Abstract: A microfluidic system for determining a response of cells comprises one or more fluid pumps. The one or more fluid pumps move a fluid across cells within a microfluidic device. The microfluidic device includes a microchannel at least partially defined by a surface having cells adhered thereto, a first port at one end of the microchannel, and a second port at an opposing end of the microchannel. The one or more fluid pumps move the fluid across the cells in a first direction toward the second port and then move the fluid across the cells in a second direction toward the first port.