Two-Way Reversible Shape Memory Polymers Containing Polydopamine Nanospheres: Light Actuation, Robotic Locomotion, and Artificial Muscles

Two-way reversible shape memory polymers (2W-SMPs), especially those that are light-responsive, are highly desirable for many applications, especially in the biomedical field, because of the convenience of indirect heating. We have designed and prepared a series of light-actuated 2W-SMP composites by incorporating very small amounts of polydopamine (PDA) nanospheres into semicrystalline polymer networks based on biodegradable poly(ε-caprolactone) copolymers. PDA nanospheres can be well dispersed in chloroform and well mixed with the polymer network. PDA nanospheres manifest good photothermal effect because of their strong absorption of light. The variation in temperature of the polymer composites can be correlated with irradiation time, light intensity, and the content of PDA nanospheres. Equations are developed to fit the temperature changes of the materials as a function of irradiation power and of the PDA particles content for a better understanding of the kinetics of the light-to-heat conversion. These polymer composites show excellent two-way reversible shape memory effects (2W-SMEs) under stress-free condition when the light is switched on and off showing a reversible angle change of 45°. The speed of angle change is larger for polymer composites irradiated with a stronger light or with a higher content of PDA nanospheres. This is the first report on 2W-SMPs using incorporated PDA nanospheres as photothermal fillers. A moving robot is designed based on photoresponsive 2W-SMP composites, which can walk on a track with triangular saw-teeth. This composite is capable of lifting and lowering a weight, acting as artificial muscles, and its actuated stress is much higher than the maximum stress yielded by most mammalian skeletal muscles. The use of biodegradable polyesters and thermal fillers made of a natural compound dopamine makes such composites potentially useful as biomaterials.