Creating your design input specification (DIS) should be the first stage of any product design process.
It is a vital tool your Design Team will use as the framework for the design and development of your medical device, and to also comply with regulatory requirements.
Wait! Just what is DIS?
In the simplest terms, your design input specification will specify the med device requirements relating to function, safety, design, and performance. The DIS should be established independent of any specific product solution, therefore allowing multiple concepts possibilities in the ideation phase which follows.
It is the collection of all the design inputs for your device. They cannot be incomplete, ambiguous, or conflict with each other.
There are many sources of design inputs, but a key input is the needs of the user (and/or patient), including risk assessment and human factors considerations.
User Needs: the Language of the Customer
Well defined user needs breathe life into your product design. They are the reasons behind each shape and function of your medical device and they represent the voice of the customer.
To make them useful for designers they must be expressed in metric form.
Specifications: the Language of the Engineer
They are the quantitative and measurable criteria that your product will comply with. They must contain a metric, a unit, and a value to be measurable, testable, unambiguous, and traceable
- Metric: the characteristic of the product that will be measured (e.g. length).
- Unit: a fixed quantity chosen as a standard of measurement (e.g. centimeter).
- Value: the numerical amount; a magnitude, quantity, or number. (e.g. 10 cm).
SO HOW DO WE TRANSLATE USER NEEDS INTO SPECIFICATIONS? The task of applying specs to something that feels immeasurable may feel intimidating. But we’re innovators—we will make it work!
For this guide, examples were drawn from the development file of a Leg Stent Deployment System (LSDS) that was designed by our team.
Step 1: Create a table
Create a table with User Needs and Metrics and the following additional headings:
- Rank – the measure of importance from least to most important (1 to 5)
- Units – standards of measurement
- Marginal Value – the limits of our value(s)
- Ideal Value – the value(s) we aim for
Step 2: Identify User Needs
Who is the customer? What are their needs?
Watch similar products in action, review benchmarked products, host focus groups, conduct interviews, and look at the FDA MAUDE database for clues. Ask users about their experience with similar products. What features do they like? What would they like to see changed?
These are the kinds of questions you ask to determine user needs.
Following are five different examples drawn from observations, surveys, interviews, and competitor analyses.
A sample of the LSDS device USER NEEDS are:
Step 3: Translate Needs into Metrics
Once you know WHAT the user needs are, you must then figure out HOW the device will meet them.
This is the trickiest part of your design input specification development.
Karl T. Ulrich & Steven D. Eppinger provide an excellent resource for product specifications in their book: “Product Design and Development, 5th Ed.” (Chapter 6).
You want to brainstorm a way to objectively measure each of your user needs. Strive for dependent variables that allow the most freedom for your Design Team.
As you will find, each user need can be measured by multiple metrics. For example, user need # 5 can be measured by 4 different metrics and possibly more.
Vice versa, metrics can measure multiple different user needs.
At the end of this guide, we will go over how to make your table easier to organize.
TO PREPARE FOR DESIGNERS, WE TURN THESE METRICS INTO SPECIFICATIONS BY ADDING UNITS AND VALUE.
Step #4: Establish Ranking Level
The rank determines the level of importance we place on our units and values. Establishing these rankings can take some research.
What user need or metric in this project design is most important? What isn’t? Where should we focus the most energy? Will there be trade-offs?
Here’s how we ranked our items:
We determined that the weight (#2) of the finished product was of mid-importance, thus won’t be a top concern in the design process.
Being able to access the lesion (#5) is very important (it is the primary purpose of a stent deployment system). And the stent must not get damaged (#4) or break. This would cause the limb-saving procedure to fail and possibly embed pieces of metal in the vascular system.
Therefore, items # 4 & 5 get a top rank of 5 and to emphasize that they will receive the greatest focus throughout the design process.
Step # 5: Find the Appropriate Units
This is quite easy for some metrics and seems impossible for others.
Keep in mind, nearly everything in this world can be calculated using universal standards of measurements, we just have to find the right one.
We decided the best way to measure control effectiveness (#1) would be to conduct a study in which participants will use the device and complete a survey or questionnaire. The results can be translated into a numerical score.
There are standard units of measurement that are followed for certain metrics, like durometer, pounds, millimeters, etc. When a yes or no is required, we list the unit as binary.
Once established, you may also need to determine the scale or precision; mm or in? Seconds or nanoseconds? Lbs or kg? Months or weeks? Etc. You can research similar products to see what the norm units are.
Step # 6: Establish Acceptance Criteria: Your Marginal and Ideal Values
It’s important to set limits and to have a goal.
Marginal Value is the unit amount limit we choose for our user need metrics.
Ideal Value it the unit amount that we strive for.
This clearly defines our acceptance criteria, while providing the design team with ‘room to move.’
For item #2, the ideal total mass is less than 200 g. If the ideal doesn’t work out, we know that it can’t be more than 250 g, which is our margin.
For item #5, it is imperative that the guidewire diameter is exactly 0.035 inches. Guidewire with this diameter is the standard product used in the radiology lab for the procedure, and the deployment catheter cannot be used without it.
The Values you choose are very important; Your finished product must meet these values. They can be the difference between success and failure.
AND THERE YOU HAVE IT: SPECIFICATIONS DERIVED FROM YOUR USER NEEDS
Final Level Tables:
I mentioned that there was an easier way to organize multiple user needs and metrics. Once you determine all user needs and metrics are matched up, give each user need a code name:
User Need #7 gets coded into “N7”. The user needs N1, N7, & N11 can all be measured using the “Effectiveness of controls” metric.
For more ease, you should have two tables, one defining all the user needs and their code names. The second table lines up all the user needs with the metrics.
User Needs Table Example
Metrics Table Example
To organize further you can assign a code name to each metric row, like M1 (Metric #1).
Now your specifications have traceability! (Metrics tied to a user need)
See how it looks clean and organized in these final tables?
It will be hard to get confused or lost with your user needs and metrics being this highly organized, and on top of it all, you also provide the required traceability to comply with FDA design control regulations!
Using these tactics will make writing your DIS easier. Happy writing!
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