The fragility of calcium polyphosphate scaffold can be improved by adjusting the calcining time
*
1
Song, W; 1Jing, X; 4Kim, J; 2Wan, CX; 3Markel, DC, 1Yang KH, 3Ralph Blasier and 1, 3 Ren, WP
1
Department of Biomedical Engineering, Wayne State University, Detroit, MI
2
Department of Biomedical Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu, China
3
Detroit Medical Center/Providence Hospital Orthopedic Surgery Residency Program, Detroit, MI
4
Department of Mechanical Engineering, College of Engineering, Cornell University, NY
Statement of Purpose:
Calcium polyphosphate (CPP) has been used as a promising bone
healing ceramics because of its biocompatibility, biomechanical strength,
and stimulation of bone growth. However, the application of CPP
scaffold in the orthopaedic field is limited, in part due to its fragility
nature. The aim of this study was to investigate the influence of the
calcining time of CPP on its fragility by using a standard three-point
bending test model.
Methods:
Preparation of CPP samples for bending test The CPP samples were
prepared by a calcining-sintering cycle. Briefly, calcium phosphate
monobasic monohydrate (Ca(H2PO4)2·H2O) were calcined at 500ºC for 5,
10 and 40h to polymerize into condensed phosphates with different
degrees of polymerization (DP), respectively. After compressing to a
solid cylindrical shape (D=0.5mm, L/D=10:1), CPP samples were
sintered at 800ºC and cooled naturally.
The bending test of CPP samples The bending test was performed in a
material tester (Instron) via a standard three-point bending test model.
The contactor at the middle point of the sample was set to move with a
speed of 1 mm/min. The stress (σ) and strain (ε) were recorded until the
sample was fractured. The Young’s modulus (E) was calculated based
on σ and ε.
X-ray diffraction (XRD) The crystallographic structure of grinded CPP
powder was characterized using powder X-ray diffractometer (pXRD).
The range of scattering angle (2θ) was 20-80° and the rotating speed
was 5°/min. The pattern of XRD was analyzed via software Jade 5.0 to
identify the crystalline type and calculate the crystallographic
parameters.
Scanning electron microscope (SEM) The CPP samples after bending
test were characterized via SEM. The fracture section of samples was
observed under 100, 1000 and 3000× magnification. Morphology was
viewed at 25kV accelerating voltage.
Fig.2 XRD patterns and calculated crystal cell volume of CPP
calcining for 5, 10 and 40h.
Results:
The calcining time influenced the fragility of CPP (Fig.1) The Young’s
modulus of CPP was increased consistently with the increase of
calcining time from 5h to 40h, which indicated the increase of fragility.
The extension of calcining time influenced the crystallographic
parameters (Fig.2) The XRD patterns of different CPP samples were
similar. The crystallographic structures of all CPP samples were
identified as β-type by fitting with standard PDF card. However, the
calculated crystal cell volume was decreased consistently with the
increase of calcining time from 5h to 40h.
The extension of calcining time influenced the formation of crystal in
CPP substrate (Fig.3) The morphology of fractured section was without
significant difference under 100× of magnification. However, CPP
substrate with 5h calcining was shown as semi-crystalline structure with
amorphous region under 3000×. From CPP5h to CPP40h, the
morphology of CPP substrate transferred into crystalline structure.
Fig.3 The surface morphology of fractured section on CPP samples
calcining for 5, 10 and 40h. (a, b, c = CPP5h, d, e, f =CPP10h, g, h, i=
CPP40h)
Discussion and Conclusions:
The fragility of CPP represents one of the most important limitations for
the application of bioceramics in bone tissue engineering. Attempts to
improve CPP material nature is clinical required and important. In this
study, we found that the fragility nature of CPP can be improved by
modification of temperature and time during the calcining-sintering
process for ceramics. We previously reported that the degree of
polymerization of CPP was linearly increased with the extension of
calcining time. The formation of condensed CPP was caused by an
intermolecular dehydration process as shown below:
n Ca(H2PO)4 xH2O
·
H－ Ca(PO 3)2 －O-H
n
+ ((2n  x) 1) H O
2
More condensed CPP molecules while less content of bound water can
be formed in substrate along with the extension of polymerization time.
During sintering process in solid phase, CPP with less content of bound
water was prone to form structure with higher crystallinity and larger
crystal grain, which also indicates higher fragility. Therefore, a
semi-crystalline structure with certain contents of bound water for
bioceramics could reduce its fragility, which can be achieved by
adjusting the calcining time during preparation steps for ceramics.
Fig.1 The Young’s modulus of CPP calcining for 5, 10 and 40h in
bending test (n=3 for each group).
ACKNOWLEDGMENTS
This work has been funded by National Natural Science Foundation of
China (grant number: 30870614) to Dr. Wan and OREF Grant to Dr.
Ren WP.
Poster No. 1915 • ORS 2011 Annual Meeting