Superhydrophobic bioactive coatings on fused deposition modeling-printed polylactic acid (PLA) and polyethylene terephthalate glycol (PETG): Wettability, surface topography and simulated body fluid (SBF) bioactivity

This study investigates the effects of three biocompatible coatings, Silicon dioxide (SiO 2 ) nanoparticles, a Polymethyl methacrylate (PMMA) + SiO 2 , and carnauba wax, over a phase-transitioned lysozyme (PTL) primer on the wettability of Fused deposition Modeling (FDM) printed PLA and PETG. A 26-run Central Composite Design (CCD) within a Response Surface Methodology (RSM) structure examined the process parameters, including layer thickness (0.1–0.3 mm), coating concentration (4–8 wt.%), and build orientation (horizontal/vertical). All three coatings converted substrates from hydrophilic water contact angle (WCA 53–73°) to hydrophobic or superhydrophobic (WCA up to 151°). PTL + wax achieved the highest WCA on PLA (151.05°, SA = 4.18°), confirming Cassie–Baxter superhydrophobicity, whereas PMMA + SiO 2 showed high sliding angles (27.71° on PLA; 25.91° on PETG) despite a moderate WCA, highlighting the critical role of the hierarchical surface texture in dynamic repellency. Surface skewness (Rsk) identified as roughness parameter strongly correlated with coated contact angle ( r = +0.848, p = 0.033), and build orientation drove a Wenzel-to-Cassie–Baxter transition upon coating. Scanning Electron Microscope (SEM) revealed denser, cauliflower-like wax morphology in PLA than in PETG, explaining WCA difference. X-ray Photoelectron Spectroscopy (XPS) revealed progressive reduction in O/C ratio across the coating sequence and confirmed the residual PTL biopolymer functionality at the wax-coated surface. A seven-day SBF immersion validated bioactive surface behaviour, with PLA exhibiting denser hydroxyapatite nucleation than PETG, consistent with its higher residual PTL activity. These findings establish a quantitative process–structure–property framework for the rational design of superhydrophobic biocompatible coatings on FDM-printed surfaces.