Microfluidic devices are described that include a rigid base layer, and an elastomeric layer on the base layer. The elastomeric layer may include at least part of a fluid channel for transporting a liquid reagent, and a vent channel that accepts gas diffusing through the elastomeric layer from the flow channel and vents it out of the elastomeric layer. The devices may also include a mixing chamber fluidly connected to the fluid channel, and a control channel overlapping with a deflectable membrane that defines a portion of the flow channel, where the control channel may be operable to change a rate at which the liquid reagent flows through the fluid channel. The devices may further include a rigid plastic layer on the elastomeric layer.