Softening catheter

A cannula structure for insertion into a living body, comprising:

• a tubular cannula having distal and proximal end portions, a central passageway therethrough from the distal end portion to the proximal end portion, the proximal end portion having larger inner and outer diameters than does the distal end portion, the cannula having a greater thickness along the proximal end portion than along the distal end portion, the proximal end portion extending to within the living body, and a transition section connecting the distal and proximal end portions whereat the diameter differences are effectuated in a gradual manner, the cannula being constructed of a material that has a softening ratio of greater than 2:1 on being contacted with an aqueous liquid and/or upon insertion into a living body.