Apparatus is described that includes a blade assembly (20) for conditioning ex vivo pericardial tissue from which fibers extend. The apparatus includes a blade head (26) that includes a blade a may (28) and a face (30) defining gaps (32) therein. The gaps are dimensioned to facilitate protrusion therethrough of fibers into the blade head, such that movement of the blade array, with respect to the face, shears the fibers. The blade assembly is configured to be used while the tissue and the blade array are submerged in a liquid (140) with the fibers facing upward, and with the face facing downward. The apparatus is configured to provide, while the blade array is submerged in the liquid, a suction force that sucks the liquid into the blade head via the gaps, and discharges a refuse liquid through a lateral exhaust hole (24). Other embodiments are also described.