Biomedical Engineering Reference
In-Depth Information
The rest of this chapter will focus on tita-
nium fi ber mesh as a scaffold material for bone
tissue replacement. As a scaffold material, tita-
nium fi ber mesh can be used in both cell-based
and growth-factor-based approaches.
the success of the fi nal three-dimensional bone
graft. Several techniques have been used by
various research groups to optimize the bone-
generating properties of scaffold materials.
These methods focus on improvement of the
seeding or loading effi cacy of the cells in the
scaffold. The major seeding methods used to
inoculate cells in a three-dimensional scaffold
are: cell chamber [
5.4 Cell-Based Approach and
Titanium Fiber Mesh
9
], spinner fl ask [
34
,
46
],
static (droplet) [
14
,
22
], cell suspension [
13
,
15
]
and perfusion systems [
]. Taken together, the
combined results indicate that a dynamic
system improves cell attachment and evens the
distribution of cells throughout the scaffold.
Static loading has cell-loading limitations.
When cells are seeded in a droplet or cell sus-
pension, cells are left on the surface of the
porous constructs and penetrate only partly
into the scaffold.
The cell density during cell seeding also
plays an important role in cell attachment and
distribution into the scaffold. Various studies
with polymers have shown an improved seeding
effi cacy when a high initial cell density is used
[
10
Mesenchymal stem cells (MSCs) can be used
for the functional repair or regeneration of
large bone defects. In this approach, a three-
dimensional scaffold material is used to deliver
the cells to the bone defect site. The fi nal success
of this tissue-engineered strategy is determined
by the number of responsive MSCs loaded in
the scaffold as well as by the material charac-
teristics of the delivery vehicle.
MSCs or osteoprogenitor cells, which are pre-
cursors of osteogenic cells, are present in bone
marrow, where they represent only a small frac-
tion of the total number of bone marrow cells
[
]. The same effect on seeding effi cacy and
cell differentiation was expected for porous
titanium scaffolds. The effect on seeding effi -
cacy was found only during the fi rst
14
]. In view of this, methods have been devel-
oped to culture-expand and select the osteo-
progenitor cell fraction from the total bone
marrow. Cultured bone marrow-derived MSCs
have proven more effective in bone formation
than total cells from fresh marrow [
12
hours
after inoculation. Thereafter, the same number
of cells was present in all scaffolds. It appears
that cells seeded in a low-cell-density suspen-
sion grew more rapidly than cells seeded in a
high-cell-density suspension. Other research
groups [
24
].
It is important to note that both the prolif-
eration and differentiation of osteoprogenitor
cells can be directed during culture by the use
of several factors, such as dexamethasone and
bone morphogenetic proteins (BMPs), that are
known to direct the differentiation of MSCs
into the osteoblast lineage in vitro [
18
] reported increased expression of
osteoblastic markers when a low-cell-density
suspension was used for seeding. This contrasts
with other results that showed increased
expression of osteoblastic markers when a
high-cell-density suspension was used [
14
]. The
addition of agents stimulating proliferation
(basic fi broblast growth factor, bFGF) and dif-
ferentiation (recombinant human BMP-
6
13
].
] focused on the
effect of seeding and loading techniques on the
osteogenic differentiation of rat bone marrow
cells grown in titanium fi ber mesh in vitro. The
meshes were seeded by various methods, i.e.,
the droplet, cell-suspension (high and low cell
density), and rotating-plate methods. Osteo-
genic cells were cultured for several days into
titanium fi ber mesh. Statistical analysis of
the results revealed that high cell density and
low rotational speed always resulted in a
signifi cantly higher DNA content. Calcium
measurements, used as a marker for matrix
mineralization, and osteocalcin analysis,
used to assess osteogenic differentiation,
Van den Dolder et al. [
38
2
,
rhBMP-
) during culture enhances the in vivo
osteogenic potential [
2
]. The osteogenic
potential of cell-loaded scaffolds can be
increased further by modifi cation of the con-
ditions used during seeding, optimization of
the number of loaded cells [
11
,
22
], and use
of dynamic rather than static loading prior to
culturing [
13
,
22
,
42
10
,
34
].
5.4.1 Cell Seeding
The method used to seed the marrow cells into
the nondegradable porous scaffold can defi ne
