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

Fig. 1 – Given the sensitive nature of many in vitro diagnostic assays, 100% solids coating provides an alternative to traditional solvent-coated adhesives.

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?

Fig. 2 – 100% solids coating can be ideal for applications where latex-free and migration-free requirements exist, such as surgical or incise drapes.
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.

Benefits of 100 percent solids coated adhesives include:

Chemical inertia, preventing their interference with assay chemistries, reactions, or culturing;

Extremely low outgassing due to the absence of residual monomers, inhibitors, or stabilizers, since impurities are stripped from the adhesive during processing;

  • Non-fluorescing properties, allowing compatibility with devices that are vulnerable to solvent interference;
  • Tailored performance to exhibit desired levels of hydrophilic/hydrophobic properties;
  • Resistance to extreme temperature, chemicals, moisture, and pH levels ranging from at least 4 to 13;
  • Precise deposition control for strictly controlled coat weights; and
  • Infrequent need for plasticizers or tackifier resins, which sometimes cause concern in medical applications.

Given that 100 percent solids coating offers many advantages over other coating technologies and requires major capital investments in infrastructure by adhesives companies, it is natural to assume it would be expensive. Perhaps surprising, however, is that the process is actually quite cost-effective compared to solvent coating. This is because 100 percent solids coating exhibits:

  • Minimal material waste (100 percent adhesive components being used versus 30 to 60 percent in solution-coated adhesives);
  • Lack of high-temperature ovens needed for drying means minimal energy consumption, making this a “green” technology;
  • None of the messy cleanup associated with solvent or emulsion solutions;
  • Low volume requirements for coating runs (with the ability to manufacture at scale as needed); and
  • Faster processing speeds and easier converting.

All in all, 100 percent solids coating is a very efficient, effective means of applying adhesives to a substrate, which is especially important as the medical industry is continually challenged to reduce the overall cost of patient care.

Fig. 3 – 100% solids coated adhesives can provide the inertness required for use in wound care, catheter securement, and other skin-contact applications.
The ability of 100 percent solids coated acrylic PSAs to provide tailored adhesive performance without interfering with device function make them widely applicable in the field of medical design. Common applications include in vitro diagnostic device construction—testing materials for urinalysis, lateral flow, microtiter plates, microfluidic, protein chain replication (PCR), and enzyme-linked immunosorbent assay (ELISA), for example. They also have wide applicability in pharmaceutical labeling, sensitive electronic component adhesion, and more. This is because 100 percent solids coated adhesives are not only cost-effective and clean, but they can be very precisely deposited on free-film, single-coated, or double-coated product constructions. As a result, 100 percent solids coated PSAs are a versatile, cost-effective choice for most any medical design application you might consider. (See Figure 3)

Getting Started

While it may be simple to see the benefits of 100 percent solids coated PSAs over other adhesive coating methods, choosing the right supplier is critical to attaining desirable results. As you are considering your options, here are a few questions to ask:

Does the supplier have the coating technology available? 100 percent solids coating requires turning a high molecular weight, viscoelastic solid of nearly infinite viscosity into a consistent, uniform coating, and highly specialized, expensive equipment is required to make this possible. Many suppliers do not have this technology.

Does the supplier have the capabilities to produce according to your requirements? Some suppliers may offer 100 percent solids coating, but may not be able to produce at the scale you need or may not have adequate wide-web, extrusion, or high-speed processing capabilities to cost-effectively produce products that suit your needs.

Does the supplier have formulation and compounding expertise in-house? End-use performance by an adhesive has to do with more than how it is coated; design and testing expertise to customize an adhesive to deliver the exact qualities you need is also valuable.

Does the supplier have supply chain reliability? Especially when dealing with acrylic adhesives and the raw materials used to create them, security of supply is important and can be a problem for certain companies that often deal with specialty chemical shortages.

Does the supplier have knowledge of the medical field? There is frequently little room for error when creating medical devices, and familiarity with the industry, regulations/standards, and performance requirements of adhesives and the end-use devices they hold together are very beneficial.

Whether regulations focused on reducing solvent use, economic drivers, or practical applications considerations drive the movement toward more 100 percent solids adhesive coating in the medical industry, do not be surprised to see the technology revolutionize the way medical devices are constructed moving forward. If you can connect with a well-equipped and collaborative partner, you can likely start realizing the benefits today.

This article was written by Steve Schroff, Business Development Chemist, Mactac, Stow, OH. For more information, Click Here .