Thin Hydrogel–Elastomer Multilayer Encapsulation for Soft Electronics

Abstract

In the exciting race to design and engineer biointegrated and body‐like electronic systems, many efforts concentrate on the integration of hydrogels in electronic assemblies. The versatility of hydrogels chemistry combined with their tissue‐mimicking properties inspires numerous demonstrations of hydrogel‐based touch panels, robots, and sensors over the years. However, their long‐term integration in a thin and functional electronic assembly remains a challenge: their sensitivity to both air‐drying and water swelling leads to important volume change of the network that is incompatible with the cohesion of a multilayer system, and has irreversible impact on the electronic properties of the assembly. To tackle this issue, proposed is a method to fabricate a hydrogel–elastomer micrometric bilayer with a stable interface, using of a low‐swelling type of hydrogel, i.e., poly(2‐hydroxyethyl methacrylate) and silicone rubber. The bilayer can sustain multiple hydration/dehydration cycles without delamination and can be kept for several months in its dry configuration. Combined with soft metallization technology, the bilayer can be readily integrated into a soft electronic circuit thereby opening a technological route for microfabricated, on‐demand morphing systems.

Double‐Hydrophobic‐Coating through Quenching for Hydrogels with Strong Resistance to Both Drying and Swelling

“…Hierarchical, hydrophobic/hydrophilic, alginate/polyacrylamide, double‐network, and polyampholyte hydrogels are some examples of recently designed advanced hydrogels with extraordinary properties. Unfortunately, because hydrogels contain a large amount of water, almost all hydrogels inevitably dry out in air, reducing their flexibility and functionality . For example, a 30 × 10 × 1.5 mm 3 Ca‐alginate hydrogel dries in air within 5 h .…”

“…Until now, only a few hydrogels have been reported that exhibit good resistance to drying or swelling . However, none of the hydrogels developed thus far have shown stability in both air and water, which limits their practical applications . Since drying and swelling are among the inherent characteristics of hydrogels, it is a significant challenge to design hydrogels with strong resistance to both drying and swelling.…”

“…Unfortunately, because hydrogels contain a large amount of water, almost all hydrogels inevitably dry out in air, reducing their flexibility and functionality. [9,13,14] For example, a 30 × 10 × 1.5 mm 3 Ca-alginate hydrogel dries in air within 5 h. [9] Additionally, most hydrogels swell significantly when immersed in water due to the hydrophilic nature of the polymers, which can decrease their mechanical properties dramatically. [15] For example, a typical as-prepared polyacrylamide (PAAm) hydrogel swells to four times its original volume in 4 h when immersed in water.…”

Cyber–Physiochemical Interfaces

“…The dehydration of hydrogel was another issue that causes hydrogel delamination and damages the stability. Using multilayer elastomer encapsulation is reported to suppress the dehydration and store for several months in the dry state, which should benefit long‐term testing …”

“…Using multilayer elastomer encapsulation is reported to suppress the dehydration and store for several months in the dry state, which should benefit long-term testing. [163] To sum up, electrophysiological signals can be detected via biopotential electrodes which convert ionic currents to electric currents at the interface between biological systems and measurement instruments. Good conformal contact helps to enhance sensitivity which can be achieved via decreasing film thickness, developing soft materials, and tissue-like hydrogel strategies.…”