Design, manufacturing and characterisation of biodegradable polylactic acid by fused filament fabrication

Polylactic acids (PLAs) are biodegradable, biocompatible and non-toxic biopolymer utilised for medical applications with good mechanical properties. Through the use of Additive Manufacturing (AM) technologies, PLAs can be deposited layer-upon-layer via Fused Filament Fabrication (FFF) to manufacture patient-tailored biomedical implants. Such devices are available in a range of sizes and thicknesses, with smaller devices capable of being realised via FFF in just a few layers. It is known that the properties of different printed layers vary because they have different thermal histories. Therefore, the principal aim of this PhD thesis was to evaluate the mechanical performance of PLAs manufactured by FFF for different numbers of layers via in vitro degradation at physiological temperature.

The mechanical performance of printed and annealed PLAs was characterised using a tensile testing machine. This was accompanied with analysis including gel permeation chromatography (GPC) to ascertain the molecular weight and differential scanning calorimetry (DSC) to determine thermal properties such as degree of crystallinity and glass transition temperature. Additionally, specimens were subjected to in vitro degradation at physiological temperature (37 oC) in phosphate-buffered solution at pH 7.4, to simulate conditions of the human body. The in vitro degradation test was performed at different time points and properties were analysed using the same testing methodology as for printed and annealed specimens. Mass change and water intake of specimens were calculated using an electronic balance. Degradation was also studied at elevated temperatures including 50 oC (below glass transition temperature) and 65 oC (above glass transition temperature) to compare against degradation behaviour at the physiological temperature.

Three different designs of tensile test specimens were evaluated, with an ISO 527-2 type 5A tensile test specimen design considered the most suitable for further investigation due to enhanced levels of mechanical performance repeatability during tensile testing. Specimens were printed using ISO 527-2 type 5A with different numbers of layers including 1, 2, 3, 4, 5, 7, and 10 layers. It was found that the molecular weight, thermal and mechanical properties differed with an increasing number of layers until 5-layer, but properties did not alter significantly above 5-layer. Therefore, 1-, 3- and 5-layer printed specimens were chosen and subjected to degradation at 37 oC. It was discovered that the shape substantially distorted during degradation. Therefore, an annealing method was applied with various temperatures and times, which enhanced the shape stability. During degradation at 37 oC, it was found that the properties of specimens were slightly different due to their initial properties and size effects. By considering the degradation at elevated temperatures, it was revealed that the change in properties was similar to degradation at 37 oC therefore supporting the use of an accelerated degradation methodology at 50 oC and 65 oC.

In summary, the data obtained within this thesis is a protocol study for the degradation of PLAs printed across different number of layers. The results demonstrated that degradation at elevated temperature is an acceptable method. Therefore, the insights and outcomes from this research could benefit future research activities to understand the effect of different process parameters of FFF on degradation behaviour.