Hybrid Artificial Muscles Advances Necessary for the Practical Application of Soft Actuators

Soft actuators and artificial muscles have long been a promising area of research. They offer many types of Human/Machine Interactions (HMI) that are not possible through conventional electric motors over a very broad range of applications. The question must be asked as to why their practical implementation still has not been realized. Unfortunately, each type of soft actuator posses a unique remarkable capability, while suffering from a material limitation that prevents its real-world use. Pump-driven pneumatics/hydraulics are impractical outside of industrial application because of the necessary support hardware. Thermo-activated organic and inorganic actuators (e.g. twisted nylon and shape memory alloy wires, respectively) have slow actuation cycles and small stroke lengths. Electroactive artificial muscles (e.g. dielectric elastomer actuator polymers and HASELs) suffer from high actuation voltages and commonly struggle to generate both forces and displacements at useful levels. Soft actuators have previously been approached by researchers looking for the development of a single type of technology that can address all needs within a parameter space. We have moved beyond this method and now hybridized distinctly different technologies into a single soft actuator. This hybridization of technologies creates original soft devices that are capable of fulfilling the application-level needs within a parameter space. They also create material requirements previously not encountered. The exterior shell of an electrostatic artificial muscle actuators traditionally is used as both a general housing and the medium that transfers the generated force. When an electrostatic artificial muscle made of polypropylene polymers is hybridized intimately with shape memory alloy wire, forces in excess of 10N are generated with ~4mm of displacement, but the actuator will destroy itself. Composite materials that incorporate ultra-high strength polyethylene fibers maintain flexibility, ease of fabrication, and raises the maximum force output without actuator damage. Use of BaTiO₃ and TiO₂ particle/polymer composites to modify dielectric constants of the electrostatic components of the hybrid muscle also tailors the performance. A further development of the hybrid muscle concept is to combine the shape memory alloy wire with an electrostatic chuck to allow duty cycle-based thermal management. New material opportunities then are presented to realize a lower voltage electrostatic chuck capable of the necessary forces in a confined space. Dielectric polymer films as thin as 500nm attached to aluminum-coated polyester, by a vacuum adhesion process, have been tested for this purpose. The top performing systems allow for actuation forces with voltages as low as 70V. This is a 100x lower actuation voltage than a typical, similarly performing electroactive artificial muscle.