Bioresorbable polymers can be very advantageous to the medical device designer owing to their unique properties and potential impact on patient health and safety. But they do present some unique challenges. While most issues can be overcome with forethought and experience, here are five tips to help engineers avoid the most common issues encountered when working with bioresorbable materials.

It is critical to not only establish how long the bioresorbable device should last in situ, but also to define what it means "to last." If the design calls for the device to degrade after six months of use, for example, does that mean that the mechanical or physical properties are gone after six months, or that all remnants of the polymer are gone? For the majority of bioresorbable materials--the bulk erosion class of polymers--water permeates the entire device. And while the mechanical properties of the material may deteriorate in six months, it may take another year for all of the material to completely resorb in the body. As a corollary to this issue, the other class of bioresorbable polymers, surface erosion polymers, do not permit water to completely permeate the device; only the surface layer is affected by the degradation mechanisms at any time. So, the mechanical and physical properties of the device erode as a function of the dimensional characteristics of the medical device.

Determination of the degradation time must be based on the final device after all processing steps have been completed. The processing method--such as extrusion, compression molding, or solvent casting--along with such factors as the number of drying cycles for the polymers and storage time, can all affect molecular weight, molecular weight distribution, and crystallinity of the polymers. In turn, molecular weight properties and crystallinity can dramatically impact degradation time. This fact may be obvious, especially for people experienced with other polymers that are sensitive to water content, such as polyurethanes. But what may be less obvious is the impact of sterilization as a final processing step. Whether the method involves water or irradiation, sterilization can reduce the molecular weight of bioresorbable polymers by as much as 50%.

The exclusion of water during all phases of handling bioresorbable materials is critical. A defining characteristic of bioresorbable materials is that they are sensitive to aqueous degradation in situ. But designers and manufacturers sometimes forget that this characteristic is also a factor when handling the material prior to actual device use. It is imperative that companies take extra care in the storage, and therefore, the exclusion of water, throughout the lifetime of these polymers. This may include storage of the raw polymers; preparation of the polymers for processing; the processing environment; storage of the work in progress; and packaging/storage of the finished goods.

Consistency is key to reproducibility. To achieve reproducible performance of the finished medical device, the properties of the bioresorbable polymer must be consistent throughout the device. As noted, these polymers are susceptible to degradation--or at the very least, changes--during processing. With this in mind, it is critical to have uniform conditions throughout the processing steps. For example, if molding is employed, temperatures should be consistent and there should not be differences in residence time in various parts of the mold.

Consider the cost factor. Bioresorbable polymers are far more expensive than most conventional elastomers or engineering plastic. So, waste minimization is a very practical concern. We strongly recommend that designers partner with experienced micromolders that are specifically experienced in this area to ensure cost-effectiveness.

Steve Coulter is the senior research associate at Fallbrook Engineering Inc. (Escondido, CA).