Developing a Thermally Actuated Soft Robot for Finger Rehabilitation

One in five Americans experience disabilities that affect their abilities to perform daily activities. The 2015 US National Health Interview Survey found that nearly 2% of adults aged 18 and over had great difficulty handling small objects with their hands, and more than 6% struggled to push/pull large objects with their arms. Such mobility impairments are commonly caused by arthritis, Cerebral Palsy, Parkinson’s Disease, or stroke. Rehabilitation through repetitive movements, which allow the brain to establish new connections among undamaged nerve cells, has been proven to promote motor recovery and reduce disability. Occupational and physical therapists assist and monitor the repeated exercise, but the extra cost and time required to travel to therapy may burden the patient in their already long journey to recovery. Hence, robotic devices that assist and monitor these repeated moments at a patient’s home could positively contribute to the patient’s rehabilitation. 

The common perception of robots is that they are stiff with limited flexibility, because most robots are made of rigid metals. However, recent advances in additive manufacturing have led to the development of soft robots, which are fabricated with highly flexible materials similar to what can be found in living organisms. Rather than metal components powered by electric motors, soft robots are generally made with polymers and actuated by pressurizing various channels of fluid flows inside of the polymer structure. Utilizing materials such as fluids, gels, and soft polymers, soft robots could have higher pliability, versatility, and comfort while conforming to the contours of the human body. Additionally, soft robots may be lighter and cost less to fabricate, improving portability and accessibility. With these potential benefits in mind, in this paper, a preliminary design of a wearable soft robots to assist finger rehabilitation will be presented.    

A soft robot that can bend with heat-induced actuation was investigated. In this soft robot design, a phase changing material (PCM) is sealed in microchannels inside a polymer structure. As heat applied, the PCM begins to change phase from solid to liquid and the pressure inside the sealed microchannels increases and “pushes” the polymer structure to a different shape to create a “movement” of the robot. Depending on the locations of the microchannels and methods the heat applied, the movements of the soft robot can be controlled with precision.

Thus far, we had demonstrated bending of a soft robotic “arm” about the length of a human finger when it was submerged in warm water (~ 40^oC). Using Stereolithography (SLA) 3D printing, molds were made to shape an elastomer found in craft stores as the soft robot’s structure. A phase changing material (PCM), paraffin wax (chosen for its melting temperature of 37^oC), was sealed inside the elastomer microchannels. The elastomer “arm” bent when the heat was applied, and the wax melted. Although the proof-of-concept design was a success in demonstrating a basic function for finger rehabilitation, research is continued for this thermally actuated soft robot to fulfill all requirements of assistive rehabilitation.