Hydrolytic degradation of polylactide in extrusion additive manufacturing

The combined use of material extrusion additive manufacturing (MEAM) and biodegradable polymers such as polylactide (PLA) is one of the most versatile and valuable manufacturing strategies for biomedical applications. MEAM enables rapid production of personalised PLA medical devices as they degrade by hydrolysis over a period of months or years. Although MEAM presents a range of opportunities, there are a number of limitations, the most critical of which is mechanical anisotropy, specifically low strength in the direction normal to the print platform (Z direction). This limits its use for long-term mechanical application. Numerous studies have attributed the diffusion of the polymer chains across the interface between layers as the main underlying mechanism of mechanical anisotropy. However, attempts to understand mechanical anisotropy of MEAM parts have resulted in considerable inconsistencies, with no consensus on the degree of anisotropy or its dependency on printing parameters.

In this thesis, experimental studies describe the development of a new microscale uniaxial tensile specimen, based on the idea of COntinuously Varied EXtrusion (CONVEX) by direct GCode scripting to reduce geometrical complexities of current testing standards and to enable improved manufacturing control. The newly devised PLA specimen comprised of stacked individual extruded filaments enabled an improved fundamental analysis of extruded filaments (F specimens, representing bulk-material properties when extruded filament printed along the print platform) and the interface (Z specimens when extruded filament printed normal to the print platform) between them. Geometrical analysis of specimens by microscopy allowed accurate cross-sectional area measurements to be used in strength calculations and generated new understanding about the effect of testing orientation on mechanical properties of MEAM parts. Mechanical and thermal characterisations of both specimen types were conducted to consider the effects of physiological temperature (PT), hydration and in-aqua testing against the control (non-hydrated specimens tested in air at room temperature). Mechanical studies showed bulk-material bond strength between layers. The filament-scale geometries in Z specimens (i.e. grooves between layers) were responsible for strain concentrations and significantly reducing strain at fracture and toughness. In contrast, for F specimens, the grooves were aligned in the direction of loading and did not impact mechanical properties. Furthermore, the importance of submerged tests at PT for PLA was confirmed by demonstrating a combined plasticisation effect of water and higher temperature, highlighting an important risk of conventional laboratory testing overestimating properties by two-fold. The testing environment has a similar effect on both F and Z specimens. Moreover, during long-term hydrolytic degradation experiments, it was found that the interface degraded in a similar manner to the bulk polymer material. Comparison of thermal and chemical properties revealed that during the early stage of hydrolytic degradation, crystallinity was the dominating factor, whilst at later stages, mechanical properties were mainly defined by the molecular weight.

The new understanding developed in this thesis highlights that for MEAM parts, the interface does not affect its long-term properties. This improves confidence in using the MEAM process for high-value applications.