Abstract: A thermal ink-jet pen cartridge including an ink reservoir for maintaining ink under negative pressure. The ink reservoir is coupled to the printhead via a discharge port. To prevent air bubbles from entering the reservoir via the discharge port and printhead after ink is expelled via the thermal process, a check valve is placed in the fluid path between the ink reservoir and the printhead at the discharge port. The check valve is a mesh having a very small mesh opening sufficient to prevent air bubbles from passing through under normal pressures. The check valve also serves the function of a particulate filter to prevent contamination of the printhead by particles from the ink reservoir.