Bioelectric Interface Technologies in Cells and Organoids

The past two decades have witnessed breakthroughs in cellular‐scale bioelectronics and their widespread applications in life sciences, creating many powerful platforms for studying organisms directly and efficiently. These advanced devices can integrate seamlessly and intimately onto/into target cells and organoids, providing unprecedented functions and revolutionary capabilities. Bioelectronics with nanostructured designs are developed to allow for long‐term, stable monitoring of electrophysiological activities. This review summarizes nanostructured bioelectronics for cell electrophysiology recording, emphasizing the crucial roles of structural designs on functions and capabilities, e.g., intracellular access, high‐density multiplexed recording, multifunctional interfaces and the conformability to curvy biological shapes. Finally, the remaining challenges and opportunities in nanostructured bioelectronics are identified, and perspectives on the future developments toward their practical applications are provided.


Introduction
Cells serve as the basic building blocks that make up all living organisms.4][5] The recordings of cell activities are therefore of pivotal importance for the understanding and prediction of advanced life behaviors.[15][16][17][18][19][20] Among these bioelectronic interfaces, the DOI: 10.1002/admi.202300550nanostructured bioelectronics, owing to their versatile, miniaturized designs and capabilities of in vivo (or in vitro) sensing of cell activities, have emerged as a very promising direction of next generation cell interfaces.
In this review, we highlight the representative nanostructured bioelectronics (Figure 1) for cell electrophysiology recording, with an emphasis on their structural designs, functions and potential applications.The next section summarizes needle type bioelectronics with a diversity of architectural designs.The content then shifts to planar type for cells, and scaffold, conformal and implantable type bioelectronics for organoids.The summary and outlook section outline opportunities and challenges that exist for future developments of nanostructured bioelectronics.

Needle Type Bioelectronics for Cells
][75][76][77][78][79][80][81][82][83][84][85] The development of such devices could not only meet the growing demand for highly sensitive and selective detection of single cells, but also provide an ideal platform to penetrate the cell membrane for intracellular detection, which is difficult to achieve with other structural configurations. [74,86]The existing penetration methods for intracellular access can be classified into two main categories, [87][88][89][90] , i.e., spontaneous penetration and assisted penetration.The spontaneous penetration usually relies on the sharp physical geometry of the nanostructure or the gravity/adhesion forces of cells, while the aided penetration requires additional chemical, electrical, or optical processes.

Nanoprobes
With conventional array-based recording devices, the contacting surfaces are typically confined to regions only near the bottom of cells.[93][94] This kind of nanoprobes could be employed for probing intracellular environment with a high spatial resolution (single cell level).Due to the nanoscale size and semi-implantable property, the nanoprobe could greatly reduce the damage of cell during membrane penetration, allowing the precise and long-term positioning into cells. [95,96]ian et al. [57] developed a novel nanoprobe based on kinked NW FETs, which allowed the localization of a point-like FET detector at the kinked tip for the realization of facile intracellular recordings (Figure 2a).The kinked Si NW modified with phospholipid modification enable spontaneous penetration of the cell membrane, without applying any external forces. [98]Meanwhile, the high surface-to-volume ratio of NWs and their operation as FETs afford excellent sensitivity to voltage signals, which ensured the real-time electrical recording of the spontaneous penetration process. [99,100]Based on a similar principle, a scalable U-shaped Si NW FET-based nanoprobe with the ability to record full amplitude intracellular action potentials was proposed, [58] as shown in Figure 2a.A key advancement is an assembly method to real-ize NW FET nanoprobe arrays with controllable tip geometry and FET size, enabling the study of the impact of the nanoprobe size and geometry on the intracellular recording. [101]It was shown that the nanoprobes with the smallest radii of curvature and FET channel lengths could facilitate the recording of full amplitude intracellular action potentials and subthreshold features. [102]In addition, arrays of these nanoprobes can be exploited to allow multiplexed recording from single cells and cell networks. [103,104]ased on the AFM platform, the electroporation of nanofountain probe technology can deliver molecules into cells in a manner that is highly efficient and gentler to cells. [105,106]The electroporation of single HeLa cells was demonstrated.Furthermore, a system for combined optical, force, and electrical measurements based on the AFM nanoprobe was developed, [107] which consists of a nanovolcano probe at the tip of a suspended microcantilever.Impedance measurements on mechanically stimulated neonatal rat cardiomyocytes were achieved using these nanovolcano probes.The nanoprobes can also be constructed with various materials and nanodevices, such as carbon nanotubes (CNTs), Si NWs, and gold nanopillars. [77,78,108]Owing to their high mechanical strength and high electrical conductivity, CNTs are attractive for cell probing, noticing that the dimensions of CNTs would potentially allow the interrogation of submicrometresized organelles deep within the cell, without causing evident damage.Singhal et al. [108] developed a multiwalled CNT-based endoscope for interrogating cells, which could transport fluids and perform optical and electrochemical diagnostics without disrupting the cell at the single organelle level.

Nanopillars
Nanopillar array-based devices can penetrate the cell membrane and enter the intracellular compartment, allowing the transfer of various biological molecules and detection of intracellular electrical and biochemical signals. [77,78]To obtain a high-fidelity intracellular signal, it is essential for the nanopillar structure to penetrate the cell membrane and form a tight coupling with the membrane.This coupling is highly dependent on the shape and size of the contact region.Generally, it has been shown that an easier intracellular access via spontaneous penetration can be achieved by decreasing the tip diameter or increasing the height. [109,110]s shown in Figure 2b-i, a vertical ultrasharp nanopillar (with a tip diameter below 10 nm) was reported to enable long-term recordings of intracellular potentials. [63]A high-performance intracellular access is crucial for the high-quality and longterm recording, while maintaining cell viability.In addition to the spontaneous poration, the electroporation (Figure 2b-ii) is the most widely-used method to facilitate the cell membrane penetration. [62,111]Due to the sharp tip (≈100 nm) of the vertical nanopillar tightly coupled to the cell membrane as shown in Figure 2b-iii, a large electric field can be generated with only a small voltage (e.g., ≈1-3 V) to locally increase the permeability of the cell membranes. [64]When a low voltage electroporation was applied on the nanopillars, nanopores were introduced in the cell membrane, and high-quality intracellular potentials could be recorded continuously for several minutes. [112]Furthermore, the optoporation was also developed as an effective method to improve the penetration, where focused high-power laser pulses  [57] Copyright 2010, American Association for the Advancement of Science.ii) U-shaped NW FET probe.Reproduced with permission. [58]Copyright 2019, Nature Publishing Group.b) Nanopillars.i) Ultra-sharp nanopillars.Reproduced with permission. [63]Copyright 2021, Wiley-VCH.ii) Membrane poration mechanisms.Reproduced with permission. [62]Copyright 2019, Wiley-VCH GmbH.iii) Vertical electrode arrays.Reproduced with permission. [64]Copyright 2012, Nature Publishing Group.iv) 3D vertical electrode arrays.Reproduced with permission. [42]Copyright 2022, Nature Publishing Group.c) Quasi-1D tubes.i) Hollow nanotube electrode.Reproduced with permission. [66]Copyright 2019, American Chemical Society.ii) Nanocrown electrodes.Reproduced with permission. [24]opyright 2022, Nature Publishing Group.iii) Nanovolcano electrode.Reproduced with permission. [67]Copyright 2019, American Chemical Society.iv) Patch clamp technique.Reproduced with permission. [97]Copyright 2019, Nature Publishing Group.
[115][116] When comparing to the electroporation, the optoporation does not interfere with spontaneous cell activity, does not imply any recording blind time, and enables a very long-term observation.
To fulfill the requirements of high-fidelity transmembrane potential recording in single cells or multicellular networks, Gu et al. reported a scalable 3D FET device (Figure 2b-iv) transformed from a 2D precursor for intra-and inter-cellular recording. [42]The 3D geometry allows the FETs to penetrate the cell membrane and record low-amplitude subthreshold signals inside the cell.On the basis of the developed devices, the intracellular action potentials and intercellular signal conductions were accurately recorded in real time.An array of the 3D FET devices with different structural designs was also demonstrated, which could interrogate cells at three different depths in a 3D microtissue.

Quasi-1D Tubes
The complex vertical quasi-1D nanotubes have been demonstrated, thanks to the fast developments of advanced manufacturing technologies at nanoscale. [24,66,67,97]Methods for intracellular delivery of molecules or nanoparticles (e.g., DNA plasmids, drug molecules, quantum dots, and gold nanoparticles) are essential for the development of modern biological and biomedical techniques, and have been widely used in single-cell studies, drug delivery, disease diagnosis, pharmaceutical therapeutics, gene regulation, and intracellular imaging. [117,118]Different from nanopillars, the hollow nanotube structures can provide a unique nanochannel that bridges the extracellular and intracellular microenvironments, facilitating the delivery of bio-cargoes into cells through the nanochannel.
[121][122] For example, an electrophoretic system based on vertical hollow nanotubes (Figure 2c-i) was developed to enable intracellular delivery of individual nanoparticles and real-time monitoring of the singleparticle delivery process. [66]The hollow tubular nanoelectrodes were shown to be able to delay the resealing of the cell membranes and induce the active membrane fusion, thereby improving the sealing resistance and prolonging the intracellular recording time. [123]Jahed et al. [24] proposed semi-hollow nanocrown electrodes with mechanical robustness, which could achieve high electroporation success rates (≈99%) with high accuracy and signal strength for cardiomyocyte intracellular access (Figure 2c-ii).The nanocrown structures could induce the cell membrane to wrap around the outer surface and adhere to the inner core, which stabilizes the membrane-electrode interface.Similarly, volcano-shaped nanoelectrode (nanovolcano) was also proposed [67] for long-term electrophysiological recordings (Figure 2c-iii).The 100-nm-thick nanovolcano wall has a nanopillar-like shape, which could induce high-curvature regions in the cell membrane, thereby increasing the chance of intracellular access.The patch-clamp technique which is originally developed to record currents of ions flowing in the cell membranes (Figure 2c-iv), has become a true stalwart of the neuroscience toolbox. [97]As the gold standard device for electrophysiology, [124,125] patch clamps could provide high-quality action potential recordings by adsorbing cell membranes to form a coupling interface with the intracellular environment. [126]eedle type bioelectronics have great advantages in penetrating the cell membrane for intracellular detection (i.e., spontaneous penetration and assisted penetration), which is difficult to achieve with other configurations of bioelectronics.Three types of needle type bioelectronics are classified due to their geometries, such as nanoprobes, nanopillars and quasi-1D nanotubes.Nanoprobe devices can detect the entire cell membrane and intracellular domain with a high spatial resolution in support of the operating platform, while such nanoprobe is inadequate in simultaneous multisite measurements.Nanopillar and quasi-1D tube devices are specially made for recording multicellular network, while the contacting surfaces are typically confined to the bottom of cells.Further advances will follow from the development of routes to high spatial resolution, multiplexed, intracellular and intercellular recording bioelectronics.

Planar Type Bioelectronics for Cells
[133][134] However, due to the 2D geometry of the electrode, it is basically not capable of directly recording the intracellular action potential.Besides, the micron-sized electrode usually records the electrical potentials of the cell population rather than a single cell, which can be well addressed by introducing nanoscale 2D structures. [135]

Nanowells
A bioelectronic system consisting of a planar microelectrode and five patterned nanowells was proposed to record the intracellular action potential of a single cardiomyocyte, [136] as shown in Figure 3a.The electroporation of nanowell electrodes could generate electric fields across the cardiomyocytes, and then penetrate the cell membrane to allow a good electrical coupling between the microelectrode and cardiomyocytes for the intracellular monitoring.The micron-sized nanowell array enabled the penetration point to stay under a single cell for the recording of permeability and action potential.Compared to those of the extracellular recording by conventional planar microelectrode array, the nanowell microelectrode array could measure the intracellular potential with higher quality, reduced noise root-mean-square, and higher signal-to-noise ratio.

Planar FETs
The function of biological organs relies on the dynamics of constituent cells and their coupling, and thus the simultaneous observation of a large number of cells over long time is essential for  [136] Copyright 2022, Nature Publishing Group.b) Planar FETs.i) Colorized electron micrograph image of a neuron on a silicon chip with a linear array of buried-channel FETs.Reproduced with permission. [137]Copyright 2005, Wiley-VCH.ii) Optical image of a cortex neuron connected to a NW device.Reproduced with permission. [138]Copyright 2006, American Association for the Advancement of Science.iii) Schematic illustration of graphene-based solution gated FETs for the detection of cell signals.Reproduced with permission. [139]Copyright 2011, Wiley-VCH.the biological research. [140]Based on the complementary metal oxide silicon (CMOS) technology, Voelker et al. [137,141] demonstrated low-noise electrolyte-oxide-silicon FETs for parallel monitoring of neurons at high spatial and temporal resolution.During the transistor recording, a neuron was directly placed onto to the exposed gate oxide of a FET, as illustrated in Figure 3bi.The source-drain current could be directly modulated by the change in the local extracellular potential in the electrolyte of the cleft between the cell and transistor.The cell-transistor junction could be regarded as a type of amplifier without interference of the electrolyte-metal contact or stray capacitances.
In addition, planar FETs based on semiconductor nanomaterials (such as Si NWs, [142][143][144] CNTs [145][146][147] and graphene, [148] MoS 2 [149] ) have attracted widespread attention, because of their capabilities of highly-sensitive and label-free detections.The application of Si NW FETs for extracellular recording from cultured neurons was first reported by the Lieber group in 2006.As shown in Figure 3b-ii, the as-fabricated Si NW FET array could be selectively passivated by patterning polylysine to promote the guided neuronal growth over Si NW FETs.Then a highly localized synapse-like junction could be formed at regions where the neuronal axon or dendrite crosses the NW device.
Moreover, the graphene electrolyte-gated FET array was developed for the detection of the cell electrophysiological activity (Figure 3b-iii), using large-area graphene films grown by chemical vapor deposition on copper foil. [139]The action potentials of these cardiomyocyte-like HL-1 cells could be effectively detected and resolved by culturing cells on the FET array.Due to the low noise and the large transconductive sensitivity, these graphene transistors offer an outstanding advantage of high signal-to-noise ratios.Through the entire transistor array, the propagation of the cell signals across the layer was successfully tracked by analyzing the multiplexed data.Integration of different types of devices to measure the same cell is also possible.The simultaneous recording of electrophysiological signals from the same cardiomyocyte was carried out by both graphene and Si NW FETs. [150]he research on planar type bioelectronics remains insufficient.Planar type bioelectronics are relatively low-cost because of their compatibility with state-of-art large-scale integrated planar fabrication techniques, thereby being applicable to highthroughput recordings.Similar to nanopillar and quasi-1D tube devices, the recording area is typically confined to the bottom of cells.The 2D geometry of the electrodes lead to a weak coupling interface.Opportunities may exist in the multiplexed extracellular recording of cell electrophysiology, which are useful for understanding intercellular communications, such as neurons.

Scaffold Type Bioelectronics for Organoids
][159] Owing to the geometrical features, such bioelectronics can provide the platforms for the structural support and growth of cells, while accommodating the spatially distributed electrodes capable of recording the cell activities.Hence, scaffold type bioelectronics have potentials in integrating a higher density of electrodes.Such potentials combined with structural designs and functionalized materials can serve as a multifunctional platform in exploring the generation and conduction of electrical impulse in cells or intercellular networks. [39,40,42,156,160]

Assembled Scaffolds
163][164][165][166][167][168][169][170][171] The excellent compatibility of such assembly methods with wellestablished planar nanofabrication and processing techniques allows their applicability to a broad set of high-performance materials.Besides, the development of routes to complex 3D structures with feature sizes in the mesoscopic range is of increasing.The purpose is to establish methods for controlling the structural designs for applications as metamaterials, offering engineered behaviors with optical, thermal, acoustic, mechanical that do not occur in the natural world, which can be summarized folding, rolling and mechanical assembly. [166]an et.al. reported a mechanically guided assembled strategy for advanced 3D designs in micro/nanomanufacturing which have potentials in many fields including biomedical engineering, metamaterials, electronics, electromechanical components. [156]ophisticated cell scaffolds can leverage high-performance components to allow interaction and communication with live cells and tissues.The developed electronic scaffolds were used for engineered dorsal root ganglion neural networks.As a demonstration, 3D bilayer nested cages of epoxy transferred onto opticalquality glass to enable high-resolution, in situ imaging, serve as growth platforms for neural networks of dorsal root ganglion cells dissociated from explants from rats.

Porous Foams
The emerging 3D cell culture systems have boosted the development of many fields, such as pathophysiological study, tissue regeneration, and drug screening.Among the various 3D cell culture systems, scaffold-based 3D cell culture systems have attracted extensive attention, [40,69,72,126,[157][158][159]172] because such porous scaffolds can simultaneously offer mechanical support and 3D microenvironments for the cell attachment and tissue formation in tissue engineering.
Qin et al. developed a stretchable and multifunctional scaffold type platform capable of 3D cell culture, mechanical loading, and electrochemical sensing. [40]The scaffold type 3D bioelectronics were fabricated by attaching the networks of gold nanotubes (Au NTs) on porous polydimethylsiloxane (PDMS), and linking Gly-Arg-Gly-Asp (GRGD) with Au NTs via covalent bond (Figure 4b).Such scaffold type 3D bioelectronics offered very good biocompatibility, excellent stretchability, and stable electrochemical sensing performance, with capabilities of mimicking the mechanotransduction of articular cartilage and monitoring the stretchinduced signaling molecules in real time.

Vertical Electrode Arrays
The shape morphing between free-standing 3D frameworks and their 2D precursors can be achieved by introducing plastic strain programming.Unlike the 3D assembly methods, [161,162,165,166,168,169] such strategy could provide freestanding 3D geometries.By exploiting the plastic deformations, Soscia et al. developed a 3D flexible spike-like multi-electrode arrays (Figure 4c), where the hinges were plastically deformed by a customized apparatus, allowing the probes to stand upright without additional supports. [39]The developed 3D multi-electrode devices containing 10 probes and 80 electrodes can be used to non-invasively interrogate the suspended neurons, with a capability of visualizing the spatial and temporal mapping of electrophysiological data across a 3D volume.
Such vertical 3D multi-electrode arrays with precisely controlled configurations can easily integrates with standard commercially available electrophysiology hardware. [39]To precisely control the bending location during actuation, a hinge region was incorporated into the structural designs.The bending region features a void in the polyimide, resulting in an inherently flexible region compared to the rest part.Polyimide was chosen as the main structural component of the probes due to its flexibility and biocompatibility.To ensure rapid and reproducible actuation of the probes, a costumed actuation apparatus was also developed.Such customized platforms are important step in facilitating noninvasive electrophysiological characterization of 3D networks of electroactive cells in vitro.
The rich diversity of accessible 3D geometric configurations is crucial to the culture of organoids.Three techniques are detailly discussed to show their design strategies, such as 3D assembly, porous foam, and plastic deformation methods.Integration of functional components (metal electrodes or FETs) on the 3D scaffold structures, enables multisite, concurrent electrophysiological monitoring within organoids.Such bioelectronics provide 3D mesostructures as electronic cell scaffolds.Reproduced with permission. [156]Copyright 2017, National Academy of Sciences.b) Porous foams.Stretchable scaffold-like 3D bioelectronics.Reproduced with permission. [40]Copyright 2021, Wiley-VCH.c) Vertical electrode arrays.3D multi-electrode arrays transformed from a 2D precursor by plastic deformations of the hinges.Reproduced with permission. [39]Copyright 2020, The Royal Society of Chemistry.
reliable, advanced platforms for studying the mechanism of selfrenewal and differentiation capacities of organoids.

Conformal Type Bioelectronics for Organoids
3][174][175] In this section, several conformal type bioelectronics for organoids and issues are introduced.The advanced devices consist of multilayered stacks of ultrathin active components, including sensors that have dimensions comparable to those of a single cell.For example, these systems can monitor specific collections of neurons in the central and peripheral .Conformal 3D bioelectronics for organoids.a) Self-bending leaflets.3D shell microelectrode arrays composed of self-bending polymer leaflets with metal coating for electrophysiology recording of brain organoids.Reproduced with permission. [43]Copyright 2022, American Association for the Advancement of Science.b) Assembled frameworks.3D frameworks as compliant, multifunctional interfaces for spheroids.Reproduced with permission. [10]Copyright 2021, American Association for the Advancement of Science.c) Self-rolling/-twining shells.i) 3D self-rolled biosensor arrays.Reproduced with permission. [44]Copyright 2019, American Association for the Advancement of Science.ii) 3D microchannels of the cuff-type implant.Reproduced with permission. [154]Copyright 2015, Wiley-VCH.iii) 3D twining electrode.Reproduced with permission. [176]Copyright 2019, American Association for the Advancement of Science.
nervous systems, as means to elaborate connections to complex behavioral responses.

Self-Bending Leaflets
Conformal multielectrode arrays can provide noninvasive recording and network mapping of extracellular potential for organoids.
Inspired by the shape of electroencephalography caps, Huang et al. developed 3D shell microelectrode arrays composed of selfbending polymer leaflets with metal coating for electrophysiology recording of brain organoids. [43]The solvent exchange between acetone and water for the gradient cross-linked SU8 triggers the controlled bending of leaflets, resulting in the 2D-to-3D transformation of shell microelectrode arrays (Figure 5a).The thickness and extent of cross-linking extent are two key design parameters for achieving different bending angles.By tuning the bending angle, shape, and electrode pattern, a customizable conformability can be realized for organoids with different sizes.
When comparing to conventional planar microelectrode arrays, such 3D microelectrode arrays can address the limitations that the recording contact area is restricted to the bottom of the 3D organoids.For a better signal-to-noise ratio, the needle type electrodes are used to provide recording access to the interior of the measured cell via penetration, while such 3D microelectrode arrays have advantages in providing noninvasive and high-speed recording and network mapping of extracellular electric field potential.

Assembled Frameworks
Mechanically-guided 3D assembly methods typically involve the 2D-to-3D geometric transformation of a patterned high-modulus thin film on a low-modulus elastomer substrate much thicker than the film, driven by compressive buckling. [162]Park et al. developed a class of compliant 3D electrical interfaces, through compressive buckling of patterned 2D electrode array consisting of polymer support and metal electrode/interconnect. [10] In particular, the 2D precursor pattern can be customized to allow the resulting 3D electronics to be matched with various different shapes of organoids, highlighting the design versatility (Figure 5b).Two examples of 3D electrical interfaces are provided to study the spreading of coordinated bursting events of an isolated cortical spheroid and of a pair of such spheroids.The multifunctional interface is capable of simultaneous electrophysiological monitoring of 16 spheroids.
With the tailored geometric features and low bending stiffnesses, the assembled 3D framework can gently envelop and hold an individual neural spheroid, with multifunctional devices on the individual wings, such as electrical, optical, chemical, and thermal interfaces.These design features, together with soft, proximity contacts to the surface of the spheroid, yields high performance in recording of neural activity. [10]Based on finite element analysis (FEA), the optimized selection of design parameters to ensure contact with extremely low forces at the soft tissue interfaces can be easily realized.Besides, the elastomeric substrate can be reversibly compressed or stretched to alter the 3D geometry to accommodate dynamic, natural changes in the size of the spheroid as it evolves and grows.

Self-Rolling/-Twining Shells
3D rolled-up nanomembrane bioelectronics can be robustly attached to a tissue to enable the enclosure of neuronal cells or nervous fibers.The rolled-up nanomembrane structures can be actuated from planar configurations mainly by light, pH, temperature, electrical or magnetic stimuli. [154,177,178]Kalmykov et al. reported 3D self-rolled biosensor arrays through use of residual stresses in the metal/polymer support bilayer structure. [44]The 3D FETs and microelectrodes were implemented to interface human cardiac spheroids for electrophysiological monitoring and investigation of the complex signal transduction in 3D cell assemblies toward an organ-on-an-electronic-chip (Figure 5c-i).The ra-dius of curvature can be controlled by the geometric design and the residual stress level.
Karnaushenko et al. reported mechanically adaptive microchannels of the cuff-type implant with integrated highperformance electronics (Figure 5c-ii), including signal amplifiers and logic circuitry based on indium gallium zinc oxide (IGZO) transistors. [154]The device can be bent, or self-assembled into a swiss roll like microtube, [178][179][180] by adjusting the environmental conditions, including the solution composition and pH.The diameter of the tubular structures can be tuned over a wide range from 50 μm to 1 mm.The differentiation and guided growth of neural stem cells in the microchannels of the 3D device were demonstrated.The integrated electronics allowed the detection of tiny amounts of ionic liquids in the microchannel, mimicking the detection of polarization/depolarization processes in neural microconduits.
[183][184] However, the mechanical and geometrical mismatches at electrode nerve interfaces and surgical implantation often induce irreversible neural damage.Zhang et.al. reported climbing-inspired twining electrodes for peripheral nerve stimulation and recording. [176]The reported 3D twining electrode has permanent shape reconfigurability, distinct elastic modulus controllability (from ≈100 MPa to ≈300 kPa), and shape memory recoverability at body temperature (Figure 5c-iii).Importantly, the 3D twining electrode is also compatible with traditional 2D planar processing.Similar to the climbing process of twining plants, the 2D stiff twining electrode can naturally self-climb onto nerves driven by 37 °C normal saline.
Conformal type bioelectronics are designed to offer high degree of flexibility and shape morphing capability.The electrodes or FETs are distributed on the surface of organoids, capable of monitoring the multisite potentials.Importantly, the 3D shape of such bioelectronics can be tailored by changing the geometries and bending stiffnesses, thereby showing potential in ondemand design for versatile organoids.The challenges and opportunities remain in morphing capability and miniaturization of such bioelectronics.

Porous 3D Mesh
To address the dimensional and mechanical mismatch of probes with the brain tissue, Xie et al. developed a 3D ultra-flexible bioelectronic network consisting of NW FET sensors to serve as minimally invasive brain probes. [48]The high flexibility of such probes precludes direct insertion into tissue, hence, rapid freezing provides sufficient rigidity to allow controlled insertion into neural tissue.Due to the high flexibility of the network configuration, the porous network probes (Figure 6a-i) can be integrated Reproduced with permission. [48]Copyright 2015, Nature Publishing Group.b) Nanofilm.Self-assembled 3D nanofilm electrode arrays as long-term neural interfaces.Reproduced with permission. [47]Copyright 2021, Wiley-VCH.c) Neuron-like probes.Bioinspired neuron-like electronics as 3D neural probes for stable, long-term recording.Reproduced with permission. [46]Copyright 2019, Nature Publishing Group.
with the brain tissue without inducing large mechanical constraints.The biocompatible porous probes were implanted into rodent brains with minimal surgical tissue damage, with capabilities of long-term stable recording.
Floch et.al. developed 3D mesh nanoelectronics consisting of highly stretchable serpentine wires(Figure 6a-ii), which can be naturally deformed to conform to the entire surface of brain organoids for long-term, multisites, continuous recording. [45]he stretchable mesh nanoelectronics can well the mechanical properties of brain organoids and be folded by the organogenetic process of progenitor or stem cells.The integrated stretchable electrode arrays show no interruption to brain organoids, adapt to the volume and morphological changes.The seamless and noninvasive coupling of electrodes to neurons enables long-term stable recording.

Nanofilm
Gao et al. presented high-density, free-standing gold nanofilm electrode arrays as a stable electrode-tissue interface. [47]The 2D nanofilm electrode arrays (10 nm in thickness) were encapsulated by biodissolvable polymer carriers, enabling their reliable implantation into deep brain tissues.After implantation and dissolution of polymer carriers, the ultrathin electrode array was released and assembled into 3D configurations to interface with neural tissues (Figure 6b).Chronically implanted nanofilm electrode arrays can form intimate and innervated interfaces with neural tissue, enabling stable recordings across multiple brain regions over several months.
Neural electrodes based on free-standing nanoscale materials offer unique opportunities to interrogate neural systems at unprecedented spatiotemporal scales, such as nanowires, nanotubes, and nanofilms.However, such ultra-flexible materials have precluded their direct implantation into the brain tissues.The methods of dissolution of polymer carriers can be extended to assemble a variety of free-standing nanoscale materials into implantable, high-density neural probes, which offer important opportunities in basic neuroscience and biomedical applications.

Neuron-Like Probes
To study the brain, bioinspired and biomimetic strategies have begun to be applied to the development of neural probes.Inspired by the structural features and mechanical properties of neurons, Yang et al. developed neuron-like electronics (NeuE) as neural probes with negligible immune responses, which exhibit seamless interpenetrating interfaces with the brain. [46]NeuE probes demonstrated a stable single-unit recording of individual cells in the nearly native physiological context without loss in recording quality (Figure 6c).NeuE are capable of mimicking the structural features (e.g., shape and size) and mechanical properties (e.g., flexibility) of neurons, representing a key advance of this work.In this biomimetic design, the sizes of the metal recording electrode and interconnect match those of the soma and neurite of the typical pyramidal neuron.The interconnect and axon have similar and the thin polymer insulation is analogous to the myelin sheath.Time-dependent histology and electrophysiology studies also reveal a functionally stable interface with the neuronal and glial networks, thus opening opportunities for next-generation brain-machine interfaces.
Implantable type bioelectronics capable of multisite long-term monitoring of tissue culture with negligible mechanical constraints.Such flexible bioelectronics are compatible with in vivo tissues and organoids with minimal surgical tissue damage, with capabilities of long-term stable recording.By customizing the structural features and mechanical properties, such implantable bioelectronics could serve as biomimetic cells, capable of electrophysiological monitoring inside in vivo tissues and organoids.

Summary and Outlook
This review summarizes the developments of three general classes of nanostructured bioelectronics for cells (Table 1), highlighting their structural designs, device functions and potential applications.First, the representative architectures (needle type) of 1D nanostructured bioelectronics are demonstrated for highly sensitive and selective intracellular detection of single cells by penetrating the cell membrane.While these 1D nanostructured bioelectronic devices offer tight coupling interfaces with cells, the invasiveness could affect the long-term recording of cell activities.Based on the well-established planar nanofabrication and processing techniques, large-area, 2D nanostructured bioelectronics with high-density integration of electrodes/FETs have been developed for multisite recording of cells, which are useful for understanding of intercellular communications.In addition, three types of representative architectures of 3D nanostructured bioelectronics are discussed, including scaffold, conformal, and implantable types.The scaffold type bioelectronics can be effectively used for studying the intercellular communication, whereas the reported devices all had a relative low electrode/FET density as compared to 2D nanostructured bioelectronics due to the complicated fabrication.The architecture of conformal type bioelectronics can be exploited to non-invasively wrap around the organoids with diverse shapes.However, it remains challenging to scale down such devices for single cell monitoring, based on the existing fabrication/assembly methods.The implantable nanostructured bioelectronics consisting of highly flexible network architecture allow for multisite long-term monitoring of tissue culture with negligible mechanical constraints.But it is difficult to achieve a precise positioning of recording sites in the implanted condition.
Bioelectric interface technologies aspire to explore the underlying mechanism of life activities in cells and organoids by bringing together the most important advances in the discipline, enhancing the comprehensive performance excellence that covers various metrics/aspects.The needle type bioelectronics have great potentials in the fields of intracellular monitoring, such as the silicon nanoprobes can be exploited to record intracellular electrical signals of a single living cell.In contrast, the patch-clamp technique could provide high-quality action potential recordings at the cell membranes.The 2D and 3D bioelectronics have more advantages in studying intercellular communications within organoids.Such platforms with unprecedented functions and revolutionary capabilities enable in vivo (or in vitro) sensing of cell activities, showing potential in revealing the fundamental mechanisms.
Looking to the future, many challenges and opportunities remain in developing and improving nanostructured bioelectronics for cells.For examples, the development of novel, efficient and low-cost methods for nanofabrication and/or assembly is the key to promote nanostructured bioelectronics toward practical applications.In view of the current research status of biological nanodevices, the design optimization of device architecture with enhanced cell-device interface coupling and fabrication feasibility still requires further efforts.Opportunities exist in the incorporation of artificial intelligence (AI) techniques to assist the design and fabrication of nanostructured bioelectronics devices.A comprehensive performance excellence that covers various metrics/aspects (e.g., high selectivity, high sensitivity, high signal-to-noise ratio, long-term stability, high density, and etc.) remains a long-term goal pursued in the development of bioelectronics.Additionally, the ability to simultaneously monitor a large number of cells over long time is essential for the biological and pathophysiological studies, but quite a few CMOS-based devices could achieve simultaneous, high-fidelity, high-throughput and large-scale intracellular recordings.Opportunities exist in the combination of cell nanoelectronics with CMOS integrated circuits, which cannot only improve the signal quality, but also provide a feasible strategy for building a large-scale, parallel monitoring platform.
To explore the diversity and complexity of life activities from cells and tissues to organisms, 3D bioelectronic devices are powerful tools for multiplexed signal collection and multi-modality studies.Multifunctional nanostructured bioelectronics that can achieve electrical/optical/chemical monitoring/stimulation as well as cell screening and intracellular delivery/extraction also represent a promising area for further explorations.The development of bioelectric interface technologies could promote the emerging of advanced nanofabrication technologies and powerful studying platforms, which are crucial to potential biomedical applications.The bioelectronics could also provide support of fundamental breakthroughs in life science.

Figure 1 .
Figure 1.A summary of nanostructured bioelectronics for cells and organoids.

Figure 3 .
Figure 3. Planar type bioelectronics for cells.a) Nanowells.Nanowell-based microelectrode for penetrating the cardiomyocyte membrane by electroporation and recording intracellular action potentials.Reproduced with permission.[136]Copyright 2022, Nature Publishing Group.b) Planar FETs.i) Colorized electron micrograph image of a neuron on a silicon chip with a linear array of buried-channel FETs.Reproduced with permission.[137]Copyright 2005, Wiley-VCH.ii) Optical image of a cortex neuron connected to a NW device.Reproduced with permission.[138]Copyright 2006, American Association for the Advancement of Science.iii) Schematic illustration of graphene-based solution gated FETs for the detection of cell signals.Reproduced with permission.[139]Copyright 2011, Wiley-VCH.

Figure 5
Figure 5. Conformal 3D bioelectronics for organoids.a) Self-bending leaflets.3D shell microelectrode arrays composed of self-bending polymer leaflets with metal coating for electrophysiology recording of brain organoids.Reproduced with permission.[43]Copyright 2022, American Association for the Advancement of Science.b) Assembled frameworks.3D frameworks as compliant, multifunctional interfaces for spheroids.Reproduced with permission.[10]Copyright 2021, American Association for the Advancement of Science.c) Self-rolling/-twining shells.i) 3D self-rolled biosensor arrays.Reproduced with permission.[44]Copyright 2019, American Association for the Advancement of Science.ii) 3D microchannels of the cuff-type implant.Reproduced with permission.[154]Copyright 2015, Wiley-VCH.iii) 3D twining electrode.Reproduced with permission.[176]Copyright 2019, American Association for the Advancement of Science.