Surface treatments
Surface treatments improve the surface quality, corrosion resistance and biocompatibility of medical devices by removing contaminants and minute imperfections introduced earlier in the manufacturing process, breaking jagged edges, rounding corners and smoothing out the surface. STI performs chemical, mechanical and electrochemical surface treatments optimized according to product specifications, materials used and the relevant ASTM standards.
SEM x1000 NiTi 100μm strut
Passivation and electropolishing treatments improve the corrosion resistance of medical implants operating inside the body and exposed to the corrosive elements present in this fluid environment. Surface treatments also remove micron-sized blemishes introduced during laser machining, including: sharp corners, jagged edges, oxide layers, burrs, slag, and recast.
Consider the SEM micrograph presented on the right. A NiTi stent magnified by 1000 demonstrates the heat and blow effect of a laser pulse on a 100 micron strut. Note the non-uniform surface, the edge burrs and dross - undesirable effects that surface treatments remove.
Chemical treatments
- Pickling - removing oxide layers, stains and molten metal effects introduced during laser cutting, as well as rust and other contaminants by immersing the object in a pickling solution (acid bath). STI uses pickling primarily to remove oxide layers from stainless steel and cobalt chrome alloys.
- Passivation - improving corrosion resistance of stainless steels, titanium and nitinol surfaces by using a mild oxidant to dissolve contaminants and sulfides and remove free iron from the surface. Passivation facilitates the rapid formation of a newer and thicker protective layer on the surface.
- Neutralization - stopping chemical reactions associated with the passivating media.
- Post-cleaning - washing the object with water or alcohol for removing any process residue.
Mechanical polishing
- Ultrasonic cleaning - removing dirt and other contaminants (oil, grease, fingerprints, dust, etc.) that adhere to the surface by immersing the object in a mild cleaning solution (water or other solvent) and then agitating the cleaning solution using high frequency sound waves. Ultrasonic cleaning enables cleaning in difficult to reach areas, including inside of small diameter tubular objects, such as: stents, guidewires, cannulae, endoscopic tubes, etc. Ultrasonic cleaning variables include the object, the cleaning solution, temperature, volume and the cleaning time.
- Honing - an abrasive machining process performed immediately after laser cutting. In stent manufacturing, for example, leftover material between struts is gently removed and rough edges smoothed out using fine files.
- Magnetic deburring - magnetic pin tumbler used for tight tolerance deburring or radiusing of sharp corners and edges introduced earlier in the fabrication process. Parts are placed in a container filled with the deburring media composed of fine stainless steel pins suspended in soapy water. As the pins circulate they strike the parts and produce the deburring action.
- Micro abrasive blasting - clean air mixed with abrasive micron-sized particles, known as the blasting media, propelled out of a nozzle at a controlled pressure. Micro abrasive blasting can be used for focused deburring, surface texturing, surface cleaning and breaking sharp corners.
Honing - stent manufacturing
Electropolishing or electrochemical polishing (EP)
Electropolishing is a controlled and repeatable electrochemical process that removes metal from the surface of complex geometrical objects by means of electrolytic dissolution. Medical devices, stents, heart valve frames and surgical tools all depend on electropolishing to achieve the required surface finish for surgical applications.
EP deburrs and polishes simultaneously, revealing the underlying metal structure without any mechanical or thermal distortions, such as: scratches, tiny cracks, internal stresses, heat affected zones and unstable oxides. EP results in a smooth, microscopically uniform, bright and reflective surface.
The following high-magnification scanning electron micrographs show cobalt chrome and stainless steel devices before and after electropolishing:
Smoother surfaces minimize contamination buildup, facilitate better passivation, and reduce friction. These electropolishing benefits improve the biocompatibility of medical implants, by minimizing the foreign body response and the ability of bacteria to grow on the implant, while improving corrosion resistance.
NiTi implants, for example, require electropolishing to selectively decrease the amount of Nickel (Ni) on the surface and produce a thin passivating layer composed of titanium oxide. Nickel is harmful to the human body due to the carcinogenic and inflammatory reactions it induces. Therefore the benefits of electropolishing go beyond deburring, edge and corner rounding and smooth surface - improving corrosion resistance and biocompatibility is essential for biomedical applications.
Process variables
Electropolishing variables include: the object's geometry, the underlying properties of the raw material, the initial surface finish (after cold working, laser cutting and heat treatment steps), the composition and saturation of the electrolytic solution, the temperature, the anode / cathode surface area ratio, the current density, the polishing time and the agitation method.