Direct ink writing of porous shape memory polyesters

Abstract

In this study, the direct ink write (DIW) additive manufacturing technique is employed to print “self-fitting” shape memory polymer (SMP) scaffolds with requisite porosity from biodegradable poly(ε-caprolactone)-diacrylate (PCL-DA)-based polymers. In contrast to cast systems, printing gives flexibility to produce scaffolds with macropores through print design, as well as bespoke geometry. Notably, the efficiency of bone scaffold implants in treating critically sized bone defects is highly dependent on conformal fit for osseointegration, and pore features for osteoinductivity. To create a suitable DIW ink, ∼40 wt% of salt particles (NaCl, <38 microns in diameter) were added to a polymer solution, endowing rheological properties required for printing- i.e., shear thinning behavior and thixotropy. The prepared ink exhibited a drop in viscosity by 2 orders of magnitude with a shear rate increase of 3 orders of magnitude, alongside thixotropy evidenced by a 50% drop in viscosity upon a 100% shear rate change, reverting upon rate normalization. After printing, inks were cured via UV light-induced cross-linking of the polymer, then the structures washed with water to remove the salt and impart microporosity; thus, the salt particles served as sacrificial rheological modifiers. The final printed structures consisted of macropores that ranged from 200–300 microns with uniformly distributed ∼10–30 micron sized micropores. Notably, degradation studies revealed a progressive increase in degradation over time, with 73% mass loss after 15 days in 0.2 M aqueous sodium hydroxide. The composition, microstructure, thermal stability, degradation, and shape memory properties of the printed and cured objects are reported. This study gives insight into the DIW printing of these polymers, as relevant to customizable bone scaffolds, and examines the ink composition to understand the effects of different factors on the properties of the printed objects.