The clinical manifestations of the EBUS-TBNA needle largely depend on the material selection. Currently, the mainstream materials are medical-grade austenitic stainless steel (304, 316L) and shape memory alloy nickel-titanium (Nitinol). Both have their own advantages in terms of hardness, elasticity, biocompatibility, and processing adaptability. This article will conduct an in-depth comparison from three dimensions: mechanical properties, ultrasonic compatibility, and fatigue resistance.

1. Balance of Hardness and Penetrating Force

According to the product specifications, the Vickers hardness (HV) range of the EBUS-TBNA needles is 200–250. This value ensures sufficient rigidity to penetrate the thickened lymph node capsule without being too hard, which could lead to brittle fracture. The typical hardness of stainless steel 304 and 316L is approximately HV 180–230, which can be increased to above HV 250 through cold work hardening or aging treatment; nitinol has a hardness of approximately HV 200–260 in the austenitic state and possesses superelasticity, being able to fully restore its shape after bending. During the puncture process, the nickel-titanium needle tip can undergo elastic deformation when encountering calcified tissue to bypass obstacles, reducing the risk of airway perforation; while stainless steel is more inclined to maintain a straight path, making it suitable for scenarios requiring precise vertical puncture.

II. Differences in Ultrasonic Echo Characteristics

The acoustic impedance of the material directly affects the contrast of the ultrasonic image. The acoustic impedance of stainless steel is approximately 45 MRayl, much higher than that of human soft tissues (about 1.5 MRayl). Theoretically, it has strong reflection, but in practice, due to the smooth surface, it produces specular reflection, resulting in excessive directionality and weak scattering. The acoustic impedance of nitinol is slightly lower (about 30 MRayl), but its unique martensitic phase transformation generates internal interfaces that can increase non-coherent scattering. However, regardless of the material, artificial textures need to be introduced through surface laser etching to enhance the echoability. Studies have shown that under the same texture parameters, the echo intensity of nitinol is approximately 15% higher than that of stainless steel, which may be related to its lower acoustic attenuation coefficient. Therefore, some high-end products tend to use nitinol as the base material and combine laser marking to achieve the best visualization.

III. Fatigue Resistance and Flexibility

The EBUS-TBNA needle needs to repeatedly pass through the curved airways (such as the main bronchus to the right upper lobe opening), enduring cyclic bending stress. The fatigue limit of stainless steel is approximately 40% of its tensile strength, while the superelasticity of nitinol enables it to undergo a strain of 8% without permanent deformation, and its fatigue life is several times higher than that of stainless steel. Additionally, the elastic modulus of nitinol (about 75 GPa) is only half that of stainless steel (about 193 GPa), meaning that at the same outer diameter, the flexibility of the nitinol needle is better, making it easier to pass through sharp-angle branches. However, the cost of nitinol is 5-10 times that of stainless steel, and its welding is difficult, so the mainstream economic product is still 316L.

IV. Biocompatibility and Regulatory Compliance

Both materials meet the ISO 10993 biocompatibility standard. 316L contains molybdenum and has better resistance to pitting corrosion than 304; the release of nickel ions in nitinol has previously raised concerns, but current surface treatments (such as electrolytic polishing + TiN coating) can reduce the ion release to below the safety threshold. In terms of certification, stainless steel follows ASTM F899, and nitinol follows ASTM F2063. Both meet the requirements of ISO 13485 quality management system.

Conclusion

Material selection essentially involves a trade-off between cost, performance, and clinical requirements. For routine diagnoses, 316L stainless steel dominates due to its mature supply chain and stable processing properties; while for complex anatomical structures or high-risk patients, the superelasticity and excellent fatigue resistance of nitinol are driving its penetration in higher-end products. In the future, with the development of coating technologies and composite materials, the material library of EBUS-TBNA needles will be further expanded.