Flexible electronics have emerged as an exciting research area in recent years, serving as ideal interfaces bridging biological systems and conventional electronic devices. Flexible electronics can not only collect physiological signals for human health monitoring but also enrich our daily life with multifunctional smart materials and devices. Conductive hydrogels (CHs) have become promising candidates for the fabrication of flexible electronics owing to their biocompatibility, adjustable mechanical flexibility, good conductivity, and multiple stimuli‐responsive properties. To achieve on‐demand mechanical properties such as stretchability, compressibility, and elasticity, the rational design of polymer networks via modulating chemical and physical intermolecular interactions is required. Moreover, the type of conductive components (eg, electron‐conductive materials, ions) and the incorporation method also play an important role in the conductivity of CHs. Electron‐CHs usually possess excellent conductivity, while ion‐CHs are generally transparent and can generate ion gradients within the hydrogel matrices. This mini review focuses on the recent advances in the design of CHs, introducing various design strategies for electron‐CHs and ion‐CHs employed in flexible electronics and highlighting their versatile applications such as biosensors, batteries, supercapacitors, nanogenerators, actuators, touch panels, and displays.image

Mechanical responsiveness is prevalent in biological systems and plays an essential role in many biomechanical processes. The past two decades have witnessed enormous effort devoted to the development of biomimetic mechano-responsive hydrogels which are capable of adapting their physical and chemical properties to external mechanical stimuli. Due to the combination of tissue similarity and mechano-responsive properties, this type of hydrogel offers great advantages for diverse biomedical applications. Strain-stiffening and self-healing hydrogels duplicate the physiological properties of biological tissues, serving as promising candidates for artificial tissues, tissue scaffolds, and wound dressings. The shear-thinning property provides the hydrogels injectability, and the regional delivery contributes to minimally invasive treatment. Mechanochromic hydrogels allow the direct visualization of mechanical stress, holding great promise in biosensing and diagnosing. This review highlights the most recent developments in mechano-responsive hydrogels with various applications in the biomedical field. Different types of mechano-responsive hydrogels are introduced with focus on their responsive mechanisms, design strategies, and in vitro/in vivo performances, providing useful insights into the understanding and future research directions of mechano-responsive hydrogels with applications in biomedical engineering.

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Coacervate is the concentrated polymer-rich liquid phase that originates from the spontaneous liquid-liquid phase separation of a colloidal system, which has been considered as "the origin of life" for its high resemblance with protoplasm, [1] precellular systems, [2] and membrane-free organelles. [3] Coacervation also plays a critical role in constructing biological tissues (e.g., forming extracellular matrices via assembling elastin with tropoelastin) [4] and developing gradient properties in materials (e.g., squid beak possessing 200 times stiffness gradient), [5] with important applications in various industrial, biological, and medical fields. [6] Coacervates are extensively employed by sessile organisms such as sandcastle worm and mussel to realize strong adhesion under turbulent seawater, [7] which inspires potential applications of coacervates as implanted biomaterials like tissue glues, wound dressings, and drug carriers. Coacervation plays a critical role in numerous biological activities such as constructing biological tissues and achieving robust wet adhesion of marine sessile organisms, which conventionally occurs when oppositely charged polyelectrolytes are mixed in aqueous solutions driven by electrostatic attraction. Here, a novel type of adhesive coacervate is reported, driven by hydrogen-bonding interactions, readily formed by mixing silicotungstic acid and nonionic polyethylene glycol in water, providing a new approach for developing coacervates from nonionic systems. The as-prepared coacervate is easily paintable underwater, show strong wet adhesion to diverse substrates, and has been successfully applied as a hemostatic agent to treat organ injuries without displaying hemolytic activity, while with inherent antimicrobial properties thus avoiding inflammations and infections due to microorganism accumulation. This work demonstrates that coacervation can occur in salt-free environments via non-electrostatic interactions, providing a new platform for engineering multifunctional coacervate materials as tissue glues, wound dressings and membrane-free cell systems.

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