Page 157 - Biomedical Engineering and Design Handbook Volume 2, Applications

136 MEDICAL DEVICE DESIGN

and didn’t like about prior models from this firm and from the competition; design engineers used
computer model methods to create a new basic design; packaging specialists worked to create an
attractive instrument that functioned according to user expectations; manufacturing people took this
instrument and developed the means to mass-produce it and the quality-assurance tests to be sure that
each device met or exceeded minimum standards; the legal and regulatory specialists developed the
data to meet government requirements in the country of use; sales personnel found potential users,
compared the new device with the competition, and adapted the device to specific user needs; then
the servicing specialists responded to customer concerns by developing quick field tests, parts kits,
and field manuals to repair defective devices on site.
This procedure was time consuming and expensive. If the manufacturing engineer found that the
device could not be produced effectively as designed, then the whole design process had to be revis-
ited. The same holds true for other steps in the process. There are legends about instruments that
would not function without failure and that could not be serviced economically. The results were
very expensive calamities.
Simultaneously considering all of these design aspects shortens the design process and allows
industry to respond much quicker to customer needs or technological breakthroughs. What once took
5 to 10 years to develop can now be done in 1 year or less. Perhaps because of this, attention has turned
to regulatory delays for device approval, which are now an unproportionately large amount of time.
Small projects by small companies may not use formal concurrent engineering teams in the way
that large projects in large companies require their use. However, the same functions need to be rep-
resented in the same parallel way. Because many medical devices are produced by small companies
with few employees, the design engineer working for one of these companies must be able to wear
many hats.

4.7.2 Technical Design
Design of medical devices employs a combination of fundamental engineering principles and empir-
ical data. Almost all respiratory devices incorporate the need to move air from one place to another.
At the least, it is usually desired to reduce airflow resistance and dead volume of the air circuit. There
is no substitute for a thorough knowledge of fluid mechanics in order to realize these goals.
Minimizing the number of sharp bends, sudden constrictions, and obstructions can reduce resistance;
reducing turbulence, tubing length, and compliant members can reduce dead volume. This knowl-
edge comes from engineering principles and can be predicted beforehand.
A field as mature as the field of respiratory devices develops around a great deal of empirical
knowledge. This information could, perhaps, be predicted from first principles, but often is deter-
mined by experimental measurement on previous devices or on prototypes of newly developed
devices. Two devices where empirical knowledge is important are the body plethysmograph and the
hospital ventilator. Each of these is a complex device that has undergone many embodiments over
the years; each has been required to become more accurate or more versatile; each has been
improved through knowledge gained by use.
The design process, then, is to begin a new device from basic engineering considerations and to
make improvements based more and more on empirical information. Computer-aided design (CAD)
and computer-aided manufacturing (CAM) programs are often constructed to incorporate the state
of knowledge in compact and easily used form. This has the advantage of allowing the information
base to be constantly accessible without dependence on the memories of any particular individual.
This makes these proprietary computer programs some of the most valuable assets of any company,
and gives companies that have been manufacturing the same kind of medical device for many itera-
tions an almost insurmountable advantage over younger companies that attempt to improve the same
kind of device. Thus, newer companies are usually found developing newer kinds of devices that do
not require as much empirical knowledge to produce. When these newer companies develop sub-
stantial amounts of their own empirical technical knowledge, they become valuable for their techni-
cal information bases, and they become acquisition targets by larger companies wishing to develop
their own devices of that type.