Highly Stretchable Flame-Retardant Skin for Soft Robotics with Hydrogel–Montmorillonite-Based Translucent Matrix

Abstract

Flame-retardant coatings are crucial for intelligent systems operating in high-temperature (300–800°C) scenarios, which typically involve multi-joint discrete or continuous kinematic systems. These multi-segment motion generation systems call for conformable yet resilient skin for dexterous work, including firefighting, packaging inflammable substances, encapsulating energy storage devices, and preventing from burning. In fire scenes, a flame-retardant soft robot shall protect integrated electronic components safely and work for navigation and surveillance effectively. Here, we establish fire-resistant robotic mechanisms with montmorillonite (MMT)-biocompatible hydrogel skin, offering effective flame retardancy (∼78°C surface temperature after 3 min in fire) and high post-fire stretchability (∼360% uniaxial tensile strain). Fatigue test results in the MMT-hydrogel polymer matrix to portray a change in post-fire energy consumption of ∼21% (between the first cycle and the 200th cycle), further indicating robustness. MMT-hydrogel synthetic skin medium is then applied to everyday household items and electronics, offering appealing protections in fire scenes (≤10% capacitance loss after 3 min and ≤14% diode light-intensity loss after 1 min in fire). We deploy shape memory alloy (SMA) actuated inchworm-, starfish-, and snail-like locomotion (average velocity ∼12 mm·min⁻¹) for translating inside fire applications. With the stretchable and flame-retardant translucent barriers, the MMT-hydrogel skinned soft robots demonstrate stable compression/relaxation cycles (25 cycles) within flames (4 min 10 s) while protecting the electronic components inside in fire scene. We solve the agility vs. endurance conundrum in this article with SMA actuation independently via Joule heating without a cross-talk from the surrounding high-temperature arena.