This study analysed a series of workshops and explored prerequisites for interdisciplinary co-design among industrial designers, design engineers and users in the development of a novel medical device. Presented as a case study, this paper focus on what affects participants’ transformative processes towards genuine participation in co-design processes. Based on activity theory, we suggest that co-design activities have to support not only users, but all participants, shifting their perspectives beyond their own domain’s rules, motives, objects and division of labour, i.e., beyond their own activity system, to support users’ participation as equal members in design teams. We propose that genuine co-design requires a holistic approach where a neutral arena, an impartial facilitator, clear rules of play, along with representational artefacts as mediating tools in the formation of a new collective activity system to foster equality, mutual value and long-term knowledge generation. Such approach requires a process over time.