Fig. 1

a) Conceptual description of pulse width modulation (PWM) technique: A target signal (e.g., red sinusoidal wave in bottom plot) is converted to a PWM signal (top plot) via a mathematical operation. Low-pass filtering the PWM signal yields the actual signal (blue ragged sinusoidal wave in bottom plot) that approximates the red sinusoidal target signal. b) Microfluidic integration of the PWM technique: A PWM signal (coding for a sinusoidal target signal as an example) electronically actuates the flow selection valve that switches between the high-pressure and low-pressure analyte reservoirs at the same concentration yet at different hydrostatic pressures due to their height differential. The PWM signal (i.e., pulse train of fast and slow flow rates at node “i”) is converted to the target flow rate signal (at node “ii”) via the filter chip. The analyte with the time-varying flow rate combines with the buffer solution at the mixer chip, effectively converting the time-varying flow rate signal to a time-varying concentration signal (note “iv”). In order to have constant flow rate at node “d”, a syringe pump withdraws the liquid at a constant flow rate from the filter chip and the resistor chip. The time-varying concentration and flow rate profiles at four different nodes (i: entering filter chip; ii: leaving filter chip and entering mixer chip; iii: entering resistor chip; iv: leaving mixer chip) are shown inside the rounded rectangular box