Biomaterial Stretches Like Muscle

Many research groups are trying to develop materials with similar properties to muscles. One of the big difficulties is creating anything with just the right muscle-like elasticity--its ability to change shape while withstanding a large strain. Now researchers at the University of British Columbia (UBC) in Vancouver, Canada, have synthesized a protein-based material that stretches exactly like the real thing.

Musclebound: Rings made from protein-based hydrogels are shown in ultraviolet light (top) and normal light (bottom). Credit: Nature

The new material achieves the elasticity of muscle by mimicking the microscopic structure of a giant muscle protein called titin. The structure of titin resembles a string with beads--globules of folded protein sequences are connected by floppy, unstructured sequences. Hongbin Li, a chemist at the UBC, and his colleagues constructed the new material that imitates this structure. They chose a mechanically stable protein sequence that folds in on itself to form globules, and another protein called resilin to serve as the floppy connectors.
The result was a "mini-titin"--a protein that resembled titin structurally but is much smaller, Li says. The researchers chemically linked the individual protein strands together to form a hydrogel--a light, solid material that consists mostly of water--and then tested the material's mechanical properties. The team describes the work in a recent issue of the journal Nature.
When they tested the material, Li and his colleagues found that it behaved much like real muscle tissue. When stretched a little bit, it bounces back like an elastic rubber band. If stretched more vigorously, the beadlike protein domains unfold, and it dissipates some energy before returning to its original state.
"It's a nice progression along the lines of building an artificial muscle," says physicist David Weitz of Harvard University, whose group studies the structure of muscle protein networks. Other groups are working on creating electroactive polymers, which contract when stimulated by an electric signal, so that the "muscle" can be controlled. The current material does not have this feature, but adding that would be "the next step," Weitz says.
Artificial muscles could one day be used as scaffolds for growing muscle to repair damage in patients; in biologically compatible devices for medical applications; even to control robots without using motors. However, since proteins tend to unravel at high temperatures and under harsh environmental conditions, this does not make them ideal for industrial applications.
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