Stiffness Graded Electroactive Artificial Muscle

In nature, hydrostatic, endo- and exo-skeletons are widely observed, and provide essential rigidity and anchoring points for the application of muscular forces. The efficient interface between a hard skeleton and soft muscle in biology is made possible by a complex hierarchy of structures and composite materials, extending from the nano- to the meso-scale. In contrast, artificial constructs which aim to bridge this hard-soft interface are prone to failure due to local discontinuities and concentrations in stress and strain which lead to material ruptures, delamination and tearing. In this article, the concept of a stiffness-graded electroactive material (SGEM) is proposed which emulates the soft-rigid interface in the nature biological systems and provides both electromechanical activity and the smooth stiffness gradient needed to bridge these two extreme states. This is achieved by programming the diffusion of a rigid filler material (polyvinyl chloride) in a liquid plasticizer (diisodecyl adipate). It is shown that the resulting stiffness gradient can match that of biological tissues such as smooth and skeletal muscles, and that the distal rigid region can be drilled and bonded and significant loads can be safely applied. Additionally, the resulting composite shows electroactive capability through graded anodophilic actuation characteristics. This protocol can be extended to numerous morphologies such as vertical or radial gradients depending on the deployment of two precursor ingredients. Finally, example applications including surface morphing and motion generation are demonstrated. The embodied stiffness gradient and electroactivity make this concept suitable for the development of bio-integrating and wearable artificial muscles systems and more effective soft robots.