Medical device designers frequently face the need to adhere products to the skin, whether it is for a few minutes or in perpetuity. The huge array of pressure-sensitive materials available can make product selection a daunting task. This article outlines some of the most basic considerations that one should take into account when choosing pressure-sensitive adhesive tapes for skin-contact applications.
Pressure-sensitive adhesives (PSAs) are used in a wide array of medical applications where it is necessary to bond a medical device to the skin. PSAs are convenient to use because they require nothing more than light pressure to become firmly attached. They are usually supplied as tapes, where thin films of adhesive have been laminated to carrier materials and release liners. A few of the more common formats are illustrated in Figure 1. When selecting an adhesive one must consider the tape as a whole because each layer is influential in determining the tape’s overall performance.
Three of the most common formats for pressure-sensitive adhesives are transfer tapes, single-coated tapes, and double-coated tapes. Transfer tapes consist simply of a PSA layer and removable liner. Single-coated tapes have a PSA layer laminated to a carrier on one side and a removable liner on the other. Double-coated tapes feature two layers of PSA (not necessarily identical) laminated to either side of a carrier and protected on one side by a removable liner. The choice of format depends on the tape’s intended purpose (e.g., a doublecoated tape might attach to a device on one side and to skin on the other).
It is often useful to categorize PSAs into broad sub-classes based on chemical composition because some basic properties can be generalized within each family. Rubber adhesives usually consist of hydrophobic polymers, such as polyisoprene or polyisobutylene, combined with small-molecule tackifiers, plasticizers, and antioxidants. They adhere aggressively to skin and are recommended for demanding applications where bond strength is the primary requirement. Acrylics are the most diverse class of PSAs. The category captures any material derived from the wide array of acrylic monomers. Depending on the choice of monomers, acrylics can be designed to meet specifications for adhesion, breathability, etc. Silicone PSAs are based on polydimethylsiloxane polymers, with or without silicate resin reinforcers and tackifiers. Soft gel silicones can offer repositionability and gentle removal, or silicone PSAs can offer performance characteristics similar to acrylics.
The tape’s backing material usually serves a purpose beyond its role as a carrier for the PSA layer. Foam or nonwoven carriers can be chosen to provide tactile comfort or cushioning effects. In the case of filmic carriers, properties to consider might include transparency, printability, color, or breathability. Regardless of what material is chosen, the stiffness of the carrier will influence the tape’s ability to conform to topologically complex surfaces and will also influence properties like peel adhesion.
Choosing the right level of adhesion is particularly critical for skin-contacting applications because there is a careful balance to be struck between secure attachment and easy removal. On the one hand, an aggressive adhesive is desirable to maintain good bond strength on the body’s curved and moving surfaces in spite of factors detrimental to adhesion, including skin roughness, surface oils, and hair. But, on the other hand, overly aggressive adhesives can traumatize skin or cause pain when they are removed. Fortunately, an adhesive’s “aggressiveness” is not one-dimensional; a good understanding of the device’s use case enables the designer to articulate the proper tradeoff between the various aspects of adhesion.
Peel, tack, and shear are the three most commonly reported facets of adhesive strength. None of these are intrinsic properties, but rather they are demonstrative data points measured under specific laboratory conditions chosen to highlight the adhesive’s resistance to failure under different types of mechanical loads. When interpreting peel, tack, and shear data one must always be conscious of how closely the laboratory conditions mimic the actual end-use conditions.
Peel adhesion is probably the most widely reported performance specification for adhesives. It gives an indication of the total interfacial bond energy, defined as the force required to remove a specimen by pulling it off of a substrate at a particular speed and angle. Peel adhesion is usually expressed in units of force per unit width, which is equivalent to energy per unit of interfacial area. Peel adhesion values depend on the substrate that is used, the time lapse between adhesive application and removal, and other details, such as ambient temperature and humidity or the application pressure.
Peel adhesion values are best used to make relative judgments within a group of adhesive candidates, as long as identical test conditions enable “apples-toapples” comparisons. Ideally, those conditions will be chosen within a reasonable approximation of the expected use. Consider how the device’s size and shape will influence the likely range of peel angles and rates that users will employ for removing the device, and choose dwell times commensurate with the device’s expected duration of use. In terms of substrates, stainless steel is the most commonly used for peel adhesion testing because it is recommended in industry standards for quality control measurements, including the Pressure Sensitive Tape Council’s “PSTC Test Methods for Pressure Sensitive Adhesive Tapes, 15th Edition,” 2007, and ASTM International Standard D3330/D3330M-04, “Standard Test Method for Peel Adhesion of Pressure- Sensitive Tape,” 2010. However, with respect to predicting end-use performance, low-energy substrates like polyethylene offer a closer approximation to human skin.
Figure 2 illustrates the importance of choosing appropriate peel conditions. Based on the three tapes’ peels on steel, one would conclude that Tapes A and B are similar and far more aggressive than Tape C. But, data on polyethylene suggest that the tapes vary in their adhesive power (Tape A > Tape C > Tape B). In this case, the polyethylene peel data correctly predict the three tapes’ relative performance on skin.
Tack is a measure of the adhesive’s initial bond strength prior to applying pressure or allowing any significant contact time to elapse. This property is often responsible for a user’s initial, qualitative impression of an adhesive’s holding power. There are several different techniques available for measuring tack, including probe tack, rolling ball tack, and “quick stick” peel tests, with loop tack being one of the most popularly reported today. Loop tack involves lightly contacting a piece of tape with a substrate (often glass) then measuring the maximum force required to remove it. As in peel adhesion testing, the values can be used to make relative judgments within a family of materials tested the same way. Since removal angle and contact time are fixed, the choice of substrate is the primary influencing variable for tack measurements.
Shear strength is the adhesive’s ability to maintain a bond under the influence of a static load. This attribute is important when securing a weighty device to the skin or when requiring a relatively stiff construction to conform to a curved surface. Shear is usually measured by using the adhesive in question to secure a weight to a steel panel, and then measuring the time it takes for the adhesive to fail and the weight to fall. By this method, longer failure times are associated with higher shear strength. The weight should be chosen to reflect loads encountered in end-use conditions, and the test can be conducted at elevated temperature, if desired, to mimic the surface temperature of skin. Just as with peel and tack, alternative substrates can also be considered instead of steel.
Moisture Handling (Breathability & Absorption)
Unless intended for only brief skin contact, medical adhesives are frequently required to have some degree of moisture handling capability. Tapes that stay in contact with skin for more than a few hours need to accommodate the skin’s natural transepidermal water loss to avoid excessive hydration of the underlying skin. The moisture vapor transmission rate (MVTR) expresses its ability to transmit water vapor and, as a general rule, it should be greater than 500 g/m2 per day to be considered breathable. The skin’s natural rate of transepidermal water loss varies by anatomic location, but is generally greater than 250 g/m2 per day. MVTR depends on the chemical composition of the adhesive and its layer thickness, as well as the breathability of the tape’s carrier. Highly breathable tapes with MVTR in excess of 1,000 g/m2 per day can be made, for example, from thin layers of acrylic PSA laminated to polyurethane films.
The capacity to absorb fluids may be important either in long-term wear applications or in cases where the device encounters bodily fluids. Adhesives designed to absorb fluid are less likely to fail under wet conditions, and can sequester irritants, keeping them out of contact with skin. Rubber adhesives incorporating absorbent material, known as hydrocolloid adhesives, can capture significant quantities of fluid while remaining adhered to the skin. Absorbent acrylics are a newly introduced class of PSAs offering a combination of both breathability and absorption in a thin, transparent material.
A unique requirement for medical adhesives is the property of “biocompatibility,” encompassing the notions that the material is non-toxic, hypoallergenic, and generally non-injurious to the biological systems with which it interacts. Consideration of whether or not an adhesive is biocompatible must be made in the context of its intended use. How frequently will the adhesive come into contact with the skin, and what is the typical duration of that contact? Will the adhesive be applied to wounded skin or mucous membranes? With an understanding of the typical use case, an assessment of biocompatibility can be made.
The International Organization for Standardization (ISO) publishes a widely- accepted set of standards for biocompatibility testing known as ISO 10993. Tests for cytotoxicity, irritation, and sensitization are recommended for all skincontacting adhesives, and additional toxicity testing may be warranted if the tape will contact anything other than intact skin.
Not all skin-contacting adhesives will undergo sterilization, but if this is a requirement one must carefully consider the method of sterilization and its potential effects on the adhesive’s performance. Moist heat, ethylene oxide, and gamma irradiation are the three most common sterilization techniques for medical devices, and different PSAs can be more or less resistant to the potential abuses inflicted by each. A summary of the potential degradative effects of sterilization may be found in Table 1.
Medical device designers sourcing non-sterile tapes are advised to test the effect of anticipated sterilization cycles on the material during the product development phase. Adhesive manufacturers can often recommend appropriate sterilization methods or caution against potential pitfalls.
Adhering a medical device to skin is a challenging proposition because the skin is a complex, dynamic, and highly variable living system. An expectation for adhesive performance in vivo cannot be captured by a single peel value or a specification for MVTR. But a comprehensive understanding of the factors at play and their connection to technical specifications can be a catalyst for critical thinking during the materials selection process.
This article was written by Neal Carty, PhD, Research Associate, Vancive Medical Technologies, Mentor, OH. For more information, Click Here