The Transport of Insulin-Like Growth Factor through Cartilage - Porous Media: Applications in Biological Systems and Biotechnology - page 404

presented in Section 11.5.2.3. This aggrecan biosynthesis model also includes
fluid-shear-induced biosynthesis and aggrecan production, transport, and
degradation.
11.2 Basic Solute Transport Model in a Deforming
Articular Cartilage
11.2.1 Introduction
The absence of blood and lymph vessels in cartilage implies that diffusive
transport must play an important role in delivering nutrients and growth fac-
tors to chondrocytes. However, important proteins such as growth factors are
large molecules that do not diffuse easily. How then, might cartilage maximize
its exposure to growth factors? Potentially, growth factor transport could be
enhanced by advective transport. When cartilage is released from a compres-
sive deformation, interstitial fluid can move into the cartilage, taking growth
factors (and other molecules) with it.
It has been speculated that physiological relevant loading enhances growth
factor transport (and subsequent matrix biosynthesis) via advection. That is,
perhaps a walking pace induces cartilage deformation and interstitial fluid
flow at a frequency and amplitude conducive to advective transport of growth
factors. This speculation arose from an observed synergy between cyclic load-
ing and growth factors on aggrecan production in cartilage explants and
chondrocyte-seeded gels [35-37]. Further, several experiments directly aimed
at measuring solute uptake or release from cartilage undergoing cyclic loading
have also indicated enhanced solute transport [38,39]. Subsequently, several
models have been constructed to examine the effect of cyclic loading on solute
transport into cartilage [28,36,40]. These models are usually based on the
theory of porous media.
To better understand this process and to introduce the underlying equa-
tions needed to describe coupled solute transport in a deforming tissue, a
basic continuum coupled solute transport; poroelastic model is first presented.
The use of porous media theory to model cartilage mechanical behavior (and
other biological tissues) is not a recent idea [41-43], however, using a coupled
reactive-transport poroelastic model to understand the transport of nutrients
through a cyclically loaded cartilage is relatively new [36,40,44]. The basic
model described in the next section is further developed in later sections, as
new behaviors are added to the model.
11.2.1.1 Modeling Cartilage Using the Theory of Porous Media
In the framework of the theory of porous media, cartilage is modeled
as an intrinsically incompressible solid phase, representing the collagen-
proteoglycan matrix, and an incompressible fluid phase. This approach