3D Interfacing between Soft Electronic Tools and Complex Biological Tissues

Living Synthelectronics: A New Era for Bioelectronics Powered by Synthetic Biology

Bioelectronics, which converges biology and electronics, has attracted great attention due to their vital applications in human-machine interfaces. While traditional bioelectronic devices utilize nonliving organic and/or inorganic materials to achieve flexibility and stretchability, a biological mismatch is often encountered because human tissues are characterized not only by softness and stretchability but also by biodynamic and adaptive properties. Recently, a notable paradigm shift has emerged in bioelectronics, where living cells, and even viruses, modified via gene editing within synthetic biology, are used as core components in a new hybrid electronics paradigm. These devices are defined as "living synthelectronics," and they offer enhanced potential for interfacing with human tissues at informational and substance exchange levels. In this Perspective, the recent advances in living synthelectronics are summarized. First, opportunities brought to electronics by synthetic biology are briefly introduced. Then, strategic approaches to designing and making electronic devices using living cells/viruses as the building blocks, sensing components, or power sources are reviewed. Finally, the challenges faced by living synthelectronics are raised. It is believed that this paradigm shift will significantly contribute to the real integration of bioelectronics with human tissues.

Electronic Skin for Health Monitoring Systems: Properties, Functions, and Applications

Electronic skin (e-skin), a skin-like wearable electronic device, holds great promise in the fields of telemedicine and personalized healthcare because of its good flexibility, biocompatibility, skin conformability, and sensing performance. E-skin can monitor various health indicators of the human body in real time and over the long term, including physical indicators (exercise, respiration, blood pressure, etc.) and chemical indicators (saliva, sweat, urine, etc.). In recent years, the development of various materials, analysis, and manufacturing technologies has promoted significant development of e-skin, laying the foundation for the application of next-generation wearable medical technologies and devices. Herein, the properties required for e-skin health monitoring devices to achieve long-term and precise monitoring and summarize several detectable indicators in the health monitoring field are discussed. Subsequently, the applications of integrated e-skin health monitoring systems are reviewed. Finally, current challenges and future development directions in this field are discussed. This review is expected to generate great interest and inspiration for the development and improvement of e-skin and health monitoring systems.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Materials-Driven Soft Wearable Bioelectronics for Connected Healthcare

In the era of Internet-of-things, many things can stay connected; however, biological systems, including those necessary for human health, remain unable to stay connected to the global Internet due to the lack of soft conformal biosensors. The fundamental challenge lies in the fact that electronics and biology are distinct and incompatible, as they are based on different materials via different functioning principles. In particular, the human body is soft and curvilinear, yet electronics are typically rigid and planar. Recent advances in materials and materials design have generated tremendous opportunities to design soft wearable bioelectronics, which may bridge the gap, enabling the ultimate dream of connected healthcare for anyone, anytime, and anywhere. We begin with a review of the historical development of healthcare, indicating the significant trend of connected healthcare. This is followed by the focal point of discussion about new materials and materials design, particularly low-dimensional nanomaterials. We summarize material types and their attributes for designing soft bioelectronic sensors; we also cover their synthesis and fabrication methods, including top-down, bottom-up, and their combined approaches. Next, we discuss the wearable energy challenges and progress made to date. In addition to front-end wearable devices, we also describe back-end machine learning algorithms, artificial intelligence, telecommunication, and software. Afterward, we describe the integration of soft wearable bioelectronic systems which have been applied in various testbeds in real-world settings, including laboratories that are preclinical and clinical environments. Finally, we narrate the remaining challenges and opportunities in conjunction with our perspectives.

Microinstrumentation for Brain Organoids

Brain organoids are three-dimensional aggregates of self-organized differentiated stem cells that mimic the structure and function of human brain regions. Organoids bridge the gaps between conventional drug screening models such as planar mammalian cell culture, animal studies, and clinical trials. They can revolutionize the fields of developmental biology, neuroscience, toxicology, and computer engineering. Conventional microinstrumentation for conventional cellular engineering, such as planar microfluidic chips; microelectrode arrays (MEAs); and optical, magnetic, and acoustic techniques, has limitations when applied to three-dimensional (3D) organoids, primarily due to their limits with inherently two-dimensional geometry and interfacing. Hence, there is an urgent need to develop new instrumentation compatible with live cell culture techniques and with scalable 3D formats relevant to organoids. This review discusses conventional planar approaches and emerging 3D microinstrumentation necessary for advanced organoid-machine interfaces. Specifically, this article surveys recently developed microinstrumentation, including 3D printed and curved microfluidics, 3D and fast-scan optical techniques, buckling and self-folding MEAs, 3D interfaces for electrochemical measurements, and 3D spatially controllable magnetic and acoustic technologies relevant to two-way information transfer with brain organoids. This article highlights key challenges that must be addressed for robust organoid culture and reliable 3D spatiotemporal information transfer.

Flexible multifunctional titania nanotube array platform for biological interfacing

Abstract

The current work presents a novel flexible multifunctional platform for biological interface applications. The use of titania nanotube arrays (TNAs) as a multifunctional material is explored for soft-tissue interface applications. In vitro biocompatibility of TNAs to brain-derived cells was first examined by culturing microglia cells-the resident immune cells of the central nervous system on the surface of TNAs. The release profile of an anti-inflammatory drug, dexamethasone from TNAs-on-polyimide substrates, was then evaluated under different bending modes. Flexible TNAs-on-polyimide sustained a linear release of anti-inflammatory dexamethasone up to ~11 days under different bending conditions. Finally, microfabrication processes for patterning and transferring TNA microsegments were developed to facilitate structural stability during device flexing and to expand the set of compatible polymer substrates. The techniques developed in this study can be applied to integrate TNAs or other similar nanoporous inorganic films onto various polymer substrates.

Impact statement

Titania nanotube arrays (TNAs) are highly tunable and biocompatible structures that lend themselves to multifunctional implementation in implanted devices. A particularly important aspect of titania nanotubes is their ability to serve as nano-reservoirs for drugs or other therapeutic agents that slowly release after implantation. To date, TNAs have been used to promote integration with rigid, dense tissues for dental and orthopedic applications. This work aims to expand the implant applications that can benefit from TNAs by integrating them onto soft polymer substrates, thereby promoting compatibility with soft tissues. The successful direct growth and integration of TNAs on polymer substrates mark a critical step toward developing mechanically compliant implantable systems with drug delivery from nanostructured inorganic functional materials. Diffusion-driven release kinetics and the high drug-loading efficiency of TNAs offer tremendous potential for sustained drug delivery for scientific investigations, to treat injury and disease, and to promote device integration with biological tissues. This work opens new opportunities for developing novel and more effective implanted devices that can significantly improve patient outcomes and quality of life.

Graphical abstract

Supplementary information

The online version contains supplementary material available at 10.1557/s43577-023-00628-y.

Tough soldering for stretchable electronics by small-molecule modulated interfacial assemblies

The rapid-developing soft robots and wearable devices require flexible conductive materials to maintain electric functions over a large range of deformations. Considerable efforts are made to develop stretchable conductive materials; little attention is paid to the frequent failures of integrated circuits caused by the interface mismatch of soft substrates and rigid silicon-based microelectronics. Here, we present a stretchable solder with good weldability that can strongly bond with electronic components, benefiting from the hierarchical assemblies of liquid metal particles, small-molecule modulators, and non-covalently crosslinked polymer matrix. Our self-solder shows high conductivity (>2×105  S  m-1), extreme stretchability (~1000%, and >600% with chip-integrated), and high toughness (~20 MJ m-3). Additionally, the dynamic interactions within our solder's surface and interior enable a range of unique features, including ease of integration, component substitution, and circuit recyclability. With all these features, we demonstrated an application as thermoforming technology for three-dimensional (3D) conformable electronics, showing potential in reducing the complexity of microchip interfacing, as well as scalable fabrication of chip-integrated stretchable circuits and 3D electronics.

Optical Integration in Wearable, Implantable and Swallowable Healthcare Devices

Recent advances in materials and semiconductor technologies have led to extensive research on optical integration in wearable, implantable, and swallowable health devices. These optical systems utilize the properties of light─intensity, wavelength, polarization, and phase─to monitor and potentially intervene in various biological events. The potential of these devices is greatly enhanced through the use of multifunctional optical materials, adaptable integration processes, advanced optical sensing principles, and optimized artificial intelligence algorithms. This synergy creates many possibilities for clinical applications. This Perspective discusses key opportunities, challenges, and future directions, particularly with respect to sensing modalities, multifunctionality, and the integration of miniaturized optoelectronic devices. We present fundamental insights and illustrative examples of such devices in wearable, implantable, and swallowable forms. The constant pursuit of innovation and the dedicated approach to critical challenges are poised to influence diverse fields.

Construction of a Flexible Optogenetic Device for Multisite and Multiregional Optical Stimulation Through Flexible µ-LED Displays on the Cerebral Cortex

Precisely delivering light to multiple locations in biological tissue is crucial for advancing multiregional optogenetics in neuroscience research. However, conventional implantable devices typically have rigid geometries and limited light sources, allowing only single or dual probe placement with fixed spacing. Here, a fully flexible optogenetic device with multiple thin-film microscale light-emitting diode (µ-LED) displays scattering from a central controller is presented. Each display is heterogeneously integrated with thin-film 5 × 10 µ-LEDs and five optical fibers 125 µm in diameter to achieve cellular-scale spatial resolution. Meanwhile, the device boasts a compact, flexible circuit capable of multichannel configuration and wireless transmission, with an overall weight of 1.31 g, enabling wireless, real-time neuromodulation of freely moving rats. Characterization results and finite element analysis have demonstrated excellent optical properties and mechanical stability, while cytotoxicity tests further ensure the biocompatibility of the device for implantable applications. Behavior studies under optogenetic modulation indicate great promise for wirelessly modulating neural functions in freely moving animals. The device with multisite and multiregional optogenetic modulation capability offers a comprehensive platform to advance both fundamental neuroscience studies and potential applications in brain-computer interfaces.

Highly Elastic and Strain Sensing Corn Protein Electrospun Fibers for Monitoring of Wound Healing

Due to the lack of sufficient elasticity and strain sensing capability, protein-based ultrafine fibrous tissue engineering scaffolds, though favorable for skin repair, can hardly fulfill on-spot wound monitoring during healing. Herein, we designed highly elastic corn protein ultrafine fibrous smart scaffolds with a three-layer structure for motion tracking at an unpackaged state. The densely cross-linked protein networks were efficiently established by introducing a highly reactive epoxy and provided the fiber substrates with wide-range stretchability (360% stretching range) and ultrahigh elasticity (99.91% recovery rate) at a wet state. With the assistance of the polydopamine bonding layer, a silver conductive sensing layer was built on the protein fibers and endowed the scaffolds with wide strain sensing range (264%), high sensitivity (gauge factor up to 210.55), short response time (<70 ms), reliable cycling stability, and long-lasting duration (up to 30 days). The unpackaged smart scaffolds could not only support cell growth and accelerate wound closure but also track motions on skin and in vivo and trigger alarms once excessive wound deformations occurred. These features not only confirmed the great potential of these smart scaffolds for applications in tissue reconstruction and wound monitoring but also proved the possibility of employing various plant protein ultrafine fibers as flexible bioelectronics.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Multifunctional tendon-mimetic hydrogels

We report multifunctional tendon-mimetic hydrogels constructed from anisotropic assembly of aramid nanofiber composites. The stiff nanofibers and soft polyvinyl alcohol in these anisotropic composite hydrogels (ACHs) mimic the structural interplay between aligned collagen fibers and proteoglycans in tendons. The ACHs exhibit a high modulus of ~1.1 GPa, strength of ~72 MPa, fracture toughness of 7333 J/m2, and many additional characteristics matching those of natural tendons, which was not achieved with previous synthetic hydrogels. The surfaces of ACHs were functionalized with bioactive molecules to present biophysical cues for the modulation of morphology, phenotypes, and other behaviors of attached cells. Moreover, soft bioelectronic components can be integrated on ACHs, enabling in situ sensing of various physiological parameters. The outstanding mechanics and functionality of these tendon mimetics suggest their further applications in advanced tissue engineering, implantable prosthetics, human-machine interactions, and other technologies.

A UV-induced self-reinforced hydrogel based on in situ hydrophobic aggregation of strained 1,2-dithiolane rings

A novel responsive hydrogel exhibiting self-reinforcement and self-healing capacity was developed based on the hydrophobic aggregation of strained 1,2-dithiolane rings. Oligomerization of 1,2-dithiolane within hydrophobic domains under UV irradiation not only reinforced the hydrogel but also maintained its dynamic cross-linked nature by converting the intraring dynamic S-S bond to an outer one.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

Robust and Multifunctional Kirigami Electronics with a Tough and Permeable Aramid Nanofiber Framework

Kirigami designs are advantageous for the construction of wearable electronics due to their high stretchability and conformability on the 3D dynamic surfaces of the skin. However, suitable materials technologies that enable robust kirigami devices with desired functionality for skin-interfaces remain limited. Here, a versatile materials platform based on a composite nanofiber framework (CNFF) is exploited for the engineering of wearable kirigami electronics. The self-assembled fibrillar network involving aramid nanofibers and poly(vinyl alcohol) combines high toughness, permeability, and manufacturability, which are desirable for the fabrication of hybrid devices. Multiscale simulations are conducted to explain the high fracture resistance of the CNFF-based kirigami structures and provide essential guidance for the design, which can be further generalized to other kirigami devices. Various microelectronic sensors and electroactive polymers are integrated onto a CNFF-based materials platform to achieve electrocardiogram (ECG), electromyogram (EMG), skin-temperature measurements, and measurement of other physiological parameters. These mechanically robust, multifunctional, lightweight, and biocompatible kirigami devices can shed new insights for the development of advanced wearable systems and human-machine interfaces.

Morphing-to-Adhesion Polysaccharide Hydrogel for Adaptive Biointerfaces

Reliable functions of medical implants highly depend on biocompatible, conformal, and stable biointerfaces for seamless biointegration with biological tissues. Though flexible biointerfaces based on synthetic hydrogels have shown promise in optimizing implant biointegration via surgical suturing, physical attachment, or manual preshaping, they still suffer from poor adaptability, such as tissue damage by surgical suturing, low bioactivity, and difficulties in conformal contact and stable fixation, especially for specific tissues of large surface curvatures. Here, we report a bilayer hydrogel-based adaptive biointerface (HAB) made of two polysaccharide derivates, N-hydroxysuccinimide (NHS) ester-activated alginate and chitosan, harnessing dual advantages of their different swelling and active groups. Leveraging on the differential swelling between the two hydrogel layers and covalent linkages with active groups at hydrogel interfaces, HABs can be programmed into sealed tubes with tunable diameters via water-induced compliable shape morphing and instant interfacial adhesion. We further demonstrate that the polysaccharide-based morphing-to-adhesion HAB possesses outstanding bioactivity in directing cellular focal adhesion and intercellular junction, versatile geometrical adaptability to diverse tubular tissues with a wide range of surface curvatures (2.8 × 10²-1.3 × 10³ m^-1), and excellent mechanical stability in high load-/shear-bearing physiological environments (blood flow volume: 85 mm·s^-1). HABs overcome the limitations of existing biointerfaces in terms of poor bioactivity and difficult biointegration with biological tissues of large surface curvatures, holding promise to open new avenues for adaptive biointerfaces and reliable medical implants.

Advancing Engineered Heart Muscle Tissue Complexity with Hydrogel Composites

A heart attack results in the permanent loss of heart muscle and can lead to heart disease, which kills more than 7 million people worldwide each year. To date, outside of heart transplantation, current clinical treatments cannot regenerate lost heart muscle or restore full function to the damaged heart. There is a critical need to create engineered heart tissues with structural complexity and functional capacity needed to replace damaged heart muscle. The inextricable link between structure and function suggests that hydrogel composites hold tremendous promise as a biomaterial-guided strategy to advance heart muscle tissue engineering. Such composites provide biophysical cues and functionality as a provisional extracellular matrix that hydrogels cannot on their own. This review describes the latest advances in the characterization of these biomaterial systems and using them for heart muscle tissue engineering. The review integrates results across the field to provide new insights on critical features within hydrogel composites and perspectives on the next steps to harnessing these promising biomaterials to faithfully reproduce the complex structure and function of native heart muscle.

Kirigami-Inspired Biodesign for Applications in Healthcare

Mechanically flexible and conformable materials and integrated devices have found diverse applications in personalized healthcare as diagnostics and therapeutics, tissue engineering and regenerative medicine constructs, surgical tools, secure systems, and assistive technologies. In order to impart optimal mechanical properties to the (bio)materials used in these applications, various strategies have been explored-from composites to structural engineering. In recent years, geometric cuts inspired by the art of paper-cutting, referred to as kirigami, have provided innovative opportunities for conferring precise mechanical properties via material removal. Kirigami-based approaches have been used for device design in areas ranging from soft bioelectronics to energy storage. In this review, the principles of kirigami-inspired engineering specifically for biomedical applications are discussed. Factors pertinent to their design, including cut geometry, materials, and fabrication, and the effect these parameters have on their properties and configurations are covered. Examples of kirigami designs in healthcare are presented, such as, various form factors of sensors (on skin, wearable), implantable devices, therapeutics, surgical procedures, and cellular scaffolds for regenerative medicine. Finally, the challenges and future scope for the successful translation of these biodesign concepts to broader deployment are discussed.

Highly Stretchable Hydrogels as Wearable and Implantable Sensors for Recording Physiological and Brain Neural Signals

Recording electrophysiological information such as brain neural signals is of great importance in health monitoring and disease diagnosis. However, foreign body response and performance loss over time are major challenges stemming from the chemomechanical mismatch between sensors and tissues. Herein, microgels are utilized as large crosslinking centers in hydrogel networks to modulate the tradeoff between modulus and fatigue resistance/stretchability for producing hydrogels that closely match chemomechanical properties of neural tissues. The hydrogels exhibit notably different characteristics compared to nanoparticles reinforced hydrogels. The hydrogels exhibit relatively low modulus, good stretchability, and outstanding fatigue resistance. It is demonstrated that the hydrogels are well suited for fashioning into wearable and implantable sensors that can obtain physiological pressure signals, record the local field potentials in rat brains, and transmit signals through the injured peripheral nerves of rats. The hydrogels exhibit good chemomechanical match to tissues, negligible foreign body response, and minimal signal attenuation over an extended time, and as such is successfully demonstrated for use as long-term implantable sensory devices. This work facilitates a deeper understanding of biohybrid interfaces, while also advancing the technical design concepts for implantable neural probes that efficiently obtain physiological information.

Microporous Multiresonant Plasmonic Meshes by Hierarchical Micro-Nanoimprinting for Bio-Interfaced SERS Imaging and Nonlinear Nano-Optics

Microporous mesh plasmonic devices have the potential to combine the biocompatibility of microporous polymeric meshes with the capabilities of plasmonic nanostructures to enhance nanoscale light-matter interactions for bio-interfaced optical sensing and actuation. However, scalable integration of dense and uniformly structured plasmonic hotspot arrays with microporous polymeric meshes remains challenging due to the processing incompatibility of conventional nanofabrication methods with flexible microporous substrates. Here, scalable nanofabrication of microporous multiresonant plasmonic meshes (MMPMs) is achieved via a hierarchical micro-/nanoimprint lithography approach using dissolvable polymeric templates. It is demonstrated that MMPMs can serve as broadband nonlinear nanoplasmonic devices to generate second-harmonic generation, third-harmonic generation, and upconversion photoluminescence signals with multiresonant plasmonic enhancement under fs pulse excitation. Moreover, MMPMs are employed and explored as bio-interfaced surface-enhanced Raman spectroscopy mesh sensors to enable in situ spatiotemporal molecular profiling of bacterial biofilm activity. Microporous mesh plasmonic devices open exciting avenues for bio-interfaced optical sensing and actuation applications, such as inflammation-free epidermal sensors in conformal contact with skin, combined tissue-engineering and biosensing scaffolds for in vitro 3D cell culture models, and minimally invasive implantable probes for long-term disease diagnostics and therapeutics.

Recent Progress in Materials Chemistry to Advance Flexible Bioelectronics in Medicine

Designing bioelectronic devices that seamlessly integrate with the human body is a technological pursuit of great importance. Bioelectronic medical devices that reliably and chronically interface with the body can advance neuroscience, health monitoring, diagnostics, and therapeutics. Recent major efforts focus on investigating strategies to fabricate flexible, stretchable, and soft electronic devices, and advances in materials chemistry have emerged as fundamental to the creation of the next generation of bioelectronics. This review summarizes contemporary advances and forthcoming technical challenges related to three principal components of bioelectronic devices: i) substrates and structural materials, ii) barrier and encapsulation materials, and iii) conductive materials. Through notable illustrations from the literature, integration and device fabrication strategies and associated challenges for each material class are highlighted.

Mechanically Guided Hierarchical Assembly of 3D Mesostructures

3D, hierarchical micro/nanostructures formed with advanced functional materials are of growing interest due to their broad potential utility in electronics, robotics, battery technology, and biomedical engineering. Among various strategies in 3D micro/nanofabrication, a set of methods based on compressive buckling offers wide-ranging material compatibility, fabrication scalability, and precise process control. Previously reports on this type of approach rely on a single, planar prestretched elastomeric platform to transform thin-film precursors with 2D layouts into 3D architectures. The simple planar configuration of bonding sites between these precursors and their assembly substrates prevents the realization of certain types of complex 3D geometries. In this paper, a set of hierarchical assembly concepts is reported that leverage multiple layers of prestretched elastomeric substrates to induce not only compressive buckling of 2D precursors bonded to them but also of themselves, thereby creating 3D mesostructures mounted at multiple levels of 3D frameworks with complex, elaborate configurations. Control over strains used in these processes provides reversible access to multiple different 3D layouts in a given structure. Examples to demonstrate these ideas through both experimental and computational results span vertically aligned helices to closed 3D cages, selected for their relevance to 3D conformal bio-interfaces and multifunctional microsystems.

Controlled Deposition of Nanostructured Hierarchical TiO2 Thin Films by Low Pressure Supersonic Plasma Jets

Plasma-assisted supersonic jet deposition (PA-SJD) is a precise technique for the fabrication of thin films with a desired nanostructured morphology. In this work, we used quadrupole mass spectrometry of the neutral species in the jet and the extensive characterization of TiO₂ films to improve our understanding of the relationship between jet chemistry and film properties. To do this, an organo–metallic precursor (titanium tetra–isopropoxide or TTIP) was first dissociated using a reactive argon–oxygen plasma in a vacuum chamber and then delivered into a second, lower pressure chamber through a nozzle. The pressure difference between the two chambers generated a supersonic jet carrying nanoparticles of TiO₂ in the second chamber, and these were deposited onto the surface of a substrate located few centimeters away from the nozzle. The nucleation/aggregation of the jet nanoparticles could be accurately tuned by a suitable choice of control parameters in order to produce the required structures. We demonstrate that high-quality films of up to several µm in thickness and covering a surface area of few cm² can be effectively produced using this PA-SJD technique.

Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics

Soft electronics are rising electronic technologies towards applications spanning from healthcare monitoring to medical implants. However, poor adhesion strength and significant mechanical mismatches inevitably cause the interface failure of devices. Herein we report a self-adhesive conductive polymer that possesses low modulus (56.1-401.9 kPa), high stretchability (700%), high interfacial adhesion (lap-shear strength >1.2 MPa), and high conductivity (1-37 S/cm). The self-adhesive conductive polymer is fabricated by doping the poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) composite with a supramolecular solvent (β-cyclodextrin and citric acid). We demonstrated the solution process-based fabrication of self-adhesive conductive polymer-based electrodes for various soft devices, including alternating current electroluminescent devices, electromyography monitoring, and an integrated system for the visualization of electromyography signals during muscle training with an array of alternating current electroluminescent devices. The self-adhesive conductive polymer-based electronics show promising features to further develop wearable and comfortable bioelectronic devices with the physiological electric signals of the human body readable and displayable during daily activities.

Graphene Microelectrodes for Real-Time Impedance Spectroscopy of Neural Cells

Understanding the changes in the electrochemical properties of neural cells upon exposure to stress factors imparts vital information about the conditions prior to their death. This study presents a graphene-based biosensor for real-time monitoring of N27 rat dopaminergic neural cells which characterizes cell adhesion and cytotoxicity factors through impedance spectroscopy. The aim was to monitor the growth of the entire cell network via a nonmetallic flexible electrode array. Therefore, a water-based graphene solution was formulized as a conductive ink, 3D-printed into a flexible substrate through an electrohydrodynamic approach, resulting in electrodes with a conductivity of 6750 s/m. The presented high-throughput method enabled microscale monitoring of the entire cell network via the design of PDMS-based growth channels. The electrical resistance of the cell network was measured continuously along with their network density, constituting a mean density of 1890 cell/mm² at full cell confluency. The results demonstrate the applicability of the impedance-based sensing of the cell network for rapid screening of the cytotoxic elements, and the real-time effect of UV exposure on dopaminergic neural cells was reported as an immediate application of the device.

A multifunctional electronic skin based on patterned metal films for tactile sensing with a broad linear response range

Electronic skins (e-skins) with multifunctional sensing functions have attracted a lot of attention due to their promising applications in intelligent robotics, human-machine interfaces, and wearable healthcare systems. Here, we report a multifunctional e-skin based on patterned metal films for tactile sensing of pressure and temperature with a broad linear response range by implementing the single sensing mechanism of piezoresistivity, which allows for the easy signal processing and simple device configuration. The sensing pixel features serpentine metal traces and spatially distributed microprotrusions. Experimental and numerical studies reveal the fundamental aspects of the multifunctional tactile sensing mechanism of the e-skin, which exhibits excellent flexibility and wearable conformability. The fabrication approach being compatible with the well-established microfabrication processes has enabled the scalable manufacturing of a large-scale e-skin for spatial tactile sensing in various application scenarios.

Recent advances in recording and modulation technologies for next-generation neural interfaces

Along with the advancement in neural engineering techniques, unprecedented progress in the development of neural interfaces has been made over the past few decades. However, despite these achievements, there is still room for further improvements especially toward the possibility of monitoring and modulating neural activities with high resolution and specificity in our daily lives. In an effort of taking a step toward the next-generation neural interfaces, we want to highlight the recent progress in neural technologies. We will cover a wide scope of such developments ranging from novel platforms for highly specific recording and modulation to system integration for practical applications of novel interfaces.

Flexible Electrodes for In Vivo and In Vitro Electrophysiological Signal Recording

A variety of electrophysiological signals (electrocardiography, electromyography, electroencephalography, etc.) are generated during the physiological activities of human bodies, which can be collected by electrodes and thus provide critical insights into health status or facilitate fundamental scientific research. The long-term stable and high-quality recording of electrophysiological signals is the premise for their further applications, leading to demands for flexible electrodes with similar mechanical modulus and minimized irritation to human bodies. This review summarizes the latest advances in flexible electrodes for the acquisition of various electrophysiological signals. First, the concept of electrophysiological signals and the characteristics of different subcategory signals are introduced. Second, the invasive and noninvasive methods are reviewed for electrophysiological signal recording with a highlight on the design of flexible electrodes, followed by a discussion on their material selection. Subsequently, the applications of the electrophysiological signal acquisition in pathological diagnosis and restoration of body functions are discussed, showing the advantages of flexible electrodes. Finally, the main challenges and opportunities in this field are discussed. It is believed that the further exploration of materials for flexible electrodes and the combination of multidisciplinary technologies will boost the applications of flexible electrodes for medical diagnosis and human-machine interface.

PEDOT:PSS-Based Bioelectronic Devices for Recording and Modulation of Electrophysiological and Biochemical Cell Signals

To understand the physiology and pathology of electrogenic cells and the corresponding tissue in their full complexity, the quantitative investigation of the transmission of ions as well as the release of chemical signals is important. Organic (semi-) conducting materials and in particular organic electrochemical transistor are gaining in importance for the investigation of electrophysiological and recently biochemical signals due to their synthetic nature and thus chemical diversity and modifiability, their biocompatible and compliant properties, as well as their mixed electronic and ionic conductivity featuring ion-to-electron conversion. Here, the aim is to summarize recent progress on the development of bioelectronic devices utilizing polymer polyethylenedioxythiophene: poly(styrene sulfonate) (PEDOT:PSS) to interface electronics and biological matter including microelectrode arrays, neural cuff electrodes, organic electrochemical transistors, PEDOT:PSS-based biosensors, and organic electronic ion pumps. Finally, progress in the material development is summarized for the improvement of polymer conductivity, stretchability, higher transistor transconductance, or to extend their field of application such as cation sensing or metabolite recognition. This survey of recent trends in PEDOT:PSS electrophysiological sensors highlights the potential of this multifunctional material to revolve current technology and to enable long-lasting, multichannel polymer probes for simultaneous recordings of electrophysiological and biochemical signals from electrogenic cells.