Researchers have developed a biocompatible material that can be 3D-printed into any shape and pre-programmed with reversible shape memory. The material is made using keratin, a fibrous protein found in hair, nails, and shells. The researchers extracted the keratin from leftover Agora wool used in textile manufacturing.
The key to keratin’s shape-changing abilities is its hierarchical structure. A single chain of keratin is arranged into a spring-like structure known as alphahelix. Two of these chains twist together to form a structure known as a coiled coil. Many of these coiled coils are assembled into protofilaments and eventually large fibers. The organization of the alpha helix and the connective chemical bonds give the material both strength and shape memory.
When a fiber is stretched or exposed to a particular stimulus, the spring-like structures uncoil and the bonds realign to form stable beta-sheets. The fiber remains in that position until it is triggered to coil back into its original shape.
To demonstrate this process, the researchers 3D-printed keratin sheets in a variety of shapes. They programmed the material’s permanent shape — the shape it will always return to when triggered — using a solution of hydrogen peroxide and monosodium phosphate. Once the memory was set, the sheet could be re-programmed and molded into new shapes; for example, one keratin sheet was folded into a complex origami star as its permanent shape. Once the memory was set, the researchers dunked the star in water, where it unfolded and became malleable. From there, they rolled the sheet into a tight tube. Once dry, the sheet was locked in as a fully stable and functional tube. To reverse the process, they put the tube back into water, where it unrolled and folded back into an origami star.
This two-step process of 3D-printing the material and then setting its permanent shapes allows for the fabrication of complex shapes with structural features down to the micron level.