In vitro biological response of plasma electrolytically oxidized and plasma-sprayed hydroxyapatite coatings on Ti–6Al–4V alloy Wing Kiu Yeung, 1 Gwendolen C. Reilly, 1,2 Allan Matthews, 1 Aleksey Yerokhin 1 1 Department of Materials Science and Engineering, The University of Sheffield, Sheffield S1 3JD, UK 2 The Kroto Research Institute, North Campus, The University of Sheffield, Sheffield S3 7HQ, UK Received 6 August 2012; revised 20 December 2012; accepted 7 January 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.32899 Abstract: Plasma electrolytic oxidation (PEO) is a relatively new surface modification process that may be used as an alter- native to plasma spraying methods to confer bioactivity to Ti alloy implants. The aim of this study was to compare physical, chemical and biological surface characteristics of two coatings applied by PEO processes, containing different calcium phos- phate (CaP) and titanium dioxide phases, with a plasma- sprayed hydroxyapatite (HA) coating. Coating characteristics were examined by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, surface profilom- etry, and wettability tests. The biological properties were determined using the human osteoblastic cell line MG-63 to assess cell viability, calcium and collagen synthesis. The tests showed that PEO coatings are significantly more hydrophilic (6%) and have 78% lower surface roughness (Ra) than the plasma-sprayed coatings. Cell behavior was demonstrated to be strongly dependent on the phase composition and surface distribution of elements in the PEO coating. MG-63 viability for the TiO 2 -based PEO coating containing amorphous CaPs was significantly lower than that for the PEO coating containing crystalline HA and the plasma-sprayed coating. However, col- lagen synthesis on both the CaP and the TiO 2 PEO coatings was significantly higher (92% and 71%, respectively) than on the plasma-sprayed coating after 14 days. PEO has been dem- onstrated to be a promising method for coating of orthopedic implant surfaces. V C 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B:000–000, 2013. Key Words: plasma electrolytic oxidation, plasma spraying, osteoblast, hydroxyapatite, collagen, Ti–6Al–4V How to cite this article: Yeung WK, Reilly GC, Matthews A, Yerokhin A. 2013. In vitro biological response of plasma electrolytically oxidized and plasma-sprayed hydroxyapatite coatings on Ti–6Al–4V alloy. J Biomed Mater Res Part B 2013:00B:000–000. INTRODUCTION Ti alloys are widely used for orthopedic implants as they pos- sess good biocompatibility, chemical stability and excellent mechanical strength. 1–3 Plasma spraying of calcium phos- phate (CaP) based ceramics onto Ti substrates, with the aim of improving bone bonding, is a common procedure. 4,5 Although plasma-sprayed CaP can increase osseointegration, the coating adhesion to the metal substrate is relatively weak, which may lead to delamination. 6 Moreover, the high temperature melting and rapid solidification processes often lead to a change in the chemical structure of the coating. 7 A relatively new electrochemical surface treatment pro- cess known as plasma electrolytic oxidation (PEO) presents a novel approach to modify Ti surfaces. PEO provides a plasma assisted electrochemical conversion of metal surfa- ces into oxide ceramic layers. 8 It operates at anodic poten- tials above the dielectric breakdown voltage of the growing oxide film, producing a ceramic coating containing elements of the metal substrate and the electrolyte. 9–12 A porous sur- face morphology is formed due to the plasma discharge and gas liberation during the PEO process, 13–15 which provides a potentially favorable structure for cell adhesion and growth. Further enhancement of bioactivity can be achieved by incorporation of Ca and P into PEO coatings, which can be implemented through either sequential or hybrid proc- essing routes. In the former, a PEO treatment of Ti is fol- lowed by either direct deposition (e.g., electrophoretic 16 or cathodic 17 ) of hydroxyapatite (HA) or by chemical and/or thermal post-treatments 18–23 to increase the Ca content and promote HA crystallization of the CaP-containing amorphous component of the PEO coating. In the latter, crystalline CaPs are codeposited and incorporated into growing PEO coating in situ from electrolytes containing suspended HA nanopar- ticles 24,25 or soluble calcium and phosphate salts 26–29 using direct current (DC), 26–28 alternative current, or pulsed bipo- lar current (PBC) 29 PEO modes. The reverse current modes promote refinement of the coating morphology and facilitate precipitation of crystalline CaPs without the need to chelate Ca 2þ cations 30 or use electrolytes with a high Ca to P ratio (>2:1). This offers substantial benefits as the phosphate forming capacity of the electrolyte is known to increase with an increasing Me to P content ratio whereas the solu- tion stability decreases 31 ; however, the coatings produced by these methods are somewhat less studied. Correspondence to: A. Yerokhin; email: A.Yerokhin@sheffield.ac.uk Contract grant sponsors: UK EPSRC (WKY and AY) V C 2013 WILEY PERIODICALS, INC. 1