Plunger rod releasably attachable to plunger head in a pump device

A mechanism for selecting an operational interaction between an axially reciprocating plunger rod of a pump device and a plunger head bidirectionally moveable between a first axial position and a second axial position in a syringe and restrained in the syringe against rotational movement, the mechanism comprising:

• a cylindrical cam body axially restrained on and rotatable about the axially reciprocating plunger rod and receivable by a one-sided open chamber concentrically formed in the plunger head by a cylindrical wall, said cylindrical cam body comprising a plurality of spaced cam surfaces circumferentially located on the cylindrical cam body;
• and
• a plurality of cam pins equiangularly protruding from the cylindrical wall towards a longitudinal axis of the plunger head, the plurality of cam pins configured to co-act with the cylindrical cam body on reciprocal movement of the plunger rod to sequentially traverse the plurality of cam surfaces to rotate the cylindrical cam body in, and relative to, the chamber to successively different angular positions corresponding to different operational interactions between the plunger rod and the plunger head.

A method of operating a pump device having a plunger rod provided with a rotatable cam body, comprising:

• engaging a disposable reservoir provided with a plunger head with a reusable part of the pump device and, while the disposable reservoir and the reusable part of the pump device are engaged
• axially moving the plunger rod in the disposable reservoir in a reciprocating manner to cause the rotatable cam body to cammingly co-act with cam pins in the plunger head to rotate the rotatable cam body to an engagement angular position in which the plunger rod is engaged with the plunger head, then through a series of alternating pushing-pulling angular positions corresponding to a series of alternating pushing forward and pulling backwards the plunger head in the disposable reservoir, and, then, to a disengagement angular position in which the plunger rod is disengageable from the plunger head.