Titania Nanotopography for Control of Cell Response to Bioactive Calcium Phosphate Coatings

Abstract

Titanium and its alloys are used extensively in the field of orthopaedic implants and are commonly coated with calcium phosphate (CaP) materials, normally hydroxyapatite (HA), to improve their bioactivity and osseointegration. In general, these coatings are designed to be stable in vivo thereby promoting an osteoconductive response. Radio frequency (RF) magnetron sputtering offers the opportunity to deposit CaP coatings with a range of chemical properties using HA and substituted HA materials at relatively low processing temperatures, compared to those for commercially applied plasma spraying. In the as-deposited (sputtered) state, these CaP coatings are amorphous and highly soluble, dissolving rapidly in aqueous solution. Control of this inherent solubility can lead to enhanced bioactivity in the surrounding environment which offers the means to engender an osteoinductive effect. Roughened titanium surfaces have been shown to offer a means to control the dissolution of CaP coatings thereon, thereby avoiding the need for thermal annealing in this regard. However, most of the processing techniques employed to date are expensive and time consuming. In this work, chemical etching of titanium surfaces to create a roughened pitted morphology capable of controlling as-deposited CaP is investigated. The resulting topography offers control of dissolution of as-deposited sputtered CaP thin films created from HA and strontium substituted hydroxyapatite (SrHA) targets, which has been determined and compared with that of a pristine titanium control and sputter deposited polycrystalline titanium layer. Detailed surface characterisation using XPS, ToFSIMS and XRD indicated that conformal thin films are produced on each substrate surface, which although removed in part are still present as a continuous layer after 7-days in aqueous culture media. In vitro studies of HA coated etched titanium showed increased cell adhesion and proliferation as seen from MTT and PicoGreen™ assay data from cells on the chemically etched surfaces compared to those on the control and comparator system. Cells on the etched surfaces were found to present with a higher coverage and had morphology indicative of pronounced surface interaction when imaged using SEM. Although still chemically intact at 7-days exposure to media, the cell culture studies for SrHA coated surfaces showed the presence of much fewer cells compared to both the HA coated and uncoated substrates. It is proposed that this is due to the higher solubility of the SrHA which causes a high degree of instability on these surfaces leading to cells being unable to adhere. A 28-day culture study confirms that morphology, with and without a HA coating, supports U-2 OS differentiation as measured by ALP and BCA assay measurements, and Alizarin red staining of excreted calcium.

Date of Award	Sept 2020
Original language	English
Supervisor	George Burke (Supervisor), Adrian Boyd (Supervisor) & Brian Meenan (Supervisor)