Introduction to Coating Selection
Coatings are integral to the design of many medical devices. They are designed to provide a variety of functions, such as managing energy delivery, providing abrasion resistance, reducing glare, offering non-stick properties, and providing thermal isolation, just to name a few. However, coatings are not a one-size-fits-all solution. When selecting a coating for a medical device, it is important to consider many factors, including those beyond just the device's intended use.
Biocompatibility Considerations
Biocompatibility refers to the ability of a device to remain in the body without causing detrimental effects to the surrounding tissue. Biocompatibility of medical devices is defined in ISO standard 10993. This standard defines the specific tests required for a medical device to demonstrate biocompatibility. Some of the specific criteria that determine the tests required include the duration of device contact with the body, whether the device will be in direct contact with blood, and whether the device serves as an implant.
Substrate Material
Substrate material refers to the surface the coating adheres to. Various coatings are formulated for or adhere better to different substrates. Examples of substrates commonly used in medical devices include surgical stainless steel, polymer catheter sheath, nitinol, or carbon fiber.
Sterilization Method Compatibility
Sterilization types vary depending on whether the device is reusable or single-use. Common sterilization types for reusable devices include steam autoclave, ethylene oxide gas, or hydrogen peroxide. Common sterilization types for single-use devices include radiation (e-beam or gamma) and ethylene oxide gas. Certain sterilization methods may interact with the polymer matrix of certain coatings and cause failures or degradation of the device. There are a number of standards that regulate sterilization, including ISO 11137 for radiation, ISO 11135 for ethylene oxide, and ISO 17665 for steam autoclave.
Single-Use vs Reusable Devices
Single-use medical devices are designed for a one-time use, whereas reusable devices are intended to be cleaned, disinfected, and sterilized for multiple uses. Whether a device can be easily cleaned often is a major factor in this decision. Specifying whether a device will be single-use or reusable usually happens very early on in the device design process, normally specified by the manufacturer.
Coating Thickness Requirements
Coating thickness is usually defined by the paramount requirement of the device coating. For example, coatings that are applied for electrical insulation in RF electrosurgery devices are usually quite thick. On the other hand, coatings applied to achieve lubricity or surface hardness are typically much thinner. Knowing the coating thickness requirement is key in the mechanical design stage in order to ensure proper fit and clearances.
Lubricity and Surface Interaction
Lubricity is often a requirement of catheter devices that are threaded through vessels in the body. Many lubricious coatings of this type fall into the hydrophilic realm, although there is a select small group of hydrophobic materials that are known for their lubricity.
Maximum Use Temperature
The maximum use temperature is often driven by the energy directed through the medical device. If the device in question is not an energy device, the maximum use temperature is often body temperature. Many devices, including electrosurgery devices, powered surgical instruments, and advanced energy systems, generate substantial heat during their use. The ability of the coating to withstand this heat, as well as the possibility that the coating acts as an insulator or conductor to impede or promote heat flow, needs to be considered.
Energy Type (Monopolar vs Bipolar)
In electrosurgery devices, the energy type is often specified as monopolar or bipolar. While bipolar devices typically operate at a lower energy level, certain coatings are susceptible to carbon tracing and other failure modes due to the proximity of the two electrodes. In monopolar electrosurgery, given the typically higher energy, criteria such as dielectric strength often become a driving factor in coating selection.
Dielectric Requirements
In electrosurgery devices, achieving the dielectric requirements is often one of the most challenging factors in coating selection. The dielectric strength requirement of a coating is often determined by the energy the device will encounter in actual use, multiplied by an appropriate safety factor. The ability of a coating to achieve this previously determined dielectric strength requirement is often verified using a hipot test, in which the voltage, current, and dwell time that a coating of a specific thickness can withstand are determined.
Additional Functional Requirements
There are many other desired functions that may be required of a coating for use in a medical device application. Some of these, described later in this document, are abrasion, scratch, and gallling resistance, inter-glare reduction, non-stick properties, thermal isolation, coating rigidity, and anti-bacterial properties. These are very specific to each device, and in the case that an existing coating does not meet the required specifications, specialized formulations can be developed.
Conclusion
Coatings on medical devices work wonders in opening the doors to what a device is capable of, but not all coatings are created equal. There are thousands of robust coatings applied to medical devices, each designed to enhance components in specific ways. Having a thorough understanding of the device use profile, including all the considerations discussed above, is integral to ensuring you choose the correct coating for the device at hand.
