Orthopaedic implants are used to restore the structure and function of damaged or diseased parts of bones or joints with the aim of improving the life quality of individuals. Orthopaedic implants intended to be used under mechanical loads should have favourable mechanical strength and long-term stability [
]. Metallic implants usually have favourable mechanical properties, for example Co–Cr alloys, stainless steel, titanium and its alloys are normally used in orthopaedic implants [
]. Although they are mechanically resilient, dissolution of metal ions from them into the body may cause inflammation around the metal implant [
]. Therefore, surface modification of metallic implants should be considered, which should improve the interfacial contact of metallic implants with surrounding tissues. In a common approach, the implants are coated with bioactive materials [
]. Common bioactive materials considered to coat metal surfaces are bioactive ceramics, such as hydroxyapatite and bioactive glasses (BGs) [
]. The applied (deposited) coatings on implants should provide various positive characteristics such as corrosion resistance, wear resistance, antibacterial properties, and they should support bone cell adhesion [
Bioactive glass (BG), invented by Hench et al. [
], facilitates the formation of a strong bond between implant and bone via the in-situ formation of a hydroxyapatite (HAp) layer similar to the mineral phase of bone. Bioactive glasses are prepared using melt quenching and sol-gel techniques [
]. Sol-gel derived glasses offer versatility in composition, size and morphology. The high surface area of sol-gel derived BGs favours the formation of an apatite layer thus leading to the effective contact between the implant and bone [
]. BG can be further modified by incorporating several biologically active metallic ions such as Cu, Ag, Au, Sr and Mn [
]. The doping of BGs with Ag and Cu ions is a well-known approach for providing potent antibacterial effect against a wide spectrum of bacteria. Nevertheless, the uncontrolled release of silver and copper ions can be toxic for osteoblast cells, which negates the mentioned benefitial antibacterial effect making the materials non-suitable for biomedical applications [
Despite the fact that BGs can provide excellent bone binding properties, BGs cannot be used in load bearing applications due to their brittle nature. Therefore, BGs are often used in combination with various synthetic polymers such as polyether ether ketone (PEEK), polymethyl methacrylate or with natural polymers (chitosan, gelatin, etc.) [
]. The combination of PEEK and BG has been investigated by various researchers during the last decade for biomedical applications [
]. PEEK can avoid stress shielding issues that may arise due to the high elastic modulus of metals in comparison to bone [
]. The combination of PEEK and BG can be tailored to achieve elastic modulus close to that of natural bone [
]. Furthermore, PEEK/BG coatings can act as a robust barrier between the bone and the metallic implant. PEEK/BG coatings have also shown cytocompatibility to osteoblast cells [
]. However, the successful incorporation of antibacterial agents into PEEK/BG composites remains a challenge. Seuss et al. [
] incorporated silver in PEEK/BG composites by electrophoretic deposition (EPD). These coatings show an antibacterial effect against Gram-positive and Gram-negative bacteria but the uncontrolled release of Ag ions represents a challenge. EPD is a simple room temperature coating technique which requires preparation of stable suspensions (of the particles/molecules which are intended to be deposited) followed by migration of the particles/molecules to the oppositely charged electrode upon the application of an electric field [
Rehman et al. [
] employed a strategy based on multilayer coating structures consisting of PEEK/BG layers (deposited via EPD) coated with silver/silica nanoclusters (deposited by radio frequency (RF) co-sputtering). The developed multi-structured coatings were bioactive and antibacterial. However, the release of Ag ions from such multi-structured coatings was not investigated. In addition, the cytotoxic effect of released silver ions should also be investigated.
Radio frequency (RF) magnetron sputtering as a coating technique provides multiple advantages including high efficiency, favourable coating-substrate bonding strength and controllable properties, as demonstrated for the fabrication of HAp bioactive coatings [
]. For example, Chernozem et al. [
] deposited CaP coatings via RF magnetron sputtering showing that the coatings improved the mechanical and biological properties of the substrate. In another study Surmenev et al. [
] showed that HA coatings can be deposited on polymeric substrates via RF magnetron sputtering, which anticipates that the technique is a suitable choice for depositing thin films on PEEK-based layers, as intended in the present study.
In this study, therefore, composite coatings based on PEEK, BG and Ag-MBGNs were developed via a single-step EPD process. Subsequently, RF co-sputtering of Ag silica nanoclusters on the previously obtained EPD coatings was employed at two different sputtering times, i.e. 20 min and 40 min.
In this study, we compared the silver release profile, antibacterial activity, adhesion strength and biocompatibility of PEEK/BG/Ag-MBGNs coatings (deposited via EPD) with multi-structured stratified coatings obtained via combination of RF co-sputtering and EPD (samples EPD/RF-20 and EPD/RF-40). The multi-structured coatings showed slower release of silver ions in comparison to the Ag-EPD layer. This controlled release of silver ions resulted in better cell viability (determined by the WST-8 assay with osteoblast-like MG-63 cells) in comparison to the Ag-EPD sample. Despite the slow release of silver ions from the EPD/RF-20 and EPD/RF-40 coatings, they showed strong antibacterial effect against Gram-positive and Gram-negative bacteria. Furthermore, EPD/RF-40 coatings developed HA crystals upon immersion in SBF, which indicates a bioactive character and potential bonding to bone. It can be concluded that the EPD/RF-20 and EPD/RF-40 coatings are suitable for further in-vivo investigations and eventually translation to clinical applications.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.