Electroactive hydrogels (EAH) that exhibit large deformation in response to an electric field have received great attention as a potential actuating material for soft robots and artificial muscle. However, their application has been limited due to the use of traditional two-dimensional (2D) fabrication methods. Here we present soft robotic manipulation and locomotion with 3D printed EAH microstructures. Through 3D design and precise dimensional control enabled by a digital light processing (DLP) based micro 3D printing technique, complex 3D actuations of EAH are achieved. We demonstrate soft robotic actuations including gripping and transporting an object and a bidirectional locomotion. 

 Stimuli-responsive hydrogels exhibiting physical or chemical changes in response to environmental conditions have attracted growing attention for the past few decades. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm. This fast, high resolution, and scalable 3D printing method for stimuli-responsive hydrogels may enable many new applications in diverse areas, including flexible sensors and actuators, bio-medical devices, and tissue engineering.

Fabric-based personal heating patches have small geometric profiles and can be attached to selected areas of garments for personal thermal management to enable significant energy savings in built environments. Scalable fabrication of such patches with high thermal performance at low applied voltage, high durability and low materials cost is critical to the widespread implementation of these energy savings. This work investigates a scalable Intense Pulsed Light (IPL) sintering process for fabricating silver nanowire on woven polyester heating patches. Just 300 microseconds of IPL sintering results in 30% lesser electrical resistance, 70% higher thermal performance, greater durability (under bending up to 2 mm radius of curvature, washing, humidity and high temperature), with only 50% the added nanowire mass compared to state-of-the-art. Computational modeling combining electromagnetic and thermal simulations is performed to uncover the nanoscale temperature gradients during IPL sintering, and the underlying reason for greater durability of the nanowire-fabric after sintering. This large-area, high speed, and ambient-condition IPL sintering process represents an attractive strategy for scalably fabricating personal heating fabric-patches with greater thermal performance, higher durability and reduced costs. 

 Biporous structures at the nano–microscale are promising candidates for controlling phase change heat transfer, through their enhanced capillary wicking and fluid transportation. However, existing methods for fabricating biporous structures involve complex process which is not suitable for small-scale thermal devices such as heat pipes, owing to their confined and non-flat inner structures. Herein, we report the biporous structures inside copper-sintered heat pipes, enabled by layer-by-layer (LbL) assembled multi-walled carbon nanotube (MWCNT)-polyethyleneimine (PEI) coating for enhanced thermal performance. The repetitive filling and removing of the oppositely charged solutions with MWCNT-PEI and carboxylic-functionalized MWCNTs assembled the nanoporous MWCNT-PEI coatings (10, 20, and 40 bilayers) on the microporous copper-sintered inner surfaces. The fiber-like MWCNT networks structurally manipulated morphology and thickness of biporous structures, while the hydrophilic PEI shells chemically optimized wettability. A reduced thermal resistance (∼14.3%) was observed for MWCNT-PEI coating in 10 bilayers, due to the enhanced capillary wicking, interfacial contact areas, and bubble dynamics, whereas the 40 bilayers did not exhibit improved thermal performance owing to the redundant nanoporous layers causing reduced volume of microporous structures and increased thermal resistance. The LbL-assembled MWCNT-PEI coatings would act as functional layers to improve the performance of miniaturized and thin-film-based thermal devices.