Medical Grade Adhesives and Sterilization Processes

In this interview, Venkat Nandivada, Manager of Technical Support at Master Bond Inc, talks to AZoM about medical grade adhesives and sterilization processes.

What are the properties to consider when deciding on an adhesive for a medical device?

Some of the desirable properties an adhesive offers, include but are not limited to, biocompatibility, non-cyotoxicity, temperature and chemical resistance, thermal and/or electrical conductivity, and optical clarity. However, the combination of properties required from medical grade adhesives ultimately depends upon the specifics of the application and the kind of sterilization process that will be used. Surgical instruments, implantable sensors, catheters, endoscopes, and ultrasound probes are just a few of the many medical products assembled with adhesives, and the conditions that these adhesives might be subjected to vary case by case.

What are some of the different methods for sterilization and what do they accomplish?

One commonly employed method is steam sterilization, often referred to as autoclaving, whereby devices are exposed to moist heat in the form of saturated steam under pressure. This will occur in a steam sterilizer or autoclave. Autoclaving rapidly eliminates most microorganisms--such as bacteria, viruses, yeasts, fungi, and spores--on the surface of an inanimate object or within a fluid. To eliminate all microorganisms, temperatures must be maintained for a minimum time, commonly 15-30 minutes at 121°C, or 3-4 minutes at 132-135°C, depending on the type of autoclaving as well as the nature of the sterilization that is required.

Some of the other methods could range from gamma to e-beam radiation, to chemical sterilants such as hydrogen peroxide, per-acetic acid, or ethylene oxide.

What Master Bond products can withstand repeated autoclaving exposure?

Autoclaving is the primary sterilization method for critical and semi-critical medical instruments that can withstand high heat and moisture. Although autoclaving cannot be used for heat sensitive objects, it is still widely used in many medical product sterilizations. Some studies claim that up to 80 % of medical devices use this as a sterilization method.

Although many compounds have been successfully tested for the U.S. Pharmacopeia (USP) Class VI requirements, not all of them are necessarily repeatedly “autoclavable”. Some select systems among them can withstand repeated autoclaving exposure. To test their resistance to autoclaving, Master Bond has conducted various experiments over the years, wherein adhesive products were cast into molds roughly 2 inches in diameter, and 0.125 inches thick. The castings were then subjected to multiple autoclaving cycles up to 100 cycles. Each autoclave cycle was around 20 minutes at 250 °F (~ 121°C) and about 15 psi of steam pressure. The weight change and the hardness of the castings were then measured periodically. When compared to the weight gain of a standard two component room temperature curable epoxy, the epoxies that have superior resistance to autoclaving typically have far less weight gain. Please refer to https://www.masterbond.com/articles/autoclave-resistance-medical-adhesives for specific data for EP62-1Med, EP30Med, EP3HTMed, UV15DC80Med, EP42HT-2Med, UV10TKMed, and EP46HT-2Med. Although some silicones were also part of this study, while the weight change was negligible, they turned extremely brittle after only a couple of autoclaving cycles. Therefore, they are not typically recommended for adhesive applications involving repeated autoclaving exposure.

EP42HT-2Med

You mentioned that there might be heat sensitive or optical devices, where autoclaving may not be an option, so what other modes of sterilization can a medical device be subjected to?

Lower temperature processes using chemical sterilants or radiation are typically used to sterilize fiber optics, electronics, and other heat sensitive critical medical devices. Ethylene oxide (EtO), glutaraldehyde, hydrogen peroxide gas plasma and peracetic acid, alcohols--isopropyl alcohol and ethanol in particular—as well as bleach related chemicals, are all examples of chemical sterilants.

EtO also is widely used for sterilization primarily in disposable medical devices for its ability to disrupt the DNA in microbes. EtO sterilization usually takes at least three hours at 30-60°C and is typically followed by 12-24 hours of aeration. Glutaraldehyde is an organic compound that destroys pathogens and can be used in solution as either a sterilant or a high level disinfectant. It requires a ten hour soak followed by a water rinse. Note that some microbes can become resistant to glutaraldehyde. In higher concentrations, hydrogen peroxide gas plasma function as a sterilant, destroying essential cell components in pathogens. In lower concentrations, it is used to disinfect ventilators, endoscopes, and other medical instruments. Peracetic (or peroxyacetic) acid is a highly effective chemical sterilant that destroys microorganisms even at lower temperatures. Peracetic acid can be used to sterilize endoscopes, arthroscopes, surgical, and dental instruments. However, it could have a corrosive effect on certain substrates. Alcohols, bleach related chemicals, a mixture of peracetic acid and hydrogen peroxide are utilzed as mid-level disinfectants on stethoscopes, thermometers, hospital pagers, and scissors. Also, they are sometimes used to disinfect external surfaces of ventilators and other medical equipment, ultrasound instruments, and medical preparation areas.

What Master Bond products resist chemical sterilants?

Some examples include UV18Med and EP41SMed, which feature excellent resistance to chemical sterilants. EP41SMed is a fast curing two part epoxy with excellent adhesion properties. UV18Med is a one part, low viscosity UV light curing system that cures after exposure to a UV light source at 365 nm.

UV18Med

Are there other forms of sterilization?

Gamma rays or high energy electrons are effective in sterilizing needles, syringes, cannulas, IV sets, and other disposables. Gamma rays are high energy photons emitted from a radioisotope source. Cobalt-60 gamma rays are effective at disrupting the DNA and other cellular structures of microorganisms. High energy electron beams (E-beams) from accelerators also can sterilize medical products by breaking the DNA chains in living organisms. Both these methods are not inexpensive, and gamma rays may leave some polymers brittle and yellow. Finally, nonionizing ultraviolet (UV) light radiation is sometimes used to sterilize medical products, particularly titanium implants. UV light sterilization is generally limited to destroying airborne microorganisms and bacteria on titanium and similar type surfaces.

Are there Master Bond polymers that can withstand sterilization by irradiation?

One example is MasterSil 151Med, which is a two component, low viscosity silicone system. It is a room temperature addition type curing system. It features unsurpassed flexibility and very low stress, particularly useful when potting and encapsulating. It also offers outstanding optical clarity.

About Venkat Nandivada

Venkat Nandivada has been the Manager of Technical Support at Master Bond Inc since 2010.

He has a Masters in Chemical Engineering from Carnegie Mellon University.

He analyzes application oriented issues and provides product solutions for companies in the aerospace, electronics, medical, optical, OEM and oil/chemical industries.