stability of complex 3D shapes and 2) achieving high-level design complexity.
The stability of complex 3D geometries depends on the materials used, programmed patterns, and the applied force-inducing pre-strain.The types of materials used and the thickness, aspect ratio, and applied pre-strain influence the applied maximum local strain, pop-up height, bending and twisting curvatures, folding angle, and force on each deformed site. [77,96,99,100,108]he appropriate selection of the parameters results in the formation of robust 3D structures from 2D structures.] Materials with high flexibility and durability without defects and hinge/bonding site design with structural stability are key strategies.Lim et al. developed foldable microstructures that exhibited excellent mechanical properties using graphene hinges following the process of reversible structuring. [112]The hinge consisted of a graphene layer and SU8 supporting polymer film exhibiting stable electrical conductivity. [112]Adverse effects are not generated in systems of varying thicknesses when the folding angle is maintained at ≤180°and under conditions of 1% tensile strain. [112]Wagner et al. compared several flexural hinges. [113]They used different materials for their studies and tuned the geometric design parameters to optimize fatigue resistance. [113]The flexural properties, load-carrying capability, and fatigue behavior of aramid fiber composite, polyamide, and photopolymer were studied, and the advantages and disadvantages of using different materials and hinge designs were revealed. [113]Rogers et al. theoretically studied the relationship between 3D structures and 2D design factors using the FEA simulation technique. [77,96,99,100,108]They studied the effect of hinge thickness, the aspect ratio of 2D materials, and the applied pre-strain of buckling-inducing elastomer on the applied strain and force. [77,96,99,100,108]High aspect ratio for shape-deforming sites and low thicknesses of hinge-induced stress concentration increase the folding angle and applied local strain. [77]The studies were conducted assuming perfect fixation of bonding sites. [77]The pre-strain exerted on the elastomer determined the strength of the compressive force exerted on the sample. [77,96,99,100,108]The compressive force is weaker than the adhesion force and results in the generation of a certain folding angle. [77,96,99,100,108]The bending/twisting curvatures associated with the out-of-plane deformed sites were also studied under these conditions. [100,108] The configured structural angles and curvatures result in diverse and unique 3D shapes. [108,100]igh-level complexity in 3D structural design can be achieved following several strategies.The selection of materials, geometry, location of hinges and bonding sites, and overall shape of 2D materials influence the degree of complexity.The usage of physical stimuli responsive materials, proper deployment of the number of hinges and bonding sites, and the design of 2D materials that exhibit a high degree of planar compactness significantly influence structural complexity. [99,108,114]Pandey et al. developed several complex self-assembled 3D polyhedra by designing a folding pathway. [114]The appropriate positioning of nickel panels and solder-integrated hinges in 2D materials enabled the construction of polyhedral. [114]The temperature was controlled to develop the appropriate structures. [114]Xu et al. studied the effects of the number and location of bonding sites on the 3D structure formed under conditions of buckling. [108]A change in the location of the bonding site results in a change in the structural design of the system formed from 2D materials. [108]Zhang et al. studied the effect of the patterns associated with 2D materials on the structural features of 3D assemblies. [99]They changed the number and shape of the cuts to arrive at the results while retaining the overall shape and the position of the bonding sites. [99]igure 12a,b present the representative 3D configuration of polyhedra exploiting functionalized hinges. [114,115]The structures were formed from 2D materials following controlled folding pathways. [114,115]Zhang et al. fabricated a reversely foldable, thermally and optically responsive actuator integrated with LDPE. [115]he actuator at the hinge site consisted of the pNIPAM system and a single-walled CNT composite. [115]The shrinkage of the pNIPAM hydrogel under the influence of thermal energy and the shrinkage of the composite following the absorption of near-IR beams by a CNT was observed. [115]Pandey et al. demonstrated self-assembling 3D structures with more than 10 faces. [114]They proposed new folding pathways following the process of computational modeling. [114]Minimal errors were made during construction when highly compact 2D materials were used, and the shortest paths for the transformation of 2D nets to 3D configurations were chosen. [114]This helped in the generation of highly complex 3D structures. [114]The reported results provided a platform for the generation of stimuli-responsive complex and programmable folding materials.
Figure 12c,d present representative examples of kirigamiinspired buckling structures formed from precut 2D materials. [77,116,117]Blees et al. produced a certain pattern on a graphene sheet and generated a 3D structure by irradiating the system with an IR laser to induce a pico-Newton scale force on the sheet. [116]Such small-scale force-controlled kirigami structures can function as mechanical metamaterials during the fabrication of stretchable electronics and robotics. [116]Yan et al. reported the bidirectional mechanical buckling of complex 3D structures. [77]This was realized under conditions of hierarchical folding. [77]Pre-designed hinges can be used to develop complex structures, such as houses, cars, multi-floor buildings, and soccer balls.The results reveal that the materials can be potentially used to develop nano-and micro-scale structuring technologies. [77]Figure 12e presents a complex 3D structural configuration. [100]The structure was fabricated starting from 2D materials following the layer-by-layer transfer technique. [100]ultilayered 2D materials are transformed into complex 3D cages following the buckling process. [100]This indicates that complex 3D structures can be developed following a functional combination of independent layers. [100]2.Performance of 3D Sensing Electronics Transformed from 2D Materials

Various Functional Electronics
Various transformation methodologies can be used to transform 2D materials into 3D structures to develop sensing electronics providing promising potential.The developed materials retain the advantages of reliable 2D fabrication technologies and possibility of unprecedented sensing ability.The 3D structures obtained from 2D materials transformation exhibit structural features that improve the sensing performance compared to the materials directly fabricated in 3D electronics.2D thin-film materials can be used to construct a skeleton-like out-of-plane deformed structure characterized by ultralow volumetric stiffness and transformation reversibility.These physical properties induce high responsiveness toward the surrounding environment and sensing source.The materials can be potentially used in the field of fabrication of 4D electronics characterized by timedomain transformable structures.Various functional electronics such as physical, optical, and electrochemical sensors can be integrated into the various 3D structures developed from 2D materials.Energy harvesting sensors fabricated following cutting-edge 2D fabrication methods can also be integrated into the systems to study the target properties of the materials.High spatial resolution can be achieved in 3D under these conditions.However, it cannot be achieved in the case of conventional 2D electronics and other 3D sensing platforms.
One of the most representative sensing targets of 3D sensing electronics is the physical properties of the nearby environmental systems.Certain 3D structures can be used to monitor human health by analyzing body movements.Various 3D structures can also be used to study the process of energy harvesting by analyzing environmental vibrations.The multimodal sensing of various external mechanical forces can also be realized using various 3D structures. [118]Kwon et al. integrated a mechanical strain sensor on a pop-up-shaped polymer structure for health monitoring. [118]The system was particularly designed to track calorie expenditure. [118]Figure 13a presents the optical image of a strain sensor and the Bluetooth-based wireless communication circuit-integrated Ecoflex-based sensor characterized by the presence of a pop-up structure at the center. [118]The strain sensor was composed of a UV-curable polyurethane acrylate/polyethylene terephthalate substrate and a 20 nm layer of sputtered Pt on the substrate. [118]Cracks induced by the tensile force were observed on the surface of the substrate. [118]The sensor containing FPCB was equipped with commercial capacitors, resistors, 8-bit microcontrollers, crystal units, rechargeable batteries, and Bluetooth modules soldered onto polyimide films. [118]The sensing system could be used to monitor jiggling, shaking, and walking Figure 12.Origami-and kirigami-inspired highly complex 3D structures formed under conditions of out-of-plane deformation.a) Illustration and optical microscopic images of cube-shaped origami-based structures based on the SWNT-pNIPAM/LDPE bilayer actuator operated under conditions of thermal energy.Reproduced with permission. [115]Copyright 2011, American Chemical Society.b) Schematic diagram of the net geometry.Properties of the melting foldable hinge (left) and SEM images of the self-folding solder-hinge-integrated 3D structures in dodecahedral, truncated octahedral, and icosahedral forms.Reproduced with permission. [114]Copyright 2011, The National Academy of Sciences.c) Optical microscopic images of paper (left top) and fabricated graphene system (left bottom) based kirigami pyramid observed using an infrared laser.Reproduced with permission. [116]Copyright 2015, Springer Nature.d) FEA prediction results and SEM images of pop-up origami structures consisting of plastic (top) and metal (Au)-polymer (SU8) bilayer (bottom) Reproduced with permission. [77]Copyright 2016, Wiley-VCH.e) Illustration of the sequential steps associated with the fabrication of 3D multi-layer mesostructures consisting of a silicon-SU8 bilayer. [100]The systems were fabricated following the transfer printing technique (left). [100]The schematic also presents the corresponding FEA results obtained for overlapped 2D precursors, the final 3D structures consisting of tri-layer materials, and the SEM images (right).Reproduced with permission. [100]Copyright 2016, AAAS.
motions based on the SNR levels (high SNR: >5). [118]The motions were monitored by analyzing the changes in the normalized resistance recorded using the strain sensor. [118]The normalized values changed up to a value of thousands, and the values were converted to obtain calorie expenditure. [118]The integration of a sensitive strain sensor into a 3D structure for the analysis of high-strain-range real-life conditions makes it possible to trace the dynamic motion of the body.This is difficult to achieve using commercial 2D strain sensors.Han et al. fabricated piezoelectric systems based on complex 3D frameworks for sensing vibrational energy-harvesting procedures. [119]Figure 13b presents the overall shape of the 3D framework. [119]The framework consists of a PVDF layer as the piezoelectric polymer. [119]The Cr/Au layers on the top and bottom surfaces functioned as the electrode metal, and the parylene-C film functioned as the neutral plane shifting supporting layer. [119]The parylene-C/Au/Cr/PVDF/Cr/Au film was used to construct a 3D structure by mechanically releasing the stress of the pre-stretched Ecoflex substrate. [119]The sensor developed for harvesting piezoelectric energy could be used to monitor the vibrational motion of a mass block. [119]This could be realized by analyzing the changes in voltage (21.5 mV; RMS value) under conditions of readily movable ultralow stiffness Various types of functional electronics-integrated shape deformable 3D structures.a) Optical images of patch-type calorie expenditure measurement device integrated with pop-up nanoscale crack-based strain sensor (left) and strain monitoring signal obtained for jiggling, shaking, and walking (right).Reproduced with permission. [118]Copyright 2019, Wiley-VCH.b) Schematic representation of the 3D PVDF mesostructure that consists of a piezoelectric layer and a sandwich electrode layer for harvesting energy (left) and the output voltage generated by the energy harvester during the process of 3D vibration.Reproduced with permission. [119]Copyright 2019, Springer Nature.c) Optical microscopic images recorded for the 2D precursor used to fabricate the multimodal sensing piezoresistive structure containing two directional four piezoresistors (left). [120]The pressure (middle) and shear force (right) were sensed as a function of four piezoresistive strain gauges.Reproduced with permission. [120]Copyright 2019, American Chemical Society.d) Optical microscopic images of the PDMS-based 3D microvascular network integrated with an array of thermistors (inset). [121]ouble-layered PDMS-based microfluidic channels enabled the flow of hot water.The temperature distribution was mapped using an infrared thermal imaging camera (right top) and an integrated array of 16 thermistors (right bottom).Reproduced with permission. [121]Copyright 2021, AAAS.e) Schematic representation and optical microscopic images of 3D multifunctional mesoscale framework integrated with a microelectrode, μ-ILED, thermal actuator and sensor, and electrochemical oxygen sensor.Reproduced with permission. [122]Copyright 2021, AAAS.f) Illustration of the pH, UV, and heavy metal indicator-integrated PLGA microfliers and the colorimetric assays conducted based on the RGB analysis of the corresponding digital images.Changes were recorded based on the changes in the pH, UV dosage, and type of heavy metals (left top, middle top, and right top, respectively).Reproduced with permission. [123]Copyright 2022, AAAS.g) Colorized SEM image recorded for the MoS 2 photodetectors consisting of MoS 2 , graphene, and SU8 materials (left) and the I-V characteristics of the 3D hemispherical photodetector showing the range of bias voltage (right).Reproduced with permission. [124]Copyright 2018, Springer Nature.
layouts. [119]The readily vibrating 3D geometries are highly sensitive and functional in the frequency range of 5-500 Hz. [119] High sensitivity in this frequency range cannot be achieved using previously reported 2D materials. [119]Won et al. reported a multimodal dynamic motion-sensing 3D piezoresistive structure integrated with four microelectromechanical sensors. [120]Figure 13c presents the schematic illustration a sensing circuit fabricated on a polyimide substrate from 2D systems. [120]The system consisted of bidirectional monocrystalline silicon nanomembranebased strain sensors. [120]The piezoresistive properties and Cr/Au conductive lines were exploited for the analysis of several mechanical stimuli. [120]Multi-mode sensing (normal force, shear force, and pressure sensing) can be achieved by characterizing the sensing response in the form of a change in normalized resistance (in the order of 10 −3 ). [120]The free movement of the sensing structure across multiple directions results in multimodal sensing, allowing the utilization of the device in a limited space. [120]uan et al. demonstrated temperature mapping with a thermistor array-integrated 3D microvascular network, which consists of double-layered PDMS. [121]Figure 13d shows the optical microscopic images of the PDMS-based 3D microfluidic system and an enlarged thermistor on the microchannel. [121]The microfabricated thermistor consisted of serpentine gold traces encapsulated with polyimide films. [121]The selective flow of hot water through each channel resulted in a certain extent of thermal distribution that could be detected by the array of 16 thermistors. [121]Such integration of electronic elements on an on-demand 3D deforming microfluidic system helps in the monitoring of the fluid transportation process. [121]omprehensive sensing and actuation of several properties, such as conductance, induced current, voltage, chemical concentration, and temperature, can be realized using the sensing and actuating component-integrated 3D structure.Kim et al. integrated UV sensors, microcontrollers, and NFC coils into 3D macrofliers to determine air cleanliness by monitoring the change in the voltage induced by the time domain. [125]A 2D precursor consisting of a shape memory polymer film, a coil-shaped etched copper foil, a commercial NFC chip, photodiodes, a set of MOSFETs, and capacitors was used to develop 3D structures. [125]ir containing a certain concentration of dust absorbs sunlight proportional to the dust concentration. [125]This affects the voltage generated (range: 0-130 mV). [125]The amount of voltage generated over 1 h increases with an increase in the air clarity. [125]ark et al. integrated various sensors into buckling-based 3D frameworks to study electrical, optical, chemical, and thermal properties to monitor multifunctional neural interfaces associated with cortical spheroids. [122]Figure 13e presents the structure of the polyimide substrate-based 3D framework integrated with several 2D functional electronics. [122]Pt black-based microelectrode was used for neural local signal sensing, commercial micro-ILED was used for optogenetic neural stimulation, serpentine Au was used for thermal actuation and sensing, and electrochemical oxygen sensors consisting of Pt black, Au, and Ag/AgCl as the working, counter, and reference electrodes, respectively, were used for monitoring the oxygen concentrations in culture media. [122]The stimulating and sensing frameworks were used to investigate the neural activities of cortical spheroids and engineered assemblies. [122]The analysis was conducted by detecting field potentials ranging from tens of microvolts to several milli-volts within the 3D state. [122]Investigation (in vitro) of the 3D asis state of brain parts can help develop the field of neuroscience and assist in researching unknown neural networks.Yoon et al. fabricated pH, UV, and heavy metal indicator-integrated 3D microflier to monitor environmental conditions, including hydrogen ion concentration, UV exposure intensity, and heavy metal concentration. [123]Figure 13f presents the representative illustration of color-changing microfliers and the corresponding data on pH, UV dosage, and heavy metal type.Concentration was monitored based on the colorimetric assays conducted following the RGB analysis method. [123]The materials used to develop 3D microfliers consisted of a PLGA-based flier body and cellulose ester membranes mixed with anthocyanin, spirooxazine, and dithizone solution (for pH, UV, and heavy metal detection, respectively). [123]The microflier enabled the monitoring of environment conditions.The image-based readout process and the processes of image capture and reconstruction (based on a color correction algorithm) were also used to obtain the results. [123]ptical properties can also be monitored by integrating photodetecting electronics into on-demand 3D structures.Lee et al. fabricated photodetector-integrated 3D architectures to realize the 3D imaging of incident light. [124]Figure 13g presents a SU8 polymer sheet-based hemispherical buckling structure integrated with a monolayer of MoS 2 photodetectors and double-layer graphene electrodes. [124]When light rays of varying power (range: 0-10 3 W m −2 ) are irradiated onto the system, the flow of 0-1 A of current is induced in the photodetector (applied voltage: −3-3 V). [124] This allows pathway mapping of the incident light. [124]he data collected from the detector array present on the hemispherical surface are used to arrive at the results. [124]The precise mapping of the pathway of the incident light can be realized using 3D sensing platforms characterized by particular features.

Versatile Sensing Performance
The construction of 3D structures and the development of integration technologies for the development of various functional electronics can help in the fabrication of 3D devices with excellent sensing and monitoring.The 3D sensing platforms can be used to control the sensing range and improve the sensitivity of the materials.
][128] A 3D pop-up height-controlled nano-crack-based strain sensor (Figure 14a) can be used to control the strain-sensing range effectively. [118]The pre-cracked Pt metal film was used as a conductive layer on a PUA-PET substrate composed of an ultrasensitive strain sensor. [118]The sensor could be used to detect a maximum of 2% tensile strain and a knee angle of 5°. [118]The sensing range increased to a knee angle of 95°with an increase in the pop-up height (up to 3.1 mm). [118]This was accompanied by a decrease in the strain on the sensor. [118]The sensing range can be widened to effectively detect dynamic motion by controlling the structural design of the materials. [118]igure 14.Versatile sensing performance of shape deformable platforms.a) Optical images of pop-up-shaped crack-based strain sensors with varying heights (top) and angle range limitations for sensing during the bending of the knee (bottom).Reproduced with permission. [118]Copyright 2019, Wiley-VCH.b) Sensing highly sensitive indentation force using a Wheatstone bridge circuit integrated pop-up 3D framework (inset) exhibiting a high degree of freedom for structural deformation.Reproduced with permission. [129]Copyright 2021, The National Academy of Sciences.c) Continuous and simultaneous monitoring of normal pressure, shear force, and bending force using mechanically-buckled multimodal sensors (inset).Reproduced with permission. [120]Copyright 2019, American Chemical Society.d) Harvesting of out-of-plane and in-plane dual mode multidirectional vibration energy using 3D serpentine mesoscale frameworks (inset).Reproduced with permission. [119]Copyright 2019, Springer Nature.e) Illustration of the 3D photodetecting platform that can be penetrated by laser beams (left), the distribution mapping of 3D photocurrent on the hemispherical surface of the platform (middle), and the process of analysis of the direction of incidence of laser beams (right).Reproduced with permission. [124]Copyright 2018, Springer Nature.
The effect of pre-strain is opposite to the effect observed when the sensing range is controlled.The introduction of pre-strain can help significantly improve the sensitivity of the system.This can be achieved by studying the changes in the sensing gauge factor with varying exerted force.Construction of 3D frameworks induced pre-strain on the Au/Cr/PI strain sensor, and this resulted in a change in the normalized voltage at the initial exertion of the indentation force, allowing highly sensitive sensing of strain induced by forces in the range of 0-0.6 mN (Figure 14b). [129]Similar results were not obtained using 2D-based sensors. [129]tralow stiffness, omnidirectional deformation, and 3D localization of the sensing source characterize 3D structured sensing electronics, and these properties help in imparting unprecedented sensing ability to the materials.The 3D piezoresistive sensing platform integrated with silicon nanomembrane-based strain sensors and Au electrodes on a patterned polyimide sheet (inset, Figure 14c) deforms freely along all directions without any spatial constraints. [120]Analysis of the changes in the normalized resistance corresponding to the four sensors enables the detection of several mechanical stimuli, such as shear stress, poking stress, pressure, and holding. [120]Another example of multimodal sensing is shown in Figure 14d, which presents a 3D microstructured piezoelectric sensing system. [119]The sensing structure consists of three layers (the Cr-Au/PVDF/Cr-Au layer as the top electrode, the piezoelectric layer, and the bottom electrode layer). [119]The energy harvesting processes associated with outof-plane and in-plane vibrations were independently studied, and the results confirmed that the devices could be used to sense the process of multi-directional energy harvesting. [119]Spatiotemporal sensing of mechanical stimuli could be realized using the systems, and this revealed that the sensing platform could be effectively used in space-constrained regions. [119]The 3D localization of the initial and final pathway points of incident light can be achieved using the photodetector array-integrated 3D hemispherical structure (Figure 14e). [124]Incident light induces the flow of current (range: 0-1 μA) in the near photo-detecting sensors consisting of a MoS 2 /SU8 bilayer. [124]The amount of current generated was proportional to the distance of the system from the incident point. [124]The interpolation process was used to identify the points of illumination, and the process was used to analyze the final pathway point. [124]Optoelectronic technologies, such as light imaging methods, involving the use of complex and scalable structures, can be used to realize the process of light source tracing efficiently.

Robotic, Bio-Interfacial, and Environmental Sensing Applications
Certain 3D structured sensing platforms can be efficiently used in the fields of fabrication of robotics, bio-interface electronics, and environment monitoring electronics as suitable physical properties can be introduced into the materials from the transformed structure.Mechanical sensing range controllability, sensitivity enhancement, and unprecedented sensing are possible compared to conventional 2D sensors.The unprecedented sensing ability of the materials can be exploited to obtain insight into certain phenomena (including in vitro phenomena).A comprehensive understanding of the material helps obtain information on unknown and multifunctional properties.Several examples of 3D sensing platforms used in the fields of robotics, bioelectronics, and environmental electronics are discussed in this section.
3D multifunctional structure on the robot arm fingertip in Figure 15a consists of asymmetrically buckled Cr-Au/PVDF/Cr-Au triple layer sheets and Ecoflex elastomer sensing external impact in terms of changes in the piezoelectric voltage. [119]Free deformation along the omni-direction and the asymmetric configuration of the system enables the analysis of multimodally sensed data obtained for mechanical stimuli. [119]Data on local pressure, normal force, directional stretching, and bending (induced by environmental sources) were obtained. [119]Multimodal mechanical sensing provides guidelines (based on space-efficiency and high-level integration) for the advancement of the field associated with the development of robotic prosthetic interfaces. [119]an et al. fabricated a microscale crab-like 3D terrestrial robot which performs submillimeter-scale locomotory activities using a temperature-dependent reversibly shape-changing SMA-based polyimide film body. [130]The terrestrial robot detected timedependent local heating and induced normal contact force on the corresponding location by exploiting the phase transition prop-erty of the SMA. [130]Such thermal sensing and micro-actuating properties reveal the application prospects of micro-robots in the field of telemetry.The materials can be used for localization and mobile sensing in a constrained space. [130]mplanted 3D piezoelectric platforms can also interact with biological systems.The sensing platform presented in Figure 15b was implanted in the hind leg of a mouse to monitor the normal locomotor activity of the organism. [119]The reduction of the supporting legs of the buckled 3D framework results in ultralow stiffness, inducing an ultrasensitive response to external forcebased vibration. [119]A prominent signal is generated when the organism trots and climbs, indicating that the contractile motions of the surrounding muscles can be identified using these systems. [119]These results could not be obtained using 2D sensors.The ultrasensitive sensing ability of the systems offers a comprehensive and fundamental understanding of certain phenomena when early-stage sensing is realized. [119]he 3D platforms can be used to realize on-demand scalability and shape tunability.On-demand programmable structuring realizes 3D state as-is measurement of sensing objects providing real-state undistorted sensing.The 3D electronic scaffolds consist of Cr/Au/TiN tri-layer-based electrode-integrated polyimide sheets and serve as a growing platform for neural networks of the dorsal root ganglion cells of rats (Figure 15c). [131]The generation of extracellular action potential in dorsal root ganglion neurons is stimulated, and the spike signals are recorded using the electrodes. [131]The results can help in understanding the fundamental biological principles associated with the processes and the application prospects of the devices providing active and 3D templated sensing platforms. [131]It is possible to expand the scales of the systems from single-cell to spheroidal.The cortical spheroidenclosing 3D multifunctional mesoscale framework (Figure 15d) consisted of an array of Pt black-based microelectrode-integrated buckled polyimide sheets. [122]This system recorded electrophysiological responses, inferring the field potentials that include the development of the field of advanced neuroscience. [122]3D sensing platforms can be used for bio-interfacial applications to realize material transport.Luan et al. reported that oxygen and macromolecular nutrients (bovine serum albumin) could be transported through a microfluidic channel in the 3D artificial microvascular system integrated with an oxygen-sensitive hydrogel and the microporous structure of the channel. [121]Liquid flow through the microfluidic channel delivers macro and micro materials.A certain concentration of the materials diffuses through the microporous PDMS layer. [121]The material-transporting ability of the system can be exploited to develop a 3D microvascular system that can be used in a tissue culture setup and for researching artificial organ-based materials. [121]Realization of an in vitro semi-real bio state helps us understand previously unexplored research fields.It also helps in the development of various research materials.
The 3D colorimetric mesoflier, including pH, heavy metals, and UV indicators, can be utilized to monitor the environmental conditions. [123]Figure 15e presents data on outdoor rain, mercury-added solution, and solar UV exposure. [123]The change in pH (from neutral to acidic), the presence of mercury with dithizone, and the exposure dose of UV at a wavelength of 365 nm resulted in sensing color transitions which are well consistent with the comparable measurements using commercial strips and Robotic, bio-interfacial, and environmental sensing applications; range from wearable/implantable to biomolecular targets.a) Optical images of the 3D piezoelectric energy harvesting and mechanical sensing platform associated with the robot surface (top left) and the magnified images (top right). [119]Monitoring of the pressing (bottom left), stretching, and bending (bottom right) motions of the robot finger by detecting the changes in the output voltage.Reproduced with permission. [119]Copyright 2019, Springer Nature.b) Illustration of the location of implantation in mice (top left) and the optical image of the implanted 3D energy harvester (top right). [119]Monitoring of the movement and behavior of mice by detecting output voltage during the processes of trotting and climbing (bottom).Reproduced with permission. [119]Copyright 2019, Springer Nature.c) Optical and confocal fluorescence microscopic images recorded for 3D electronic scaffolds consisting of eight separated electrodes associated with dorsal root ganglion neural networks (top). [131]Stimulation recording of extracellular action potentials associated with the dorsal root ganglion neurons and the specific response spikes (bottom).Reproduced with permission. [131]Copyright 2017, The National Academy of Sciences.d) Optical and confocal microscopic images of 3D multifunctional mesoscale frameworks containing cortical spheroids for recording electrophysiological (top) and action potentials at each microelectrode (bottom).Reproduced with permission. [122]Copyright 2021, AAAS.e) Optical microscopic images of the pH level (left), heavy metal type, heavy metal concentration (middle), and UV intensity (right) indicating that 3D colorimetric mesofliers detecting acidic rain, Hg concentration, and UV exposure intensity, respectively. [123]The insets present the comparison of the results obtained by conducting colorimetric assays and using commercial tools.Reproduced with permission. [123]Copyright 2022, AAAS.
kits. [123]Each color transition was induced by pH-, Hg-, and UVdependent structural changes in anthocyanin, dithizone chelate, and a photochromic dye. [123]The real-time sensing data and the mapping and monitoring results represent the great advances made in the field of large-area monitoring technology. [123]

Outlook
Various designs and diversely constructed 3D sensing platforms have been developed over the last few years.Numerous researchers emphasized the necessity of developing 3D materials that can be used to 1) fabricate on-demand complex and conformal form factors, 2) achieve high 3D spatial density under conditions of high degree integration, and 3) realize the highly efficient configuration of structural materials.3D platforms integrated with versatile sensing electronics present diverse functionalities and can be used for advanced applications.The developed 3D platforms can be used to conduct research related to 1) diverse applicability including customized implantable devices for monitoring/diagnosis/treatment and robotic devices for sensing/actuating, 2) availability of multi-functioning in a limited space, and 3) measurability of unprecedented properties based on the 3D configuration of materials.
Mainstream of current 3D sensing electronics includes 3D printing electronics and 2D to 3D shape deformable electronics that are developed in perspective of structure formation, sensing and supporting materials, materials composition, and properties of developed systems.Versatile properties and characteristics of the materials have been reported.However, the application prospects of the materials are limited by the problems associated with material selection and 3D structure-inducing features.Herein, we present four challenges associated with the field that need to be addressed in the future.
Various materials should be developed for performance diversification of 3D sensing electronics specifically related to the 3D printing technology.A limited choice of materials hinders the application prospects of the materials in the field of developing sensing platforms.Previously used printing fillers consisted of solid-state conductive nanoparticles in the resin matrix, and these could not be used for the development of gas-state or hightemperature operable materials.Further research should be conducted to optimize the properties of various unused printing materials in their composite state.The homogeneous properties of the materials should be studied regardless of the device dimension, gravity, and temperature effects.
It is challenging to overcome the fundamental structural limitations of 3D electronics.The elastomer substrate, attached to the skeleton-like configuration of 2D to 3D transformable electronics, hinder the application prospects of the materials.Most buckling-based structures consist of elastomeric substrates under complex programmed structures.This hinders the application prospects of the materials and the process of integration of the system into various sensing targets.The free-standing structuring of complex 3D geometries and the optimization of compatible shape-fixing thin film materials should be focused on in the near future.Elastomer substrate-free simple buckling structures based on shape-memory polymer sheets have been recently developed.Structures fabricated following the deposition of thick oxide layers on overall structures have also been reported. [125,130]he skeleton-like configuration imparts low volumetric density.A high degree of 3D spatial integration cannot be achieved under these conditions.Research should be conducted to maximize the utilization of 3D space and integrate multiplexing 2D electronics to address these problems.
Mechanical and electrical stability of structural materials is critical challenges on overall 3D sensing electronics.The currently used 3D printing methods involve the random percolation of conductive materials into the base resin.This results in strainstimuli-sensitive vulnerable conductivity.Multilayered combinations of nanowires, flakes, and organic conductors which solidify under optimized conditions can be potentially used to fabricate the appropriate systems.Folding-based 3D sensing electronics also presents limitations related to the mechanical stability of hinges and creases.The generation of an excess of local strain can be observed under these conditions.Appropriate materials should be identified, and optimal structural design should be proposed to realize the stable operation of integrated electronics with reliable mechanical and electrical properties.Some research groups have reported electrically stable materials and structural designs to address the problem of mechanical fatigue.The results reveal that further research should be conducted to develop versatile 2D electronics-integrated 3D sensing platforms. [112,113]t is critical to achieve high spatial resolution at the microscale level for the 3D systems to realize microscopic applications.Challenges related to scalability at the nanoscale can be addressed if the morphologies of the systems fabricated following the process of 3D printing are understood.The problems associated with the systems can be attributed to the bottom-up extrusion (conducted through nozzles) and photo-curing processes.The development of nanoscale filler extrusion and light irradiation techniques can help advance the field of 3D printing research.Scalability issue also emerged in 2D to 3D transformable electronics showing controllability issue of selective adhesion between the elastomer substrate and 2D materials in the case of buckling.The selective adhesion force associated with the bonding/non-bonding sites should be controlled to achieve the desired results.The reported buckling structures have limitations in under micro-scale fabrication because exertion of adhesion forces of exact position and size are difficult in nano-scale dimension.The development of highresolution adhesion force-exerting fabrication, including control of unnecessary van der Waals force and capillary force, is the remaining area of the 3D electronics.
Previous works of 3D sensing electronics help in the expansion of the spatial sensing dimension, and following the promising direction of the sensing electronics is in extendibility related to the time domain.Time-dependent modulation of material properties and structural design imparts unprecedented characteristics to the sensing platform.The spatiotemporal 4D electronics exhibits various properties, such as on-demand reversible and repeatable configuration transformability, material degradability, and on-demand decomposability and recyclability.The developed sensing electronics can be used to study in vitro and in vivo targets.These can also find their applications in the field of robotics.The use of these materials can improve the quality of life of humans as these can be used to realize unprecedented sensing, monitoring, and actuating properties.

Figure 1 .
Figure 1.Conceptual schematics of 3D electronic sensors and methodological classification for the next-generation sensing applications.

Figure 2 .
Figure 2. Characteristics of 3D-printed electronics and comparison between 3D-printed electronics and 2D electronics.

Figure 7 .
Figure 7. Applications of sensing and actuation using 3D-printed soft robotics.a) Sensorized tip-integrated robot with several growing roots composed of a magnetic encoder and a motor for the exploration of soil.Reproduced with permission.[69]Copyright 2017, Mary Ann Liebert, Inc. b) 3D-printed soft robotic fingers fully embedded with actuators and sensors for artificial perception.Reproduced with permission.[60]Copyright 2015, The American Society of Mechanical Engineers.c) Soft robotic gripper system for biomimetic somatosensitive actuators (SSAs) fabricated following an embedded 3D printing method.Reproduced with permission.[61]Copyright 2018, Wiley-VCH.

Figure 8 .
Figure 8. 3D-printed sensing platforms for bio-interface applications.a) Bionic ear for transmitting and responding to RF signals fabricated followingan additive manufacturing process.Reproduced with permission.[28]Copyright 2013, American Chemical Society.b) Heart patches fabricated using conductive and dielectric bio-inks and cardiac cell-containing ECM hydrogels to record extracellular potentials.Reproduced with permission.[46]Copyright 2021, Wiley VCH.c) Cardiac microphysiological device fabricated using six functional inks for the fabrication of piezoresistive, high-conductance, and biocompatible soft materials that can be used for data acquisition and long-term functional studies.Reproduced with permission.[62]Copyright 2017, Springer Nature.d) 3D-printed capacitive sensor to measure pressure and shear stress to study pressure and shear loading properties on a stumpsocket interface composed of an elastomeric material (TangoBlack).Reproduced with permission.[71]Copyright 2015, Elsevier.e) Platforms designed to wirelessly harvest and remotely analyze sweat on the skin surface under conditions of real-time monitoring.Reproduced with permission.[72]Copyright 2016, Wiley VCH.

Figure 11 .
Figure11.Various pop-up structures formed under conditions of buckling in the presence of dimension-and shape-controlled 2D precursors.a) Schematic illustration of the buckling process conducted under the action of external mechanical compressive stress in the presence of designed 2D precursors and elastomeric substrate.Reproduced with permission.[98]Copyright 2021, Elsevier.b) FEA results and SEM images of buckled mesostructures consisting of metal (Au) and polymeric (SU8) bilayer.The partial thickness ratio and length ratio of the 2D precursor were controlled.Reproduced with permission.[77]Copyright 2016, Wiley-VCH.c) SEM and optical microscopic images of 3D starfish-like mesostructures reveal the presence of polyimide sheets that span the submicron to meter range.Reproduced with permission.[78]Copyright 2017, Elsevier.d) Design of 2D precursors, red bonding site, FEA results, and SEM images of complex pop-up structures formed by controlling the numbers and locations of the bonding sites (left) and the overall shape (right) of the 2D precursor.Reproduced with permission.[99,108]Copyright 2015, The National Academy of Sciences.Copyright 2015, AAAS.e) SEM images of the complex 3D mesostructures constructed under conditions of omnidirectional buckling.Reproduced with permission.[108]Copyright 2015, AAAS.

Figure 13 .
Figure 13.Various types of functional electronics-integrated shape deformable 3D structures.a) Optical images of patch-type calorie expenditure measurement device integrated with pop-up nanoscale crack-based strain sensor (left) and strain monitoring signal obtained for jiggling, shaking, and walking (right).Reproduced with permission.[118]Copyright 2019, Wiley-VCH.b) Schematic representation of the 3D PVDF mesostructure that consists of a piezoelectric layer and a sandwich electrode layer for harvesting energy (left) and the output voltage generated by the energy harvester during the process of 3D vibration.Reproduced with permission.[119]Copyright 2019, Springer Nature.c) Optical microscopic images recorded for the 2D precursor used to fabricate the multimodal sensing piezoresistive structure containing two directional four piezoresistors (left).[120]The pressure (middle) and shear force (right) were sensed as a function of four piezoresistive strain gauges.Reproduced with permission.[120]Copyright 2019, American Chemical Society.d) Optical microscopic images of the PDMS-based 3D microvascular network integrated with an array of thermistors (inset).[121]Double-layered PDMS-based microfluidic channels enabled the flow of hot water.The temperature distribution was mapped using an infrared thermal imaging camera (right top) and an integrated array of 16 thermistors (right bottom).Reproduced with permission.[121]Copyright 2021, AAAS.e) Schematic representation and optical microscopic images of 3D multifunctional mesoscale framework integrated with a microelectrode, μ-ILED, thermal actuator and sensor, and electrochemical oxygen sensor.Reproduced with permission.[122]Copyright 2021, AAAS.f) Illustration of the pH, UV, and heavy metal indicator-integrated PLGA microfliers and the colorimetric assays conducted based on the RGB analysis of the corresponding digital images.Changes were recorded based on the changes in the pH, UV dosage, and type of heavy metals (left top, middle top, and right top, respectively).Reproduced with permission.[123]Copyright 2022, AAAS.g) Colorized SEM image recorded for the MoS 2 photodetectors consisting of MoS 2 , graphene, and SU8 materials (left) and the I-V characteristics of the 3D hemispherical photodetector showing the range of bias voltage (right).Reproduced with permission.[124]Copyright 2018, Springer Nature.

Figure 15 .
Figure 15.Robotic, bio-interfacial, and environmental sensing applications; range from wearable/implantable to biomolecular targets.a) Optical images of the 3D piezoelectric energy harvesting and mechanical sensing platform associated with the robot surface (top left) and the magnified images (top right).[119]Monitoring of the pressing (bottom left), stretching, and bending (bottom right) motions of the robot finger by detecting the changes in the output voltage.Reproduced with permission.[119]Copyright 2019, Springer Nature.b) Illustration of the location of implantation in mice (top left) and the optical image of the implanted 3D energy harvester (top right).[119]Monitoring of the movement and behavior of mice by detecting output voltage during the processes of trotting and climbing (bottom).Reproduced with permission.[119]Copyright 2019, Springer Nature.c) Optical and confocal fluorescence microscopic images recorded for 3D electronic scaffolds consisting of eight separated electrodes associated with dorsal root ganglion neural networks (top).[131]Stimulation recording of extracellular action potentials associated with the dorsal root ganglion neurons and the specific response spikes (bottom).Reproduced with permission.[131]Copyright 2017, The National Academy of Sciences.d) Optical and confocal microscopic images of 3D multifunctional mesoscale frameworks containing cortical spheroids for recording electrophysiological (top) and action potentials at each microelectrode (bottom).Reproduced with permission.[122]Copyright 2021, AAAS.e) Optical microscopic images of the pH level (left), heavy metal type, heavy metal concentration (middle), and UV intensity (right) indicating that 3D colorimetric mesofliers detecting acidic rain, Hg concentration, and UV exposure intensity, respectively.[123]The insets present the comparison of the results obtained by conducting colorimetric assays and using commercial tools.Reproduced with permission.[123]Copyright 2022, AAAS.