The performance of adhesives used for wearable medical device applications is critical to the efficacy of the final product, as an improperly affixed device is unlikely to achieve the desired result when applied to the patient. Aside from medical device applications, accurately predicting how an adhesive will be impacted by the sterilization process in order to make informed choices is also important for advanced wound care and consumer wellness applications such as first aid.
It is essential that adhesives remain securely affixed to dry, perspiring, or sensitive human skin — even during showering and physical exertion — yet be removable without causing skin damage or leaving residue. Therefore, understanding and accurately anticipating how an adhesive will perform is pivotal. But even with a thorough understanding of the formulation of the adhesive material, there are factors such as sterilization that can alter its properties and, ultimately, its effectiveness.
Sterilization for Skin-Contact Adhesives
In cases where a medical product's indications include “use on broken skin,” sterilization is necessary. Historically, the two most common sterilization methods have been ethylene oxide (EO) gas and gamma irradiation. However, there has recently been a rise in the utilization of electron-beam (e-beam) sterilization, which is preferable when nonpermeable packaging is indicated and/or when delicate electronics are incorporated into the device.
E-beam sterilization requires an electron beam accelerator to create a beam of fast-moving electrons that penetrate and ionize materials as the materials pass by the source on a conveyor belt. Electrons are able to penetrate the medical device's packaging to sterilize the product within. It is a carefully controlled and highly precise process that is particularly favorable for medical products because the high dosage rates means that sterilization is completed more quickly than by other methods, reducing the exposure time necessary — during which polymers may be affected. Another advantage is that no residual impurities are created or left behind. Therefore, after the sterilization process is complete, the already-packaged product is ready for shipping and use.
While there are many advantages to e-beam sterilization, it has been known to affect acrylics through both the formation of cross-links and through bond/chain breaking. Because skin-contact adhesives for wearable medical device applications are most commonly formulated using acrylics, this has the potential to impact efficacy. It is expected that different formulations of acrylic adhesives may experience lesser e-beam effects on physical properties than others, so understanding how an adhesive will be affected is critical to final product functionality.
Examining the Effects of e-Beam Sterilization
Different doses of e-beam sterilization were used to explore effects on the physical properties of four different, commonly used acrylic skin-contact adhesives (I to IV) to identify which formulations may be least affected by e-beam sterilization. A selection of medical-grade skin adhesives was coated onto substrates. Physical properties (shear, adhesion to release, 180° peel, and moisture vapor transmission rate [MVTR]) were tested before and after exposure to e-beam sterilization of 27 and 40 kGy.
Materials, Testing, and Results
The four adhesives were transfer coated via knife over roll onto a 76-lb double-silicone-coated liner. Each adhesive was laminated onto a 2-mil thick polyethylene terephthalate (PET) film and a 45-gsm nonwoven PET fabric. The physical properties of the adhesive laminates were tested prior to e-beam exposure and after both 27- and 40-kGy radiation. The data obtained is detailed below with initial (pre-e-beam) measurements being compared with post-e-beam values.
180° Peel. A strip measuring 1 × 7 in. was cut from the PET film laminate and ~6-in. length was adhered to a stainless-steel plate. The laminate was pressed onto the plate using a 2-kg roller and allowed to rest for 20 minutes before being removed at an angle of 180° at a rate of 12 in./min. The average peel was measured in lbf/in. (see Table 1).
4.4 psi Static Shear. A strip measuring 1× 3 in. was cut from the PET film laminate and was adhered to a stainless-steel plate such that an area of ½ × 1 in. of laminate was contacting the plate. The laminate was pressed onto the plate using a 2-kg roller and allowed to rest for 20 minutes. A 1-kg weight was hung from the bottom of the laminate strip and the time until the adhesive released from the plate was measured in minutes (see Table 2).
Adhesive Release (AR). A strip measuring 1 × 7 in. was cut from the PET film laminate, and the nonadherent side of the PU film was adhered to a stainless-steel plate using 6-in. length of double sided tape. The liner is removed at an angle of 135° at a rate of 300 in./min. The average peel was measured in g/in. (see Table 3).
MVTR. Moisture vapor transmission rate was measured by cutting out 3-in. diameter circles of the nonwoven PET laminates and placing them onto upright Thwing-Albert Vapometer MVTR cups containing 100 mL of water. The cups were then placed into a chamber at 98.6 °F/10% RH and weighed after 1 and 24 hours to calculate MVTR in g/24hr/m2 (see Table 4).
180° Peel. All adhesives, except for adhesive II, displayed a trend of a greater change in peel with increase in e-beam exposure (see Figure 1). Adhesive II displayed a reverse of this trend. There was little change (1–13 percent) in the 180° peel values after e-beam sterilization for adhesives I, II, and III, whether the e-beam exposure was 27 or 40 kGy. Adhesive IV did display a greater decrease in 180° peel value: 16 percent after 27 kGy exposure and 25 percent after 40 kGy exposure.
4.4 psi Static Shear. All adhesives except for adhesive III displayed a trend of a greater change in static shear with increase in e-beam exposure (see Figure 2 [Log Scale]). Adhesive III displayed very little difference in change dependent on the e-beam exposure. There were great differences is static shear for adhesives I and IV: from 274 percent increase to 5546 percent increase, while adhesives II and III had much less increase in shear: from a 0.8 percent decrease to a 51 percent increase for adhesive II and from a 46 to 49 percent decrease for adhesive III. In order to be able to observe all of the data on one graph, these values were plotted on a logarithmic scale.
Adhesive Release (AR). All adhesives displayed a trend of a greater change in AR with increase in e-beam exposure, with adhesives II and III displaying the least change in AR and adhesives I and IV displaying greater changes in AR (see Figure 3).
The increase in AR is not unexpected. The exposure of silicone-coated liners to irradiation is known to cause the silicone to cross-link, leading to higher release forces. To lessen this effect, it is common to specify an easy-release silicone-coated liner for products that are expected to undergo radiation sterilization.
MVTR. All adhesives, except for adhesive IV, displayed small (<7 percent) changes in MVTR. Adhesive IV displayed a 21–24 percent increase in MVTR (see Figure 4).
Although e-beam technology provides a relatively quick, effective, and safe way to sterilize medical device products, it is important to anticipate how it may impact the medical acrylic adhesive and to select an adhesive option least likely to be degraded during the process.
During testing of four different medical acrylic adhesives, two of the adhesives showed considerably less effect from e-beam radiation than the others. The main test data that set these two adhesives (II and III) apart from adhesives I and IV were static shear and MVTR. The MVTR change due to irradiation was not as significant as the difference observed for shear testing. This change in shear testing results could translate into large differences in wear time for wearable healthcare products for adhesives I and IV.
Adhesives II and III are likely to work well in applications where acrylic adhesives are exposed to e-beam irradiation, as they experienced minimal change during sterilization.
This article was written by John Bobo, Associate Director, R&D, for Scapa Healthcare, Windsor, CT. For more information, Click Here.