Medical Device Surface Treatment
Surface roughness level is a parameter of surface quality, commonly measured as the average deviation of the surface profile from the mean line (Ra). Surface smoothness is one of the parameters used to evaluate medical device functionality, safety and quality. Smooth surfaces will be always easier to clean and will minimize dirt accumulation and contamination.
Moreover, improper surface roughness of a medical device may lead to blood clots and tissue reactions. Smooth surfaces assist in preventing platelet activation and aggregation. In the pharmaceutical industry, surface material and roughness levels are very important in product contact materials for achieving the required cleanliness level and minimizing the chances of contamination.
There are several common methods of surface treatment, which include acid pickling, electrochemical polishing and passivation.
Acid pickling is a chemical treatment for the removal physical and chemical contamination from the surfaces of metallic materials by immersion in an acid solution. Contamination may be present as result of heat treatment, welding or other physical processes. Where colored oxide deposits can be visually detected, there is a chromium-depleted layer underneath.
The lower chromium layer gives lower corrosion resistance. To restore this corrosion resistance, the outer layer should be removed by exposing the fully alloyed stainless steel surface. Mechanical removal may leave high roughness, or other particles, on the surface. Pickling solutions include high concentrations of nitric acid (HNO3) and hydrofluoric acid (HF) to remove the scale and the depleted chromium layer.
The duration and temperature of the treatment may be optimized. Pickling is used for metal removal and can cause the metal surface to “shine”. Pickling solutions are suitable for stainless steel, are highly corrosive to carbon steel and have a greater effect on metal surfaces than passivation.
In this process, a metal surface is smoothed by submerging a piece of metal medical device (anode) and a cathode into an electrolyte solution bath and applying DC power. The anode is connected to the DC positive terminal and the cathode is connected to the negative terminal.
When electropolishing chromium-nickel steel alloys, the concentration of chromium on the surface increases (iron and nickel are being etched at a faster rate), so the surface becomes much harder. The anode device is oxidized and dissolves in the electrolyte, resulting in a smooth surface.
Advantages of electropolishing over other techniques are:
- its ability to reach various and difficult-to-access areas on stainless steel surfaces.
- it does not “fold” the stainless steel bumps on the surface. Instead, it disconnects and removes them from the treated surface.
- that coating the outer stainless steel surface with a layer of chromium and nickel improves the durability of the stainless steel.
- it removes relatively low amounts of metal from the surface and thus will not damage the metal and/or change its mechanical properties.
- it gives a beautiful aesthetic result (shiny surfaces).
A process of dissolving any carbon steel contamination from the surface of stainless steel medical devices or pharmaceutical machinery. Passivation does not typically go below the surface of the metal and does not change the properties of the metal.
Passivation may be done with nitric acid (HNO3) or citric acid (C6H8O7) solutions which are not as aggressive (less concentrated) as the acids used in pickling. The use of passivation is intended to target contamination and aid in creating a passive oxide film on the surface. Passivation does not usually result in a marked change in appearance of the steel surface.
Pickling and passivation involve use of dangerous acids and thus adequate precautions must be taken. Additional information can be found in ASTM A380, Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
The use of biocompatible materials in medical devices involves the consideration of the intended use, time of exposure and location (skin surface/topical, implant, external communication, blood circulation, etc.). The use of implants for the human body began in the late 1950s; those uses require assurance that the materials and devices are safe and effective.
Implantable medical devices are expected to function for many years, stay attached to its relevant area/organ and not cause any adverse response from tissue that could compromise the performance of the device or the health of the patient. The biocompatibility of a medical device material depends on corrosion, degradation or a specific biological response.
Examples of such materials include corrosion resistant alloys (titanium, cobalt, platinum), inert oxide ceramics (alumina, zirconia), and biostable polymers (polyethylene, polypropylene, polytetrafluoroethylene). Biomaterial safety requirements include minimizing interactions with human body tissue and development of bodily resistance.
Biomaterials must pass biological safety tests in in vitro and in vivo studies. Coatings used may be a key factor in implantable devices. Often the use of polymer coating is used to improve surface roughness, enhance lubricity, and improve resistance to friction, chips and device impact protection, inhibition of blood coagulation, hydroscopic or hydrophobic surfaces.
Biological medical devices must undergo characterization analyses of biological and toxicological risks from extractable and leachable materials, including cleaning and sterilization residues, and the potential risk to the patient must be evaluated.
See ISO 10993-17:2002, Biological evaluation of medical devices – Part 17: Establishment of allowable limits for leachable substances, for details about why risk assessment is an essential part of material biocompatibility and necessary for ensuring patient health and safety.