Thermal Responsive Shape Memory Polymers for Biomedical Applications

Abstract

Shape memory polymers (SMPs) are a type of polymeric materials that can be programmed to memorize a less-constrained shape/configuration, subsequently assume a strained temporary shape/configuration, and then revert to the memorized shape/configuration upon triggering by an external stimulus(Lendlein & Kelch, 2002). Such responsiveness to stimuli is reminiscent of the adaptive responses universally observed in living organisms. Based on the nature of the external stimulus, SMPs can be categorized into light-responsive SMPs, chemical-responsive SMPs, magnetic field-responsive SMPs, and thermal-responsive SMPs, etc. Thermal-responsive SMPs are one of the most studied systems and will be the focus of this chapter. Shape memory effect is not a universal property of polymeric materials. Polymer networks comprising of both net-points and reversible switching components, which are responsible for maintaining the dimensional stability and determining reversible thermal response of the polymer, respectively, have the potential to be programmed to exhibit shape memory effect. Processing histories and programming conditions also impact the shape memory effect of an SMP. The intrinsic mechanism for shape memory behavior in thermal responsive SMPs is the reversible freezing and activation of polymeric chain motion in the switching segments below and above the transition temperature (Ttrans), respectively. Shape memory properties have been reported in a wide range of polymers including, but not limited to, polyurethanes, epoxies, polyolefins and polyesters (Behl & Lendlein, 2007a; Behl, 2010; Lendlein & Kelch, 2002; Liu, 2007; Mather, 2009). The earliest report of shape memory effect dated back to the 1940s (Liu, 2007). Industrial applications of the shape memory effect took place in the 1950s when chemist Paul Cook, founder of Raychem Corporation, invented heat shrink tubings using radiation-crosslinked polyethylenes (Dole, 1981). The term “shape memory polymer” became better known as shape memory poly(norborene), developed by the French company CDF-Chimie, was commercialized in Japan by Nippon Zeon Company in the 1980s under the brand name Norsorex. Poly(trans-isoprene) and poly(styrene-butadiene) with shape memory effect were subsequently developed by two other Japanese companies (Leng, 2010). These events ushered in the first golden age of SMP research, which had primarily focused on polyenebased systems until segmental polyurethane-based SMPs were introduced by Mitsubishi