Natural coatings on titanium surfaces to improve their biological response

Biomolecules and extracts from natural products are gaining increasing interest due to their beneficial properties for human health, low toxicity, environmental compatibility and sustainability. In this work, keratin, chitosan and peppermint essential oil have been used for the preparation of coatings on titanium substrates for biomedical implants or devices. All these coatings were obtained from local natural products or byproducts: keratin from discarded wool, chitosan from shrimps shells and peppermint essential oils from a local production. This approach supports a sustainable use of the resources and the sustainment of local economies with transformation of byproducts in high added values products. Keratin was chosen for its ability to stimulate soft tissue adhesion and to be easily doped with metal ions in order to confer even antibacterial activity. Keratin coatings were realized as electrospun oriented/random submicrometric fibres or continuous films. Chitosan was selected for its anti-inflammatory and antibacterial properties. Continuous coatings were obtained with different grafting strategies (direct grafting, tresyl chloride activation or polydopamine addition). Peppermint essential oil was chosen for its antibacterial activity and used for the obtainment of continuous coatings based on the self polymerizing ability of terpenes. The coatings were characterized by means of SEM-EDS, FTIR, zeta potential, wettability, tape and scratch tests, cell and bacteria cultures. Coatings were successfully obtained for all the considered natural substances. Good adhesion to titanium substrates was reached through the optimization of the surface preparation and grafting process. All the coatings were chemically stable in water and the continuous coating were mechanically resistant and protective for the metallic substrate. The keratin coatings were hydrophilic while mint oil and chitosan coatings were hydrophobic. At physiological pH, keratin and mint oil coatings were negatively charged while chitosan ones were positively charged. The oriented keratin fibres were able to drive fibroblast alignment. Ag-doped keratin fibres and mint coating showed antibacterial properties.