  Molecular Communication (MC) is a bio-inspired communication modality that
utilizes chemical signals in the form of molecules to exchange information
between spatially separated entities. Pulse shaping is an important process in
all communication systems, as it modifies the waveform of transmitted signals
to match the characteristics of the communication channel for reliable and
high-speed information transfer. In MC systems, the unconventional
architectures of components, such as transmitters and receivers, and the
complex, nonlinear, and time-varying nature of MC channels make pulse shaping
even more important. While several pulse shaping methods have been
theoretically proposed for MC, their practicality and performance are still
uncertain. Moreover, the majority of recently proposed experimental MC testbeds
that rely on microfluidics technology lack the incorporation of programmable
pulse shaping methods, which hinders the accurate evaluation of MC techniques
in practical settings. To address the challenges associated with pulse shaping
in microfluidic MC systems, we provide a comprehensive overview of practical
microfluidic chemical waveform generation techniques that have been
experimentally validated and whose architectures can inform the design of pulse
shaping methods for microfluidic MC systems and testbeds. These techniques
include those based on hydrodynamic and acoustofluidic force fields, as well as
electrochemical reactions. We also discuss the fundamental working mechanisms
and system architectures of these techniques, and compare their performances in
terms of spatiotemporal resolution, selectivity, system complexity, and other
performance metrics relevant to MC applications, as well as their feasibility
for practical MC applications.
