The present invention relates to a fluid vessel insertion device comprising a) a puncture member arranged for location within a cannulation sheet and comprising an interior lumen； b) a retractor arranged to actuate the puncture member relative to the cannulation sheet in use between i) a first situation in which a distal end of the puncture member protrudes beyond an end of the cannulation sheet, and ii) a second situation in which the distal end of the puncture member is within the cannulation sheet； c) a chamber in fluid communication with an exterior of the device only via the lumen and distal end ofthe puncture member； and d) a sheet of material located in the chamber to prevent the proximal end of the puncture member from moving； wherein the distal end of the puncture member is configured as a puncturing end having a distal n end opening for insertion into a fluid vessel when in use, and wherein the retractor triggers retraction of the puncture member between the first and the second situation automatically in response to wetting of the sheet of material by a fluid entering the puncture member lumen, whereby the sheet of material is comprising a material whose tensile strength is reduced upon contact with the fluid, and breaks under the pressure of the retractor.
