Practical limits on the maximal speed of solution exchange for patch clamp experiments

Studying ligand-gated ion channels often requires the ability to change solutions quickly. Using finite element models, I have examined the practical limitations of how fast solutions can be exchanged an an outside-out patch using a dual stream switcher. The primary factors controlling the speed of response are the flow velocity, proximity of the patch to the exit ports, the width of the partition between the two streams, the velocity with which the streams can be moved across the patch, and the viscosity of the solutions. The practical limit seems to be a rise time of similar to 20 mu s. The rate-limiting step is the velocity of the (usually piezo) motor that translates the streams across the patch. increasing the perfusate viscosity improves speed by slowing dissipation of the concentration gradients. A flow switcher can also be used for bipolar temperature jumps with a rise time of similar to 100 mu s.