Shape Memory Performance of Thermoplastic Amphiphilic Triblock Copolymer poly(D,L-lactic acid- co-ethylene glycol- co-D,L-lactic acid) (PELA)/Hydroxyapatite Composites

Abstract

Biodegradable polymer/hydroxyapatite (HA) composites are desired for skeletal tissue engineering. When engineered with thermal-responsive shape memory properties, they may be delivered in a minimally invasive temporary shape and subsequently triggered to conform to a tissue defect. Here we report the shape memory properties of thermoplastic amphiphilic poly(D,L-lactic acid-co-ethylene glycol-co-D,L-lactic acid) (PELA, 120 kDa) and HA-PELA composites. These materials can be cold-deformed and stably fixed into temporary shapes at room temperature and undergo rapid shape recovery (< 3 s) at 50 °C. Stable fixation (>99% fixing ratio) of large deformations is achieved at -20 °C. While the shape recovery from tensile deformations slows with higher HA contents, all composites (up to 20 wt% HA) achieve high shape recovery (>90%) upon 10-min equilibration at 50 °C. The permanent shapes of HA-PELA can be reprogramed at 50 °C, and macroporous shape memory scaffolds can be fabricated by rapid prototyping.

Keywords: biodegradable; hydroxyapatite; rapid prototyping; shape memory polymer; tissue engineering.

Figure 1

Scanning electron micrographs of the bottom surface (left) and cross-sections (right) of solvent-cast PELA films with 0–20 wt% of HA. Scale bars = 50 μm.

Figure 2

Elastic moduli (n=3) of PELA films with 0–20 wt% of HA at (A) 25 °C or (B) 37 °C. Specimens (5.3 mm × 35 mm × ~0.2 mm) were ramped at 100 mm/min (25 °C) on an MTS mechanical testing system or at 1 N/min (37 °C) on a Q800 dynamic mechanical analyzer. * p < 0.05 (One way ANOVA with Tukey post-hoc).

Figure 3

Temperature-dependent storage moduli of PELA films with 0–20 wt% of HA. Specimens (5.3 mm × 35 mm × ~0.2 mm) were subjected to 0.02% strain at a frequency of 1 Hz while temperature was ramped at 2 °C/min on a Q800 dynamic mechanical analyzer.

Figure 4

Shape memory behavior determined by (A) strain-controlled and (B) stress-controlled cyclic thermal mechanical testing of PELA films with 0–20 wt% of HA. Three consecutive cycles for each specimen (5.3 mm × 35 mm × ~0.2 mm) are shown.

Figure 5

Reprogrammable shape memory of HA-PELA films (20 wt% HA). (A) Cold-deformation and fixation into a temporary spiral at room temperature (r.t.) and rapid shape recovery to permanent flat bar shape (as cast) at 50 °C. (B) Reprogramming the flat bar into a permanent spiral shape at 50 °C, cold-deformation and fixation into a temporary flat bar at r.t., and subsequent rapid recovery back to reprogrammed permanent spiral.

Figure 6

Shape memory properties of a rapid prototyped macroporous cylindrical HA-PELA (25 wt% HA) scaffold. (A) CAD image (left) and stereomicroscopy image (right) of the rapid prototyped HA-PELA scaffold. (B) Cold-pressing and fixation of the HA-PELA scaffold into a collapsed disc at room temperature. (C) Rapid shape recovery of the collapsed disc into the original cylindrical shape in a 50 °C water bath. Scale bars = 3 mm.