Adhesives and sealants troubleshooting tips from Dr. Dave Dunn.

Question:We are developing a new dental instrument and will use several adhesives to assemble it. What types of adhesives have the lowest toxicity, and do we need to find adhesives that are FDA approved?

Answer: The U.S. Food & Drug Administration (FDA) does not approve adhesives used in medical devices. Only the final device needs approval, and it makes a big difference whether the adhesive comes into contact with the body or body fluids. However, you obviously want to choose adhesives that will not cause approval problems in your instrument.

Several manufacturers supply adhesives that are approved or certified to national or international standards. In the U.S., the standard has traditionally been called USP Class VI, but many manufacturers are now moving to the ISO 10993 international standard. Both standards involve testing an adhesive to show it is non-toxic and biocompatible. The companies that invest in these certifications also have quality assurance programs in place to ensure batch-to-batch reproducibility of their products.

Several different adhesive technologies are used in medical device adhesives. An important point to remember is that, while a liquid adhesive may appear quite hazardous when you read its Material Safety Data Sheet (MSDS), the toxicity testing is always carried out on the cured adhesive. Epoxies, cyanoacrylates, light- or UV-cured acrylics, polyurethanes. and silicones are all candidate adhesives and sealants.

Your choice will depend on the specific materials you are bonding and your assembly method. For example, epoxy adhesives have been used quite widely but are sometimes not the perfect choice where very high assembly speeds are required. They take quite a long time to cure, and UV-cured acrylics have replaced them in some applications because they can be cured in seconds. Another important consideration is whether your instrument requires sterilization before use or between uses, and what type of sterilization is used. Steam sterilization is usually the most difficult situation for adhesive bonds, and you need to make sure you do the appropriate testing.