Traditional user-centred design processes frequently marginalize vulnerable users. Their perspectives are thus not well represented in discussions of the future of the medicines and technologies on which they rely. We present PDFi, a method that responds to this issue in the context of Urinary Tract Infections (UTIs). PDFi was developed through a collaboration with a medical device development company. The method marries Participatory Design (PD) strategies with Design Fictions (DFi) to personally ground, inspire and reveal values and imaginaries of vulnerable individuals who rely on medical technologies for their health and wellbeing. By more fully including these vulnerable users in shaping their medical technology futures, PDFi gives voice to those most impacted by such futures; it elicits emotions, discussion and debate, and powerfully reshapes current approaches to user-centred design.