Scope of tissue engineering - Biomedical Engineering Desk Reference - page 699

Cell aggregates generated in vitro (in either suspension
cultures or stirred bioreactors) that exceed about 1 mm
in size invariably develop necrotic cores. Culture condi-
tions in RWV bioreactors are unique in that the fluid
dynamics of the system allow for efficient mass transfer
of nutrients and oxygen diffusion. In this environment,
dissociated cells can assemble into macroscopic tissue
aggregates several mm in size, which are largely devoid of
such necrotic cores. With the possible exception of
avascular tissue of low cellularity and slow metabolism,
such as cartilage, all tissues, including those growing in
RWV bioreactors, will eventually require internal, blood-
vessel-like conduits for the delivery of oxygen and nu-
trients as well as for the removal of waste products. At-
tempts to generate 3-D constructs as replacement tissues
will necessitate combining RWV technology with in-
novative methods for creating bioengineered blood con-
duits such as growing endothelial cells on the inside and
outside of tubular scaffolds.

primary mass transport property of interest, at least ini-
tially, is diffusion. In a scaffold, the rate and distance
a molecule diffuses depend on both the material and
the molecule characteristics and interactions. As a con-
sequence, diffusion rates will be affected by the MWand
size of the diffusion species (defined by Stokes radii)
compared to the pore of scaffolds. For example, mole-
cules such as glucose, oxygen, and vitamin B 12 , with
MWs less than 1300 Da and Stokes radii less than 1 nm,
are able to freely diffuse into and from ionically cross-
linked hydrogels. Gel properties such as polymer fraction,
polymer size, and crosslinker concentration determine
the gel nanoporous structure. However, higher MW
molecules such as albumin and fibrinogen are not able to
freely diffuse, and their rate of diffusion is further de-
creased by increases in polymer concentration, in cross-
linker concentration, and/or in the extent of gelation.
Ultimately, diffusion requirements and subsequent ma-
terial choice depend on the scaffold application.
The mass transfer of nutrients and waste between
a scaffold and the surrounding medium follows the
equation below:

7.2.6.3.4 Kinetics
The success of scaffolds for tissue engineering is typically
coupled to the appropriate transport of gases, nutrients,
proteins, cells, and waste products into, out of, and/or
within the scaffold. To obtain fluid flow and nutrient flux
in porous, 3-D scaffolds, dynamic culture methods
( Fig. 7.2-24 ) [15] have been employed. However, the

J s ¼DS d C
d x

where J s is the diffusion of a solute, D is the diffusivity of
the solute, S is the surface area normal to the direction of

Spinner flask bioreactors Rotating bioreactors Direct flow systems

Three-dimensional fluid flow

V

∞

K

Δ P

∇ P

V = -

V

Fig. 7.2-24 Representative overview of dynamic culture methods used to obtain fluid flow and nutrient flux in porous 3-D scaffolds.
In spinner flask bioreactors, culture medium is stirred around scaffolds at fixed positions within the vessel. In rotating bioreactors,
scaffolds are maintained in continuous free fall through the culture medium. In a direct flow system, scaffolds are fixed in position and
perfused with culture medium by a direct application of a uniform fluid pressure head.