A tissue thickness compensator can comprise a compensator body and at least one vessel contained in the compensator body. The vessel can define an inner cavity which can comprise an inner atmosphere having a pressure which is lower than the atmospheric pressure of the atmosphere surrounding the tissue thickness compensator. In at least one embodiment, the vessel and the compensator body can be maintained in a collapsed state until staples are fired through the vessel. At such point, the vessel can re-expand and apply a biasing force to tissue captured within the staples.