Polymeric coatings improve mechanical and biological properties of resin composites: An in vitro study

This study aimed to investigate the effects of polymeric coatings on the physical, chemical, and biological properties of resin composites (RCs).

A total of n = 10 RC discs were fabricated for each experimental group and coated with polydopamine (PDA), polyethylene glycol (PEG), or polyetheretherketone (PEEK) using a spin-coating technique. Three surface preparation protocols were applied: prior to polymerization, after polymerization with an oxygen-inhibition layer (OIL), and after polymerization without OIL. Surface characterization was performed using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). Oral fibroblast viability was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay at 24 h and 48 h. Scratch resistance, surface roughness, water contact angle, water sorption, solubility, and microhardness were also measured. Data normality was tested with the Shapiro-Wilk test. For normally distributed data, one-way and two-way analysis of variance (ANOVA) with Tukey's HSD post-hoc test was applied. For non-normally distributed data, the Kruskal-Wallis test with Dunn's post-hoc test or a two-way robust ANOVA with Bonferroni correction was performed. A significance level of p < 0.05 was considered statistically significant.

At 24 h, the post-polymerized PEEK group showed the highest cell viability, while PDA without OIL showed the lowest (p < 0.001). At 48 h, PDA without OIL exhibited the highest viability, whereas post-polymerized PDA showed the lowest (p < 0.001). PEEK coatings demonstrated the highest scratch resistance (p = 0.035) and significantly improved microhardness (p = 0.019). All coatings significantly reduced water contact angles, enhancing hydrophilicity (p < 0.05). No significant differences were found in water sorption and solubility. Surface roughness was influenced by the substrate, not the coating type.

Within the limitations of this short-term in vitro study, spin-coated PEEK, PDA, and PEG coatings improved the mechanical, biological, and hydrolytic performance of RCs. Among these, PEEK conferred the greatest mechanical reinforcement, significantly enhancing scratch resistance and microhardness. PDA exhibited a distinct time-dependent biological effect, with reduced fibroblast adhesion at 24 h but markedly increased proliferation at 48 h, highlighting its dynamic bioactivity. PEG, although consistently associated with lower fibroblast adhesion due to its antifouling properties, contributed to enhanced surface hydrophilicity. Collectively, these findings indicate that polymeric surface coatings can beneficially modulate RC behavior; however, without standardized aging protocols (e.g., thermocycling, long-term water storage, simulated mechanical loading) and in vivo validation, the results should be regarded as preliminary and not yet clinically conclusive.

Based on short-term in vitro data, this study suggests that PEEK and PDA coatings may have the potential to reinforce the biological seal at the soft tissue-restoration interface. Such an effect could, in the long term, support gingival adaptation, papillary fill, and embrasure contouring, thereby contributing to both aesthetic and functional outcomes. Nevertheless, these inferences should be regarded as preliminary; without standardized aging protocols and in vivo validation, immediate clinical applicability cannot be asserted. Future studies are required to determine whether these coatings can indeed improve soft tissue integration and biological seal stability at the clinical level.