A diffusion cell includes: a donor chamber having bottom; a receptor chamber below and in fluid communication with the donor chamber, and having upper and lower ends; a membrane between and in contact with the bottom of the donor chamber and upper end of the receptor chamber, and adapted to diffuse some of the media in a liquid from the donor chamber to the receptor chamber; a conduit having a first port near the lower end of the receptor chamber and a second port above the first port and near the upper end of the receptor chamber; and a bubble trap in fluid communication with the upper end of the receptor chamber and having a third port higher than the second port; wherein circulation of a flow of fluid through the conduit and the receptor chamber removes bubbles from underneath the membrane and transports bubbles to the bubble trap.