Rapid coating of AZ31 magnesium alloy with calcium deficient hydroxyapatite using microwave energy Yufu Ren a, ⁎, Huan Zhou a,b , Maryam Nabiyouni c , Sarit B. Bhaduri a,d a Department of Mechanical, Industrial and Manufacturing Engineering, The University of Toledo, Toledo, OH, USA b Institute of Biomedical Engineering and Health Sciences, Changzhou University, Changzhou, Jiangsu, China c Department of Bioengineering, The University of Toledo, Toledo, OH, USA d Division of Dentistry, The University of Toledo, Toledo, OH, USA abstract article info Article history: Received 4 September 2014 Received in revised form 6 December 2014 Accepted 8 January 2015 Available online 10 January 2015 Keywords: AZ31 magnesium alloy Calcium deficient hydroxyapatite coating Biocompatible coating Microwave Due to their unique biodegradability, magnesium alloys have been recognized as suitable metallic implant mate- rials for degradable bone implants and bioresorbable cardiovascular stents. However, the extremely high degra- dation rate of magnesium alloys in physiological environment has restricted its practical application. This paper reports the use of a novel microwave assisted coating technology to improve the in vitro corrosion resistance and biocompatibility of Mg alloy AZ31. Results indicate that a dense calcium deficient hydroxyapatite (CDHA) layer was uniformly coated on a AZ31 substrate in less than 10 min. Weight loss measurement and SEM were used to evaluate corrosion behaviors in vitro of coated samples and of non-coated samples. It was seen that CDHA coatings remarkably reduced the mass loss of AZ31 alloy after 7 days of immersion in SBF. In addition, the prompt precipitation of bone-like apatite layer on the sample surface during immersion demonstrated a good bioactivity of the CDHA coatings. Proliferation of osteoblast cells was promoted in 5 days of incubation, which indicated that the CDHA coatings could improve the cytocompatibility of the AZ31 alloy. All the results suggest that the CDHA coatings, serving as a protective layer, can enhance the corrosion resistance and biological response of magne- sium alloys. Furthermore, this microwave assisted coating technology could be a promising method for rapid sur- face modification of biomedical materials. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Recently, magnesium and its biodegradable alloys have been exten- sively studied as revolutionary implant materials due to their unique properties such as high strength/weight ratio, comparable mechanical properties with natural bone and excellent biocompatibility [1–5]. Fur- thermore, the Mg 2+ ion has been demonstrated to stimulate initial os- teogenesis and promote new bone formation [6]. However, Mg and its alloys degrade much too quickly in physiological environment due to the high concentration of chloride ions [1,7,8]. Moreover, hydrogen gas evolved as a corrosion by-product may cause problems against the surrounding tissues, thus hindering the attachment of osteoblasts to the implant surface [9]. To successfully employ these alloys, some kind of protective coating is desirable. To date, there have been many technologies developed to produce such coatings on magnesium and its alloys [10–12]. The most common methods are electroplating, conversion coating, anodizing, or- ganic coating and thermal spraying [13–17]. Although electroless nickel plating and chromate conversion coatings have shown promising corro- sion resistance, the toxicity in the nature of these treatment processes restricts their application in the human body. Oxide films produced by anodizing coatings have shown adverse results in fatigue behavior of substrate and poor biocompatibility [18,19]. Furthermore, pinholes and blisters formed in organic polymer coating processes and inherent porosity produced by thermal spraying facilitate galvanic corrosion be- tween the porous coating and magnesium substrate [10]. Besides the disadvantages addressed above, poor biocompatibility of the deposited coating is another shortcoming in these techniques [10]. Therefore, re- cent publications have focused on bioactive ceramic coatings such as calcium phosphate coatings, which presented good corrosion resistance as well as outstanding biocompatibility and bioactivity [5,8,20–22]. Calcium phosphates (CaP), especially hydroxyapatite/calcium defi- cient hydroxyapatite (HA/CDHA, Ca 10 (PO 4 ) 6 (OH) 2 ) have been broadly used as biomedical materials for their chemical similarity to bone min- eral. In addition, CaP exhibits non-toxicity, high biocompatibility and superior ability to induce physicochemical bonds between the implant and cortical bone i.e. osseointegration [23–26]. To date, several forms of calcium phosphate coating have been developed on magnesium and its alloys by various techniques, such as chemical precipitation [19,27–32], electrodeposition [33–35], sol–gel [36] and hydrothermal method [37,38]. For example, Yang et al. [29] produced hydroxyapatite (HA) coatings on AZ31 alloy by immersing the AZ31 substrate in super- saturated calcification solutions and applying heat treatments afterwards. Materials Science and Engineering C 49 (2015) 364–372 ⁎ Corresponding author. E-mail address: Yufu.Ren@rockets.utoledo.edu (Y. Ren). http://dx.doi.org/10.1016/j.msec.2015.01.046 0928-4931/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec