Hybrid Hydrogel-Magnet Actuated Capsule for Automatic Gut Microbiome Sampling

Objective: Non-invasive, pill-sized capsules can provide intestinal fluid sampling to easily retrieve site-specific gut microbiome samples for studies in nutrition and chronic diseases. However, capsules with both automatic sampling and active locomotion are uncommon due to limited onboard space. This paper presents a novel hybrid hydrogel-magnet actuated capsule featuring: i) pH-responsive hydrogels that will automatically trigger fluid sampling at an environmental pH of $>$ 6 and ii) active locomotion by an external rotating magnetic field. Method: Two capsule designs were fabricated (Design A: 31 $\mu$L sampling volume with dimensions 8 mm × 19 mm, Design B: 41 $\mu$L sampling volume with dimensions 8 mm × 21 mm). They were immersed in simulated gastric (pH = 1.2) and simulated intestinal fluid (pH = 6.8) to test for automatic intestinal fluid sampling. An external rotating magnetic field was applied to test for active locomotion. Finally, seal tests were performed to demonstrate sample contamination mitigation. Results: Preliminary experiments showed that sampling occurred quickly and automatically in simulated intestinal fluid at 6–15 hours, active locomotion via rotation, rolling, and tumbling were possible at magnetic field magnitudes $<$ 10 mT, oil piston seals were better at mitigating sample contamination than water piston seals, and minimum o-ring seal pressures limits of 1.95 and 1.69 kPa for Design A and B respectively were sufficient against intra-abdominal pressures. Significance: This work presents the ability to impart capsule multi-functionality in a compact manner without onboard electronics or external triggering for sampling.