Biomedical Engineering Reference
In-Depth Information
4.15 Various Medtronic lead designs from 1980 to 2000. Over time,
technology andmarket forces have lead to the development of smaller diameter
leads (courtesy of Medtronic, Inc.).
Many device failure examples indicate that device problems are often caused
by unexpected failure mechanisms. Testing is one of the most critical steps to
ensure reliability in product development. Prior to testing, a failure mode and
effects analysis is conducted for an intended application. Cross-interactions
among the components of the same devices are considered. A full or partial
biocompatibility assessment in accordance with ISO 10993 and relevant in vitro
and in vivo chemical and mechanical stability tests are performed.
Commonly used factors for chemical stability tests include pH, lipid, metal
ions (relevant to metals in the same devices), enzymes, per-oxidative agents (for
inflammation), biological agents (combination products), and corrosion products
(for metals). Mechanical tests include material property assessments (e.g.,
tensile, fracture, and bending) and long-term stability assessments (e.g., creep
and stress relaxation, environmental cracking, and wearing). These screening
tests provide valuable information to grade the acceptability of a material for its
intended application.
If the materials pass the test criteria, they are made into the intended parts and
assembled with other parts into individual modules or a whole device, and tested
in vitro following the application requirements. If passed, the whole devices are
tested in vivo for safety and efficacy. Upon passing these tests in this sequence,
the development work moves on to clinical trials. This is the general process for
introducing completely new materials or devices, and is required by regulatory
bodies worldwide before a device may be released into the market. But, research
does not stop here. Manufacturers continue to conduct post-market surveillance
on products implanted in the patients for a number of years to monitor product
safety. This leads to a deeper and broader understanding of the device perform-
ance and provides feedback for the device manufacturers to continuously
improve their devices.
Except for ISO 10993 and a few other vertical device guidance documents,
there are few other standard procedures (Tables 4.1 and 4.2). Testing protocols
and in vivo models are designed according to specific device applications,
though none has been found to be entirely predictive of human applications.
Applying sound rationales and having experience can be invaluable when
interpreting some of these standards. In many cases, test work can be easier if
the development work is to replace an existing material or component with an
