Biomedical Engineering Reference
In-Depth Information
reaggregate is the stiffness gradient [ 63 ] which is probably formed when soft
reaggregate comes into contact with the stiffer biomaterial. Stiff material gen-
erally attracts cells. We suggested to take the surface that is covered by the
outgrown cells as an index of bioactivity. In addition, the cell morphology of the
outgrown cells can be assessed as an additional parameter. The point which
might affect the outcome is the difference in the oxygen tension gradient in vivo,
with probably the lowest tension at the implant surface, and in this in vitro
system where the lowest oxygen tension is not at the reaggregate-test material
surface but in the centre of the reaggregate. Oxygen tension gradients may
greatly affect cell migration activity and cell proliferation rate [ 47 , 111 ].
• From culture medium to body fluid mimicry. Culture medium contains a mixture
of salts, amino acids and dilute quantities of serum proteins (mostly heat-
inactivated fetal calf serum, 10% concentration), and specific growth factors.
Under in vivo conditions, the cell environment is crowded with proteins
resulting in a reduction of diffusion and local high concentrations of released
cell products. In this environment, nonspecific reactions play an important role.
Unlike the in vivo environment, procollagen produced by the cells, for instance,
only slowly coverts to collagen under common in vitro conditions. One solution
for this nonphysiological situation is given by the team of Raghunath. They
could show that, by the addition to the medium of charged macromolecules with
a large hydrodynamic radius, the situation is changed and collagen is formed as
in vivo [ 79 ]. Additionally, differentiation of various cell types is promoted in
such an environment [ 45 ]. These effects show the biological relevance of the
excluded volume effect by macromolecules and suggest that by adding mac-
romolecules the original microenvironment can be better mimicked [ 45 ].
• From static to dynamic. The in vivo situation is characterized by the presence of
microcirculation in the vicinity of the implanted material [ 59 ]. Dolder and
co-workers reported that proliferation and differentiation towards osteoblasts
(ALP activity, calcium deposition) of rat bone marrow stromal cells is (or at
least tends to be) increased in the presence of a fluid flow [ 50 ]. This effect might
be due to locally increased nutrients and oxygen tension, or to fluid-flow-
induced mechanical loading [ 12 ]. It has been shown that fluid flow affects the
outcome of material evaluation (e.g. scaffold mesh size [ 69 ]). By developing an
in vitro set-up, the question arises which situation should be mimicked, since the
microcirculation is strongly dependent on the site of implantation. Besides fluid
flow variation, fluctuations in hydrodynamic compression and strain are nor-
mally occurring within the body, affecting cell performance [ 144 ].
• Oxygen tension. Control and adaptation of oxygen tension in cell culture is
hardly ever considered even though it is a very important factor in how cells
react [ 53 ]. While hypoxia is a pathological condition for most cell types, it is a
key factor for chondrocyte development and behaviour. In native tissue, chon-
drocytes are exposed to oxygen concentrations ranging from 1-5%, since the
distance to the vessels supplying the synovial membrane is exceptionally large
compared to other tissues [ 163 ]. Cells react to a hypoxic environment by
switching their metabolism from aerobic to anaerobic, up-regulating stress
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