Characterization of Hydrogels for Nucleus Pulposus Replacement

Haley ThompsonMentor: Dr. Skip Rochefort, CBEE

Oregon State University8/20/2008

http://images.google.com/imgres?imgurl=http://www.3dscience.com/img/Products/3D_Models/Human_Anatomy/Skeletal/Human_Spine/3d_model_anat_spine_web1.jpg&imgrefurl=http://www.3dscience.com/3D_Models/Human_Anatomy/Skeletal/Human_Spine.php&h=300&w=300&sz=59&hl=en&start=9&tbnid=PAnjhO-ugBaxNM:&tbnh=116&tbnw=116&prev=/images%3Fq%3Dspine%26hl%3Den%26rlz%3D1T4GGLR_enUS235

Back Pain and Herniated Disks

Components of a spinal disk:

What happens when a disk is herniated? • Tear in annulus fibrosus•Nucleus pulposus squeezes out•Disk loses cushioning function

Bader, Rebecca. "Development and Characterization of Novel Hydrogels for Nucleus Pulposus Replacement." Oregon State University, Corvallis. 15 Dec. 2006.

Treatments•Discectomy• Spinal Fusion

Disadvantages: - short term solution - decreased mobility - not a spinal disk repair

http://brispine.com.au/images/posterior_fusion_x_ray.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>.

FillersSodium Alginate BeadsDerived from seaweedFormed by chemical crosslinking of Sodium Alginate and Calcium ChlorideSodium Alginate FDA approved

Darlon FibersUsed in tying fliesThin 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 Filler1.0-1.3 mm diameter

33% filler by volume(low)

33% filler by volume(low)

50% filler by volume(high)

50% filler by volume(high)

Darlon Filler0.2in long

.02g per 6ml solution(low)

.02g per 6ml solution(low)

.04g per 6ml solution(med)

.04g per 6ml solution(med)

.06g per 6ml solution(high)

.06g per 6ml solution(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

sample

rotating plate

fixed plate sample

rotating plate

fixed plate

Parallel Plate Geometry

γω

Frequency Sweep

G’ is the elastic or ‘storage’ modulus

G’’ is the viscous or ‘loss’ modulus

G’

G’’

Agar and Agarose Comparison

Strain Sweep

Strain applied to the gel

Linear Region

Yielding region

Nonlinear Region

Agar and Agarose Comparison

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

Sample G’ G’’ Rank by G’

Rank by G’’

Overall Rank

sheep NP 8,540 3,2100 0 0

1% agar med concentration .2in fibers 9,930 1,770

3 5 1

1% agar high concentration .2in fibers 8,430 1,730

1 6 2

1% agarose med concentration .2in fibers 10,160 1,860

4 4 3

1% agar low concentration .5gbeads 8,070 705

2 9 4

1% agarose low concentration .2in fibers 13,430 2,310

8 3 5

1% agar low concentration .2in fibers 6,600 1,035

5 8 6

1% agarose low concentration .5gbeads 13,730 1,570

10 7 7

1% agarose high concentration 1gbeads 16,210 2,775

11 2 8

1% agarose high concentration .2in fibers 17,360 3,540

12 1 9

1% Agarose 6,460 3506 11 10

1% agar high concentration 1gbeads 4,245 415

7 10 11

1% Agar 3,590 1809 12 12

Dynamic Oscillation Results

•Addition of fillers positively effects the moduli of the materials in the dynamic oscillatory test

Rheology Instron Rheology Instron

stress at linear strain

max stress strain

stress at yield strain

stress at linear strain max stress strain

stress at yield strain

AGAR AGAROSE

unfilled 280 12 580 53.5 84750 2.8 unfilled 380 9 700 38 345500 4.8low beads 390 9 725 37 70100 1.4 low beads 745 10 1180 53.5 139250 1.25

high beads 245 10.5 605 85.5 44625 1.3 high beads 865 11 1520 51.5 86700 1.4

low fiber 340 10 560 68 62150 1.2 low fiber 800 11 1400 70 170300 1.5

med fiber 600 11.5 1055 90 60700 1.1 med fiber 600 11 1040 67 152000 1.4

high fiber 430 11 710 78 62800 0.85 high fiber 890 10 1580 87.5 157950 1.6

Strain Comparison Results

• 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% 2F2004&_rdoc=3&_fmt=high&_orig=browse&_srch=docinfo(%23toc%236632%232004%23999959993.8998%23567049%23FLA%23display%23Volume)&_cdi=6632&_sort=

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-4ae0-

ac9f-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>.