The Crucial Divide between Hydrophobic and Hydrophilic Coatings
Knowing the advantages, characteristics and functions of a coating allows design engineers to make the right choice for their applications. Dr Josh Simon of Biocoat explains the important differences between hydrophobic and hydrophilic coatings.
It is easy to get the words "hydrophobic" and "hydrophilic" confused with one another because the two words are so similar. When distinguishing between hydrophobic and hydrophilic coatings for medical devices, the differences and similarities are especially important to remember. How can a person remember the main differentiating factors? For those inclined towards word origins, "phobic" invokes the Greek word for fear, so a hydrophobic coating "fears" water, and thus does not interact with it well. It may even repel the water. "Philic" invokes the Greek word for love, and by inference a hydrophilic coating is "water-loving." It readily interacts with water.
With these definitions, it now becomes easier to point out the similarities and differences. There are three major areas to think about: functions, applications and business models involved with the coatings. This article gives an overview of each area.
When it comes to function, there are similarities and differences between hydrophobic and hydrophilic coatings. Comparing functionality involves looking at physical properties such as contact angle, coefficient of friction, degree of cross-linking and durability.
The chief parameter for distinguishing between hydrophobic and hydrophilic surfaces is contact angle. This is the angle a droplet of fluid, usually water, makes at the point of contact with a rigid solid surface. For hydrophobic coatings, this angle is always greater than 90 degrees, and it can be as high as 150 degrees. Hydrophilic coatings always have contact angles less than 90 degrees and usually less than 50 degrees. A "perfect" hydrophilic surface would give a contact angle of zero degrees, that is, the water would sheet evenly on the surface.
Although both types of coatings have relatively low coefficients of friction compared with common substrates found in medical devices such as polyurethane or nylon, hydrophilic coatings tend to be an order of magnitude more lubricious. Some of the best hydrophobic coatings offer coefficients of friction in the range of approximately 0.15 to 0.3. By contrast, hydrophilic coatings that claim to be exceptionally lubricious have coefficients of friction in the range of 0.005 to 0.2. This advantage comes at a cost, however. Hydrophilic coatings, by their nature, must be wet in order to exhibit lubricity, whereas low friction hydrophobic coatings do not need to be wet. A dry hydrophobic coating is more lubricious than a dry hydrophilic coating in most cases.
One area where both types of coating are similar is how they are affected by cross-linking. Generally, there is an inverse relationship between lubricity and degree of cross-linking: the more cross-linked a material is, the less likely it is to be slippery. Cross-linking also affects basic polymeric properties such as glass transition temperature, stiffness and solubility. These factors, together with polar interactions, all relate to how a surface interacts with water because they control how easily a material absorbs or repels water. Physically, materials that are cross-linked also have higher durability.
Often, materials used in medical devices do not require durability to be as high as materials used in industrial applications. For example, a catheter used for accessing the coronary arteries for stent delivery is a disposable device. A hydrophilic coating on that catheter would typically need to withstand one or two cycles of sterilisation and up to ten passes through the vasculature. The same coating on an outdoor lighting system exposed to sun, rain and temperature fluctuations for permanent duration would fail. A design engineer would choose a hydrophobic coating to withstand those conditions, a higher coefficient of friction not being as relevant.
In the medical device industry there is considerable overlap between applications that use hydrophobic and hydrophilic coatings. Sometimes, a project will start out using a hydrophobic coating only to find that it is not lubricious enough, and efforts will then focus on switching to a hydrophilic coating. Conversely, a hydrophilic coating may prove not to be durable enough for an application and the project team will entertain the idea of a semi-lubricious hydrophobic coating with higher durability.
Apart from this consideration, there are some clear cases where one type of coating is required over another. For example, project teams will need to understand their device requirements with regard to water barriers and optics.
A primary purpose of hydrophobic coatings such as polytetrafluoroethylene or polyxylylene is to act as a barrier against water. If a device must be sealed so that moisture does not get inside, one of these hydrophobic coatings will work to repel water from the device's surface and act as a sealant over areas where water can penetrate to the interior.
Hydrophilic coatings do the opposite. They imbibe water, and most of them are in fact comprised of more than 90% water when wet. Continuous diffusion of water across the thickness of these coatings occurs, and if this would be detrimental to the device, a hydrophilic coating is contraindicated. Granted, most medical hydrophilic coatings rely on primer coats or base coats for adhesion to a surface, and these primers tend to be relatively hydrophobic, which could cause them to act as water barriers too. However, often they are not designed for these purposes and would not constitute as good a barrier as a material such as polytetrafluoroethylene or polyxylylene.
For devices that require lenses or other optics that interact with water, a hydrophobic coating could be a liability. Consider a newly waxed car with a hydrophobic finish. After a rainstorm, beads of water can be seen on the surface. On a camera lens or sensor in a medical instrument this beading can impair imaging. If these surfaces are instead coated with hydrophilic material, the opposite effect would occur: the water would spread evenly over the surface, producing a nominal anti-fogging effect.
Different business models
There are crucial differences in business models for hydrophobic and hydrophilic coatings. Partially, these differences may be due to the maturity of the two industries, the hydrophobic coatings segment being much older. However, they also differ due to the level of know-how required, which stems from the physical realities of applying the coatings.
Essentially, hydrophobic coatings have matured into commodities since their early experimental usage before the 1940s. They are usually applied using standard vapour deposition or spray equipment that can coat nearly any surface morphology with materials such as polytetrafluoroethylene or polyxylylene. Like most other commodities, the business model entails placing an order for which the vendor generates a quote. Suppliers can be contacted for quotes on machinery for applying the coating or for contract coating services.
In stark contrast, hydrophilic coatings, especially as used in the medical field since the early 1980s, are not a commodity. They are an advanced, specialised, heavily regulated part of the overall good manufacturing practice developed for making medical devices.
Although the two industries are at different levels of evolution, there are indeed differences in the learning curve and level of know-how needed to effectively apply the two technologies. With some small amount of training and validation on pre-fabricated coating process equipment, a manufacturer could create usable products with hydrophobic coatings. This is because standard processing equipment for hydrophobic coatings is suitable for multiple projects without too much modification.
In contrast, hydrophilic coatings always require fine-tuning of process parameters and extensive modifications to process equipment. If a hydrophilic coating is sold as a bulk product, the client would run into significant delays when trying to develop and validate a process. Eventually, a usable process may emerge, but without help from a knowledgeable hydrophilic coating vendor, process development will be significantly delayed. Therefore, many idiosyncrasies exist with each hydrophilic coating that is on the market and even a coatings expert would take time to sort through them. Selling bottles of coating solution at near cost is not a particularly viable business model for hydrophilic coatings. The business model that is used for selling hydrophilic coatings employs a closely integrated relationship between client and vendor and is consequently more complex.
Initially, before solidifying a sale or relationship with a hydrophilic coating company, the client will need to work with the vendor to develop the application and determine if the coating is appropriate. That means sending the vendor samples of the medical device to have them coated as a test. It may take several rounds of testing back and forth to see if the coating can work. Not everything works on the first try.
Because hydrophilic coating methods include dipping and spraying, as opposed to chemical vapour deposition methods seen with some hydrophobic coatings, they are more sensitive to technique and differences in device configuration. Due to higher technical support and time requirements by hydrophilic coating clients, hydrophilic coating vendors often charge a licence fee and royalty. Some hydrophilic coatings are patented; with others their patents have expired. Either way, a licence fee will cover the patented material and/or the know-how. For this licence fee, the client should get substantial technical support, technology transfer help and a "can-do" customer service attitude.
Some differences among the various hydrophilic coating vendors occur in this "courtship process. Some vendors will charge high fees for "development." They will require a development contract with a down payment and milestone payments. Other vendors will not do this unless the project is so complex it would require significant time and resources from the coatings vendor to determine how to apply the coating. Another difference will occur with royalty payments. The actual royalty rate that each company charges is usually confidential. The combination of licence fee, reagent costs and royalties is unique to each relationship and often strikes a balance between volume and margins.
Given the differences in functions, applications and business models for hydrophobic and hydrophilic coatings, it is easy to suggest that the two industries will coexist indefinitely. Although some of the clientele overlap, there are definite niches of users that do not. Medical device design engineers should be aware of the advantages, characteristics and functions available in coatings to make the most suitable choice for the final application.
Josh Simon, MBA, PhD,
is Senior Product Manager at Biocoat Inc,
211 Witmer Road, Horsham, Pennsylvania 19044, USA,
tel. +1 215 734 0888,
Hydrophilic Coatings Blog
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