Hydrogel-based conductive materials for smart wearable devices have attracted increasing attention due to their excellent flexibility, versatility, and outstanding biocompatibility. This review presents the recent advances in multifunctional conductive hydrogels for electronic devices. First, conductive hydrogels with different components are discussed, including pure single network hydrogels based on conductive polymers, single network hydrogels with additional conductive additives (i.e., nanoparticles, nanowires, and nanosheets), double network hydrogels based on conductive polymers, and double network hydrogels with additional conductive additives. Second, conductive hydrogels with a variety of functionalities, including self-healing, super toughness, self-growing, adhesive, anti-swelling, antibacterial, structural color, hydrophobic, anti-freezing, shape memory and external stimulus responsiveness are introduced in detail. Third, the applications of hydrogels in flexible devices are illustrated (i.e., strain sensors, supercapacitors, touch panels, triboelectric nanogenerator, bioelectronic devices, and robot). Next, the current challenges facing hydrogels are summarized. Finally, an imaginative but reasonable outlook is given, which aims to drive further development in the future.

Recent advances in conductive hydrogels: classifications, properties, and applications.

Biological organisms present marvelous morphing behaviors from the quiescent blooming of flowers to the energetic wing‐flapping of birds that have always inspired humans to design better‐engineered products. The diversity of natural motion is attributed primarily to the intricate and hierarchical structure of actuators that are self‐assembled from nanoscale structures to superstructures. Compared to the biological actuators, their manmade counterparts, often with significantly limited capabilities, are fabricated from various materials with relatively simple structures using limited fabrication techniques. With the rapid developments in technologies that require soft robotics and human‐machine interfaces, there is increasing demand for soft actuators with improved capabilities such as larger output force, repeatability, and a more comprehensive range of motion. Biological actuators provide critical insights into the structure‐function relationship and offer exciting concepts to advance the science and technology of artificial soft actuators. Here, the design approaches found in natural actuation systems are discussed from the nanoscale to the highest levels in the structural hierarchy and the physical principles involved in their diverse actuation capabilities. In that context, finally, the fabrication techniques that have been utilized for manmade soft actuators, with a focus on the advantages, challenges, and concepts for potential future developments are reviewed.

Bioinspired Structures for Soft Actuators

Multimaterial additive manufacturing has important applications in various emerging fields. However, it is very challenging due to material and printing technology limitations. Here, we present a resin design strategy that can be used for single-vat single-cure grayscale digital light processing (g-DLP) 3D printing where light intensity can locally control the conversion of monomers to form from a highly stretchable soft organogel to a stiff thermoset within in a single layer of printing. The high modulus contrast and high stretchability can be realized simultaneously in a monolithic structure at a high printing speed (z-direction height 1 mm/min). We further demonstrate that the capability can enable previously unachievable or hard-to-achieve 3D printed structures for biomimetic designs, inflatable soft robots and actuators, and soft stretchable electronics. This resin design strategy thus provides a material solution in multimaterial additive manufacture for a variety of emerging applications. 

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Single-vat single-cure grayscale digital light processing 3D printing of materials with large property difference and high stretchability

http://www.cell.com/article/S2589004222009464/pdf

Fluid-driven hydrogel actuators with an origami structure

Designs perfected through evolution have informed bioinspired animal‐like robots that mimic the locomotion of cheetahs and the compliance of jellyfish; biohybrid robots go a step further by incorporating living materials directly into engineered systems. Bioinspiration and biohybridization have led to new, exciting research, but humans have relied on biotic materials—non‐living materials derived from living organisms—since their early ancestors wore animal hides as clothing and used bones for tools. In this work, an inanimate spider is repurposed as a ready‐to‐use actuator requiring only a single facile fabrication step, initiating the area of “necrobotics” in which biotic materials are used as robotic components. The unique walking mechanism of spiders—relying on hydraulic pressure rather than antagonistic muscle pairs to extend their legs—results in a necrobotic gripper that naturally resides in its closed state and can be opened by applying pressure. The necrobotic gripper is capable of grasping objects with irregular geometries and up to 130% of its own mass. Furthermore, the gripper can serve as a handheld device and innately camouflages in outdoor environments. Necrobotics can be further extended to incorporate biotic materials derived from other creatures with similar hydraulic mechanisms for locomotion and articulation.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202201174

Necrobotics: Biotic Materials as Ready‐to‐Use Actuators

Soft robotics greatly benefits from nature as a source of inspiration, introducing innate means of safe interaction between robotic appliances and living organisms. In contrast, the materials involved are often nonbiodegradable or stem from nonrenewable resources, contributing to an ever-growing environmental footprint. Furthermore, conventional manufacturing methods, such as mold casting, are not suitable for replicating or imitating the complexity of nature’s creations. Consequently, the inclusion of sustainability concepts alongside the development of new fabrication procedures is required. We report a customized 3D-printing process based on fused deposition modeling, printing a fully biodegradable gelatin-based hydrogel (biogel) ink into dimensionally stable, complex objects. This process enables fast and cost-effective prototyping of resilient, soft robotic applications from gels that stretch to six times their original length, as well as an accessible recycling procedure with zero waste. We present printed pneumatic actuators performing omnidirectional movement at fast response times (less than a second), featuring integrated 3D-printed stretchable waveguides, capable of both proprio- and exteroception. These soft devices are endowed with dynamic real-time control capable of automated search-and-wipe routines to detect and remove obstacles. They can be reprinted several times or disposed of hazard-free at the end of their lifetime, potentially unlocking a sustainable future for soft robotics. Description A method for 3D printing of biodegradable hydrogels for sensor-clad, omnidirectional actuators is reported.

/pdf/3d-printing-of-resilient-biogels-for-omnidirectional-and-5cj99iaq.pdf

E. Woritzka

Papers

3D printing of resilient biogels for omnidirectional and exteroceptive soft actuators