[Memory effect of resorbable polymers]

Influence of Layer Thickness, Raster Angle, Deformation Temperature and Recovery Temperature on the Shape-Memory Effect of 3D-Printed Polylactic Acid Samples

The success of the 3D-printing process depends upon the proper selection of process parameters. However, the majority of current related studies focus on the influence of process parameters on the mechanical properties of the parts. The influence of process parameters on the shape-memory effect has been little studied. This study used the orthogonal experimental design method to evaluate the influence of the layer thickness H, raster angle θ, deformation temperature Td and recovery temperature Tr on the shape-recovery ratio Rr and maximum shape-recovery rate Vm of 3D-printed polylactic acid (PLA). The order and contribution of every experimental factor on the target index were determined by range analysis and ANOVA, respectively. The experimental results indicated that the recovery temperature exerted the greatest effect with a variance ratio of 416.10, whereas the layer thickness exerted the smallest effect on the shape-recovery ratio with a variance ratio of 4.902. The recovery temperature exerted the most significant effect on the maximum shape-recovery rate with the highest variance ratio of 1049.50, whereas the raster angle exerted the minimum effect with a variance ratio of 27.163. The results showed that the shape-memory effect of 3D-printed PLA parts depended strongly on recovery temperature, and depended more weakly on the deformation temperature and 3D-printing parameters.

An Experimental Study of Heat Adaptation of Bioabsorbable Craniofacial Meshes and Plates

Intraoperative heating of bioabsorbable plates and mesh panels to above the glass transition temperature is commonly performed to assist their adaptation to bone. Some studies suggest that once heat-adapted, such implants under certain conditions tend to partially revert to their preadapted shape, termed a "memory effect." We investigated this phenomenon by using heat-adapted 82:18 poly-L-lactic acid:polyglycolic acid copolymer mesh panel and plate specimens with a glass transition temperature of 57 degrees C. The specimens retained limited malleability even at temperatures as low as 45 degrees C, substantially below the nominal glass transition temperature, as measured by shape relaxation experiments. At 40 degrees C to 42 degrees C, however, shape relaxation was not observed. A three-dimensional synthetic bone construct was also fixated using 90 degrees C heat-adapted plates, then incubated in a 37 degrees C buffer bath for 4 weeks, with periodic measurement of the shape of the construct. No changes in shape were recorded over this interval, suggesting that heat-adapted bioabsorbable implants forming a three-dimensional fixation network with multiple bone fragments cannot independently shape relax with the overall interconnectedness of the network ensuring its stability over time.