Page 34 - Biomedical Engineering and Design Handbook Volume 2, Applications

MEDICAL PRODUCT DESIGN 13

available? It is highly probable that the idea that emerges from the selection process with the highest
score will have some yet-to-be-discovered flaw. This stage is much too early to commit all of the
resources to a single concept. It is a time when options should be kept open.
Conclude this process by documenting the selection. Include all of the scoring sheets, concept
descriptions, and related data. This will become part of the medical device master file, and perhaps
play a role in getting product approval and it will be useful when concepts need to be reconsidered
later in order to make final decisions.

1.12 ARCHITECTURE/SYSTEM DESIGN

It was mentioned early that the term medical device describes a wide variety of products. The process
of proceeding from concept through system design and detail design will vary greatly thoughout the
spectrum of products. Design development should in general follow a path similar to that of physi-
cally similar nonmedical items. The major accommodation is in documentation. It is important to
maintain a detailed record of the decisions made and their basis. This will be important through the
product’s manufacturing start-up and afterward, when alterations are proposed for manufacturing
reasons, to accommodate later product upgrades, etc. The ability to return to well-kept documentation
and follow the original logic will provide guidance, sometimes supporting the change, often indi-
cating its inappropriateness. There is a tendency in these situations to repeat the same mistakes.
A change in a medical device is a much more expensive undertaking than it would be in other objects
of equivalent complexity because of the qualification, verification, and testing that is so often
required. Good documentation can often prevent the initiation of some misguided efforts.
The objective in the system-level design is to deconstruct the product into separate elements that
can be considered independently. Once this has been done, the various team members can proceed
to define the elements of the product. The system design must define the interfaces where the
separate elements meet. This includes shapes, dimensions, and connection characteristics, such as
currents, digital signals, fluid pressures, and forces. Decisions made at this stage have a profound
effect on the complete design. The team and the leader must exercise judgment here. In a sense
these decisions are allocating a budget. In certain examples this is obvious, as in space allocated or
power consumption limits. Sometimes it may be subtler than this, but much of the budgetary sense
will remain.
The actual relationships defined in the system design are called the architecture. There are two
extremes that are considered in system architecture, integrated and modular. Modular systems are
made up of components or modules that have clear interfaces with each other, and are easily visualized
as systems. Integrated products, on the other hand, appear to be a single element, with what inter-
faces exist being so soft and blurred as to be hard to identify. Each of these styles has its advantages.
Integration allows the device to be technically more efficient. With the overall device optimized,
there is no loss due to inefficiencies of connections, etc.
Modularity provides flexibility at several levels, which is often extremely desirable. As men-
tioned earlier, it can make the design effort simpler. It also allows for modifications and upgrades. If
a system component is changed, one can test the new component for performance more efficiently
than testing the entire system. All of the engineering issues tend to be less challenging in this mod-
ular environment. In addition, a modular design enables element replacement. This is important in
several ways. An element of the system can be designed with the expectation that its life will be
shorter than that of the system, and it will be replaced when needed. The clearest case of this in med-
ical devices is a single-use or disposable element such as a blade. An appropriate architecture allows
a very efficient design, replacing the elements that cannot be reused without wasting those that can.
Modularity also enables variety in design by permitting variation in one or more components.
This is a way to provide devices in various sizes, or sometimes at various levels of performance. In
some cases, this is accomplished by selling the components of the system separately, in others the
unit can be thought of as a platform product with several different products, all based on the same
system and sharing a large number of elements. 8