Hierarchically Rough rGO/HAp Nanocoatings on SLM Ti-6Al-4 V Implants for Enhanced Biointegration

Additive manufacturing of Ti-6Al-4V implants enables patient-specific design, but process-induced surface and microstructural variations can strongly affect biological performance. In this work, Ti64 samples with varying volumetric energy densities (VEDs) were fabricated by selective laser melting to examine the combined effects of processing and surface modification on cell response and antibacterial behavior. The specimens were surface activated by controlled acid etching and coated with hydroxyapatite/reduced graphene oxide (HAp/rGO) composite via electrophoretic deposition. Surface morphology, chemistry, and porosity were assessed using Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy, profilometry, micro-CT. Micro-CT showed higher internal porosity at lower VED, while a balance between densification and structural integrity was achieved at VED of 107 J mm^{- 3}. In vitro cytocompatibility demonstrated >90% viability for all samples, meeting ISO 10993-5 requirements. Despite this, cell attachment depends on VED, the E63 condition exhibited the most uniform coverage after HAp/rGO coating, indicating an improved surface-cell interactions at lower VED. Antibacterial assays showed VED dependence; high-VED-E320 samples produced the highest biofilm formation even after coating, whereas low and intermediate VED conditions significantly reduced bacterial adhesion. Overall, biological functionality is observed to be governed by both coating and SLM parameters, with 107 J·mm^{- 3} offering the best combined structural, cytocompatible, and antibacterial performance.