Soft and elastic hydrogel-based microelectronics for localized low-voltage neuromodulation

From lab to wearables: Innovations in multifunctional hydrogel chemistry for next-generation bioelectronic devices

Functional hydrogels have emerged as foundational materials in diagnostics, therapy, and wearable devices, owing to their high stretchability, flexibility, sensing, and outstanding biocompatibility. Their significance stems from their resemblance to biological tissue and their exceptional versatility in electrical, mechanical, and biofunctional engineering, positioning themselves as a bridge between living organisms and electronic systems, paving the way for the development of highly compatible, efficient, and stable interfaces. These multifaceted capability revolutionizes the essence of hydrogel-based wearable devices, distinguishing them from conventional biomedical devices in real-world practical applications. In this comprehensive review, we first discuss the fundamental chemistry of hydrogels, elucidating their distinct properties and functionalities. Subsequently, we examine the applications of these bioelectronics within the human body, unveiling their transformative potential in diagnostics, therapy, and human-machine interfaces (HMI) in real wearable bioelectronics. This exploration serves as a scientific compass for researchers navigating the interdisciplinary landscape of chemistry, materials science, and bioelectronics.

Rapid prototyping and facile customization of conductive hydrogel bioelectronics based on all laser process

Proper customization in size and shape is essential in implantable bioelectronics for stable bio-signal recording. Over the past decades, many researchers have heavily relied on conventional photolithography processes to fabricate implantable bioelectronics. Therefore, they could not avoid the critical limitation of high cost and complex processing steps to optimize bioelectronic devices for target organs with various sizes and shapes. Here, we propose rapid prototyping using all laser processes to fabricate customized bioelectronics. PEDOT:PSS is selectively irradiated by an ultraviolet (UV) pulse laser to form wet-stable conductive hydrogels that can softly interact with biological tissues (50 μm line width). The encapsulation layer is selectively patterned using the same laser source by UV-curing polymer networks (110 μm line width). For high stretchability (over 100%), mesh structures are made by the selective laser cutting process. Our rapid prototyping strategy minimizes the use of high-cost equipment, using only a single UV laser source to process the electrodes, encapsulation, and substrates that constitute bioelectronics without a photomask, enabling the prototyping stretchable microelectrode array with an area of 1 cm2 less than 10 min. We fabricated an optimized stretchable microelectrode array with low impedances (∼1.1 kΩ at 1 kHz) that can effectively record rat's cardiac signals with various health states.

Stretchable Tissue-Like Gold Nanowire Composites with Long-Term Stability for Neural Interfaces

Soft and stretchable nanocomposites can match the mechanical properties of neural tissue, thereby minimizing foreign body reactions to provide optimal stimulation and recording specificity. Soft materials for neural interfaces should simultaneously fulfill a wide range of requirements, including low Young's modulus (<<1 MPa), stretchability (≥30%), high conductivity (>> 1000 S cm-1), biocompatibility, and chronic stability (>> 1 year). Current nanocomposites do not fulfill the above requirements, in particular not the combination of softness and high conductivity. Here, this challenge is addressed by developing a scalable and robust synthesis route based on polymeric reducing agents for smooth, high-aspect ratio gold nanowires (AuNWs) of controllable dimensions with excellent biocompatibility. AuNW-silicone composites show outstanding performance with nerve-like softness (250 kPa), high conductivity (16 000 S cm-1), and reversible stretchability. Soft multielectrode cuffs based on the composite achieve selective functional stimulation, recordings of sensory stimuli in rat sciatic nerves, and show an accelerated lifetime stability of >3 years. The scalable synthesis method provides a chemically stable alternative to the widely used AgNWs, thereby enabling new applications within electronics, biomedical devices, and electrochemistry.

Non-Invasive Detection of Early-Stage Fatty Liver Disease via an On-Skin Impedance Sensor and Attention-Based Deep Learning

Early-stage nonalcoholic fatty liver disease (NAFLD) is a silent condition, with most cases going undiagnosed, potentially progressing to liver cirrhosis and cancer. A non-invasive and cost-effective detection method for early-stage NAFLD detection is a public health priority but challenging. In this study, an adhesive, soft on-skin sensor with low electrode-skin contact impedance for early-stage NAFLD detection is fabricated. A method is developed to synthesize platinum nanoparticles and reduced graphene quantum dots onto the on-skin sensor to reduce electrode-skin contact impedance by increasing double-layer capacitance, thereby enhancing detection accuracy. Furthermore, an attention-based deep learning algorithm is introduced to differentiate impedance signals associated with early-stage NAFLD in high-fat-diet-fed low-density lipoprotein receptor knockout (Ldlr-/-) mice compared to healthy controls. The integration of an adhesive, soft on-skin sensor with low electrode-skin contact impedance and the attention-based deep learning algorithm significantly enhances the detection accuracy for early-stage NAFLD, achieving a rate above 97.5% with an area under the receiver operating characteristic curve (AUC) of 1.0. The findings present a non-invasive approach for early-stage NAFLD detection and display a strategy for improved early detection through on-skin electronics and deep learning.

Stretchable ionic-electronic bilayer hydrogel electronics enable in situ detection of solid-state epidermal biomarkers

Continuous and in situ detection of biomarkers in biofluids (for example, sweat) can provide critical health data but is limited by biofluid accessibility. Here we report a sensor design that enables in situ detection of solid-state biomarkers ubiquitously present on human skin. We deploy an ionic-electronic bilayer hydrogel to facilitate the sequential dissolution, diffusion and electrochemical reaction of solid-state analytes. We demonstrate continuous monitoring of water-soluble analytes (for example, solid lactate) and water-insoluble analytes (for example, solid cholesterol) with ultralow detection limits of 0.51 and 0.26 nmol cm-2, respectively. Additionally, the bilayer hydrogel electrochemical interface reduces motion artefacts by a factor of three compared with conventional liquid-sensing electrochemical interfaces. In a clinical study, solid-state epidermal biomarkers measured by our stretchable wearable sensors showed a high correlation with biomarkers in human blood and dynamically correlated with physiological activities. These results present routes to universal platforms for biomarker monitoring without the need for biofluid acquisition.

Biocompatible Electrical and Optical Interfaces for Implantable Sensors and Devices

Implantable bioelectronics hold tremendous potential in the field of healthcare, yet the performance of these systems heavily relies on the interfaces between artificial machines and living tissues. In this paper, we discuss the recent developments of tethered interfaces, as well as those of non-tethered interfaces. Among them, systems that study neural activity receive significant attention due to their innovative developments and high relevance in contemporary research, but other functional types of interface systems are also explored to provide a comprehensive overview of the field. We also analyze the key considerations, including perforation site selection, fixing strategies, long-term retention, and wireless communication, highlighting the challenges and opportunities with stable, effective, and biocompatible interfaces. Furthermore, we propose a primitive model of biocompatible electrical and optical interfaces for implantable systems, which simultaneously possesses biocompatibility, stability, and convenience. Finally, we point out the future directions of interfacing strategies.

Multifunctional Dopamine-Based Hydrogel Microneedle Electrode for Continuous Ketone Sensing

Diabetic ketoacidosis (DKA), a severe complication of type 1 diabetes (T1D), is triggered by production of large quantities of ketone bodies, requiring patients with T1D to constantly monitor their ketone levels. Here, a skin-compatible hydrogel microneedle (HMN)-continuous ketone monitoring (HMN-CKM) device is reported. The sensing mechanism relies on the catechol-quinone chemistry inherent to the dopamine (DA) molecules that are covalently linked to the polymer structure of the HMN patch. The DA serves the dual-purpose of acting as a redox mediator for measuring the byproduct of oxidation of 3-beta-hydroxybutyrate (β-HB), the primary ketone bodies; while, also facilitating the formation of a crosslinked HMN patch. A universal approach involving pre-oxidation and detection of the generated catechol compounds is introduced to correlate the sensor response to the β-HB concentrations. It is further shown that real-time tracking of a decrease in ketone levels of T1D rat model is possible using the HMN-CKM device, in conjunction with a data-driven machine learning model that considers potential time delays.

Skin-inspired, sensory robots for electronic implants

Drawing inspiration from cohesive integration of skeletal muscles and sensory skins in vertebrate animals, we present a design strategy of soft robots, primarily consisting of an electronic skin (e-skin) and an artificial muscle. These robots integrate multifunctional sensing and on-demand actuation into a biocompatible platform using an in-situ solution-based method. They feature biomimetic designs that enable adaptive motions and stress-free contact with tissues, supported by a battery-free wireless module for untethered operation. Demonstrations range from a robotic cuff for detecting blood pressure, to a robotic gripper for tracking bladder volume, an ingestible robot for pH sensing and on-site drug delivery, and a robotic patch for quantifying cardiac function and delivering electrotherapy, highlighting the application versatilities and potentials of the bio-inspired soft robots. Our designs establish a universal strategy with a broad range of sensing and responsive materials, to form integrated soft robots for medical technology and beyond.

Wireless Bioelectronics for In Vivo Pressure Monitoring with Mechanically-Compliant Hydrogel Biointerfaces

Recent electronics-tissues biointefacing technology has offered unprecedented opportunities for long-term disease diagnosis and treatment. It remains a grand challenge to robustly anchor the pressure sensing bioelectronics onto specific organs, since the periodically-varying stress generated by normal biological processes may pose high risk of interfacial failures. Here, a general yet reliable approach is reported to achieve the robust hydrogel interface between wireless pressure sensor and biological tissues/organs, featuring highly desirable mechanical compliance and swelling resistance, despite the direct contact with biofluids and dynamic conditions. The sensor is operated wirelessly through inductive coupling, characterizing minimal hysteresis, fast response times, excellent stability, and robustness, thus allowing for easy handling and eliminating the necessity for surgical extraction after a functional period. The operation of the wireless sensor has been demonstrated with a custom-made pressure sensing model and in vivo intracranial pressure monitoring in rats. This technology may be advantageous in real-time post-operative monitoring of various biological inner pressures after the reconstructive surgery, thus guaranteeing the timely treatment of lethal diseases.

Conducting polymer scaffolds: a new frontier in bioelectronics and bioengineering

Conducting polymer (CP) scaffolds have emerged as a transformative tool in bioelectronics and bioengineering, advancing the ability to interface with biological systems. Their unique combination of electrical conductivity, tailorability, and biocompatibility surpasses the capabilities of traditional nonconducting scaffolds while granting them access to the realm of bioelectronics. This review examines recent developments in CP scaffolds, focusing on material and device advancements, as well as their interplay with biological systems. We highlight applications for monitoring, tissue stimulation, and drug delivery and discuss perspectives and challenges currently faced for their ultimate translation and clinical implementation.

Advances in conductive hydrogels for neural recording and stimulation

The brain-computer interface (BCI) allows the human or animal brain to directly interact with the external environment through the neural interfaces, thus playing the role of monitoring, protecting, improving/restoring, enhancing, and replacing. Recording electrophysiological information such as brain neural signals is of great importance in health monitoring and disease diagnosis. According to the electrode position, it can be divided into non-implantable, semi-implantable, and implantable. Among them, implantable neural electrodes can obtain the highest-quality electrophysiological information, so they have the most promising application. However, due to the chemo-mechanical mismatch between devices and tissues, the adverse foreign body response and performance loss over time seriously restrict the development and application of implantable neural electrodes. Given the challenges, conductive hydrogel-based neural electrodes have recently attracted much attention, owing to many advantages such as good mechanical match with the native tissues, negligible foreign body response, and minimal signal attenuation. This review mainly focuses on the current development of conductive hydrogels as a biocompatible framework for neural tissue and conductivity-supporting substrates for the transmission of electrical signals of neural tissue to speed up electrical regeneration and their applications in neural sensing and recording as well as stimulation.

Polymer Bioelectronics: A Solution for Both Stimulating and Recording Electrodes

The advent of closed-loop bionics has created a demand for electrode materials that are ideal for both stimulating and recording applications. The growing complexity and diminishing size of implantable devices for neural interfaces have moved beyond what can be achieved with conventional metallic electrode materials. Polymeric electrode materials are a recent development based on polymer composites of organic conductors such as conductive polymers. These materials present exciting new opportunities in the design and fabrication of next-generation electrode arrays which can overcome the electrochemical and mechanical limitations of conventional electrode materials. This review will examine the recent developments in polymeric electrode materials, their application as stimulating and recording electrodes in bionic devices, and their impact on the development of soft, conformal, and high-density neural interfaces.

Materials and Structural Designs toward Motion Artifact-Free Bioelectronics

Bioelectronics encompassing electronic components and circuits for accessing human information play a vital role in real-time and continuous monitoring of biophysiological signals of electrophysiology, mechanical physiology, and electrochemical physiology. However, mechanical noise, particularly motion artifacts, poses a significant challenge in accurately detecting and analyzing target signals. While software-based "postprocessing" methods and signal filtering techniques have been widely employed, challenges such as signal distortion, major requirement of accurate models for classification, power consumption, and data delay inevitably persist. This review presents an overview of noise reduction strategies in bioelectronics, focusing on reducing motion artifacts and improving the signal-to-noise ratio through hardware-based approaches such as "preprocessing". One of the main stress-avoiding strategies is reducing elastic mechanical energies applied to bioelectronics to prevent stress-induced motion artifacts. Various approaches including strain-compliance, strain-resistance, and stress-damping techniques using unique materials and structures have been explored. Future research should optimize materials and structure designs, establish stable processes and measurement methods, and develop techniques for selectively separating and processing overlapping noises. Ultimately, these advancements will contribute to the development of more reliable and effective bioelectronics for healthcare monitoring and diagnostics.

Permeable Bioelectronics toward Biointegrated Systems

Bioelectronics integrates electronics with biological organs, sustaining the natural functions of the organs. Organs dynamically interact with the external environment, managing internal equilibrium and responding to external stimuli. These interactions are crucial for maintaining homeostasis. Additionally, biological organs possess a soft and stretchable nature; encountering objects with differing properties can disrupt their function. Therefore, when electronic devices come into contact with biological objects, the permeability of these devices, enabling interactions and substance exchanges with the external environment, and the mechanical compliance are crucial for maintaining the inherent functionality of biological organs. This review discusses recent advancements in soft and permeable bioelectronics, emphasizing materials, structures, and a wide range of applications. The review also addresses current challenges and potential solutions, providing insights into the integration of electronics with biological organs.

Multifunctional Nanocomposite Yield-Stress Fluids for Printable and Stretchable Electronics

Compliant materials are crucial for stretchable electronics. Stretchable solids and gels have limitations in deformability and durability, whereas active liquids struggle to create complex devices. This study presents multifunctional yield-stress fluids as printable ink materials to construct stretchable electronic devices. Ionic nanocomposites comprise silica nanoparticles and ion liquids, while electrical nanocomposites use the natural oxidation of liquid metals to produce gallium oxide nanoflake additives. These nanocomposite inks can be printed on an elastomer substrate and stay in a solid state for easy encapsulation. However, their transition into a liquid state during stretching allows ultrahigh deformability up to the fracture strain of the elastomer. The ionic inks produce strain sensors with high stretchability and temperature sensors with high sensitivity of 7% °C-1. Smart gloves are further created by integrating these sensors with printed electrical interconnects, demonstrating bimodal detection of temperatures and hand gestures. The nanocomposite yield-stress fluids combine the desirable qualities of solids and liquids for stretchable devices and systems.

Stretchable Functional Nanocomposites for Soft Implantable Bioelectronics

Material advances in soft bioelectronics, particularly those based on stretchable nanocomposites─functional nanomaterials embedded in viscoelastic polymers with irreversible or reversible bonds─have driven significant progress in translational medical device research. The unique mechanical properties inherent in the stretchable nanocomposites enable stiffness matching between tissue and device, as well as its spontaneous mechanical adaptation to in vivo environments, minimizing undesired mechanical stress and inflammation responses. Furthermore, these properties allow percolative networks of conducting fillers in the nanocomposites to be sustained even under repetitive tensile/compressive stresses, leading to stable tissue-device interfacing. Here, we present an in-depth review of materials strategies, fabrication/integration techniques, device designs, applications, and translational opportunities of nanocomposite-based soft bioelectronics, which feature intrinsic stretchability, self-healability, tissue adhesion, and/or syringe injectability. Among many, applications to brain, heart, and peripheral nerves are predominantly discussed, and translational studies in certain domains such as neuromuscular and cardiovascular engineering are particularly highlighted.

Implantable Neural Microelectrodes: How to Reduce Immune Response

Implantable neural microelectrodes exhibit the great ability to accurately capture the electrophysiological signals from individual neurons with exceptional submillisecond precision, holding tremendous potential for advancing brain science research, as well as offering promising avenues for neurological disease therapy. Although significant advancements have been made in the channel and density of implantable neural microelectrodes, challenges persist in extending the stable recording duration of these microelectrodes. The enduring stability of implanted electrode signals is primarily influenced by the chronic immune response triggered by the slight movement of the electrode within the neural tissue. The intensity of this immune response increases with a higher bending stiffness of the electrode. This Review thoroughly analyzes the sequential reactions evoked by implanted electrodes in the brain and highlights strategies aimed at mitigating chronic immune responses. Minimizing immune response mainly includes designing the microelectrode structure, selecting flexible materials, surface modification, and controlling drug release. The purpose of this paper is to provide valuable references and ideas for reducing the immune response of implantable neural microelectrodes and stimulate their further exploration in the field of brain science.

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

Flexible, Biodegradable, and Wireless Magnetoelectric Paper for Simple In Situ Personalization of Bioelectric Implants

Bioelectronic implants delivering electrical stimulation offer an attractive alternative to traditional pharmaceuticals in electrotherapy. However, achieving simple, rapid, and cost-effective personalization of these implants for customized treatment in unique clinical and physical scenarios presents a substantial challenge. This challenge is further compounded by the need to ensure safety and minimal invasiveness, requiring essential attributes such as flexibility, biocompatibility, lightness, biodegradability, and wireless stimulation capability. Here, a flexible, biodegradable bioelectronic paper with homogeneously distributed wireless stimulation functionality for simple personalization of bioelectronic implants is introduced. The bioelectronic paper synergistically combines i) lead-free magnetoelectric nanoparticles (MENs) that facilitate electrical stimulation in response to external magnetic field and ii) flexible and biodegradable nanofibers (NFs) that enable localization of MENs for high-selectivity stimulation, oxygen/nutrient permeation, cell orientation modulation, and biodegradation rate control. The effectiveness of wireless electrical stimulation in vitro through enhanced neuronal differentiation of neuron-like PC12 cells and the controllability of their microstructural orientation are shown. Also, scalability, design flexibility, and rapid customizability of the bioelectronic paper are shown by creating various 3D macrostructures using simple paper crafting techniques such as cutting and folding. This platform holds promise for simple and rapid personalization of temporary bioelectronic implants for minimally invasive wireless stimulation therapies.

Sensing, Imaging, and Therapeutic Strategies Endowing by Conjugate Polymers for Precision Medicine

Conjugated polymers (CPs) have promising applications in biomedical fields, such as disease monitoring, real-time imaging diagnosis, and disease treatment. As a promising luminescent material with tunable emission, high brightness and excellent stability, CPs are widely used as fluorescent probes in biological detection and imaging. Rational molecular design and structural optimization have broadened absorption/emission range of CPs, which are more conductive for disease diagnosis and precision therapy. This review provides a comprehensive overview of recent advances in the application of CPs, aiming to elucidate their structural and functional relationships. The fluorescence properties of CPs and the mechanism of detection signal amplification are first discussed, followed by an elucidation of their emerging applications in biological detection. Subsequently, CPs-based imaging systems and therapeutic strategies are illustrated systematically. Finally, recent advancements in utilizing CPs as electroactive materials for bioelectronic devices are also investigated. Moreover, the challenges and outlooks of CPs for precision medicine are discussed. Through this systematic review, it is hoped to highlight the frontier progress of CPs and promote new breakthroughs in fundamental research and clinical transformation.

Closed-Loop Wearable Device Network of Intrinsically-Controlled, Bilateral Coordinated Functional Electrical Stimulation for Stroke

Innovative functional electrical stimulation has demonstrated effectiveness in enhancing daily walking and rehabilitating stroke patients with foot drop. However, its lack of precision in stimulating timing, individual adaptivity, and bilateral symmetry, resulted in diminished clinical efficacy. Therefore, a closed-loop wearable device network of intrinsically controlled functional electrical stimulation (CI-FES) system is proposed, which utilizes the personal surface myoelectricity, derived from the intrinsic neuro signal, as the switch to activate/deactivate the stimulation on the affected side. Simultaneously, it decodes the myoelectricity signal of the patient's healthy side to adjust the stimulation intensity, forming an intrinsically controlled loop with the inertial measurement units. With CI-FES assistance, patients' walking ability significantly improved, evidenced by the shift in ankle joint angle mean and variance from 105.53° and 28.84 to 102.81° and 17.71, and the oxyhemoglobin concentration tested by the functional near-infrared spectroscopy. In long-term CI-FES-assisted clinical testing, the discriminability in machine learning classification between patients and healthy individuals gradually decreased from 100% to 92.5%, suggesting a remarkable recovery tendency, further substantiated by performance on the functional movement scales. The developed CI-FES system is crucial for contralateral-hemiplegic stroke recovery, paving the way for future closed-loop stimulation systems in stroke rehabilitation is anticipated.

Self-Healing Hydrogel Bioelectronics

Hydrogels have emerged as powerful building blocks to develop various soft bioelectronics because of their tissue-like mechanical properties, superior bio-compatibility, the ability to conduct both electrons and ions, and multiple stimuli-responsiveness. However, hydrogels are vulnerable to mechanical damage, which limits their usage in developing durable hydrogel-based bioelectronics. Self-healing hydrogels aim to endow bioelectronics with the property of repairing specific functions after mechanical failure, thus improving their durability, reliability, and longevity. This review discusses recent advances in self-healing hydrogels, from the self-healing mechanisms, material chemistry, and strategies for multiple properties improvement of hydrogel materials, to the design, fabrication, and applications of various hydrogel-based bioelectronics, including wearable physical and biochemical sensors, supercapacitors, flexible display devices, triboelectric nanogenerators (TENGs), implantable bioelectronics, etc. Furthermore, the persisting challenges hampering the development of self-healing hydrogel bioelectronics and their prospects are proposed. This review is expected to expedite the research and applications of self-healing hydrogels for various self-healing bioelectronics.

Direct electron beam patterning of electro-optically active PEDOT:PSS

The optical and electronic tunability of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has enabled emerging applications as diverse as bioelectronics, flexible electronics, and micro- and nano-photonics. High-resolution spatial patterning of PEDOT:PSS opens up opportunities for novel active devices in a range of fields. However, typical lithographic processes require tedious indirect patterning and dry etch processes, while solution-processing methods such as ink-jet printing have limited spatial resolution. Here, we report a method for direct write nano-patterning of commercially available PEDOT:PSS through electron-beam induced solubility modulation. The written structures are water stable and maintain the conductivity as well as electrochemical and optical properties of PEDOT:PSS, highlighting the broad utility of our method. We demonstrate the potential of our strategy by preparing prototypical nano-wire structures with feature sizes down to 250 nm, an order of magnitude finer than previously reported direct write methods, opening the possibility of writing chip-scale microelectronic and optical devices. We finally use the high-resolution writing capabilities to fabricate electrically-switchable optical diffraction gratings. We show active switching in this archetypal system with >95 % contrast at CMOS-compatible voltages of +2 V and -3 V, offering a route towards highly-miniaturized dynamic optoelectronic devices.

Flexible Peripheral Nerve Interfacing Electrode for Joint Position Control in Closed-Loop Neuromuscular Stimulation

Addressing peripheral nerve disorders with electronic medicine poses significant challenges, especially in replicating the dynamic mechanical properties of nerves and understanding their functionality. In the field of electronic medicine, it is crucial to design a system that thoroughly understands the functions of the nervous system and ensures a stable interface with nervous tissue, facilitating autonomous neural adaptation. Herein, we present a novel neural interface platform that modulates the peripheral nervous system using flexible nerve electrodes and advanced neuromodulation techniques. Specifically, we have developed a surface-based inverse recruitment model for effective joint position control via direct electrical nerve stimulation. Utilizing barycentric coordinates, this model constructs a three-dimensional framework that accurately interpolates inverse isometric recruitment values across various joint positions, thereby enhancing control stability during stimulation. Experimental results from rabbit ankle joint control trials demonstrate our model's effectiveness. In combination with a proportional-integral-derivative (PID) controller, it shows superior performance by achieving reduced settling time (less than 1.63 s), faster rising time (less than 0.39 s), and smaller steady-state error (less than 3 degrees) compared to the legacy model. Moreover, the model's compatibility with recent advances in flexible interfacing technologies and its integration into a closed-loop controlled functional neuromuscular stimulation (FNS) system highlight its potential for precise neuroprosthetic applications in joint position control. This approach marks a significant advancement in the management of neurological disorders with advanced neuroprosthetic solutions.

Control of polymers' amorphous-crystalline transition enables miniaturization and multifunctional integration for hydrogel bioelectronics

Soft bioelectronic devices exhibit motion-adaptive properties for neural interfaces to investigate complex neural circuits. Here, we develop a fabrication approach through the control of metamorphic polymers' amorphous-crystalline transition to miniaturize and integrate multiple components into hydrogel bioelectronics. We attain an about 80% diameter reduction in chemically cross-linked polyvinyl alcohol hydrogel fibers in a fully hydrated state. This strategy allows regulation of hydrogel properties, including refractive index (1.37-1.40 at 480 nm), light transmission (>96%), stretchability (139-169%), bending stiffness (4.6 ± 1.4 N/m), and elastic modulus (2.8-9.3 MPa). To exploit the applications, we apply step-index hydrogel optical probes in the mouse ventral tegmental area, coupled with fiber photometry recordings and social behavioral assays. Additionally, we fabricate carbon nanotubes-PVA hydrogel microelectrodes by incorporating conductive nanomaterials in hydrogel for spontaneous neural activities recording. We enable simultaneous optogenetic stimulation and electrophysiological recordings of light-triggered neural activities in Channelrhodopsin-2 transgenic mice.

Evolving Therapeutic Landscape of Intracerebral Hemorrhage: Emerging Cutting-Edge Advancements in Surgical Robots, Regenerative Medicine, and Neurorehabilitation Techniques

Intracerebral hemorrhage (ICH) is the most serious form of stroke and has limited available therapeutic options. As knowledge on ICH rapidly develops, cutting-edge techniques in the fields of surgical robots, regenerative medicine, and neurorehabilitation may revolutionize ICH treatment. However, these new advances still must be translated into clinical practice. In this review, we examined several emerging therapeutic strategies and their major challenges in managing ICH, with a particular focus on innovative therapies involving robot-assisted minimally invasive surgery, stem cell transplantation, in situ neuronal reprogramming, and brain-computer interfaces. Despite the limited expansion of the drug armamentarium for ICH over the past few decades, the judicious selection of more efficacious therapeutic modalities and the exploration of multimodal combination therapies represent opportunities to improve patient prognoses after ICH.

Evolution of Musculoskeletal Electronics

The musculoskeletal system, constituting the largest human physiological system, plays a critical role in providing structural support to the body, facilitating intricate movements, and safeguarding internal organs. By virtue of advancements in revolutionized materials and devices, particularly in the realms of motion capture, health monitoring, and postoperative rehabilitation, "musculoskeletal electronics" has actually emerged as an infancy area, but has not yet been explicitly proposed. In this review, the concept of musculoskeletal electronics is elucidated, and the evolution history, representative progress, and key strategies of the involved materials and state-of-the-art devices are summarized. Therefore, the fundamentals of musculoskeletal electronics and key functionality categories are introduced. Subsequently, recent advances in musculoskeletal electronics are presented from the perspectives of "in vitro" to "in vivo" signal detection, interactive modulation, and therapeutic interventions for healing and recovery. Additionally, nine strategy avenues for the development of advanced musculoskeletal electronic materials and devices are proposed. Finally, concise summaries and perspectives are proposed to highlight the directions that deserve focused attention in this booming field.

Aldehyde group pendant-grafted pectin-based injectable hydrogel

Periodate oxidation has been the widely accepted route for obtaining aldehyde group-functionalized polysaccharides but significantly influenced the various physicochemical properties due to the ring opening of the backbone of polysaccharides. The present study, for the first time, presents a novel method for the preparation of aldehyde group-functionalized polysaccharides that could retain the ring structure and the consequent rigidity of the backbone. Pectin was collected as the representative of polysaccharides and modified with cyclopropyl formaldehyde to obtain pectin aldehyde (AP), which was further crosslinked by DL-lysine (LYS) via the Schiff base reaction to prepare injectable hydrogel. The feasibility of the functionalization was proved by FT-IR and 1H NMR techniques. The obtained hydrogel showed acceptable mechanical properties, self-healing ability, syringeability, and sustained-release performance. Also, as-prepared injectable hydrogel presented great biocompatibility with a cell proliferation rate of 96 %, and the drug-loaded hydrogel exhibited clear inhibition of cancer cell proliferation. Overall, the present study showed a new method for the preparation of aldehyde group-functionalized polysaccharides, and the drug-loaded hydrogel has potential in drug release applications.

Low-impedance tissue-device interface using homogeneously conductive hydrogels chemically bonded to stretchable bioelectronics

Stretchable bioelectronics has notably contributed to the advancement of continuous health monitoring and point-of-care type health care. However, microscale nonconformal contact and locally dehydrated interface limit performance, especially in dynamic environments. Therefore, hydrogels can be a promising interfacial material for the stretchable bioelectronics due to their unique advantages including tissue-like softness, water-rich property, and biocompatibility. However, there are still practical challenges in terms of their electrical performance, material homogeneity, and monolithic integration with stretchable devices. Here, we report the synthesis of a homogeneously conductive polyacrylamide hydrogel with an exceptionally low impedance (~21 ohms) and a reasonably high conductivity (~24 S/cm) by incorporating polyaniline-decorated poly(3,4-ethylenedioxythiophene:polystyrene). We also establish robust adhesion (interfacial toughness: ~296.7 J/m2) and reliable integration between the conductive hydrogel and the stretchable device through on-device polymerization as well as covalent and hydrogen bonding. These strategies enable the fabrication of a stretchable multichannel sensor array for the high-quality on-skin impedance and pH measurements under in vitro and in vivo circumstances.

Wireless Battery-free and Fully Implantable Organ Interfaces

Advances in soft materials, miniaturized electronics, sensors, stimulators, radios, and battery-free power supplies are resulting in a new generation of fully implantable organ interfaces that leverage volumetric reduction and soft mechanics by eliminating electrochemical power storage. This device class offers the ability to provide high-fidelity readouts of physiological processes, enables stimulation, and allows control over organs to realize new therapeutic and diagnostic paradigms. Driven by seamless integration with connected infrastructure, these devices enable personalized digital medicine. Key to advances are carefully designed material, electrophysical, electrochemical, and electromagnetic systems that form implantables with mechanical properties closely matched to the target organ to deliver functionality that supports high-fidelity sensors and stimulators. The elimination of electrochemical power supplies enables control over device operation, anywhere from acute, to lifetimes matching the target subject with physical dimensions that supports imperceptible operation. This review provides a comprehensive overview of the basic building blocks of battery-free organ interfaces and related topics such as implantation, delivery, sterilization, and user acceptance. State of the art examples categorized by organ system and an outlook of interconnection and advanced strategies for computation leveraging the consistent power influx to elevate functionality of this device class over current battery-powered strategies is highlighted.

3D spatiotemporally scalable in vivo neural probes based on fluorinated elastomers

Electronic devices for recording neural activity in the nervous system need to be scalable across large spatial and temporal scales while also providing millisecond and single-cell spatiotemporal resolution. However, existing high-resolution neural recording devices cannot achieve simultaneous scalability on both spatial and temporal levels due to a trade-off between sensor density and mechanical flexibility. Here we introduce a three-dimensional (3D) stacking implantable electronic platform, based on perfluorinated dielectric elastomers and tissue-level soft multilayer electrodes, that enables spatiotemporally scalable single-cell neural electrophysiology in the nervous system. Our elastomers exhibit stable dielectric performance for over a year in physiological solutions and are 10,000 times softer than conventional plastic dielectrics. By leveraging these unique characteristics we develop the packaging of lithographed nanometre-thick electrode arrays in a 3D configuration with a cross-sectional density of 7.6 electrodes per 100 µm2. The resulting 3D integrated multilayer soft electrode array retains tissue-level flexibility, reducing chronic immune responses in mouse neural tissues, and demonstrates the ability to reliably track electrical activity in the mouse brain or spinal cord over months without disrupting animal behaviour.

One Pot Photomediated Formation of Electrically Conductive Hydrogels

Electrically conductive hydrogels represent an innovative platform for the development of bioelectronic devices. While photolithography technologies have enabled the fabrication of complex architectures with high resolution, photoprinting conductive hydrogels is still a challenging task because the conductive polymer absorbs light which can outcompete photopolymerization of the insulating scaffold. In this study, we introduce an approach to synthesizing conductive hydrogels in one step. Our approach combines the simultaneous photo-cross-linking of a polymeric scaffold and the polymerization of 3,4-ethylene dioxythiophene (EDOT), without additional photocatalysts. This process involves the copolymerization of photo-cross-linkable coumarin-containing monomers with sodium styrenesulfonate to produce a water-soluble poly(styrenesulfonate-co-coumarin acrylate) (P(SS-co-CoumAc)) copolymer. Our findings reveal that optimizing the [SS]:[CoumAc] ratio at 100:5 results in hydrogels with the strain at break up to 16%. This mechanical resilience is coupled with an electronic conductivity of 9.2 S m-1 suitable for wearable electronics. Furthermore, the conductive hydrogels can be photopatterned to achieve micrometer-sized structures with high resolution. The photo-cross-linked hydrogels are used as electrodes to record stable and reliable surface electromyography (sEMG) signals. These novel photo-cross-linkable polymers combined with one-pot PEDOT (poly-EDOT) polymerization open possibilities for rapidly prototyping complex bioelectronic devices and creating custom-designed interfaces between electronics and biological systems.

Electrochemical and Electrical Biosensors for Wearable and Implantable Electronics Based on Conducting Polymers and Carbon-Based Materials

Bioelectronic devices are designed to translate biological information into electrical signals and vice versa, thereby bridging the gap between the living biological world and electronic systems. Among different types of bioelectronics devices, wearable and implantable biosensors are particularly important as they offer access to the physiological and biochemical activities of tissues and organs, which is significant in diagnosing and researching various medical conditions. Organic conducting and semiconducting materials, including conducting polymers (CPs) and graphene and carbon nanotubes (CNTs), are some of the most promising candidates for wearable and implantable biosensors. Their unique electrical, electrochemical, and mechanical properties bring new possibilities to bioelectronics that could not be realized by utilizing metals- or silicon-based analogues. The use of organic- and carbon-based conductors in the development of wearable and implantable biosensors has emerged as a rapidly growing research field, with remarkable progress being made in recent years. The use of such materials addresses the issue of mismatched properties between biological tissues and electronic devices, as well as the improvement in the accuracy and fidelity of the transferred information. In this review, we highlight the most recent advances in this field and provide insights into organic and carbon-based (semi)conducting materials' properties and relate these to their applications in wearable/implantable biosensors. We also provide a perspective on the promising potential and exciting future developments of wearable/implantable biosensors.

Recent Advances in Implantable Neural Interfaces for Multimodal Electrical Neuromodulation

Electrical neuromodulation plays a pivotal role in enhancing patient outcomes among individuals suffering from neurological disorders. Implantable neural interfaces are vital components of the electrical neuromodulation system to ensure desirable performance; However, conventional devices are limited to a single function and are constructed with bulky and rigid materials, which often leads to mechanical incompatibility with soft tissue and an inability to adapt to the dynamic and complex 3D structures of biological systems. In addition, current implantable neural interfaces utilized in clinical settings primarily rely on wire-based techniques, which are associated with complications such as increased risk of infection, limited positioning options, and movement restrictions. Here, the state-of-art applications of electrical neuromodulation are presented. Material schemes and device structures that can be employed to develop robust and multifunctional neural interfaces, including flexibility, stretchability, biodegradability, self-healing, self-rolling, or morphing are discussed. Furthermore, multimodal wireless neuromodulation techniques, including optoelectronics, mechano-electrics, magnetoelectrics, inductive coupling, and electrochemically based self-powered devices are reviewed. In the end, future perspectives are given.

Influence of biopolymer composition and crosslinking agent concentration on the micro- and nanomechanical properties of hydrogel-based filaments

Hydrogel filaments were manufactured using wet spinning technique, incorporating variations in the concentrations of sodium alginate, gelatin, and calcium chloride (crosslinking agent). The combination of biopolymer concentrations was determined using design of experiments (DoE) approach. The resulting filaments were produced from the developed hydrogels. Tensile and vertical strength analyses of the filaments were conducted using an electromechanical extensor. Atomic force microscopy was employed to evaluate the roughness, viscoelasticity, retraction, and deflection of the hydrogels. By employing DoE, a total of seventeen different combinations of biopolymers and crosslinkers were generated to construct the hydrogels. The filaments exhibited variations in electromechanical traction (measured in kPa) and produced distinct stress peaks. Furthermore, diverse roughness values were observed among the tested materials, with the combinations featuring higher concentrations of sodium alginate displaying the highest Young's modulus. This study demonstrates that manipulating the concentrations of biopolymers and crosslinking agents can modulate the micro and nanomechanical properties of biopolymeric filaments.

A Nanozyme-Based Electrode for High-Performance Neural Recording

Implanted neural electrodes have been widely used to treat brain diseases that require high sensitivity and biocompatibility at the tissue-electrode interface. However, currently used clinical electrodes cannot meet both these requirements simultaneously, which hinders the effective recording of electronic signals. Herein, nanozyme-based neural electrodes incorporating bioinspired atomically precise clusters are developed as a general strategy with a heterogeneous design for multiscale and ultrasensitive neural recording via quantum transport and biocatalytic processes. Owing to the dual high-speed electronic and ionic currents at the electrode-tissue interface, the impedance of nanozyme electrodes is 26 times lower than that of state-of-the-art metal electrodes, and the acquisition sensitivity for the local field potential is ≈10 times higher than that of clinical PtIr electrodes, enabling a signal-to-noise ratio (SNR) of up to 14.7 dB for single-neuron recordings in rats. The electrodes provide more than 100-fold higher antioxidant and multi-enzyme-like activities, which effectively decrease 67% of the neuronal injury area by inhibiting glial proliferation and allowing sensitive and stable neural recording. Moreover, nanozyme electrodes can considerably improve the SNR of seizures in acute epileptic rats and are expected to achieve precise localization of seizure foci in clinical settings.

Neuromorphic hardware for somatosensory neuroprostheses

In individuals with sensory-motor impairments, missing limb functions can be restored using neuroprosthetic devices that directly interface with the nervous system. However, restoring the natural tactile experience through electrical neural stimulation requires complex encoding strategies. Indeed, they are presently limited in effectively conveying or restoring tactile sensations by bandwidth constraints. Neuromorphic technology, which mimics the natural behavior of neurons and synapses, holds promise for replicating the encoding of natural touch, potentially informing neurostimulation design. In this perspective, we propose that incorporating neuromorphic technologies into neuroprostheses could be an effective approach for developing more natural human-machine interfaces, potentially leading to advancements in device performance, acceptability, and embeddability. We also highlight ongoing challenges and the required actions to facilitate the future integration of these advanced technologies.

Phase-separated stretchable conductive nanocomposite to reduce contact resistance of skin electronics

Skin electronics, facilitating a high-quality interface between external devices and human skin for recording physiological and/or electrophysiological signals as well as delivering external electrical and/or mechanical energy into the human body, has shown significant progress. However, achieving mechanically conformal contact and electrically low contact resistance at the device-skin interface remains challenging. Here, we propose a material strategy to potentially address such an issue by using phase separation of silver nanowires and silver nanoparticles (Ag NWs and Ag NPs) within a stretchable conductive nanocomposite (NC). This phase-separated NC ensures low contact resistance and high conductivity, which are key requirements in skin electronics, while maintaining excellent mechanical contact with the skin. To achieve phase separation, we hydrophobically treated the surfaces of Ag NWs and Ag NPs. Then, as the NC solidified, the solvent contained in the NC was slowly evaporated to sufficiently precipitate Ag NPs within the NC. As a result, the phase-separated NC exhibited high conductivity (~ 18,535 S cm-1), excellent stretchability (~ 80%), and low contact resistance on both the top and bottom NC surfaces (average ~ 0.132 Ω). The phase-separated NC has enabled implementation of high performance skin-mounted devices, including strain sensors, electrophysiological sensors, and a wearable heater.

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.

Coaxial Electrohydrodynamic Printing of Microscale Core-Shell Conductive Features for Integrated Fabrication of Flexible Transparent Electronics

Reliable insulation of microscale conductive features is required to fabricate functional multilayer circuits or flexible electronics for providing specific physical/chemical/electrical protection. However, the existing strategies commonly rely on manual assembling processes or multiple microfabrication processes, which is time-consuming and a great challenge for the fabrication of flexible transparent electronics with microscale features and ultrathin thickness. Here, we present a novel coaxial electrohydrodynamic (CEHD) printing strategy for the one-step fabrication of microscale flexible electronics with conductive materials at the core and insulating material at the outer layer. A finite element analysis (FEA) method is established to simulate the CEHD printing process. The extrusion sequence of the conductive and insulating materials during the CEHD printing process shows little effect on the morphology of the core-shell filaments, which can be achieved on different flexible substrates with a minimum conductive line width of 32 ± 3.2 μm, a total thickness of 53.6 ± 4.8 μm, and a conductivity of 0.23 × 107 S/m. The thin insulating layer can provide the inner conductive filament enough protection in 3D, which endows the resultant microscale core-shell electronics with good electrical stability when working in different chemical solvent solutions or under large deformation conditions. Moreover, the presented CEHD printing strategy offers a unique capability to sequentially fabricate an insulating layer, core-shell conductive pattern, and exposed electrodes by simply controlling the material extrusion sequence. The resultant large-area transparent electronics with two-layer core-shell patterns exhibit a high transmittance of 98% and excellent electrothermal performance. The CEHD-printed flexible microelectrode array is successfully used to record the electrical signals of beating mouse hearts. It can also be used to fabricate large-area flexible capacitive sensors to accurately measure the periodical pressure force. We envision that the present CEHD printing strategy can provide a promising tool to fabricate complex three-dimensional electronics with microscale resolution, high flexibility, and multiple functionalities.

Doping Engineering of Conductive Polymers and Their Application in Physical Sensors for Healthcare Monitoring

Physical sensors have emerged as a promising technology for real-time healthcare monitoring, which tracks various physical signals from the human body. Accurate acquisition of these physical signals from biological tissue requires excellent electrical conductivity and long-term durability of the sensors under complex mechanical deformation. Conductive polymers, combining the advantages of conventional polymers and organic conductors, are considered ideal conductive materials for healthcare physical sensors due to their intrinsic conductive network, tunable mechanical properties, and easy processing. Doping engineering has been proposed as an effective approach to enhance the sensitivity, lower the detection limit, and widen the operational range of sensors based on conductive polymers. This approach enables the introduction of dopants into conductive polymers to adjust and control the microstructure and energy levels of conductive polymers, thereby optimizing their mechanical and conductivity properties. This review article provides a comprehensive overview of doping engineering methods to improve the physical properties of conductive polymers and highlights their applications in the field of healthcare physical sensors, including temperature sensors, strain sensors, stress sensors, and electrophysiological sensing. Additionally, the challenges and opportunities associated with conductive polymer-based physical sensors in healthcare monitoring are discussed.

Soft Bioelectronics for Neuroengineering: New Horizons in the Treatment of Brain Tumor and Epilepsy

Soft bioelectronic technologies for neuroengineering have shown remarkable progress, which include novel soft material technologies and device design strategies. Such technological advances that are initiated from fundamental brain science are applied to clinical neuroscience and provided meaningful promises for significant improvement in the diagnosis efficiency and therapeutic efficacy of various brain diseases recently. System-level integration strategies in consideration of specific disease circumstances can enhance treatment effects further. Here, recent advances in soft implantable bioelectronics for neuroengineering, focusing on materials and device designs optimized for the treatment of intracranial disease environments, are reviewed. Various types of soft bioelectronics for neuroengineering are categorized and exemplified first, and then details for the sensing and stimulating device components are explained. Next, application examples of soft implantable bioelectronics to clinical neuroscience, particularly focusing on the treatment of brain tumor and epilepsy are reviewed. Finally, an ideal system of soft intracranial bioelectronics such as closed-loop-type fully-integrated systems is presented, and the remaining challenges for their clinical translation are discussed.

Injectable smart stimuli-responsive hydrogels: pioneering advancements in biomedical applications

Hydrogels have established their significance as prominent biomaterials within the realm of biomedical research. However, injectable hydrogels have garnered greater attention compared with their conventional counterparts due to their excellent minimally invasive nature and adaptive behavior post-injection. With the rapid advancement of emerging chemistry and deepened understanding of biological processes, contemporary injectable hydrogels have been endowed with an "intelligent" capacity to respond to various endogenous/exogenous stimuli (such as temperature, pH, light and magnetic field). This innovation has spearheaded revolutionary transformations across fields such as tissue engineering repair, controlled drug delivery, disease-responsive therapies, and beyond. In this review, we comprehensively expound upon the raw materials (including natural and synthetic materials) and injectable principles of these advanced hydrogels, concurrently providing a detailed discussion of the prevalent strategies for conferring stimulus responsiveness. Finally, we elucidate the latest applications of these injectable "smart" stimuli-responsive hydrogels in the biomedical domain, offering insights into their prospects.

Soft ferroelectret ultrasound receiver for targeted peripheral neuromodulation

Bioelectronic medicine is a rapidly growing field where targeted electrical signals can act as an adjunct or alternative to drugs to treat neurological disorders and diseases via stimulating the peripheral nervous system on demand. However, current existing strategies are limited by external battery requirements, and the injury and inflammation caused by the mechanical mismatch between rigid electrodes and soft nerves. Here we report a wireless, leadless, and battery-free ferroelectret implant, termed NeuroRing, that wraps around the target peripheral nerve and demonstrates high mechanical conformability to dynamic motion nerve tissue. As-fabricated NeuroRing can act as an ultrasound receiver that converts ultrasound vibrations into electrostimulation pulses, thus stimulating the targeted peripheral nerve on demand. This capability is demonstrated by the precise modulation of the sacral splanchnic nerve to treat colitis, providing a framework for future bioelectronic medicines that offer an alternative to non-specific pharmacological approaches.

Soft and Conductive Polyethylene Glycol Hydrogel Electrodes for Electrocardiogram Monitoring

The measurement of biosignals in the clinical and healthcare fields is fundamental; however, conventional electrodes pose challenges such as incomplete skin contact and skin-related issues, hindering accurate biosignal measurement. To address these challenges, conductive hydrogels, which are valuable owing to their biocompatibility and flexibility, have been widely developed and explored for electrode applications. In this study, we fabricated a conductive hydrogel by mixing polyethylene glycol diacrylate (PEGDA) with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) polymers dissolved in deionized water, followed by light-triggered crosslinking. Notably, this study pioneered the use of a PEGDA-PEDOT:PSS hydrogel for electrocardiogram (ECG) monitoring- a type of biosignal. The resulting PEGDA-PEDOT:PSS hydrogel demonstrated remarkable conductivity while closely approximating the modulus of skin elasticity. Additionally, it demonstrated biocompatibility and a high signal-to-noise ratio in the waveforms. This study confirmed the exceptional suitability of the PEGDA-PEDOT:PSS hydrogel for accurate biosignal measurements with potential applications in various wearable devices designed for biosignal monitoring.

Manipulation of cross-linking in PEDOT:PSS hydrogels for biointerfacing

Conducting hydrogels can be used to fabricate bioelectronic devices that are soft for improved cell- and tissue-interfacing. Those based on conjugated polymers, such as poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS), can be made simply with solution-based processing techniques, yet the influence of fabrication variables on final gel properties is not fully understood. In this study, we investigated if PEDOT:PSS cross-linking could be manipulated by changing the concentration of a gelling agent, ionic liquid, in the hydrogel precursor mixture. Rheology and gelation kinetics of precursor mixtures were investigated, and aqueous stability, swelling, conductivity, stiffness, and cytocompatibility of formed hydrogels were characterized. Increasing ionic liquid concentration was found to increase cross-linking as measured by decreased swelling, decreased non-network fraction, increased stiffness, and increased conductivity. Such manipulation of IL concentration thus afforded control of final gel properties and was utilized in further investigations of biointerfacing. When cross-linked sufficiently, PEDOT:PSS hydrogels were stable in sterile cell culture conditions for at least 28 days. Additionally, hydrogels supported a viable and proliferating population of human dermal fibroblasts for at least two weeks. Collectively, these characterizations of stability and cytocompatibility illustrate that these PEDOT:PSS hydrogels have significant promise for biointerfacing applications that require soft materials for direct interaction with cells.

Effective mechanical properties of frozen hydrogel with ice inclusions

Hydrogel exhibits attractive mechanical properties that can be regulated to be extremely tough, strong and resilient, adhesive and fatigue-resistant, thus enabling diverse applications ranging from tissue engineering scaffolds, flexible devices, to soft machines. As a liquid-filled porous material composed of polymer networks and water, the hydrogel freezes at subzero temperatures into a new material composed of polymer matrix and ice inclusions: the frozen hydrogel displays dramatically altered mechanical properties, which can significantly affect its safety and reliability in practical applications. In this study, based upon the theory of homogenization, we predicted the effective mechanical properties (e.g., Young's modulus, shear modulus, bulk modulus and Poisson ratio) of a frozen hydrogel with periodically distributed longitudinal ice inclusions. We firstly estimated its longitudinal Young's modulus, longitudinal Poisson ratio and plane strain bulk modulus using the self-consistent method, and then its longitudinal and transverse shear modulus using the generalized self-consistent method; further, the results were employed to calculate its transverse Young's modulus and transverse Poisson ratio. We validated the theoretical predictions against both finite element (FE) simulation and experimental measurement results, with good agreement achieved. We found that the estimated transverse Poisson ratio ranges from 0.3 to 0.53 and, at low volume fraction of ice inclusions, exhibits a value larger than 0.5 that exceeds the Poisson ratios of both the polymer matrix and the ice inclusion (typically 0.33-0.35). Compared with other homogenization methods (e.g., the rule of mixtures, the Halpin-Tsai equations, and the Mori-Tanaka method), the present approach is more accurate in predicting the effective mechanical properties (in particular, the transverse Poisson ratio) of frozen hydrogel. Our study provides theoretical support for the practical applications of frozen liquid-saturated porous materials such as hydrogel.

Water-responsive supercontractile polymer films for bioelectronic interfaces

Connecting different electronic devices is usually straightforward because they have paired, standardized interfaces, in which the shapes and sizes match each other perfectly. Tissue-electronics interfaces, however, cannot be standardized, because tissues are soft1-3 and have arbitrary shapes and sizes4-6. Shape-adaptive wrapping and covering around irregularly sized and shaped objects have been achieved using heat-shrink films because they can contract largely and rapidly when heated7. However, these materials are unsuitable for biological applications because they are usually much harder than tissues and contract at temperatures higher than 90 °C (refs. 8,9). Therefore, it is challenging to prepare stimuli-responsive films with large and rapid contractions for which the stimuli and mechanical properties are compatible with vulnerable tissues and electronic integration processes. Here, inspired by spider silk10-12, we designed water-responsive supercontractile polymer films composed of poly(ethylene oxide) and poly(ethylene glycol)-α-cyclodextrin inclusion complex, which are initially dry, flexible and stable under ambient conditions, contract by more than 50% of their original length within seconds (about 30% per second) after wetting and become soft (about 100 kPa) and stretchable (around 600%) hydrogel thin films thereafter. This supercontraction is attributed to the aligned microporous hierarchical structures of the films, which also facilitate electronic integration. We used this film to fabricate shape-adaptive electrode arrays that simplify the implantation procedure through supercontraction and conformally wrap around nerves, muscles and hearts of different sizes when wetted for in vivo nerve stimulation and electrophysiological signal recording. This study demonstrates that this water-responsive material can play an important part in shaping the next-generation tissue-electronics interfaces as well as broadening the biomedical application of shape-adaptive materials.

A fully integrated, standalone stretchable device platform with in-sensor adaptive machine learning for rehabilitation

Post-surgical treatments of the human throat often require continuous monitoring of diverse vital and muscle activities. However, wireless, continuous monitoring and analysis of these activities directly from the throat skin have not been developed. Here, we report the design and validation of a fully integrated standalone stretchable device platform that provides wireless measurements and machine learning-based analysis of diverse vibrations and muscle electrical activities from the throat. We demonstrate that the modified composite hydrogel with low contact impedance and reduced adhesion provides high-quality long-term monitoring of local muscle electrical signals. We show that the integrated triaxial broad-band accelerometer also measures large body movements and subtle physiological activities/vibrations. We find that the combined data processed by a 2D-like sequential feature extractor with fully connected neurons facilitates the classification of various motion/speech features at a high accuracy of over 90%, which adapts to the data with noise from motion artifacts or the data from new human subjects. The resulting standalone stretchable device with wireless monitoring and machine learning-based processing capabilities paves the way to design and apply wearable skin-interfaced systems for the remote monitoring and treatment evaluation of various diseases.