Logan, NJ; (2016) Strategies to improve the bioactivity of cobalt chromium molybdenum using surface modifcation techniques. Doctoral thesis , UCL (University College London).

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Abstract

Titanium and its alloys are widely accepted as the current gold standard for orthopaedic and dental applications due to its good biocompatibility. Other materials, such as the cobalt alloy, cobalt chromium molybdenum (CoCrMo), can often be preferred due to its mechanical strength and resistance to wear, but despite these advantageous properties, CoCrMo does not have the biocompatibility of titanium alloys. This project examined novel strategies of enhancing the biocompatibility of CoCrMo. Firstly this involved coating the surface of CoCrMo in a thin, durable, layer of anatase titanium oxide (TiO2) using atmospheric pressure chemical vapour deposition (CVD). TiO2 is the same oxide layer as found on titanium and its alloys, and consequently, markers of osteogenic differentiation and adhesion were enhanced (p < 0.05) in human mesenchymal stem cells on TiO2 surfaces in vitro. This early data implied that there was an enhanced rate of bone formation occurring on TiO2 coated surfaces. Following this, functionalization of the TiO2 layer using a process called ultraviolet photofunctionalization was investigated. Whilst the majority of cellular processes were not affected, markers of cell adhesion were significantly improved on functionalised surfaces. Ultraviolet photofunctionalization of the TiO2 coated CoCrMo caused the substrates to transition from hydrophobic to superhydrophilic, which resulted in enhanced cell retention due to larger cells with increased actin and vinculin expression (p < 0.05). The topographical effect of CoCrMo was then examined by creating four topographies, ranging from smooth to moderately rough (SMO, AE, SLA50, SLA250). It was found that the moderately rough SLA250 surface was advantageous for osteogenic applications, by enhancing the osteogenic differentiation and retention of human mesenchymal stem cells (p < 0.05). Lastly, the combination of the topographically modified SLA250 surface and the TiO2 coating was investigated. It was of interest to ascertain if the TiO2 CoCrMo SLA surface could stimulate a similar cellular response to titanium SLA. It was found that a large amount of cellular processes were similar between all groups, although markers of osteogenic differentiation were significantly enhanced on the TiO2 SLA CoCrMo surface to a comparable level as seen on titanium SLA (p < 0.05). This data implies that the application of the TiO2 SLA surface to implants formed of CoCrMo, could promote enhanced bone formation and ultimately lead to reduced healing times following surgery and increased implant stability.

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