Photoresponsive Bilayer Coating Integrating a Porous Zinc Oxide Layer and a Chitosan–Antibiotic Drug Delivery System for Antimicrobial Photodynamic Therapy in Biomedical Implants

Abstract Peri-implant infections remain a major cause of implant failure, highlighting the need for antimicrobial implant surfaces capable of suppressing biofilm formation while preserving the physicochemical and biological features needed for osseointegration. Here, we engineered a photoresponsive bilayer coating on titanium by anchoring a photosensitive antibiotic, demeclocycline (DMC), loaded into a polymeric chitosan film onto a zinc-doped porous oxide layer produced by plasma electrolytic oxidation (PEO). The inner PEO/Zn layer generated a porous ZnO-containing TiO2 matrix that provided mechanical interlocking for the polymeric film. The outer polymeric chitosan film deposition reduced surface roughness and hydrophilicity and enabled DMC incorporation. Chemical analyses confirmed the stepwise construction of the bilayer coating, revealing TiO2/ZnO porous oxide layer formation and chitosan and DMC incorporation. ROS generation under blue light was confirmed by the DCFH-DA fluorescence assay. Mechanically, the bilayer coating decreased the friction coefficient by approximately 75%, with the chitosan layer acting as a lubricating barrier under tribological stress. DMC release was sustained for 21 days with lysozyme-mediated degradation accelerating early drug release. Against saliva-derived polymicrobial biofilms formed for 96 h, the DMC-loaded coating reduced viable counts by approximately 2-log units under dark conditions, confirming intrinsic antibacterial activity. Upon blue light irradiation, the reduction reached approximately 3-log units, indicating on-demand photodynamic enhancement of the antimicrobial effect mediated by ROS. This response was accompanied by reduced metabolic activity, lower biofilm mass and matrix protein content, increased membrane-compromised cells assessed by live/dead, and suppression of more pathogenic microbial complexes. The coating also retained antibiofilm activity after microbial recolonization and the reusability of photodynamic therapy. Beyond antimicrobial activity, the coating preserved preosteoblastic cell viability and supported hydroxyapatite nucleation. Proteomic analysis showed an enriched immune, coagulation, protease-regulatory, and structural pathway proteomic profile. Overall, this bilayer platform integrates mechanical stability, sustained antibiotic delivery, and light-activated antimicrobial photodynamic therapy, representing a promising multifunctional coating for the control of implant infection.