Material and Mechanical Properties of Tr
Proceedings of the ASME 2009 Summer Bioengineering Conference (SBC2009)
June 17-21, Resort at Squaw Creek, Lake Tahoe, CA, USA
SBC2009-204934
MATERIAL AND MECHANICAL PROPERTIES OF TRICALCIUM PHOSPHATE-BASED
(TCP) SCAFFOLDS
Juan Vivanco (1), Sylvana García-Rodríguez (1), Everett L. Smith (2), Heidi-Lynn Ploeg (1)
(1) Department of Mechanical Engineering and
Material Science Program, University of
Wisconsin-Madison
(2) Department of Population Health Sciences,
University of Wisconsin-Madison
INTRODUCTION
Bioactive ceramic materials like tricalcium phosphates (TCP)
have been emerging as alternative and valid approaches to the current
therapies of bone scaffolding to target major diseases such as fracture
healing, osteoporosis, and osteoarthritis [1]. These scaffolds can
induce bone formation by acting as carriers or guides for enhanced
bone regeneration by cell migration, proliferation, and differentiation.
Previous studies have shown that both material and architectural
characteristics influence the mechanical strength of bone scaffolds and
their biological functionality [2]. In this study, the physical and
mechanical properties of three sets of TCP scaffolds were analyzed.
The physical properties that were evaluated included volume, mass,
density and porosity. The scaffold compressive mechanical properties
were evaluated in air at room temperature and in saline at 37°C.
filled to level H with the ethanol solution and weighed (W1) (Figure
1). After the scaffolds were water-saturated, they were immersed in
the container with ethanol, and the volume of alcohol was adjusted so
that the container with the scaffold was completely filled again level
H. For each specimen the ethanol was agitated until no air bubbles
emerged from the scaffold and all pores were filled. This new volume
is composed of the scaffold (Vscaffold) and the remaining ethanol (V2).
The the scaffold’s volume is given by Equation 1. Using the displaced
fluid volume method, material volume, density and porosity
measurements for the three sets (n = 8 per set) were determined (Table
1).
Vscaffold = V1 – V2 = (W1 – W2 + Wscaffold) / ρethanol
METHODS
Physical Properties
The volume and mass of TCP bone scaffolds fabricated with
three different sintering temperatures (defined as A, B, and C in order
of increasing temperatures) were evaluated (n=50 per set). The dry
bulk dimensions of the scaffolds were measured using a digital vernier
caliper. Dry and wet weights were measured with a digital balance
(Series PB-S, Mettler Toledo, Columbos, OH). To evaluate water
absorption, specimens were submerged in distilled water for 24 hrs
and centrifuged (Model TJ-6, Beckman, Fullerton, CA) at 3000 rpm
for 3 minutes to eliminate excess water on their surface. The wet
scaffolds were then weighed, Wscaffold.
Material volume was measured using a technique based on
displacement of a known-density fluid (ethanol solution, ρethanol).
Previous validations studies found a mean error of 0.975% when
measuring known volumes. A container of volume V1 was completely
(1)
Figure 1. Measurement of scaffold material volume
Mechanical Testing
Mechanical testing was performed using the ZETOS Bone
Loading System, a custom-made device that uses a piezoelectric
actuator (PZA) to apply compressive displacement to a specimen [3].
Applied force and specimen deformation were recorded during the test
and were processed to find specimen stiffness or apparent elastic
1
Copyright © 2009 by ASME
modulus. All the scaffolds underwent two preconditioning trials
(preload of 10 N followed by compressive displacement of 5 µm). Due
to the maximum displacement range of the PZA (70 µm), a series of
tests were performed on each scaffold until the specimen failed.
Displacement was applied gradually at approximately 0.50 µm/s until
either the maximum force (1500 N) or maximum displacement
(70 µm) was reached. The linear region of the force-displacement
curve (Figure 2b) was chosen to calculate stiffness, K, defined as the
slope of the linear fit of the region. Stiffness was converted to apparent
elastic modulus Eapp using Hooke’s law, Eapp = KL/A (where F is the
applied force, d is the compressive displacement, A is the bulk area of
the top surface and L is the bulk length of the specimen in the
dimension parallel to the loading direction) (Figure 2a). Failure, or
partial cracking, was identified in the force-displacement curve as a
sudden decrease in force.
Figure 3. Mean scaffold stiffness (N/µ
µm)
L
K=
Table 2. Stiffness and apparent elastic modulus for each
set and test environment, mean (std dev)
F
d
Set
(a)
A
B
C
K
(N/µm)
(b)
Figure 2. (a) Definition of loading direction. (b) Typical
force-displacement scaffold curve
Set
A
B
C
CSE (GPa)
Density (g/cm3)
2.5 (0.14)
2.9 (0.19)
3.2 (0.29)
30.1
Dry, room Temperature
CSE (GPa)
32.3 (29.0)
39.0 (51.9)
58.6 (41.8)
43.3
Saline, body Temperature
CSE (GPa)
8.70 (3.26)
28.4 (30.6)
45.0 (35.8)
27.4
Mean CSE
(GPa)
20.5
33.7
51.8
REFERENCES
1. Schenberger D. and Bafan M., 2008. , "Bioceramics: The Future
of Joint Healing" Med-Tech Precision Magazine, Spring.
2. Hutmacher DW., 2000, "Scaffolds in tissue engineering bone and
cartilage," Biomaterials, 21(24):2529-2543.
3. García S., Smith E., and Ploeg H., 2008, "A calibration procedure
for a bone loading system," Journal of Medical Devices, 2:01061-01106-6, March.
Table 1. TCP scaffold physical properties, mean (std dev)
Volume (mm3)
85.9 (6.45)
68.6 (4.97)
59.6 (4.51)
Mean K
(N/µm)
24.4
42.2
36.1
DISCUSSION AND CONCLUSIONS
In this study, the mechanical behavior of TCP scaffolds was
affected by the sintering temperature of the fabrication process as well
as the test environment. TCP scaffolds sintered at the lowest
temperature had the highest bulk volume, lowest material density,
lowest apparent modulus and absorbed the most water. The structural
stiffness for all the TCP scaffolds was reduced when tested at 37°C in
saline. Dry, room temperature conditions are practical for
characterization of the structure, but future applications will be at 37°C
and in a saline environment. It is therefore important to investigate the
scaffolds mechanical properties under these conditions.
Factors such as surface quality, ductility of the structure, cellular
response, characteristics of the culture medium and general conditions
of the test environment are only a few factors that need be to be
considered in future investigations of the scaffold-cell system.
RESULTS
A paired t-test (α = 0.05) was performed to determine if there
was a difference in weight as a consequence of a dry or wet state. Only
the weight of set C was not significantly different due to water
absorption.
Statistical analyses (ANOVA and Tukey Test, α = 0.05) found a
difference between the scaffold sets regarding material volume and
material density (p-value < 0.05 for both). On the other hand, porosity
was not different due to sintering temperature (Table 1). Set A was the
least stiff (22.5 N/µm in saline at 37°C) and sets B and C were the
stiffest (34.4 and 33.7 N/µm, respectively) (Figure 3 and Table 2). A
two-way ANOVA and Tukey Test (α = 0.05) found that both sintering
temperature and testing environment, affected the scaffold
compressive stiffness (p-value < 0.05 for both factors).
Wet weight (g)
0.2147 (0.006)
0.1982 (0.002)
0.1913 (0.004)
38.4
Saline, body Temperature
Eapp (MPa)
K (N/µm)
22.5 (7.81)
2810 (973)
34.4 (8.03)
4080 (2170)
33.7 (4.94)
4720 (681)
Table 3. Cumulative strain energy for each set and test
environment, mean (std dev)
Strain energy was calculated by finding the area under the stress-strain
curve until failure. Most scaffolds failed during the first load cycles.
To obtain a quantitative measure and take the number of load cycles
into account, the parameter cumulative strain energy (CSE) was
defined as the sum of strain energy to failure. Thus, if failure was
found in the second load cycle, the cumulative strain energy was the
sum of the strain energy from the first two load cycles. This must not
be considered as the strain energy to failure; it is only a measure for
comparison purposes, and it does not have the same meaning as total
strain energy to failure. The compressive mechanical properties of the
three sets of scaffolds (A, B and C) were determined in air at room
temperature (n=4 per set) and wet with saline at 37°C (n=6 per set).
Set
A
B
C
Dry, room Temperature
Eapp (MPa)
K (N/µm)
29.9 (10.7)
3730 (1330)
47.6 (3.27)
6490 (440)
37.8 (5.23)
5290 (746)
Porosity, %
50.7 (3.63)
49.1 (3.70)
52.1 (3.77)
2
Copyright © 2009 by ASME
June 17-21, Resort at Squaw Creek, Lake Tahoe, CA, USA
SBC2009-204934
MATERIAL AND MECHANICAL PROPERTIES OF TRICALCIUM PHOSPHATE-BASED
(TCP) SCAFFOLDS
Juan Vivanco (1), Sylvana García-Rodríguez (1), Everett L. Smith (2), Heidi-Lynn Ploeg (1)
(1) Department of Mechanical Engineering and
Material Science Program, University of
Wisconsin-Madison
(2) Department of Population Health Sciences,
University of Wisconsin-Madison
INTRODUCTION
Bioactive ceramic materials like tricalcium phosphates (TCP)
have been emerging as alternative and valid approaches to the current
therapies of bone scaffolding to target major diseases such as fracture
healing, osteoporosis, and osteoarthritis [1]. These scaffolds can
induce bone formation by acting as carriers or guides for enhanced
bone regeneration by cell migration, proliferation, and differentiation.
Previous studies have shown that both material and architectural
characteristics influence the mechanical strength of bone scaffolds and
their biological functionality [2]. In this study, the physical and
mechanical properties of three sets of TCP scaffolds were analyzed.
The physical properties that were evaluated included volume, mass,
density and porosity. The scaffold compressive mechanical properties
were evaluated in air at room temperature and in saline at 37°C.
filled to level H with the ethanol solution and weighed (W1) (Figure
1). After the scaffolds were water-saturated, they were immersed in
the container with ethanol, and the volume of alcohol was adjusted so
that the container with the scaffold was completely filled again level
H. For each specimen the ethanol was agitated until no air bubbles
emerged from the scaffold and all pores were filled. This new volume
is composed of the scaffold (Vscaffold) and the remaining ethanol (V2).
The the scaffold’s volume is given by Equation 1. Using the displaced
fluid volume method, material volume, density and porosity
measurements for the three sets (n = 8 per set) were determined (Table
1).
Vscaffold = V1 – V2 = (W1 – W2 + Wscaffold) / ρethanol
METHODS
Physical Properties
The volume and mass of TCP bone scaffolds fabricated with
three different sintering temperatures (defined as A, B, and C in order
of increasing temperatures) were evaluated (n=50 per set). The dry
bulk dimensions of the scaffolds were measured using a digital vernier
caliper. Dry and wet weights were measured with a digital balance
(Series PB-S, Mettler Toledo, Columbos, OH). To evaluate water
absorption, specimens were submerged in distilled water for 24 hrs
and centrifuged (Model TJ-6, Beckman, Fullerton, CA) at 3000 rpm
for 3 minutes to eliminate excess water on their surface. The wet
scaffolds were then weighed, Wscaffold.
Material volume was measured using a technique based on
displacement of a known-density fluid (ethanol solution, ρethanol).
Previous validations studies found a mean error of 0.975% when
measuring known volumes. A container of volume V1 was completely
(1)
Figure 1. Measurement of scaffold material volume
Mechanical Testing
Mechanical testing was performed using the ZETOS Bone
Loading System, a custom-made device that uses a piezoelectric
actuator (PZA) to apply compressive displacement to a specimen [3].
Applied force and specimen deformation were recorded during the test
and were processed to find specimen stiffness or apparent elastic
1
Copyright © 2009 by ASME
modulus. All the scaffolds underwent two preconditioning trials
(preload of 10 N followed by compressive displacement of 5 µm). Due
to the maximum displacement range of the PZA (70 µm), a series of
tests were performed on each scaffold until the specimen failed.
Displacement was applied gradually at approximately 0.50 µm/s until
either the maximum force (1500 N) or maximum displacement
(70 µm) was reached. The linear region of the force-displacement
curve (Figure 2b) was chosen to calculate stiffness, K, defined as the
slope of the linear fit of the region. Stiffness was converted to apparent
elastic modulus Eapp using Hooke’s law, Eapp = KL/A (where F is the
applied force, d is the compressive displacement, A is the bulk area of
the top surface and L is the bulk length of the specimen in the
dimension parallel to the loading direction) (Figure 2a). Failure, or
partial cracking, was identified in the force-displacement curve as a
sudden decrease in force.
Figure 3. Mean scaffold stiffness (N/µ
µm)
L
K=
Table 2. Stiffness and apparent elastic modulus for each
set and test environment, mean (std dev)
F
d
Set
(a)
A
B
C
K
(N/µm)
(b)
Figure 2. (a) Definition of loading direction. (b) Typical
force-displacement scaffold curve
Set
A
B
C
CSE (GPa)
Density (g/cm3)
2.5 (0.14)
2.9 (0.19)
3.2 (0.29)
30.1
Dry, room Temperature
CSE (GPa)
32.3 (29.0)
39.0 (51.9)
58.6 (41.8)
43.3
Saline, body Temperature
CSE (GPa)
8.70 (3.26)
28.4 (30.6)
45.0 (35.8)
27.4
Mean CSE
(GPa)
20.5
33.7
51.8
REFERENCES
1. Schenberger D. and Bafan M., 2008. , "Bioceramics: The Future
of Joint Healing" Med-Tech Precision Magazine, Spring.
2. Hutmacher DW., 2000, "Scaffolds in tissue engineering bone and
cartilage," Biomaterials, 21(24):2529-2543.
3. García S., Smith E., and Ploeg H., 2008, "A calibration procedure
for a bone loading system," Journal of Medical Devices, 2:01061-01106-6, March.
Table 1. TCP scaffold physical properties, mean (std dev)
Volume (mm3)
85.9 (6.45)
68.6 (4.97)
59.6 (4.51)
Mean K
(N/µm)
24.4
42.2
36.1
DISCUSSION AND CONCLUSIONS
In this study, the mechanical behavior of TCP scaffolds was
affected by the sintering temperature of the fabrication process as well
as the test environment. TCP scaffolds sintered at the lowest
temperature had the highest bulk volume, lowest material density,
lowest apparent modulus and absorbed the most water. The structural
stiffness for all the TCP scaffolds was reduced when tested at 37°C in
saline. Dry, room temperature conditions are practical for
characterization of the structure, but future applications will be at 37°C
and in a saline environment. It is therefore important to investigate the
scaffolds mechanical properties under these conditions.
Factors such as surface quality, ductility of the structure, cellular
response, characteristics of the culture medium and general conditions
of the test environment are only a few factors that need be to be
considered in future investigations of the scaffold-cell system.
RESULTS
A paired t-test (α = 0.05) was performed to determine if there
was a difference in weight as a consequence of a dry or wet state. Only
the weight of set C was not significantly different due to water
absorption.
Statistical analyses (ANOVA and Tukey Test, α = 0.05) found a
difference between the scaffold sets regarding material volume and
material density (p-value < 0.05 for both). On the other hand, porosity
was not different due to sintering temperature (Table 1). Set A was the
least stiff (22.5 N/µm in saline at 37°C) and sets B and C were the
stiffest (34.4 and 33.7 N/µm, respectively) (Figure 3 and Table 2). A
two-way ANOVA and Tukey Test (α = 0.05) found that both sintering
temperature and testing environment, affected the scaffold
compressive stiffness (p-value < 0.05 for both factors).
Wet weight (g)
0.2147 (0.006)
0.1982 (0.002)
0.1913 (0.004)
38.4
Saline, body Temperature
Eapp (MPa)
K (N/µm)
22.5 (7.81)
2810 (973)
34.4 (8.03)
4080 (2170)
33.7 (4.94)
4720 (681)
Table 3. Cumulative strain energy for each set and test
environment, mean (std dev)
Strain energy was calculated by finding the area under the stress-strain
curve until failure. Most scaffolds failed during the first load cycles.
To obtain a quantitative measure and take the number of load cycles
into account, the parameter cumulative strain energy (CSE) was
defined as the sum of strain energy to failure. Thus, if failure was
found in the second load cycle, the cumulative strain energy was the
sum of the strain energy from the first two load cycles. This must not
be considered as the strain energy to failure; it is only a measure for
comparison purposes, and it does not have the same meaning as total
strain energy to failure. The compressive mechanical properties of the
three sets of scaffolds (A, B and C) were determined in air at room
temperature (n=4 per set) and wet with saline at 37°C (n=6 per set).
Set
A
B
C
Dry, room Temperature
Eapp (MPa)
K (N/µm)
29.9 (10.7)
3730 (1330)
47.6 (3.27)
6490 (440)
37.8 (5.23)
5290 (746)
Porosity, %
50.7 (3.63)
49.1 (3.70)
52.1 (3.77)
2
Copyright © 2009 by ASME