A material that can shrink to fit around internal organs without crushing them could be used to monitor electrical activity in the body. It contracts to less than half of its initial length when wet, and can conform around the organs’ irregular shapes.
Junqi Yi at Nanyang Technological University in Singapore and her colleagues made this material, called WRAP, from a petroleum-derived polymer similar to plastic. They tuned its molecular structure to mimic that of spider silk, which shortens when wet. The shrinkage happens in both these materials because water disrupts bonds between chains of stretched molecules that then recoil.
Across several experiments, when the researchers added water to WRAP, it become softer and stretchier, contracting up to 65 per cent in length. For example, a dry loop of the material with a diameter of 17 centimetres shrank to 8 millimetres in diameter.
Yi says that WRAP achieves as much of a reduction in size as plastic packaging films that shrink when heated. But for use inside the body, adding water is a more convenient way to cause shrinking.
She and her colleagues built soft electrodes into a loop of WRAP, then placed it around the heart of a rat. The moisture in the animal’s chest cavity made WRAP shrink and conform around the organ. They then used it to record the electrical signals within the heart, similar to the those recorded by a conventional epicardial electrogram. For a monitoring method that requires implanting a device inside of the body, the process was minimally invasive, but the electrode had to be replaced after about two weeks because it started to degrade, says Yi.
The fact that WRAP shrinks so much and within a few seconds is a notable advancement over existing materials, says Ximin He at the University of California, Los Angeles. She says that not having to pre-shape devices made from WRAP is also an advantage because it could lead to more personalised devices.
Christopher Bettinger at Carnegie Mellon University in Pennsylvania says that shrinking is generally a useful tool in biomedicine, and a material that does so on demand could also be used to clamp blood vessels or could be added to other materials to make them responsive to water in their environment.