Hydrodynamics in Porous Media with Applications to Tissue Engineering - Porous Media: Applications in Biological Systems and Biotechnology - page 84

transport can be enhanced by means of perfusion through the scaffold with
a mean velocity u 0 . At the pore scale the dynamics of the perfusion fluid
flow (solvent fluid momentum transfer) is then characterized by the Reynolds
number defined by

u 0 / φ

ρ × a ×

Re = (3.4)

η

Here ρ and η are, respectively, the density and the dynamic viscosity of
the culture fluid. Moreover, for problems in solute nutrient mass transport,
it is pertinent to similarly introduce the Peclet number Pe , which represents
the ratio of the convective and diffusive effects implied at the length scale a
of the pores and given by

u 0 / φ
D

Pe = a

×

(3.5)

3.3.3 Effects of Mechanical Loading: Cell and
Tissue Mechanobiology
Increasing evidence suggests that mechanical forces, which are known to be
important modulators of cell physiology, might increase the biosynthetic activ-
ity of cells in bioartificial matrices and, thus, possibly improve or accelerate
tissue regeneration in vitro (Butler et al . 2000). Various studies have demon-
strated the validity of this principle, particularly in the context of muscu-
loskeletal tissue engineering. For example, cyclical mechanical stretch was
found to (1) enhance proliferation and matrix organization by human heart
cells seeded on gelatin-matrix scaffolds (Akhyari et al . 2002), (2) improve
the mechanical properties of tissues generated by skeletal muscle cells sus-
pended in collagen or Matrigel (Powell et al . 2002), and (3) increase tissue
organization and expression of elastin by smooth muscle cells seeded in poly-
meric scaffolds (Kim et al . 1999). Pulsatile radial stress of tubular scaffolds
seeded with smooth muscle cells improved structural organization and suture
retention of the resulting engineered blood vessels, and enabled the vessels
to remain open for 4 weeks following in vivo grafting (Niklason et al . 1999).
Dynamic deformational loading or shear of chondrocytes embedded in a three-
dimensional environment stimulated GAG synthesis (Davisson et al . 2002)
and increased the mechanical properties of the resulting tissues (Mauck et al .
2000; Waldman et al . 2003). Strains in elongation and torsion on collagen
gels embedding mesenchymal progenitor cells induced cell alignment, forma-
tion of orientated collagen fibers, and upregulation of ligament-specific genes
(Altman et al . 2002). This study provided evidence that specific mechanical
forces applied to three-dimensional cellular constructs might not only enhance
the development of an engineered tissue but also direct the differentiation of
multipotent cells along specific lineages.
Despite numerous proof-of-principle studies showing that mechanical con-
ditioning can improve the structural and functional properties of engineered