Diffusion insert, in particular for maintaining a membrane, said diffusion insert comprising at least one cup defining an axial donor compartment intended to receive a donor solution, said cup having a generally cylindrical shape provided with a radial return, the inner portion of which defines a radial bearing surface suitable for the periphery of said membrane placed in said cup to bear thereon, said radial return defining at least one cylindrical axial orifice intended to be closed by the central portion of said membrane, said diffusion insert comprising at least: - a clamping element suitable for being housed in said cup, above said membrane, and for cooperating with said cup in order to clamp the periphery of said membrane, - blocking means arranged to block said clamping element with respect to said cup at least axially, and - a deformation element comprising an axial deformation surface suitable for deforming said membrane axially and radially and for cooperating with said clamping element in order to stretch said membrane at least radially with respect to said clamping surface, said deformation element being suitable for cooperating with said cup in order to preserve a radial lateral clearance directly tangent to said deformation surface of said deformation element covered by said stretched membrane, - a return element provided between said clamping element and said blocking element, said return element being formed by a tab inclined with respect to the axis of said diffusion insert and being elastically deformable between a "rest" state in which it does not tend to move said clamping element away from said blocking element and a "bent" state in which it tends to move said clamping element away from said blocking element in order to stress, deform and radially stretch said membrane in a controlled manner, - a pressing ring forming the clamping element, the deformation element and the return element, the outer dimensions of the pressing ring being smaller than the inner dimensions of the opening of the cup, in its lower portion, the pressing ring is provided with an axial extension forming a deformation ring defining the deformation element, said deformation ring having an outer diameter smaller than the inner diameter of the axial orifice and a height greater than the thickness of the return, thus when the pressing ring in the "use" position being housed at the bottom of the cup, the deformation ring passes through the axial orifice until it extends under the cup. - a clamping element suitable for being housed in said cup, above said membrane, and for cooperating with said cup in order to clamp the periphery of said membrane, - blocking means arranged to block said clamping element with respect to said cup at least axially, and - a deformation element comprising an axial deformation surface suitable for deforming said membrane axially and radially and for cooperating with said clamping element in order to stretch said membrane at least radially with respect to said clamping surface, said deformation element being suitable for cooperating with said cup in order to preserve a radial lateral clearance directly tangent to said deformation surface of said deformation element covered by said stretched membrane, - a return element provided between said clamping element and said blocking element, said return element being formed by a tab inclined with respect to the axis of said diffusion insert and being elastically deformable between a "rest" state in which it does not tend to move said clamping element away from said blocking element and a "bent" state in which it tends to move said clamping element away from said blocking element in order to stress, deform and radially stretch said membrane in a controlled manner, - a pressing ring forming the clamping element, the deformation element and the return element, the outer dimensions of the pressing ring being smaller than the inner dimensions of the opening of the cup, in its lower portion, the pressing ring is provided with an axial extension forming a deformation ring defining the deformation element, said deformation ring having an outer diameter smaller than the inner diameter of the axial orifice and a height greater than the thickness of the return, thus when the pressing ring in the "use" position being housed at the bottom of the cup, the deformation ring passes through the axial orifice until it extends under the cup.