  This paper considers the scenario of a targeted drug delivery system, which
consists of deploying a number of biological nanomachines close to a biological
target (e.g. a tumor), able to deliver drug molecules in the diseased area.
Suitably located transmitters are designed to release a continuous flow of drug
molecules in the surrounding environment, where they diffuse and reach the
target. These molecules are received when they chemically react with compliant
receptors deployed on the receiver surface. In these conditions, if the release
rate is relatively high and the drug absorption time is significant, congestion
may happen, essentially at the receiver site. This phenomenon limits the drug
absorption rate and makes the signal transmission ineffective, with an
undesired diffusion of drug molecules elsewhere in the body. The original
contribution of this paper consists of a theoretical analysis of the causes of
congestion in diffusion-based molecular communications. For this purpose, it is
proposed a reception model consisting of a set of pure loss queuing systems.
The proposed model exhibits an excellent agreement with the results of a
simulation campaign made by using the Biological and Nano-Scale communication
simulator version 2 (BiNS2), a well-known simulator for molecular
communications, whose reliability has been assessed through in-vitro
experiments. The obtained results can be used in rate control algorithms to
optimally determine the optimal release rate of molecules in drug delivery
applications.
