Hybrid Hydrogel‐Magnet Actuators with pH‐Responsive Hydrogels for Gastrointestinal Microrobots

Abstract

Limited space on millimeter‐scale devices for biomedical applications makes it challenging to incorporate bulky actuators and power for onboard mechanical actuation. Stimuli‐responsive hydrogels, such as pH‐responsive hydrogels, provide a solution to automatically sense and actuate in the gastrointestinal tract. However, hydrogels are often nonload bearing and slow in actuation. To overcome these challenges, a new type of hybrid actuator is developed which utilizes a pH‐responsive hydrogel with magnets to trigger magnetic springs (i.e., permanent magnets with repulsive, spring‐like forces) to quickly initiate rotational and translational movements at pH > 6. The agar‐poly(acrylic acid) hydrogel undergoes a large volume transition at pH > 6 and exhibits large nominal blocking stress of 610–819 kPa for a 3–4 mm diameter cylinder hydrogel. Moreover, the scaling of hydrogel force and response times are experimentally confirmed. Based on the hydrogel properties, an analytical hydrogel model is developed to predict hydrogel force and displacement under varying magnetic loads and wall constraints in simulated gastric fluid (SGF, pH 1.2) and simulated intestinal fluid (SIF, pH 6.8), and the experimental data validate the model. Finally, an innovative hybrid hydrogel‐magnet actuator that triggers rotational and translational motion without external activation is demonstrated.