Aim. Prosthetic joint infections pose a major clinical challenge. Developing novel material surface technologies for orthopedic implants that prevent bacterial adhesion and biofilm formation is essential. Antimicrobial coatings applicable to articulating implant surfaces are limited, due to the articulation mechanics inducing wear, coating degradation, and toxic particle release. Noble metals are known for their antimicrobial activity and high mechanical strength and could be a viable coating alternative for orthopaedic implants [1]. In this study, the potential of thin platinum-based metal alloy coatings was developed, characterized, and tested on cytotoxicity and antibacterial properties. Method. Three platinum-based metal alloy coatings were sputter-coated on medical-grade polished titanium discs. The coatings were characterized using optical topography and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Ion release was measured using inductively coupled plasma optical emission spectrometry (ICP-OES). Cytotoxicity was tested according to ISO10993-5 using mouse fibroblasts (cell lines L929 and 3T3). Antibacterial surface activity, bacterial adhesion, bacterial proliferation, and biofilm formation were tested with gram-positive Staphylococcus aureus ATCC 25923 and gram-negative Escherichia coli ATCC 25922. Colony forming unit (CFU) counts, live-dead fluorescence staining, and SEM-EDS images were used to assess antibacterial activity. Results. Three different platinum-based metal alloys consisting of platinum-iridium, platinum-copper, and platinum-zirconium. The coatings were found 80 nm thick, smooth (roughness average < 60 nm), and non-toxic. The platinum-copper coating showed a CFU reduction larger than one logarithm in adherent bacteria compared to uncoated titanium. The other coatings showed a smaller reduction. This data was confirmed by SEM and live-dead fluorescence images, and accordingly, ICP-OES measurements showed low levels of metal ion release from the coatings. Conclusions. The platinum-copper coating showed low anti-adhesion properties, even with extremely low metal ions released. These platinum-based metal alloy coatings cannot be classified as antimicrobial yet. Further optimization of the coating composition to induce a higher ion release based on the galvanic principle is required and copper looks most promising as the antimicrobial compound of choice. Acknowledgments. This publication is supported by the DARTBAC project (with project number NWA.1292.19.354) of the research program NWA-ORC which is (partly) financed by the Dutch Research Council (NWO); and the AMBITION project (with project number NSP20–1-302), co-funded by the PPP Allowance made available by Health-Holland, Top Sector Life Sciences & Health to ReumaNederland

INTRODUCTION. Trabecular Titanium. ™. (TT) is a novel material with a structure similar to trabecular bone, already used for prosthetic clinical applications. Being the bone-implant interface the weakest point during the initial healing period, the association of TT with a hydrogel enriched with progenitor cells and osteoinductive factors may represent a promising strategy to improve prosthesis osteointegration. In a previous in vitro study we evaluated the ability of an ammidated carboxymethylcellulose hydrogel (CMCA) and of TT enriched with CMCA to support bone marrow mesenchymal stem cells (BMSCs) viability and osteogenic differentiation [1]. The aim of this study was to evaluate in vivo if the association of TT with CMCA enriched with strontium chloride (SrCl. 2. ) and BMSCs could ameliorate TT osteointegration. METHODS. This study combines TT with CMCA, SrCl. 2. and BMSCs. To mimic prosthesis-bone implants, TT discs were seeded with human BMSCs predifferentiated in osteogenic medium, then press-fit into engineered bone. A total of 36 athymic mice were implanted subcutaneously, each animal received 2 constructs as un-seeded TT and TT+CMCA or cell seeded TT+BMSCs and TT+CMCA+BMSCs. After 4, 8 and 12 weeks, osteodeposition, bone mineral density (BMD) and osteointegration were evaluated by fluorescence imaging, micro-CT, SEM, histology and pull-out tests. RESULTS. Micro-CT analysis demonstrated the homogeneity of the engineered bone in all experimental groups, supporting the reproducibility of our novel engineered model. Macroscopic evaluation of explanted constructs after 4 weeks revealed their integration with mice subcutaneous structures. In pull-out biomechanical tests, increases in extraction energy and peak force from 4 to 12 weeks were observed in all the experimental groups, except TT+CMCA. TT+CMCA+BMSCs showed the highest value of peak force and the greatest increase in comparison to samples explanted at 4 weeks. In vivo fluorescence imaging showed osteodeposition activity inside the constructs, observation confirmed by the ex-vivo analyses revealing a higher activity in TT+BMSCs and in TT+CMCA+BMSCs in comparison to acellularized TT samples. SEM evaluation of ECM deposition at the interface between bone scaffolds and TT disks revealed a significant difference between TT+CMCA+BMSCs and the other experimental groups with the former showing an almost complete filling of the space between the integration surfaces already after 4 weeks. In histomorphometric analyses of tissue ingrowth at 8 weeks, TT+BMSCs and TT+CMCA+BMSCs showed a greater tissue ingrowth compared to TT and TT+CMCA samples. DISCUSSION. Several efforts have been made to improve osteointegration with particular attention to critical cases such as implant revision surgeries. The association of porous structures with osteoinductive factors enriched hydrogels and stem cells represents a novel and promising strategy for more effective osteointegration to reduce prosthesis mobilization risks. Our results demonstrate that the association of Trabecular Titanium. ™. with a SrCl. 2. enriched hydrogel and BMSCs increases the production of ECM and may thus represent a valid approach to accelerate prosthesis osteointegration. Further validation of these data will include construct implantation in large animal orthotopic models to better mimic surgical procedures