**1. Introduction**

For many years, 316L stainless steel has been widely used for orthopedic and orthodontic applications due to its excellent mechanical properties, good biocompatibility, and high corrosion resistance [1,2]. Metallic implants themselves are not bioactive; therefore, medical doctors and scientists are always concerned with achieving acceptable integration of implants into living bone. The main problem with metallic implants is the possible formation of fibrous tissue around the gues<sup>t</sup> body implanted into bone [3]. Fibrous tissue formation is caused by the absorption of protein and other organic molecules on the hydrophobic surface of metals. Hydrophobic surfaces attract these organic molecules with enhanced formation of the above-mentioned biofilms [4,5]. It was suggested that formation of protein capsules may cause inflammation at the implant-bone interface, followed by the rejection of an implant [6]. Therefore, modified surfaces with antibacterial properties have been developed [7–9].

Implants are foreign bodies, and therefore, surface morphology and other properties have an impact on the behavior of bone cells that come into contact with implants [10,11]. Of course, the immune system of an individual patient is very important for the response to implanting biomaterials [12]. Bioglasses and other bioceramics such as calcium phosphates bond to natural bone and support the proliferation of new cells [2,13–15]. Surface wettability enhances osteoblast adhesion of human cells and also cell proliferation [16–18]. The problems associated with the application of metallic implants might be solved by the formation of calcium hydroxyapatite (Ca10(PO4)6(OH)2; CHA) on the surface [19,20]. CHA is a well-known biomaterial for its bioactivity, biocompatibility, and ability to induce growth of a new bone tissue. After implantation of a ceramic-coated, metal implant, the CHA forms strong chemical bonds with natural bone tissue due to its structural and chemical similarity to the bone [21,22].

Application of biphasic calcium phosphate in biocomposites or hydroxyapatite showed no fibrous encapsulation of the particles and composites after implantation allowing further infiltration of cells within the sample implants [23–25].

Previously we described sol-gel chemistry processing for the preparation of calcium hydroxyapatite films [26–28]. The CHA thin films were fabricated on substrates including silicon, quartz, and modified titanium. Both dip-coating and spin-coating techniques were compared. The same sol-gel method was used for the synthesis of CHA coatings on stainless steel substrates [29]. The quality of thin films obtained using the spin-coating technique was slightly better than when using dip-coating. However, the adhesion of CHA coatings obtained by both processes were less than desired. Recently, the e ffects of surface roughness on the adhesive properties of thin films were discussed and investigated [30]. In our study, the CHA thin films were applied to roughened stainless steel substrates using the spin-coating technique. Transverse and longitudinal roughness was patterned on the metal. The bioactivity of sol-gel derived calcium hydroxyapatite coatings in simulated body fluid are presented.
