Date of Award

12-2010

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Bioengineering

Advisor

Vyavahare, Narendra

Committee Member

Laberge , Martine

Committee Member

Nagatomi , Jiro

Abstract

Valvular heart diseases lead to over 290,000 heart valve replacements worldwide each year, and approximately half of these involve replacement with a bioprosthetic heart valve (BHV) [1]. BHVs exhibit excellent hemocompatibility, but suffer from inadequate long-term durability with most adult implanted valves failing within 12 to 15 years after implantation [2]. Although this may be adequate for some individuals, BHV implantation may be contraindicated in younger individuals to avoid reoperation. Even in elder recipients, valve dysfunction can still cause death or reoperation that could be avoided with increased BHV durability. Therefore, investigation into methods of increasing BHV durability can not only improve the quality of life for BHV recipients, but widen the accessible patient demographic as well.
All BHVs are currently treated with glutaraldehyde, which is used to stabilize collagen through crosslinking as well as reduce tissue immunogenicity. However, glutaraldehyde crosslinking potentiates BHV calcification and also causes undesirable changes to tissue properties that are conducive to structural degeneration [3,4,5]. Considering that calcification and structural degradation are the foremost causes of BHV failure, there has been interest in alternative, non-glutaraldehyde fixation methods [3].
One phenomenon that can contribute to structural degeneration is the loss of glycosaminoglycans (GAGs) from cuspal tissue [6,7]. GAGs are large highly hydrophilic molecules that form a gel-like sheet within the center layer of valve cusps. This layer aids in stress absorption and reduction of shearing between the other tissue layers [8]. GAGs lack the amine functionalities to be crosslinked by glutaraldehyde and under current fixation methods are lost during preparation, in vivo implantation and storage [7,8,9]. Carbodiimide crosslinking with EDC and NHS reacts with available carboxyl groups in addition to amine groups allowing for the crosslinking of both GAGs and collagen. These crosslinks may aid in GAG preservation and thereby contribute to improved mechanical function. However, it has been shown that crosslinking alone is not sufficient to fully preserve GAGs, as they are still prone to loss through enzymatic degradation [10].
Neomycin, a hyaluronidase inhibitor, has been shown previously to prevent the enzymatic degradation of GAGs [11]. Neomycin also contains amine functionalities that enable stable incorporation into carbodiimide-initiated crosslinks. This study investigates the effects of carbodiimide crosslinking in combination with neomycin on the stability of structural proteins, GAG preservation, calcification, and biomechanical properties.
The incorporation of neomycin into EDC, and NHS fixation (NEN) was found to improve GAG retention, fully preventing loss during storage, implantation, and direct enzymatic digestion. Additionally, the use of neomycin enhanced the elastin and collagen stability of EDC and NHS crosslinking alone (EDC). When compared to glutaraldehyde crosslinked tissues (GLUT), NEN treated tissues demonstrated similar collagen and elastin stability but far improved GAG retention. EDC and NEN crosslinking also yielded tissues with reduced stiffness and increased extensibility when compared to GLUT crosslinked tissues. These results suggest that NEN fixation and resulting GAG preservation may improve the mechanics of BHV tissues. In doing so, tissue stress and shock loading may be reduced during function, preventing the onset of collagen damage and increasing the lifespan of BHVs.
1. Dasi LP, et al. Clinical and Exp Pharm and Phys. (2009) 36:225-237.
2. Siddiqui RF, et al. Histopathology. (2009) 55:135-144.
3. Schoen FJ, et al. J Biomed Mater Res. (1999) 47:439-465.
4. Schoen FJ, Levy RJ. Ann Thorac Surg. (2005) 78:1072-1080.
5. van Noort R, et al. Biomaterials. (1982) 3:21-26.
6. Vyavahare N, et al. J Biomed Mat Res. (1999) 46:44-50.
7. Grande-Alen KJ, et al. J Biomed Mat Res. (2003) 65A:251-259.
8. Lovekamp J, et al. Biomaterials (2006) 27:1507-1518.
9. Simionescu DT, et al. J Heart Val Dis. (2003) 12:226-234.
10. Mercuri JJ, et al. Biomaterials (2007) 28:496-503.
11. Raghavan D, et al. Biomaterials (2007) 28:2861-2868.

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