Sterilization can greatly influence the integrity of a medical device, especially those bonded with adhesives. Therefore, it is critical that device manufacturers select the adhesive best suited for the substrate materials, end use environment, and sterilization technique.

The data shown in Charts 1 and 2 reflect the post-sterilization performance of three light cure acrylic adhesives used to bond 22-gauge stainless steel cannulas to either polycarbonate or plasma treated polypropylene hubs.

The pull strength data from this study on actual devices shows post-sterilization performance similar to that of the untreated control group. Often there is actually an improvement in strength. This improvement may be the result of elevated temperatures enhancing the adhesive’s cross-linking reaction, or of annealing that actually relaxes stress on the component. But what happens when we eliminate both substrates and joint design from the test and look

only at the properties of the adhesive itself and how it is affected by sterilization? To answer this question, let’s focus on the performance of typical light cure acrylic and light cure silicone adhesives when exposed to autoclave, ethylene oxide, and gamma irradiation sterilization.

Adhesives studied

This study tested the Loctite light cure acrylic adhesives referenced in Charts 1 and 2:

• 3921, a highly fluorescent, 150 cP (centipoise) adhesive that provides bond strength on dissimilar materials. This material is commonly used in needle bonding applications.

• 3933, a 3,250 cP adhesive that provides bond strength to polycarbonate and other thermoplastics with minimal stress cracking. This material is typically used for bonding thermoplastic substrates in fluid devices such as housings for filters and fittings.

• 3972, a 4,500 cP adhesive that provides tack-free curing capabilities at wavelengths greater than 254 nm including the visible wavelengths in excess of 405 nm. This material is used in devices where there is significant adhesive exposed in the bond line, for example in tube fittings where the bond line is not encapsulated between two substrates and a tack-free surface is required to prevent contamination.

These light-cure acrylic adhesives were selected primarily due to their glass transition temperature (T_g) and their ability to be cured under ultraviolet (UV) and/ or visible (Vis) wavelengths. T_g is the temperature at which a substance changes from a glassy solid to a rubbery soft material. Loctite 3921 has a T_g of 82ºC, while 3933 and 3972 have T_gs of 54ºCand 49ºC respectively.

Glass transition temperature is critical to the performance of a medical device exposed to sterilization. Adhesives are far less likely to support a load once its T_g has been exceeded as the material becomes soft and pliable, losing its rigidity and strength. While the adhesive’s ability to support a load returns after cooling down, any assembly that is under stress during the sterilization cycle may fail at sterilization temperatures.

The study also included testing two silicone adhesives formulations. For this, the cure method was the most important performance criteria in the selection process:

• 5056, a 2,200 cP light cure silicone, offers superior heat and moisture resistance and bonds well to silicone tubing and polycarbonate or thermoplastic fittings. This traditional light-cure adhesive will not cure in areas that are not exposed to light of the appropriate wavelength and intensity during the curing process.

• 5240, a 25,000 cP dual-cure silicone, cures on exposure to light and moisture and offers high tear strength. The secondary moisture cure mechanism allows the adhesive to cure in shadowed areas where light cannot reach.

Both of these adhesives would typically be used for bonding respiratory devices such as masks or breathing circuits, or for assembling components made from silicone substrates.

Glass Transition temperature was not a critical factor in the selection of silicone adhesives as the T of silicones is typically in the -40ºC range and all of the tested sterilization methods operated above the T_g of silicones.

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