Generation of dynamic chemical signals with microfluidic C-DACs

The utilization of microfluidic "lab-on-a-chip'' devices in fundamental medical research, drug discovery and clinical diagnostics has rapidly increased in the past decade. Lab-on-a-chip devices process small volumes of analytes and reagents through on- chip microfluidic signal processing circuits. This paper discusses the implementation of a basic microfluidic circuit block, the concentration digital- to- analog converter ( or C- DAC) which produces discretized chemical concentrations in a constant stream of solvent. The chemical concentration is controlled by a time- varying digital word; hence C- DACs are suitable for on- chip generation of arbitrary chemical signals. A 4- bit continuous- flow C- DAC was fabricated in two- level PDMS technology and tested. Several chemical waveforms ( sawtooth, cosine, and ramp) were generated at flow rates of 2 mu L min(-1) and frequencies of 0.6 - 4 mHz. The frequency cut off of this C- DAC was similar to 500 mHz.