The number of adhesive products subjected to biocompatibility testing continues to grow. Master Bond has more than 30 grades of USP Class VI-certified adhesives, encapsulants, and coatings.
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Adhesive selection continues to be an afterthought within the medical design community, resulting in avoidable manufacturing difficulties.

It’s not that engineers don’t know that some material families— polyolefins representing a classic example— are difficult to bond. It’s that they believe there’s always an ideal bonding solution available no matter what the substrate.

And they’re right- to an extent. There is a bonding solution for just about every substrate combination because the adhesives industry continues to formulate products that work with toughto- bond and dissimilar materials. But the solution may not be “ideal” in the sense that it requires expensive tradeoffs. Olefinic materials, for example, can be adhesively bonded but only if a primer or surface treatment figures into the equation, adding time and cost to the assembly process.

The point isn’t to avoid tough-to-bond materials altogether. Sometimes they’re the best choice for the job. Instead, the point is there are many more bonding options available when materials and adhesives are evaluated as a system early in the design process, when engineers dig deep into issues related to biocompatibility, sterilization resistance, bonding performance, and ease of assembly.

Answering three basic questions as part of this process will help aid in the selection of the best timeand cost-efficient adhesive for the job.

1. Is the adhesive biocompatible? One common misconception about medical adhesives involves their biocompatibility. Engineers sometimes believe that the FDA regulations require that adhesives be tested for biocompatibility before they can be incorporated in a medical device. Yet the FDA actually considers only the biocompatibility of the finished medical device, not its individual materials. Engineers have the freedom to select and combine materials as they please, as long as the finished device passes muster with the FDA.

In practice, though, sticking with materials known to be biocompatible make it more likely that the finished device will be biocompatible. So device engineers favor adhesives and other materials that have been “prescreened” by passing industry standard biocompatibility tests. In North America, the most widely accepted test standard is United States Pharmacopeia (USP) Class VI.

Intended to evaluate the suitability of polymeric materials for direct and indirect patient contact, the USP Class VI regimen consists of a series of in vivo reactivity and toxicity tests. These tests involve injecting and implanting samples of the material under consideration into mice and rabbits, which are evaluated at fixed time intervals for any ill effects.

Passing USP Class VI testing doesn’t guarantee that a device will gain FDA approval. All it tells an engineer is that a material has low levels of in vivo toxicity under the test conditions. Yet given that a large number of standard adhesives cannot be considered biocompatible, USP Class VI testing represents a valuable tool for identifying products suitable for medical applications.

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