"Introduction: Several molecules of natural origin are of great interest to add specific surface functionalities to implant biomaterials: phenolic compounds and keratin can be used for different targets, as an example. They are derived from industrial processing of respectively plants or animals derivatives through a valorization process of waste and a sustainable circular economy approach. The modified surfaces enhance and fast the tissue integration, fight or reduce the risk of infections, guide the tissue growth, modulate the inflammatory response.
Different processing can be followed: grafting of a molecular monolayer (functionalization), thin or thick continuous coatings. The selection of the processing must be guided by the chemistry of the surface and ligands as well as by the expected mechanism of action of the biomolecule: progressive release or permanent link to the surface. Functionalization or coating can be coupled to different surface topography of the substrate for a synergic chemical-physical effect on the host response.
Post-processing (packaging, sterilization, storage) of the final products must be adapted to the presence of the biomolecules, too.
Potential applications are in orthopedic, dental, cardiovascular implants.
Methodology: Polyphenols (phenolic acids, flavonoids, and condensed tannins) are extracted from organic red grape pomace. The functionalization process is performed at pH = 7.4 with the addition of calcium ions, which act as a bridge between the substrate and polyphenols. The presence, amount (semi-quantitative), distribution, release, and type of bonding to the surface of the grafted polyphenols have been assessed.
Using electro-spinning, mirror-polished Ti disks were uniformly coated with keratin fibers obtained from discarded wool via sulfitolysis; surface functionalization with keratin molecules has been released, too. The keratin modified surfaces were then doped with silver (Ag) to introduce antibacterial properties.
The resulting specimens were characterized in terms of morphology and chemical composition by FESEM, FTIR, zeta potential titration curves, KPFM, and XPS. The antibacterial properties of the Ag-doped specimens were tested against a multidrug-resistant Staphylococcus aureus biofilm through morphology (FESEM) and metabolic assay. Lastly, the cytocompatibility of the specimens was confirmed using human primary gingival fibroblasts and mesenchymal cells.
Results: The functionalized samples have a homogeneous distribution of polyphenols as a continuous layer and micro-sized agglomerates. The grafted polyphenols maintain redox chemical and radical scavenging ability. A fraction of polyphenols is released into water in one day, while a firmly grafted layer remains on the surface even after four weeks. A larger release can occur in case of an environment with pH of 4–5 (e.g. inflammation). The functionalized surfaces can be sterilized by gamma irradiation without significant damage of the grafted polyphenols.
Concerning keratin, the Ag surface enrichment was effective in reducing viability and maturation of S. aureus biofilm, without compromising human cell viability. The cell spread was found to be very sensitive to keratin fiber stimulation.
Conclusions: Both the strategies thus appear to be very promising to introduce surface features in line with the main requirements for transmucosal and bone implants and it is of great interest to compare them in terms of efficacy and target application."