Page 31 - Biomedical Engineering and Design Handbook Volume 2, Applications

10 MEDICAL DEVICE DESIGN

necessitating numerous repetitions of the measurement. It will be much more convenient to have
easily determined objectives. It is almost always possible to reduce the issue to easily examined
variables. One exception would be a reliability specification where many units would have to be
tested to arrive at an acceptable failure rate.
Once the list of metrics has been set, the values to be required must be determined. Here we have
several leads to go on. We have the results of our user studies, which might have set some specific
guidelines. We have the existing products that our competitors or we market and products that will
be perceived to fall into the same category. These must be examined for similar user characteristics,
sizes, operating forces, etc. What are the current accepted values? What are the values we hope to
achieve? It is a good idea to obtain samples of all these products if it has not yet been done, even
those that are in no way competitive, if they are going to be thought of by the user as in the same
class. If there is a range of quality here, the level that is being aimed for must be determined and the
values set accordingly. Having reviewed all the pertinent information available, specification values
should be set. (See Table 1.1.)

TABLE 1.1 Idealized Example of Some User Needs Translated to Specifications for the External
Fixation System

User comment Specification Units Value
Make it from aluminum or The system mass will be no more than X Kilograms 0.75
carbon fiber
It must be easy to install Installation time will be less than X Minutes 20
It must not weaken the bone Holes in bones must have diameters no Millimeters 4.0
too much more than X

The documentation of this process is important because in most cases the target specifications
arrived at here will be unobtainable. (You find that you cannot make the laproscopic tool strong
enough and still fit it through a 7-mm port!) Later in the project there will be an effort to reach some
compromises and having at hand the details of how the existing values were chosen will save more
than half of the discussion time and, more often, allow the discussion to proceed in a more civilized
manner, preserving the team morale.

1.10 CONCEPT DEVELOPMENT

With the specifications in place, it is now time to seriously look for solutions. The concept devel-
opment phase is the time to search out ideas that will meet the need. Much has been written about
ways to generate new ideas, and some of them will be touched on here, but more important than
method is motivation. The objective is to generate an absolutely superb product, one that meets all
the requirements in a cost-effective, safe, elegant way, and to do it quickly and efficiently. Unfor-
tunately, at this stage of the process, quickly and efficiently often dominate the first part of that
sentence in a manner that is very shortsighted. There is a tendency to proceed with the first idea that
appears to have merit. In the general scheme of things, a little extra time at this stage might very
well produce a much better idea, or perhaps even a scheme that is only slightly better. Consider the
trade-off.
Time spent on concept generation is relatively inexpensive. Prototypes are not being built, clinical
trials aren’t being conducted. A great deal of paper may get expended and even some computer time,
but if one looks at the rate at which resources are consumed, the concept phase of a development
project is on the lower end of the scale. If one thinks of the concept phase as a search for the best