  Molecular communications is a promising framework for the design of
controlled-release drug delivery systems. In this framework, drug carriers are
modeled as transmitters, the diseased cells as absorbing receivers, and the
channel between transmitter and receiver as diffusive channel. However,
existing works on drug delivery systems consider only simple drug carrier
models, which limits their practical applicability. In this paper, we
investigate diffusion-based spherical matrix-type drug carriers, which are
employed in practice. In a matrix carrier, the drug molecules are dispersed in
the matrix 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 validate the results through
particle-based simulations. Moreover, we show that a transparent spherical
transmitter, with the drug molecules uniformly distributed over the entire
volume, is as special case of the considered matrix system. For this case, we
provide an analytical expression for the channel response. Finally, we compare
the channel response of the matrix transmitter with those of point and
transparent spherical transmitters to reveal the necessity of considering
practical models.
