From securing the simplest bandage to adhering components of the most complex diagnostic devices, pressure-sensitive adhesives (PSAs) have long had a crucial role to play in the medical industry. PSAs are versatile, capable of being formulated to stick well to practically any substrate, and are low-profile, delivering appropriate levels of permanent adhesion or removability without compromising device function or aesthetics. As ubiquitous as they may be in medical design, however, PSAs have a drawback that has kept them from being truly ideal for use in this field at times—the side effects of the method in which they are coated onto substrates.
Solvent: A Problem
For some time, solvents have been the primary means of adhesive coating for medical devices. Solvent coating is a widely used process in which adhesive polymers are diluted with a solvent to create a solution that is then coated onto a substrate. Solvent coating is used frequently with silicone-, acrylic-, and rubber-based adhesives. This is an effective process from a manufacturing standpoint, and has been the only way to coat acrylic and silicon for 25-plus years. The problem with solvent coating becomes apparent, however, when you consider the residual effects of using harsh solvents in a manufacturing process that will ultimately yield sensitive end-use medical equipment.
Solvents, by definition, must exhibit properties that allow them to dissolve another substance to create a solution. As such, they sometimes must be fairly harsh to break down certain compounds, and can require careful handling. This may seem like a concern primarily for adhesives manufacturers and converters that deal with solvents firsthand, and it is. But even after solvent-coated PSAs have been dried out by ovens during the manufacturing process, the PSAs still contain some level of solvent within them. As a result, solvent-coated products have can sometimes outgas volatile organic chemicals (VOCs), toxic residual monomers, and, potentially, even carcinogens, making them a less-than-ideal choice for certain medical uses.
While some solvents are safer than others, and in many cases the minimal levels of VOCs left in coated PSAs are not cause for major concern with regard to deleterious human health effects, outgassing of residual chemicals can remain a problem for other reasons. Consider, for example, a sensitive in vitro diagnostic assay device designed to test for minute chemical changes. While a solvent-coated adhesive used in this application may not be overtly dangerous, chemical migration from the PSA could be enough to throw off precise measurements used to make important healthcare decisions. In this case, testing devices are undermined by their very construction. Even in less sensitive medical applications, some may prefer to avoid the risk of any form of toxins being present in the first place. (See Figure 1)
Adding to mounting pressure to reconsider widespread solvent use in adhesive coating is a trend toward regulation of solvent applications across industries. Regulations in certain states and foreign countries already deter the use of solvents in market segments like paint and structural adhesives, and many believe it is likely only a matter of time before the appeal of solvent coating is a thing of the past.
So, you may wonder, where does this leave us? The industry clearly cannot afford to replace PSAs as a fastener technology on a broad scale. Are there other options?
There are, though until the past several years there were not many other great options for widespread medical adhesive coating. Rubber-based adhesives can be applied via a hot-melt process, requiring only elevated heat levels to soften them enough for coating. Rubber-based adhesives are only ideal for sticking to certain types of substrates, however, and they often lack some of the design flexibility that acrylic-based adhesives yield. Water-based emulsion coating methods are sometimes used to apply acrylic adhesives, but this method may not always meet the latex-free requirements of some medical device applications, limiting their usefulness on a major scale. (See Figure 2)
There is another coating option, however, that many overlook simply because many adhesive companies have not made the significant investments in technology necessary to offer it. And that option is 100 percent solids adhesive coating.
A Solid Choice for Adhesive Coating
100 percent solids coating is essentially what it sounds like—coating of solid adhesive material onto substrates without the need for a liquid carrier such as those used in solvent or emulsion coating. Unlike hot melt coating, where extreme heat is used to melt rubber-based adhesives, coating 100 percent solids acrylic requires far lower temperatures and the use of high-energy radiation to cross-link and cure the adhesives so that they reach a specified molecular weight. The result is an adhesive that is ultra-clean and does not suffer from many of the problems inherent in solvent coating.