Programmed water-induced shape-memory of bioabsorbable poly(D,L-lactide): activation and properties in physiological temperature.

This study reports of the novel water-induced shape-memory of bioabsorbable poly(D,L-lactide). We have developed an orientation-based programming process that generates an ability for poly(D,L-lactide) to transform its shape at 37°C in an aqueous environment without external energy and to adapt to a predefined stress level by stress generation or relaxation. In this orientation-programming process, polymer material is deformed and oriented at an elevated temperature and subsequently cooled down while retaining its deformed shape, tension, and polymer chain entanglements. At body temperature and in an aqueous environment, the shape-memory is activated by the plasticizing effect of water molecules diffused into the polymer matrix causing an entropy-driven directed relaxation of oriented and preloaded polymer chains. This plasticizing effect is clearly seen as a decrease of the onset glass transition temperature by 10-13°C. We found that γ-irradiation used for sterilizing the orientation-programmed materials strongly affected the shape-recovery rate, but not the recovery ratio. Both non-γ-irradiated and γ-irradiated sample materials showed excellent shape-recovery ratios during a ten-week test period: 94 and 97%, respectively. The orientation-programmed materials generated a predefined load in a 37°C aqueous environment when their shape-recovery was restricted, but when external tension was applied to them, they adapted to the predefined level by stress relaxation. Our results show that functionality in terms of shape-memory can be generated in bioabsorbable polymers without tailoring the polymer chain structure thus shortening the time from development of technology to its utilization in medical devices.