Controllable Fabrication Of Biomimetic Flexible Fiber Actuators

The muscle-driven motions of creatures are extremely dexterous and elegant.The dexterity comes from the combinations of softness,highly addressable and fast response,and directional deformation of muscles.These intriguing properties are rooted in the hierarchical fibrous structure of biological muscles.The muscle fiber,as a motor unit,is a fiber actuator that generates a linear actuation.The nervous system transforms simple fiber length changes into complex movements and deformations through the recruitment of multiple motor units.Among the various types of artificial muscles that aim to achieve muscle-like movements,electrically driven dielectric elastomers and pressure-driven pneumatic artificial muscles have received much attention because of their addressability,which enables them to precisely modulate individual actuators in an actuator cluster.And they also possess excellent actuation performance.However,the fabrication of biomimetic flexible fiber actuators based on dielectric elastomers and pneumatic artificial muscles has only been studied in a few exploratory studies.Limited by the availability of materials,the actuators have poor actuation performance and single deformation mode.To address the above issues,we designed and synthesized block copolymer thermoplastic elastomers using reversible addition fragmentation chain transfer(RAFT)emulsion polymerization to take advantage of their excellent processing properties.The research is carried out in terms of material design and preparation,fiber processing methods,and asymmetric fiber structure design.The high performance and biomimetic controllable complex deformation of fiber actuators are achieved by introducing asymmetry in material microstructure and macroscopic fiber structure,as well as fiber bundle construction and programmed actuation.The main innovative results obtained are as follows:(1)A new method for strengthening the axial actuation performance of dielectric elastomer fiber actuators was created.A theoretical model of the electroactuation of anisotropic dielectric elastomer fiber actuators was developed and it was found that a moderate orthogonal modulus ratio(～3)is sufficient to maximize the strain of the rolled fiber actuators in the axial direction,achieving a strong directional output with a 100%actuation strain enhancement.Guided by the theory,poly(styrene-b-butyl acrylate-b-styrene)(SBAS)thermoplastic elastomers were thermally relaxed to produce a highly oriented asymmetric structure of the internal molecular chains to fabricate intrinsically anisotropic SBAS films.Then the anisotropic SBAS films were rolled to obtain the anisotropic dielectric elastomer fiber actuators.The axial strain of the anisotropic fiber actuators was twice as high as that of the isotropic fiber actuators,which is consistent with the theoretical model.(2)SBAS dielectric elastomer fiber actuators can be self-adhesive assembled into fiber bundles to achieve linear amplification of the output force,while generating muscle-like multidegree-of-freedom deformation and motion like extension,bending,and rotation through addressable actuation.(3)Solution spinning processing technology for block copolymer thermoplastic elastomers was investigated,and poly(styrene-b-(2-ethylhexyl acrylate)-b-styrene)(SEHAS)block copolymers with low chain entanglement density were found to have better spinnability.Based on this,a co-extrusion process of dielectric elastomer materials and electrode materials was established to prepare transparent,fully flexible underwater-driven dielectric elastomer fiber actuators in a continuous manner.The eccentric structure of the extruded fibers can be regulated by changing the shape of the extrusion head.The coaxial fiber actuator produced axial elongation with a maximum linear strain of 12.1%when actuated.The eccentric fiber actuator had an asymmetric structure,and actively produced large bending deformation in a single actuator without the need for a passive layer.By changing the topology of the eccentric fiber through knotting,the asymmetry of the eccentric position is further created,and then a single fiber can be actuated to produce a complex deformation in the "S" shape with double bending direction.(4)Based on the thermoplasticity of block copolymers,processing and post-processing techniques for elastomeric hollow eccentric fibers were invented to endow single pneumatic fiber actuators with complex actuation deformation capabilities.Pneumatic fiber actuators with a hollow eccentric structure were prepared continuously by co-extrusion,whose basic pneumatic deformation mode is bending due to the asymmetry of the structure.The maximum bending curvature of the fiber actuator of length 25 mm was 150.8 m-1 at an air pressure of 13 kPa,and the bending angle reached 360°.Under the influence of gravity,the fiber actuator of 85 mm in length produced a three-dimensional out-of-plane spiral-shrinkage deformation,the shrinkage reached a highest value of 80%at an air pressure of 21 kPa.The eccentric position is spatially discontinuous by knotting,resulting in an "S" shaped double bending direction deformation of the knotted fiber actuator.Twisting the fiber based on the processability of the block copolymer creates a continuous change in the eccentric position,resulting in a complex out-of-plane multiple "S" shaped actuation deformation of the twisted fiber actuator.