Previous approaches to studying heart valve biomechanics have included:
- Selective enzymatic digestion and biomechanical testing
- Materials testing and constitutive modeling
- Histologic and electron microscopy
New areas of investigation have built upon this basic understanding of valvular function and have explored computational modeling as a tool for the study of soft tissue biomechanics, and tissue engineering as a means of fabricating the complex biological structures directly in the laboratory.
Also notable has been the work towards developing tissue-engineered heart valves, using the biological materials found in the natural tissues–elastin, collagen and glycosaminoglycans. These molecules can be synthesized by cells in culture or purified from tissues, then manipulated to mimic the normal structural framework of the aortic valve. This approach is particularly relevant to connective tissues that have limited ability for self-repair, like the cardiac valves. Recent successes include the fabrication of tissue-engineered mitral valve chordae with a strength and stiffness ten times greater than any other material produced using similar approaches. Much of this success lies in the use of optimized cell and collagen ratios, nutrition fortified media and special purpose-built dynamic bioreactors that stretch the tissue-engineered chordae during maturation in vitro.
The main approach of the Heart Valve Laboratory is to study the structure/function relationship of heart valve tissues so that we can determine the failure mechanisms and find ways to improve the design and fabrication of existing artificial heart valves. Our ultimate goal is to develop a bioprosthetic valve that completely mimics the function of the natural valve it must replace. As biomedical engineers, our research makes use of engineering approaches to study and understand the aortic and mitral valves. This involves materials testing, mathematical modeling, and microscopy and biochemical analysis.
Micro mechanical testing of the valve tissue is being done to determine the functional relationships between the fibrosa and the ventricularis, and the roles played by collagen and Elastin. This is done at very high speed to simulate physiological loading conditions. We are also using selective enzymatic degradation to remove certain valve tissue structural proteins (collagen, Elastin, Gag’s), and then measure the mechanics of the resulting material to determine how each constituent contributes to the mechanics of the whole tissue.
We use a video image processing technique to measure biaxial strains of the valve materials in their intact state. Such alternative materials testing techniques are needed to describe completely the material properties of such highly deformable and anisotropic fibrous materials.
We are using mathematical modeling to analyze the viscoelastic nature of the heart valve tissue, and to establish a closer link between testing and analysis. Through such a link, difficult-to-measure material parameters can be estimated, constitutive models verified, and difficult-to-perform tests simulated. Thus far, we have developed extensions to Fung’s original Quasilinear Viscoelastic theory, that enable us to extract QLV parameters from conventional, medium speed materials tests, that are remarkably predictive of the long term cyclic loading behavior of the heart valve tissue. In collaboration with researchers at NASA, we are studying a fiber composite model in its ability to predict the mechanics of heart valves tissues.
Computerized video densitometry, polarized light microscopy, specific antibody staining, and 3-D reconstruction of serial sections all have been used to quantify the content and distribution of collagen, elastin and glycosaminoglycans throughout the aortic valve cusps.
The mechanical, microscopic and biochemical techniques developed on aortic valves, have recently been applied to the study of myxomatous mitral valve disease. This is a disease characterized by thickening of the valve tissues and stretching of the leaflets and chordae causing the valve to leak. Although it can be corrected surgically, the outcomes are not always satisfactory, and the disease itself is not at all understood. By understanding the cause of the disease, we can better treat the patient. To that end, we have been studying the specific biochemical and mechanical changes that occur in these tissues. Our results suggests that it is a disease more of the chordae, rather than the leaflets, as was previously thought. We are also beginning to explore the genetic determinants of the disease, to better understand its cause.
A long-term project is ongoing to engineer a viable tissue valve implant consisting of materials currently found in the natural tissues; the elastin, collagen and glycosaminoglycans. These molecul es can be synthesized by cells in culture or purified from tissues, and then manipulated to mimic the normal structural framework of the aortic valve. Living cells harvested from the intended recipient of this device can then be cultured on these materials to transform an essentially dead, inert material, to one that behaves biomechanically more like the native valve, and may be capable of repair and regeneration. This process is referred to as tissue engineering, and our approach is specifically focused on replacing connective tissues that are not capable of repair on their own.
This research is funded by grants from the NIH and the U.S. Army. We are actively seeking ways of augmenting our research funds by participating in technology transfer, through research contracts from Industry.
Ongoing research projects include:
- Analysis of immunoglobulin genes used by organ recipients to target transplanted xenografts
- Analysis of the molecular structure of xenoantibody-antigen binding, making use of site-directed mutagenesis and computer modeling
- Development of treatment strategies to prevent graft rejection, including the application of anti-idiotypic antibodies and small molecular inhibitors designed to specifically target the xenoantibody binding site
- The application of gene therapy to induce transplant tolerance
Success in any of these research projects will offer greater quality of life to organ transplant recipients, as they will reduce the severe side-effects associated with currently available immunosuppression therapies. During the 2003-2004 research period, Dr. Mary Kearns-Jonker received a number of new research grants and made significant advances towards long-term survival of transplanted hearts in the mouse model. The xenotransplantation laboratory recently reported the successful application of gene therapy using lentiviral vectors to induce chimerism and long-term heart graft survival in a mouse model. The long-term goals of this work are to apply gene therapy to induce transplantation tolerance.
At the Saban Research Institute of Children’s Hospital Los Angeles, cardiovascular research is carried out by independent principal investigators. In a broad sense, these three areas comprise:
- Heart Valve Biomechanics and Tissue Engineering
- Molecular Aspects of Organ Graft Rejection
- Mechanisms of Graft Fibrosis
The immediate goals of the program are to establish the world-class presence of heart valve research in the Saban Research Institute, expand research in tissue engineering and computational modeling and identify new synergies within the group for continuing our pioneering work in basic and applied cardiovascular research.