A needleless fluid injection system include a working shaft (14) with a proximal end and a distal end, an injection shaft with a proximal end and a distal end, and a tissue tensioning mechanism (40) positioned at the distal end of at least one of the working shaft and the injection shaft. Embodiments of the tissue tensioning mechanism can include one or more of multiple radially expanding arcuate members (44), a linearly compressible element (60) with first and second plates at opposite ends of the linearly compressible element, an arcuate member (70) having a first end engaged within the lumen of the injection shaft and a second end moveable relative to the first end, first and second wedges (90a, b) that are slideable relative to each other, an inflatable bladder (110) with a foam material within a body opening of the bladder, at least two electromagnetic coils (146,148), or an electroactive polymer portion (166).