Fig. 2.
Microfluidic device for studying frequency response of single cells. (A) One of the input arms of a Y-shaped flow chamber is fed by a solution of 1 M sorbitol in SC medium (in red) from reservoir R3 at a hydrostatic pressure head at P0. The other arm is fed by SC (in blue) from one of two reservoirs, R1 at a hydrostatic pressure head at P+ and R2 at P−. The choice between the two reservoirs, R1 or R2, is made by a programmable three-way electrovalve. When reservoir R2 is chosen, the fluid from reservoir R3 fills most of the chamber, whereas when R1 is chosen, the fluid from R1 fills the chamber. Periodic changes in the state of the electrovalve allow a change in the environment of the cells at a tunable period T. (B) To characterize the device, water was filled in reservoirs R1 and R2, and a dilute solution of fluorescein was filled in R3. The state of a cross-section of the flow chamber was imaged by fluorescent microscopy. This state is shown as a function of flow-cell width (along the x axis) and time (along the y axis) for both T = 1-s oscillations (Left) and T = 0.5-s oscillations (Right). The bright regions indicate fluorescein solution, and the dark regions indicate water. The sharp interfaces between the dark and bright regions show the efficiency of our device in changing conditions rapidly. (C) The fluorescence intensity for a typical point in the middle of the channel is shown for oscillation periods ranging from T = 0.5 s to T = 4 s. The curves have been offset along the y axis for clarity, and for each curve, time has been scaled by its input frequency ω. All the curves oscillate between the same minimum and maximum fluorescence intensities. The transition from one medium to the other gets less sharp as T = 0.5 s is approached, indicating the limit of resolution for our experimental device.