Abstract: A heat and moisture exchanger (HMX) device may be used with a patient interface system to treat a patient with a flow of air at a therapeutic pressure. The HMX device may include: a frame; and at least one layer of HMX material supported by the frame, the HMX material being configured to adsorb water vapour from gases exhaled by the patient and desorb water vapour into the flow of air at the therapeutic pressure, the HMX material being flexible, the at least one layer of HMX material having a flat shape in an undeformed state, and the at least one layer of HMX material having at least one slit that is configured to be widened when the at least one layer of HMX material is deformed into a non-flat shape.