Characterization of
Hydrogels for Nucleus
Pulposus Replacement
Haley Thompson
Mentor: Dr. Skip Rochefort, CBEE
Oregon State University
8/20/2008
http://images.google.com/imgres?imgurl=http://www.3dscience.com/img/
ts/3D_Models/Human_Anatomy/Skeletal/Human_Spine/3d_model_anat_
web1.jpg&imgrefurl=http://www.3dscience.com/3D_Models/Human_Anat
keletal/Human_Spine.php&h=300&w=300&sz=59&hl=en&start=9&tbnid=
OugBaxNM:&tbnh=116&tbnw=116&prev=/images%3Fq%3Dspine%26hl%
Back Pain and Herniated Disks
Components of a spinal disk:
Bader, Rebecca. "Development and Characterization of Novel Hydrogels for Nucleus
Pulposus Replacement." Oregon State University, Corvallis. 15 Dec. 2006.
What happens when a disk is herniated?
• Tear in annulus fibrosus
•Nucleus pulposus squeezes out
•Disk loses cushioning function
Treatments
•Discectomy
• Spinal Fusion
Disadvantages:
- short term solution
- decreased mobility
- not a spinal disk repair
http://brispine.com.au/images/posterior_fusion_x_ra
y.jpg
Disk Replacements
SB Charite Disk
•Complete disk replacement
http://biomed.brown.edu/Courses/BI108/BI108_
2002_Groups/discs/Charite.htm
PDN® Prosthetic Disc-Nucleus
• Nucelus pulposus replacement
• Hydrogel encased in polyethylene
jacket
DASCOR
• Nucelus pulposus replacement
• gel-inflated polyurethane balloon
Agar and Agarose Gels
Common
electrophoresis gels
that are:
•body-friendly
•easy to produce
•fairly cheap
"Gel Structure of Agarose." MBMB 451A Section One - Fall 2005. 1 Sept. 2005. Southern Illinois University
School of Medicine. 30 June 2008
<http://web.siumed.edu/~bbartholomew/images/protein_methods/gel_structure_of_agarose.gif>.
Fillers
Sodium Alginate Beads
Derived from seaweed
Formed by chemical crosslinking of
Sodium Alginate and Calcium Chloride
Sodium Alginate FDA approved
Darlon Fibers
Used in tying flies
Thin thread similar to nylon
http://www.charliesflyboxinc.com/flybox//details.cfm?
parentID=156
Gels and Filler Combinations
1% Agarose
1% Agar
Unfilled
No filler
No filler
Bead Filler
33% filler by volume
(low)
33% filler by volume
(low)
50% filler by volume
(high)
50% filler by volume
(high)
1.0-1.3 mm diameter
Darlon Filler
0.2in long
.02g per 6ml solution .02g per 6ml solution
(low)
(low)
.04g per 6ml solution .04g per 6ml solution
(med)
(med)
.06g per 6ml solution .06g per 6ml solution
(high)
(high)
Using Rheometry to Characterize Gel
What is Rheometry?
The study of deformation and flow
of materials
TA AR 2000EX
Test Procedure
Dynamic oscillation:
Frequency sweep
• 0.1-10Hz
•γ=1%
Strain sweep
•1-100% strain
•ω=0.1Hz
ω
γ
Parallel Plate
Geometry
rotating plate
fixed plate
sample
Frequency Sweep
G’ is the elastic or ‘storage’ modulus
G’’ is the viscous or ‘loss’ modulus
1% Agar Gel
10000
G’
Modulus (Pa)
1000
100
G’’
1% Agar G'
1%Agar G''
10
1
0.1
1
Frequency (Hz)
10
Agar and Agarose Comparison
Moduli Comparison of Materials
100000
Modulus (Pa)
10000
Sheep NP avg G'
1000
Sheep NP avg G''
1% Agar G'
100
1%Agar G''
1% Agarose G'
1% Agarose G''
10
1
0.1
1
Frequency (Hz)
10
Strain Sweep
1% Agar Strain Sweep
1000
Yielding region
900
800
Osc Stress (Pa)
700
600
Nonlinear Region
500
400
1% Agar
Strain Sweep
300
Linear Region
200
100
0
0
20
40
60
% Strain
80
100
Strain applied to the gel
Agar and Agarose Comparison
1% Agar Strain Sweep
1000
1000
900
900
800
800
700
700
600
500
400
run 1 take 2
300
Osc Stress (Pa)
Osc Stress (Pa)
1% Agarose Strain Sweep
600
500
400
200
200
100
100
0
run 1
300
0
0
20
40
% Strain
60
80
0
20
40
60
% Strain
80
100
Instron Extrusion Testing
•Find a gel that is resistant to extrusion
•Less likely to rupture again
Extrusion Tester
Side View
Top View
Sample of Results:
1% Agarose 1g gel beads
First peak is initial break point
10
10
10
10
8
8
8
8
6
6
6
6
4
2
0
0.075
-2
4
2
0.175
Displacement (In)
0
0.075
-2
0.125
0.175
0.225
Displacement (In)
12
Ft-lb
Ft-lb
12
Ft-lb
12
Ft-lb
12
4
4
2
2
0
0.075
-2
0.175
Displacement (In)
0
0.075
-2
0.175
Displacement (In)
Dynamic Oscillation Results
G’
G’’
sheep NP
8,540
3,210
1% agar med
concentration .2in fibers
9,930
1,770
1% agar high concentration
.2in fibers
8,430
1,730
1% agarose med
concentration .2in fibers
10,160
1,860
8,070
705
13,430
2,310
6,600
1,035
1% agar low concentration
.5gbeads
1% agarose low
concentration .2in fibers
1% agar low concentration
.2in fibers
1% agarose low
concentration .5gbeads
13,730
1,570
1% agarose high
concentration 1gbeads
16,210
2,775
1% agarose high
concentration .2in fibers
17,360
3,540
1% Agarose
6,460
350
1% agar high concentration
1gbeads
1% Agar
4,245
3,590
Rank
by G’
Rank
by G’’
Overall
Rank
0
0
0
3
5
1
1
6
2
Sheep NP G'
Sheep NP G''
4
4
3
2
9
4
8
3
5
5
8
6
10
7
7
11
2
8
12
1
9
6
11
10
7
10
11
9
12
12
415
180
Best Dynamic Oscillation Samples
100000
1% Agar med
concentration .2'' fiber G'
1% Agar med
concentration .2'' fiber G''
1% Agarose med
cocentration .2'' fiber G'
1% Agarose med
concentration .2'' fiber G''
1% Agar G'
10000
Modulus (Pa)
Sample
1% Agar G''
1000
1% Agarose G'
1% Agarose G''
100
0.1
1
10
Frequency (Hz)
•Addition of fillers positively effects the moduli of the materials in the dynamic oscillatory test
Strain Comparison Results
Rheology
stress at
max
linear
strain stress
Instron
stress at
strain yield
strain
AGAR
Rheology
Instron
stress at
stress at
linear
strain max stress strain yield
strain
AGAROSE
unfilled
low beads
280
390
12
9
580 53.5
725
37
84750
70100
2.8 unfilled
1.4 low beads
380
745
9
10
700
1180
38 345500 4.8
53.5 139250 1.25
high beads
245
10.5
605 85.5
44625
1.3 high beads
865
11
1520
51.5
low fiber
340
10
560
68
62150
1.2 low fiber
800
11
med fiber
600
11.5
1055
90
60700
1.1 med fiber
600
high fiber
430
11
710
78
62800
0.85 high fiber
890
86700
1.4
1400
70 170300
1.5
11
1040
67 152000
1.4
10
1580
87.5 157950
1.6
• In dynamic oscillatory shear experiments fillers increase the modulus as evidenced by the increase in the
maximum yield stress
•In Instron compression tests the modulus of the unfilled material is always higher than filled materials
•Relative moduli of individual gels (max stress at yield) correlates well with the magnitudes of the gel
stiffness in the compression test
•Although dynamic oscillatory shear tests are useful for general screening of materials, compression tests
are important for real world applications
Summary of Research
• Got a new rheological instrument on line.
•Designed strain sweep test
•Developed techniques to make gels and forms
• Added fibers and gel beads to gels
• Tested in Instron apparatus
•Determined areas for focus of future research
Conclusions
• Addition of fillers increases the strength and the resiliency of the gels
• Fillers did not have the anticipated effect on the performance in the
instron test
• While the dynamic oscillatory strain sweep is good for screening gels,
the compression test is important because it more closely mimics the
true conditions in the back
• Instron test indicates that bonding between gels and matrix are the
‘weak link’
• Syneresis of Agarose affects performance in the Instron test
Future Work
• Address issues with syneresis of the Agarose gels
• Try to enhance interstitial strength of the filler and the gel matrix
• Continue to test a variety of concentrations of new and existing fillers
Acknowledgements
• HHMI Thank you for funding this internship and supporting research for
undergraduates.
• Dr. Kevin Ahern Thank you for heading this program and helping
at every step.
• Dr. Skip Rochefort Thank you for giving me the opportunity to
work in your lab and gain valuable experience.
• Nikki Buck, Jessica McKiernan, Coralie
Backlund for being valuable friends and research partners
• Will Beattie and Rebecca Bader for sharing their
research on the subject
Citations
Bono, Christopher M., and Steven R. Garfin. "History and evolution of disc replacement." The Spine Journal 4 (2004):
145-50. 12 Nov. 2004. ScienceDirect. Oregon State University, Corvallis. 1 July2008<http://www.science
direct.com/science?_ob =ArticleURL&_udi=B6W7P4DSGJ9014&_user=576687&_coverDate=11%2F01%
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Cloyd, Jordan M., Neil R. Malhotra, Lihui Weng, Weiliam Chen, Robert L. Mauck, and Dawn M. Elliot. "Material properties in
unconfined compression of human nucleus." European Spine Journal 16 (2007): 1892-898. 28 July 2007.
Springer-verlag. Oregon State University, Corvallis. 14 July 2008
<http://www.springerlink.com/content/4734757161711440/fulltext.pdf>.
Deyo, Richard A. Scientific American vol. 279, issue 2, (1998): 49-49. EBSCO. Oregon State University, Corvallis. 20 June 2008
<http://0web.ebscohost.com.oasis.oregonstate.edu/ehost/detail?vid=1&hid=103&sid=42fb68ce-d1bf-4ae0ac9f-b84ddd59b470%40sessionmgr107>.
Thomas, Jonathan, Anthony Lowman, and Michele Marcolongo. "Novel Associated Hydrogels for Nucleus Pulposus
Replacement.“ Journal of Biomedical Materials Research Part A 67 (2003): 1329-337. 2003. InterScience.
Oregon State University, Corvallis. 23 June 2008
<http://www3.interscience.wiley.com/cgibin/fulltext/106563998/htmlstart>.