A pressure controller (Fluigent LU-FEZ) with an attached flow-rate meter is used control flow rates for the experiments. Each inlet tubing is flushed with the desired solution before the tubing is connected to the microfluidic device. Driving pressure ∆P is recorded every 100ms for the duration of each experiment, which last typically at least 15 minutes but in some cases as long as nearly 6 hours (Table ). ∆P can be measured to a maximum of 2000 mbar, with a resolution of 1 mbar. The response time of the pressure controller is 2 seconds. Flow rate is controlled independently for each of the two inlet streams. The flow rate ratio of alginate and calcium channels is held constant at 4:1. The flow rates Q are typically 0.96 µL/min and 0.24 µL/min for alginate and calcium streams, respectively. However, the variable flow rate studies are obtained with Q varying from 0.96 µL/min to 9.6 µL/min for the alginate stream. We refer to the condition with Q = 0.96µL/min for alginate and 0.24 µL/min for calcium in terms of the total flow rate, Q T = 1.2µ L/min.