Self-sensing artificial muscles made with graphene fibers play piano and move robotic eyes

Researchers at the University of Pennsylvania have developed artificial muscle fibers that sense their own contraction state and hold intermediate positions on demand, a capability that has largely eluded soft actuators. The fibers, called CoilLCE, combine a liquid crystal elastomer (LCE) with multilayer graphene powder and an ultrathin copper coil embedded along the fiber’s length, and translate electrical signals into muscle-like motion.

Ziyun Zhang, co-lead author of the study, with the robotic eyes and hand used to demonstrate the CoilLCE artificial muscles. Image credit: Penn Engineering

The fibers contain just 0.25 wt.% graphene. At that loading the composite reached a toughness of 632 MJ/m³, more than three times the 192 MJ/m³ of a pure LCE fiber, and its stiffness decreased rather than increased, bringing it closer to the Young’s modulus of skeletal muscle, 0.1 to 0.3 MPa. The researchers attribute this to graphene sheets acting as physical barriers between oligomer chains, lowering the effective crosslink density without disrupting the liquid crystal alignment that the actuation depends on. At 0.75 wt.% the alignment was substantially impaired, and both actuation strain and breaking stress declined. The reduced stiffness also lowers the force a passively stretched fiber exerts against its contracting partner. In antagonistic pairs the graphene-loaded fibers reached an actuation strain of 16.1%, compared with 11.2% for pure LCE, an improvement of about 44%. The graphene also improved the uniformity of heating along the fiber.

Ziyun Zhang, co-lead author of the study, with the robotic eyes and hand used to demonstrate the CoilLCE artificial muscles. Image credit: Penn EngineeringThe fibers contain just 0.25 wt.% graphene. At that loading the composite reached a toughness of 632 MJ/m³, more than three times the 192 MJ/m³ of a pure LCE fiber, and its stiffness decreased rather than increased, bringing it closer to the Young’s modulus of skeletal muscle, 0.1 to 0.3 MPa. The researchers attribute this to graphene sheets acting as physical barriers between oligomer chains, lowering the effective crosslink density without disrupting the liquid crystal alignment that the actuation depends on. At 0.75 wt.% the alignment was substantially impaired, and both actuation strain and breaking stress declined. The reduced stiffness also lowers the force a passively stretched fiber exerts against its contracting partner. In antagonistic pairs the graphene-loaded fibers reached an actuation strain of 16.1%, compared with 11.2% for pure LCE, an improvement of about 44%. The graphene also improved the uniformity of heating along the fiber.

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