Bioadhesive polymer semiconductors and transistors for intimate biointerfaces

Lithium Bond-Mediated Molecular Cascade Hydrogel for Injury-Free and Repositionable Adhesive Bioelectronic Interfaces

Flexible bioelectronic interfaces with adhesive properties are essential for advancing modern medicine and human-machine interactions. However, achieving both stable adhesion and non-damaging detachment remains a significant challenge. In this study, a lithium bond-mediated molecular cascade hydrogel (LMCH) for bioelectronic interfaces is designed, which facilitates robust adhesion at the tissue level and permits atraumatic detachment for repositioning as required. By integrating the adhesive properties of the molecular cascade structure with the elastic characteristics of the hydrogel interface, the LMCH interface not only achieved a high adhesion strength (197 J m-2) on the skin, but also significantly extended the cracking cycles on the tissue surface during the peeling process from 4 to 380, marking an enhancement of nearly two orders of magnitude. Furthermore, with Young's modulus similar to that of human tissue (25 kPa), exceptional stretchability (1080%), and high ionic conductivity (7.14 S m-1), the LMCH interface demonstrates outstanding tissue compatibility, biocompatibility, and stable detection capabilities for electrocardiogram (ECG) and electromyogram (EMG) signals. This study presents new insights and potential for advancing bioelectronics and implantable interface technologies.

Jellyfish-Inspired Ultrastretchable, Adhesive, Self-Healing, and Photoswitchable Fluorescent Ionic Skin Enabled by a Supramolecular Zwitterionic Network

Ionic hydrogels suit ionic skins, but advanced hydrogels are challenging. Inspired by jellyfish, we developed an ionic hydrogel with ultrastretchability, conductivity, adhesion, self-healing, and photoswitchable fluorescence via a supramolecular zwitterionic network. This hydrogel consists of silk fibroin, zwitterionic betaine analogue, biomineral calcium salts, and spiropyran in a dynamically cross-linked macromolecular network. Calcium ions facilitate electrical signal transmission and ionic interactions, while spiropyran enables photoswitchable color and fluorescence. Density functional theory and Fourier transform infrared analysis reveal abundant hydrogen bonding, ionic associations, and van der Waals forces, contributing to stretchability, adhesion, and self-healing, making them ideal for epidermal electrodes. The hydrogel also shows potential in optical printing and anti-counterfeiting applications due to spiropyran's reversible photochromic and photoluminescent behaviors. Moreover, a jellyfish-like robot capable of electric-driven movement is created by using these features. This study enhances understanding of dynamic noncovalent interactions in zwitterionic networks, enriching hydrogel design principles and advancing intelligent ionic skins.

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.

Boosting hydrogel conductivity via water-dispersible conducting polymers for injectable bioelectronics

Bioelectronic devices hold transformative potential for healthcare diagnostics and therapeutics. Yet, traditional electronic implants often require invasive surgeries and  are mechanically incompatible with biological tissues. Injectable hydrogel bioelectronics offer a minimally invasive alternative that interfaces with soft tissue seamlessly. A major challenge is the low conductivity of bioelectronic systems, stemming from poor dispersibility of conductive additives in hydrogel mixtures. We address this issue by engineering doping conditions with hydrophilic biomacromolecules, enhancing the dispersibility of conductive polymers in aqueous systems. This approach achieves a 5-fold increase in dispersibility and a 20-fold boost in conductivity compared to conventional methods. The resulting conductive polymers are molecularly and in vivo degradable, making them suitable for transient bioelectronics applications. These additives are compatible with various hydrogel systems, such as alginate, forming ionically cross-linkable conductive inks for 3D-printed wearable electronics toward high-performance physiological monitoring. Furthermore, integrating conductive fillers with gelatin-based bioadhesive hydrogels substantially enhances conductivity for injectable sealants, achieving 250% greater sensitivity in pH sensing for chronic wound monitoring. Our findings indicate that hydrophilic dopants effectively tailor conducting polymers for hydrogel fillers, enhancing their biodegradability and expanding applications in transient implantable biomonitoring.

Immune-compatible designs of semiconducting polymers for bioelectronics with suppressed foreign-body response

One of the greatest obstacles to achieving implantable electronics with long-term functionality and minimized inflammatory reactions is the immune-mediated foreign-body response (FBR). Recently, semiconducting polymers with mixed electron-ion conductivity have been demonstrated as promising candidates to achieve direct electrical interfacing on bio-tissues. However, there is limited understanding of their immune compatibility in vivo, and strategies for minimizing the FBR through molecular design remain underexplored. Here we introduce a set of molecular design strategies for enhancing the immune compatibility of semiconducting polymers. Specifically, we show that selenophene, when incorporated in the backbone, can mitigate the FBR by suppressing macrophage activation. In addition, side-chain functionalization with immunomodulatory groups decreases the FBR further by downregulating the expression of inflammatory biomarkers. Together, our synthesized polymers achieve suppression of the FBR by as much as 68% (as indicated by the collagen density). In the meantime, these immune-compatible designs still provide a high charge-carrier mobility of around 1 cm2 V-1 s-1. We anticipate that such immune-compatible design principles can be translated to a variety of conjugated polymers to suppress the FBR for implantable applications.

Advances in Electrically Conductive Hydrogels: Performance and Applications

Electrically conductive hydrogels are highly hydrated 3D networks consisting of a hydrophilic polymer skeleton and electrically conductive materials. Conductive hydrogels have excellent mechanical and electrical properties and have further extensive application prospects in biomedical treatment and other fields. Whereas numerous electrically conductive hydrogels have been fabricated, a set of general principles, that can rationally guide the synthesis of conductive hydrogels using different substances and fabrication methods for various application scenarios, remain a central demand of electrically conductive hydrogels. This paper systematically summarizes the processing, performances, and applications of conductive hydrogels, and discusses the challenges and opportunities in this field. In view of the shortcomings of conductive hydrogels in high electrical conductivity, matchable mechanical properties, as well as integrated devices and machines, it is proposed to synergistically design and process conductive hydrogels with applications in complex surroundings. It is believed that this will present a fresh perspective for the research and development of conductive hydrogels, and further expand the application of conductive hydrogels.

Vertically Phase-Separated PEDOT:PSS Film via Solid-Liquid Interface Doping for Flexible Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) are seen as some of the most promising devices in organic bioelectronics. Recent interest in OECTs is sparked by the high performance of an organic semiconductor channel material, i.e., poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The capability of ion penetration and charge transport of the channel determines the performance of the OECTs. However, the uniform structure of the PEDOT:PSS channel always makes it difficult to achieve a well-balanced between the two functions. Here, we report a novel PEDOT:PSS film with a vertical phase separation structure (VPSS-P), where PSS accumulates at the surface, and PEDOT enriches at the bottom of the film. Such a unique structure improves the electrochemical stability and reduces the contact resistance, significantly enhancing OECT performance with high transconductance (70.5 mS), product of mobility (μ) and volumetric capacitance (C*) (μC* ∼ 479 F cm-1 V-1 s-1), and ultralow contact resistance (∼0.79 Ω cm). Flexible OECT devices with VPSS-P show robust performance against deformation. Our findings highlight a new class of high-performance transistors and provide guidelines for designing state-of-the-art channel materials.

Toughening hydrogels through a multiscale hydrogen bonding network enabled by saccharides for a bio-machine interface

Hydrogels have considerably emerged in a variety of fields, but their weak mechanical properties severely restrict the wide range of implementation. Herein, we propose a multiscale hydrogen bonding toughening strategy using saccharide-based materials to optimize the hydrogel network. The monosaccharide (glucose) at the molecular scale and polysaccharide (cellulose nanofibrils) at the nano/micro scale can effectively form hydrogen bonds across varied scales within the hydrogel network, leading to significantly enhanced mechanical properties. Besides, the toughened hydrogels present excellent environmental resilience and bad solvent resistance, allowing them to retain their performance in various severe environments. Notably, after being exchanged with a bad solvent such as ethanol, the alcogel exhibits strain-depended vivid interference color, allowing it to function as a mechano-optical sensor. Finally, this strategy has been shown to be adaptable across multiple material systems, and the resulting hydrogels have potential as a bioelectronic interface for long-term stable recording of physiological signals, highlighting the potential of sustainable biomaterials in designing high-quality hydrogels for advanced 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.

Polyelectrolyte-based wireless and drift-free iontronic sensors for orthodontic sensing

The real-time monitoring of health conditions of humans is a long-lasting topic, but there are two major challenges. First, many biomedical applications accept only implanted sensors. Second, tissue-like soft sensors often suffer from viscoelasticity-induced signal drift, causing inaccurate measurements. Here, we report a wireless and drift-free sensory system enabled by a low-creep polyelectrolyte elastomer. The system consists of the iontronic pressure sensors incorporating inductance-capacitance (LC) oscillators, exhibiting combined low drift ratio, high Q factor, high robustness to interferences, and wide-range measurement, superior to other capacitive sensors using regular dielectrics or ionogels. We have recorded 14-day orthodontic loads of two subjects using the system, showing pressure decreasing from 300 to 50 kPa and torque from 12.5 to 0.5 N·mm. The wireless, drift-free sensory system may be extended to other implants for long-term and accurate sensing.

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.

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.

The Rising of Flexible Organic Electrochemical Transistors in Sensors and Intelligent Circuits

Flexible electronic devices in biomedicine, environmental monitoring, and brain-like computing have garnered significant attention. Among these, organic electrochemical transistors (OECTs) have been spotlighted in flexible sensors and neuromorphic circuits for their low power consumption, high signal amplification, excellent biocompatibility, chemical stability, stretchability, and flexibility. However, OECTs will also face some challenges on the way to commercialized applications, including the need for improved long-term stability, enhanced performance of N-type materials, integration with existing technologies, and cost-effective manufacturing processes. This review presents the device physics of OECTs in detail, including the evaluation of their various properties and the introduction of different configurations of the aforementioned OECTs. Subsequently, the components of this device and their roles are explained in depth, and the main ways to design and fabricate flexible OECTs are summarized. Following this, we summarize and analyze the principles and applications of OECTs for electrophysiological signal sensing, chemical sensing, biosensing, and sensor arrays. In addition, the concepts of OECT-based digital and neuromorphic circuits and their applications are presented. Finally, the paper summarizes the opportunities and challenges of OECT-based flexible electronics.

Matching P- and N-type Organic Electrochemical Transistor Performance Enables a Record High-gain Complementary Inverter

The charge transport of channel materials in n-type organic electrochemical transistors (OECTs) is greatly limited by the adverse effects of electrochemical doping, posing a long-standing puzzle for the community. Herein, an n-type conjugated polymer with glycolated side chains (n-PT3) is introduced. This polymer can adapt to electrochemical doping and create more organized nanostructures, mitigating the adverse effects of electrochemical doping. This unique characteristic gives n-PT3 excellent charge transport in the doped state and reversible ion storage, making it highly suitable as an n-type organic mixed ionic-electronic conducting (OMIEC) material. n-PT3 exhibits a high electron mobility of µ ≈ 1.0 cm2 V-1 s-1 and a figure of merit value of µC* ≈ 100 F cm-1 V-1 s-1, representing one of the best results for n-type OMIEC materials. A new p-type OMIEC polymer has been synthesized as the channel material for constructing a complementary inverter to match the n-type OECT channel layer based on n-PT3. As a result, a voltage gain value of up to 307 VV-1 has been achieved, which is a record value for sub-1 V complementary inverters based on OECTs. This work offers valuable insights into designing electrochemical doping adaptive n-type OMIEC materials and fabricating high-gain organic complementary inverters.

Molecular engineering of supramolecular polymer adhesive with confined water and a single crown ether

Here, we report a water-induced supramolecular polymer adhesive formed from confined water and an intrinsically amphiphilic macrocyclic self-assembly in a nanophase-separated structure. The selenium-containing crown ether macrocycle, featuring a strong hydrophilic hydrogen-bond receptor (selenoxide) and a synergistic hydrophobic selenium-substituted crown core, confines water within a segregated, interdigitated architecture. While water molecules typically freeze around 0 °C, the confined water in this supramolecular polymer remains in a liquid-like state down to -80 °C. Previous studies suggested that multiple crown ether units are required to generate confined water; however, in this case, a single unit is sufficient to control the formation and disappearance of confined water, driving supramolecular polymerization. Typically, the DC conductivity of water follows an Arrhenius temperature dependency (ln σ DC ∝ 1/T). In contrast, this new crown ether unit maintains water in confined states, exhibiting Vogel-Fulcher-Tammann behavior (ln σ DC ∝ 1/(T - T 0)) at temperatures above the glass transition. Moreover, this water-induced supramolecular polymer demonstrates remarkable adhesion to hydrophilic surfaces, maintaining strong adhesion even at low temperatures. These findings illustrate how a single small macrocycle can control the complex structure and functionality of water in supramolecular systems.

Integration of Functional Materials in Photonic and Optoelectronic Technologies for Advanced Medical Diagnostics

Integrating functional materials with photonic and optoelectronic technologies has revolutionized medical diagnostics, enhancing imaging and sensing capabilities. This review provides a comprehensive overview of recent innovations in functional materials, such as quantum dots, perovskites, plasmonic nanomaterials, and organic semiconductors, which have been instrumental in the development of diagnostic devices characterized by high sensitivity, specificity, and resolution. Their unique optical properties enable real-time monitoring of biological processes, advancing early disease detection and personalized treatment. However, challenges such as material stability, reproducibility, scalability, and environmental sustainability remain critical barriers to their clinical translation. Breakthroughs such as green synthesis, continuous flow production, and advanced surface engineering are addressing these limitations, paving the way for next-generation diagnostic tools. This article highlights the transformative potential of interdisciplinary research in overcoming these challenges and emphasizes the importance of sustainable and scalable strategies for harnessing functional materials in medical diagnostics. The ultimate goal is to inspire further innovation in the field, enabling the creation of practical, cost-effective, and environmentally friendly diagnostic solutions.

On-site biosignal amplification using a single high-spin conjugated polymer

On-site or in-sensor biosignal transduction and amplification can offer several benefits such as improved signal quality, reduced redundant data transmission, and enhanced system integration. Ambipolar organic electrochemical transistors (OECTs) are promising for this purpose due to their high transconductance, low operating voltage, biocompatibility, and suitability for miniaturized amplifier design. However, limitations in material performance and stability have hindered their application in biosignal amplification. Here, we propose using high-spin, hydrophilic conjugated polymers and a computational screening approach to address this challenge. We designed a high-spin polymer, namely P(TII-2FT), which exhibits satisfactory, stable, and balanced ambipolar OECT performance. The figure-of-merits achieved by the P(TII-2FT) devices surpass those of the current leading materials by 5 to 20 times, resulting in remarkable voltage gains while maintaining a compact form factor. Based on this amplifier, we have successfully achieved on-site capture and amplification of various electrophysiological signals with greatly enhanced signal quality.

A supramolecular hydrogel leveraging hierarchical multi-strength hydrogen-bonds hinged strategy achieving a striking adhesive-mechanical balance

To obtain high-performance tissue-adhesive hydrogel embodying excellent mechanical integrity, a supramolecular hydrogel patch is fabricated through in situ copolymerization of a liquid-liquid phase separation precursor composed of self-complementary 2-2-ureido-4-pyrimidone-based monomer and acrylic acid coupled with subsequent corporation of bioactive epigallocatechin gallate. Remarkably, the prepared supramolecular hydrogel leverages hierarchical multi-strength hydrogen-bonds hinged strategy assisted by alkyl-based hydrophobic pockets, broadening the distribution of binding strength of physical junctions, striking a canonical balance between superb mechanical performance and robust adhesive capacity. Ultimately, the fabricated supramolecular hydrogel patch stands out as a high stretchability (1500 %), an excellent tensile strength (2.6 MPa), a superhigh toughness (12.6 MJ m-3), an instant and robust tissue adhesion strength (263.2 kPa for porcine skin), the considerable endurance under cyclic loading and reversible adhesion, a superior burst pressure tolerance (108 kPa) to those of commercially-available tissue sealants, and outstanding anti-swelling behavior. The resultant supramolecular hydrogel patch demonstrates the rapid hemorrhage control within 60 s in liver injury and efficient wound closure and healing effects with alleviated inflammation and reduced scarring in full-thickness skin incision, confirming its medical translation as a promising self-rescue tissue-adhesive patch for hemorrhage prevention and sutureless wound closure.

Vertical-Structure Overcomes the Strain Limit of Stretchable Organic Electrochemical Transistors

Intrinsically stretchable organic electrochemical transistors (IS-OECTs), utilizing organic mixed ionic-electronic conductors (OMIECs) as their channel materials, have drawn great attention recently because of their potential to enable seamless integration between bioelectronic devices and living systems. However, the fabrication of IS-OECTs presents challenges due to the limited availability of OMIEC materials that possess the desired combination of mechanical and electrical properties. In this work, 1) we report the first successful fabrication of a vertical intrinsically stretchable OECT (VIS-OECT), achieved by using elastoadhesive electrodes; 2) we experimentally proved that vertical architecture can push the strain limit of an IS-OECT from 20% to 50%; and 3) the above finding introduces an unconventional design concept: the strain limit of an IS-OECT can surpass the intrinsic stretchability of the constituent OMIECs by employing vertical structure.

Regulating Optoelectronic and Thermoelectric Properties of Organic Semiconductors by Heavy Atom Effects

Heavy atom effects can be used to enhance intermolecular interaction, regulate quinoidal resonance properties, increase bandwidths, and tune diradical characters, which have significant impacts on organic optoelectronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), etc. Meanwhile, the introduction of heavy atoms is shown to promote charge transfer, enhance air stability, and improve device performances in the field of organic thermoelectrics (OTEs). Thus, heavy atom effects are receiving more and more attention. However, regulating heavy atoms in organic semiconductors is still meeting great challenges. For example, heavy atoms will lead to solubility and stability issues (tellurium substitution) and lack of versatile design strategy and effective synthetic methods to be incorporated into organic semiconductors, which limit their application in electronic devices. Therefore, this work timely summarizes the unique functionalities of heavy atom effects, and up-to-date progress in organic electronics including OFETs, OPVs, OLEDs, and OTEs, while the structure-performance relationships between molecular designs and electronic devices are clearly elucidated. Furthermore, this review systematically analyzes the remaining challenges in regulating heavy atoms within organic semiconductors, and design strategies toward efficient and stable organic semiconductors by the introduction of novel heavy atoms regulation are proposed.

Molecularly Designed and Nanoconfined Polymer Electronic Materials for Skin-like Electronics

Stretchable electronics have seen substantial development in skin-like mechanical properties and functionality thanks to the advancements made in intrinsically stretchable polymer electronic materials. Nanoscale phase separation of polymer materials within an elastic matrix to form one-dimensional nanostructures, namely nanoconfinement, effectively reduces conformational disorders that have long impeded charge transport properties of conjugated polymers. Nanoconfinement results in enhanced charge transport and the addition of skin-like properties. In this Outlook, we highlight the current understanding of structure-property relationships for intrinsically stretchable electronic materials with a focus on the nanoconfinement strategy as a promising approach to incorporate skin-like properties and other functionalities without compromising charge transport. We outline emerging directions and challenges for intrinsically stretchable electronic materials with the aim of constructing skin-like electronic systems.

Functional Hydrogels for Aqueous Zinc-Based Batteries: Progress and Perspectives

Aqueous zinc batteries (AZBs) hold great potential for green grid-scale energy storage due to their affordability, resource abundance, safety, and environmental friendliness. However, their practical deployment is hindered by challenges related to the electrode, electrolyte, and interface. Functional hydrogels offer a promising solution to address such challenges owing to their broad electrochemical window, tunable structures, and pressure-responsive mechanical properties. In this review, the key properties that functional hydrogels must possess for advancing AZBs, including mechanical strength, ionic conductivity, swelling behavior, and degradability, from a perspective of the full life cycle of hydrogels in AZBs are summarized. Current modification strategies aimed at enhancing these properties and improving AZB performance are also explored. The challenges and design considerations for integrating functional hydrogels with electrodes and interface are discussed. In the end, the limitations and future directions for hydrogels to bridge the gap between academia and industries for the successful deployment of AZBs are discussed.

Design Strategy, On-Demand Control, and Biomedical Engineering Applications of Wet Adhesion

The adhesion of tissues to external devices is fundamental to numerous critical applications in biomedical engineering, including tissue and organ repair, bioelectronic interfaces, adhesive robotics, wearable electronics, biomedical sensing and actuation, as well as medical monitoring, treatment, and healthcare. A key challenge in this context is that tissues are typically situated in aqueous and dynamic environments, which poses a bottleneck to further advancements in these fields. Wet adhesion technology (WAT) presents an effective solution to this issue. In this review, we summarize the three major design strategies and control methods of wet adhesion, comprehensively and systematically introducing the latest applications and advancements of WAT in the field of biomedical engineering. First, single adhesion mechanism under the frameworks of the three design strategies is systematically introduced. Second, control methods for adhesion are comprehensively summarized, including spatiotemporal control, detachment control, and reversible adhesion control. Third, a systematic summary and discussion of the latest applications of WAT in biomedical engineering research and education were presented, with a particular focus on innovative applications such as tissue-electronic interface devices, ingestible devices, end-effector components, in vivo medical microrobots, and medical instruments and equipment. Finally, opportunities and challenges encountered in the design and development of wet adhesives with advanced adhesive performance and application prospects are discussed.

Continuous monitoring with a shake

Oscillating signals rapidly reset a sensor for real-time protein detection in the body.

Recent Advances in Multifunctional Naturally Derived Bioadhesives for Tissue Engineering and Wound Management

Recent advancements in naturally derived bioadhesives have transformed their application across diverse medical fields, including tissue engineering, wound management, and surgery. This review focuses on the innovative development and multifunctional nature of these bioadhesives, particularly emphasizing their role in enhancing adhesion performance in wet environments and optimizing mechanical properties for use in dynamic tissues. Key areas covered include the chemical and physical mechanisms of adhesion, the incorporation of multi‐adhesion strategies that combine covalent and non‐covalent bonding, and bioinspired designs mimicking natural adhesives such as those of barnacles and mussels. Additionally, the review discusses emerging applications of bioadhesives in the regeneration of musculoskeletal, cardiac, neural, and ocular tissues, highlighting the potential for bioadhesive‐based therapies in complex biological settings. Despite substantial progress, challenges such as scaling lab‐based innovations for clinical use and overcoming environmental and mechanical constraints remain critical. Ongoing research in bioadhesive technologies aims to bridge these gaps, promising significant improvements in medical adhesives tailored for diverse therapeutic needs.

Microscale droplet assembly enables biocompatible multifunctional modular iontronics

Hydrogel iontronic devices can emulate biological functions and communicate with living matter. But the fabrication of miniature, soft iontronic devices according to modular designs has not been achieved. In this work, we report the use of surfactant-supported assembly of freestanding microscale hydrogel droplets to construct various iontronic modules, circuits, and biointerfaces. Chemical modifications of silk fibroin produced a pair of oppositely charged hydrogels. Microscale assembly of various combinations of hydrogel droplets produced iontronic diodes, npn- and pnp-type transistors, and diverse reconfigurable logic gates. Through the incorporation of poly(amino acid)s, we have demonstrated a droplet-based synthetic synapse with ionic polymer-mediated long-term plasticity. Further, our iontronic transistor can serve as a biocompatible sensor to record electrophysiological signals from sheets of human cardiomyocytes, paving a way to the building of miniature bioiontronic systems.

Electro-Optically Configurable Synaptic Transistors With Cluster-Induced Photoactive Dielectric Layer for Visual Simulation and Biomotor Stimuli

The integration of visual simulation and biorehabilitation devices promises great applications for sustainable electronics, on-demand integration and neuroscience. However, achieving a multifunctional synergistic biomimetic system with tunable optoelectronic properties at the individual device level remains a challenge. Here, an electro-optically configurable transistor employing conjugated-polymer as semiconductor layer and an insulating polymer (poly(1,8-octanediol-co-citrate) (POC)) with clusterization-triggered photoactive properties as dielectric layer is shown. These devices realize adeptly transition from electrical to optical synapses, featuring multiwavelength and multilevel optical synaptic memory properties exceeding 3 bits. Utilizing enhanced optical memory, the images learning and memory function for visual simulation are achieved. Benefiting from rapid electrical response akin to biological muscle activation, increased actuation occurs under increased stimulus frequency of gate voltage. Additionally, the transistor on POC substrate can be effectively degraded in NaOH solution due to degradation of POC. Pioneeringly, the electro-optically configurability stems from light absorption and photoluminescence of the aggregation cluster in POC layer after 200 °C annealing. The enhancement of optical synaptic plasticity and integration of motion-activation functions within a single device opens new avenues at the intersection of optoelectronics, synaptic computing, and bioengineering.

Surface-Grafted Biocompatible Polymer Conductors for Stable and Compliant Electrodes for Brain Interfaces

Durable and conductive interfaces that enable chronic and high-resolution recording of neural activity are essential for understanding and treating neurodegenerative disorders. These chronic implants require long-term stability and small contact areas. Consequently, they are often coated with a blend of conductive polymers and are crosslinked to enhance durability despite the potentially deleterious effect of crosslinking on the mechanical and electrical properties. Here the grafting of the poly(3,4 ethylenedioxythiophene) scaffold, poly(styrenesulfonate)-b-poly(poly(ethylene glycol) methyl ether methacrylate block copolymer brush to gold, in a controlled and tunable manner, by surface-initiated atom-transfer radical polymerization (SI-ATRP) is described. This "block-brush" provides high volumetric capacitance (120 F cm─3), strong adhesion to the metal (4 h ultrasonication), improved surface hydrophilicity, and stability against 10 000 charge-discharge voltage sweeps on a multiarray neural electrode. In addition, the block-brush film showed 33% improved stability against current pulsing. This approach can open numerous avenues for exploring specialized polymer brushes for bioelectronics research and application.

Soft hydrogel semiconductors with augmented biointeractive functions

Hydrogels, known for their mechanical and chemical similarity to biological tissues, are widely used in biotechnologies, whereas semiconductors provide advanced electronic and optoelectronic functionalities such as signal amplification, sensing, and photomodulation. Combining semiconducting properties with hydrogel designs can enhance biointeractive functions and intimacy at biointerfaces, but this is challenging owing to the low hydrophilicity of polymer semiconductors. We developed a solvent affinity-induced assembly method that incorporates water-insoluble polymer semiconductors into double-network hydrogels. These semiconductors exhibited tissue-level moduli as soft as 81 kilopascals, stretchability of 150% strain, and charge-carrier mobility up to 1.4 square centimeters per volt per second. When they are interfaced with biological tissues, their tissue-level modulus enables alleviated immune reactions. The hydrogel's high porosity enhances molecular interactions at semiconductor-biofluid interfaces, resulting in photomodulation with higher response and volumetric biosensing with higher sensitivity.

Advances in Biointegrated Wearable and Implantable Optoelectronic Devices for Cardiac Healthcare

With the prevalence of cardiovascular disease, it is imperative that medical monitoring and treatment become more instantaneous and comfortable for patients. Recently, wearable and implantable optoelectronic devices can be seamlessly integrated into human body to enable physiological monitoring and treatment in an imperceptible and spatiotemporally unconstrained manner, opening countless possibilities for the intelligent healthcare paradigm. To achieve biointegrated cardiac healthcare, researchers have focused on novel strategies for the construction of flexible/stretchable optoelectronic devices and systems. Here, we overview the progress of biointegrated flexible and stretchable optoelectronics for wearable and implantable cardiac healthcare devices. Firstly, the device design is addressed, including the mechanical design, interface adhesion, and encapsulation strategies. Next, the practical applications of optoelectronic devices for cardiac physiological monitoring, cardiac optogenetics, and nongenetic stimulation are presented. Finally, an outlook on biointegrated flexible and stretchable optoelectronic devices and systems for intelligent cardiac healthcare is discussed.

Single ambipolar OECT-based inverter with volatility and nonvolatility on demand

Organic electrochemical transistor (OECT)-based inverter introduces new prospects for energy-efficient brain-inspired artificial intelligence devices. Here, we report single-component OECT-based inverters by incorporating ambipolar p(gDPP-V). Notably, p(gDPP-V) shows state-of-the-art ambipolar OECT performances in both conventional (p/n-type mode transconductance of 29/25 S cm-1) and vertical (transconductance of 297.2/292.4 μS μm-2 under p/n operation) device architectures. Especially, the resulting highly stable vertical OECT-based inverter shows a high voltage gain of 105 V V-1 under a low driving voltage of 0.8 V. The inverter exhibits undiscovered voltage-regulated dual mode: volatile receptor and nonvolatile synapse. Moreover, applications of physiology signal recording and demonstrations of NAND/NOR logic circuits are investigated within the volatile feature, while neuromorphic simulations with a convolutional neural network and image memorizing capabilities are explored under the nonvolatile behavior. The ambipolar OECT-based inverter, capable of both volatile and nonvolatile operations, provides possibilities for the applications of reconfigurable complementary logic circuits in novel neuromorphic computing paradigms.

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.

3D printed multi-coupled bioinspired skin-electronic interfaces with enhanced adhesion for monitoring and treatment

Skin-electronic interfaces have broad applications in fields such as diagnostics, therapy, health monitoring, and smart wearables. However, they face various challenges in practical use. For instance, in wet environments, the cohesion of the material may be compromised, and under dynamic conditions, maintaining conformal adhesion becomes difficult, leading to reduced sensitivity and fidelity of electrical signal transmission. The key scientific issue lies in forming a stable and tight mechanical-electronic coupling at the tissue-electronic interface. Here, inspired by octopus sucker structures and snail mucus, we propose a strategy for hydrogel skin-electronic interfaces based on multi-coupled bioinspired adhesion and introduce an ultrasound (US)-mediated interfacial toughness enhancement mechanism. Ultimately, using digital light processing micro-nano additive manufacturing technology (DLP 3D), we have developed a multifunctional, diagnostic-therapeutic integrated patch (PAMS). This patch exhibits moderate water swelling properties, a maximum deformation of up to 460%, high sensitivity (GF = 4.73), and tough and controllable bioadhesion (shear strength increased by 109.29%). Apart from outstanding mechanical and electronic properties, the patch also demonstrates good biocompatibility, anti-bacterial properties, photothermal properties, and resistance to freezing at -20 °C. Experimental results show that this skin-electronic interface can sensitively monitor temperature, motion, and electrocardiogram signals. Utilizing a rat frostbite model, we have demonstrated that this skin-electronic interface can effectively accelerate the wound healing process as a wound patch. This research offers a promising strategy for improving the performance of bioelectronic devices, sensor-based educational reforms and personalized diagnostics and therapeutics in the future. STATEMENT OF SIGNIFICANCE: Establishing stable and tight mechanical-electronic coupling at the tissue-electronic interface is essential for the diverse applications of bioelectronic devices. This study aims to develop a multifunctional, diagnostic-therapeutic integrated hydrogel skin-electronic interface patch with enhanced interfacial toughness. The patch is based on a multi-coupled bioinspired adhesive-enhanced mechanism, allowing for personalized 3D printing customization. It can be used as a high-performance diagnostic-therapeutic sensor and effectively promote frostbite wound healing. We anticipate that this research will provide new insights for constructing the next generation of multifunctional integrated high-performance bioelectronic interfaces.

Fast-response fiber organic electrochemical transistor with vertical channel design for electrophysiological monitoring

Fiber organic electrochemical transistors (OECTs) hold significant promise for in vivo bio-signal amplification due to their minimally invasive and seamless integration with biological tissues. However, their use in monitoring rapid physiological changes, such as electrophysiological signals, has been constrained by slow response time, arising from their extensive channel dimensions. Here, we introduce a novel fiber OECT designed with a micro-scale vertical channel (F-vOECT) that substantially reduces the response time by an order of magnitude to 12 ms and achieves a maximum transconductance of 16 mS at zero gate bias, marking a substantial improvement over previous fiber OECTs. This compact and flexible fiber device demonstrates robust performance under cyclic switching, dynamic deformation and exhibits excellent biocompatibility. When subcutaneously implanted in rats, the F-vOECT enables stable, continuous electrocardiogram monitoring for 7 days, successfully identifying episodes of atrioventricular block. These capabilities illustrate its potential for clinical electrophysiological diagnostics. The design strategy of F-vOECT opens new avenues for developing fast-responsive fiber bioelectronic 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.

Stretchable and biodegradable self-healing conductors for multifunctional electronics

As the regenerative mechanisms of biological organisms, self-healing provides useful functions for soft electronics or associated systems. However, there have been few examples of soft electronics where all components have self-healing properties while also ensuring compatibility between components to achieve multifunctional and resilient bio-integrated electronics. Here, we introduce a stretchable, biodegradable, self-healing conductor constructed by combination of two layers: (i) synthetic self-healing elastomer and (ii) self-healing conductive composite with additives. Abundant dynamic disulfide and hydrogen bonds of the elastomer and conductive composite enable rapid and complete recovery of electrical conductivity (~1000 siemens per centimeter) and stretchability (~500%) in response to repetitive damages, and chemical interactions of interpenetrated polymer chains of these components facilitate robust adhesion strength, even under extreme mechanical stress. System-level demonstration of soft, self-healing electronics with diagnostic/therapeutic functions for the urinary bladder validates the possibility for versatile, practical uses in biomedical research areas.

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.

Synchronous Switching Strategy to Enhance the Real-Time Powering and Charging Performance of Triboelectric Nanogenerator

Triboelectric nanogenerators (TENGs) are of great significance as sustainable power sources that harvest energy from the human body and environment. Nevertheless, due to TENG's impedance-dependent output voltage characteristics, in traditional strategy (TS), real-timely powering a sensor with TENG has a poor sensing on/off ratio (or response), and directly charging a capacitor with TENG shows a low charging efficiency. This degraded real-time powering and charging performance of TENG compared to a commercial constant voltage source is a huge challenge of the TENG field for a long time. This work proposes a synchronous switching strategy (SSS) for TENG to real-timely power sensors or charge capacitors without degrading its performance. Compared with TS, this new strategy enables sensors to have 5-7 times sensing on/off ratio enhancement when using TENG as a power source, reaching the powering ability of a commercial constant voltage source, it makes the powering performance of TENG stable under different driving frequency, improving the powering robustness of TENG. In addition, compared with TS, SSS can also enhance the charging efficiency of TENG in every charging cycle by up to 2.4 times when charging capacitors. This work contributes to real-timely powering or charging the distributed, mobile and wireless electronics using TENG.

In-Sensor Organic Electrochemical Transistor for the Multimode Neuromorphic Olfactory System

The olfactory system is one of the six basic sensory nervous systems. Developing artificial olfactory systems is challenging due to the complexity of chemical information decoding and memory. Conventional chemical sensors can convert chemical signals into electric signals to decode gas information but they lack memory functions. Additional storage and processing units would significantly increase the complexity and power consumption of the devices, especially for portable and wearable devices. Here, an olfactory-inspired in-sensor organic electrochemical transistor (OI-OECT) is proposed, with the integrated functions of chemical information decoding, tunable memory level, and selectivity of vapor sensing. The ion-gel electrolyte endows the OI-OECT with the function of tunable memory levels and a low operating voltage. Typical synaptic behaviors, including inhibitory postsynaptic current and paired-pulse facilitations, are successfully achieved. Importantly, the gas memory level can be effectively modulated by the gate voltages (0 and -1 V), which realized the transformation of volatile and nonvolatile memory. Furthermore, benefiting from the recognition of multiple gases and ability to detect cumulative damage caused by gases, the OI-OECT is demonstrated for early warning system targeting leakage detection of two gases (NH3 and H2S). This work achieves the integrated functions of chemical gas information decode, tunable gas memory level, and selectivity of gas in a single device, which provides a promising pathway for the development of future artificial olfactory systems.

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.

Glucose-responsive, self-healing, wet adhesive and multi-biofunctional hydrogels for diabetic wound healing

Diabetic wounds are serious clinical complications which manifest wet condition due to the mass exudate, along with disturbed regulation of inflammation, severe oxidative stress and repetitive bacterial infection. Existing treatments for diabetic wounds remain unsatisfactory due to the lack of ideal dressings that encompass mechanical performance, adherence to moist tissue surfaces, quick repair, and diverse therapeutic benefits. Herein, we fabricated a wet adhesive, self-healing, glucose-responsive drug releasing hydrogel with efficient antimicrobial and pro-healing properties for diabetic wound treatment. PAE hydrogel was constructed with poly(acrylic acid-co-acrylamide) (AA-Am) integrated with a dynamic E-F crosslinker, which consisted of epigallocatechin gallate (EGCG) and 4-(2-acrylamidoethylcarbamoyl)-3-fluorophenylboronic acid (AFPBA). Due to the dynamic crosslinking nature of boronate esters, abundant catechol groups and hydrogen bonding, PAE hydrogel demonstrated excellent mechanical properties with about 1000 % elongation, robust adhesion to moist tissues, fast self-healing, and absorption of biofluids of 10 times of its own weight. Importantly, PAE hydrogel exhibited sustained and glucose-responsive release of EGCG. Together, the bioactive PAE hydrogel had effective antibacterial, antioxidative, and anti-inflammatory properties in vitro, and accelerated diabetic wound healing in rats via reducing tissue-inflammatory response, enhancing angiogenesis, and reprogramming of macrophages. Overall, this versatile hydrogel provides a straightforward solution for the treatment of diabetic wound, and shows potential for other wound-related application scenarios.

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.

Highly Adaptive Kirigami-Metastructure Adhesive with Vertically Self-Aligning Octopus-like 3D Suction Cups for Efficient Wet Adhesion to Complexly Curved Surfaces

An essential requirement for biomedical devices is the capability of conformal adaptability on diverse irregular 3D (three-dimensional) nonflat surfaces in the human body that may be covered with liquids such as mucus or sweat. However, the development of reversible adhesive interface materials for biodevices that function on complex biological surfaces is challenging due to the wet, slippery, smooth, and curved surface properties. Herein, we present an ultra-adaptive bioadhesive for irregular 3D oral cavities covered with saliva by integrating a kirigami-metastructure and vertically self-aligning suction cups. The flared suction cup, inspired by octopus tentacles, allows adhesion to moist surfaces. Additionally, the kirigami-based auxetic metastructure with a negative Poisson's ratio relieves the stress caused by tensile strain, thereby mitigating the stress caused by curved surfaces and enabling conformal contact with the surface. As a result, the adhesive strength of the proposed auxetic adhesive is twice that of adhesives with a flat backbone on highly curved porcine palates. For potential application, the proposed auxetic adhesive is mounted on a denture and performs successfully in human subject feasibility evaluations. An integrated design of these two structures may provide functionality and potential for biomedical applications.

Stretchable Functional Nanocomposites for Soft Implantable Bioelectronics

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

Recent Advances in Materials, Devices and Algorithms Toward Wearable Continuous Blood Pressure Monitoring

Continuous blood pressure (BP) tracking provides valuable insights into the health condition and functionality of the heart, arteries, and overall circulatory system of humans. The rapid development in flexible and wearable electronics has significantly accelerated the advancement of wearable BP monitoring technologies. However, several persistent challenges, including limited sensing capabilities and stability of flexible sensors, poor interfacial stability between sensors and skin, and low accuracy in BP estimation, have hindered the progress in wearable BP monitoring. To address these challenges, comprehensive innovations in materials design, device development, system optimization, and modeling have been pursued to improve the overall performance of wearable BP monitoring systems. In this review, we highlight the latest advancements in flexible and wearable systems toward continuous noninvasive BP tracking with a primary focus on materials development, device design, system integration, and theoretical algorithms. Existing challenges, potential solutions, and further research directions are also discussed to provide theoretical and technical guidance for the development of future wearable systems in continuous ambulatory BP measurement with enhanced sensing capability, robustness, and long-term accuracy.

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.

Electrically weldable conductive elastomers

Flexible and stretchable electronic devices are subject to failure because of vulnerable circuit interconnections. We develop a low-voltage (1.5 to 4.5 V) and rapid (as low as 5 s) electric welding strategy to integrate both rigid electronic components and soft sensors in flexible circuits under ambient conditions. This is achieved through the design of conductive elastomers composed of borate ester polymers and conductive fillers, which can be self-welded and generate welding effects to various materials including metals, hydrogels, and other conductive elastomers. The welding effect is generated through the electrochemical reaction-triggered exposure of interfacial adhesive promotors or the cleavage/reformation of dynamic bonds. Our strategy can ensure both mechanical compliance and conductivity at the circuit interfaces and easily produce welding strengths in the kilopascal to megapascal range. The as-designed conductive elastomers in combination with the electric welding technique provide a robust platform for constructing standalone flexible and stretchable electronic devices that are detachable and assemblable on demand.

Silicon Vacancies Diamond/Silk/PVA Hierarchical Physical Unclonable Functions for Multi-Level Encryption

Physical unclonable functions (PUFs) have emerged as a promising encryption technology, utilizing intrinsic physical identifiers that offer enhanced security and tamper resistance. Multi-level PUFs boost system complexity, thereby improving system reliability and fault tolerance. However, crosstalk-free multi-level PUFs remain a persistent challenge. In this study, a hierarchical PUF system that harnesses the spontaneous phase separation of silk fibroin /PVA blend and the random distribution of silicon-vacancy diamonds within the blend is presented. The thermodynamic instability of phase separation and inherent unpredictability of diamond dispersion gives rise to intricate random patterns at two distinct scales, enabling time-efficient hierarchical authentication for cryptographic keys. These patterns are complementary yet independent, inherently resistant to replication and damage thus affording robust security and reliability to the proposed system. Furthermore, customized authentication algorithms are constructed: visual PUFs authentication utilizes neural network combined structural similarity index measure, while spectral PUFs authentication employs Hamming distance and cross-correlation bit operation. This hierarchical PUF system attains a high recognition rate without interscale crosstalk. Additionally, the coding capacity is exponentially enhanced using M-ary encoding to reinforce multi-level encryption. Hierarchical PUFs hold significant potential for immediate application, offering unprecedented data protection and cryptographic key authentication capabilities.

Ultrastable N-Type Semiconducting Fiber Organic Electrochemical Transistors for Highly Sensitive Biosensors

Organic electrochemical transistors (OECTs) have attracted increasing attention due to their merits of high transconductance, low operating voltage, and good biocompatibility, ideal for biosensors. However, further advances in their practical applications face challenges of low n-type performance and poor stability. Here, it is demonstrated that wet-spinning the commercially available n-type conjugated polymer poly(benzimidazobenzophenanthroline) (BBL) into highly aligned and crystalline fibers enhances both OECT performance and stability. Although BBL is only soluble in high-boiling-point strong acids, it can be wet-spun into high-quality fibers with adjustable diameters. The BBL fiber OECTs exhibit a record-high area-normalized transconductance (gm,A) of 2.40 µS µm-2 and over 10 times higher figure-of-merit (µC*) than its thin-film counterparts. More importantly, these fiber OECTs exhibit remarkable stability with no noticeable performance attenuation after 1500 cycles over 4 h operation, outperforming all previously reported n-type OECTs. The superior performance and stability can be attributed to shorter π-π stacking distance and ordered molecular arrangement in the fibers, endowing the BBL fiber OECT-based biosensors with outstanding sensitivity while keeping a miniaturized form factor. This work demonstrates that, beyond new material development, developing new fabrication technology is also crucial for addressing the performance and stability issues in n-type OECTs.

Multi-stimulus perception and visualization by an intelligent liquid metal-elastomer architecture

Multi-stimulus responsive soft materials with integrated functionalities are elementary blocks for building soft intelligent systems, but their rational design remains challenging. Here, we demonstrate an intelligent soft architecture sensitized by magnetized liquid metal droplets that are dispersed in a highly stretchable elastomer network. The supercooled liquid metal droplets serve as microscopic latent heat reservoirs, and their controllable solidification releases localized thermal energy/information flows for enabling programmable visualization and display. This allows the perception of a variety of information-encoded contact (mechanical pressing, stretching, and torsion) and noncontact (magnetic field) stimuli as well as the visualization of dynamic phase transition and stress evolution processes, via thermal and/or thermochromic imaging. The liquid metal-elastomer architecture offers a generic platform for designing soft intelligent sensing, display, and information encryption systems.