  The design of components with molecular communication (MC) functionalities
can bring an opportunity to enable some emerging applications in fields from
personal healthcare to modern industry. In this paper, we propose the designs
of the microfluidic transmitter and receiver with quadruple concentration shift
keying (QCSK) modulation and demodulation functionalities. To do so, we first
present an AND gate design, and then apply it to the QCSK transmitter and
receiver design. The QCSK transmitter is capable of modulating two input
signals to four different concentration levels, and the QCSK receiver can
demodulate a received signal to two outputs. More importantly, we also
establish a mathematical framework to theoretically characterize our proposed
microfluidic circuits. Based on this, we first derive the output concentration
distribution of our proposed AND gate design, and provide the insight into the
selection of design parameters to ensure an exhibition of desired behaviour. We
further derive the output concentration distributions of the QCSK transmitter
and receiver. Simulation results obtained in COMSOL Multiphysics not only show
the desired behaviour of all the proposed microfluidic circuits, but also
demonstrate the accuracy of the proposed mathematical framework.
