A transdermal
drug delivery device includes a reservoir, a transdermal membrane, a
piston, a
control rod, a spring, and a rotational
cam. The reservoir is configured to hold a formulation. The transdermal membrane is configured to allow the formulation from the reservoir to pass therethrough. The
piston is configured to move into the reservoir. The
control rod is attached to the
piston and includes a plurality of teeth thereon. The spring is configured to apply force to the
control rod in the direction of the reservoir. The rotational
cam has a first camming surface and a second camming surface that are configured to engage with the plurality of teeth. The rotational
cam, when rotated, is configured to disengage the first camming surface from a first tooth of the plurality of teeth, thereby allowing the spring to advance the piston into the reservoir to expel the formulation onto the transdermal membrane.