A microfluidic device for analyzing permeability of substances through a membrane (M), wherein the substances are disposed in a respective fluid flow (F1,F2), the microfluidic device comprising - a housing ; - a first flow channel for passing a first fluid flow (F1) through the housing between a first input connector and a first output connector ; - a second flow channel for passing a second fluid flow (F2) through the housing between a second input connector and a second output connector ; - an access cavity extending from outside into the housing through the first flow channel and into the second flow channel for accessing an inside of the housing to place the membrane (M) over a cavity opening forming an exclusive fluid interconnection between overlapping areas of the flow channels ; and - a clamping ring comprising a connection structure configured to engage a corresponding connection structure of the housing inside the access cavity for holding the sample membrane (M) in place over the cavity opening while the membrane (M) is, in use, exposed to the respective fluid flows (F1,F2) through the flow channels on either sides of the membrane (M), wherein the cavity opening on one side of the membrane (M) and a ring opening through the clamping ring on another side of the membrane (M) are configured to expose the membrane (M) to the respective fluid flows (F1,F2) in the flow channels , wherein the clamping ring is configured to form part of the first flow channel , characterised in that the clamping ring comprises a sloped and/or rounded transition between its inner and outer diameters getting gradually thinner towards its center where the membrane (M) is to be exposed to the first fluid flow (F1). - a housing ; - a first flow channel for passing a first fluid flow (F1) through the housing between a first input connector and a first output connector ; - a second flow channel for passing a second fluid flow (F2) through the housing between a second input connector and a second output connector ; - an access cavity extending from outside into the housing through the first flow channel and into the second flow channel for accessing an inside of the housing to place the membrane (M) over a cavity opening forming an exclusive fluid interconnection between overlapping areas of the flow channels ; and - a clamping ring comprising a connection structure configured to engage a corresponding connection structure of the housing inside the access cavity for holding the sample membrane (M) in place over the cavity opening while the membrane (M) is, in use, exposed to the respective fluid flows (F1,F2) through the flow channels on either sides of the membrane (M), wherein the cavity opening on one side of the membrane (M) and a ring opening through the clamping ring on another side of the membrane (M) are configured to expose the membrane (M) to the respective fluid flows (F1,F2) in the flow channels , wherein the clamping ring is configured to form part of the first flow channel , characterised in that the clamping ring comprises a sloped and/or rounded transition between its inner and outer diameters getting gradually thinner towards its center where the membrane (M) is to be exposed to the first fluid flow (F1). <EOS>