Surface Finish & Biocompatibility

A laser-cut edge is never directly implantable. The recast layer (re-solidified metal), HAZ, and oxidation products from cutting must be removed through post-processing to achieve a biocompatible surface that won't trigger adverse tissue reactions or accelerate corrosion in the body.

Quick Answer

The standard post-laser finishing sequence for implants: 1) Acid pickling (remove recast/slag), 2) Electropolishing (smooth surface to Ra < 0.4μm), 3) Passivation(rebuild protective oxide layer), 4) Final cleaning (ultrasonic + DI water rinse). The finished surface must pass ISO 10993 biocompatibility testing (2025 revision enhances risk-based approach) for the intended contact duration and type.

Post-Laser Finishing Sequence

Step	Process	Purpose	Typical Ra After
0. As-cut	—	Baseline (recast + HAZ present)	1–5μm Ra
1. Acid pickling	HF/HNO₃ immersion	Remove recast, oxides, slag	0.5–2μm Ra
2. Electropolishing	Anodic dissolution in electrolyte	Smooth surface, remove micro-burrs	0.1–0.4μm Ra
3. Passivation	Citric/nitric acid treatment	Form protective Cr₂O₃ or TiO₂ layer	Unchanged
4. Final cleaning	Ultrasonic + DI water + dry	Remove all chemical residues	Unchanged

Material-Specific Finishing

Stainless Steel (316LVM)

• Electropolish: H₃PO₄/H₂SO₄ electrolyte, 40–60°C
• Passivation: citric acid per ASTM A967
• Target Ra: < 0.25μm for implant contact
• Cr/Fe ratio > 1.5 verified by XPS
• Blue passivation test per ASTM F1089

Nitinol

• Critical: minimize Ni surface concentration
• Shape-set before electropolish (preserve geometry)
• Electropolish: methanol/HClO₄, −30°C to −40°C
• Target: TiO₂-rich surface layer with Ni < 2 at%
• Corrosion testing per ASTM F2129

Titanium (Ti-6Al-4V ELI)

• Acid etch: HF/HNO₃ to remove alpha-case
• No standard electropolishing (anodizing optional)
• Micro-blasting for textured osseointegration surfaces
• Target Ra depends on application (smooth vs textured)
• Passivation per ASTM F86

CoCr Alloys (L-605)

• Electropolish: H₃PO₄-based at 50–70°C
• Very hard material — longer electropolish cycle
• Target Ra: < 0.2μm for stent applications
• Co ion release testing per ISO 10993-12
• High corrosion resistance in body fluids