  Arterial blood pressure is a key vital sign for the health of the human body.
As such, accurate and reproducible measurement techniques are necessary for
successful diagnosis. Blood pressure measurement is an example of molecular
communication in regulated biological systems. In general, communication in
regulated biological systems is difficult because the act of encoding
information about the state of the system can corrupt the message itself. In
this paper, we propose three strategies to cope with this problem to facilitate
reliable molecular communication links: communicate from the outskirts; build
it in; and leave a small footprint. Our strategies---inspired by communication
in natural biological systems---provide a classification to guide the design of
molecular communication mechanisms in synthetic biological systems. We
illustrate our classification using examples of the first two strategies in
natural systems. We then consider a molecular link within a model based on the
Michaelis-Menten kinetics. In particular, we compute the capacity of the link,
which reveals the potential of communicating using our leave a small footprint
strategy. This provides a way of identifying whether the molecular link can be
improved without affecting the function, and a guide to the design of synthetic
biological systems.
