IN OUTGASSING, ONE OR MORE VAPOR PHASE CONTAMINANTS that have been trapped in a product or component are released. At first glance, this might seem like a non-problem. After all, if a contaminant is adsorbed into the materials of construction and then released, the product becomes cleaner as outgassing progresses. In that sense, one might think of outgassing as the obverse of airborne molecular contamination (AMC) [1]. However, outgassing itself has the potential for contamination.

Depending on the environment and on the use of the product, outgassing may result in catastrophic product failure. For one thing, the released vapor is not always removed from the vicinity of the component or product. If the component is in a sealed environment, the outgassing chemical may react with other materials of construction, resulting in materials compatibility problems or corrosion. As a general rule, outgassing and reactivity are enhanced as the temperature increases. Decreased pressure also enhances the removal of vapors. Outgassing problems have been observed in sealed navigation systems where solvent residue reacts with a flotation fluid or with metal parts. Outgassing can produce clouding of optics. Avoiding outgassing is essential in deep-space exploration, where the systems must perform reliably over long periods of time and where possibilities for rework are at best problematic. Obviously, one must also minimize outgassing from medical devices.

Whether or not the product is sealed, release of solvents from implantable medical devices is inherently undesirable. During our recent Medtec workshop, we provided an example where a proposed solvent was considered preferable to the currently-used solvent because, under similar conditions, less of the proposed solvent was found to outgas using gas chromatography. An attendee correctly commented that the lower level of solvent was not per se necessarily sufficient to demonstrate acceptability, given the variable differences in toxicity among solvents. This is an important point. Further, one cannot necessarily extrapolate potential impact of outgassing solvents based on toxicity characteristics. Even vapors of mild or relatively safe solvents can exhibit undesirable effects.

While a contaminant has very occasionally been shown to have beneficial effects, it is probably prudent to assume that solvents should be removed to as low a level as possible. It would also probably be prudent to supplement analytical testing with at least in vitro studies, particularly for very critical biomedical applications. By the way, relatively non-volatile (i.e. high boiling point) solvents and water are included in this discussion.