Biomedical Engineering Reference
In-Depth Information
In addition, it is desirable for the scaffold to possess mechanical properties similar to those of
bone, and to have the capability to encapsulate and deliver bone cells for osteogenic differentiation
and bone regeneration. During CPC setting, it undergoes a dissolution and re-precipitation process
to form nano-sized hydroxyapatite
[31-34]
. Scanning electron microscopy (SEM) shows a micro-
graph of elongated nano-sized hydroxyapatite crystals that make up the CPC matrix (
Figure 12.1A
).
Figure 12.1B
plots the crystal length distribution. Based on 300 crystals measured with SEM, the
apatite crystal length ranged between 75 and 550 nm (median length
299 nm), and the crystal
width ranged between 50 and 150 nm (median width
87 nm).
Macropores have been built into biomaterials to facilitate bony ingrowth and implant fixation
[14-19]
. One advantage of a macroporous CPC is that it can form macroporous hydroxyapatite
implants
in situ
without high-temperature sintering and machining. Particulate and fibrous porogens
can be mixed in the CPC paste to create interconnected macropores
[32,33,37]
. One approach was
to combine slow-dissolving fibers with fast-dissolving porogens to develop strong scaffolds with con-
trolled strength histories and tailored macropore formation rates
[33]
. The rationale was that CPC
containing fast-dissolving porogen and slow-dissolving fibers would have two stages of macropore for-
mation
in vivo
. The fast porogen would dissolve quickly upon contact with the physiological solution
to form macropores to start tissue ingrowth, while the fibers would provide the needed early strength
to the graft. After significant new bone ingrowth into the macropores thus increasing the CPC strength,
the fibers would then dissolve and create the second group of macropores for further tissue ingrowth.
The fiber degradation rate can be controlled to match the new bone formation rate for specific applica-
tions. In one study, an absorbable polyglactin fiber (Vicryl™ suture, Ethicon, Somerville, NJ) was cut
to 8-mm-long filaments
[32]
. This suture consisted of fibers braided into a bundle with a diameter of
approximately 322 μm, suitable for producing macropores after fiber degradation
[32]
.
Figure 12.1C
shows a macropore in CPC formed by the dissolution of a fast-dissolving porogen,
mannitol crystals (CH
2
OH[CHOH]
4
CH
2
OH, Sigma, St. Louis, MO)
[38]
. The letter “O” refers to the
pre-osteoblastic cells (MC3T3-E1) that infiltrated into the macropore in CPC.
Figure 12.1D
shows
examples of macropore channels in CPC created from the dissolution of absorbable polyglactin
fibers. These pores were approximately 300 μm in diameter and 8 mm in length. A high magnification
in
Figure 12.1E
shows a human umbilical cord mesenchymal stem cell (hUCMSC) attaching to the
nano-apatite in CPC via the cytoplasmic extensions “E.” These studies demonstrate the feasibility of
developing macroporous, CPC-based nano-apatite scaffolds which are promising for cell delivery and
tissue engineering.
12.3
CELL INFILTRATION INTO SCAFFOLD
To investigate cell infiltration into the macropores of the CPC scaffold,
in vitro
cell culture was per-
formed on the new CPC scaffolds. MC3T3-E1 mouse pre-osteoblastic cells were cultured following
established protocols
[39]
. Traditional CPC control and the new, mechanically stronger CPC scaffold
composites were tested. Fifty thousand cells diluted into 2 ml of media were added to each well con-
taining a specimen or to an empty well of tissue culture polystyrene (TCPS, as a biocompatible con-
trol), and incubated for 1 or 14 days.
As shown in
Figure 12.2
, the pore in the CPC scaffold was large enough for the osteoblast cell
“O,” and the cell had developed long cytoplasmic extensions “E” anchoring to the pore bottom. Cells
