Mechanoreceptive soft robotic molluscoids made of granular hydrogel-based organoelectronics

Abstract

• Double network granular hydrogels combine low hysteresis and 1.4 MPa Young’s modulus. • Their sensor signal response exhibits low drift and relaxation. • The granular structure enables direct ink writing for enhanced sensor sensitivity. • The sensors allow a molluscoid soft robot to detect its position and surroundings. The viscoelasticity of many soft polymers renders flexible sensors susceptible to hysteresis and signal drift that limit their accuracy. To reduce the impact of viscoelasticity on the signal without compromising the softness of the material, we introduce electrically conductive double network granular hydrogel sensors. These sensors are composed of polyelectrolyte microgels that are covalently connected through a percolating hydrogel. To impart electrical conductivity to the granular material, the interstitial spaces are functionalized with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). These electrically conductive granular hydrogels exhibit a Young’s modulus up to 1.4 MPa, an ultimate strength up to 2.9 MPa and a stress–strain hysteresis below 6 %. This combination of mechanical properties results in an unprecedently low signal relaxation of 2 %. Indeed, the signal drifts less than 0.1 % if 10 times stretched to 50 %. The granular structure of these materials offers an additional benefit: it is made from jammed microgels that exhibit rheological properties ideal for direct ink writing (DIW). We leverage this feature to 3D print conductive paths into a soft molluscoid robot to render it proprioceptive and exteroceptive. We foresee this hydrogel-based electrically conductive material to enable the fabrication of the next generation of flexible electronics that can reliably link sensor readings to soft robot performance.