Coatings exist in multiple industries, aside from medical device coatings. In fact, there is an index of all kinds of coating methods on Wikipedia that is enlightening. When it comes to medical device coatings, I tend to think that the industry is behind on its technology, when compared with circuit board coating technology, for instance. Most of the coating processes in that index are never used in medical device manufacturing, as near as I can tell. The main reason for this is demand. Not all of those coating methods are necessary for medical devices, even though they are required for circuit boards or electronics. So, what are the most-used methods for coating medical devices?
One of the most common processes for coating medical devices is dip coating. There are generally five steps to it: 1) Surface preparation/washing 2) Submersion of the device in a coating liquid (with a certain dwelling time in some instances) 3) Withdrawal from the coating liquid, i.e. coating application/deposition 4) Drying and/or Curing of the coating (via heat or UV) 5) Post processing, if any. Especially with heat curing, this is a batch process. With proper equipment, the batches can be quite large, giving a decent effective throughput.
Spray coat systems use a special nozzle and driver to nebulize the coating solution and apply it to the surface as a mist. Some of these systems, like the one from Sono-Tek use ultrasound transducers to control spray droplet size, whcih can impact the thickness and quality of the coating. In general, spray coating systems can be set up as continuous processes. While they are often too slow for mass-produced electronics, they can work well for most normal medical device volumes. (Of course, that is not the case if you are in the business of medical disposables sold by the millions per week.)
Reel-to-Reel coatings are most often seen for guide wires and films. They are usually not applicable to small intricate devices. Essentially, this method is literally what it says it is. One reel of wire or film is unraveled and travels through a resevoir of coating solution and then into an oven for drying/curing, before being rolled up onto the second reel. As far as continuous processing goes, reel-to-reel coating is effective. The trick is to align it with the idiosyncracies of a given coating. It can be difficult to make a reel go slow enough or through a big enough oven to cure a coating, for example.
This method is used mostly on complicated shapes, like stents. Tiny nozzles directed robotically can trace along struts and other structures with precision. By dialing in the viscosity of the coating solution, a set amount of coating with a set thickness can cover the surface. If all things are equal with the type of coating, this process is amenable to a continuous system.
Of all methods, brush coating is most primitive. Using a brush, a skilled operator applies a coating to a surface. Since this is a manual process, there can be a lot of variability in coating quality. However, with skilled operators it can be quite acceptable. Naturally, as one may suspect, this process is not amenable to large volumes. However, it is surprising to inspect some facilities and see 300+ individuals brushing on coating at a rate of several devices per minute. It deserves some consideration.
In this method, a flat surface is set onto a plate and locked down, usually with a strong vacuum or magnet. A motor then spins the surface quickly, while a droplet of coating solution is deposited at the exact center of the spinning surface. Centrifugal force instantly draws the droplet out away from the axis of rotation, causing the surface to be coated. Parameters like solution viscosity and angular velocity are important for determining coating thickness and quality. It can be difficult to produce large numbers of devices with this method.
Inevitably, there will be other methods for coating medical devices. At this time, these seem to be the most popular. Each method has inherent pros and cons, and not all methods are applicable to all devices or materials.