We have used formal methods technology to investigate software and user interface design issues that may induce use error in medical devices. Our approach is based on mathematical models that capture safety concerns related to the use of a device. We analysed nine commercial medical devices from six manufacturers with our approach, and precisely identified 30 design issues. All identified issues can induce use errors that could lead to adverse clinical consequences, such as numbers being incorrectly entered. An issue with formal approaches is in making results accessible to developers, human factors experts and clinicians. In this paper, we use our tool PVSio-web to demonstrate the identified issues: PVSio-web allows us to generate realistic and interactive user interface prototypes from the same mathematical models used for analysis. Users can explore the behaviour of the prototypes by pressing buttons on realistic user interfaces that reproduce the functionality and visual representation of the real devices. Users can examine the device behaviour resulting from any interaction. Key sequences identified from analysis can be used to explore in detail the identified design issues in an accessible way.