In vitro and in vivo bioactivity of CoBlast hydroxyapatite coating and the effect of impaction on its osteoconductivity Fei Tan a, b, c , Mariam Naciri a, b , Denis Dowling d , Mohamed Al-Rubeai a, b, ⁎ a School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland b Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland c School of Medicine and Medical Science, University College Dublin, Belfield, Dublin 4, Ireland d School of Electrical, Electronic and Mechanical Engineering, University College Dublin, Belfield, Dublin 4, Ireland abstract article info Article history: Received 24 February 2011 Received in revised form 26 April 2011 Accepted 13 July 2011 Available online 23 July 2011 Keywords: Biomaterial Hydroxyapatite coating CoBlast Mesenchymal stem cells Osteoconduction Osseointegration mRNA PCR The novel non-thermal CoBlast process has been used recently to create a hydroxyapatite coating on metallic substrates with improved biological response compared to an uncoated implant. In this study, we compared the biological effect of coatings deposited by this process and the industrial standard technique — plasma- spray. Physicochemical properties of these two coatings have been found to be significantly different in that CoBlast HA is less rough but more hydrophilic than the plasma-spray HA as evidenced by data obtained from profilometry and goniometry. Mesenchymal stem cell attachment and adhesion are enhanced on CoBlast HA. Analysis by a combination of EDX and ICP suggests that the higher crystallinity retained by the CoBlast HA result in slower coating dissolution. Detailed in vitro evaluation reveals that plasma-spray HA might induce slightly faster cell proliferation and earlier osteogenic differentiation, but CoBlast HA becomes equivalent to it by the late osteogenic stage. PCR array facilitated the identification of differentially regulated genes involved in various functional aspects of in vitro osteogenesis by the CoBlast HA coating. The expression level of the functional protein products of these genes are in agreement with the PCR data. Coating metallic screws with HA significantly improves the in vivo osseointegration. By measuring of removal force using torque measurement instrument and analyzing the patterns found in X-ray images it is demonstrated that the two HA coatings elicit comparable osseointegration. Using simulated impaction model, CoBlast HA is shown to maintain better performance in cell attachment and mineralization than plasma-spray HA, especially following significant impactions. This might indicate a potentially greater osteoconductivity of CoBlast HA coating in shear-stress associated surgical applications. Collectively, it was demonstrated that CoBlast HA is an effective alternative to plasma-spray HA coating and a promising replacement for specialized surgical applications. © 2011 Elsevier Inc. All rights reserved. 1. Introduction Hydroxyapatite (HA), being chemically similar to the inorganic component of bone mineral, is one of the most popularly used bioactive ceramics in the surgical repair of hard tissue trauma and disease (Paital and Dahotre, 2009). HA's successful applications have been witnessed in a range of surgical specialties: bone substitute in bony defects restoration in orthopaedic surgery (Koshino et al., 2001), sinus obliteration (Moeller et al., 2010) and ossicular chain recon- struction (Pasha et al., 2000) in otolaryngological surgery, as well as craniofacial augmentation in plastic surgery (Quatela and Chow, 2008). In addition, HA has been extensively used as a thin film coating on titanium (Ti) alloys in load-bearing scenarios (Jaffe and Scott, 1996) and shear stress-susceptible applications (Sandén et al., 2002). The underlying rationale is to combine the high strength/weight ratio of the metallic alloy and the osteoconductivity and dissolubility of HA to achieve improved osseointegration. The outcome would be accelerated fixation of the implant by the adjacent newly formed bone tissue (Landor et al., 2007). Although various well-studied techniques exist to deposit HA onto Ti alloy substrate, plasma thermal spraying has been the industrial benchmark process owing to its high deposition rate, good biocorrosion resistance and substrate fatigue resistance of the coating, and capability to obtain various coating thickness (Sun et al., 2001). Nevertheless, the high thermal energy utilized in the plasma-spray process is its main drawback as described in the following series of events: (1) inevitable and unadjustable precipitation of crystal phases such as tricalcium phosphate (TCP) and tetracalcium phosphate (TTCP), (2) decreased crystallinity resulted in increased solubility of the coating, and (3) separation of the coating and possibly unsatisfactory in vivo bone fixation (Sun et al., 2003; Xue et al., 2004). Furthermore, the high temperature encountered Biotechnology Advances 30 (2012) 352–362 ⁎ Corresponding author at: School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland. Tel.: +353 1 7161862; fax: +353 1 716 1177. E-mail address: m.al-rubeai@ucd.ie (M. Al-Rubeai). 0734-9750/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2011.07.008 Contents lists available at ScienceDirect Biotechnology Advances journal homepage: www.elsevier.com/locate/biotechadv