Complementary Surface Characterization Methods for Qualitative Detection of Oil Residues on Metallic Biomaterials

Residual machining oils on metallic medical device components may pose potential biocompatibility risks and often trigger extensive chemical characterization in accordance with ISO 10993-18. For metallic biomaterials, however, such residues are typically surface-confined and substantially reduced by post-machining treatments and cleaning processes, making comprehensive chemical analysis potentially overly conservative. Accordingly, efficient surface-based screening approaches are needed to determine whether further analytical evaluation is warranted. In this study, we developed and evaluated complementary surface characterization methods integrating laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) for the qualitative detection of machining oil residues on metallic biomaterials. White paraffin oil (WPO, mineral origin) and canola oil (CO, vegetable origin) were used as representative model systems and microdeposited at systematically varied surface loadings onto polished 316L stainless steel substrates. LSCM enabled visualization of oil residues through characteristic bright-dark contrast, multicolored thin-film interference bands, Newton's rings, and discrete droplet morphologies, with reliable detection down to approximately 2 μg cm^-2. SEM imaging optimized using low accelerating voltage, larger spot sizes, and imaging at oil-substrate interfaces, provided enhanced surface sensitivity and achieved detection limits of approximately 0.05 μg cm^-2 for WPO and 0.4 μg cm^-2 for CO. FTIR confirmed the presence of oil residues through oil-specific spectral features, with detection limits of approximately 0.10-0.20 μg cm^-2 depending on oil type and residue morphology. Collectively, these results demonstrate that complementary surface characterization methods provide an effective qualitative screening workflow for identifying residual machining oils on metallic biomaterials. Beyond demonstrating technical detection capabilities, the proposed workflow offers practical value for guiding subsequent analytical decision making, specifically by informing whether comprehensive ISO 10993-18 chemical characterization is warranted, and for supporting cleaning verification and risk-based biocompatibility assessment strategies.