Medical Device Polymers: Designing for Degradation

The degradation rates of the polymers used in many medical devices are a function of both morphology and time. In the following exchange, Dennis Jamiolkowski, Distinguished Research Fellow at Ethicon Inc. (Somerville, NJ), a Johnson & Johnson company, explains this relationship, which has a critical bearing on the design and development of many implantable applications.

MPMN: How does polymer degradation affect the design and functionality of medical devices?

Jamiolkowski: One must first consider that everything is driven by the intended function of the device. Once a basic concept for the design has been created, a polymer candidate can be selected to meet the intended product profile. Considerations include key mechanical properties such as tensile or shear strength. Remember that other mechanical properties, such as yield stress or modulus, may be just as important. It all depends on the failure mode of a given design.

Mechanical properties depend not only on the given polymer but also on the polymer morphology--the shape of the chains--resulting from the thermal and shear forces to which the polymer is exposed. For example, if a manufacturer fabricates a device using injection molding, the strength the device develops will depend in part on the amount of molecular orientation and the level of crystallinity achieved during processing.

In the case of synthetic absorbable polyesters, polymer degradation is usually based on chemical hydrolysis. After a device is implanted in the body, it reacts with water present in tissue, leading to chain cleavage and a loss of molecular weight. This phenomenon, in turn, leads to a loss of mechanical properties.

However, not all mechanical properties follow the same loss profile; a loss of modulus is usually different from a loss of strength. It therefore follows that a deep understanding of failure mechanisms as a function of time is critical. Often, this correlation is initially modeled utilizing in vitro experiments. Medical device designers must be acutely aware of what will be needed in a clinical situation. For example, will a modulus retention of 50% at five weeks postimplantation be sufficient?

A given polymer with a given polymer morphology and a specific device geometry will absorb in the body within a given period of time. One must therefore ensure that the tissue reaction elicited during that time frame will be acceptable.

Based on initial results, further design iterations are usually required. Note the interconnectivity: Design changes alter polymer morphology, which then changes mechanical properties and mechanical property loss profiles once a device is implanted.

MPMN: How do environmental factors affect the physical properties of polymers, and what can the medical device design engineer do to adjust for these effects?

Jamiolkowski: Environmental considerations begin with the storage of the resin to be thermally formed into the device. Improper storage can cause the sensitive polyester to lose molecular weight prematurely or pick up small, unwanted amounts of water. If a part is manufactured using such thermal processing methods as compression or injection molding, even 30 ppm of water can lead to significant hydrolysis during part formation. And at elevated temperatures, the water reacts very quickly to reduce a polymer's molecular weight, resulting in unstable processes. What can one do? Keep the resin religiously dry and be aware of the possibility of thermal degradation. Above all, characterize the part to understand what is happening, purposefully or unintentionally.

The topic of environmental factors is multifaceted. One consideration, for example, is how the device will be sterilized. Will it be sterilized using ethylene oxide (EO)? If so, the EO may act as a plasticizer of the absorbable polyester and cause all sorts of problems, including warpage and distortion. If one intends to use gamma irradiation, it may be important to consider the package environment in order to minimize the loss of molecular weight usually associated with this type of sterilization.

Bob Michaels is managing editor of Medical Product Manufacturing News.