Since this is a medical appliance and not a cell phone we needed to have some key goals/rules that are unique to the medical device domain.
Rule #1: Don't kill, maim or hurt anyone
To accomplish this goal we needed to prevent user error. The key use error identified was unintended actuation or prevent the user from activating features without their intent. We needed to prevent the accidental modification of the patient's therapy when they weren't using it, like when they put it in their pocket or carried it around in their purse. We applied the following design mitigations to prevent the use errors. I call them mitigations because they each have a usability draw back.
- Locking mechnasim - the patient can lock the handheld when they are finished or the system will automatically lock after a period on non-activity, but this means you will need to unlock the handheld each time you want to use it.
- Resistive touchscreen rather than a capacitive touchscreen - the patient uses their finger or other fleshy protruberance to activate onscreen targets, just bumping your handheld into other hard surfaces won't activate the targets, but this also means you need to take your gloves off to make an onscreen request.
- Multi-modal interactions for therapy modifications - the patient makes a therapy adjustment request onscreen and then confirms it with a hardware button keypress. This protects the patient, but may cause interaction confusion when switching between touchscreen and hardware interaction combinations.
- Use target size and target spacing appropriate for patients with movement disorders - we used a 1/2" target with 1/2" spacing, but this reduced the navigation options and information space available on the screen.
Some other great guidelines for you to use on your touchscreen are available at the Information and Communication Technology (ICT)Accessibility site.