Smart polymeric hydrogels

Abstract

In nature, most biological tissues exist in a quasi-solid hydrogel state, adapting their properties in response to environmental changes via natural evolution. Encouraged by these natural hydrogels, significant research efforts over the past two decades have been devoted to creating their synthetic counterparts, known as smart polymeric hydrogels (SPHs). Nowadays, SPHs have attracted a global research focus due to their unique and adaptable properties; however, creating SPHs that meet specific demands remains one of the major scientific challenges. Through rational designs, SPHs can be engineered to exhibit remarkable adaptability in both structure and function beyond imagination for many unprecedented applications. In light of these advancements, this special issue of SmartMat hosted by Prof. Tao Chen, Prof. Feng Zhou, Prof. Ximin He, and Prof. Jie Zheng, featuring the topic of “Smart Polymeric Hydrogels (SPHs)” selectively collects one Review and 11 Research Articles from leading experts. This collection aims to highlight recent several key aspects of material design and synthesis, functional advancement, and diverse applications of SPHs. In the modern era, there is a growing interest in flexible wearable electronics that can mimic the remarkable mechanical properties of biological tissues and skin-like sensations. These devices have huge potential in health monitoring, soft robotics, prosthetics, and more. Polymeric hydrogels, which mimic human tissues in mechanical modulus, softness, and moisture, are believed to be promising materials for these applications. The key step in creating SPH-based flexible electronics lies in the design of SPHs with responsive conductive properties. To this end, Jie Zheng and coworkers (https://doi.org/10.1002/smm2.1160) introduced conductive polypyrrole (PPy) into polyvinyl alcohol (PVA) matrix to produce a type of double-network PVA/PPy hydrogels via in situ ultrafast gelation followed by a freezing/thawing process. Peiyi Wu and colleagues (https://doi.org/10.1002/smm2.1228) proposed to leverage the rapid self-initiated polymerization of conductive MXene nanomaterials with abundant hydrophilic functional –OH/–F groups and cationic methyl chloride quarternized N,N-dimethylamino ethylacrylate monomers to produce a new MXene-based nanocomposite hydrogel through both hydrogen bonds and electrostatic interactions. Benefiting from the incorporation of PPy and MXene as conductive additives, which have been extensively tested for their sensitivity to strain, pressure, or temperature changes, the resultant SPHs have achieved excellent conductivity for diverse applications, including the continuous monitoring of human respiration under physiological conditions, limb movement, and body temperature. Furthermore, electroencephalography, a critical physiological signal, has been addressed by Jianxin Tang et al. (https://doi.org/10.1002/smm2.1173), who reported a self-adhesive and low-contact impedance PVA/polyacrylamide (PAAm) double-network hydrogel used as a promising semidry electrode for real-world brain–computer interfaces. Another important strategy for creating smart conductive polymeric gels involves the integration of ionic liquids or deep eutectic solvents into the voids of three-dimensional (3D) cross-linked polymer networks. This integration forms 3D ion channels within the polymer matrix, enabling ions to directionally migrate and redistribute under external electric fields, thus endowing the polymeric gels with intrinsic ionic conductivity. Unlike traditional ionic hydrogels, these polymeric ionogels can effectively address the long-standing issues of water evaporation, offering the potential for enhanced long-term measurement stability in flexible sensory devices. To highlight the recent progress on polymer ionogels and their applications in flexible ionic devices, Tengling Ye and coworkers (https://doi.org/10.1002/smm2.1253) were invited to contribute a review article. This article thoroug