https://pmc.ncbi.nlm.nih.gov/articles/PMC4115763/

After the vacuum-assisted breast biopsy needle undergoes mechanical processing, it must undergo a series of surface treatments to meet medical-grade requirements. The three processes of passivation, electro-polishing, and ultrasonic cleaning together form the "golden triangle" of surface quality. This article will analyze the principles and functions of each of these processes one by one.

1. Passivation: Constructing an Invisible Rust-Proof Armor

Passivation is a process that uses chemical methods to form a dense oxide film on the surface of metals. For SS316 stainless steel, it is common to immerse it in a citric acid solution (with a concentration of 10–20% and a temperature of 50–60°C) for 20–30 minutes. Citric acid chelates free iron ions and simultaneously promotes the formation of the Cr₂O₃ passivation film. This film is only 2–5 nm thick, but it can effectively prevent the penetration of chloride ions and increase the corrosion potential by 200–300 mV. After passivation, a blue spot test (using potassium ferricyanide solution to detect free iron) is required. The qualified standard is that no blue spots appear within 30 seconds. This step ensures the corrosion resistance of the needle body both inside and outside the body, avoiding inflammatory reactions caused by the release of metal ions.

II. Electro-Polishing: Mirror-Like Luster and Low Friction Surface

Electro-polishing utilizes the principle of electrochemical anode dissolution. The workpiece is immersed in a mixed electrolyte of phosphoric acid and sulfuric acid. After connecting a direct current, the areas with higher surface protrusions have a higher current density and a faster dissolution rate, thereby achieving a smooth and shiny surface. Electro-polishing can remove micro-cracks, burrs, and stress layers left by mechanical processing, reducing the surface roughness from Ra 0.8 μm to Ra ≤ 0.2 μm. For VABB needles, the smooth surface brings two major benefits: first, it reduces puncture resistance and minimizes tissue tearing; second, it reduces bacterial adhesion, making cleaning easier. After electro-polishing, the workpiece needs to be thoroughly rinsed with deionized water to remove residual electrolyte and undergo a salt spray test in accordance with ASTM B912 to verify corrosion resistance.

III. Ultrasonic Cleaning: The Ultimate Method for Achieving Nanoscale Cleanliness

Even after electro-polishing, the surface of the needle body may still adsorb oil stains, dust, or metal debris. Ultrasonic cleaning utilizes 20–40 kHz high-frequency vibrations to generate cavitation effects in the cleaning solution - bubbles form in the negative pressure zone and collapse rapidly in the positive pressure zone, releasing local high temperature and high pressure (approximately 5000 K, 1000 atm), and forcefully removing contaminants. The cleaning process typically includes: alkaline degreasing → deionized water rinsing → alcohol dehydration → hot air drying. After cleaning, the concentration of particles in the cleaning solution (≥10 μm particles ≤ 100 per mL) needs to be detected using a particle counter to ensure compliance with ISO 14644 cleanroom standards. For instruments like VABB needles that come into direct contact with living tissues, ultrasonic cleaning is an indispensable final line of defense.

IV. The Value of the Three-Step Collaboration

Passivation, electro-polishing, and ultrasonic cleaning are not independent operations but rather complement each other. Passivation provides chemical protection, electro-polishing improves the physical surface, and ultrasonic cleaning removes residues. When these three are implemented together, the VABB needle simultaneously possesses high corrosion resistance, low friction, and ultra-high cleanliness, meeting the strict requirements of ISO 13485 for the cleaning verification of medical devices.

Conclusion

The surface treatment may seem simple, but it actually involves multiple interdisciplinary fields such as electrochemistry, fluid mechanics, and acoustic chemistry. It is these three "invisible" processes that give the vacuum-assisted breast biopsy needle the ability to work safely and efficiently within the body, becoming an important pillar of the reliability of minimally invasive biopsy technology.