Gut microbiome composition and its metabolic pathways have significant effect on chronic diseases and nutrition uptake. Stool sampling is often used to study the microbiome. However, site-specific microbiome information is lost during gut transit. Samples can be directly retrieved through ileostomy or colonoscopy, but are highly invasive. Consequently, a noninvasive, ingestible capsule with fluid sampling abilities was designed and its performance was investigated. The novel pH-responsive hydrogel-magnet actuator enables the automatic sampling of gut microbiome in intestinal fluid.
Initially, the capsule is closed. When the pH-responsive hydrogel reaches a critical hydrogel height (hcrit), the capsule slides open, a piston pulls fluid into the capsule for sampling, and the capsule closes. We estimate the maximum swelling height of the hydrogel in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) using a hydrogel model. The hcrit must fall in between these heights to trigger sampling only in intestinal fluid. The hcrit of the magnetic mechanism can be tuned by changing the size and distances among all magnets within the capsule. Magnetic modeling is used to design the open-sample-close magnetic mechanism at hcrit. Two capsule designs were prototyped for testing.
The capsules had sampling volumes of 31 μL and 41 μL with dimensions 8 × 19 mm and 8 × 21 mm respectively. Under a rotating magnetic field, the capsule tumbled, rolled, and rotated for locomotion. The capsules’ o-ring seals had minimum seal pressure limits of 1.95 kPa and 1.69 kPa which were sufficient against intra-abdominal pressure to prevent sample contamination. However, the hydrogel swelling time to reach hcrit was 6 to 15 hours in SIF. Further studies can be performed to optimize the hydrogel swelling rate. We successfully demonstrated that hydrogel-magnet actuators enable the design of compact capsules with multifunctional capabilities without the need for onboard electronics or enteric coatings.