The self-propelled tiny capsules remove the outer shell and transport the drug directly to the tumor cells. The role of these micro-robots was confirmed in the mouse intestines. Perhaps one day, they will become targeted treatments for cancer and reach some hard-to-reach areas of the body. Related results were published in Science-Robotics.

"When the capsule reaches the tumor, we can activate it, rupture the capsule and release the micromotor, which will move in the tumor area. This move is very important for drug delivery," said Wei Gao of the California Institute of Technology.

He and the team members created a multi-layer micromotor. The first is magnesium particles having a diameter of about 20 microns. Magnesium is wrapped in a layer of gold and then wrapped in a layer of hydrogel containing anticancer drugs. Finally, several of these micromotors are contained in gelatin capsules.

Gas escapes from the hole and powers the movement of the capsule. Image source: Wu, et al.

The team fed the capsules to mice with melanoma cells in the gut. Melanoma is used because these cells absorb near-infrared light well, so researchers can better use photoacoustic computed tomography to track capsule effects. This technology delivers near-infrared light into the tissue. In tissue, near-infrared light is converted into sound and an ultrasound image is returned.

Gao and colleagues tracked the capsule as it entered the mouse's intestine and approached the cancer cell. Once they reach the target location, they illuminate a capsule with a strong infrared light. The capsule heats the gold and releases the drug, while also releasing magnesium, which produces hydrogen bubbles by chemical reaction with the intestinal fluid. This gas is released from the 2 nm hole left in the outer shell, powering the capsule around the intestine, just as a tiny balloon releases air while flying.

Researchers say that the delivery of drugs in the gastrointestinal tract is a tricky problem because everything is moving, so the drug is washed away before it is treated. "We need long-term release. Micromotors move around the tumor, so they can penetrate the tumor tissue and release the drug slowly over a long period of time," Gao said.