The jet stream is usually generated by the pressure of a piston in an enclosed liquid-filled chamber. The piston is usually pushed by the release of a compressed metal spring, although investigational devices may use piezoelectric effects and other novel technologies to pressurize the liquid in the chamber. The springs of currently-marketed and historical devices may be compressed by operator muscle power, hydraulic fluid, built-in battery-operated motors, compressed air or gas, and other means. Gas-powered and hydraulically-powered devices may involve hoses that carry compressed gas or hydraulic fluid from separate cylinders of gas, electric air pumps, foot-pedal pumps, or other components to reduce the size and weight of the hand-held part of the system and to allow faster and less-tiring methods to perform consecutive vaccinations.