Use errors, where medical devices work to specification but lead to the clinicians making mistakes resulting in patient harm, is a critical problem. Manufacturers need tools to help them find such design flaws at an early stage and regulators need tools to help check devices are safe to approve for market. We have developed a prototyping tool, PVSio-web, to help check the safety of medical device interface and interaction design. It supports a model-based design process: that is, it is based on precise mathematical descriptions of the device's behaviour. This allows sophisticated proof and model checking technology to be used to verify that devices meet essential safety requirements. The architecture allows for the flexible addition of `plug-in' modules to extend its functionality giving different views of the design that allow different stakeholders to work together. Working with the US regulator, the Food and Drug Administration (FDA), our tool has helped identify problems in a series of commercial medical devices. Hospitals have used it as part of training programmes highlighting safety-related design issues. In ongoing work we are developing plug-ins that support the verification and validation of interoperable medical systems.