HOW CAN AN IMPLANTABLE MEDICAL DEVICE be optimized? Sterilization is a given. In addition, implantable medical devices exhibit specific, often dynamic interaction with the host tissue. Given the number of variables involved, processes are based in part on historical clinical success. However, progress in achieving harmonious interaction of the device with the host depends on critical cleaning,surface characterization, and surface optimization.

Issues and questions posed at a recent presentation¹ and at an exhibition and conference^2,3 illustrate the growing awareness of the potential impact of surface quality or attributes, contamination or residue, and cleanliness technique on the performance of medical devices.

Beyond Sterilization and Cleaning 101
Whether in academia or in industry, when cleanliness is mentioned in conjunction with medical devices, the first response is sterilization. The inherent suspicion that all contamination problems could be solved with aggressive sterilization is being supplanted by the understanding that critical cleaning and definedsurface quality represent the path forward.

The medical device community is moving far beyond “Cleaning 101,” to address such issues as the correct ultrasonic parameters for a given set ofsoils, materials of construction, and product configuration and the appropriate use of final cleaning techniques such as supercritical CO₂. Beyond minimizing residue level and quantifying each residue species, there is the issue of toxicity. Estimating residue toxicity is a major challenge; and techniques to characterize potential residue toxicity, based on ISO 10993-17were discussed.³

The potential contribution of the process, including the cleaning process, on product degradation is recognized for both permanent implantable devices and for single-use devices that are cleaned and reused.³ Materials compatibility is often thought of in gross, visible terms. However, for medical devices, subtle surface changes may produce marked changes in the nature of interaction with the host. Avoiding undesirable changes means defining not only the residue, but also the surface.

Critical Cleaning by Sub-vendors
The key to successful cleaning and to minimizing contamination is to implement controlled environments and controlled processes in and out of the cleanroom. In the past, fabricators involved in initial phases of medical device assembly might assume that their customer, the medical device manufacturer, would conduct the critical cleaning steps. Such assumptions are counterproductivebecause soils that are allowed to remain on a part tend to become more adherentover time. A cleanroom cannot correct a contaminated product.

Based on comments at the conference and the exhibition,^2,3 fabricators and operators of job shops are taking an active role in achieving the cleaning and contamination control of medical devices.

Complex Interactions
For implantable devices, understanding the interaction with the ultimate, in-use environment is a complex issue. Certainly, the physical properties of materials of construction must be well understood. In addition, for complex, miniature, and micro-devices, understanding and optimizing surface properties becomes increasingly important.⁴ The surface and near-surface of the device can interact with the host; this has positive and negative aspects. In some cases, it is desirable for the implant to meld with the host tissue. For example, osseointegration (active meshing of the implant with bone tissue) is essential for the success of dental implants; and the attributes of the surface (texture, base material, coating, and process residue) may all have positive impacts on the process. There have been many thoughtful studies characterizing the surface properties of failed dental implants.^5,6 Given the dizzying array of possible physical, chemical, biochemical, and biological interactions after implantation, the interpretation of such failure analyses will be enhanced when undesirable surface properties at the starting point are more clearly defined. Identifying an undesirable surface prior to implantation and optimizing the surface priorto implantation would be expected to reduce the need for such failure analysis.