Biomedical Engineering Reference
In-Depth Information
12.4
BIOMIMETIC NANO-APATITE-COLLAGEN FIBER SCAFFOLD
Human bone is comprised of collagen fibers (mainly type I) and nano-sized hydroxyapatite crystals.
Hence, synthetic hydroxyapatite/collagen fiber composites are promising in mimicking and replac-
ing bone. Only a few studies have incorporated collagen into moldable, self-setting calcium phosphate
cements
[40,41]
. In one study, collagen decreased the strength of the cement
[40]
, whilst in another study,
collagen decreased the cell activities
in vitro
[41]
. Therefore, there is a need to develop an
in situ
-setting
collagen-calcium phosphate cement composite with increased cell attachment and increased toughness.
In a recent study, a type I bovine collagen fiber (Sigma, St. Louis, MO) was added to CPC to
develop an
in situ
-setting, bone-mimicking, nano-apatite-collagen composite
[42]
. Bovine Achilles ten-
don was cleaned of all noncollagenous tissue and extracted at a temperature of 0°C with a 3% Na
2
HPO
4
solution to remove soluble proteins. To remove mucopolysaccharides, the tendon was extracted with a
25% KCl solution. The resultant collagen was washed with water, dehydrated with absolute alcohol, and
air dried. The collagen was in the form of elongated, flexible bundles, with a bundle diameter ranging
from approximately 0.1 to 3 μm. The length of the bundles ranged from about 20 to 100 μm
[42]
.
The CPC powder was mixed with distilled water to form a paste to which the collagen fibers
were added and incorporated by mixing. The following collagen mass fractions of collagen/
(collagen
CPC) were used: 0%, 2.5%, and 5.0%.
Figure 12.3A
shows collagen fibers covered with
small hydroxyapatite crystals that make up the CPC matrix
[42]
. This indicates an intimate contact
between collagen fibers and hydroxyapatite crystals. The work-of-fracture (or toughness, which mea-
sures the energy required to fracture the specimen) (
Figure 12.3B
) was increased from (9.4
3.1)
J/m
2
with 0% collagen to (430
103) J/m
2
with 5% collagen (
P
0.05) at a powder: liquid ratio
(P/L) of 2.5. At a P/L of 3, the work-of-fracture was increased from (22.2
3.8) J/m
2
without col-
lagen to (381
119) J/m
2
with 5% collagen (
P
0.05).
Figure 12.3C
shows cell attachment for 1 day
on CPC-collagen composite. Increasing the collagen content increased the cell number per specimen
area (
P
0.05). The number of cells/area was 382
99 cells/mm
2
at 5% collagen, which was higher
than (
P
0.05) that at 0% collagen (173
42 cells/mm
2
)
[42]
. Hence, the incorporation of collagen
fibers into CPC not only improved its toughness but also increased the osteoblastic cell attachment.
12.5
FAST FRACTURE OF NANO-APATITE SCAFFOLD
To improve the mechanical properties for load-bearing dental and orthopedic applications, chitosan and
absorbable fibers were used to reinforce CPC. Chitosan and its derivatives are natural polymers that are
biocompatible, biodegradable, and osteoconductive
[43]
. Chitosan has been shown to strengthen and
increase toughness of CPC
[44]
, and resist the washout of CPC paste in physiological solution as well
as accelerate CPC setting
[33]
. Chitosan lactate (referred to as chitosan; VANSON, Redmond, WA)
was dissolved in water at concentrations of 0%, 5%, 10%, or 15% to form CPC liquids.
Polyglactin fibers (Ethicon) were mixed into the CPC paste to form a composite paste, which was
placed into a rectangular mold of 3 mm
4 mm
25 mm
[32]
. A fiber volume fraction of 20% was
used to obtain a CPC-chitosan-fiber paste that was readily mixed and not dry. The fiber volume frac-
tion was equal to the volume of fibers divided by the volume of the entire specimen. The paste in the
mold was set in a humidor with 100% relative humidity at 37°C.
Figure 12.4
plots the results from the single-load, three-point flexure (mean
SD;
n
6)
[45]
.
CPC with 15% chitosan reached strength of 16.2
3.0 MPa, higher (
P
0.05) than MPa for CPC
