Researchers at the Georgia Institute of Technology are developing so-called shape-memory polymers that temporarily stretch or compress. Heat, light, or local chemical environment then transforms them back into their original or permanent shape.

The mechanical properties of these polymers make them attractive for many biomedical applications. “Metal stents are frequently covered in plastic anyway, so we set out to remove the metal leaving just a polymer sheath,” explains Ken Gall, a mechanical engineering professor. “Also, polymers are more flexible and don’t stress artery walls like metal.”

Gall’s research group has designed a shapememory polymer stent that can be compressed and fed into a blocked artery through a tiny hole in the body, just like conventional stents. Then, the warmth of the body triggers the polymer’s expansion into its permanent shape, deploying without auxiliary devices.

For another project, Gall has been investigating how altering a polymer’s chemistry changes its properties, such as stretchiness. “You can tailor the polymer to moderate its strength, stiffness, stretchiness, and expansion rate,” he says.

Gall is also exploring how these polymers might act as neuronal probes. “We’re looking for smart materials that can be synthesized in the 100-micron range, about the size of a strand of hair, and then inserted into brain tissue,” he explains. “This type of probe would need to slowly change shape inside the brain to avoid disturbing surrounding tissue.”

Another project examines the use of shapememory polymers for the spine. Most spinal surgeries are not performed arthroscopically, so Gall sees benefits in using shape-memory materials for minimally invasive spinal surgery.

Georgia Tech professor Ken Gall reaches inside a thermomechanical test frame that measures polymer properties under conditions simulating the human body. Photo by Gary Meek for Georgia Tech.

Inside a thermomechanical test frame, a shape-memory polymer fractures as it stretches past its deformation limit. This testing lets researchers determine the maximum possible change from permanent to temporary shape and vice versa.