3D Printing of Polylactic Acid and Zinc Oxide Nanocomposites for Biomedical Applications

Poly(lactic acid) (PLA) is one of the most widely used biopolymers in biomedical applications, prized for its biodegradable and biocompatible properties. However, its usage in clinical practice is severely limited by its susceptibility to bacterial contamination. One promising solution involves combining PLA with antibacterial nanofillers, resulting in PLA nanocomposites that possess inherent antibacterial functionalities. Zinc oxide (ZnO) nanoparticles stand out due to their affordability and broad antibacterial spectrum, demonstrating effectiveness against hospital-acquired strains like Escherichia coli and Staphylococcus aureus. Yet, traditional manufacturing methods have constrained PLA-ZnO nanocomposites to simple applications, such as sutures, primarily due to challenges in producing complex geometries. To expand the biomedical applications of these materials, this research integrates additive manufacturing (AM, a.k.a. three-dimensional (3D) printing), specifically the material extrusion (MEX) technique known as fused filament fabrication (FFF or fused deposition modelling, FDM). This method is particularly efficient for creating intricate structures needed for biomedical devices, such as implants and scaffolds. The simplicity and low operational cost of FFF make it an accessible approach for producing PLA-ZnO nanocomposites that are both cost-effective and inherently resistant to bacterial contamination. Despite the advantages of AM, its adoption presents challenges that may undermine the viability of these nanocomposites in healthcare. A significant issue is the absence of standardised testing protocols for assessing the mechanical performance of additively manufactured polymers. This lack of benchmark makes it difficult to evaluate the mechanical properties of printed nanocomposites against existing materials, hindering their acceptance in the marketplace. Moreover, the interaction between ZnO and PLA is critical; ZnO can catalyse the degradation of PLA, especially under high temperatures and shear stresses. This degradation, which is exacerbated during the filament extrusion and FFF process (which involves multiple thermal cycles), can deteriorate the mechanical properties of the nanocomposites and alter their antibacterial efficacy and biocompatibility. Additionally, FFF parts are more porous than those made through conventional methods due to their layered construction, leading to a higher specific surface area that may affect the antibacterial response and cytotoxicity of the materials. Furthermore, existing literature primarily focuses on the antibacterial properties of PLA-ZnO nanocomposites while often neglecting their biocompatibility. Achieving antibacterial efficacy typically requires high filler loadings, which can increase cytotoxicity. The same mechanisms that confer antibacterial activity to ZnO nanoparticles may also contribute to potential cytotoxic effects, complicating the printability of the nanocomposites. Evidently, overcoming these challenges is fundamental to fully realise the potential of these additively manufactured nanocomposites and enable their acceptance for medical applications. Following on from the preambular chapters presenting a general introduction to the present research (Chapter 1), a thorough analysis of the state of the art (Chapter 2), and a detailed description of the applied methodologies (Chapter 3), Chapters 4-6 dive into the experimental activities conducted to address the challenges described in the paragraphs above. In Chapter 4, a case study centred on PLA, the benchmark material, aimed to establish a reliable method for measuring the anisotropic mechanical properties of the printed nanocomposites. The tensile properties were assessed using ASTM D638 and ASTM D3039, two widely recognized standards for FFF parts. The chapter addressed challenges with these standards, particularly the incompatibility of the dog-bone specimen geometry in ASTM D638 for FFF parts, and the absence of standardised specimen size and raster orientation in ASTM D3039. It was concluded that rectangular specimen geometries would be more effective than dog-bone shapes for accurately evaluating the tensile behaviour of FFF parts in this research. Overall, the preliminary experiments highlighted the necessity of reporting specific setup and printing parameters to ensure test repeatability, even when adhering to international standards. In Chapter 5, the feasibility of using FFF for producing PLA-ZnO nanocomposites was explored through three strategies to reduce PLA thermal degradation: master-batching (pre-mixing high ZnO concentrations with PLA), silane treatment of ZnO nanoparticles, and precise adjustment of ZnO loading (1-5 wt.%). The chapter detailed the entire manufacturing process, highlighting that accelerated matrix degradation from ZnO posed a significant challenge. However, employing melt-mixing improved processability, while keeping filler loading below 2 wt.% and treating ZnO with silane proved effective. Notably, FFF printing did not significantly affect thermal stability, with filament extrusion identified as the main challenge. Overall, the research confirmed the viability of using FFF to create complex geometries with stable mechanical properties. Finally, in Chapter 6, the experimental campaign assessed the safe adoption of the additively manufactured PLA-ZnO nanocomposites in clinical settings. This involved evaluating their degradation rates in various media, along with their antibacterial properties and cytotoxicity. Remarkably, the nanocomposites demonstrated over 99% bacterial reduction, even at filler loadings lower than those typically reported in the literature. Additionally, the nanocomposites with the lowest filler loading exhibited biocompatibility comparable to neat PLA. Thus, the antibacterial functionalities were confirmed to be retained, and potentially enhanced, post-printing, likely due to the porous hierarchical structure of FFF parts, which facilitated water penetration. This directly increased the degradation rates of the matrix, leading to higher concentrations of ZnO and Zn²⁺, both critical for the antibacterial response of the nanocomposites. The ability to achieve excellent antibacterial functionality at low filler loadings is advantageous for maintaining biocompatibility. Low filler loadings also reduce the extent of ZnO-induced degradation of the PLA matrix. Furthermore, the silane treatment did not adversely affect the nanocomposites' antibacterial or biocompatibility profiles. Collectively, these findings confirm the suitability of PLA-ZnO nanocomposites for producing non-toxic, antibacterial biomedical devices through additive manufacturing. In conclusion, this research is pivotal for advancing the use of FFF in producing antibacterial and biocompatible PLA-ZnO nanocomposites, facilitating broader biomedical applications. Ultimately, by addressing mechanical, processing, and biological challenges, this work paves the way for the integration of these nanocomposites into clinical settings.<p></p>