We propose two new injection techniques for use in electrophoresis microchips, which we call 'Front Gate Pressure Injection' and 'Back Gate Pressure Injection'. Both techniques enable a controlled and variable size sample introduction with reduced bias compared to electrokinetic gated injection. A continuous flow of sample and buffer solution is electrokinetically driven near to the entrance of the separation channel, using a single voltage that is constant in time. A short sample plug is then injected in the separation channel by a 0.1 sec pressure pulse. The latter is generated using the mechanical deflection of a poly(dimethylsiloxane) membrane that is loosely placed on a dedicated chip reservoir. Back Gate Pressure Injection was found to significantly decrease the injection bias compared to a classical gate flow injection while keeping the separation efficiency for fluorescein / rhodamine B solutions.