A system (10) for stimulating the healing of tissue is described. The system comprises a porous pad (11)； an airtight dressing (13)； a means for connecting a distal end (16b) of a conduit (16) through the dressing (13)； a canister (18) removably connected to a proximal end (16a) of the conduit (16)； means (14) for applying negative pressure to a wound site； a first filter (20) positioned between said canister (18) and said means (14) for applying negative pressure； a second filter (22) positioned between said first filter (20) and said means (14) for applying negative pressure； and a means for varying said negative pressure over a time interval comprising means for adjusting actual pressure to meet a varying target pressure, characterised in that said varying target pressure oscillates between a target maximum and a target minimum pressure.
