MilliFlex: A Multi-legged Millirobot with pH-Responsive Polymers for Targeted Drug Delivery

The ongoing miniaturization of robotic systems has led to groundbreaking applications in biomedicine recently. The major challenge of this advancement is shrinking components such as sensors, actuators, and power sources to the millimeter scale while maintaining functionality and reliability. Rather than relying on difficult-to-scale hard materials, this paper proposes and implements the first pH-responsive untethered soft millirobot named MilliFlex sized 14 millimeters long, 9 millimeters wide, and 3 millimeters tall, which can be further reduced based on the fabrication availability. MilliFlex is invented with a novel dual layer architecture of positive-tone legs by applying a magnetic field to a silicone/Fe mixture and negative-tone craters by displacing a silicone/Fe mixture with polyethylene beads. The positive-tone legs provide enhanced mobility for targeted drug delivery while the negative-tone craters carry the drug load. MilliFlex's design leverages pH-responsive polymers to release approximately 59 cubic millimeters of drugs at specific points within the gastrointestinal system. The system level test indicates that the drugs were successfully deposited at the designed pH value location within a human stomach model. In addition, MilliFlex is non-electronic, non-invasive to the human body, and biocompatible, offering promising prospects for enhancing the precision and effectiveness of drug therapies.