Practical Challenges for Medical Device Coatings
Before selecting a device coating, physical properties and provided benefits are not the only consideration for device designers. There are other critical factors that can significantly impact efficacy. This article reviews variables such as sterilization methods, shelf-life, and aging which can all have an adverse impact on the coating.
Within the world’s $300+ billion medical device market is a dizzying array of interventional and invasive devices suitable to an equally dizzying array of ailments. Each device is crafted with clinical input and thoughtful design decisions with regard to components in order to optimize form and function. An often overlooked component that is now in widespread usage is the device coating; there are several prominent types: hydrophilic, hydrophobic, drug eluting, anti-microbial, anti-fouling/non-stick, and combinations and sub-types there of. Active development programs at coatings manufacturers over the last three decades have pushed the barriers of what is possible with existing systems, but they have also presented new challenges that engineers must tackle and understand. Particularly, methods of sterilization and factors that affect shelf-life can interact with coating integrity or properties. These “benchtop” thought experiments have serious implications for transport and usage of coated medical devices in the field as well.
As an example, in the case of most tissue-contacting devices, sterility is a non-negotiable requirement. The method of sterilization may cause unanticipated, but crucial, interactions with coatings. Traditionally, there are several types of sterilization processes in use today for medical devices: ethylene oxide (ETO), e-beam, autoclaving, gamma irradiation, and hydrogen peroxide vapor (HPV). Each one of these methods possesses the capacity to chemically react with most polymeric coatings. Design engineers must view coating performance data after sterilization as a more reliable indicator of clinical performance than un-sterilized coating data. For drug eluting and antimicrobial coatings, there is the additional factor of having the sterilization method interact with the active pharmaceutical ingredient (API). For example, e-beam and gamma sterilization can induce radicals in the API that may cause it to react with itself or the excipient in an undesirable fashion. For non-polymeric coatings, such as titanium oxide and other metallics, which seem to be currently in their infancy for purposes of drug elution, there are advantages for using autoclaving because the coating is unlikely to be destroyed by the high humidity and temperature. However, elution of the API can be triggered by the steam process, thereby creating uncertain dosages in the device. Essentially, the effect of sterilization can be an important hurdle to overcome, but it is not the only one.
Aging and shelf-life are other factors important for medical device coating functionality. At time zero, any given type of coating can display exceptional functionality. However, with time, the performance can decrease, even with proper storage. For the most part, this decrease in performance is expected and “normal” with most coatings. The only determinant of whether or not functionality is “good enough” is whether or not the device can still meet its specifications. Nevertheless, engineers must be aware that data they read in information sheets can potentially be biased toward unaged material, and the ravages of aging on the coating may not be found out until it is too late. It is important to inquire about aging during initial coating evaluations. Importantly, designers must also not overlook the effects of both aging and sterilization together. The most operationally relevant data for a coating will display performance after sterilization and maximum intended aging.
Finally, the practical implications of sterilization and aging must come into consideration. Benchtop and verification testing ideally addresses the conditions of actual use, but sometimes the assumption of actual use consists of having the device sit on a shelf in a hospital at room temperature, for example. That assumption does not take into account the day to day transport and handling of the coated device before it is sold. Frequently, devices are distributed through sales representatives who store the coated devices in the trunks of their vehicles. If it is summer time, or if the sales official is located in Arizona, temperatures in that trunk may exceed the recommended storage limits and severely decrease the performance of the coating. Before reaching the sales representative, the coating may be subjected to uncontrolled warehouse conditions. If the warehouse is located in Puerto Rico, and it is not air conditioned, the API or excipients may undergo chemical degradation or reaction. Thus, it is not only important to consider the effects of sterilization and aging in isolation, but also in conjunction with practical environments that the coating must pass through in order to reach the final user.
Proper design and evaluation of any given type of medical-grade coating is not a trivial exercise in medical device development. It should employ a sophisticated and crucial set of requirements that must be met in order to create a component that functions in real life situations. Diligent testing and design with regards to the effects of sterilization and aging on medical device coatings serve to enrich the ability of the coated medical device to perform, thereby contributing to the health and welfare of patients under treatment.