Chemically Powered Polymer Artificial Muscles

Abstract

Designing, fabricating, and operating active materials that mimic skeletal muscles have been ambitious challenges. Polymeric artificial muscles and actuators have been of particular interest in research and development in soft robotics due to their large actuation, flexibility, lightweight nature, and biomimetic capabilities. Stimulus types which trigger the shape change in actuating polymers are diverse, with each having different actuation mechanisms, affecting performance, functions, and usage in certain conditions. While scientists are still in pursuit of an ideal material and stimulus system, nature's complex series of chemical reactions that control the human body provide inspiration for developing the actuation mechanism of biomimetic actuators. Similar to biological reactions, chemically responsive polymers can transform chemical energy into mechanical energy, creating force and displacement.

This dissertation enhances the performance of chemically-driven polymer fiber actuators by exploiting their chemistry and geometry. It also proposes a novel actuation medium and stimuli type, utilizing redox reactions induced by fuels to power fiber actuators. The first phase demonstrates leveraging the architecture of pH-responsive fibers via twisting and coiling to amplify actuation stroke and work capacity. This approach resulted in a contractive actuation stroke of up to 43%, which is six times greater than that of aligned fibers. This increase is due to significant changes in bending and torsional stiffness, resulting from alterations in elastic modulus and fiber diameter.

The research also enhances the actuation rates and magnitude of humidity-driven artificial muscles by incorporating nanomaterials into a polymer matrix. Adding 0.5%wt. graphene to the matrix led to a 24% actuation, a 40% increase over neat fibers. The study highlights the crucial role of graphene nanoplatelet (GNP) dispersion in enhancing actuation performance by altering ion exchange capacity and water uptake. By leveraging the tunability of actuation magnitude with various graphene concentrations. A humidity-responsive self-walking robot is designed and operated.

Lastly, a novel chemical actuation mechanism for redox polymers is proposed. This method involves H2 and O2 gas environments reacting with the Pt-coated redox actuator, resulting in reversible length changes. This technique offers electrolyte and electrode-free, all-solid polymer actuators, exhibiting a catch-state feature. The polymer fibers, fabricated through the wet-spinning process, provide scalable actuators with up to 3.8% contraction and can withstand maximum actuation stress of 12 MPa. The maximum work capacity generated was 120 J/kg, which is 15 times higher than that of biological muscles. Finally, the practical application of these fuel-driven fibers was demonstrated by embedding them into a humanoid hand, showcasing their potential in real-world applications.