The invention forms micropores by an off-axis holographic exposure process. The radiopaque polymer may comprise a polymer that is inherently radiopaque or a polymer that has been made radiopaque by the addition of an additive that provides radiopaque characteristics. A layer of over molded sealing material provided on the support plate is in sealing engagement with the edge portions of the adjacent cover portions when they are in their closed position. The value of t\u2032peak is determined as a function of a mobility term that is determined during a potentiometry phase following the amperometric measurements.