In healthcare, translating evidence into changed practice remains challenging. Novel interventions are being used to address these challenges, including the use of "knowledge brokers." But how sustainable these roles might be, and the consequences for the individual of enacting such roles, are unknown. We explore these questions by drawing on qualitative data from case studies of full-time roles in research-practice collaboration. We suggest that structural issues around professional boundaries, organisational norms and career pathways may make such roles difficult to sustain in the long term, but highlight interventions that might improve their feasibility.