  Molecular communications is a promising framework for the design of
controlled-release drug delivery systems. Under this framework, drug carriers,
diseased cells, and the channel are modeled as transmitters, absorbing
receivers, and diffusive channel, respectively. In this paper, we investigate
diffusion-based spherical matrix-type drug carriers, which are employed in
medical applications. In a matrix carrier, the drug molecules are dispersed in
the matrix core and diffuse from the inner to the outer layers of the carrier
once immersed in a dissolution medium. We derive the channel response of the
matrix carrier transmitter for an absorbing receiver and compare our results
with commonly used point and transparent spherical transmitters to highlight
the necessity of considering practical models. Furthermore, we derive a
criterion for evaluating whether the release process or the channel dynamics
are more important for the characteristics of the channel response of a drug
delivery system. For the limiting regimes, where only the release process or
the channel determine the behavior of the end-to-end system, we propose
closed-form approximations for the channel response. Finally, we investigate
the channel responses for the release of common therapeutic drugs, e.g.,
doxorubicin, from a diblock copolymer micelle acting as drug carrier.
