mHealth and Telehealth technologies are increasingly used to provide personalised, interactive and timely access to health data, thereby helping patients take a more active role in their care process. However, similar to any intervention, the use of these technologies has to be assured to justify that they do not compromise patient safety. In this paper, we discuss the development of a safety case for MediPi; a research prototype for a low-cost open-source digital platform that collects physiological data from patients, at home, and makes it available to decision-support systems used by clinicians. We identify potential hazardous failures associated with the use of MediPi and examine current risk controls. We also explore the modular structure of the overall safety case of the platform. We conclude with a discussion of patient safety challenges related to the unsupervised nature of the care setting and the use of commercial off-the-shelf personal devices.