A Closer Look at Medical Adhesive Tapes for Diagnostic Applications
How they differ from skin applications and more
When someone mentions medical adhesives often the first thought to come to mind is skin adhesion. It’s understandable since nearly everyone has worn some type of medical adhesive product, from a simple bandage to an electrode, athletic tape, IV cover, and so on. But there is a whole other world of medical adhesives that goes unnoticed by many – diagnostics.
What exactly do we mean by medical diagnostic adhesives? Specifically, we are referring to adhesive tapes used in the construction or assembly of in-vitro diagnostic devices. These devices are used either by consumers at home or by medical professionals to perform a series of tests or “Assays” to help achieve a diagnosis. One of the most commonly known diagnostic devices is the at-home pregnancy test. The user places the sampling end of the device in the urine stream, waits a few minutes, and obtains a result (diagnosis). Another common and widely used example is a blood glucose test or diabetic test strips. According to (bit complicated, in 2011 we had a report saying 11 billion were used. So, I did the math with today's numbers for CDC and Mexico & Canada figures, and the 15 million diabetics using insulin and testing 3x average per day works out to 16.5 billion for North America. ), approximately 17 billion test strips were used in North America in 2020, with an average price per strip at the pharmacy of $1.50. Revenues from blood glucose testing strips alone are greater than several skin adhesive markets combined.
What exactly do we mean by medical diagnostic adhesives? Specifically, we are referring to adhesive tapes used in the construction or assembly of in-vitro diagnostic devices. These devices are used either by consumers at home or by medical professionals to perform a series of tests or “Assays” to help achieve a diagnosis. One of the most commonly known diagnostic devices is the at-home pregnancy test. The user places the sampling end of the device in the urine stream, waits a few minutes and obtains a result (diagnosis). Another common and widely used example is a blood glucose test, or diabetic test strips. According to (bit complicated , in 2011 we had a report saying 11 billion were used. So, I did the math with todays numbers for CDC and Mexico & Canada figures and the 15 million diabetics using insulin and testing 3x average per day works out to 16.5 billion for North America. ), approximately 17 billion test strips were used in North America in 2020, with an average price per strip at the pharmacy of $1.50. Revenues from blood glucose testing strips alone are greater than several skin adhesive markets combined.
In addition to home applications, there are endless diagnostic applications that take place behind the scenes. For example, COVID-19 tests and blood samples from patient blood draws. These specimens are taken to a lab and placed onto a device where a series of assay’s occur and report the result or diagnosis. Through the analyzation of blood, saliva, urine or tissue, there are hundreds of tests that can be performed to provide insight into bodily function or presence of disease. Diagnoses can range from drug screening to cancer detection.
Several different types of diagnostic devices vary in size, functionality and complexity – and nearly all utilize adhesives in some manner. Examples include:
- Urinalysis strips
- Lateral flow devices – pregnancy tests
- Micro-fluidic devices – blood glucose, blood testing
- Micro-titer plates – used in drug discovery, lab work
- Microwells – Microbial and pathogen detection
- Microarray – Genetic analysis
- PCR (polymerase chain reaction) – DNA amplification
Although each type of diagnostic device is unique in its workings, when it comes to adhesive requirements, they share many similarities.
Adhesive requirements
One of the first questions asked of medical diagnostic adhesives experts is, “What regulatory requirements are associated with diagnostic adhesives?” Unlike skin adhesives, which must have ISO:10993 or USP Class VI to even be considered, there is no standardized testing requirements that must be met. Patient safety isn’t a concern for the adhesive since it’s not going to be used on a patient. However, many people are surprised to find that, although it’s extremely rare, an adhesive approved for skin contact can also be suitable for use in diagnostic devices. Skin adhesives rarely have the necessary level of cleanliness required for use in diagnostic applications. Skin adhesive applications have a much wider breadth of materials that are suitable for skin contact. The primary function of the skin is to act as a protective barrier against the harsh external environment and protect the delicate chemistries and biological process occurring inside the body. With in-vitro diagnostics, however, those chemistries and biological workings are being taken from inside the protective environment of the body and placed into a much less favorable environment of a diagnostic device.
The requirements for diagnostic adhesives depend on application, but as mentioned, there are many similarities shared among device types. Importantly, the adhesive must not interfere with any of the delicate chemistries or biological processes occurring within the device. Because of this requirement, diagnostic adhesives are the cleanest adhesives used in tapes. They have extremely low outgassing, VOCs (volatile organic compounds) and adhesive component migration. The sensitive assay chemistries are easily corrupted by even the slightest impurity that could be imparted from the adhesive. In many instances, the volatile and migratory components of adhesives are comprised sites of chemical functionality or reactivity. It is important that diagnostic adhesives be as inert as possible with little to no residue chemical reactivity. Any residue monomer, cross-linker, vinyl functionality, reactive sites or acid functionality can hinder or interfere with the device.
This is one of the main reasons water-based acrylic or rubber-based adhesives are not used in this market space. With all the surfactants, wetting and defoaming agents, anti-microbial compounds and more, water-based adhesives simply cannot meet even the least stringent of cleanliness requirements for diagnostic devices. Rubber based adhesives are suitable in a few applications, but the large degree of unsaturation and antioxidant often interfere with various types of detection methods utilized. The pressure-sensitive adhesives (PSAs) that are used by tape manufactures for diagnostic devices are pure acrylics or silicone adhesives with no additives or modifiers. These adhesives can be cast from solvent or coated using more modern ultra-violet (UV) curing techniques. It is necessary to apply extra drying and reaction/curing for these adhesives to ensure the cleanest most inert adhesive possible.
Some types of assays rely a form of optical detection or analysis involving light. These devices are commonly referred to as auto fluorescence. Light at a particular wavelength is shined onto the device and excites the target analyte, which in turn emits (fluoresces) light back at another wavelength. The machine then measures to provide the result. Envision the effect of a blacklight shining on a white fabric or neon paint or ink. Adhesive tapes for these types of devices must not only be optically clear to the naked eye, but also the wavelengths of light being used, to excite and fluoresce back. As an example, the highest end ultra-clear PET (polyethylene terephthalate) adhesive laminates used in high-end optical applications such as TV screens, wouldn’t be suitable in these devices due to the way they absorb ligh, which would obscure the results. Adhesives for these applications must not only be clean, but of specific monomer composition as to not interfere with the transmission of light wavelengths.
Diagnostic adhesives can be exposed to different environmental conditions, including prolonged exposure to acids, bases, alcohols and many kinds of reagents and buffer solutions. Certain methods require the device be exposed to elevated temperatures either continuously or cycled. It is essential that the adhesive maintain its chemical stability and adhesion integrity. In many devices, the adhesive is used to seal wells or channels containing fluids. If the adhesive bond begins to fail this can lead to leakages and/or cross contamination compromising the functionality of the device.
While it is necessary for the adhesive to aggressively bond to the many different types of plastics used in devices to prevent leakage, it’s also important that the adhesive doesn’t have too much flow. Many times, these properties work in opposition to each other. Devices, such as those that are microfluidic, work by transporting small amounts of biological fluids through very small channels through the device to reagents sites. Often, the flow of fluids relies on capillary action alone. If an adhesive has excessive creep, it’s very easy for the microchannels to become occluded, restricting capillary flow and ultimately leading to device failure. This can be exaggerated at elevated temperatures.
Difference in adhesion challenge
Although there are rigorous requirements that diagnostic adhesives must meet in terms of cleanliness and inertness, when it comes adhesion and bonding for these tapes, requirements are much simpler than tapes that are used to adhere to skin. Skin is by the far the most difficult and frustrating surface for which to design an adhesive tape. Skin is not a static material, but a complex, living and fluid system comprised of many different parts. It’s constantly passing moisture, excreting oils, and regenerating/shedding itself. It’s also responsible for the emotional responsiveness of the human sense of touch, or put simply, it’s the primary source of pain response. Skin adhesion is the only application for adhesive products in which pain must be considered during product development. The problem is pain is subjective and hard to quantify. Every person has a different threshold for pain and discomfort. Even on the same patient, pain responses can vary based on the area of the body. The rib cage and face area are more sensitive that the arms. Additionally, seasonality can also influence how a patient reacts to an adhesive tape as pain response is greater during the cold, dry winter months. And, with device manufacturers seeking to extend wear times of devices, balancing the Moisture Vapor Transmission Rates, wear times, aggressiveness and pain/trauma upon removal becomes increasing difficult.
Another challenge product developers face with skin adhesion is generating tangible results. Even with the tight design window for cleanliness for diagnostic adhesives, it’s relatively easy to measure adhesive performance accurately, repeatedly and with great accuracy. Analytical techniques such as headspace GC and FTIR analysis can be used to preciously measure performance, and test devices can always be made and evaluated for functionality. Also, adhesion testing for diagnostic tapes is easily performed. Samples can simply be placed in an Instron and peel force is accurately and reproducibly reported. While there may be some variations in the plastics being used by suppliers, relativity speaking, adhesion to materials such as high impact polystyrene, ABS, PE, PP and TPO is straight-forward and well understood.
Skin on the other hand, is much more difficult to test. There are no adequate predictive materials for skin adhesion. Even cadaver skin does not work well for lab evaluation purposes because there are too many differences when compared to living skin. Unlike adhesion testing on rigid inanimate objects, it’s not practical to test peel adhesion off of skin using Instron or a similar type of tape tester. When designing adhesive tape products, scientists rely on practical testing and subjective user feedback, which must then be performed across a large data population. Many times, these evaluations need to be repeated multiple times to ensure reproducibility and validate results. This can take upwards of months, or even years, in some cases, and at significant expense.