Multi-layer cannula structure

A swellable cannula is formed of concentric inner and outer hydrophilic and substantially non-hydrophilic layer structures (12, 14, 40, 42, 44). The hydrophilic layer structure (12, 14, 40) comprises at least 2/3rds of the cross-sectional area of the wall of the cannula. The substantially non-hydrophilic layer structure (14, 12, 42, 44) is either substantially water impermeable or allows water to pass through at a controlled rate. The substantially non-hydrophilic layer structure may also control the diffusion of medicaments. The cannula has control softenability and swellability characteristics.