Biomedical Engineering Reference
In-Depth Information
4.3.2.2 Cells
(P(PF-co-EG)) incorporated with bovine chon-
drocytes found both increasing cell number
and glycosaminoglycan (GAG) production over
the
Many types of cells are responsible for produc-
ing and maintaining the extracellular matrix
essential to the function of all musculoskeletal
tissues. For this reason, many research efforts
have focused on developing cell carriers to aid
orthopedic tissue regeneration [
]. A variety of other
hydrophilic polymers, such as collagen, chito-
san, and PEG-based materials, have also been
investigated for cell-delivery applications [
8
-day culture period [
26
12
,
28
,
35
,
99
,
16
,
30
].
100
].
Scaffolds used as cell carriers generally have
interconnected pore structures formed by
various methods such as phase separation,
solvent casting/particulate leaching, or electro-
spinning [
4.4 Requirements
of Biodegradable
Orthopedic Implants
]. Pore morphology is espe-
cially important in the preparation of scaffolds
made of hydrophobic materials, because in
these cases the pore structure is a main means
of providing void space for nutrient exchange
and cell attachment [
15
,
61
,
63
As mentioned above, scaffold materials must
fulfi ll critical requirements before they can be
used in orthopedic tissue engineering. The cri-
teria include biocompatibility, biodegradabil-
ity, relevant biological properties, appropriate
mechanical properties, and material process-
ability. These criteria are discussed individu-
ally below.
]. PLGA scaffolds
with different pore sizes have been used suc-
cessfully in bone-formation experiments in
vitro, resulting in osteoblast growth and dif-
ferentiated cell function in
15
,
63
,
72
]. In
another study, knitted PLGA scaffolds seeded
with bone marrow cells were employed to
bridge a gap in the rabbit tendon [
52
days [
48
4.4.1 Biodegradability
]. The use
of porous PGA scaffolds seeded with bovine
chondrocytes also resulted in the formation
of cartilaginous tissue in over
75
The degradation of implanted materials in
orthopedic tissue engineering is essential
because it eliminates the need for implant
removal in a second surgical intervention, and
provides space for native tissue growth. There-
fore, this degradation should be achieved at a
rate that will enable native tissue to be gener-
ated in the defect site. In the meantime, par-
tially degraded scaffolds should maintain their
mechanical integrity until the newly formed
tissues have suffi cient strength to replace them
[
weeks. The
compressive modulus of PGA-chondrocyte
constructs reached the same order of magni-
tude as that of normal bovine cartilage in
12
9
weeks and a similar aggregate modulus was
achieved in
].
Unlike PLGA, PLA, and PGA, many other
biodegradable polymers, both natural and syn-
thetic, are hydrophilic, leading to the forma-
tion of hydrogels [
12
weeks [
68
]. Hydrogels
have an advantage over porous hydrophobic
scaffolds in that hydrogels often have mechani-
cal and structural properties similar to the
extracellular matrix of soft tissues and are easy
to process in terms of the incorporation of cells
and bioactive molecules [
61
,
63
,
88
,
93
]. However, this strategy may not
be ideal for patients with enhanced catabolic
diseases, although ideal for healthy persons.
Material degradation occurs by several mecha-
nisms, including hydrolysis and enzymatic
degradation. Most synthetic polymers are
degraded by hydrolysis of their ester linkages.
This degradation generally occurs by bulk or
surface erosion mechanisms, depending on the
water permeability of the scaffold [
8
,
30
,
49
,
92
]. In addition, the
high water content of hydrogels eliminates the
need for pores to facilitate nutrient diffusion
deep within the construct. As with carriers for
bioactive molecules, hydrogels that include
cells can be injected into the tissue defect in the
form of a liquid solution and subsequently
cross-linked into gel constructs. This strategy
simplifi es the procedure of cell transplantation
[
62
]. On the
other hand, many natural materials and some
polymers, including degradable peptide
sequences, are degraded by enzymatic mecha-
nisms [
56
for specifi c
examples of materials that degrade by each of
these means).
32
,
33
,
85
] (see Section
3
.
5
]. Recently, an in vitro study with
poly(propylene fumarate-co-ethylene glycol)
16
,
26
,
93
,
94
