Advanced Control Strategies of Light-Responsive Polymers for Soft Robotics

Soft robotics is a rapidly developing research field that has triggered a significant amount of research effort during the past few years. The field aims at providing new technical innovations to overcome the challenges encountered in conventional hard-bodied robotic systems constructed using rigid joints and links, such as lack of flexibility and adaptability. Among the most promising materials for the fabrication of soft robots are smart stimuli-responsive polymers that can be triggered with external energy sources to undergo various chemical and physical changes such as mechanical motions like contraction or bending. Among different classes of stimuli-responsive polymers, photomechanical actuators are of particular interest as they provide a route to harness light energy to remotely fuel mechanical motions. Today, most photochemical actuators are based on reversible photochemical switching of photochromic molecules between two forms with distinct structural and photochemical properties. On the other hand, photoactuation can also be driven photothermally using light absorption by organic dyes or inorganic moieties for heat generation, which stimulates the shape changes of the polymer.

In this thesis we use liquid crystal networks and hydrogels as materials platforms to devise photoactuators and soft robots that can be controlled and powered remotely with light producing reversible shape changes. Liquid crystal networks enable pre-programmable shape changes and hence several actuation modes can be achieved within one material. In hydrogels, complex shape changes can be programmed by tuning materials properties locally after fabrication. By utilizing both photothermal and photochemical effects, we use three advanced light control strategies to power photomechanical actuation: self-sustained motion, multicolor functions, and reconfigurability. By using these strategies, we demonstrate sophisticated photoactuators exhibiting self-oscillation, non-reciprocal motions, logic gate actuation, reconfigurable gripping, and shape-morphing between Gaussian curvatures. The results of this thesis deepen the understanding on the role of photothermal and photochemical effects in controlling photomechanical actuation, and present new pathways and control strategies for soft micro-robotics.