Effect of Poly-L-Lysine coating on titanium osseointegration: from characterization to in vivo studies
Dental implant prostheses cannot preclude a correct and stable implant osseointegration, which is still a challenge and greatly depends on biomaterial-cell interface. Titanium (Ti) coating using polyelectrolyte poly-L-lysine (PLL) may represent an interesting and simple approach, to provide a charged surface net able to improve cell adherence. However, in vitro and in vivo effects of Ti coated with PLL have been poorly investigated. The aims of the present study are (1) to obtain and characterize, chemically and physically, Ti disks coated with PLL (TiPLL); (2) to perform in vitro studies on osteoblast cell lines' cytocompatibility and functionality (alkaline phosphatase [ALP] activity, calcium deposition, proinflammatory interleukin 6 production); (3) to obtain in vivo evidence of osseointegration, using a sheep animal model. XPS, AFM, and contact-angle analyses demonstrated that the Ti disk was successfully covered with PLL, providing higher hydrophilicity to the Ti disk. No cellular toxicity, enhanced calcium deposition, and a decreased tendency toward interleukin-6 production were observed in the osteoblast seeded onto TiPLL. In vivo experiments showed cortical bone microhardness at 3 months significantly improved in the presence of the PLL coating. PLL coating on Ti implants seemed to safely enhance calcium deposition and implant early osseointegration in animals, suggesting promising evidence to optimize the surface properties of dental implants.

X-ray photoelectron spectroscopy (XPS). N1s, C1s, and O1s high-resolution (HR) spectra are reported for samples with and without PLL coating (upper and lower, respectively), showing the typical peaks of organic material/protein: HR-N1s showed a single peak close to 400 eV, HR-C1s had 285-286-288 eV peaks, and HR-O1s displayed 530-532-533 eV peaks, suggesting the presence of PLL coating.

Figure 2. Alizarin Red assay (a), performed at 28 days of cell culture, revealed how TiPLL disks induced a significantly higher amount of calcified secreted matrix (P < .05, Student's t test), not only in comparison to Ti disks. Pro-inflammatory Il-6 production (b). IL-6 expression showed a reduced trend in osteoblasts seeded on TiPLL disks than on Ti, also under LPS stimulation, even if without a statistical significance. Figure 3. Vicker Microhardness evaluation (after 12 weeks from implantation): trabecular and cortical refer to peri-implant bone, while native bone refers to bone around natural tooth. A higher mean value of cortical bone around implants coated with PLL (TiPLL) can be found, as compared to uncoated implants (Ti; P < .05; Student's t test).
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