Investigation of the biodegradation kinetics and associated mechanical properties of 3D-printed polycaprolactone during long-term preclinical testing.

Abstract

Polycaprolactone (PCL) is a bioresorbable polymer increasingly utilized for customized tissue reconstruction as it is readily 3D printed. A critical design requirement for PCL devices is determining the in vivo bioresorption rate and the resulting change in device mechanics suited for target tissue repair applications. The primary challenge with meeting this requirement involves accurate prediction of degradation in the target tissues. PCL undergoes bulk hydrolytic degradation following first order kinetics until an 80-90 % drop in the starting number average molecular weight (Mn) after 2-3 years in vivo. However, initial polymer architecture, composite incorporation, manufacturing modality, device architecture, and target tissue can impact degradation. In vitro models do not fully capture device degradation, and the limited long-term (2-3 year) models primarily utilize subcutaneous implants. In this study, we investigate the degradation rate of 3D-printed airway support devices (ASDs) comprised of PCL and 4 % hydroxyapatite (HA) when implanted on Yucatan porcine tracheas for two years. After one year of degradation, we report a mass loss of less than 1 % and Mn reduction of 25 %. After two years, mass and Mn decreased by 10 % and 50 % respectively. These changes are accompanied by an increase in elastic modulus from 146.7 ± 5.2 MPa for freshly printed ASDs to 291.7 ± 16.0 MPa after one year and 362.5 ± 102.4 MPa after two years. Additionally, there was a decrease in yield strain, and increase in yield stress from implantation to 1-year (p < 0.001). Plastic strain completely diminished by two years, resulting in brittle failure at a yield stress of 12.5 MPa. The significantly lower rate of hydrolysis coupled with hydrolytic embrittlement indicates alternate degradation kinetics compared to subcutaneous models. Fitting a new model for degradation and predicting elastic and damage properties of this new degradation paradigm provide significant advancements for 3D-printed device design in clinical repair applications.