Biomedical Engineering Reference
In-Depth Information
cellular elements, or their combined presence. Factors that may affect the
biocompatibility and biostability of devices primarily include material
properties, device designs, contacting tissues, and implantation techniques.
At the chemical and cellular level, biocompatibility and biostability of a
device or material depend on the type of chemicals and cellular responses that
occur at the host±device interfaces. Implanted devices must withstand the
humoral (bulk) environment within the body that consists of water, electrolytes,
proteins, fatty acids, glucose, urea, lactic acid, creatinine and more. The
inflammation or `foreign body response' of the host to the device is a term that
describes cellular reactions to the implant. Once implanted, a cascade of reac-
tions starts and results in the device being covered with a layer of foreign body
giant cells and/or macrophages, and a fibrotic capsule composed primarily of
collagen. The cellular components of the foreign body reaction can have
significant effects on the biostability of the materials in the device. These cells
can release a number of enzymes and oxidants to destroy the foreign body
(device and materials). The compounds in tissues that seem to have the greatest
effect on polymer biostability are water, oxidants and free radicals, and
hydrolytic enzymes. Along with the chemical and cellular environment around
the implant, the body also exerts mechanical loads (forces) upon the device by
interactions or juxtaposition with bones, muscles, and organs. A patient's own
physical activity and occasional inadvertent actions can also affect the device.
Therefore, an implanted device must withstand routine physical activity as well
as the mechanical environment within the body.
A great deal of research has been done to understand the biological reactions
to materials including protein adsorption, blood coagulation and thrombosis,
inflammation, cellular reactions, and tissue remodeling. Yet, no material has
been discovered and recognized to have a zero or null inflammatory reaction. On
this point researchers have learned a number of important lessons: first, a certain
level of biological reactions, such as an inflammatory response or thrombotic
reaction, may not be adverse. Biological reactions at certain levels are accept-
able although the levels depend on specific tissues and applications; second,
combining drugs with the devices can be an effective means to control bio-
logical reactions compared with inventing new materials or new surface
treatment methods. Steroid releasing leads are an excellent example of using a
drug to minimize the inflammation reaction to the pacing electrodes. Such drugs
incorporated in the tips of lead electrode effectively reduce the inflammatory
response that gives rise to high pacing thresholds; third, although bio-
compatibility is always considered with high priority, in reality the majority
of device failures are due to poor biostability of materials, especially when
unexpected interactions among different materials and biological factors are
present in the biological environment.
Most materials pass biocompatibility testing as long as they do not release
toxic levels of additives, monomers, contaminants, degradable products,
