Body-temperature programmable ultra-soft shape memory elastomers for comfort fitting

Abstract

Shape memory materials can be utilized to realize fast shape customization for personalized comfort fitting experiences. In this paper, a series of body-temperature programmable shape memory elastomers are developed for comfort fitting. Unlike the reported ones, in which the shape switch is realized due to body-temperature phase transition (around 37 °C), these newly developed ones are fabricated with silicone rubber (as elastic component) and a kind of polycaprolactone (PCL, as transition component) that has melting point of about 56 °C and much lower crystallization temperature. The thermal properties, shape memory performances, mechanical behaviors under uni-axial tension at room temperature and at human body temperature (37 °C) are investigated. The specialty of these shape memory elastomers is that the programming is not carried out at around melting point of the transition component. Instead, it could be done upon cooling to 37 °C. The temporary shape is fixed only after completion of PCL crystallization. As the melting point of PCL is much higher (56 °C), the mechanical properties of these shape memory elastomers are ensured rather stable near 37 °C, which eliminates the concern of shaping/fitting failure when ambient temperature fluctuates. Besides, ultra-softness and high stretchability at room temperature are revealed as well. Excellent shape memory performance of these elastomers at 37 °C is also confirmed. The shape fixity ratio is over 90% when PCL content reaches wt% of 35%. In such a way, these elastomers are able to well duplicate the local shapes of human body in actual fitting. Upon reheating to 80 °C, they also can completely recover to their original shapes. The fitting time window at 37 °C is revealed around 5 min, which is rather appropriate in practice. The yielding behaviors during tension and long-term shaping/fitting performance of these elastomers are also discussed. A simple wearable orthotic device for hallux valgus is developed for proof-of-concept. It is concluded that these newly developed shape memory elastomers are well suited candidates for comfort fitting.