Development of borate glass-reinforced hydroxyapatite bioceramic powders – selective laser sintering, a feasibility study

Softas, Christos (2023). Development of borate glass-reinforced hydroxyapatite bioceramic powders – selective laser sintering, a feasibility study. University of Birmingham. Ph.D.

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Abstract

Over the past three decades, composite biomaterials comprising of hydroxyapatite (HA) as the main phase and bioactive glasses (BG) as the second phase have gained increased attention. This class of materials, glass reinforced hydroxyapatite composites (GR-HA), exhibits superior mechanical and biological performance compared to pure HA. Since the emergence of the topic in the mid-90s, mainly phosphate and silicate-based GR-HA systems were investigated. Two common issues concerning the utilisation of these glass systems are crystallisation and HA decomposition phenomena that can be deleterious both in terms of bioactivity and mechanical performance. Considerably fewer studies utilising borate-based glasses in GR-HA composites have been carried out besides their biocompatibility and enhanced thermal stability.

This work focused on the development of a novel GR-HA feedstock, resistant to crystallisation and HA dehydroxylation phenomena, utilising a borate-based glass system, 50B\(_2\)O\(_3\)-20SiO\(_2\)-20Na\(_2\)O-(10-x)CaO-xLa\(_2\)O\(_3\) (0 ≤ x ≤ 2.5). A parametric study investigating the optimal composition and processing parameters, such as glass content (0 – 10 wt.%) and sintering temperature (1200 °C – 1300 °C) was carried out; in tandem the effect of lanthanum oxide was studied, too. 10 wt.% was identified as the optimal glass loading since no thermal degradation was observed via X-ray diffraction (XRD). Additionally, enhanced densification and overall mechanical performance was recorded via microcomputed tomography (μCT), density and microhardness measurements. Small glass additions (≤ 5 wt.%) acted as heterogeneous nuclei sites or triggering points that induced limited HA dehydroxylation products, accruing for less than 27% of the total composition. The main secondary phase detected was beta tricalcium phosphate (β-TCP) that reverted to alpha tricalcium phosphate (α-TCP) with increasing temperature. No antimicrobial efficacy was detected against S. Epidermidis or P. Aeruginosa, regardless of the glass content. No cytotoxic behaviour was recorded during SaoS-2 human osteosarcoma cell attachment studies. GR-HA specimens containing 10 wt.% of glass content exhibited enhanced cell viability compared to pure HA specimens, while similar or slightly worse biological performance was recorded for composites containing smaller additions of glass. Lastly, there are strong indications that lanthanum oxide potentially enhances the overall mechanical and biological performance of GR-HA samples. The samples containing the highest amount of lanthanum oxide content, exhibited the best performance in terms of densification, microhardness and cell culture studies.

In parallel, the utilisation of the most promising GR-HA composition was investigated as a powder feedstock during a selective laser sintering (SLS) feasibility study. The novel GR-HA feedstock containing 10 wt.% of 50B\(_2\)O\(_3\)-20SiO\(_2\)-20Na\(_2\)O-7.5CaO-2.5La\(_2\)O\(_3\) exhibited better processability and thermal stability compared to pure HA during SLS processing. Two different geometries were successfully printed with the glass containing powder feedstock compared to one geometry with its pure HA counterpart. Moreover, miniscule traces of secondary phases were recorded in the as-printed GR-HA specimens (< 2%), whereas significant thermal degradation was observed in the pure HA samples (~ 40% of the total composition). A post-processing heat treatment cycle was successful in partially reversing any hydroxyapatite dehydroxylation products to HA and enhanced densification.

The developed GR-HA system containing 10 wt.% of the novel borate-based glass composition, 50B\(_2\)O\(_3\)-20SiO\(_2\)-20Na\(_2\)O-(10-x)CaO-xLa\(_2\)O\(_3\) (0 ≤ x ≤ 2.5), is a promising biomaterial for tissue engineering applications. No crystallisation or HA thermal degradation were observed during heat treatment (conventional sintering or SLS processing), while the GR-HA composites exhibited enhanced biological performance compared to pure HA samples.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):