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What’s the Difference Between Grinding Materials?

Feb. 28, 2019
Materials have different properties, meaning each has its own quirks when processing. Both in precision applications and well-regulated industries such as in medical, understanding these behaviors becomes a matter of life or death.

Here are several of the typical kinds of materials used for medical products and how they are prepared using high-volume grinding-polishing systems.

Titanium alloys. These alloys are difficult to prepare due to the development of a deformed surface layer. Sectioning damage is a common problem attributed to the alloy’s sensitivity to high temperatures when coolant is not properly directed at the workpiece, or else through cold working from an aggressive machining process. This deformation can result in grain twinning and strain-induced transformation structures, while high temperature can cause phase distribution changes.

There are a variety of implants. Titanium is well known as an implant material, but each material and patients have their own material properties that may alter what would make the best fit.

Polishing with finer diamond can cause high deformation in titanium samples that is difficult to remove. This remnant deformation is effectively removed through chemo-mechanical polishing using oxide polishing suspension.

Nitinol. This is a nickel titanium alloy employed in endovascular stents. The super-elastic nature of nitinol is very suitable to stent application due to its microstructure transformation and ability to retain some strain after deployment.

The structure observed before implantation is austenitic in nature. When compressed and fitted on a catheter, the structure changes to martensitic. Once fitted in the body, the stent expands—creating a reverse martensite to austenite transformation—but since it’s constrained within arterial walls, complete strain recovery doesn’t happen. This a benefit often referred to as biased stiffness.

There are automated or semi-automated grinding machines that can reduce labor and increase the quality of grinding samples. Manual grinding takes an employee of more complex tasks that could slow down production.

Stainless steels. Manufacturers conventionally use stainless steels such as AISI 316L in orthopedics, owing to its advantages of being low in cost, offering good mechanical properties, and being easy to process. Stainless steel alloys can, however, pose challenges such as incompatibility with surrounding bone tissue (due to having a higher density) and the likelihood of the alloy corroding in a bodily fluid environment over time. This is a major concern as the resultant effect of corrosion byproducts on human tissue can pose a severe health risk.

Nickel ions, for instance, are known to act as allergens that result in inflammations; they may lead to carcinogenicity in the body, hence the development of low-nickel or nickel-free steels. However, stainless steels are still commonly used for surgical implants and instruments. They can be found in stents, fracture fixation plates and screws, spinal implant devices, and aneurysm clips, to give a few examples. The prolonged use has been attributed to surface modification to significantly improve surface passivity and through an electropolishing process to improve surface Cr concentration for better passivity.

Stents help keep coronary arteries open and reduce the chance of a heart attack. A stent is inserted into the clogged artery with a balloon catheter. The balloon is inflated, and the stent expands and locks in place. This holds the artery open and allows blood to flow more freely.

Cobalt-based alloys. Cobalt alloys are structurally tough materials with excellent wear and corrosion resistance and good biocompatibility. They are costly to manufacture through conventional machining processes compared to iron- or titanium-based medical alloys, hence the preference for medical applications. They are commonly found as artificial hips and knee condyles but also used in acetabular cups and as tibial trays.

Medical Products Depend on Accurate Analysis

In the quality assurance process for manufacturing products used to replace human body parts, the grinding-polishing process involves multiple steps that require the selection of proper surfaces and abrasives at each point in the process. The goal is to provide an acceptable surface for examination. While any kind of grinding-polishing system can provide this kind of finish, semi-automatic machines can enable the quality assurance process to keep pace with the high-volume manufacturing happening on the plant floor.

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