A staple cartridge assembly for use with a surgical stapler. The assembly has a cartridge body having a support portion with a plurality of staple cavities with openings. There is also a plurality of staples, wherein at least a portion of each the staple is removably stored within a the staple cavity. Each the staple is movable between an unfired position and a fired position, and is deformable between an unfired configuration and a fired configuration. The assembly also includes a compressible tissue thickness compensator configured to be captured within the staples. The compressible tissue thickness compensator at least partially covers the staple cavity openings. The compressed tissue thickness compensator is configured to assume different compressed heights within different the staples. The compressible tissue thickness compensator comprising a lyophilized foam having an oxygen generating agent embedded therein.