Optimisation of Plasma Coatings for Bone-Replacement Scaffolds Using Mesenchymal Stem Cells.

Musculoskeletal disease is a leading cause of morbidity across the world, associated with pain, immobility, deformities and in some cases, death. Disease prevalence increases with age. In the coming years, the incidence of disorders affecting the skeleton will rise, causing huge healthcare and socioeconomic burden. Current treatments are typically restricted to pain management followed by end-stage total joint replacement. Cell-based therapies are an appealing biological option in orthopaedics, as they may provide long-lasting restoration of skeletal tissue function by exploiting the intrinsic capacity of mesenchymal stem cells (MSCs) to differentiate into bone and cartilage, often in association with a biomimetic support material to enable 3D reconstruction.

Although many materials have been investigated to enhance bone repair, none have seen as much clinical use as the calcium phosphates, the most prominent being hydroxyapatite (HA). This material has been reported as both osteoconductive (support osteoprogenitors and new bone deposition) and potentially osteoinductive (stimulate differentiation of osteoprogenitors towards bone-forming osteoblasts). This cellular response to HA scaffolds or HA-coated implants has been demonstrated in vitro and in vivo using MSCs and terminally differentiated osteoblasts. MSCs are a multipotent progenitor cell population found in many tissues throughout the body including bone marrow; they can be isolated and induced to differentiate into many different cell types, including osteoblasts.

The inherent variability in HA scaffold/coating production leads to large variation in the final material characteristics. This is particularly important as cell responses to these material surfaces have been demonstrated to rely on multiple factors. These include surface topography, (micro)structure and composition, which are in some part determined by initial production process variables (such as time, temperature, feed rate). Whilst recent developments in computational topography and solid free-form fabrication make it possible to generate components with control over gross architecture, allowing patient-specific design, the determination of the optimal conditions for cellular responses (e.g. attachment, proliferation, differentiation) still relies on inefficient, time-consuming and expensive assays commonly using a 'One Factor At a Time' (OFAT) approach. This process does not account for the significant between-factor interactions within the chosen process, nor the influence of these factor variables on the cellular response.

The purpose of this study is to implement statistical Design of Experiments (DOE) techniques using multivariate analysis to assess multiple input parameter effects on osteogenic responses of MSCs to HA in parallel experimental runs; therefore avoiding confounding results due to between-factor interactions of traditional OFAT approaches. In this instance the experimental methodology will be implemented around the HA plasma spraying of orthopaedic implant devices, a complex production process known to enhance cellular responses and bone growth, but have a large degree of variation in relation to input parameters that will influence clinical effect.
Specifically, we propose a 4-factor DOE screening of HA plasma-coating process input parameters in conjunction with assays of osteogenic capacity. This will allow rapid screening of multiple HA material output parameters and their effects at a cellular level. Leading HA formulations will be taken forward for further analysis of osteogenic induction and mechanism of action. This proposal will rely equally on cross-institutional expertise in materials science (Leeds, DePuy), process engineering, DOE screening (DePuy), MSC biology, cell differentiation and cell-biomaterial interaction (York), making it a truly interdisciplinary approach to expedite optimised biomimetic scaffold development for clinical applications.