Abstract: A diffusion insert (1) for a membrane (5), includes a cup (2) defining a donor compartment (20) receiving a donor solution, the base thereof defining an axial port (22) sealed by the membrane (5), a clamping element (3) to pin down the membrane (5), an element for axially locking (4) the clamping element (3), a deformation element (6) capable of deforming the membrane (5) and of protecting the lateral movement directly tangential to the deformation element (6), characterized in that the insert includes a return element (7) for deforming and radially stretching the membrane (5) in a controlled manner. A kit, a cell, a diffusion unit, and a diffusion method for implementing the diffusion insert are also disclosed.