A method for transferring a solid body across a surface of a biological body includes (i) applying an electrical input to a controllable electromagnetic actuator; (ii) producing with the electromagnetic actuator a mechanical force corresponding to the electrical input; and (iii) applying the mechanical force to a reservoir coupled at one end to a nozzle, the mechanical force producing a pressure within the reservoir, a magnitude of the pressure varying with the mechanical force and causing ejection of a fluid from the reservoir to drive the solid body into the biological body. A method for delivering a substance to a target body includes (i) positioning a needle-free injector proximate to a surface of the target body; (ii) injecting the substance into the target body; and (iii) while injecting, moving the needle-free injector along the surface, thereby sweeping the surface.