August 1, 2008 -- For the past few years, medical researchers have been trying to develop ways to peer painlessly inside the human body, from a swallowable sensor to a magnetically controlled image-snapping capsule. Now, a group at Carnegie Mellon University (CMU) has shown that a tiny capsule robot is adhesive enough to anchor inside an intestine and yet gentle enough not to tear soft tissue.
The anchoring robot would be swallowed like a normal pill and move through the body until it reached the gut. Then a doctor, using a wireless control, would tell the robot when to expand its legs and anchor. It would be good not only for snapping images, but also potentially for biopsies, drug delivery, heat treatment, and other treatment applications.
While doctors have, for the past several years, used a camera pill that transmits images of the intestines, being able to control the movement of such a device would have many benefits, says Mark Schattner, a gastroenterologist at the Memorial Sloan-Kettering Cancer Center, who was not involved in the work. "The number-one use would be biopsy," says Schattner. "The other would be control of bleeding--if you could cauterize or laser a source of bleeding, that would be [a] major therapeutic use." While the CMU robot is not yet ready for such uses, its ability to securely and safely anchor in the body is the first step in achieving more-advanced applications.
The trick to making the robot was finding an adhesive that would "stick repeatedly to tissues like intestines, esophagus, stomach, heart, and kidney surfaces," says Metin Sitti, a professor and principal investigator of the NanoRobotics Lab at CMU. Although strong biomedical adhesives exist, they stick once and cannot be removed. Other attempts to create removable adhesives utilized clamps and hooks, which could potentially damage tissue. By developing a strong adhesive that can attach and reattach many times, Sitti hopes to build a robot that can actually crawl inside the human body for therapeutic purposes without causing harm.
Sitti and his lab group looked to beetles, which secrete oil-like liquids along their foot hairs in order to stick securely to surfaces. They coated their robot's feet with a similarly viscous liquid to "help get more adhesion by giving them a surface-tension component," says Sitti. Aside from increasing capillary and intermolecular forces, secretions help feet adhere to rough surfaces by filling in the gaps, he adds.
The group attached three robotic legs to a standard capsule camera and covered microscopic fibers on the adhesion pads with biocompatible silicone oil. The capsule robot is one centimeter in diameter and three centimeters in length, with 1.5-centimeter-long feet that open on demand and press into the surface of the tissue to increase friction and anchor the device, says Sitti. In a recent paper published in the Journal of Adhesion Science and Technology, Sitti showed that the oil increased adhesion up to 25 percent over a dry attempt on a smooth surface. On a slightly rough surface, the oily layer improved adhesion by almost 6 times. Recently, the team demonstrated that the capsule robot can successfully anchor on animal intestines in vitro, says Sitti, as well as on an animal esophagus.
"Clearly, a capsule that you can control in real time is going to be the next major advance for capsule-based systems," says Schattner. "The current capsule technology is not controllable: you're at the mercy of what the body is doing. The only thing you can do is image. You can't do anything therapeutic." Doctors have used the capsules to image the esophagus, colon, and--mainly--small intestine.
Sitti's group is also mimicking gecko feet. Geckos have angled hairs on their feet that allow them to pull in one direction to adhere more securely, and in another direction to detach. "We made some angled fibers [where] in one direction friction is very high, and the other direction it's low," says Sitti. The group plans to put the angled fibers on the capsule robot in the future.