Nitinol vs Stainless Steel vs Cobalt-Chromium: Complete Comparison for Medical Stents

Overview of Stent Materials

Nitinol (NiTi) — Shape Memory Alloy

Nitinol is an intermetallic compound consisting of approximately 50-51% nickel and 49-50% titanium. Its key advantage is the shape memory effect and superelasticity, enabling self-expanding stent designs.

Нержавеющая сталь 316L

316L is an austenitic chromium-nickel steel containing 16-18% chromium, 10-14% nickel, and 2-3% molybdenum. Low carbon content (<0.03%) ensures high corrosion resistance after welding.

Cobalt-Chromium Alloys (L605, MP35N)

L605 contains 49-53% cobalt, 19-21% chromium, 14-16% tungsten, and 9-11% nickel. MP35N composition includes 33-37% cobalt, 33-37% nickel, 19-21% chromium, and 9-10.5% molybdenum. Both alloys demonstrate superior strength and biocompatibility.

Mechanical Properties Comparison

*Nitinol elastic modulus varies with temperature and phase: 28-41 GPa in martensite, 75-83 GPa in austenite.

Key Mechanical Insights

• Strength: MP35N demonstrates highest strength (up to 2070 MPa), with L605 second. Nitinol shows wide range depending on processing.
• Stiffness: CoCr alloys and 316L have elastic modulus ~200-240 GPa. Nitinol is significantly softer (28-83 GPa), providing better flexibility for self-expanding stents.
• Сверхэластичность: Only Nitinol recovers up to 8-10% strain without permanent deformation at body temperature.
• Weight: Nitinol is 20-30% lighter than CoCr and steel, important for large implants.

Biocompatibility and Clinical Safety

Nitinol: The Nickel Question

The primary concern with Nitinol is high nickel content (50-51%), which is a known allergen. However, surface passivation forms a protective TiO₂ layer 5-20 nm thick that prevents nickel ion release (less than 1 μg/cm²/week per ISO 10993).

Clinical data: Long-term studies show that properly passivated Nitinol stents have allergic reaction rates <0.1%, comparable to 316L steel.

316L: Gold Standard

Stainless steel 316L is considered the biocompatibility benchmark for implants over 40+ years. Nickel content (10-14%) is lower than Nitinol, and chromium forms a stable oxide film.

Cobalt-Chromium Alloys: Superior Inertness

L605 and MP35N demonstrate exceptional biocompatibility due to high chromium content (19-21%) and absence of toxic elements. Frequently used in drug-eluting stents requiring long-term stability.

*Requires quality surface passivation

Corrosion Resistance in Physiological Environment

Corrosion resistance is critical for long-term implant functionality, as stents operate in an aggressive environment: 0.9% NaCl physiological solution, pH 7.4, 37°C temperature, cyclic loading.

Corrosion Assessment Methods

• Pitting Potential (Epit): Measured in Ringer’s solution per ASTM F2129
• Corrosion Currents (icorr): Potentiodynamic polarization method
• Long-term Immersion: 6-12 months in artificial biofluids

Comparative Results

• МП35Н: Highest corrosion resistance. Epit >+800 mV vs SCE, icorr <0.1 μA/cm²
• Л605: Excellent resistance. Epit >+600 mV, icorr <0.2 μA/cm²
• Nitinol: Good resistance with proper passivation. Epit >+400 mV, icorr <0.5 μA/cm²
• 316L: Satisfactory. Epit >+300 mV, icorr 0.3-1.0 μA/cm². Pitting risk in high-stress zones

Conclusion: For critical applications (small diameter, high cyclic loads), CoCr alloys are preferred. For peripheral stents, Nitinol or 316L suffice.

Manufacturing Requirements and Laser Cutting

Nitinol Processing

Nitinol laser cutting requires minimal heat-affected zone (HAZ less than 20 μm) to preserve shape memory effect. Process includes:

• Fiber laser 1064 nm, 10-100 ns pulses
• Nitrogen or argon assist gas
• Multi-pass cutting with low energy per pass
• Mandatory heat treatment after cutting (450-550°C) for shape “programming”
• Electropolishing to remove recast layer and passivate

316L Processing

• Easier to cut compared to Nitinol and CoCr
• HAZ less critical (up to 50 μm acceptable)
• High-speed cutting possible
• Passivation in HNO₃ required to restore oxide layer

L605 and MP35N Processing

• High hardness requires higher laser energies
• Work hardening tendency during cold deformation
• Mandatory annealing after cutting (1150-1175°C for L605)
• Electropolishing difficulty due to hardness

Manufacturing Complexity: 316L (simple) < Nitinol (moderate) < L605 (complex) < MP35N (very complex)

Applications in Different Stent Types

Self-Expanding Stents

Material of Choice: Nitinol

Nitinol’s shape memory effect allows the stent to expand autonomously upon reaching body temperature of 37°C. Applied in:

• Peripheral arteries: Femoral, popliteal, carotid (diameter 5-10 mm)
• TAVI procedures: Valve stents for aortic valve
• Venous stents: Deep veins, vena cava
• Biliary/ureteral stents: Minimally invasive interventions

Balloon-Expandable Stents

Materials: 316L, L605, MP35N

Plastic deformation under balloon pressure (8-16 atm) fixes the stent in expanded position. Used in:

• Coronary arteries: 316L for bare-metal stents (diameter 2.5-4.0 mm)
• Drug-eluting stents: L605 or MP35N for thin-walled designs (wall thickness 60-80 μm)
• Renal arteries: High radial force with L605/MP35N
• Intracranial stents: MP35N for small vessels

Bioresorbable Scaffolds

Typically made from polymers, though thin metallic scaffolds from magnesium or iron alloys are sometimes used. Traditional Nitinol/CoCr/316L not applicable as they don’t resorb.

Cost Analysis: Material Cost vs Performance

Raw Material Cost (Relative)

• 316L Steel: Baseline cost = 1.0x
• Nitinol: 3-5x more expensive than steel
• L605 CoCr: 4-6x more expensive than steel
• MP35N CoCr: 6-8x more expensive than steel

Manufacturing Costs

Laser cutting and post-processing costs significantly impact final device cost:

• 316L: Simple processing, short cycle time
• Nitinol: Requires heat treatment + electropolishing, +30-50% time
• L605/MP35N: Complex processing of hard material, +50-100% time

Total Cost of Ownership (TCO)

When evaluating full cost of ownership, consider:

• Clinical outcomes: Fewer reinterventions with DES (L605/MP35N) vs BMS (316L) reduces long-term healthcare costs
• Implant lifespan: CoCr alloys last 15-20+ years vs 10-15 years for 316L in some applications
• Profile reduction: MP35N enables thinner walls → less material → partial compensation for high price

Selection Recommendations

• Budget solutions: 316L for simple balloon-expandable stents
• Mid-range: Nitinol for peripheral and self-expanding stents
• Premium segment: L605/MP35N for thin-walled DES and critical applications

Frequently Asked Questions (FAQ)

Can Nitinol be used for balloon-expandable stents?

Technically possible but not recommended. Nitinol’s superelasticity causes the stent to attempt returning to its original shape after balloon pressure removal. This reduces radial force and may cause elastic recoil. Balloon-expandable stents require materials with plastic deformation: 316L, L605, MP35N.

Why is MP35N more expensive than L605 when both are cobalt-chromium alloys?

MP35N contains 9-10.5% molybdenum and 33-37% nickel, making the alloy more expensive and difficult to process. Additionally, MP35N undergoes special thermomechanical treatment to achieve peak properties (ultimate strength up to 2070 MPa). L605 is less alloyed and simpler to manufacture, yet still exceeds 316L.

What wall thickness is typical for stents from different materials?

• 316L BMS: 100-140 μm
• L605 DES: 80-100 μm
• MP35N DES: 60-80 μm (thinnest)
• Nitinol peripheral: 200-400 μm (thicker due to self-expansion)

Are there Nitinol allergies in patients?

Nickel allergy occurs in ~10-15% of the population, but clinically significant reactions to Nitinol stents are extremely rare (<0.1%). The passivated TiO₂ surface effectively blocks nickel ion release. However, for patients with confirmed severe nickel allergy, stents from 316L or CoCr are recommended.

Can patients with stents undergo MRI?

Yes, all three materials — 316L, Nitinol, CoCr — are considered MR Conditional per ASTM F2503. This means safe MRI up to 3 Tesla when following manufacturer guidelines. Ferromagnetic artifacts are minimal due to austenitic structure and weak magnetic response.

Which material is better for diabetic patients?

Diabetes increases the risk of in-stent restenosis. For such patients, drug-eluting stents from L605 or MP35N with antiproliferative coatings are preferred. Bare-metal stents from 316L show worse outcomes. Nitinol peripheral stents have also shown good efficacy in diabetic populations.