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.

Large-area stretchable electronics are critical for progress in wearable computing, soft robotics and inflatable structures. Recent efforts have focused on engineering electronics from soft materials-elastomers, polyelectrolyte gels and liquid metal. While these materials enable elastic compliance and deformability, they are vulnerable to tearing, puncture and other mechanical damage modes that cause electrical failure. Here, we introduce a material architecture for soft and highly deformable circuit interconnects that are electromechanically stable under typical loading conditions, while exhibiting uncompromising resilience to mechanical damage. The material is composed of liquid metal droplets suspended in a soft elastomer; when damaged, the droplets rupture to form new connections with neighbours and re-route electrical signals without interruption. Since self-healing occurs spontaneously, these materials do not require manual repair or external heat. We demonstrate this unprecedented electronic robustness in a self-repairing digital counter and self-healing soft robotic quadruped that continue to function after significant damage.

An autonomously electrically self-healing liquid metal-elastomer composite for robust soft-matter robotics and electronics.

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.

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

PDMS with designer functionalities—Properties, modifications strategies, and applications

Owing to their structural advantages in comparison with bulk polydimethylsiloxane (PDMS), porous 3D substrates, which are also known as PDMS sponges, possess immense potential in energy generation, transport and storage, absorption, separation, photocatalysis, wearable electronics and life science applications. Extensive effort has therefore been devoted to the design and synthesis of porous PDMS sponges. The key to excellent performance in high-value applications is a deep knowledge of the beneficial aspects of their porosity, structure and morphology for a specific application. Several new routes for the fabrication of PDMS sponges, as well as novel applications, have been developed recently. Therefore, this review focuses on advances in the field of the fabrication and application of PDMS sponges since 2011. After a concise introduction, which includes remarks on the benefits of PDMS in comparison with other materials, strategies for the synthesis/fabrication of PDMS sponges are presented and their advantages and drawbacks are concisely discussed. A summary of the potential fabrication procedures will be beneficial for those entering the field, as it gives a quick overview of the feasible applications of a PDMS sponge fabricated via a specific method. Afterwards, different high-value applications of PDMS sponges are described in conjunction with specific and detailed examples of all the abovementioned applications. In this regard, suggestions are also made regarding which fabrication method is most beneficial for the application being discussed. Finally, recent advances in the fabrication and application of PDMS sponges are summarized, as well as issues, such as surface functionalization and pore size limitations, which are impeding reasonable future applications. We envision that this review will be helpful to those entering the field, as well as experienced researchers who need rapid access to recent literature, and will give new stimuli for the development of novel porous materials.

Recent progress in fabrication and application of polydimethylsiloxane sponges

Liquid metal sponges were developed by loading liquid metals (GaInSn) into elastomer sponges. The elasticity of 3D-interconnected networks and the fluidic nature of liquid metals led to the formation of all-soft structures for electrical conductors with high electrical conductivity and mechanical flexibility.

Liquid metal sponges for mechanically durable, all-soft, electrical conductors

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

A bio-inspired, leaf-like pumping strategy by mimicking the transpiration process through leaves is developed for autonomous and continuous liquid transport enabled by durable hydrophilic sponges. Without any external power sources, flows are continuously generated ascribed to the combination of capillary wicking and evaporation of water. To validate this method, durable hydrophilic polydimethylsiloxane sponges modified with polyvinyl alcohol via a "dip-coat-dry" method have been fabricated, which maintains hydrophilicity more than 2 months. The as-made sponges are further applied to achieve stable laminar flow patterns, chemical gradients, and "stop-flow" manipulation of the flow in microfluidic devices. More importantly, the ease-of-operation and excellent pumping capacity have also been verified with over 24 h's pumping and quasi-stable high flow rates up to 15 µL min-1. The present strategy can be easily integrated to other miniaturized systems requiring pressure-driven flow and should have potential applications, such as cell culture, micromixing, and continuous flow reaction.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700028

Deyong Zhu

Papers

Recent progress in fabrication and application of polydimethylsiloxane sponges

Liquid metal sponges for mechanically durable, all-soft, electrical conductors

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

Hydrophilic Sponges for Leaf-Inspired Continuous Pumping of Liquids.