Lili Jiang

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.

Human–machine interface (HMI) electrodes enable interactions between humans and bioelectronic devices by facilitating electrical stimulation and recording neural activity. However, reconciling the soft, hydrated nature of living human tissues with the rigid, dry properties of synthetic electronic systems is inherently challenging. Overcoming these significant differences, which is critical for developing compatible, effective, and stable interfaces, has become a key research area in materials science and technology. Recently, hydrogels have gained prominence for use in HMI electrodes because these soft, hydrated materials are similar in nature to human tissues and can be tuned through the incorporation of nanofillers. This review examines the functional requirements of HMI electrodes and highlights recent progress in the development of polyphenol‐mediated multifunctional hydrogel‐based HMI electrodes for bioelectronics. Furthermore, aspects such as mussel‐inspired and polyphenol‐mediated adhesion, underlying mechanisms, tissue‐matching mechanical properties, electrochemical performance, biocompatibility, biofouling resistance, stability under physiological conditions, anti‐inflammatory, and antioxidant properties are discussed. Finally, applications in bioelectronics and further perspectives are outlined. Advances in HMI hydrogel electrodes are expected to facilitate the unprecedented integration of biological systems and electronic devices, potentially revolutionizing various biomedical fields and enhancing the capabilities and performance of bioelectronic devices.

Stretchable and conductive hydrogels are rapidly emerging as new generation candidates for wearable devices. However, the poor electroactivity and bioadhesiveness of traditional conductive hydrogels has limited their applications. Herein, a mussel-inspired strategy is proposed to prepare a specific core–shell redox-active system, consisting of a polydopamine (PDA) modified zeolitic imidazolate framework 71 (ZIF-71) core, and a poly 3,4-ethylenedioxythiopene (PEDOT) shell. Owing to the abundant catechol groups, PEDOT can be assembled on the surface of ZIF-71 to create a redox-active system. The core–shell nanoparticles could act as a redox-active nanofiller to develop a conductive polyacrylamide (PAM) hydrogel with energy-storage properties. The core–shell PEDOT@PZIF-71 system provides a mussel-inspired environment in the hydrogel matrix and endows the hydrogel with stretchability and adhesiveness. The hydrogel can be applied as a functional electrode for both bioelectronics and supercapacitors. Moreover, this hydrogel exhibits favorable biocompatibility and can be implanted in vivo for biosignal measurement without causing inflammation. This redox-active core–shell PEDOT@PZIF-71 system provides a promising strategy for the design of hydrogel-based wearable electronic devices.

The increasing development of biomedicine and bioelectronics has highlighted the requirement for smart materials that can respond to changes in physical and chemical properties under external environments, such as magnetic fields, electric fields, and temperature. Accordingly, hydrogels have been widely evaluated as promising candidates for smart materials owing to their intriguing structures comprising a cross‐linked network of polymer chains with interstitial spaces filled with solvent water. This feature endows hydrogels with soft and wet characteristics, which not only induce high tissue affinity but also allow the introduction of environmentally responsive nanoparticles to release specific smart properties. Herein, we reviewed novel smart hydrogels that can be applied in biomedicine and bioelectronics, and highlighted and discussed existing challenges in current technologies and research.

To satisfy the requirements of social power development, it is urgently necessary to develop innovative and sustainable new energy storage devices. Supercapacitors have attracted considerable attention as a new type of energy storage device, owing to their high energy density, high power density, fast charging and discharging speeds, and long cycle life. The electrode material is an important factor in determining the electrochemical performance of supercapacitors. In recent years, researchers explored the application of metal‐organic frameworks (MOFs) and their derivatives as electrode materials for supercapacitors. In this paper, the preparation of monometallic, bimetallic, and conductive MOFs, and their derivatives for application in supercapacitors are reviewed. In addition, challenges facing MOFs in the field of supercapacitors and their future development prospects are discussed.

Functional hydrogel-based electrodes and electrolytes for multifunctional supercapacitors for wearable bioelectronic devices.

Nanoscale composites for high‐performance electrodes employed in flexible, all‐solid‐state supercapacitors are being developed. A series of binder‐free composites, each consisting of a transition bimetal oxide, a metal oxide, and a metal nitride grown on N‐doped reduced graphene oxide (rGO)‐wrapped nickel foam are obtained by using a universal strategy. Three different transition metals, Co, Mo, and Fe, are separately compounded with nickel ions, which originate from the nickel foam, to form three composites, NiCoO₂@Co₃O₄@Co₂N, NiMoO₄@MoO₃@Mo₂N, and NiFe₂O₄@Fe₃O₄@Fe₂N, respectively. These as‐prepared active materials have similar regular variation patterns in their properties, including better conductivity and battery‐mimicking pseudocapacitance, which result in their high whole‐electrode capacitance performance [2598.3 F g⁻¹ (39.85 F cm⁻²), 3472.6 F g⁻¹ (41.43 F cm⁻²) and 1907.5 F g⁻¹ (3.41 F cm⁻²) for the composites incorporating Co, Mo, and Fe, respectively]. The as‐assembled flexible, all‐solid‐state NiCoO₂@Co₃O₄@Co₂N//KOH/PVA//NiCoO₂@Co₃O₄@Co₂N device can be easily bent and exhibits high energy density and power density of 92.8 Wh kg⁻¹ and 1670.4 W kg⁻¹, respectively. The universality of this design strategy could allow it to be employed in producing hybrid materials for high‐performance energy‐storage devices.

The past decade has witnessed an emerging and fast-growing field on mechanically soft systems, ranging from soft materials (e.g. hydrogels) to soft devices (e.g. flexible electronics), and to soft functional systems (e.g. soft robotics). It is envisioned that soft systems will continue to thrive in the coming decades in academia and industrial communities. The launch of Soft Science is to provide the first dedicated platform for publishing research findings and to share the knowledge among the communities.