Hydrogels for biomedical applications.

Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile-based nanogenerators (NGs), which will inevitably promote the rapid development and widespread applications of next-generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self-powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric-based NGs with both excellent electrical output properties and outstanding textile-related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric-based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large-scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric-based NGs.

Fiber/Fabric-Based Piezoelectric and Triboelectric Nanogenerators for Flexible/Stretchable and Wearable Electronics and Artificial Intelligence.

Textiles have been concomitant of human civilization for thousands of years. With the advances in chemistry and materials, integrating textiles with energy harvesters will provide a sustainable, environmentally friendly, pervasive, and wearable energy solution for distributed on-body electronics in the era of Internet of Things. This article comprehensively and thoughtfully reviews research activities regarding the utilization of smart textiles for harvesting energy from renewable energy sources on the human body and its surroundings. Specifically, we start with a brief introduction to contextualize the significance of smart textiles in light of the emerging energy crisis, environmental pollution, and public health. Next, we systematically review smart textiles according to their abilities to harvest biomechanical energy, body heat energy, biochemical energy, solar energy as well as hybrid forms of energy. Finally, we provide a critical analysis of smart textiles and insights into remaining challenges and future directions. With worldwide efforts, innovations in chemistry and materials elaborated in this review will push forward the frontiers of smart textiles, which will soon revolutionize our lives in the era of Internet of Things.

Smart Textiles for Electricity Generation.

Conductive MXene Nanocomposite Organohydrogel for Flexible, Healable, Low‐Temperature Tolerant Strain Sensors

3D printing alias additive manufacturing can transform 3D virtual models created by computer-aided design (CAD) into physical 3D objects in a layer-by-layer manner dispensing with conventional molding or machining. Since the incipiency, significant advancements have been achieved in understanding the process of 3D printing and the relationship of component, structure, property and application of the created objects. Because hydrogels are one of the most feasible classes of ink materials for 3D printing and this field has been rapidly advancing, this Review focuses on hydrogel designs and development of advanced hydrogel-based biomaterial inks and bioinks for 3D printing. It covers 3D printing techniques including laser printing (stereolithography, two-photon polymerization), extrusion printing (3D plotting, direct ink writing), inkjet printing, 3D bioprinting, 4D printing and 4D bioprinting. It provides a comprehensive overview and discussion of the tailorability of material, mechanical, physical, chemical and biological properties of hydrogels to enable advanced hydrogel designs for 3D printing. The range of hydrogel-forming polymers covered encompasses biopolymers, synthetic polymers, polymer blends, nanocomposites, functional polymers, and cell-laden systems. The representative biomedical applications selected demonstrate how hydrogel-based 3D printing is being exploited in tissue engineering, regenerative medicine, cancer research, in vitro disease modeling, high-throughput drug screening, surgical preparation, soft robotics and flexible wearable electronics. Incomparable by thermoplastics, thermosets, ceramics and metals, hydrogel-based 3D printing is playing a pivotal role in the design and creation of advanced functional (bio)systems in a customizable way. An outlook on future directions of hydrogel-based 3D printing is presented.

3D printing of hydrogels: Rational design strategies and emerging biomedical applications

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti-freezing, non-drying CPA-based organohydrogels with long-term stability by partially displacing water molecules within the pre-fabricated hydrogels. CPA-based Ca-alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA-based organohydrogels remain unfrozen and mechanically flexible even up to -70 °C and are stable under ambient conditions or even vacuum.

Rational Fabrication of Anti-Freezing, Non-Drying Tough Organohydrogels by One-Pot Solvent Displacement.

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Non-stick, ultra-elastic liquid metal droplets were fabricated by coating polytetrafluoroethylene (PTFE) particles onto the surface of NaOH-treated liquid metal droplets. The liquid metal droplets consisted of a liquid metal core, a thin anti-oxidation layer to maintain the high surface tension of the liquid metal and a particle-interpenetrated shell to enhance the mobility of the droplet.

Liquid metal droplets with high elasticity, mobility and mechanical robustness

Robust Fabrication of Nonstick, Noncorrosive, Conductive Graphene-Coated Liquid Metal Droplets for Droplet-Based, Floating Electrodes

Recently, temperature-resistant hydrogels, hydrogels which are freezing- and dehydration-resistant, have garnered considerable attention in the scientific community as they extend the rage of application of hydrogels to arid and/or cold environments. Besides, these hydrogels exhibit tunable conductivity and mechanical performance while offering excellent biocompatibility and flexibility, making them interesting candidates for flexible and wearable electronics and (bio)sensors. Several biomimetic strategies were developed to fabricate anti-freezing and anti-dehydration hydrogels with a diversity of merits, such as high strain resistance and conductivity, even at sub-zero temperatures, and employed as (bio)sensors, electrodes, and energy-storage devices. This review summarizes the recent advances in the preparation and application of temperature-resistant hydrogels, indicates issues of the state-of-the-art hydrogels, and offers potential future research directions.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cphc.201900545

Xiaohu Zhou

Papers

Rational Fabrication of Anti-Freezing, Non-Drying Tough Organohydrogels by One-Pot Solvent Displacement.

Liquid Metal‐Based Transient Circuits for Flexible and Recyclable Electronics

Liquid metal droplets with high elasticity, mobility and mechanical robustness

Robust Fabrication of Nonstick, Noncorrosive, Conductive Graphene-Coated Liquid Metal Droplets for Droplet-Based, Floating Electrodes

Biomimetic Extreme‐Temperature‐ and Environment‐Adaptable Hydrogels