A dialysis device (10) embodies a hermetically sealed sample chamber formed by a gasket (12) with dialysis membranes (14, 16) affixed to each side in substantially parallel relationship. The gasket (12) is impermeable to the sample being dialyzed, but is penetrable and reusable such that a needle can be inserted through the gasket (12) into the thickness and then withdrawn without sample being permitted to leak. The gasket (12) is of sufficient thickness to accommodate insertion of a needle. The device (10) is fitted into a rigid housing (20) containing windows (24, 26) exposing the dialysis membranes (14, 16) and a channel (34) parallel to the dialysis membranes (14, 16) for directing a needle into the gasket (12) in a direction substantially perpendicular to the edge of the gasket (12) so that the needle can access the chamber without contacting the surface of the membranes (14, 16).