Ultrathin Hydrogel Films toward Breathable Skin-Integrated Electronics

Liquid metal-based stretchable bioelectronic fiber for electrical stimulation and drug delivery in minimally invasive cardiac therapy

Cardiovascular diseases, such as ventricular arrhythmias and heart failure, require timely and effective treatment to prevent disease progression and improve patient outcomes. Current therapeutics, including electrical shock and emergent cardiovascular medications, have significantly contributed to managing these conditions. However, due to their systemic side effects, there are ongoing demands for highly effective localized therapies. In this regard, a soft implantable device has been considered for cardiac applications, but invasiveness in their implantation procedure and difficulty in compact integration of multiple functions are unmet challenges. To address these issues, we develop a stretchable, multi-functional fiber designed for emergent cardiac intervention, offering electrogram recording, electrical modulation, and drug therapy directly at the epicardial surface. With temperature-dependent phase shifting properties of the liquid metal inside the fiber, the stiffened fiber can be implanted into the thoracic cavity without invasive surgery. Once implanted, the softened fiber provides multimodal therapies (e.g., chemotherapy and electrical therapy) tailored to the patient's condition. By tuning the delivery parameters based on continuous electrogram recording, effective and urgent cardiac interventions for severe arrhythmias are demonstrated in an in vivo rat model.

Unperceivable Designs of Wearable Electronics

Wearable smart electronics are taking an increasing part of the consumer electronics market, with applications in advanced healthcare systems, entertainment, and Internet of Things. The advanced development of flexible, stretchable, and breathable electronic materials has paved the way to comfortable and long-term wearables. However, these devices can affect the wearer's appearance and draw attention during use, which may impact the wearer's confidence and social interactions, making them difficult to wear on a daily basis. Apart from comfort, one key condition for user acceptance is that these new technologies seamlessly integrate into our daily lives, remaining unperceivable to others. In this review, strategies to minimize the visual impact of wearable devices and make them more suitable for daily use are discussed. These new devices focus on being unperceivable when worn and comfortable enough that users almost forget their presence, reducing psychological discomfort while maintaining accuracy in signal collection. Materials selection is crucial for developing long-term and unperceivable wearable devices. Recent developments in these unperceivable electronic devices are also covered, including sensors, transistors, and displays, and mechanisms to achieve unperceivability are discussed. Finally, the potential applications are summarized and the remaining challenges and prospects are discussed.

High performance breathable conductive hydrogel sensor based on sodium alginate and polyacrylamide with cross-linked dual network structures

Conductive hydrogels have attracted immense interest in the field of flexible electronic devices and biosensors. However, issues such as poor breathability, insufficient adhesion and water retention properties still remain and limit their applications as wearable sensors. To address these issues, a porous conductive hydrogel (SA/PAM/CTAB-GO) with high breathability (5.97 mg.cm-2·h-1), favorable conductivity (2.85 S/m), desirable adhesion (1.39 KPa) and moisture retention properties was synthesized by polymerization and crosslinking using sodium alginate and polyacrylamide as gel substrates, cetyltrimethylammonium bromide modified graphene oxide as foam stabilizer and Ca2+ as crosslinking agent. The hydrogel exhibited excellent mechanical properties, water retention characteristics, and high breathability comparable to cotton. Moreover, the SA/PAM/CTAB-GO hydrogel presented excellent sensing sensitivity, fast response ability (225 ms), and favorable endurance, which monitored human motions sensitively and realized the accurate, sensitive and reliable detection of human ECG and EMG signals. The hydrogel sensor was applied in human-computer interaction and sensitively controlled robot arms and virtual characters. The design of dual network mechanism of calcium crosslinking and foam porosity method improved the sensing properties, breathability, adhesion, and stability of the hydrogel making it possesses high performance as wearable sensor and has promising application prospect in flexible electronics.

Nanoarchitectonics of a Skin-Like Polymeric Hydrogel with High Anti-Swelling and Self-Adhesion Performance for Underwater Communication

Hydrogels are flexible materials characterized by a 3D network structure, which possess high water content and adjustable physicochemical properties. They have found widespread applications in tissue engineering, electronic skin, drug delivery, flexible sensors, and photothermal therapy. However, hydrogel networks often exhibit swelling behavior in aqueous environments, which can result in structural degradation and a loss of gel performance. In this study, polyacrylic acid is utilized as the primary network structure with the incorporation of the natural polymer chitosan. Furthermore, a conductive hydrogel exhibiting good mechanical strength similar to human skin and excellent anti-swelling properties is developed by integrating phytic acid into the hydrogel network. The as-prepared hydrogels exhibited maximum swelling in pure water, achieving an equilibrium swelling rate of 15%. Additionally, a dopamine-grafted polyacrylic acid binder is synthesized through a coupling reaction to enhance the adhesion of the hydrogels to various substrates. The hydrogels demonstrated strong adhesion properties with different substrates. Whether in the air or underwater, the hydrogel sensor effectively monitors human movement behaviors. Furthermore, by utilizing the sensing signals to send Morse code, the hydrogel sensor can facilitate underwater communication. This type of hydrogel sensor is anticipated to play a significant role in wearable sensing applications and underwater communication.

Hydrogel tissue adhesive: Adhesion strategy and application

Hydrogel tissue adhesives have emerged as a promising alternative to conventional wound closure methods such as sutures and staples due to their operational simplicity demonstrated biocompatibility and capacity for multifunctional integration. However, complex and variable tissue microenvironments and dynamic adhesion surfaces still challenge the actual adhesion performance of adhesives, especially natural polymer-based adhesives. In addition, to expand the application of adhesives in biomedical fields, there is an urgent need to further improve tissue adhesion performance through composition design, adhesion mechanism research and bioeffect development. This review focuses on the adhesive properties of adhesives and their applications in biomedical fields. Adhesion-cohesion equilibria, forms of adhesion failure, methods for improving cohesion and various interfacial adhesion mechanisms are presented. Moreover, practical biomedical applications of tissue adhesives are reviewed, focusing on skin, heart, stomach, liver, and cornea. Finally, this review looks ahead to a new generation of multi-functional, strong adhesion tissue adhesives, in the hope of providing inspiration to those working in the field.

An Implantable In-Hydrogel Wireless Supercapacitor-Activated Neuron System Enables Bidirectional Modulation

The bidirectional modulation of cerebral neurons in the brain possesses enhancement and inhibition of neural activity, which is of great interest in the treatment of motor nerve disorders and emotional disorders, and cognitive defects. However, existing approaches usually rely on electrical/electrochemical stimulations, which show low security by implanting metal probes and unidirectional currents with single modulation. Herein, an implantable in-hydrogel wireless supercapacitor-activated neuron system consisting of the coil, diode bridge circuit, in-hydrogel supercapacitor, and stimulation electrodes is fabricated, which provides a bidirectional and adjustable ion diffusion current to safely and effectively excite and inhibit brain neurons. The designed in-hydrogel supercapacitor exhibits a high storage charge ability of ≈90 times larger than the devices without hydrogel encapsulation, owing to the in situ radical addition mechanism. Moreover, the in-hydrogel electrodes are implanted into the thalamus, amygdala, and prefrontal lobes of the brain to evoke the corresponding changes in potential intensity and frequency through the external chargeable coil and diode bridge circuit, which verifies the potential of the multimodule supercapacitor in amelioration and treatment Parkinson's, severe depression, and Alzheimer's disease.

Moisture-responsive ultralow-hysteresis polymer ionogels for adhesion-switchable strain sensing

Adhesion-switchable ultralow-hysteresis polymer ionogels are highly demanded in soft electronics to avoid debonding damage and signal distortion, yet the design and fabrication of such ionogels are challenging. Herein, we propose a novel method to design switchable adhesive ionogels by using binary ionic solvents with two opposite-affinity ionic components. The obtained ionogels exhibit moisture-induced phase separation, facilitating switchable adhesion with a high detaching efficiency (>99%). Moreover, before and after phase separation, the viscoelastic behavior of the ionogels is maintained in the rubbery plateau region within common frequency ranges with ultralow mechanical hysteresis (∼3%) under large strain, enabling accurate and stable strain and pressure sensing. Accordingly, the ionogel films can be used as functional elements in a smart clamp to realize flytrap-like selective activation, based on high sensitivity to the vibration intensity from the targeted prey. This work may inspire future research on the development of advanced soft electronics.

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.

Multifunctional Hydrogel Electronics for Synergistic Therapy and Visual Monitoring in Wound Healing

To overcome the limitations of precise monitoring and inefficient wound exudate management in wound healing, an advanced multifunctional hydrogel electronics (MHE) platform based on MXene@MOF/Fe3O4@C photonic crystal hydrogel is developed. This platform combines optical/electrical sensing, synergistic therapy, and real-time visual monitoring into a single, efficient system, offering a comprehensive solution for wound healing. Under photothermal stimulation, the hydrogel releases metal ions that generate hydroxyl radicals, effectively eliminating antibiotic-resistant bacteria. Beyond its antibacterial efficacy, this system offers unprecedented real-time monitoring through temperature-responsive visualization, while structural color changes upon wound exudate absorption provide a clear indication for dressing replacement. By integrating these functionalities, MHE platform allows for precise control of the therapeutic process, significantly improving wound healing and treatment monitoring. The platform's optical/electrical sensing capabilities further broaden its potential applications across other biomedical fields. This breakthrough technology provides clinicians with a powerful tool to optimize therapeutic outcomes, marking a major advancement in wound care and biomedical applications.

One-step fabrication of ultrathin porous Janus membrane within seconds for waterproof and breathable electronic skin

It remains a challenge for a simple and scalable method to fabricate ultrathin porous Janus membranes for stretchable on-skin electronics. Here, we propose a one-step droplet spreading phase separation strategy to prepare an ultrathin and easily collected Janus thermoplastic polyurethane (TPU) membrane within seconds. The metal-ion solvation structure mitigated migration kinetics to delay TPU solution demixing, promoting the further penetration of the coagulating solvent. Consequently, the developed membranes, with an average preparation rate of 25.2 cm2 s-1, had a thickness of 5 μm, and the water vapor transmission rate was determined to be 663 g m-2 d-1. The small pore layer having an average pore size of 1.7 μm effectively blocked external liquid water. The porous Janus TPU membrane coated by liquid metal served as a building block to develop a new generation of monolithic stretchable electronics with simultaneous high permeability and waterproofness.

Antifreezing Ultrathin Bioionic Gel-Based Wearable System for Artificial Intelligence-Assisted Arrhythmia Diagnosis in Hypothermia

Cardiovascular disease (CAD) is a major global public health issue, with mortality rates being significantly impacted by cold temperatures. Stable and reliable electrocardiogram (ECG) monitoring in cold environments is crucial for early detection and treatment of CAD. However, existing skin sensor struggle to balance freeze resistance, breathability, flexibility, conductivity and adhesion at cold temperatures. Here, we introduce a solvent cross-linking strategy and an in situ transfer method to prepare ultrathin bioionic gels, featuring a freezing point below -80 °C and a thickness of only 12.6 μm. The strong and abundant interactions between the ionic liquid solvent and the zwitterionic polymer effectively suppress low-temperature crystallization, forming a toughened and highly adhesive network structure. This network enables the in situ formation of an ultrathin morphology, which can be seamlessly transferred onto various substrates. Furthermore, the solvent-cross-linked network maintains a large interpolymer chain spacing, facilitating rapid ion transport pathways. Even at subzero temperatures, the gel maintains its multifunctionality, demonstrating tissue-like softness (34.6 kPa), high ionic conductivity (10.06 mS cm-1), excellent stretchability (360%), high transparency, robust adhesive strength (175.3 kPa) and interfacial toughness (1146 J m-2). Integrated into a flexible wearable device, the ultrathin gel ensures excellent skin conformity, user comfort, and high signal-to-noise ECG signal acquisition. Leveraging an artificial neural network, the system analyzes bradycardia ECG signals and achieves 96.88% accuracy in arrhythmia detection under cold conditions. This bioionic gel-based system presents a promising solution for early CAD diagnosis and prediction in extreme environments.

Optimized synergistic effects in sweat glucose detection with a Pt single-atom catalyst on NiO for fingertip wearable biosensors

For the approximately 8.5 % of the global population living with diabetes, puncture-based glucose testing is often an unpleasant experience. Non-invasive sweat glucose testing not only reduces pain and the risk of wound infection but also offers a more suitable method for real-time glucose monitoring. In this study, we developed a fingertip wearable biosensor (FWB) capable of continuously measuring glucose levels in sweat, providing valuable data for assessing glucose concentrations in humans. We successfully synthesized NiO/Pt single-atom catalysts (NiO/Pt SAs) using a UV reduction technique, achieving a detection range of 5 μM to 2 mM that encompasses the full spectrum of physiological glucose levels. Additionally, incorporating 0.075 g of starch enhanced the hydrogel's water absorption and swelling properties, allowing it to absorb over 832 % of its dry weight without breaking, thereby improving sweat absorption efficiency. We also designed an annular microfluidic channel for rapid sweat transport. The circular design fits snugly on the fingertip surface, minimizing footprint and increasing comfort. This makes the device more stable in real-world use and minimizes the effects of external movements or environmental changes. Experimental results confirmed the feasibility of using the FWB to detect glucose in sweat samples from volunteers. We believe our research holds significant promise for advancements in sweat analysis and health monitoring, presenting a novel and efficient approach for continuous glucose monitoring.

Hydrogels empowered mesenchymal stem cells and the derived exosomes for regenerative medicine in age-related musculoskeletal diseases

As the population ages, musculoskeletal diseases (MSK) have emerged as a significant burden for individuals, healthcare systems, and social care systems. Recently, regenerative medicine has exhibited vast potential in age-related MSK, with mesenchymal stromal cells (MSCs) and their derived exosomes (Exos) therapies showing distinct advantages. However, these therapies face several limitations, including issues related to ensuring stability and effective distribution within the body. Hydrogels, acting as an ideal carrier, can enhance the therapeutic effects and application range of MSCs and Exos derived from MSCs (MSC-Exos). Therefore, this review comprehensively summarizes the application progress of MSCs and MSC-Exos combined with hydrogels in age-related MSK disease research. It aims to provide a detailed perspective, showcasing the functional enhancement of MSCs and MSC-Exos when incorporated into hydrogels. Additionally, this review explores their potential and challenges in treating age-related MSK diseases, offering references for future research directions and potential innovative strategies.

Gel-Based Electrolytes for Organic Electrochemical Transistors: Mechanisms, Applications, and Perspectives

Organic electrochemical transistors (OECTs) have emerged as the core component of specialized bioelectronic technologies due to their high signal amplification capability, low operating voltage (<1 V), and biocompatibility. Under a gate bias, OECTs modulate device operation via ionic drift between the electrolyte and the channel. Compared to common electrolytes with a fluid nature (including salt aqueous solutions and ion liquids), gel electrolytes, with an intriguing structure consisting of a physically and/or chemically crosslinked polymer network where the interstitial spaces between polymers are filled with liquid electrolytes or mobile ion species, are promising candidates for quasi-solid electrolytes. Due to relatively high ionic conductivity, the potential for large-scale integration, and the capability to suppress channel swelling, gel electrolytes have been a research highlight in OECTs in recent years. This review summarizes recent progress on OECTs with gel electrolytes that demonstrate good mechanical as well as physical and chemical stabilities. Moreover, various components in forming gel electrolytes, including different mobile liquid phases and polymer components, are introduced. Furthermore, applications of these OECTs in the areas of sensors, neuromorphics, and organic circuits, are discussed. Last, future perspectives of OECTs based on gel electrolytes are discussed along with possible solutions for existing challenges.

Polydopamine-Mediated Nanofillers Reinforced Zwitterion Hydrogel Electrodes for Supercapacitors in Bioelectronics

Supercapacitors that can function when in direct contact with human tissue are of paramount importance for wearable bioelectronics but face mismatching with biological tissue and its movement. Herein, we developed a zwitterion hydrogel elastomer electrode-based all-hydrogel supercapacitor (AHSC) characterized by good energy storage properties, bioadhesion, body movement-matching mechanical properties, and biocompatibility. These functions were realized by integrating a [2-(methacryloyloxy)ethyl]dimethyl-(3-propylsulfonate)ammonium hydroxide (DMAPS) and hydroxyethyl acrylate (HEA)-copolymerized zwitterion hydrogel electrode (DMAPS-HEA) with redox-active nanofillers. This hydrogel electrode endowed AHSC with body movement-matching mechanical properties and biocompatibility. Redox-active nanofillers were designed with the structure of a reduced graphene oxide (rGO)-anchored cobalt/nickel bimetallic metal-organic framework (Co/Ni MOF) using polydopamine (PDA). The Co/Ni MOF contributes to the high energy storage performance. rGO enhances the conductivity, whereas PDA introduces catechol groups, contributing to the bioadhesion. This AHSC serves as a flexible alternative to traditional rigid and low-tissue-affinity power supply devices in bioelectronics.

Paintable, Fast Gelation, Highly Adhesive Hydrogels for High-fidelity Electrophysiological Monitoring Wirelessly

High-fidelity wireless electrophysiological monitoring is essential for ambulatory healthcare applications. Soft solid-like hydrogels have received significant attention as epidermal electrodes because of their tissue-like mechanical properties and high biocompatibility. However, it is challenging to develop a hydrogel electrode that provides robust contact and high adhesiveness with glabrous skin and hairy scalp for high-fidelity, continuous electrophysiological signal detection. Here, a paintable, fast gelation, highly adhesive, and conductive hydrogel is engineered for high-fidelity wireless electrophysiological monitoring. The hydrogel, consisting of gelatin, gallic acid, sodium citrate, lithium chloride, glycerol, and Tris-HCl buffer solution exhibits a reversible thermal phase transition capability, which endows it with the attributes of on-skin applicability and fast in situ gelation with 15 s, thereby addressing the aforementioned limitations. The introduction of gallic acid enhances the adhesive properties of the hydrogel, facilitating secure electrode attachment to the skin or hairy scalp. To accentuate the potential applications in at-home electrophysiological health monitoring, the hydrogel electrodes are demonstrated for high-fidelity electrocardiogram recording for one hour during various daily activities, as well as in simultaneous electroencephalogram and electrocardiogram recording during a 30 min nap.

Highly Self-Adhesive and Biodegradable Silk Bioelectronics for All-In-One Imperceptible Long-Term Electrophysiological Biosignals Monitoring

Skin-like bioelectronics offer a transformative technological frontier, catering to continuous and real-time yet highly imperceptible and socially discreet digital healthcare. The key technological breakthrough enabling these innovations stems from advancements in novel material synthesis, with unparalleled possibilities such as conformability, miniature footprint, and elasticity. However, existing solutions still lack desirable properties like self-adhesivity, breathability, biodegradability, transparency, and fail to offer a streamlined and scalable fabrication process. By addressing these challenges, inkjet-patterned protein-based skin-like silk bioelectronics (Silk-BioE) are presented, that integrate all the desirable material features that have been individually present in existing devices but never combined into a single embodiment. The all-in-one solution possesses excellent self-adhesiveness (300 N m-1) without synthetic adhesives, high breathability (1263 g h-1 m-2) as well as swift biodegradability in soil within a mere 2 days. In addition, with an elastic modulus of ≈5 kPa and a stretchability surpassing 600%, the soft electronics seamlessly replicate the mechanics of epidermis and form a conformal skin/electrode interface even on hairy regions of the body under severe perspiration. Therefore, coupled with a flexible readout circuitry, Silk-BioE can non-invasively monitor biosignals (i.e., ECG, EEG, EOG) in real-time for up to 12 h with benchmarking results against Ag/AgCl electrodes.

Ultrafast enzyme-responsive hydrogel for real-time assessment and treatment optimization in infected wounds

Monitoring wound infection and providing appropriate treatment are crucial for achieving favorable outcomes. However, the time-consuming nature of laboratory culture tests may delay timely intervention. To tackle this challenge, a simple yet effective HDG hydrogel, composed of hydrogen peroxide (H₂O₂), dopamine, and GelMA polymer, is developed for the ultrafast detection and treatment of Staphylococcus aureus (SA) infections. The HDG hydrogel detects SA by exploiting its secreted catalase to catalyze H₂O₂, producing oxygen, which in turn accelerates the polymerization of colorless dopamine into deep brown polydopamine (PDA). The bacterial detection process takes only 10 min with high sensitivity, and the results can be readily recognized by the naked eye or quantified using a cell phone-based digital analysis. Moreover, the HDG hydrogel provides a dual antibacterial mechanism through chemical and photothermal therapies via the generated PDA, significantly improving bacterial clearance. In animal experiments, the HDG hydrogel demonstrated promising capabilities in monitoring and eliminating bacteria, enhancing collagen deposition, reducing inflammation, and promoting the healing of infected wounds. This multifunctional design offers an enzyme-responsive strategy for the rapid assessment and management of infections, simplifying infection evaluation and facilitating the development of advanced wound dressings.

Multi-Level High Entropy-Dissipative Structure Enables Efficient Self-Decoupling of Triple Signals

The theory of high entropy-dissipative structure is confined to high-entropy alloys and their oxide materials under harsh conditions, but it is very difficult to obtain high entropy-dissipative structure for smart sensors based on polymers and metal oxides under mild conditions. Moreover, multiple signal coupling effect heavily hinder the sensor applications, and current multimodal integrated devices can solve two signal-decoupling, but need very complicated process way. In this work, new synthesis concept is the first time to fabricate high entropy-dissipative conductive layer of smart sensors with triple-signal response and self-decoupling ability within poly-pyrrole/zinc oxide (PPy/ZnO) system. The sensor (SPZ20) amplifies pressure (17.54%/kPa) and gas (0.37%/ppm), reduces humidity (0.41%/% RH) and temperature (0.12%/°C) signals, simultaneously achieving the triple self-decoupling effect of pressure and gas in the complex temperature-humidity field because of the enlarged pressure-contact area, enhanced gas-responsive sites, altered vapor path and its own heat insulation function. Additionally, it inherits the strong robustness (500 rubbing, washing, and heating or freezing cycles) and endurance (10 000 photo-purification cycles) of traditional high-entropy materials for information transmission and smart alarms in emergencies or harsh environments. This work gives a new insight into the multiple-signal response and smart flexible electronic design from natural fibers.

In-Ear Electronics with Mechanical Adaptability for Physiological Sensing

Significant developments have been made in the field of wearable healthcare by utilizing soft materials for the construction of electronic sensors. However, the lack of adaptability to complex topologies, such as ear canal, results in inadequate sensing performance. Here, we report an in-ear physiological sensor with mechanical adaptability, which softens upon contact with the ear canal's skin, thus reducing the sensor-skin mechanical mismatch and interface impedance. An efficient strategy of mechanical adjustment and switching is exploited to increase the softness of the device, leading to a significant decrease in Young's modulus from 30.5 MPa of thermoplastic polyurethane (TPU) to 0.86 MPa of TPU/Ecoflex foam (TEF).The mechanical adaptability at body temperature endows the in-ear device improved device-canal contact area and interface stability. As a result, the TEF-based in-ear device demonstrates reliable sensing, low motion artifact, and high comfort in electroencephalography (EEG) and core body temperature sensing. High quality EEG signals of alpha, beta, delta, and gamma are measured during different activities. Moreover, the TEF-based in-ear device exhibits high reusability for over 4 months, which makes it suitable for long-term healthcare monitoring.

Customized surface adhesive and wettability properties of conformal electronic devices

Conformal and body-adaptive electronics have revolutionized the way we interact with technology, ushering in a new era of wearable devices that can seamlessly integrate with our daily lives. However, the inherent mismatch between artificially synthesized materials and biological tissues (caused by irregular skin fold, skin hair, sweat, and skin grease) needs to be addressed, which can be realized using body-adaptive electronics by rational design of their surface adhesive and wettability properties. Over the past few decades, various approaches have been developed to enhance the conformability and adaptability of bioelectronics by (i) increasing flexibility and reducing device thickness, (ii) improving the adhesion and wettability between bioelectronics and biological interfaces, and (iii) refining the integration process with biological systems. Successful development of a conformal and body-adaptive electronic device requires comprehensive consideration of all three aspects. This review starts with the design strategies of conformal electronics with different surface adhesive and wettability properties. A series of conformal and body-adaptive electronics used in the human body under both dry and wet conditions are systematically discussed. Finally, the current challenges and critical perspectives are summarized, focusing on promising directions such as telemedicine, mobile health, point-of-care diagnostics, and human-machine interface applications.

Hydrogels in wearable neural interfaces

The integration of wearable neural interfaces (WNIs) with the human nervous system has marked a significant progression, enabling progress in medical treatments and technology integration. Hydrogels, distinguished by their high-water content, low interfacial impedance, conductivity, adhesion, and mechanical compliance, effectively address the rigidity and biocompatibility issues common in traditional materials. This review highlights their important parameters-biocompatibility, interfacial impedance, conductivity, and adhesiveness-that are integral to their function in WNIs. The applications of hydrogels in wearable neural recording and neurostimulation are discussed in detail. Finally, the opportunities and challenges faced by hydrogels for WNIs are summarized and prospected. This review aims to offer a thorough examination of hydrogel technology's present landscape and to encourage continued exploration and innovation. As developments progress, hydrogels are poised to revolutionize wearable neural interfaces, offering significant enhancements in healthcare and technological applications.

Graphical Abstract

Engineering robust and transparent dual-crosslinked hydrogels for multimodal sensing without conductive additives

Conductive hydrogels are pivotal for the advancement of flexible sensors, electronic skin, and healthcare monitoring systems, facilitating transformative innovations. However, issues such as inadequate intrinsic compatibility, mismatched mechanical properties, and limited stability curtail their potential, resulting in compromised device efficacy and performance degradation. In this research, we engineered functional hydrogels featuring a dual-crosslinked network composed of (PA/PVA)-P(AM-AA) to address these challenges. This design eliminates the need for conductive additives, thereby enhancing intrinsic compatibility. Notably, the hydrogels exhibit exceptional mechanical properties, with high tensile strength (∼700 %), Young's modulus (∼5.33 MPa), increased strength (∼2.46 MPa) and toughness (∼6.59 MJ m-3). They also achieve a compressive strength of ∼7.33 MPa at 80 % maximal compressive strain and maintain about 89 % transparency. Moreover, flexible sensors derived from these hydrogels demonstrate enhanced multimodal sensing capabilities, including temperature, strain, and pressure detection, enabling precise monitoring of human movements. The integration of multiple hydrogels into a three-dimensional sensor array facilitates detailed spatial pressure distribution mapping. By strategically applying dual-crosslinked network engineering and eliminating conductive additives, we have streamlined the design and manufacturing of hydrogels to meet the rising demand for high-performance wearable sensors.

HSPiP and QbD oriented optimized stearylamine-elastic liposomes for topical delivery of ketoconazole to treat deep seated fungal infections: In vitro and ex vivo evaluations

The study explored stearylamine containing cationic elastic liposomes to improve topical delivery and efficacy of ketoconazole (KETO) to treat deeply seated fungal infections. Stearylamine was used for dual functionalities (electrostatic interaction and flexibility in lipid bilayer). Hansen solubility program (HSPiP) estimated Hansen solubility parameters (HSP) based on the SMILE file and structural properties followed by experimental solubility study to validate the predicted values. Various formulations were developed by varying phosphatidylcholine and surfactants (tween 80 and span 80) concentration. To impart cationic properties, stearylamine (1.0 %) was added into the organic phase. Using quality by design (QbD) method, we optimized the formulations and evaluated for vesicle size, polydispersity index, zeta potential, morphology (scanning electron microscopy), in vitro drug release (%), and ex vivo permeation profiles. Result showed that there is a good correlation (0.65) between HSPiP predicted and actual experimental solubility of KETO in water, chloroform, S80, and tween 80. Spherical OKEL1 showed an established correlation between the predicted and the actual formulation parameters (size, zeta potential, and polydispersity index) (259 nm vs 270 nm, +2.4 vs 0.21 mV, and 0.24 vs 0.27). OKEL1 was associated with the highest value of %EE (83.1 %) as compared to liposomes. Finally, OKEL1 exhibited the highest % cumulative permeation (49.9 %) as compared to DS (13 %) and liposomes (25 %). Moreover, OKEL1 resulted in 4-fold increase in permeation flux as compared to DS which may be attributed to vesicular mediated improved permeation and gel based compensated trans epidermal water loss in the skin. The drug deposition elicited OKEL1 and OKEL1-gel as suitable carriers for maximum therapeutic benefit to treat deeply seated fungal infections.

Multifunctional Porous Soft Composites for Bimodal Wearable Cardiac Monitors

Wearable heart monitors are crucial for early diagnosis and treatment of heart diseases in non-clinical settings. However, their long-term applications require skin-interfaced materials that are ultrasoft, breathable, antibacterial, and possess robust, enduring on-skin adherence-features that remain elusive. Here, we have developed multifunctional porous soft composites that meet all these criteria for skin-interfaced bimodal cardiac monitoring. The composite consists of a bilayer structure featuring phase-separated porous elastomer and slot-die-coated biogel. The porous elastomer ensures ultrasoftness, breathability, ease of handling, and mechanical integrity, while the biogel enables long-term on-skin adherence. Additionally, we incorporated ε-polylysine in the biogel to offer antibacterial properties. Also, the conductive biogel embedded with silver nanowires was developed for use in electrocardiogram sensors to reduce contact impedance and ensure high-fidelity recordings. Furthermore, we assembled a bimodal wearable cardiac monitoring system that demonstrates high-fidelity recordings of both cardiac electrical (electrocardiogram) and mechanical (seismocardiogram) signals over a 14-day testing period.

A Flexible Organomagnetic Single-Layer Composite Film with Built-In Multistimuli Responsivity

Materials possessing multiple properties and functionalities, that can be controlled or modulated by external stimuli, are a central focus of current research in materials sciences due to their potential to significantly enhance various future technological applications. Herein, we report a significant advancement in this field through the development of a smart, multifunctional organomagnetic composite material. By utilizing a thin layer of polydimethylsiloxane (PDMS) and polypyrrole (PPy) precursors, doped with nickel nanoparticles (NiNPs), we have created an innovative organomagnetic, PDMS/PPy/NiNPs (PPN), single-layer composite film that displays multistimuli responsivity. The study presents the first demonstration of a multifunctional flexible, three-component film structure integrating the structural and flexible PDMS component, together with a conductive polymer component and metal-based nanoparticles into a single-layer design, which displays enhanced and unprecedented responsivity properties against multiple different stimuli. Unlike typical stacked multilayered structures, that exhibit one or two functionalities at most, this novel configuration exhibits multiple functionalities, including magnetoresistance, mechanical stress response, piezoresistivity, and temperature change sensitivity. The as-prepared film demonstrates notable magnetoresistance responsivity, with a relative electrical resistance, ΔR/R0, changing under a weak magnetic field and under ambient conditions. The significance of our study lies in the film's versatility, stability, and sensitivity, especially within the physiological temperature range, making it highly relevant for future biomedical applications. Furthemore, the film's sensitivity to mechanical deformation reveals an impressive piezoresistance behavior. Unlike existing multilayer architectures of higher complexity, our single-layer thin film offers a simpler, more flexible, and reliable solution with a broad range of stimuli-sensing capabilities. The significance of this novel multiresponsive composite material is underscored by the growing demand for advanced materials in biomedical devices, magnetic switches, sensors, electronic skin, transistors, and organic spintronic devices. These promising organomagnetic self-standing layers provide a robust platform for future technological innovations.

Programmable and flexible wood-based origami electronics

Natural polymer substrates are gaining attention as substitutes for plastic substrates in electronics, aiming to combine high performance, intricate shape deformation, and environmental sustainability. Herein, natural wood veneer is converted into a transparent wood film (TWF) substrate. The combination of 3D printing and origami technique is established to create programmable wood-based origami electronics, which exhibit superior flexibility with high tensile strength (393 MPa) due to the highly aligned cellulose fibers and the formation of numerous intermolecular hydrogen bonds between them. Moreover, the flexible TWF electronics exhibit editable multiplexed configurations and maintain stable conductivity. This is attributed to the strong adhesion between the cellulose-based ink and TWF substrate by non-covalent bonds. Benefiting from its anisotropic structure, the programmability of TWF electronics is achieved through sequentially folding into predesigned shapes. This design not only promotes environmental sustainability but also introduces its customizable shapes with potential applications in sensors, microfluidics, and wearable electronics.

Advancements in Flexible Electronics Fabrication: Film Formation, Patterning, and Interface Optimization for Cutting-Edge Healthcare Monitoring Devices

Flexible electronics can seamlessly adhere to human skin or internal tissues, enabling the collection of physiological data and real-time vital sign monitoring in home settings, which give it the potential to revolutionize chronic disease management and mitigate mortality rates associated with sudden illnesses, thereby transforming current medical practices. However, the development of flexible electronic devices still faces several challenges, including issues pertaining to material selection, limited functionality, and performance instability. Among these challenges, the choice of appropriate materials, as well as their methods for film formation and patterning, lays the groundwork for versatile device development. Establishing stable interfaces, both internally within the device and in human-machine interactions, is essential for ensuring efficient, accurate, and long-term monitoring in health electronics. This review aims to provide an overview of critical fabrication steps and interface optimization strategies in the realm of flexible health electronics. Specifically, we discuss common thin film processing methods, patterning techniques for functional layers, interface challenges, and potential adjustment strategies. The objective is to synthesize recent advancements and serve as a reference for the development of innovative flexible health monitoring devices.

Mechanical Regulation of Polymer Gels

The mechanical properties of polymer gels devote to emerging devices and machines in fields such as biomedical engineering, flexible bioelectronics, biomimetic actuators, and energy harvesters. Coupling network architectures and interactions has been explored to regulate supportive mechanical characteristics of polymer gels; however, systematic reviews correlating mechanics to interaction forces at the molecular and structural levels remain absent in the field. This review highlights the molecular engineering and structural engineering of polymer gel mechanics and a comprehensive mechanistic understanding of mechanical regulation. Molecular engineering alters molecular architecture and manipulates functional groups/moieties at the molecular level, introducing various interactions and permanent or reversible dynamic bonds as the dissipative energy. Molecular engineering usually uses monomers, cross-linkers, chains, and other additives. Structural engineering utilizes casting methods, solvent phase regulation, mechanochemistry, macromolecule chemical reactions, and biomanufacturing technology to construct and tailor the topological network structures, or heterogeneous modulus compositions. We envision that the perfect combination of molecular and structural engineering may provide a fresh view to extend exciting new perspectives of this burgeoning field. This review also summarizes recent representative applications of polymer gels with excellent mechanical properties. Conclusions and perspectives are also provided from five aspects of concise summary, mechanical mechanism, biofabrication methods, upgraded applications, and synergistic methodology.

Flexible electronics for cardiovascular monitoring on complex physiological skins

Cardiovascular diseases (CVDs) pose a significant global health threat, responsible for a considerable portion of worldwide mortality. Flexible electronics enable continuous, noninvasive, real-time, and portable monitoring, providing an ideal platform for personalized healthcare. Nevertheless, challenges persist in sustaining stable adherence across diverse and intricate skin environments, hindering further advancement toward clinical applications. Strategies such as structural design and chemical modification can significantly enhance the environmental adaptability and monitoring performance of flexible electronics. This review delineates processing techniques, including structural design and chemical modification, to mitigate signal interference from sebaceous skin, motion artifacts from the skin in motion, and infection risks from fragile skin, thereby enabling the accurate monitoring of key cardiovascular indicators in complex physiological environments. Moreover, it delves into the potential for the strategic development and improvement of flexible electronics to ensure their alignment with complex physiological environment requirements, facilitating their transition to clinical applications.

Multifunctional Conductive Hydrogel Interface for Bioelectronic Recording and Stimulation

The past few decades have witnessed the rapid advancement and broad applications of flexible bioelectronics, in wearable and implantable electronics, brain-computer interfaces, neural science and technology, clinical diagnosis, treatment, etc. It is noteworthy that soft and elastic conductive hydrogels, owing to their multiple similarities with biological tissues in terms of mechanics, electronics, water-rich, and biological functions, have successfully bridged the gap between rigid electronics and soft biology. Multifunctional hydrogel bioelectronics, emerging as a new generation of promising material candidates, have authentically established highly compatible and reliable, high-quality bioelectronic interfaces, particularly in bioelectronic recording and stimulation. This review summarizes the material basis and design principles involved in constructing hydrogel bioelectronic interfaces, and systematically discusses the fundamental mechanism and unique advantages in bioelectrical interfacing with the biological surface. Furthermore, an overview of the state-of-the-art manufacturing strategies for hydrogel bioelectronic interfaces with enhanced biocompatibility and integration with the biological system is presented. This review finally exemplifies the unprecedented advancement and impetus toward bioelectronic recording and stimulation, especially in implantable and integrated hydrogel bioelectronic systems, and concludes with a perspective expectation for hydrogel bioelectronics in clinical and biomedical applications.

Interfacing Neuron-Motor Pathways with Stretchable and Biocompatible Electrode Arrays

In the field of neuroscience, understanding the complex interactions within the intricate neuron-motor system depends crucially on the use of high-density, physiological multiple electrode arrays (MEAs). In the neuron-motor system, the transmission of biological signals primarily occurs through electrical and chemical signaling. Taking neurons for instance, when a neuron receives external stimuli, it generates an electrical signal known as the action potential. This action potential propagates along the neuron's axon and is transmitted to other neurons via synapses. At the synapse, chemical signals (neurotransmitters) are released, allowing the electrical signal to traverse the synaptic gap and influence the next neuron. MEAs can provide unparalleled insights into neural signal patterns when interfacing with the nerve systems through their excellent spatiotemporal resolution. However, the inherent differences in mechanical and chemical properties between these artificial devices and biological tissues can lead to serious complications after chronic implantation, such as body rejection, infection, tissue damage, or device malfunction. A promising strategy to enhance MEAs' biocompatibility involves minimizing their thickness, which aligns their bending stiffness with that of surrounding tissues, thereby minimizing damage over time. However, this solution has its limits; the resulting ultrathin devices, typically based on plastic films, lack the necessary stretchability, restricting their use to organs that neither stretch nor grow. For practical deployments, devices must exhibit certain levels of stretchability (ranging from 20 to 70%), tailored to the specific requirements of the target organs. In this Account, we outline recent advancements in developing stretchable MEAs that balance stretchability with sufficient electrical conductivity for effective use in physiological research, acting as sensors and stimulators. By concentrating on the neuron-motor pathways, we summarize how the stretchable MEAs meet various application needs and examine their effectiveness. We distinguish between on-skin and implantable uses, given their vastly different requirements. Within each application scope, we highlight cutting-edge technologies, evaluating their strengths and shortcomings. Recognizing that most current devices rely on elastic films with a Young's modulus value between ∼0.5 and 5 MPa, we delve into the potential for softer MEAs, particularly those using multifunctional hydrogels for an optimizing tissue-device interface and address the challenges in adapting such hydrogel-based MEAs for chronic implants. Additionally, transitioning soft MEAs from lab to fab involves connecting them to a rigid adapter and external machinery, highlighting a critical challenge at the soft-rigid interface due to strain concentration, especially in chronic studies subject to unforeseen mechanical strains. We discuss innovative solutions to this integration challenge, being optimistic that the development of durable, biocompatible, stretchable MEAs will significantly advance neuroscience and related fields.

Screening of Optimal Konjac Glucomannan-Protein Composite Gel Formulations to Mimic the Texture and Appearance of Tripe

This study aimed to develop a product that closely replicates the texture and appearance of tripe. The effect of three different proteins (soy protein isolate (SPI), pea protein isolate (PPI), and whey protein isolate (WPI)) at different protein levels and processing conditions (heating (90 °C, 1 h) followed by cooling (4 °C, 12 h) and heating (90 °C, 1h) followed by freezing (-18 °C, 12 h)) of konjac glucomannan (KGM) was analyzed. The optimal formulations for simulating tripe were screened by examining their similarity to real tripe in terms of texture, color, and sensory experience. The screened formulations were also subjected to a preliminary mechanistic investigation. The results show that all three proteins improved the gel's textural properties to varying degrees. At the same concentration, the hardness and chewiness of the KGM/WPI composite gel were significantly higher than those of the other two KGM/protein composite gels, among which the composite gel obtained by adding 8% WPI and 5% KGM heating-frozen (FWK4) had the greatest hardness and chewiness of 4338.07 g and 2313.76, respectively, and the springiness differences in all of the composite gels were small. In addition, the addition of protein increased the whiteness of the hybrid gels, with WPI having the most significant effect on the whiteness of the composite gels (whiteness increased from 30.25 to 62.80 as the concentration of WPI increased from 0 to 10%). Freezing increased composite gel hardness and chewiness, but reduced gel springiness and whiteness. Cluster analysis showed that the composite gel obtained by heating-cooling 8% WPI and 5% KGM (WK4) was very similar to the real tripe in terms of chewiness and whiteness, and WK4 had the highest sensory scores for color, tissue morphology, tactile sensation, taste, and odor. The acceptability score in terms of tissue morphology reached 4.3. Meanwhile, the characterization results of WK4 indicate the presence of large junction areas in the gel network. Fourier transform infrared spectroscopy (FTIR) analysis, X-ray diffraction, and intermolecular force contributions indicated that the incorporation of WPI promoted integral interactions, and that hydrophobic interactions and disulfide bonding played a key role in the WK4 composite gel system. Moreover, scanning electron microscopy (SEM) also showed that the combination of WPI and konjac glucan resulted in a more compact gel structure. This study is informative for the development of the field of bionic tripe processing.

An ultraflexible energy harvesting-storage system for wearable applications

The swift progress in wearable technology has accentuated the need for flexible power systems. Such systems are anticipated to exhibit high efficiency, robust durability, consistent power output, and the potential for effortless integration. Integrating ultraflexible energy harvesters and energy storage devices to form an autonomous, efficient, and mechanically compliant power system remains a significant challenge. In this work, we report a 90 µm-thick energy harvesting and storage system (FEHSS) consisting of high-performance organic photovoltaics and zinc-ion batteries within an ultraflexible configuration. With a power conversion efficiency surpassing 16%, power output exceeding 10 mW cm-2, and an energy density beyond 5.82 mWh cm-2, the FEHSS can be tailored to meet the power demands of wearable sensors and gadgets. Without cumbersome and rigid components, FEHSS shows immense potential as a versatile power source to advance wearable electronics and contribute toward a sustainable future.

OECT - Inspired electrical detection

Organic Electrochemical Transistors (OECTs) are integral in detecting human bioelectric signals, attributing their significance to distinct electrochemical properties, the utilization of soft materials, compact dimensions, and pronounced biocompatibility. This review traverses the technological evolution of OECT, highlighting its profound impact on non-invasive detection methodologies within the biomedicalfield. Four sensor types rooted in OECT technology were introduced: Electrocardiogram (ECG), Electroencephalogram (EEG), Electromyography (EMG), and Electrooculography (EOG), which hold promise for integration into wearable detection systems. The fundamental detection principles, material compositions, and functional attributes of these sensors are examined. Additionally, the performance metrics and delineates viable optimization strategies for assorted physiological electrical detection sensors are discussed. The overarching goal of this review is to foster deeper insights into the generation, propagation, and modulation of electrophysiological signals, thereby advancing the application and development of OECT in medical sciences.

Flexible Conformally Bioadhesive MXene Hydrogel Electronics for Machine Learning-Facilitated Human-Interactive Sensing

Wearable epidermic electronics assembled from conductive hydrogels are attracting various research attention for their seamless integration with human body for conformally real-time health monitoring, clinical diagnostics and medical treatment, and human-interactive sensing. Nevertheless, it remains a tremendous challenge to simultaneously achieve conformally bioadhesive epidermic electronics with remarkable self-adhesiveness, reliable ultraviolet (UV) protection ability, and admirable sensing performance for high-fidelity epidermal electrophysiological signals monitoring, along with timely photothermal therapeutic performances after medical diagnostic sensing, as well as efficient antibacterial activity and reliable hemostatic effect for potential medical therapy. Herein, a conformally bioadhesive hydrogel-based epidermic sensor, featuring superior self-adhesiveness and excellent UV-protection performance, is developed by dexterously assembling conducting MXene nanosheets network with biological hydrogel polymer network for conformally stably attaching onto human skin for high-quality recording of various epidermal electrophysiological signals with high signal-to-noise ratios (SNR) and low interfacial impedance for intelligent medical diagnosis and smart human-machine interface. Moreover, a smart sign language gesture recognition platform based on collected electromyogram (EMG) signals is designed for hassle-free communication with hearing-impaired people with the help of advanced machine learning algorithms. Meanwhile, the bioadhesive MXene hydrogel possesses reliable antibacterial capability, excellent biocompatibility, and effective hemostasis properties for promising bacterial-infected wound bleeding.

Wireless Technologies in Flexible and Wearable Sensing: From Materials Design, System Integration to Applications

Wireless and wearable sensors attract considerable interest in personalized healthcare by providing a unique approach for remote, noncontact, and continuous monitoring of various health-related signals without interference with daily life. Recent advances in wireless technologies and wearable sensors have promoted practical applications due to their significantly improved characteristics, such as reduction in size and thickness, enhancement in flexibility and stretchability, and improved conformability to the human body. Currently, most researches focus on active materials and structural designs for wearable sensors, with just a few exceptions reflecting on the technologies for wireless data transmission. This review provides a comprehensive overview of the state-of-the-art wireless technologies and related studies on empowering wearable sensors. The emerging functional nanomaterials utilized for designing unique wireless modules are highlighted, which include metals, carbons, and MXenes. Additionally, the review outlines the system-level integration of wireless modules with flexible sensors, spanning from novel design strategies for enhanced conformability to efficient transmitting data wirelessly. Furthermore, the review introduces representative applications for remote and noninvasive monitoring of physiological signals through on-skin and implantable wireless flexible sensing systems. Finally, the challenges, perspectives, and unprecedented opportunities for wireless and wearable sensors are discussed.

Nature-Inspired Molecular-Crowding Enabling Wide-Humidity Range Applicable, Anti-Freezing, and Robust Zwitterionic Hydrogels for On-Skin Electronics

Hydrogels are currently in the limelight for applications in soft electronics but they suffer from the tendency to lose water or freeze when exposed to dry environments or low temperatures. Molecular crowding is a prevalent occurrence in living cells, in which molecular crowding agents modify the hydrogen bonding structure, causing a significant reduction in water activity. Here, a wide-humidity range applicable, anti-freezing, and robust hydrogel is developed through the incorporation of natural amino acid proline (Pro) and conductive MXene into polyvinyl alcohol (PVA) hydrogel networks. Theoretical calculations reveal that Pro can transform "free water" into "locked water" via the molecular-crowding effect, thereby suppressing water evaporation and ice forming. Accordingly, the prepared hydrogel exhibits high water retention capability, with 77% and 55% being preserved after exposure to 20 °C, 28% relative humidity (RH) and 35 °C, 90% RH for 12 h. Meanwhile, Pro lowers the freezing temperature of the hydrogel to 34 °C and enhances its stretchability and strength. Finally, the PVA/Pro/MXene hydrogels are assembled as multifunctional on-skin strain sensors and conductive electrodes to monitor human motions and detect tiny electrophysiological signals. Collectively, this work provides a molecular crowding strategy that will motivate researchers to develop more advanced hydrogels for versatile applications.

Emerging Trends of Nanofibrous Piezoelectric and Triboelectric Applications: Mechanisms, Electroactive Materials, and Designed Architectures

Over the past few decades, significant progress in piezo-/triboelectric nanogenerators (PTEGs) has led to the development of cutting-edge wearable technologies. Nanofibers with good designability, controllable morphologies, large specific areas, and unique physicochemical properties provide a promising platform for PTEGs for various advanced applications. However, the further development of nanofiber-based PTEGs is limited by technical difficulties, ranging from materials design to device integration. Herein, the current developments in PTEGs based on electrospun nanofibers are systematically reviewed. This review begins with the mechanisms of PTEGs and the advantages of nanofibers and nanodevices, including high breathability, waterproofness, scalability, and thermal-moisture comfort. In terms of materials and structural design, novel electroactive nanofibers and structure assemblies based on 1D micro/nanostructures, 2D bionic structures, and 3D multilayered structures are discussed. Subsequently, nanofibrous PTEGs in applications such as energy harvesters, personalized medicine, personal protective equipment, and human-machine interactions are summarized. Nanofiber-based PTEGs still face many challenges such as energy efficiency, material durability, device stability, and device integration. Finally, the research gap between research and practical applications of PTEGs is discussed, and emerging trends are proposed, providing some ideas for the development of intelligent wearables.

Biodegradable active composite hydrogel packaging for postharvest climacteric bananas preservation

Climacteric bananas are susceptible to endogenous ethylene and temperature, resulting in dehydration, accelerated senescence and deterioration. The widely-used plastic cling films is particularly complicated due to their high consumption and non-degradability. Herein, this study proposed to fabricate a carboxymethyl cellulose/polyvinyl alcohol/pyrazoic acid (CPP) hydrogel for postharvest banana preservation. The hydrogel demonstrated excellent potential as a packaging film, including natural degradability (complete degradation within 50 days), high tensile performance, transparent visibility and biosafety. As a validation experiment, bananas in a 30 °C environment confirmed the effectiveness of CPP hydrogels in banana postharvest preservation. Compared with the blank control and CP hydrogel, CPP packaging film delayed the processes of browning, dehydration, softening, nutrients loss, ripening and senescence in bananas, thereby maintaining their commercial value. Accordingly, this study demonstrates the potential of hydrogel materials as an alternative strategy to climacteric fruit preservation and plastic film.

Biocompatible Electronic Skins for Cardiovascular Health Monitoring

Cardiovascular diseases represent a significant threat to the overall well-being of the global population. Continuous monitoring of vital signs related to cardiovascular health is essential for improving daily health management. Currently, there has been remarkable proliferation of technology focused on collecting data related to cardiovascular diseases through daily electronic skin monitoring. However, concerns have arisen regarding potential skin irritation and inflammation due to the necessity for prolonged wear of wearable devices. To ensure comfortable and uninterrupted cardiovascular health monitoring, the concept of biocompatible electronic skin has gained substantial attention. In this review, biocompatible electronic skins for cardiovascular health monitoring are comprehensively summarized and discussed. The recent achievements of biocompatible electronic skin in cardiovascular health monitoring are introduced. Their working principles, fabrication processes, and performances in sensing technologies, materials, and integration systems are highlighted, and comparisons are made with other electronic skins used for cardiovascular monitoring. In addition, the significance of integrating sensing systems and the updating wireless communication for the development of the smart medical field is explored. Finally, the opportunities and challenges for wearable electronic skin are also examined.

Structuring and Shaping of Mechanically Robust and Functional Hydrogels toward Wearable and Implantable Applications

Hydrogels possess unique features such as softness, wetness, responsiveness, and biocompatibility, making them highly suitable for biointegrated applications that have close interactions with living organisms. However, conventional man-made hydrogels are usually soft and brittle, making them inferior to the mechanically robust biological hydrogels. To ensure reliable and durable operation of biointegrated wearable and implantable devices, mechanical matching and shape adaptivity of hydrogels to tissues and organs are essential. Recent advances in polymer science and processing technologies have enabled mechanical engineering and shaping of hydrogels for various biointegrated applications. In this review, polymer network structuring strategies at micro/nanoscales for toughening hydrogels are summarized, and representative mechanical functionalities that exist in biological materials but are not easily achieved in synthetic hydrogels are further discussed. Three categories of processing technologies, namely, 3D printing, spinning, and coating for fabrication of tough hydrogel constructs with complex shapes are reviewed, and the corresponding hydrogel toughening strategies are also highlighted. These developments enable adaptive fabrication of mechanically robust and functional hydrogel devices, and promote application of hydrogels in the fields of biomedical engineering, bioelectronics, and soft robotics.

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.

3D-Printed Soft Temperature Sensors Based on Thermoelectric Effects for Fast Mapping of Localized Temperature Distributions

We propose a novel design of thermoelectric (TE) effect-based soft temperature sensors for directly monitoring localized subtle temperature stimuli. This design integrates rheology-engineered three-dimensional (3D) printing of high-performance carbon-based TE materials and polymer-based viscoelastic materials with low thermal conductivity. Rheological engineering of carbon nanotube (CNT) TE inks ensures the 3D printing of highly sensitive TE sensing units on directly written 3D soft platforms. Additionally, we pre-dope CNT inks with p- and n-type organic dopants to achieve high sensitivity and a fast response to temperature changes. The introduced 3D soft platforms with low thermal conductivity lead to an efficient thermal gradient on TE sensing units in the out-of-plane direction. Furthermore, encapsulating the temperature sensor array with the same polymer-based materials as the 3D soft platforms facilitates independent detection of localized temperature stimuli by minimizing thermal interaction between sensing units, resulting in precise temperature mapping by localized detection. Our 3D-printed soft temperature sensors exhibit high sensitivity to relatively small temperature changes, with a minimum sensing resolution of 0.1 K within tens of milliseconds. Moreover, the temperature sensor array not only detects localized temperature stimuli by imaging the temperature distribution but also demonstrates remarkable mechanical reliability against repetitive deformation with high accuracy.

All-solution-processed ultraflexible wearable sensor enabled with universal trilayer structure for organic optoelectronic devices

All-solution-processed organic optoelectronic devices can enable the large-scale manufacture of ultrathin wearable electronics with integrated diverse functions. However, the complex multilayer-stacking device structure of organic optoelectronics poses challenges for scalable production. Here, we establish all-solution processes to fabricate a wearable, self-powered photoplethysmogram (PPG) sensor. We achieve comparable performance and improved stability compared to complex reference devices with evaporated electrodes by using a trilayer device structure applicable to organic photovoltaics, photodetectors, and light-emitting diodes. The PPG sensor array based on all-solution-processed organic light-emitting diodes and photodetectors can be fabricated on a large-area ultrathin substrate to achieve long storage stability. We integrate it with a large-area, all-solution-processed organic solar module to realize a self-powered health monitoring system. We fabricate high-throughput wearable electronic devices with complex functions on large-area ultrathin substrates based on organic optoelectronics. Our findings can advance the high-throughput manufacture of ultrathin electronic devices integrating complex functions.

Self-healing and adhesive MXene-polypyrrole/silk fibroin/polyvinyl alcohol conductive hydrogels as wearable sensor

Conductive hydrogels become increasing attractive for flexible electronic devices and biosensors. However, challenges still remain in fabrication of flexible hydrogels with high electrical conductivity, self-healing capability and adhesion property. Herein, a conductive hydrogel (PSDM) was prepared by solution-gel method using MXene and dopamine modified polypyrrole as conductive enhanced materials, polyvinyl alcohol and silk fibroin as gel networks, and borax as cross-linking agent. Notably, the PSDM hydrogels not only showed high permeability (13.82 mg∙cm-2∙h-1), excellent stretch ability (1235 %), high electrical conductivity (11.3 S/m) and long-term stability, but also exhibited high adhesion performance and self-healing properties. PSDM hydrogels displayed outstanding sensing performance and durability for monitoring human activities including writing, finger bending and wrist bending. The PSDM hydrogel was made into wearable flexible electrodes and realized accurate, sensitive and reliable detection of human electromyographic and electrocardiographic signals. The sensor was also applied in human-computer interaction by collecting electromyography signals of different gestures for machine learning and gesture recognition. According to 480 groups of data collected, the recognition accuracy of gestures by the electrodes was close to 100 %, indicating that the PSDM hydrogel electrodes possessed excellent sensing performance for high precision data acquisition and human-computer interaction interface.

Full integration of highly stretchable inorganic transistors and circuits within molecular-tailored elastic substrates on a large scale

The emergence of high-form-factor electronics has led to a demand for high-density integration of inorganic thin-film devices and circuits with full stretchability. However, the intrinsic stiffness and brittleness of inorganic materials have impeded their utilization in free-form electronics. Here, we demonstrate highly integrated strain-insensitive stretchable metal-oxide transistors and circuitry (442 transistors/cm2) via a photolithography-based bottom-up approach, where transistors with fluidic liquid metal interconnection are embedded in large-area molecular-tailored heterogeneous elastic substrates (5 × 5 cm2). Amorphous indium-gallium-zinc-oxide transistor arrays (7 × 7), various logic gates, and ring-oscillator circuits exhibited strain-resilient properties with performance variation less than 20% when stretched up to 50% and 30% strain (10,000 cycles) for unit transistor and circuits, respectively. The transistors operate with an average mobility of 12.7 ( ± 1.7) cm2 V-1s-1, on/off current ratio of > 107, and the inverter, NAND, NOR circuits operate quite logically. Moreover, a ring oscillator comprising 14 cross-wired transistors validated the cascading of the multiple stages and device uniformity, indicating an oscillation frequency of ~70 kHz.

A three-dimensional liquid diode for soft, integrated permeable electronics

Wearable electronics with great breathability enable a comfortable wearing experience and facilitate continuous biosignal monitoring over extended periods1-3. However, current research on permeable electronics is predominantly at the stage of electrode and substrate development, which is far behind practical applications with comprehensive integration with diverse electronic components (for example, circuitry, electronics, encapsulation)4-8. Achieving permeability and multifunctionality in a singular, integrated wearable electronic system remains a formidable challenge. Here we present a general strategy for integrated moisture-permeable wearable electronics based on three-dimensional liquid diode (3D LD) configurations. By constructing spatially heterogeneous wettability, the 3D LD unidirectionally self-pumps the sweat from the skin to the outlet at a maximum flow rate of 11.6 ml cm-2 min-1, 4,000 times greater than the physiological sweat rate during exercise, presenting exceptional skin-friendliness, user comfort and stable signal-reading behaviour even under sweating conditions. A detachable design incorporating a replaceable vapour/sweat-discharging substrate enables the reuse of soft circuitry/electronics, increasing its sustainability and cost-effectiveness. We demonstrated this fundamental technology in both advanced skin-integrated electronics and textile-integrated electronics, highlighting its potential for scalable, user-friendly wearable devices.

E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

Electrically Powered Dissipative Hydrogel Networks Reveal Transient Stiffness Properties for Out-of-Equilibrium Operations

Living systems use dissipative processes to enable precise spatiotemporal control over various functions, including the transient modulation of the stiffness of tissues, which, however, is challenging to achieve in soft materials. Here, we report a new platform to program hydrogel films with tunable, time-dependent mechanical properties under out-of-equilibrium conditions, powered by electricity. We show that the lifetime of the transient network of a surface-confined hydrogel film can be effectively controlled by programming the generation of an electrochemically oxidized mediator in the presence of a chemical or photoreducing agent in solution. It is, therefore, electrically possible to direct the transient stiffening or softening of the hydrogel film, enabling high modularity of the material functions with precise spatiotemporal control. Temporally controlled operations of the hydrogel films are demonstrated for the on-demand, dose-controlled release of multiple model protein payloads from electrode arrays using the present electrically powered dissipative system. This demonstration of electrically driven transient modulation of the stiffness properties of hydrogel films represents an important step toward the engineering of dissipative materials for developing future biomedical applications that can harness the temporal, adaptive properties of this new class of materials.