  Chemical reactions-based microfluidic circuits are expected to provide new
opportunities to perform signal processing functions over molecular domain. To
realize this vision, in this article, we exploit and present the digital signal
processing capabilities of chemical reactions-based microfluidic circuits.
Aiming to facilitate microfluidic circuit design, we describe a microfluidic
circuit using a five-level architecture: 1) Molecular Propagation; 2) Chemical
Transformation; 3) Microfluidic Modules; 4) Microfluidic Logic Gates; and 5)
Microfluidic Circuits. We first identify the components at Levels 1 and 2, and
present how their combinations can build the basic modules for Level 3. We then
assemble basic modules to construct five types of logic gate for Level 4,
including AND, NAND, OR, NOR, and XOR gates, which show advantages of
microfluidic circuits in reusability and modularity. Last but not least, we
discuss challenges and potential solutions for designing, building, and testing
microfluidic circuits with complex signal processing functions in Level 5 based
on the digital logic gates at Level 4.
