a wafer for use in the selective connecting and disconnecting of plastic tubes incorporates a fuse in an aperture . the fuse is made of a three layer laminate having light transmission characteristics which change after the wafer and its laminate have been heated . the three layers comprise two outer transparent layers and an intermediate layer having limited light transmission characteristics . when the wafer is heated the intermediate layer shrinks thereby changing the light transmission characteristics by providing a path for light to pass through the laminate without passing through the intermediate layer .