Medical device surface treatment
Surface roughness level is a parameter of the 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 devices functionality, safety and quality. Smooth surface will be always easier to clean and will minimize dirt accumulation and contamination. Moreover, improper surface roughness of medical device may lead to blood clot and tissue reaction. Smooth surface will assist preventing platelet activation and aggregation. In the pharmaceutical industry, surface material and roughness level are very important in product contact material for achieving the required cleanness level and minimize the chances for contamination.
There are several common methods of surface treatment which include Electrochemical polishing, Acid Pickling and Passivation.
A method for chemical treatment for surface physical and chemical contamination removal from metallic materials by immersion in an acid solution. Contamination may be present as result of heat treatment, welding or other physical processes. Were colored oxides deposits can be visually detected, there is underneath a chromium depleted layer. The lower chromium layer gives lower corrosion resistance. In order to restore the corrosion resistance, the layer should be removed exposing the fully alloyed stainless steel surface. Mechanical removal may leave high roughness surface or other particles. Pickling solutions include high concentration 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 and are highly corrosive to carbon steel and have a greater effect on metal surface then Passivation.
The process in which a metal surface is smoothed by submerging a piece of metal medical device (Anode) and a Cathode into an electrolyte bath and applying DC power. The anode is connected to the positive terminal of the DC and the cathode is connected to the negative terminal. When electro polishing chromium-nickel steel alloys the concentration of chromium on the surface increases (iron and nickel are being etched with a faster rate), so the surface becomes much harder. The anode device is oxidized and dissolves in to the electrolyte results in a smooth surface.
The advantages of electro-polishing over other techniques are:
- Accessibility to various areas and difficult to access over the stainless steel surfaces.
- Doesn’t “fold” the stainless steel bumps on the surface. Instead, disconnect and remove them from the treated surface.
- Coating the outer stainless steel surface with a layer of chromium and nickel that improve the durability of stainless steel features
- Relatively, remove low amounts of metal from the surface and thus won’t damage metal and/or change its mechanical properties.
- In electro-polishing, we receive a beautiful aesthetic result (shiny parts surface).
A process of dissolving any carbon steel contamination from the surface of the stainless steel medical devices/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 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 a 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 the utilization of dangerous acids and 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 material in medical devices involves the consideration of intendent use, time of exposure and location (surface skin, implant, external communicating, blood circulation etc.). The use of implants for the human body began in the late 50’s, those uses required to ensure the materials and devices to be safe and effective. The Implantable medical devices are expected to function for many years, stay attached to its relevant area/organ and do not cause any adverse response from tissue that can compromise the performance of the device or health to the patient. The biocompatibility of the medical device material depends on corrosion, degradation or specific biological response. Examples of corrosion resistant alloys (titanium, cobalt, platinum), inert of oxide ceramics (alumina, zirconia), and biostable polymers (polyethylene, polypropylene, polytetrafluoroethylene). Biomaterials safety requirement is to have minimum interact human body tissue and resistance. Biomaterials must pass biological safety tests as part of in vitro and in vivo studies. A coating is 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 impact device protection, inhibition of blood coagulation, hydroscopic or hydrophobic surface. Biological medical devices must undergo analysis characterization of biological and toxicological risk from extractable including cleaning and sterilization residues and evaluate the potential risk to patient. 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 are necessary for ensuring patient health and safety.