Physical and Chemical Characterization of Medical Device Materials

From programmable pacemakers and laser surgical devices to inactive medical devices such as intraocular lenses, resorbable bone screws or intravascular catheters, today’s state-of-the-art medical devices are a vital tool in the diagnosis and treatment of diseases and illnesses. Despite their numerous benefits, however, all medical devices present some degree of risk to a patient, even when used appropriately. For this reason, a risk assessment of any medical device must include an evaluation of the inherent safety of a device and its components. A particular safety concern associated with the use of medical devices is the risk presented by simple contact with the human body. Even when used as designed, medical devices and their constituent materials may have the potential to produce unintended local or systemic effects. Depending on the duration of contact, some materials may also be harmful or toxic to the body, or may have adverse developmental or reproductive effects. Further, processes such as manufacturing, packaging, shipping and routine sterilization may adversely affect a material’s composition, rendering an otherwise safe device potentially harmful. Biocompatibility testing represents a series of staged assessments to determine the potential harmful effects that can result from human contact with a medical device or component, and is an essential aspect of the overall product safety assessment required for global regulatory approval. The physical and chemical characterization of materials is the first stage of biocompatibility testing, and involves the analysis of substances that can potentially leach from a medical device during normal, anticipated use. When conducted in advance of other biocompatibility testing, material characterization testing may reduce overall testing time and speed regulatory approval. This UL white paper discusses the physical and chemical characterization of medical device materials in the context of an overall biocompatibility assessment. Beginning with background information on biocompatibility issues associated with medical devices, the white paper reviews specific physical and chemical effects and the testing specified under ISO 10993-18 dealing with the chemical characterization of materials. The white paper then outlines a structured approach to the development of an effective material equivalency program to speed material selection, and concludes with a discussion of the benefits of maintaining such a program throughout a product’s development and production cycle.

Modern medical devices are composed of a diverse range of materials and components, each with their own physical and chemical characteristics. Although these materials may pose minimal risk when incorporated in products intended for general use, their inclusion in medical devices that come in contact with the human body expands the scope of potential safety considerations. These considerations can include contamination from the device, the breakdown or decomposition of device materials, the migration of device materials to other parts of the body, and consequences from intended device degradation. Even when materials incorporated into a medical device have been assessed for their biocompatibility, new biocompatibility risks can be introduced through manufacturing and post-production processes that can have an adverse effect on these materials. For example, contact with lubricants or other chemicals during the production process can compromise the chemical integrity of a material. Similarly, sterilization and packaging may adversely affect material composition. Leachable substances from the packaging materials, like residual solvents or ink from a package label, may interact with a biocompatible material. In addition to direct risks to human safety, the physical and chemical characteristics of materials used in medical devices can indirectly interfere with device functionality. For instance, chemicals that leach from device materials can potentially alter the mechanical or electronic properties of the device itself. In these cases, a critical device such as a pacemaker might malfunction or cease to operate altogether, thereby placing a pacemaker recipient at significant risk. In other cases, vibration, shock or temperature variations encountered during shipment of a packaged device may damage a device. Biocompatibility testing and evaluation looks at all of the potential adverse effects on a human body that may result from the materials used in a medical device. The complete range of testing conducted as part of a biocompatibility assessment is detailed in the standard ISO 10993-1:2009, Biological Evaluation of Medical Devices—Part 1: Evaluation and Testing Within a Risk Management System. However, in determining which specific tests to conduct, special consideration is given to how a medical device will actually be used and the degree and duration of contact between the device and the patient. Table 1 provides additional details on the criteria used to determine which potential biological effects must be considered when evaluating a given medical device. This framework outlines an assessment program for the final evaluation of the biocompatibility of a device under evaluation. However, a detailed risk analysis is required to identify the actual tests to be performed and any additional test required for the evaluation of other biological aspects. For example, tests for chronic toxicity, carcinogenicity, biodegradation, toxicokinetics, immunotoxicity, reproductive/developmental toxicity or other organ-specific toxicities should be considered in addition to those tests identified in Table 1, based on the intended use of the medical device.