Mode shifting shape memory polymer and hydrogel composite fiber actuators for soft robots

Soft robotics is a promising new field offering robot systems that mimic the versatility and complexity of movement and propulsion seen in natural organisms. Such systems require complex actuation including length expansion/contraction, bending and torsion. To date, such actuators have been configured individually. Here, it is shown that composite fibers formed from a shape memory polymer and a thermo-sensitive hydrogel could be configured into any one of the tensile, torsional and flexural actuators. Furthermore, the programmed mode of actuation could be thermally erased and the same fiber re-programmed into a different type of actuator. The fibers were prepared from poly(N-isopropylacrylamide) with polycaprolactone and the fiber composition was tuned to optimize both shape fixity and the degree of actuation. The fibers could be programmed by heating to 60 ℃ and then cooled under tensile, flexural or torsional load. The fiber was then conditioned by immersing in water at room temperature which induced swelling of the hydrogel phase and shape deformation. The pre-programmed shape of the fiber was then restored when the fiber was heated at 35 ℃ in water to de-swell the thermo-sensitive poly(N-isopropylacrylamide) hydrogel. Reversible actuation was observed through multiple cycles of heating and cooling. The optimized fibers generated torsional strokes of 10 turn/m; bending curvature changes of 0.4 mm−1 or tensile strokes of up to 92%. The composite fibers offer a convenient means for generating a variety of different actuation types for soft robotics.