Understanding transport phenomena and degradation of bioresorbable medical polymers - Durability and Reliability of Medical Polymers - page 84

and in particular, the well-known zero-order drug delivery, constant over
time. Clearly, owing to the diffusion effect, the mass transfer becomes a
decreasing trend. During drug diffusion, concentration decreases with the
driving force. in order to counterbalance the Fick law effect, companies have
proposed many alternatives such as double release through a patch. in these
systems, the drug is highly concentrated within particles loaded in a larger
matrix (the patch) (Mauri, 2005). The drug initially diffuses through the
matrix where its concentration varies more slowly than in the particles. it can
then be transported into the target tissue. However, this system is not always
effective because the drug delivery decreases over time. Researchers have
therefore explored many alternatives aiming to provide constant drug delivery
in which diffusion is coupled with a transport phenomenon. an example of
drug delivery controlled by enzymes is as follows. The decreasing trend of
the diffusive drug fl ow is counterbalanced with an increase in porosity of the
matrix, owing to enzymatic erosion. Therefore, the diffusion force decreases
but the diffusion coeffi cient increases. It is possible to use a bio-artifi cial
polymeric matrix composed of natural and synthetic polymers such as amide
and polyvinyl alcohol. By entrapping amylase in the composite matrix, it is
possible to manage and control the effects of the driving forces and diffusion
coeffi cients. The most important physical properties of a membrane are its
hydraulic permeability and effective diffusion coeffi cient. The permeability
k measures the facility of crossing the membrane and may be approximated
with Darcy's law:

k
L

[4.29]

V = m m L D D P P

where V is the fl ow velocity, D P the difference of pressure, m the viscosity and
L the membrane depth. Using the Blake-Kozeny semiempirical correlation,
the permeability could be established as:

￿ ￿ ￿ ￿ ￿ ￿

23

d 23 d 23

[4.30] k = 150(1 - )

2

e

where e is the matrix porosity and d the dimensions of the pores. in general,
k increases as the porosity and dimensions increase. The other quantity
involved is the effective diffusivity that measures solute diffusion through the
composite material. it is assumed that on one side of the membrane there is a
fl uid with a solute at low concentration C s , and a pure fl uid on the other side.
This is a common situation in drug delivery, where the clearance does not
allow the formation of an equilibrium state. in the absence of D P (convection
phenomena) the solute massive fl ow J s is determined by Fick's law:

D
L C

J =

s

=

L C

C s

L

[4.31]