Chemists have developed a nanomaterial that they can trigger to shape-shift — from flat sheets to tubes and back to sheets again — in a controllable fashion. The nanomaterial, which in sheet form is 10,000 times thinner than the width of a human hair, is made of synthetic collagen. Naturally occurring collagen is the most abundant protein in humans, making the new material intrinsically biocompatible.
Collagen is the main structural protein in the body’s connective tissue such as cartilage, bones, tendons, ligaments, and skin. It is also abundant in blood vessels, the gut, muscles, and in other parts of the body. Collagen taken from other mammals, such as pigs, is sometimes used for wound healing and other medical applications in humans. The collagen protein is composed of a triple helix of fibers that wrap around one another like a three-stranded rope. The strands are not flexible — they’re stiff like pencils and they pack together tightly in a crystalline array.
A collagen sheet is one large, two-dimensional crystal but because of the way the peptides pack, it is like a bundle of pencils with half the pencils having the leads pointing up and the other half having the eraser end pointing up. The scientists sought to refine the collagen sheets so that each side would be limited to one functionality — one surface of the sheet would be all lead points and the other surface would be all erasers. The ultimate goal was to develop collagen sheets that could be integrated with a medical device by making one surface compatible with the device and the other surface compatible with functional proteins in the body.
When the researchers engineered the separate types of surfaces into single collagen sheets, however, it caused the sheets to curl up like scrolls. They then found that the shape-shifting transition was reversible — they could control whether a sheet was flat or scrolled simply by changing the pH of the solution it was in. They also demonstrated that they could tune the sheets to shape-shift at particular pH levels in a way that could be controlled at the molecular level through design.
The condition around which the transition occurs is a physiological condition that opens up the potential to find a way to load a therapeutic into a collagen tube under controlled, laboratory conditions. The collagen tube could then be tuned to unfurl and release the drug molecules it contains after it enters the pH environment of a human cell.