Biomedical Engineering Reference
Developing Your Product
We have already looked at the importance of a good specification. It is the cornerstone of any
good design. In this chapter we shall take a look at the basics of a product design specification
(hereafter called a PDS). First we shall examine how to develop a good statement of need;
subsequently we shall expand this to a full-blown PDS.
I cannot stress enough how important the PDS is. It is the essential input to meet FDA,
Medical Device Directive (MDD), and ISO 13485 requirements. But, and more importantly,
it is your only weapon in the battle between you and your customers. Too often the customer
forgets what they asked for, doesn't understand what they really want, and just keeps
changing their minds. The PDS is your weapon that enables you to say “this is what you
agreed to last time.” More importantly, and as we have seen earlier, the more effort you put
into understanding what is “really needed” the easier everything else becomes.
A good rule of thumb for a PDS is that you should be able to give a completed PDS to
another designer (of equivalent skills) and without any further conversation they should be
able to understand what is needed, fully. That is our task for this chapter: to be able to write a
fully populated PDS.
Before we start, there's one last thing to note. Too often a PDS is confused with the
“specification” one finds in sales literature. They are not the same thing. In sales the
specification describes the characteristics of a product after it has been designed, and
customers use it to discern between competing products. Do not use these as the basis for
5.2 Developing the Statement of Need (or Brief)
As described in previous sections, the starting point of the design process is the identification
of a need. This need has to be articulated and approved before any further effort (and hence
costs) are attributed to the project. We have met the types of need, and it is immaterial where
the seed comes from - how we define the need is the same.