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A group of researchers from China and the U.S. have enabled a foil paper doll to do sit-ups, by adding a crystalline organic material to a polymer to make it more flexible. The polymer covalent organic framework, or polyCOF, was used by the scientists to build their version of an artificial muscle.
To make conventional COFs, the scientists linked simple carbon-containing molecules with covalent bonds to make a porous crystalline powder. The crystalline structure imparts mechanical properties useful for drug delivery and catalysis, for example. But trying to make membranes or sheets using regular COFs has proven difficult, because the structure is brittle and breaks before it can bend.
The scientists wondered if mechanical properties would improve if linear polymers were used as one of the building blocks. They added polyethylene glycol to the reactant mix to build the COF compound, which made the final polyCOF malleable enough to be twisted and stretched without damage.
The team then demonstrated how polyCOFs could be used as a type of artificial muscle. They built a doll, using the membrane for the waist and aluminum foil for other parts. When exposed to ethynol vapors, the doll sat up. When the gases were withdrawn, the doll laid down again. The steps were repeated several times, making the doll do sit-ups.
Actuators, or similar devices that mimic the contraction, expansion and rotation of real muscle, are a rich area of scientific development. Artificial muscle could be useful in a wide range of potential applications including robots, prosthetic limbs and powered exoskeletons.
The scientists’ findings are published in “PolyCOFs: A New Class of Freestanding Responsive Covalent Organic Framework Membranes with High Mechanical Performance,” Zhifang Wang, et al., American Chemical Society, June 25, 2019.
The authors acknowledge the financial support from the National Natural Science Foundation of China (21601093), Tianjin Natural Science Foundation of China (17JCZDJC37200), and NSF (CBET-1706025).