Biomedical Engineering Reference
In-Depth Information
solute diffusion, and d C /d x is the concentration gradi-
ent. However, diffusion is just one of many parameters
that are important in the cell-culturing process.
To evaluate the role of nutrient diffusion and con-
sumption within cell-seeded scaffolds in the absence of
flow, Botchwey et al. developed a 1-D glucose diffusion
model. Such a quantitative analysis will provide a basis
for development of new dynamic culture methodologies
to overcome the limitations of passive nutrient diffusion
in 3-D cell-scaffold composite systems. However, many
factors still remain to be quantified, such as the transport
of oxygen, other nutrients, cell-cell signaling molecules,
growth factors, metabolic wastes, and cell chemotactic
factors in dynamic culture, all of which may have an
effect on cell function and tissue synthesis.
enhance the rate and quality of tissue growth by re-
creating in vitro some of the same conditions that the
tissue experiences in vivo. Articular cartilage is amenable
to such an approach, as it is a tissue defined not only by its
cellular constituents but also by its dynamic physical
environment. Therefore, appropriate mechanical stimu-
lation has been applied to cells during their culture, es-
pecially in case cells in the human body live in an
environment heavily influenced by mechanical forces
such as in load-bearing tissues. This is the major reason
why chondrocyte has been very often selected as a cell
model for tissue engineering under mechanical stimula-
tion. Another reason is that the cartilage in which chon-
drocytes live is an avascular tissue and receives oxygen and
nutrients from the synovial fluid. Chondrocyte is known
to be one of the most robust cells and to develop dif-
ferently on the basis of what culturing processes are used.
The presence of mechanical forces such as hydrostatic
pressure or direct compression stimulates chondrocytes
to secrete more ECM as compared with static culture.
There are two major methods for mechanically stim-
ulating cells outside of their culturing environment to
enhance their growth. One is the bioprocessing which
uses mechanical stimuli only at intervals during the cul-
turing period. The other is the bioreactor system which
uses a constant mechanical force to stimulate the cells.
The advantage of these approaches is the introduction of
a mechanical stimulus and an increase in diffusion
through the porous scaffold. In the bioprocess, me-
chanical stimuli are given to cells via either hydrostatic
pressure or direct compression. When cartilage located in
articular joints is loaded during walking, running, or
shifting weight while standing, the force is transmitted
throughout the tissue which contains water by 75-80
wt%, being absorbed primarily by the fluid. The pressure
produced by the compressed fluid acts uniformly on the
chondrocytes within ECM. This hydrostatic pressure
ranges between 7 and 10 MPa during normal activity.
One technique that uses hydrostatic pressure as a me-
chanical stimulus is to employ a two-step process that
separates culturing from force application. The cells are
kept mostly in static culture medium, where they are
nourished. At prescribed times, the cells are moved to
a hydrostatic chamber where a specified load is placed on
them. Another technique uses a semicontinuous perfu-
sion system that feeds the cells and applies hydrostatic
pressure in the same device. In this case the cell cultures
do not have to be moved as much, which reduces the
possibility of contamination, and the process can be au-
tomated to run for long periods of time without any need
for manual labor. The production of proteoglycans and
type II collagen is currently the main experimental in-
dicator of positive mechanical stimulation, although
a few studies also report DNA synthesis or aggregate
moduli. Table 7.2-7 summarizes representative results of
7.2.6.4 Externally applied mechanical
stimulation
Some tissues exist in a mechanically dynamic environ-
ment. Blood vessels are continuously exposed to me-
chanical forces that lead to adaptive remodeling. Although
there have been many studies characterizing the re-
sponses of vascular cells to mechanical stimuli, the pre-
cise mechanical characteristics of the forces applied to
cells to elicit these responses are not clear. Soft muscu-
loskeletal tissues also adapt to immobilization and realize
strengthening in response to exercise. Tendons are in
a continuous state of dynamic remodeling. Cells
suspended within a 3-D network of ECM respond to
external changes via receptor-ligand interactions that
relay signals from outside the cell to the cytoskeletal
domain and thereby influence subsequent cellular func-
tion such as attachment, migration, differentiation, and
apoptosis. As natural tissues, especially those which
should resist against mechanical loading and pressured
such as cartilages, bones, ligaments, cardiac muscle, and
blood vesselsdare subjected to mechanical stimuli
during development, it will be reasonable to assume that
mechanical stimuli, pulsed or unpulsed, should be given
to cell-scaffold constructs aimed at least at in vitro tissue
engineering of musculoskeletal and cardiovascular tissues.
Indeed, numerous studies have shown local mechanical
signals to be a key factor directing the development,
growth, repair, and maintenance of bone and cartilage.
Since some cells can sense their mechanical environment
such as endogenously generated tension, exogenous
stimulus may be used as a conditioning modality to in-
fluence the efficacy of tissue engineered replacements for
load-bearing tissues. Thus, researchers have employed
functional tissue engineering approaches that place spe-
cial focus on using physical stimuli to encourage the de-
velopment of a biomechanically functional tissue. A
premise of in vitro tissue engineering is that one may
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