A staple cartridge assembly , comprising: a cartridge body comprising a deck ; a plurality of staples ; and an implantable layer positioned over said deck, wherein said implantable layer is characterised by comprising, either: (i) a first plurality of fibers comprised of a first material having a first thermal transition temperature; and a second plurality of fibers comprised of a second material having a second thermal transition temperature, wherein said second thermal transition temperature is lower than said first thermal transition temperature, wherein said second material is intermixed with said first material, and wherein said second fibers are contracted within said layer during a process which exposes said layer to a process temperature which exceeds said second thermal transition temperature; or: (ii) a mesh comprised of a first material having a first thermal transition temperature; and fibers comprised of a second material having a second thermal transition temperature, wherein said first thermal transition temperature is lower than said second thermal transition temperature, wherein said fibers are interwoven with said mesh, and wherein said mesh is constricted during a process which exposes said layer to a process temperature which exceeds said first thermal transition temperature. a cartridge body comprising a deck ; a plurality of staples ; and an implantable layer positioned over said deck, wherein said implantable layer is characterised by comprising, either: (i) a first plurality of fibers comprised of a first material having a first thermal transition temperature; and a second plurality of fibers comprised of a second material having a second thermal transition temperature, wherein said second thermal transition temperature is lower than said first thermal transition temperature, wherein said second material is intermixed with said first material, and wherein said second fibers are contracted within said layer during a process which exposes said layer to a process temperature which exceeds said second thermal transition temperature; or: (ii) a mesh comprised of a first material having a first thermal transition temperature; and (i) a first plurality of fibers comprised of a first material having a first thermal transition temperature; and a second plurality of fibers comprised of a second material having a second thermal transition temperature, wherein said second thermal transition temperature is lower than said first thermal transition temperature, wherein said second material is intermixed with said first material, and wherein said second fibers are contracted within said layer during a process which exposes said layer to a process temperature which exceeds said second thermal transition temperature; or: (ii) a mesh comprised of a first material having a first thermal transition temperature; and fibers comprised of a second material having a second thermal transition temperature, wherein said first thermal transition temperature is lower than said second thermal transition temperature, wherein said fibers are interwoven with said mesh, and wherein said mesh is constricted during a process which exposes said layer to a process temperature which exceeds said first thermal transition temperature. <EOS>