Apparatus for filtering liquids, comprising flat membrane pockets arranged parallel and means for pressure-tight connection of the membrane pockets to one another and for coupling an extraction means. A bracket is attached to the side of a cushion seat back which is laying flat. In another embodiment, the di-tyrosine cross-linked polypeptide or polypeptide chain has enhanced stability compared to the same polypeptide or polypeptide chain in the absence of di-tyrosine cross-linking.