Achieving tissue-level softness on stretchable electronics through a generalizable soft interlayer design

Recent Progress of Soft and Bioactive Materials in Flexible Bioelectronics

Materials that establish functional, stable interfaces to targeted tissues for long-term monitoring/stimulation equipped with diagnostic/therapeutic capabilities represent breakthroughs in biomedical research and clinical medicine. A fundamental challenge is the mechanical and chemical mismatch between tissues and implants that ultimately results in device failure for corrosion by biofluids and associated foreign body response. Of particular interest is in the development of bioactive materials at the level of chemistry and mechanics for high-performance, minimally invasive function, simultaneously with tissue-like compliance and in vivo biocompatibility. This review summarizes the most recent progress for these purposes, with an emphasis on material properties such as foreign body response, on integration schemes with biological tissues, and on their use as bioelectronic platforms. The article begins with an overview of emerging classes of material platforms for bio-integration with proven utility in live animal models, as high performance and stable interfaces with different form factors. Subsequent sections review various classes of flexible, soft tissue-like materials, ranging from self-healing hydrogel/elastomer to bio-adhesive composites and to bioactive materials. Additional discussions highlight examples of active bioelectronic systems that support electrophysiological mapping, stimulation, and drug delivery as treatments of related diseases, at spatiotemporal resolutions that span from the cellular level to organ-scale dimension. Envisioned applications involve advanced implants for brain, cardiac, and other organ systems, with capabilities of bioactive materials that offer stability for human subjects and live animal models. Results will inspire continuing advancements in functions and benign interfaces to biological systems, thus yielding therapy and diagnostics for human healthcare.

Programmable and Spatial Stiffness Gradient Substrates for Highly Robust Artificial Skins

Stretchable artificial skins have garnered great interest for their potential applications in real-time human-machine interaction and equipment operation status monitoring. The local stiffer structure areas on the substrates for the functional elements have been verified to improve the robustness of the artificial skins, but it remains challenging to achieve robust sensing performance under mechanical deformation due to large mechanical mismatch and the intricate fabrication process. Herein, we propose an easy strategy for fabricating a substrate with spatial and programmable stiffness gradients to greatly decrease strain interference and increase the robustness under stretching and bending. The substrate was fabricated by direct writing PDMS with laser gelation, where the sensing elements lay on the place with higher stiffness. The modulus of the substrates varied up to 10-fold, and they also show excellent adhesive properties and durability. This configuration of the spatial stiffness gradient effectively inhibits the deformation strain effect of stretching and bending on the sensing elements. Prototype flexible sensors and light-emitting diodes can be integrated into stretchable artificial skins to exhibit highly robust performance during dynamic deformations, demonstrating an efficient pathway for fabricating robust stretchable electronics, especially for real-time health surveillance.

Photo-crosslinkable organic materials for flexible and stretchable electronics

As technology advances to enhance human perceptual experiences of the surrounding environment, significant research on stretchable electronics is actively progressing, spanning from the synthesis of materials to their applications in fully integrated devices. A critical challenge lies in developing materials that can maintain their electrical properties under substantial stretching. Photo-crosslinkable organic materials have emerged as a promising solution due to their ability to be precisely modified with light to achieve desired properties, such as enhanced durability, stable conductivity, and micropatterning. This review examines recent research on photo-crosslinkable organic materials, focusing on their components and integration within stretchable electronic devices. We explore the essential characteristics required for each device component (insulators, semiconductors, and conductors) and explain how photo-crosslinking technology addresses these needs through its principles and implementation. Additionally, we discuss the integration and utilization of these components in real-world applications, including physical sensors, organic field-effect transistors (OFETs), and organic solar cells (OSCs). Finally, we offer a concise perspective on the future directions and potential challenges in ongoing research on photo-crosslinkable organic materials.

Directly Printed 3D Soft Microwave Plasmonic Enhanced-Q Resonators by Decoupling from Lossy Media

Soft electronic components are essential building blocks for realizing form-factor-free applications; however, most designs are confined to 2D or 2.5D structures due to challenges in maintaining 3D structural integrity. This limitation is particularly critical for electromagnetic devices, such as resonators, where dielectric losses from elastomeric substrates severely hinder high-performance functionality. Here, directly printed 3D electromagnetic soft plasmonic enhanced-quality(Q) factor resonators are proposed, using highly conductive composites. By incorporating an immiscible solvent into an elastomer matrix, emulsion phases are formed that significantly enhance the storage modulus, enabling the fabrication of 3D-printed structures while improving their electrical conductivity. 3D microwave plasmonic resonators with a high degree of design freedom, such as pillars and hooks are demonstrated. These structures exhibit improved resistance to dielectric interference by leveraging the resonance in lossless air. Moreover, integrating a coplanar ground plane further decouples the resonators from lossy substrates, resulting in a 3.4-fold enhancement in the Q-factor (octupole mode) compared to 2D resonators. This improvement enables stable operation on high-permittivity surfaces, such as human skin. Additionally, a single 3D resonator demonstrates wireless deformation-sensing capabilities, facilitating the simultaneous detection of strain amplitude and orientation. This result can pave the way for advanced sensing applications in soft electronics.

Simultaneous Isotropic Omnidirectional Hypersensitive Strain Sensing and Deep Learning-Assisted Direction Recognition in a Biomimetic Stretchable Device

Omnidirectional strain sensing and direction recognition ability are features of the human tactile sense, essential to address the intricate and dynamic requirements of real-world applications. Most of the current strain sensors work by converting uniaxial strain into electrical signals, which restricts their use in environments with multiaxial strain. Here, the first device with simultaneous isotropic omnidirectional hypersensitive strain sensing and direction recognition (IOHSDR) capabilities is introduced. By mimicking the human fingers from three dimensions, the IOHSDR device realizes a novel heterogeneous substrate that incorporates the involute of a circle, resulting in isotropic behavior in the radial direction and anisotropic property in the involute direction for hypersensitive strain sensing. With the assistance of a deep learning-based model, the IOHSDR device accomplishes an impressive accuracy of 99.58% in recognizing 360° stretching directions. Additionally, it exhibits superior performance in the typical properties of stretchable strain sensors, with a gauge factor of 634.12, an ultralow detection limit of 0.01%, and outstanding durability exceeding 15 000 cycles. The demonstration of radial artery pulse and throat vibration applications highlights the IOHSDR's unique characteristics of isotropic omnidirectional sensing and precise direction detection unleashing new classes of wearable health monitoring devices.

Mussel-Like Silk Fibroin Hydrogel With Skin Compliance Soft Electrode for Wearable Devices

Flexible wearable electronic devices, capable of real-time physiological monitoring for personalized health management, are increasingly recognized for their convenience, comfort, and customization potential. Despite advancements, challenges persist for soft electrodes due to the skin's complex surface, biocompatibility demands, and modulus mismatch. In response, a mussel-inspired polydopamine-nanoclay-silk fibroin hydrogel (DA-C-SFH) is introduced, synthesized via a two-step process. The initial polydopamine oxidation introduces free catechol groups through polydopamine-incorporated nanoclay, followed by integration with silk fibroin, refining the fibroin network at the mesoscopic scale. This DA-C-SFH exhibits low modulus, high elasticity, adhesive properties, and biocompatibility, enabling conformal skin adhesion. It effectively detects subtle signals, such as pulse waves, and serves as a soft epidermal electrode, capable of recording various electrophysiological signals, including electrocardiograms and electromyograms, thus underscoring its potential in medical electronics and health monitoring applications.

Autonomous Bioelectronic Devices Based on Silk Fibroin

The development of autonomous bioelectronic devices capable of dynamically adapting to changing biological environments represents a significant advancement in healthcare and wearable technologies. Such systems draw inspiration from the precision, adaptability, and self-regulation of biological processes, requiring materials with intrinsic versatility and seamless bio-integration to ensure biocompatibility and functionality over time. Silk fibroin (SF) derived from Bombyx mori cocoons, has emerged as an ideal biomaterial with a unique combination of biocompatibility, mechanical flexibility, and tunable biodegradability. Adding autonomous features into SF, including self-healing, shape-morphing, and controllable degradation, enables dynamic interactions with living tissues while minimizing immune responses and mechanical mismatches. Additionally, structural tunability and environmental sustainability of SF further reinforce its potential as a platform for adaptive implants, epidermal electronics, and intelligent textiles. This review explores recent progress in understanding the structure-property relationships of SF, its modification strategies, and its great potential for integration into advanced autonomous bioelectronic systems while addressing challenges related to scalability, reproducibility, and multifunctionality. Future opportunities, such as AI-assisted material design, scalable fabrication techniques, and the incorporation of wireless and personalized technologies, are also discussed, positioning SF as a key material in bridging the gap between biological systems and artificial technologies.

Advances in conducting nanocomposite hydrogels for wearable biomonitoring

Recent advancements in wearable biosensors and bioelectronics have led to innovative designs for personalized health management devices, with biocompatible conducting nanocomposite hydrogels emerging as a promising building block for soft electronics engineering. In this review, we provide a comprehensive framework for advancing biosensors using these engineered nanocomposite hydrogels, highlighting their unique properties such as high electrical conductivity, flexibility, self-healing, biocompatibility, biodegradability, and tunable architecture, broadening their biomedical applications. We summarize key properties of nanocomposite hydrogels for thermal, biomechanical, electrophysiological, and biochemical sensing applications on the human body, recent progress in nanocomposite hydrogel design and synthesis, and the latest technologies in developing flexible and wearable devices. This review covers various sensor types, including strain, physiological, and electrochemical sensors, and explores their potential applications in personalized healthcare, from daily activity monitoring to versatile electronic skin applications. Furthermore, we highlight the blueprints of design, working procedures, performance, detection limits, and sensitivity of these soft devices. Finally, we address challenges, prospects, and future outlook for advanced nanocomposite hydrogels in wearable sensors, aiming to provide a comprehensive overview of their current state and future potential in healthcare applications.

A Printed Microscopic Universal Gradient Interface for Super Stretchable Strain-Insensitive Bioelectronics

Stretchable electronics capable of conforming to nonplanar and dynamic human body surfaces are central for creating implantable and on-skin devices for high-fidelity monitoring of diverse physiological signals. While various strategies have been developed to produce stretchable devices, the signals collected from such devices are often highly sensitive to local strain, resulting in inevitable convolution with surface strain-induced motion artifacts that are difficult to distinguish from intrinsic physiological signals. Here all-printed super stretchable strain-insensitive bioelectronics using a unique universal gradient interface (UGI) are reported to bridge the gap between soft biomaterials and stiff electronic materials. Leveraging a versatile aerosol-based multi-materials printing technique that allows precise spatial control over the local stiffnesses with submicron resolution, the UGI enables strain-insensitive electronic devices with negligible resistivity changes under a 180% uniaxial stretch ratio. Various stretchable devices are directly printed on the UGI for on-skin health monitoring with high signal quality and near-perfect immunity to motion artifacts, including semiconductor-based photodetectors for sensing blood oxygen saturation levels and metal-based temperature sensors. The concept in this work will significantly simplify the fabrication and accelerate the development of a broad range of wearable and implantable bioelectronics for real-time health monitoring and personalized therapeutics.

Toward Stretchable Flexible Integrated Sensor Systems

Skin-like flexible sensors hold great potential as the next generation of intelligent electronic devices owing to their broad applications in environmental monitoring, human-machine interfaces, the Internet of Things, and artificial intelligence. Flexible electronics inspired by human skin play a vital role in continuous and real-time health monitoring. This review summarizes recent progress in skin-mountable electronics developed by designing flexible electrodes and substrates into different structures, including serpentine, microcrack, wrinkle, and kirigami. Furthermore, this review briefly discusses advances in wearable integrated sensor systems that mimic the flexibility of human skin, as well as multisensing functions. In the future, innovations in stretchable integrated sensor systems will be crucial to develop next-generation intelligent skin-based sensors for practical applications such as medical diagnosis, treatment, and environment monitoring.

Implantable Soft Neural Electrodes of Liquid Metals for Deep Brain Stimulation

Stimulating large volumes of neural networks using macroelectrodes can modulate disorder-associated brain circuits effectively. However, conventional solid-metal electrodes often cause unwanted brain damage due to their high mechanical stiffness. In contrast, low-modulus liquid metals provide tissue-like stiffness while maintaining macroscale electrode dimensions. Here, we present implantable soft macroelectrodes made from biocompatible liquid metals for brain stimulation. These probes can be easily fabricated by simply filling polymeric tubes with a liquid metal, offering a straightforward method for creating brain stimulation devices. They can be customized in various lengths and diameters and also serve as recording microelectrodes. The electrode tips are enhanced with platinum nanoclusters, resulting in low impedance and effective charge injection while preventing liquid metal leakage into brain tissue. In vivo experiments in neuropathic pain rat models demonstrate the stability and effectiveness of these probes for simultaneous neural stimulation and recording, demonstrating their potential for pain alleviation and behavioral control.

Organic Artificial Nerves: Neuromorphic Robotics and Bioelectronics

Neuromorphic electronics are inspired by the human brain's compact, energy-efficient nature and its parallel-processing capabilities. Beyond the brain, the entire human nervous system, with its hierarchical structure, efficiently preprocesses complex sensory information to support high-level neural functions such as perception and memory. Emulating these biological processes, artificial nerve electronics have been developed to replicate the energy-efficient preprocessing observed in human nerves. These systems integrate sensors, artificial neurons, artificial synapses, and actuators to mimic sensory and motor functions, surpassing conventional circuits in sensor-integrated electronics. Organic synaptic transistors (OSTs) are key components in constructing artificial nerves, offering tunable synaptic plasticity for complex sensory processing and the mechanical flexibility required for applications in soft robotics and bioelectronics. Compared to traditional sensor-integrated electronics, early implementations of organic artificial nerves (OANs) incorporating OSTs have demonstrated a higher signal-to-noise ratio, lower power consumption, and simpler circuit designs along with on-device processing capabilities and precise control of actuators and biological limbs, driving progress in neuromorphic robotics and bioelectronics. This paper reviews the materials, device engineering, and system integration of the OAN design, highlights recent advancements in neuromorphic robotics and bioelectronics utilizing the OANs, and discusses current challenges and future research directions.

Biointerface engineering of flexible and wearable electronics

Biointerface sensing is a cutting-edge interdisciplinary field that merges conceptual and practical aspects. Wearable bioelectronics enable efficient interaction and close contact with biological components such as tissues and organs, paving the way for a wide range of medical applications, including personal health monitoring and medical intervention. To be applicable in real-world settings, the patches must be stable and adhere to the skin without causing discomfort or allergies in both wet and dry conditions, as well as other desirable features such as being ultra-soft, thin, flexible, and stretchable. Biosensors have emerged as promising tools primarily used to directly detect biological and electrophysiological signals, enhancing the efficacy of personalized medical treatments and enabling accurate tracking of human well-being. This review highlights the engineering of skin-tissue surfaces/interfaces and their interactions with wearable patches, aiming for both a broad and in-depth understanding of the mechanical and physicochemical properties required for the advancement of flexible and wearable skin patches. Specifically, the advantages of flexible bioelectronics and sensors with optimized surface geometry for long-term diagnosis are discussed. This insight aims to guide the future development of functional materials that can interact with human tissue in a controlled manner. Finally, we provide perspectives on the challenges and potential applications of biointerface engineering in wearable devices.

Recent Progress in Intrinsically Stretchable Sensors Based on Organic Field-Effect Transistors

Organic field-effect transistors (OFETs) are an ideal platform for intrinsically stretchable sensors due to their diverse mechanisms and unique electrical signal amplification characteristics. The remarkable advantages of intrinsically stretchable sensors lie in their molecular tunability, lightweight design, mechanical robustness, solution processability, and low Young's modulus, which enable them to seamlessly conform to three-dimensional curved surfaces while maintaining electrical performance under significant deformations. Intrinsically stretchable sensors have been widely applied in smart wearables, electronic skin, biological detection, and environmental protection. In this review, we summarize the recent progress in intrinsically stretchable sensors based on OFETs, including advancements in functional layer materials, sensing mechanisms, and applications such as gas sensors, strain sensors, stress sensors, proximity sensors, and temperature sensors. The conclusions and future outlook discuss the challenges and future outlook for stretchable OFET-based sensors.

A soft, ultra-tough and multifunctional artificial muscle for volumetric muscle loss treatment

The escalating prevalence of skeletal muscle disorders highlights the critical need for innovative treatments for severe injuries such as volumetric muscle loss. Traditional treatments, such as autologous transplants, are constrained by limited availability and current scaffolds often fail to meet complex clinical needs. This study introduces a new approach to volumetric muscle loss treatment by using a shape-memory polymer (SMP) based on block copolymers of perfluoropolyether and polycaprolactone diol. This SMP mimics the biomechanical properties of natural muscle, exhibiting a low elastic modulus (2-6 MPa), high tensile strength (72.67 ± 3.19 MPa), exceptional toughness (742.02 ± 23.98 MJ m-3) and superior biocompatibility, thereby enhancing skeletal muscle tissue integration and regeneration within 4 weeks. Moreover, the polymer's shape-memory behavior and ability to lift >5000 times its weight showcase significant potential in both severe muscle disorder treatment and prosthetic applications, surpassing existing scaffold technologies. This advancement marks a pivotal step in the development of artificial muscles for clinical use.

Hydrogel Strain Sensors for Integrating Into Dynamic Organ-on-a-Chip

Current hydrogel strain sensors have never been integrated into dynamic organ-on-a-chip (OOC) due to the lack of sensitivity in aqueous cell culture systems. To enhance sensing performance, a novel strain sensor is presented in which the MXene layer is coated on the bottom surface of a pre-stretched anti-swelling hydrogel substrate of di-acrylated Pluronic F127 (F127-DA) and chitosan (CS) for isolation from the cell culture on the top surface. The fabricated strain sensors display high sensitivity (gauge factor of 290.96), a wide sensing range (0-100%), and high repeatability. To demonstrate its application, alveolar epithelial cells are cultivated on the top surface of the hydrogel strain sensor forming alveolar barriers, and then integrated into dynamic lung-on-a-chip (LOC) systems. This system can sensitively monitor normal physiological breathing, pathological inflammation stimulated by lipopolysaccharide (LPS), and alleviated inflammation through drug intervention.

Anisotropic hydrogel microelectrodes for intraspinal neural recordings in vivo

Creating durable, motion-compliant neural interfaces is crucial for accessing dynamic tissues under in vivo conditions and linking neural activity with behaviors. Utilizing the self-alignment of nano-fillers in a polymeric matrix under repetitive tension, here, we introduce conductive carbon nanotubes with high aspect ratios into semi-crystalline polyvinyl alcohol hydrogels, and create electrically anisotropic percolation pathways through cyclic stretching. The resulting anisotropic hydrogel fibers (diameter of 187 ± 13 µm) exhibit fatigue resistance (up to 20,000 cycles at 20% strain) with a stretchability of 64.5 ± 7.9% and low electrochemical impedance (33.20 ± 9.27 kΩ @ 1 kHz in 1 cm length). We observe the reconstructed nanofillers' axial alignment and a corresponding anisotropic impedance decrease along the direction of cyclic stretching. We fabricate fiber-shaped hydrogels into bioelectronic devices and implant them into wild-type and transgenic Thy1::ChR2-EYFP mice to record electromyographic signals from muscles in anesthetized and freely moving conditions. These hydrogel fibers effectively enable the simultaneous recording of electrical signals from ventral spinal cord neurons and the tibialis anterior muscles during optogenetic stimulation. Importantly, the devices maintain functionality in intraspinal electrophysiology recordings over eight months after implantation, demonstrating their durability and potential for long-term monitoring in neurophysiological studies.

Ultrasensitive Wearable Pressure Sensors with Stress-Concentrated Tip-Array Design for Long-Term Bimodal Identification

The great challenges for existing wearable pressure sensors are the degradation of sensing performance and weak interfacial adhesion owing to the low mechanical transfer efficiency and interfacial differences at the skin-sensor interface. Here, an ultrasensitive wearable pressure sensor is reported by introducing a stress-concentrated tip-array design and self-adhesive interface for improving the detection limit. A bipyramidal microstructure with various Young's moduli is designed to improve mechanical transfer efficiency from 72.6% to 98.4%. By increasing the difference in modulus, it also mechanically amplifies the sensitivity to 8.5 V kPa-1 with a detection limit of 0.14 Pa. The self-adhesive hydrogel is developed to strengthen the sensor-skin interface, which allows stable signals for long-term and real-time monitoring. It enables generating high signal-to-noise ratios and multifeatures when wirelessly monitoring weak pulse signals and eye muscle movements. Finally, combined with a deep learning bimodal fused network, the accuracy of fatigued driving identification is significantly increased to 95.6%.

Recent advances in soft, implantable electronics for dynamic organs

Unlike conventional rigid counterparts, soft and stretchable electronics forms crack- or defect-free conformal interfaces with biological tissues, enabling precise and reliable interventions in diagnosis and treatment of human diseases. Intrinsically soft and elastic materials, and device designs of innovative configurations and structures leads to the emergence of such features, particularly, the mechanical compliance provides seamless integration into continuous movements and deformations of dynamic organs such as the bladder and heart, without disrupting natural physiological functions. This review introduces the development of soft, implantable electronics tailored for dynamic organs, covering various materials, mechanical design strategies, and representative applications for the bladder and heart, and concludes with insights into future directions toward clinically relevant tools.

Isolated Perfused Hearts for Cardiovascular Research: An Old Dog with New Tricks

In modern cardiovascular research, isolated perfused hearts have become cost-effective and highly reproducible tools to investigate the mechanisms of cardiovascular diseases (CVDs). Since they were first introduced in the nineteenth century, isolated perfused hearts have been extensively used for testing novel therapies, elucidating cardiac metabolic and electrophysiological activities, and modeling CVDs, including ischemic heart disease, arrhythmias, and hyperacute rejection. In recent years, ex vivo heart perfusion (EVHP) has shown potential in cardiac transplantation by allowing prolonged preservation and reconditioning of donor hearts. In this review, we summarize the evolution of the isolated perfused heart technique and its applications in cardiovascular research to help researchers comprehensively understand the capabilities of isolated heart models and provide guidance to use them to investigate various CVDs.

Increasing the stability of electrolyte-gated organic synaptic transistors for neuromorphic implants

Electrolyte-gated organic synaptic transistors (EGOSTs) can have versatile synaptic plasticity in a single device, so they are promising as components of neuromorphic implants that are intended for use in neuroprosthetic electronic nerves that are energy-efficient and have simple system structure. With the advancement in transistor properties of EGOSTs, the commercialization of neuromorphic implants for practical long-term use requires consistent operation, so they must be stable in vivo. This requirement demands strategies that maintain electronic and ionic transport in the devices while implanted in the human body, and that are mechanically, environmentally, and operationally stable. Here, we cover the structure, working mechanisms, and electrical responses of EGOSTs. We then focus on strategies to ensure their stability to maintain these characteristics and prevent adverse effects on biological tissues. We also highlight state-of-the-art neuromorphic implants that incorporate these strategies. We conclude by presenting a perspective on improvements that are needed in EGOSTs to develop practical, neuromorphic implants that are long-term useable.

Soft Bioelectronics for Heart Monitoring

Cardiovascular diseases (CVDs) are a predominant global health concern, accounting for over 17.9 million deaths in 2019, representing approximately 32% of all global fatalities. In North America and Europe, over a million adults undergo cardiac surgeries annually. Despite the benefits, such surgeries pose risks and require precise postsurgery monitoring. However, during the postdischarge period, where monitoring infrastructures are limited, continuous monitoring of vital signals is hindered. In this area, the introduction of implantable electronics is altering medical practices by enabling real-time and out-of-hospital monitoring of physiological signals and biological information postsurgery. The multimodal implantable bioelectronic platforms have the capability of continuous heart sensing and stimulation, in both postsurgery and out-of-hospital settings. Furthermore, with the emergence of machine learning algorithms into healthcare devices, next-generation implantables will benefit artificial intelligence (AI) and connectivity with skin-interfaced electronics to provide more precise and user-specific results. This Review outlines recent advancements in implantable bioelectronics and their utilization in cardiovascular health monitoring, highlighting their transformative deployment in sensing and stimulation to the heart toward reaching truly personalized healthcare platforms compatible with the Sustainable Development Goal 3.4 of the WHO 2030 observatory roadmap. This Review also discusses the challenges and future prospects of these devices.

Intrinsically stretchable fully π-conjugated polymer film via fluid conjugated molecular external-plasticizing for flexible light-emitting diodes

Fully π-conjugated polymers with rigid aromatic units are promising for flexible optoelectronic devices, but their inherent brittleness poses a challenge for achieving high-performance, intrinsically stretchable fully π-conjugated polymer. Here, we are establishing an external-plasticizing strategy using semiconductor fluid plasticizers (Z1 and Z2) to enhance the optoelectronic, morphological, and stretchable properties of fully π-conjugated polymer films for flexible light-emitting diodes. The synergistic effect of hierarchical structure and optoelectronic properties of Z1 in poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films enable excellent stretchable deformability (~25%) and good conductivity. PLEDs based on F8BT/Z1 films show stable electroluminescence and efficiency under 15% stretch and 100 cycles at 10% strain, revealing outstanding stress tolerance. This strategy is also improving the stretchable properties of polymers like poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) and poly(2-methoxy-5(2'-ethyl)hexoxy-phenylenevinylene) (Super Yellow), demonstrating its general applicability. Therefore, this strategy can provide effective guidance for designing high-performance stretchable fully π-conjugated polymers films for flexible electronic devices.

Highly Conductive and Stretchable Hydrogel Nanocomposite Using Whiskered Gold Nanosheets for Soft Bioelectronics

The low electrical conductivity of conductive hydrogels limits their applications as soft conductors in bioelectronics. This low conductivity originates from the high water content of hydrogels, which impedes facile carrier transport between conductive fillers. This study presents a highly conductive and stretchable hydrogel nanocomposite comprising whiskered gold nanosheets. A dry network of whiskered gold nanosheets is fabricated and then incorporated into the wet hydrogel matrices. The whiskered gold nanosheets preserve their tight interconnection in hydrogels despite the high water content, providing a high-quality percolation network even under stretched states. Regardless of the type of hydrogel matrix, the gold-hydrogel nanocomposites exhibit a conductivity of ≈520 S cm-1 and a stretchability of ≈300% without requiring a dehydration process. The conductivity reaches a maximum of ≈3304 S cm-1 when the density of the dry gold network is controlled. A gold-adhesive hydrogel nanocomposite, which can achieve conformal adhesion to moving organ surfaces, is fabricated for bioelectronics demonstrations. The adhesive hydrogel electrode outperforms elastomer-based electrodes in in vivo epicardial electrogram recording, epicardial pacing, and sciatic nerve stimulation.

Soft bioelectronics for diagnostic and therapeutic applications in neurological diseases

Physical and chemical signals in the central nervous system yield crucial information that is clinically relevant under both physiological and pathological conditions. The emerging field of bioelectronics focuses on the monitoring and manipulation of neurophysiological signals with high spatiotemporal resolution and minimal invasiveness. Significant advances have been realized through innovations in materials and structural design, which have markedly enhanced mechanical and electrical properties, biocompatibility, and overall device performance. The diagnostic and therapeutic potential of soft bioelectronics has been corroborated across a diverse array of pre-clinical settings. This review summarizes recent studies that underscore the developments and applications of soft bioelectronics in neurological disorders, including neuromonitoring, neuromodulation, tumor treatment, and biosensing. Limitations and outlooks of soft devices are also discussed in terms of power supply, wireless control, biocompatibility, and the integration of artificial intelligence. This review highlights the potential of soft bioelectronics as a future platform to promote deciphering brain functions and clinical outcomes of neurological diseases.

Needle-Like Multifunctional Biphasic Microfiber for Minimally Invasive Implantable Bioelectronics

Implantable bioelectronics has attracted significant attention in electroceuticals and clinical medicine for precise diagnosis and efficient treatment of target diseases. However, conventional rigid implantable devices face challenges such as poor tissue-device interface and unavoidable tissue damage during surgical implantation. Despite continuous efforts to utilize various soft materials to address such issues, their practical applications remain limited. Here, a needle-like stretchable microfiber composed of a phase-convertible liquid metal (LM) core and a multifunctional nanocomposite shell for minimally invasive soft bioelectronics is reported. The sharp tapered microfiber can be stiffened by freezing akin to a conventional needle to penetrate soft tissue with minimal incision. Once implanted in vivo where the LM melts, unlike conventional stiff needles, it regains soft mechanical properties, which facilitate a seamless tissue-device interface. The nanocomposite incorporating with functional nanomaterials exhibits both low impedance and the ability to detect physiological pH, providing biosensing and stimulation capabilities. The fluidic LM embedded in the nanocomposite shell enables high stretchability and strain-insensitive electrical properties. This multifunctional biphasic microfiber conforms to the surfaces of the stomach, muscle, and heart, offering a promising approach for electrophysiological recording, pH sensing, electrical stimulation, and radiofrequency ablation in vivo.

Anisotropic Hydrogel Microelectrodes for Intraspinal Neural Recordings in vivo

Creating durable, motion-compliant neural interfaces is crucial for accessing dynamic tissues under in vivo conditions and linking neural activity with behaviors. Utilizing the self-alignment of nano-fillers in a polymeric matrix under repetitive tension, here, we introduce conductive carbon nanotubes with high aspect ratios into semi-crystalline polyvinyl alcohol hydrogels and create electrically anisotropic percolation pathways through cyclic stretching. The resulting anisotropic hydrogel fibers (diameter of 187 ± 13 µm) exhibit fatigue resistance (20,000 cycles at 20% strain) with a stretchability of 64.5 ± 7.9%, and low electrochemical impedance (900 ± 149 kΩ @ 1kHz). We observe the re-constructed nanofillers' axial alignment and a corresponding anisotropic impedance decrease along the direction of cyclic stretching. We fabricate fiber-shaped hydrogels into bioelectronic devices and implant them into wild-type and transgenic Thy1-ChR2-EYFP mice to record electromyographic signals from muscles in anesthetized and freely moving conditions. These hydrogel fibers effectively enable the simultaneous recording of electrical signals from ventral spinal cord neurons and the tibialis anterior muscles during optogenetic stimulation. Importantly, the devices maintain functionality with repeatable recording results over eight months after implantation, demonstrating their durability and potential for long-term monitoring in neurophysiological studies.

Materials, Structure, and Interface of Stretchable Interconnects for Wearable Bioelectronics

Since wearable technologies for telemedicine have emerged to tackle global health concerns, the demand for well-attested wearable healthcare devices with high user comfort also arises. Skin-wearables for health monitoring require mechanical flexibility and stretchability for not only high compatibility with the skin's dynamic nature but also a robust collection of fine health signals from within. Stretchable electrical interconnects, which determine the device's overall integrity, are one of the fundamental units being understated in wearable bioelectronics. In this review, a broad class of materials and engineering methodologies recently researched and developed are presented, and their respective attributes, limitations, and opportunities in designing stretchable interconnects for wearable bioelectronics are offered. Specifically, the electrical and mechanical characteristics of various materials (metals, polymers, carbons, and their composites) are highlighted, along with their compatibility with diverse geometric configurations. Detailed insights into fabrication techniques that are compatible with soft substrates are also provided. Importantly, successful examples of establishing reliable interfacial connections between soft and rigid elements using novel interconnects are reviewed. Lastly, some perspectives and prospects of remaining research challenges and potential pathways for practical utilization of interconnects in wearables are laid out.

Vertically stacked skin-like active-matrix display with ultrahigh aperture ratio

Vertically stacked all-organic active-matrix organic light-emitting diodes are promising candidates for high-quality skin-like displays due to their high aperture ratio, extreme mechanical flexibility, and low-temperature processing ability. However, these displays suffer from process interferences when interconnecting functional layers made of all-organic materials. To overcome this challenge, we present an innovative integration strategy called "discrete preparation-multilayer lamination" based on microelectronic processes. In this strategy, each functional layer was prepared separately on different substrates to avoid chemical and physical damage caused by process interferences. A single interconnect layer was introduced between each vertically stacked functional layer to ensure mechanical compatibility and interconnection. Compared to the previously reported layer-by-layer preparation method, the proposed method eliminates the need for tedious protection via barrier and pixel-defining layer processing steps. Additionally, based on active-matrix display, this strategy allows multiple pixels to collectively display a pattern of "1" with an aperture ratio of 83%. Moreover, the average mobility of full-photolithographic organic thin-film transistors was 1.04 cm2 V-1 s-1, ensuring stable and uniform displays. This strategy forms the basis for the construction of vertically stacked active-matrix displays, which should facilitate the commercial development of skin-like displays in wearable electronics.

A universal packaging substrate for mechanically stable assembly of stretchable electronics

Stretchable electronics commonly assemble multiple material modules with varied bulk moduli and surface chemistry on one packaging substrate. Preventing the strain-induced delamination between the module and the substrate has been a critical challenge. Here we develop a packaging substrate that delivers mechanically stable module/substrate interfaces for a broad range of stiff and stretchable modules with varied surface chemistries. The key design of the substrate was to introduce module-specific stretchability and universal adhesiveness by regionally tuning the bulk molecular mobility and surface molecular polarity of a near-hermetic elastic polymer matrix. The packaging substrate can customize the deformation of different modules while avoiding delamination upon stretching up to 600%. Based on this substrate, we fabricated a fully stretchable bioelectronic device that can serve as a respiration sensor or an electric generator with an in vivo lifetime of 10 weeks. This substrate could be a versatile platform for device assembly.

Unveiling Trends: Nanoscale Materials Shaping Emerging Biomedical Applications

The realm of biomedical materials continues to evolve rapidly, driven by innovative research across interdisciplinary domains. Leveraging big data from the CAS Content Collection, this study employs quantitative analysis through natural language processing (NLP) to identify six emerging areas within nanoscale materials for biomedical applications. These areas encompass self-healing, bioelectronic, programmable, lipid-based, protein-based, and antibacterial materials. Our Nano Focus delves into the multifaceted utilization of nanoscale materials in these domains, spanning from augmenting physical and electronic properties for interfacing with human tissue to facilitating intricate functionalities like programmable drug delivery.

Dielectric Design of High Dielectric Constant Poly(Urea-Urethane) Elastomer for Low-Voltage High-Mobility Intrinsically Stretchable All-Solution-Processed Organic Transistors

Stretchable organic transistors for skin-like biomedical applications require low-voltage operation to accommodate limited power supply and safe concerns. However, most of the currently reported stretchable organic transistors operate at relatively high voltages. Decreasing their operational voltage while keeping the high mobility still remains a key challenge. Here, the study presents a new dielectric design to achieve high-dielectric constant poly(urea-urethane) (PUU) elastomer, by incorporating a flexible small-molecular diamine crosslinking agent 4-aminophenyl disulfide (APDS) into the main chain of (poly (propylene glycol), tolylene 2,4-diiso-cyanate terminated) (PPG-TDI). Compared with commercial elastomers, the PUU elastomer as dielectric of the stretchable organic transistors shows the outstanding advantages including lower surface roughness (0.33 nm), higher adhesion (45.18 nN), higher dielectric constant (13.5), as well as higher stretchability (896%). The PUU dielectric enables the intrinsically stretchable, all-solution-processed organic transistor to operate at a low operational voltage down to -10 V, while preserving a substantial mobility of 1.39 cm2 V-1 s-1. Impressively, the transistor also demonstrates excellent electrical stability under repeated switching of 10 000 cycles, and remarkable mechanical robustness when stretched up to 100%. The work opens up a new molecular engineering strategy to successfully realize low-voltage high-mobility stretchable all-solution-processed organic transistors.

Morphology-Controlled Ion Transport in Mixed-Orientation Polymers

Advancing iontronics with precisely controlled ion transport is fundamentally important to bridge external organic electronics with the biosystem. This long-standing goal, however, is thus far limited by the trade-off between the active ion electromigration and idle diffusion leakage in the (semi)crystalline film. Here, we presented a mixed-orientation strategy by blending a conjugated polymer, allowing for simultaneously high ion electromigration efficiency and low leakage. Our studies revealed that edge-on aggregation with a significant percolative pathway exhibits much higher ion permeability than that of the face-on counterpart but encounters pronounced leakage diffusion. Through carefully engineering the mixed orientations, the polymer composite demonstrated an ideal switchable ion-transport behavior, achieving a remarkably high electromigration efficiency exceeding one quadrillion ions per milliliter per minute and negligible idle leakage. This proof of concept, validated by drug release in a skin-conformable organic electronic ion pump (OEIP), offers a rational approach for the development of multifunctional iontronic devices.

A Gradient Stiffness-Programmed Circuit Board by Spatially Controlled Phase-Transition of Supercooled Hydrogel for Stretchable Electronics Integration

Due to emerging demands in soft electronics, there is an increasing need for material architectures that support robust interfacing between soft substrates, stretchable electrical interconnects, and embedded rigid microelectronics chips. Though researchers have adopted rigid-island structures to solve the issue, this approach merely shifts stress concentrations from chip-conductor interfaces to rigid-island-soft region interfaces in the substrate. Here, a gradient stiffness-programmed circuit board (GS-PCB) that possesses high stretchability and stability with surface mounted chips is introduced. The board comprises a stiffness-programmed hydrogel substrate and a laser-patterned liquid metal conductor. The hydrogel simultaneously obtains a large stiffness disparity and robust interfaces between rigid-islands and soft regions. These seemingly contradictory conditions are accomplished by adopting a gradient stiffness structure at the interfaces, enabled by combining polymers with different interaction energies and a supercooled sodium acetate solution. By integrating the gel with laser-patterned liquid metal with exceptional properties, GS-PCB exhibits higher electromechanical stability than other rigid-island research. To highlight the practicality of this approach, a finger-sensor device that successfully distinguishes objects by direct physical contact is fabricated, demonstrating its stability under various mechanical disturbances.

Transparent and Conformal Microcoil Arrays for Spatially Selective Neuronal Activation

Micromagnetic stimulation (μMS) using small, implantable microcoils is a promising method for achieving neuronal activation with high spatial resolution and low toxicity. Herein, we report a microcoil array for localized activation of cortical neurons and retinal ganglion cells. We developed a computational model to relate the electric field gradient (activating function) to the geometry and arrangement of microcoils, and selected a design that produced an anisotropic region of activation <50 μm wide. The device was comprised of an SU-8/Cu/SU-8 tri-layer structure, which was flexible, transparent and conformal and featured four individually-addressable microcoils. Interfaced with cortex or retina explants from GCaMP6-expressing mice, we observed that individual neurons localized within 40 μm of a microcoil tip could be activated repeatedly and in a dose- (power-) dependent fashion. These results demonstrate the potential of μMS devices for brain-machine interfaces and could enable routes toward bioelectronic therapies including prosthetic vision devices.

Determining the appropriate natural fibers for intelligent green wearable devices made from biomaterials via multi-attribute decision making model

Intelligent and green wearable technology becomes essential for new modern societies. This work introduces a multi criteria decision making model to properly assess and compare relative desired criteria for selecting the most suitable constituents for green body wearable bio-products made from bio-based materials. It aims to enhance the sustainability of intelligent green wearable devices by providing support in the selection process of lightweight, eco-friendly materials suitable for personal body wearable bio-products made of natural fiber composites to improve qualities that may help in better monitoring human vital signs and thereby address the health care concern. The relative intrinsic characteristic and merits of various natural fibers were utilized to compare and evaluate their relative performance in bio-composites. The model considered several evaluation factors like mechanical performance including tensile strength and modulus of elasticity, comfortability including size and weight, availability, fiber orientation, cellulose content, and cost. Results have demonstrated different priorities of the considered natural fibers relative to each evaluation factor. However, the model was capable of properly evaluating and ranking the best fibers relative to the whole conflicting evaluation criteria simultaneously. The closeness of priorities in several cases emphasizes upon using such decision making models to be able to judge the relative merits of natural fibers for such applications. It can also help designers to avoid bias during determining the best alternatives considering several conflicting evaluation criteria.

Self-compliant ionic skin by leveraging hierarchical hydrogen bond association

Robust interfacial compliance is essential for long-term physiological monitoring via skin-mountable ionic materials. Unfortunately, existing epidermal ionic skins are not compliant and durable enough to accommodate the time-varying deformations of convoluted skin surface, due to an imbalance in viscosity and elasticity. Here we introduce a self-compliant ionic skin that consistently works at the critical gel point state with almost equal viscosity and elasticity over a super-wide frequency range. The material is designed by leveraging hierarchical hydrogen bond association, allowing for the continuous release of polymer strands to create topological entanglements as complementary crosslinks. By embodying properties of rapid stress relaxation, softness, ionic conductivity, self-healability, flaw-insensitivity, self-adhesion, and water-resistance, this ionic skin fosters excellent interfacial compliance with cyclically deforming substrates, and facilitates the acquisition of high-fidelity electrophysiological signals with alleviated motion artifacts. The presented strategy is generalizable and could expand the applicability of epidermal ionic skins to more complex service conditions.

Unconstrained Piezoelectric Vascular Electronics for Wireless Monitoring of Hemodynamics and Cardiovascular Health

The patient-centered healthcare requires timely disease diagnosis and prognostic assessment, calling for individualized physiological monitoring. To assess the postoperative hemodynamic status of patients, implantable blood flow monitoring devices are highly expected to deliver real time, long-term, sensitive, and reliable hemodynamic signals, which can accurately reflect multiple physiological conditions. Herein, an implantable and unconstrained vascular electronic system based on a piezoelectric sensor immobilized is presented by a "growable" sheath around continuously growing arterial vessels for real-timely and wirelessly monitoring of hemodynamics. The piezoelectric sensor made of circumferentially aligned polyvinylidene fluoride nanofibers around pulsating artery can sensitively perceive mechanical signals, and the growable sheath bioinspired by the structure and function of leaf sheath has elasticity and conformal shape adaptive to the dynamically growing arterial vessels to avoid growth constriction. With this integrated and smart design, long-term, wireless, and sensitive monitoring of hemodynamics are achieved and demonstrated in rats and rabbits. It provides a simple and versatile strategy for designing implantable sensors in a less invasive way.

Stretchable and Lithography-Compatible Interconnects Enabled by Self-Assembled Nanofilms with Interlocking Interfaces

Stretchable interconnects with miniature widths are vital for the high-density integration of deformable electronic components on a single substrate for targeted data logic or storage functions. However, it is still challenging to attain high-resolution patternability of stretchable conductors with robust circuit fabrication capability. Here, we report a self-assembled silver nanofilm firmly interlocked by an elastomeric nanodielectric that can be photolithographically patterned into microscale features while preserving high stretchability and conductivity. Both silver and dielectric nanofilms are fabricated by layer-by-layer assembly, ensuring wafer-scale uniformity and meticulous control of thicknesses. Without any thermal annealing, the as-fabricated nanofilms from silver nanoparticles (AgNPs) exhibit conductivity of 1.54 × 106 S m-1 and stretchability of ∼200%, which is due to the impeded crack propagation by the underlying PU nanodielectrics. Furthermore, it is revealed that AgNP microstrips defined by photolithography show higher stretchability when their widths are downscaled to 100 μm owing to confined cracks. However, further scaling restricts the stretchability, following the early development of cracks cutting across the strip. In addition, the resistance change of these silver interconnects can be decreased using serpentine architectures. As a demonstration, these self-assembled interconnects are used as stretchable circuit boards to power LEDs.

Highly stable and strain-insensitive metal film conductors via manipulating strain distribution

Metal film-based stretchable conductors are essential elements of flexible electronics for wearable, biomedical, and robotic applications, which require strain-insensitive high conductivity over a wide strain range and excellent cyclic stability. However, they suffer from serious electrical failure under monotonic and cyclic tensile loading at a small strain due to the uncontrolled film cracking behavior. Here, we propose a novel in-plane crack control strategy of engineering hierarchical microstructures to achieve outstanding electromechanical performance via harnessing the strain distribution in metal films. The wrinkles delay the crack initiation at undercuts which should be the most vulnerable sites during the stretching process. The surface protrusions/grooves/undercuts inhibit the crack propagation because of the effective strain redistribution. In addition, hierarchical microstructures significantly improve cyclic stability due to the strong interfacial adhesion and stable crack patterns. The metal film-based conductors exhibit ultrahigh strain-insensitive conductivity (1.7 × 107 S m-1), negligible resistance change (ΔR/R0 = 0.007) over an ultra-wide strain range (>200%), and excellent cyclic strain durability (>15 000 cycles at 100% strain). A range of metal films was explored to establish the universality of this strategy, including ductile copper and silver, as well as brittle molybdenum and high entropy alloy. We demonstrate the strain-insensitive electrical functionality of a metal film-based conductor in a flexible light-emitting diode circuit.