Effect of environment on mechanical properties of 3D printed polylactide for biomedical applications

In this study, the importance of the testing environment for correct assessment of tensile strength of polylactide (PLA) is investigated. A novel design of tensile specimen was developed to test the anisotropic mechanical properties of additively manufactured specimens. The effects of three environmental factors were considered: physiological temperature (37 °C), hydration (specimens stored in solution for 48 h) and in-aqua testing (specimens submerged in solution). For the first time, these factors were studied both individually and combined, and were evaluated against a control point (non-hydrated specimens tested in air at room temperature). The tensile strength and elastic modulus of hydrated specimens tested submerged at 37 °C were reduced by 50.1% and 20.3%, respectively, versus the control. In contrast, testing the hydrated polymer in air at room temperature, which is commonly used to refer to wet strength in literature, only showed an 18.3% reduction in tensile strength with a negligible change in elastic modulus. To assess transferability of the results, additively manufactured specimens were also tested normal to the interface between 3D printed layers, and they demonstrated similar reductions in strengths and moduli. The results demonstrate the importance of using an appropriate methodology for tensile testing; otherwise, mechanical properties may be overestimated by two-fold.