Hydrogel‐Based Multifunctional Soft Electronics with Distributed Sensing Units: A Review

Soft electronics have attracted great interest owing to their potential applications in electronic skins, implanted devices, soft robotics, etc. Among the various soft materials, hydrogels are recognized as an ideal building block of soft electronics due to their tissue‐like physicochemical properties, abundant stimuli‐responses, and excellent mechanical compliances. Compared to elastomers, hydrogels containing large amounts of water exhibit better mechanical match to soft tissues and permeability to hydrophilic molecules. To date, most hydrogel‐based soft electronics (HSE) are facilely developed using a bulk conductive gel as the sensing unit, different from the elastomer‐based soft electronics with sophisticated integration of electronic elements. To advance their applications in engineering and biomedical fields, it is significant to devise hydrogel electronics with distributed sensing units. This Review summarizes the fabrication and applications of HSE, focusing on the multifunctional HSE with patterned conductive circuits and distributed sensing units. First, the fabrication of single‐functional soft electronics is briefly introduced with a bulk gel as the building block, including strain, temperature, chemical, and proximity sensors. Then, the approaches to integrating multiple sensing units into one hydrogel are summarized with examples of applications. Finally, perspectives are given on future directions and potential challenges in this field.


Introduction
Soft electronics that can integrate with the human body and collect information are central for developing the Internet of Everything. To obtain high-quality and stable signals, the soft electronics should be capable of forming a seamless interface with human body and having mechanical compliance with repeated DOI: 10.1002/adsr.202200069 deformations of skins or biotissues. Therefore, soft electronics need good stretchability to conform complex surfaces with non-Gaussian curvature. To impart stretchability into soft electronics, elastomers are usually exploited as the matrix of conductive fillers or the substrate of sensing elements, which enables versatile stretchable electronics with broad applications such as in eskins, intelligent healthcare, smart display, and neuromodulation. [1][2][3][4] However, the mechanical discrepancies between elastomers with soft and wet biological tissues and their poor permeability to hydrophilic molecules and nutrients hamper their applications as implanted devices.
Compared to elastomers, hydrogels consist of a hydrophilic network and a large amount of water, showing high similarity to soft biotissues, multiple responses to external stimuli, and good permeability to water and nutrients. [5][6][7][8][9][10][11][12][13][14] After the breakthrough in the design of tough gels, hydrogels with tunable mechanical properties are recognized as an ideal material for devising stretchable soft electronics. [11][12][13][14] Sensing functions can be achieved in hydrogels using i) conductive hydrogels as active sensing elements or ii) non-conductive as the passive substrate of electronic components. Conductive hydrogels can be classified into ionically conductive gels and electronically conductive gels. Ionically conductive gels are developed by incorporating salts (e.g., LiCl) into the gel matrixes. [15,16] Directional migration of mobile ions in the hydrogel activated by an electrical field enables sensing functions with applications such as strain sensors. Considering the unique scenario of bioelectronic activities with ion migration in the human body, ionically conductive hydrogel is an ideal candidate to fabricate implanted devices. However, the relatively low conductivity (10 −3 -10 1 S m −1 ) and release of ions to body fluids are two major challenges for practical implanted applications. Electronically conductive gels can be developed by incorporating conductive fillers or conductive polymers into the hydrogel to form percolated network. [17] However, high content of fillers is required to obtain satisfied conductivity, compromising the composite hydrogels' mechanical compliance. Regarding the incorporation of conductive polymers, one obstacle is the low conductivity (10 −1 -10 3 S m −1 ) resulting from the non-conductive hydrogel network's electrical insulation. For hydrogel-based soft electronics (HSE) Figure 1. Characteristics of hydrogels. a) Highly stretchable and flexible hydrogels. Reproduced with permission. [5] Copyright 2014, Nature Publishing Group. b) Self-healing ability of dynamically cross-linked hydrogels. Reproduced with permission. [8] Copyright 2018, American Chemical Society. c) Degradability of biopolymer-based hydrogels. Reproduced with permission. [9] Copyright 2020, Nature Publishing Group. d) Biomimetic 4D-printed shape-morphing of hydrogels based on temperature responsiveness. Reproduced with permission. [10] Copyright 2016, Nature Publishing Group. e) Tough poly(2-acrylamido-2-methylpropane-sulfonic acid)/poly(acrylamide) (PAMPS/PAAm) double-network hydrogel (left) with cutter-slicing resistance. Reproduced with permission. [11] Copyright 2003, Wiley-VCH. Tough alginate/PAAm double-network hydrogels (right) crosslinked by Ca 2+ with notchinsensitive stretchability. Reproduced with permission. [12] Copyright 2012, Nature Publishing Group.
with non-conductive hydrogels as substrate, sensing functions are realized by integrating electronic components on or in hydrogels that are connected with stretchable conductors. This strategy allows us to develop various sophisticated conductive patterns on hydrogels to achieve integrated multifunctional HSE. Several features such as high stretchability, compliance, self-healing ability, biocompatibility, degradability, permeability, and stimuliresponsiveness also bring tremendous advantages to HSE compared with elastomer-based soft electronics (Figure 1). [18][19][20][21][22] For HSE with conductive hydrogel as sensing elements, a bulk conductive hydrogel is usually directly used to detect strain, temperature, etc. Some examples show a hydrogel device fabricated by printing ionic conductive hydrogels on both sides of dielectric elastomer with a triple-layer structure can detect pressure and temperature simultaneously by separately testing the capacitance of two gel layers and the resistance of the single gel layer. [23] However, the relatively simple structure of such HSE restricts the application in detecting spatially distributed www.advancedsciencenews.com www.advsensorres.com stimuli and integrating more functions into one HSE. [22] In contrast, commercial electronics made with rigid materials and state-of-the-art soft electronics comprising various electronic components such as resistors, capacitors, inductors, and transistors are multifunctional and highly integrated. [1] To broaden the impact of HSE, hydrogel-based multifunctional soft electronics with distributed sensing elements deserve more attention.
This Review summarizes the recent advances in multifunctional HSE with patterned conductive circuits and distributed sensing elements. We first briefly introduce the representative types of hydrogel sensors with conductive hydrogels as sensing elements. Then, we consider strategies for preparing integrated multifunctional HSE by two approaches: structural design of rigid conductors and innovations in intrinsically stretchable conductors. Additional functions, such as adhesiveness and selfshaping ability that benefited from the unique features of hydrogels are also discussed. Finally, the key challenges and potential solutions of integrated multifunctional HSE are discussed.

HSE Sensors Based on Conductive Hydrogels
Understanding the basic working mechanisms of hydrogel sensors with simple structure and function is crucial to prepare integrated multifunctional devices. The function of sensors is realized by converting external stimuli (e.g., strain, temperature, pH) to electrical signals that can be quantitatively analyzed. Sensitivity is an important criterion to evaluate the performance of the sensors and is strongly dependent on material selection and structural design. Macroscopic physical properties such as changes in color, volume, and mechanical properties can also be used as metrics for sensing soft electronics with hydrogels as sensing elements. In the following parts, we briefly summarize several kinds of representative simple hydrogel-based sensors and highlight example applications where possible, which should be the basis for the fabrication of integrated multifunctional devices.

Strain/Pressure Sensors
Hydrogel-based strain/pressure sensors convert mechanical stimuli to electrical signals. Their sensitivity and working range are highly dependent on the materials and working mechanisms of the sensors. The hydrogel-based strain/pressure sensors can be divided into two categories based on their working mechanisms. The mechanism of resistance-type strain/pressure sensor is based on the resistance change during stretch or compression, as shown in Figure 2a. [24][25][26][27][28][29][30] For ionic hydrogels, the network deformation has negligible influence on the ion conductivity. Therefore, the resistance change of ionic hydrogels under external stretching is between the conductivity-constant conductors and resistance-constant conductors. Such mechanoelectrical response hampers ionic hydrogel as strain/pressure sensor with a high gauge factor (usually used as the sensitivity metric). For electronically conductive hydrogels with conductive fillers or polymers, the morphologies change of percolated network during stretching usually causes a large change in resistance, thus enabling strain sensors with high gauge factor. However, the irreversible loss in electrical performance due to the evolution of percolated networks under repeated stretching cycles might present a challenge for achieving strain sensors with stable performances.
Another type of strain/pressure sensing mechanism is based on capacitance. A conventional capacitance pressure sensor is designed by sandwiching a dielectric elastomer between two conductive hydrogels (Figure 2b). The decrease in distance between two hydrogel layers under applied pressure increases capacitance. [31][32][33][34] Another super-capacitive type hydrogel pressure sensor comprises an ionic hydrogel covered with two electronic conductors on both sides (Figure 2c). [35][36][37] The interfaces of ionic hydrogel and electrodes generate interfacial capacitance that is considered as electrical double-layer (EDL) capacitance. Ultrahigh capacitance in the interfaces is formed after an external voltage is applied. The change in the contact area between the ionic hydrogel and the electronic conductor causes a significant increase in capacitance. The ultrahigh unit area capacitance of super-capacitive pressure sensors enables enhanced sensitivity and sensing range compared to traditional parallel-plate capacitive pressure sensors.
Strain/pressure-based sensors can be further used to monitor the physical contact between an object and the sensor. [38][39][40] For example, a surface-capacitive touch sensor based on EDL principle is shown in Figure 2d. [38] The same voltage is applied at two ends of an ionic hydrogel that generates a uniform electrostatic field in the hydrogel. When a grounded conductor contacts the surface of the hydrogel, the potential difference between electrodes and the touch point results in current flow from electrodes to the finger. The current magnitude is associated with the distance between electrodes and the touch site, which can be used to determine the relative location of the touch point. A soft and stretchable touch panel that can be conformally attached to human skin is realized by connecting four electrodes with each corner of a rectangular ionic hydrogel to write words and play games.

Temperature Sensors
Temperature affects the mobility of ions in hydrogel or the swelling capacity of hydrogels, which can be used to devise temperature sensors. As shown in Figure 2e, a temperature sensor is developed by using two junctions between mobile ions and mobile electrons. [41] Each junction involves an electronic conductor and an ionic hydrogel in contact, which serves as a sensing and reference end, respectively. The temperature difference between the sensing and the reference end results in a voltage between the two electrodes and can be used for quantitative measurement. For temperature measurement, such stretchable hydrogel temperature sensors can be attached to objects with a complex curved surface (e.g., eggs). For another type of hydrogel-based thermometry, the temperature-triggered volume change can be converted into an optical signal by Bragg diffraction, thus allowing to monitor the ambient temperature change. [42,43]

Chemical Sensors
Functional additives, such as small molecules and living cells, can be involved into hydrogel matrix. By exploiting the diffusion capability of hydrogels to various materials, communication between functional additives and external chemical stimuli can be  Versatile hydrogel-based sensors. a) Schematic illustration of a resistance-type strain sensor. Resistance of hydrogel increases as the sensor is stretched under an applied force. Reproduced with permission. [24] Copyright 2019, Wiley-VCH. b) Sensing mechanism of a traditional parallel-plate capacitive sensor. Capacitance of two hydrogel layers increases as the sensor is compressed under applied pressure. Reproduced with permission. [37] Copyright 2021, Wiley-VCH. c) Sensing mechanism of an ultra-capacitive sensor. Under external pressure, ions in the hydrogel are attracted to the hydrogel-electrode interfaces, resulting in a dramatic increase in capacitance. d) Schematic and applications of an ionic touch sensor. Reproduced with permission. [38] Copyright 2016, American Association for the Advancement of Science. e) Schematic and application of an ionotronic thermometry comprising of two electronic conductor-ionic hydrogel junctions. Reproduced with permission. [41] Copyright 2022, National Academy of Sciences. f) Schematic illustration of the hydrogel chemical sensor prepared by involving living cells into hydrogel matrix. Reproduced with permission. [47] Copyright 2017, National Academy of Sciences. The living cells can generate fluorescence after exposure to cognate inducers, enabling the smart glove to detect inducers. g) Schematic diagrams illustrating the sensing mechanism of the proximity sensor that can detect human movements in real-time. Reproduced with permission. [48] Copyright 2021, American Association for the Advancement of Science.  Figure 2f). [44][45][46] By embedding chemical-sensitive living cells, the hydrogel sensor can convert chemical signals to fluorescent signals that can be transmitted through the transparent hydrogel, allowing for rapid qualitative measurement of chemical inducers. [44] In addition, glucose, lactic acid and other sweat components can also be detected by including corresponding enzymes in hydrogels. [47]

Proximity Sensors
Humans can collect and record information from their surroundings wirelessly. Although wireless communication systems have been well developed using integrated circuit (IC) chips as electrical elements, soft and stretchable proximity sensors are still lacking. Based on electrostatic induction, Song et al. prepared proximity sensors by exploiting an ionic hydrogel as a stretchable electric field receiver that can detect the electric field originating from static charges on the surface of an object ( Figure 2g). [48,49] The approaching and departure of the target object result in the rearrangement of charges in the hydrogel receiver, thus inducing a voltage across the external load between the ground and the hydrogel receiver. The hydrogel proximity sensor can detect the approaching and receding of a person walking from meters away.

Multifunctional HSE with Distributed Sensing Elements
Although substantial progresses have been made in HSE based on conductive hydrogels, the absence of patterned conductive circuits and distributed sensing units hammers their further applications in multi-site detection and multifunctional soft electronics. In addition, the low conductivity of conductive hydrogels also restrict their applications in electrical circuits with high performance ( Table 1). An effective strategy is to embed various electronic components in hydrogel and use stretchable conductors as electrical interconnects. The development of stretchable conductors is the cornerstone for fabricating such electronics. Generally, two approaches have been established to impart stretchability to electrical materials: by making non-stretchable materials (e.g., metal wire) into wavy, bulked, or helical structures [50] or by developing intrinsically stretchable materials. [1] In the following parts, we will discuss how to prepare integrated multifunctional HSE with these two strategies.

Multifunctional HSE with Rigid Materials as Electrical Interconnects
In recent years, scientists have developed a series of elastomerbased (i.e., polydimethylsiloxane, PDMS) stretchable electronics with bulked, wavy, or helical metal wires as electrical interconnects of IC chips by compressive-induced mechanical instability (Figure 3a). [50,51] Such a strategy can also be used in the field of HSE by embedding wavy metal wires in gel matrix (Figure 3b). [52][53][54] However, the significant dissimilarity in Young's modulus between metal conductor and gel matrix will result in stress concentration at the interface of these two materials when a device is stretched, thus causing failure of the interface or fragmentation of the hydrogel matrix ( Figure 3c). This issue can be alleviated from three aspects: i) using tough hydrogels with excellent mechanical properties as substrates; ii) improving the interface interactions between rigid conductor and gel substrate; iii) spatially modulating substrate stiffness in the locations of IC chips. For example, as shown in Figure 3d, stretchable multifunctional HSE with a reliable substrate-conductor interface is fabricated using wavy silanized titanium wires as interconnects of distributed functional electronic components and robust polyacrylamide-alginate (PAAm-alginate) hydrogel as substrate. [52] The modified titanium wires can form robust interface bonding with the hydrogel matrix, thus enabling HSE with high stretchability and stability. Drug-delivery channels, reservoirs, and temperature sensors are further integrated into the hydrogel matrix to realize temperature sensing and programmable release of mock drugs. This work provides a simple approach to design stretchable and multifunctional HSE using tough hydrogel substrate and surface-modified metal conductor.
Another method is to reduce stress concentration by spatially modulating the mechanical properties of the substrates. When stretching, strain is concentrated in the soft areas while the hard areas have imperceptible deformation, which can prevent delamination or detachment of rigid electronic components (integrated in hard areas) with the substrate. Figure 3e shows a multimodal HSE that can detect temperature, humidity, and strain  [51] Copyright 2017, Nature Publishing Group. b) Stretchable integrated HSE embedded with wavy metal wires as electrical interconnects. c) Finite-element simulation shows a maximum principal strain of the hydrogel matrix during stretching of HSE in (b). d) HSE embedded with distributed functional electronic components for temperature sensing and controlled drug delivery. Reproduced with permission. [52] Copyright 2016, Wiley-VCH. e) Multifunctional HSE, which can detect temperature, humidity, and strain. Different colors represent different modulus of the hydrogel. Reproduced with permission. [9] Copyright 2020, Nature Publishing Group. f) Modulating stiffness of the hydrogel substrate by additional crosslinking with multivalent ions in the specific areas thus enabling HSE integrated with rigid commercial electronic components. Reproduced with permission. [55] Copyright 2020, Royal Society of Chemistry.
simultaneously. [9] The substrate is composed of biogels with different stiffness, where the location of the strain sensor is soft, and the locations of temperature sensor and humidity sensor are hard. Such structure design can increase the sensitivity of the strain sensor and prevent stress concentration at the temperature and humidity sensor regions, thus reducing the impact of stretching on the detection accuracy of these two sensors. In addition, the stiffness of the hydrogel matrix can also be modulated by posttreatment. For example, a PAAm-alginate hydrogel substrate is locally stiffened by additional crosslinking with multivalent ions in the specific regions that settled with IC chips (Figure 3f). The resultant HSE can be stretched to 150% strain without component detachment and functional degradation. [55] The strategy to locally modulate the stiffness of the soft substrate provides a simple and effective way to fabricate integrated multifunctional HSE with rigid conductors and commercial chips.
A big advantage of soft electronics with rigid materials as conductive circuits is that the electrical performances (e.g., the re-sistance of the conductive circuits) are almost decoupled from stretching before failure. However, in pursuit of multifunctional HSE with structural engineering-induced stretchability of metal wires as conductors, several major aspects still need to be improved: i) more investigations and optimizations are required to enhance the adhesion between soft and hard interfaces under large strain; ii) the decline of electrical performance resulted from corrosion of metal wires in water and ion-rich environment of hydrogel should be resolved; iii) new technologies need to be developed to integrated rigid IC chips into soft gel matrix for scalable fabrication. [56]

Multifunctional HSE with Intrinsically Stretchable Materials as Electrical Interconnects
To improve integration capability of electrical conductors with soft substrate, an effective alternative way is to fabricate multifunctional HSE by employing intrinsically stretchable conductors as building blocks. Due to the intrinsic ionic conductivity (10 −3 -10 0 S m −1 ) and ion permeability of hydrogels, it is almost impossible to prepare HSE with distributed sensing elements by using ionic conductors (e.g., ionic hydrogels) as electrical circuits in the gel substrate. The use of electronically conductive intrinsically stretchable conductors (ECSCs) with conductivity much higher than that of hydrogel substrate is a viable solution. Recently, significant advances have been made to develop ECSCs with high conductivity, covering a broad range of materials, including liquid metals (LMs), conductive polymers, and nanocomposite materials. [57][58][59][60][61][62][63] In the Introduction, we briefly described how to prepare electrically conductive hydrogel, which is usually directly used as a bulk hydrogel sensor in most application scenarios. In this section, we will summarize the advanced integrated multifunctional HSE with distributed sensing elements by using ECSCs as electrical circuits and discuss their advantages and disadvantages in this field.
Among the various ECSCs, LMs are the best combination of conductivity and deformability, thus enabling them an ideal candidate for developing stretchable electronics. [57][58][59][60][64][65][66][67][68][69][70] Galliumbased LM, such as gallium-indium alloy (EGaIn, 75% gallium and 25% indium) and galinstan (68.5% gallium, 21.5% indium, and 10% tin) are most investigated due to their metal-like conductivity (3-6 × 10 6 S m −1 ), low toxicity, deformability (Young's modulus: 1-10 Pa), and unique surface chemistry (forming a thin gallium oxide layer (Ga 2 O 3 ) on the surface instantly when exposed to air) (Figure 4a). [59] However, the high surface tension (>600 mN m −1 ) of Ga-based LM results in poor patternability. Many advanced patterning technologies, such as direct writing, 3D printing, stencil printing, and injection molding have been proposed to fabricate stretchable electronics with LM as conductor and elastomer as substrate ( Table 2). [64][65][66][67][68][69][70] Considering the high similarity of hydrogels to biotissues and the high conductivity of LM, it is promising to combine these two kinds of materials to develop HSE with LM as patterned conductive circuits or sensing elements. Hao et al. reported multifunctional hydrogelbased e-skin with iron powders dopped LM as electrical interconnections of sensing elements and biopolymer hydrogel as substrate, which can detect strain, pH, temperature, electrocardiogram (ECG), and achieve controllable iontophoretic drug delivery (Figure 4b). [69] Although this work attempts to solve the substantial problem in the field of HSE, that is, transforming bulk hydrogels into integrated multifunctional electronics just like a commercial one, the stability of LM in an aqueous environment may restrict their further practical applications. In addition to e-skin, implantable HSE is also demonstrated by injecting LM into predesigned microfluidic channels of double-network tough hydrogels, which can monitor the heartbeats of rabbits in real-time. [70] However, problems such as the absence of a wireless data transmission system, the safety of LM in vivo, and the stability of hydrogels in physiological environments need further investigation for next-generation implanted HSE.
As described above, the real applications of hydrogel-LM-based soft electronics still have many limitations. Considering the excellent fluidity of LM, the electrical stability of LM during longterm use and repeated stretching is the first challenge that needs to be solved. For integrated HSE, forming a reliable connection between LM circuits and IC microchips is difficult, especially un-der large strain. It is also worth mentioning that the commonly used printable LM doped with metal nanoparticles will have an inevitable electrochemical reaction in the hydrogel matrix, which creates grand challenges for the stability of the device. Overall, metal-like conductivity, deformability, and patternability of LM will bring tremendous opportunities to integrated HSE after the above-mentioned problems are well resolved.
There are usually two approaches to fabricate hydrogel-CPs hybrid soft electronics with distributed sensing units. The most straightforward way to combine these two materials is directly patterning CPs on hydrogel substrate. For example, PEDOT micropatterns are polymerized onto agarose gel substrate by electrochemical reaction to fabricate fully organic, soft, wet, and stretchable electrode. [77] Considering the similarity of network structure between hydrogels and CPs, another approach is developed by physically mixing or chemically polymerizing CPs into the hydrogel matrix to prepare electrically conductive hydrogels. Conductive patterns can be prepared by selectively polymerizing PAni network on hydrogel matrix with a stencil mask (Figure 4c and Table 2). [73] However, the resolution of the conductive pattern is poor with line width at a millimeter scale. Liu et al. found that doping ionic liquid into PEDOT:PSS can promote the aggregation of PEDOT polymer chains, thus enabling ion gels with high conductivity, which can be transformed to hydrogel by further removing the ionic liquid with simple water exchange process. [74] The resultant electrically conductive hydrogels exhibit low modulus (<10 kPa), high water content (≈85%), unprecedented electrical conductivity (≈4700 S m −1 ), and can be patterned to any geometries with line width down to 5 μm via photolithography. The above-mentioned features bestow a perfect bioelectronics interface with low impedance for localized low-voltage neuromodulation.
CPs exhibit matching modulus to hydrogels and can be processed in similar ways with gels. Therefore, they have better integration capability with gel substrate when compared to rigid conductors and LMs. CPs can form more robust interfaces with the gel matrix, thus enhancing device reliability. Future investigations should concentrate on the aspects of increasement in electrical conductivity, developing micropatterning methods, and innovations in scalable integration of CPs with hydrogel matrix.
Nanomaterials-based soft electronics are usually fabricated by mixing conductive fillers into a stretchable polymer matrix. [78][79][80][81] The nanofillers are connected to form a conductive percolated network. The commonly used nanomaterials including 0D nanoparticles, 1D nanowires or nanotubes, and 2D nanosheets. Soft materials such as PDMS, Ecoflex, and polyvinylalcohol  [59] Copyright 2008, Wiley-VCH.) and its commonly used patterning methods. (Reproduced with permission. [66,69,68] Copyright 2019, American Association for the Advancement of Science; Copyright 2022, Wiley-VCH; Copyright 2018, Wiley-VCH.) b) A multifunctional HSE with biopolymer hydrogel as substrate and LM as electrical interconnects of functional hydrogel electrodes. Reproduced with permission. [69] Copyright 2022, Wiley-VCH. c) Fabrication of patterned conductive PAni pathways on the surface of tough polyion complex hydrogel by a stencil mask. Reproduced with permission. [73] Copyright 2018, American Chemical Society. d) HSE with micropatterned electrically conductive hydrogel as electrodes for low-voltage neuromodulation. Reproduced with permission. [74] Copyright 2019, Nature Publishing Group. e) Hydrogel with high electrical conductivity fabricated by partial dehydration of the composite comprising of Ag flakes and PAAm-alginate hydrogel. Reproduced with permission. [86] Copyright 2021, Nature Publishing Group. f) HSE with patterned AgNWs electrodes for spatially pressure detection. Reproduced with permission. [87] Copyright 2020, Royal Society of Chemistry. www.advancedsciencenews.com www.advsensorres.com Requires high concentration of conductive polymers to achieve high conductivity [78][79][80][81][82][83][84][85][86][87] Chemically polymerizing [71][72][73][74][75][76] (PVA) are usually used as matrix to offer stretchability and recoverability to the percolated network. [82] The electrical conductivity of the nanomaterials-based soft conductors is generally associated with the type and volume fraction of the nanofillers and subsequent post-treatments, such as annealing. [83,84] To obtain integrated multifunctional HSE with patterned nanomaterials-based stretchable conductors as electronical circuits, the conductors should have high conductivity, stretchability, and patternability. [85] However, high electrical conductivity usually requires high volume fraction of nanofillers, which will result in poor mechanical compliance and stretchability of the resultant conductors. Moreover, the annealing process that can greatly improve the conductivity of nanocomposites may not be suitable for hydrogel matrix because of the loss in flexibility and stretchability of hydrogels during these processes. Therefore, obtaining integrated HSE with high conductive patterned nanomaterials-based stretchable conductors with low filler doping concentration is difficult. As an exciting example, the low volume fraction of Ag flakes is mixed with polyacrylamide-alginate hydrogel, followed by a dehydration process to remove partial water, thus inducing the percolation of conductive pathways (Figure 4e). [86] The resultant hydrogel conductor exhibits incredible electrical conductivity up to 34 700 S m −1 , while Young's modulus is lower than 10 kPa matching that of soft biotissues. The precursor of the conductive Ag-hydrogel composite is viscoelastic and can be printed to desired patterns with a stencil mask to fabricate integrated soft electronics or robots. Another approach is to prepare nanomaterial conductive patterns directly on the hydrogel surface. Zhu et al. deposed processable AgNWs dispersion on the surface of polyion complex (PIC) hydrogel plasticized with concentrated sodium chloride (NaCl) solution (Figure 4f). [87] HSE with robust mechanical properties and high conductivity is obtained after evaporating the solvent of AgNWs dispersion and dialyzing out ions from the gel. The facile pattern ability of AgNWs and the excellent mechanical properties of the gel matrix enables the design of integrated HSE with multi-site detec-tion ability. Although printing nanomaterials on hydrogel can achieve integrated HSE with patterned circuits, the interface between nanomaterials and hydrogel may fail during cyclic stretching, and the different recover ratios of AgNWs and hydrogel after the stretching-releasing process may cause structural instability of the conductive layer, such as surface wrinkling. These works represent substantial progress in HSE with patterned circuits and distributed sensing elements using nanomaterials composite as stretchable conductors. However, the irreversible change in electrical performance due to the morphologies change of percolated network under cyclic stretching remains a challenge for HSE with nanomaterials-based conductors.
One essential and specific concern of HSE with distributed sensing elements is the durability during long-term use. The durability of HSE can be considered from the following aspects: mechanical properties of the gel matrix; the interface bonding of matrix and conductor; dehydration; anti-freezing, etc. For mechanical properties, the development of tough hydrogels in recent years solved this problem to a large extent. The interface bonding of matrix and conductive components can be improved by chemical modification of material surfaces or modulation of matrix' modulus, thus enabling strong, tough, and fatigueresistant devices capable of carrying thousands of cyclic loadings. The unique point when discussing HSE's durability is the dehydration or freezing of water in hydrogels which will cause failure of devices. Strategies to develop anti-dehydration hydrogels include introducing highly hydratable salts, glycerol, and designing surface hydrophobic coatings. Incorporating salts is a facile and effective approach to improve water retention capacity. [88][89][90][91] The water molecules inside the hydrogels' network can form hydrated ions with salt cations/anions, compared to free water molecules which will evaporate naturally, the formed hydrates need to break the bonds to evaporate. The surface coating prevents air-drying and avoids affecting the bulk structure of hydrogels, although selectively modifying the surface is still a challenge. [92][93][94][95] Meanwhile, durability is also reflected in the anti-freezing performance Figure 5. Additional functions of HSE. a) Self-healing of HSE with LM as conductor. Reproduced with permission. [60] Copyright 2020, Wiley-VCH. b) Hydrogel electronics with adhesion ability that enables the formation of a seamless interface with biotissues. Reproduced with permission. [104] Copyright 2022, Wiley-VCH. c) Hydrogel-elastomer hybrid electronics with conductive and adhesive hydrogels as electrical connects of IC chips. d) Ultra-thin HSE with kirigami structure that can be wrapped on the surface of objects with non-zero Gaussian curvature, such as orange and finger. Reproduced with permission. [114] Copyright 2021, Wiley-VCH. e) Self-shaping HSE which can deform to a 3D configuration from a flat state to fix a rabbit's heart to detect the heartbeat. Reproduced with permission. [117] Copyright 2021, Wiley-VCH.
at low temperatures, where ice growth can be inhibited by replacing water inside the hydrogels with other solvents. [96,97] Chen et al. displaced water within the hydrogels by glycerol, glycol, or sorbitol. [96] Owing to the ice-inhibiting effect of alcohols, both water retention capacity and anti-freezing properties have been improved. Although many works on HSE durability have been carried out, several challenges remain. The introduction of salts can only mitigate water loss with limited improvement, while coating still requires more work for designing a robust interface between hydrogel matrix and the coating. Further direction should not only be to delay water loss, but also to consider how to retain the basic device function like flexibility and conductivity even after dehydration.
With the development of intrinsically stretchable conductors, a higher requirement is to achieve various fully stretchable electronic components, such as a resistor, capacitor, transistor, inductor, etc. The electrical performance, stability, and integration density of newly reported stretchable electronic components are far lower than the rigid commercial ones. [50] For HSE with a high degree of integration, many efforts need to be made to develop high-performance stretchable electronic components with easy integration into the hydrogel.
Adv. Sensor Res. 2023, 2, 2200069 Figure 6. Soft electronics towards real-world applications. a) Schematic to show next-generation soft electronics that can be seamlessly combined with organs to record and stimulate. Reproduced with permission. [118] Copyright 2022, American Association for the Advancement of Science. b) A soft and wireless smart glove with multiple electronic modules can detect the location, amplitude, and strain type. Reproduced with permission. [127] Copyright 2020, American Association for the Advancement of Science.

Additional Functions of HSE
Besides high stretchability and biocompatibility, hydrogels offer soft electronics with other intriguing functions. Self-healing allows soft electronics to restore their original functions after physical damage. [98] The self-healing of hydrogels is usually realized by introducing non-covalent interactions or dynamic bonds into hydrogels. [99] The dynamic rearrangement of crosslinkers allows the hydrogels to recover their original mechanical properties spontaneously or under external stimuli (e.g., light, heat, pH). However, almost all the reported self-healable HSE are partially healable, there are rare reports about integrated HSE that all the components, including the substrate, conductive circuits, and electrodes can undergo self-healing. The HSE comprising of selfhealable hydrogel substrate and embedded LM circuits exhibit both mechanical and electrical self-healing ability, but the LM only serves as conductive wires (Figure 5a). [60,69] To obtain fully self-healable integrated HSE, the future investigation should focus on developing self-healable conductive circuits and electronic components with easy integration into the hydrogel.
Recently, scientists have developed various strategies to promote tough interface bonding between hydrogels and other materials such as metal, elastomer, and biotissues, which are essential for the fabrication of multilayer integrated HSE. [100][101][102][103] The bonding of hydrogels with other materials is usually realized by physical or chemical interactions between hydrogels and target materials. The tough bonding of HSE with biotissues results in a reliable human-machine interface that enables stable information collection (Figure 5b). [104] Tough bonding of hydrogels with elastomers is developed by using benzophenone as an initiator to form robust chemical bonding between hydrogel and elastomer. Integrated hydrogel-elastomer hybrid soft electronics with pattern ionic gels as conductive circuits and PDMS as the substrate is fabricated thanks to such excellent interface bonding (Figure 5b). [94] The established bonding strategies of hydrogels to other materials greatly facilitate the integration capability of multifunctional hydrogel devices.
A grand challenge in soft electronics is improving the conformability of 2D devices with 3D organs. [105,106] Traditional approaches include reducing the thickness of devices, introducing open mesh or kirigami structure into the devices, etc. [107,108] However, it becomes hard to handle when the device is too thin. The use of tape or glue, as well as self-adhesion of the substrate, may cause damage to the biotissues. [109][110][111][112][113] The kirigami structure in weak hydrogel may result in mechanical defects that cause failure of the materials. Yu et al. fabricated tough metallosupramolecular hydrogel films of poly(acrylic acid) (PAA) with robust carboxyl-Zr 4+ coordination bonds as crosslinkers (Figure 5d). [114] The hydrogel can encode the Kirigami structure due to the fast photopolymerization process. After being printed with LM as stretchable conductor, the resultant HSE can be conformably attached to human skin with sophisticated geometry (e.g., elbow) due to the additional deformation freedom bestowed by kirigami structure to detect human motions. Although Young's modulus of the tough hydrogel is several tens of megapascals, the too-thin thickness (<100 μm) and the kirigami structure make it difficult to handle the HSE. A cleverer way to wrap 2D devices on the surface of 3D organs is to use stimuli-responsive materials as the substrate of soft electronics. [115,116] Considering the abundant responsiveness of hydrogels, they are ideal materials to develop HSE with self-shaping ability. Using patterned hydrogels with different swelling ratios as the substrate of LM conductor, the HSE can undergo fast shape-shifting to self-wrap on the rabbit heart to detect the artificial heartbeats (Figure 5e). [117] The active morphing HSE should also pave the way for using responsive elastomer as the building block to fabricate innovative soft electronics.

Conclusion and Outlook
In this Review, we aim to summarize hydrogels in the applications of integrated multifunctional soft electronics with distributed sensing elements. From the perspective of sensor type, the HSE can be categorized to strain sensor, temperature sensor, Adv. Sensor Res. 2023, 2, 2200069 chemical sensor, electrocardiogram electrode, etc. From the view of building blocks of soft electronics, hydrogels can be served as substrates, conductive circuits, and electrodes. By integrating multiple electronic components into one hydrogel device, the resultant HSE should have combined functions including recording, stimulation, and therapy (Figure 6a). [118] Although grand advances have been made in multifunctional HSE, many challenges still need to be overcome.
First, the coupling between different sensors should be minimized. For example, the resistance of conductive circuits may increase during the stretching process, affecting the detection accuracy of the strain sensor and the others. [109] Second, the electrical component density of current-reported integrated HSE is much lower than that of commercial rigid electronics. To solve this problem, advanced technologies need be developed to create fine conductive lines with high conductivity on the surface or embedded in hydrogels. Another possible solution is to fabricate multilayer HSE with vertical interconnect circuits or develop 3D devices for assembling more electronic components. [120] Third, for multifunctional hydrogel bioelectronics, although hydrogels have long been considered to have excellent biocompatibility, their long-term stability and safety (e.g., immune reaction) in living organisms need to be further proven. [121] The implantation of HSE also requires a sufficiently small device size, which is still a big challenge in HSE up to now, mainly due to the lack of strategies for micropatterning of electrical circuits with high conductivity on hydrogel substrate. Fourth, as a common challenge in soft electronics, the soft power system is still in the laboratory development stage with low energy density and power budget. [122] Moreover, the water-rich environment that HSE is also applied to poses a substantial challenge for the power system package. There are some efforts to introduce energy harvest systems, such as triboelectric nanogenerators (TENGs), into soft electronics. However, technologies need to be developed to improve the power budget for long-term implantation and integrate such energy supply systems with energy storage devices into multifunctional HSE. [123,124] Fifth, wireless data read systems such as near field communication (NFC) or radio frequency identification (RFID) are still absent in HSE. Traditional data transmission mode with electric cables dramatically limits the use of HSE in portable wearables and implantable devices. Some examples show LM-based stretchable antennas have the potential for wireless data transmission but are far away from real applications. [125] To solve the grand challenges in this burgeoning field, great efforts need to be made from cross-disciplinary researchers including material scientists, chemists, biologists, and physicists. To broaden the influence of this field, new and real-world applications [126][127][128][129] that integrated with power system, flexible display, sensors, and signal processing system should be demonstrated (Figure 6b), instead of the stereotyped finger bending detection by using a bulk hydrogel that connected to the huge signal collection and processing devices with a whole tangle of wires. In the future, highly integrated and multifunctional HSE should merit the design of next-generation soft electronics and robotics.