A method for injecting a substance through a biological body surface includes providing a needle-free transdermal transport device configured to inject the substance through the surface. The substance is injected into the biological body with the transport device while a parameter of the injection is sensed and a servo-controller is used to dynamically adjust at least one injection characteristic based on the sensed parameter. The substance is injected for (i) a first time period during which a first portion of a volume of the substance is injected at a first injection pressure, and (ii) a second time period during which a remainder of the volume of the substance is injected at a second injection pressure. A viscosity of the substance may be determined, and a pressure calculated for injecting the substance based on the viscosity. The substance may be injected with the transport device by using the calculated pressure.