Accurate measurement is essential to the practice of good medicine. Our collective ability to properly diagnose what is happening to the body has been a key driver of medical advancement throughout the years, but without reliable tests and testing equipment, such diagnoses would often be impossible. Because physicians, lab technicians, and other medical professionals must be committed to making data-driven decisions if they are to be successful, healthcare providers rely heavily on specialized diagnostic devices to provide the information they need to draw informed conclusions.

Maintaining the validity of assay results sometimes requires careful test protocol orchestration and very sensitive equipment. In other cases, the assay is simple enough, and accurate measurement only requires the avoidance of sample contamination — a danger of which most people are aware. What may be less obvious, however, is the potential for assay results to be compromised without any outside interference when the very composition of the diagnostic device undermines the equipment’s function.

A component as simple as the pressure-sensitive adhesive (PSA) binding the equipment together can affect readings of sensitive in vitro testing equipment if care is not taken to select a PSA optimized for the application. Therefore, it is important for medical technology designers to have at least a cursory knowledge of the types of PSAs used in medical device construction to ensure wise decisions are made. The good news is that improvements in coating technology are making it easier (and more economical) than ever to make wise adhesive choices in the medical field.

Adhesive Use in Diagnostic Devices
The use of PSAs within in vitro diagnostic (IVD) equipment is widespread, as these adhesives provide consistent, convenient bonding of a wide range of materials that comprise device components both large and small. PSAs can also be formulated to exhibit properties that make them compelling for practically any application, granting designers flexibility they may not have with other fastening methods. For IVD applications — such as urinalysis strips, lateral flow assays (e.g., pregnancy tests), microfluidic testing (e.g., glucose measurement), protein chain replication (PCR), or micro-titer plate construction — PSAs can also exhibit properties necessary to facilitate proper equipment function. Examples of desirable properties for PSAs used in IVD equipment include:

• Chemical inertness, with no residual monomers
• A lack of outgassing of volatile organic chemicals (VOCs) and other compounds
• Physical compatibility with test procedures
• Stability over a wide range of environmental conditions
• Proper hydrophobic or hydrophilic properties
• Appropriate electrical conductivity
• Necessary optical properties to facilitate test functionality
• High quality control during manufacturing
• Tight thickness tolerances to allow precise measurement

While PSAs can adhere to the strict standards outlined previously for IVD device bonding, it does not mean they always do. Even with careful selection of materials and precise formulation, the method of coating the PSA to a substrate may be all it takes to compromise end-use device accuracy. Consider, for example, the fact that many PSAs used in the medical industry are still coated onto surfaces using solvents. While convenient for manufacturers because of the investments they have made in solvent-coating technology, this method is less than ideal for IVD construction for several reasons.

First, solvents are, by their very nature, harsh chemicals. They may contain VOCs and even carcinogens in certain circumstances, and thus, there is danger of outgassing or leaching of dangerous residual compounds from PSAs coated using these solvents. Besides any health hazards these solvent-derived compounds could cause, they can interfere with sensitive diagnostic equipment, yielding validity issues with assay results. From a process standpoint, solvent coating can also be a messy and slow process. The solution-based adhesive coated to the substrate is only comprised of 30 to 60 percent true adhesive components; the rest is solvent that must be dried off, minimizing the speed at which machinery can run while the adhesive is cured.

It is also worth noting that regulations limiting the use of solvents in manufacturing are already in effect in certain foreign countries and under Proposition 65 in California, though most regulations to date focus on handling and application of solvents. We have seen regulation of solvent usage in the paint and structural adhesive industry, however, and it is likely only a matter of time before more mandates limiting the use of solvent coatings appear for PSA production.

The Case for 100 Percent Solids
There is another option for PSA coating in medical (and other) applications that yields an ultra-clean adhesive at a competitive price point: 100 percent solids coating. In this process, no solvent is used — only pure adhesive components that are applied consistently at a specified molecular weight to substrates by using high-energy beams to crosslink compounds. This process can take place at room temperature, meaning no energy-consuming drying ovens or challenging cleanup is required. Unlike relatively expensive solvent-coated acrylic adhesives, 100 percent solids-coated acrylic adhesives can be produced much more cost-effectively. These PSA products are inert, non-outgassing, non-fluorescing, non-migratory, and usable in applications requiring very precise deposition control, making them practically perfect for IVD equipment manufacturing. They also rarely require the use of tackifier resins or plasticizers to create, making them safe for use in applications where there is sensitivity to these materials.

At this point, the question may be, “If 100 percent solids coating is so ideal for medical applications from a performance and price standpoint, why does anyone still use other coating methods?” The answer lies in simple infrastructure realities. The technology required to perform 100 percent solids coating — especially at scale — requires investments of tens of millions of dollars, and most adhesives companies simply do not have the equipment in place to coat in this manner. Those that do have the technology, however, can now coat adhesives onto substrates more cleanly, quickly, and cost effectively than previously possible.

Find the Right Solution
For designers who have never thought about how adhesive choice could affect assays, now is a great time to start. Consider 100 percent solids coating as a means of applying those adhesives; IVD equipment manufacturers would be well served to do so moving forward, as there are abundant advantages to relying on this process. Start by seeking out suppliers that offer this coating method, and then work with them to ensure the adhesive selected is formulated, coated, and handled in a manner that suits the application. Through this process, an IVD equipment company will be best positioned to provide not only a well-constructed product, but trustworthy assay results that can be relied upon when making important decisions about people’s health and wellness.