An investigation into laser sintering of nano-hydroxyapatite coated powders for bone tissue applications

Exploratory routes were undertaken to deposit Nanoparticles (NPs) such as nanohydroxyapatite (nHA) onto control substrate particles namely Polyamide 12 (PA12) and biodegradable particles such as Poly Lactic Acid (PLA). This was to exploit the benefits of additive manufacturing for the production of an osteoconductive nanocomposite for the potential use in bone tissue applications. This was achieved by adapting a previously developed method that was used to deposit multi-walled carbon nanotubes (MWCNT) onto PA12 particles by Bai et al. [1]. The advantages of using this coating method include its relative simplicity and the ability to produce large quantities of powder over a relatively short period. Furthermore, this method was shown to leave nanoparticles exposed on the surface of the final part without the need for post-processing, which may be highly advantageous for guiding cell-material interaction. The overall aim was to produce a nanocomposite which was suitable as a bone substitute material and served to further expand the current range of materials available for laser sintering, specifically for biomedical applications, and provide further insight into the effects that nanocoated powders can have on the laser sintering process. Nanoparticle loadings from 0.5 to 5.0 wt% were tested to establish the maximum surface coverage of the polymer particles. The resulting nanocoated powders were subject to morphological and physical examination using scanning electron microscopy, laser diffraction size analysis, powder rheometry and scanning differential calorimetry, to assess the suitability of the powders for laser sintering. The nanocoated powders were initially processed in a commercial (Formiga P100) laser sintering system operating at 10.6 μm. However, given the large volume of powder required for each test (minimum of 1-2 kg per run), a consumer (Sintratec) laser sintering machine was also used for the investigation. The laser wavelength used in the consumer system (445 nm) was not suitable for the processing of PA12 powders. Therefore, MWCNT (0.5 wt% for PA12 powders and 1.0 wt% for PLA powders) in addition to nHA was used to coat the powders to enable their processing. The PA12-nHA nanocomposites containing 0.5 wt% and 1.0 wt% nHA, which were processed on the commercial system, exhibited a significant increase in tensile modulus (18% and 13% respectively) and tensile strength (33% and 9% respectively) when compared to the neat PA12 parts. However, as the loading was increased from 1.0 to 1.5 wt% the presence of nHA on the surface of the polymer particles led to significant processing issues in both systems. Part warping became more pronounced as the nHA content was increased, which ultimately led to build failure as the parts were pulled off the powder bed by the re-coating mechanism. It was found that part warping could be reduced by decreasing the overall energy density imparted onto the powder bed from 0.11 J mm2 to 0.032 J mm2 for PA12-nHA powders in the commercial system and 0.19 J mm2 to 0.12 J mm2 for PA12-MWCNT-nHA powders in the consumer system. However, if lowered too much, the resulting parts had a high degree of porosity, poor handleability and poor overall mechanical properties. The PLA-MWCNT-nHA powders on the other hand displayed an opposite trend, requiring a higher energy density input (from 0.10 J mm2 to 0.19 J mm2) to prevent significant warping. However, parts produced were mechanically weak with poor handlability. As such, a heat treatment process was also investigated to improve the final part density of the PLA nanocomposites. This proved successful by substantially increasing the tensile modulus of the part by 44% for parts containing 2.0 wt% nHA and 327% for parts containing 5 wt% nHA. A post processing heat treatment may be an effective method for improving the consolidation of parts processed from powders which are sensitive to the effects of warping, and that have a narrow processing window. In-vitro cellular assessment of the nanocomposite materials, produced by laser sintering, indicated cytocompatibility after 7 days of direct culture. This was established using Alamar Blue and Hoechst DNA 33258 assays and cellular morphological assessment via scanning electron microscopy. Osteoconductivity analysis, via Alkaline Phosphatase (ALP) and Osteocalcin assays using an immortalised version of Human Mesenchymal Stem Cells (hiMSCs) cultured directly onto the nanocomposites, revealed an improvement in the osteoconductive response for materials containing the highest loading of nHA after 21 days. Thus, the powders investigated in this work were capable of being processed by laser sintering and the resulting nanocomposites were cytocompatible and demonstrated a higher bioactive potential when compared to the base polymer material