Optimisation of plasma sprayed hydroxyapatite coatings using a design of experiments approach

Hip revision surgery, where a failed hip replacement is replaced and damage is repaired, presents a significant socioeconomic burden. Improved fixation and longevity of hip stems can alleviate this problem, thereby improving patient quality of life. This is achieved by modulating hydroxyapatite (HA) surfaces to provide a more favourable environment for regeneration in situ by resident mesenchymal stromal cells (MSCs). By changing the variables that go into application of the HA surface by plasma spraying, material properties can be changed and a model to predict material changes can be created using Design of Experiments (DoE). Plasma spraying is a highly complex, multifactorial technique working on the principle of introducing HA powder into a high temperature plasma jet, propelling molten and partially molten particles of HA onto a substrate. Five factors (A-E) were explored using a DoE approach to assess the effect of each on crystallinity and roughness. A model of crystallinity and roughness response to changing factor settings was created, allowing for prediction of material properties in response to changing plasma sprayer settings. MSC response to different surfaces was assessed in terms of proliferation, osteogenic response, and Wnt signalling. Depending on the surface, Wnt signalling and proliferation can be enhanced, and HA can have osteoinductive properties. Optimal plasma sprayer settings for maximising proliferation, osteogenic response, and Wnt signalling were determined and material properties for each predicted, enabling the production of HA-coated hip stems with fine-tuneable MSC responses and material properties. The use of foetal bovine serum (FBS) in biomaterial studies brings with it ethical and scientific issues, chiefly among them the undefined nature of protein adsorption to biomaterial surfaces. A chemically defined medium was developed using DoE, which supported growth of MSCs on biomaterial surfaces, as well MSC expansion and extracellular vesicle production in a clinical grade perfusion bioreactor.