Demonstrating the influence of water on shape-memory polymer networks based on poly[(rac-lactide)-co-glycolide] segments in vitro.

Thermally-responsive shape-memory polymers (SMP) are highly promising implant materials for applications in minimally-invasive surgery since the shape-memory effect (SME) enables the implantation of a bulky device in a compressed temporary state through a small incision. When heated to a temperature exceeding the material switching temperature (Tsw), the device recovers its original bulky shape. Therefore, SMP implants with Tsw ~ 37 °C are required for such applications because the body cannot withstand excessive applications of heat. Here, Tsw of networks based on poly[(rac-lactide)-co-glycolide] star-shaped macrotriol or macrotetrols with 19-22 wt% glycolide content, varying oligomer molecular weight (Mn = 3000-10000 g·mol-1), and netpoint functionality (f = 3 or 4) were lowered from 55-66 °C to below body temperature via the uptake of water, which also induced SME at body temperature. Programmed samples kept their temporary shape at room temperature in water as well as at 37 °C under dry conditions but recovered in 37 °C water. Water uptake/swelling studies and FTIR analysis indicated that the mechanism of solvent-induced SME involved the plasticization of water in switching domains as opposed to changes in swelling or hydrogen bonding. This indirect actuation of SME by using a combination of solvent and heat could be exploited in easy-to-handle shape-memory implant with slower degradation kinetics.