Thesis (doctoral)--Lund University, 1996. / Added t.p. with thesis statement inserted.

Thesis (M.S.)--University of Missouri-Columbia, 2004. / Typescript. Includes bibliographical references (leaves 100-104). Also available on the Internet.

Hundreds of thousands of vascular bypass grafts are implanted in the United States every year, but there has yet to be an ideal graft material to substitute for one’s own autologous vessel. Many synthetic materials have been shown to be successful vessel replacements; however, none have been proven to exhibit the same mechanical properties as native vessels, one of the most important criteria in selecting a vascular graft material. Part of this issue is due to the fact that, currently, there is no “gold standard” for testing the longitudinal and transverse tensile properties of small diameter tubular materials. While there are ASTM and ISO standards that suggest ways to test tubes in their original form, many researchers have published tensile strength data based on cutting the tube and testing it as a flat sample. Thus, it was the aim of this thesis to understand, establish, and implement accurate tensile testing methods of small diameter polymers in their original, tubular state on Cal Poly’s campus. Two test fixtures were created based on specified design criteria in order to test materials in their tubular form in both the longitudinal and transverse directions. Both fixtures were successful in testing PLGA and ePTFE samples, and statistical data was gathered for the transverse test fixture. The new transverse test fixture was tested against the current method of testing, and a significant (α = 0.05) difference between methods was established for ultimate tensile strength. This analysis, however, cannot determine which test method is more accurate, thus more extensive testing is required to verify the design of both fixtures. By developing a method for testing small diameter polymers in tubular form on Cal Poly’s campus, it allows for more testing of various small diameter tubes and more comparative data to validate each design. It also demonstrates a need for a more detailed and widespread standardization of testing for small diameter tubes, especially in vascular substitute applications where the ideal vessel replacement has yet to be found.

Regenerative medicine involves formation of tissue or organ for replacement of a wounded or dysfunctional tissue. Healthy cells extracted from the patient are expanded and are seeded on a three-dimensional biodegradable scaffold. The structure is then placed in a bioreactor and is provided with nutrients for the cells, which proliferate and migrate throughout the scaffold to eventually form a desired to tissue that can be transplanted into the patient's body.  Inability to monitor this complex process of regeneration in real-time makes control and optimization of this process extremely difficult. Histology, the gold standard used for tissue structural assessment, is a static technique that only provides "snapshots" of the progress and requires the specimen to be sacrificed. This inefficiency severely limits our understanding of the biological processes associated with tissue growth during the in vitro pre-conditioning phase. Optical Coherence Tomography (OCT) enables imaging of cross sectional structure in biological tissues by measuring the echo time delay of backreflected light. OCT has recently emerged as an important method to assess the structures of physiological, pathological as well as tissue engineered blood vessels. The goal of the present study is to develop an imaging system for non-destructive monitoring of blood vessels maturing within a bioreactor. Non-destructive structural imaging of tissue-engineered blood vessels cultured in a novel bioreactor was performed using free-space and catheter-based OCT imaging, while monitoring of the endothelium development was performed using a fluorescence imaging system that utilizes a commercial OCT catheter. The project included execution of three specific aims. Firstly, we developed OCT instrumentation to determine geometrical and optical properties of porcine and human skin in real-time. The purpose of the second aim was to assess structural development of tissue-engineered blood vessels maturing in a bioreactor. We constructed a novel quartz-based bioreactor that will permit free space and catheter-based OCT imaging of vascular grafts. The grafts were made of biodegradable PCL-collagen and seeded with multipotent mesenchymal cells. We imaged the maturing grafts over 30 days to assess changes in graft wall thickness. We also monitored change in optical properties of the grafts based on free-space OCT scanning.   Finally, in order to visualize the proliferation of endothelial cells and development of the endothelium, we developed an imaging system that utilizes a commercial OCT catheter for single-cell-level imaging of the growing endothelium of a tissue-engineered blood vessel. We have developed two modules of an imaging system for non-destructive monitoring of maturing bioengineered vascular grafts. The first module provides the ability to non-destructively examine the structure of the grafts while the second module can track the progress of endothelialization. As both modules use the same endoscope for imaging, when operated in sequence, they will produce high-resolution, three-dimensional, structural details of the graft and two-dimensional spatial distribution of ECs on the lumen. This non-destructive, multi-modality imaging can be potentially used to monitor and assess the development of luminal bioengineered constructs such as colon or trachea. / Ph. D.

Suportes eletrofiados para crescimento celular são de interesse para a engenharia de tecidos, principalmente em função de sua estrutura em forma de rede tridimensional de fibras de diâmetro nanométrico. Esta arquitetura especial permite a geração de elevada área superficial e porosidade, características importantes para a adesão, proliferação e infiltração de células para o interior do suporte. A utilização de um suporte eletrofiado como enxerto vascular necessita ainda que este apresente excelentes propriedades mecânicas, associadas a uma elevada biocompatibilidade. Neste trabalho mostramos que estas propriedades podem ser alcançadas a partir da eletrofiação de uma co-solução de PET e colágeno gerando um material híbrido, visto que PET apresenta excelentes propriedades mecânicas e o colágeno é o principal componente da matriz extracelular. A obtenção dos suportes eletrofiados de PETcolágeno mostrou ser possível utilizando-se como solvente HFIP e HFIP/TFA 7:2. No entanto, neste último, o colágeno é completamente degradado durante o processo de solubilização. Fixando-se os parâmetros de eletrofiação, a morfologia da malha obtida mostrou ser dependente da relação massa PET/massa colágeno, concentração total da solução e solvente utilizado. Foram obtidos materiais com distribuição de diâmetros unimodal e bimodal, além de materiais com formato em fitas e teias entre as fibras. Ainda, PET e colágeno formam malhas de composição complexa, nas quais são encontradas fibras compostas de materiais puros, mas também formam blendas em que os dois materiais encontram-se misturados em uma mesma fibra. Os materiais S8,2 S4,6 foram caracterizados química-, mecânica- e biologicamente. Observou-se que, para filmes planos, estes materiais apresentaram energia de superfície mais próxima da do colágeno, o que justifica a melhor adesão celular em S8,2 e S4,6 do que no PET. S8,2 mostrou ter valores de módulo de elasticidade e elongação máxima próximos ao da artéria femoral, enquanto que S4,6 apresentou-se como um material quebradiço. Os ensaios de crescimento celular utilizando fibroblastos, um modelo de tecido conjuntivo (linhagem 3T3-L1) e células endoteliais, um modelo de tecido arterial e venoso (HUVECs) comprovaram a excelente adesão e proliferação celular nos suportes celulares. S8,2 apresentou-se como o melhor material frente às células HUVECS, enquanto que S4,6 foi o melhor material frente às células 3T3-L1. Propõe-se a utilização de S8,2 como um biomaterial para enxertia vascular e S4,6 como material de recobrimento de próteses já utilizadas. / Scaffolds obtained by electrospinning for cellular growth are of interest for materials engineering, especially considering its structure in the form of a three-dimensional fiber mesh of nanometric diameter. This special architecture allows the generation of larger surface areas and higher porosity structures, and also important characteristics for the adhesion, proliferation and infiltration of cells into the scaffold. The use of an electrospun scaffold as a vascular graft additionally requires excellent mechanical properties, associated with a high biocompatibility level. In this study we demonstrate that these properties can be achieved by means of electrospinning of PET and collagen co-solution producing a hybrid material, considering that PET possesses excellent mechanical properties and that collagen is the principal component of the extracellular matrix. The production of electrospun scaffolds of PET/collagen is shown to be possible using HFIP and HFIP/TFA 7:2 as solvents. However, in this last one, the collagen is completely degraded during the solubilization process. If the electrospinning parameters are maintained constant, the morphology of the mesh obtained was found to be dependent on the ratio of PET/collagen (w/w), total concentration of the solution and solvent employed. Materials were obtained with unimodal and bimodal diameter distribution, as well as material in the form of ribbons and mesh between the fibers. In addition, PET and collagen form a mesh of complex composition, in which fibers composed by pure and blended materials were found. The materials PET/collagen 80:20 (S8,2) and PET/collagen 40:60 (S4,6) were characterized chemically, mechanically and biologically. It was observed that, for spincoated films, these materials present a surface energy closer to that of collagen, explaining the better cellular adhesion in S8,2 e S4,6 than for PET. S8,2 presents very similar elasticity and elongation modulus values to the femoral artery, while S4,6 is a brittle material. The cellular growth experiments using fibroblasts as a model of conjunctive tissue (3T3-L1) and endothelial cells as a model of arterial and venous tissue (HUVEC) proved the excellent adhesion and cellular proliferation on the cellular PET/collagen scaffolds. S8,2 was shown to be the best material considering HUVEC cells, while S4,6 was the best material considering 3T3-L1 cells. According to the results obtained, the use of S8,2 is proposed as a biomaterial for vascular grafts and S4,6 as a material for a coating for vascular grafts prostheses.

The growing incidences of coronary artery bypass graft surgeries have triggered a need to engineer a viable small diameter blood vessel substitute. An ideal tissue engineered vascular graft should mimic the microenvironment of a native blood vessel, while providing the adequate compliance post-implantation. Current vascular graft technologies lack the ability to promote vascular ECM deposition, leading to a compliance mismatch and ultimately, graft failure. Hence, in order to engineer suitable vascular grafts, this thesis describes the synthesis and characterization of novel elastin mimetic peptides, EM-19 and EM-23, capable of promoting vascular ECM deposition within a poly(ethylene glycol) diacrylate (PEG-DA) hydrogel. By combining the material properties of a synthetic and bio-inspired polymer, a suitable microenvironment for cell growth and ECM deposition can be engineered, leading to improved compliance. As such, characterization of EM-19 and EM-23 was conducted in human vascular smooth muscle cell (SMC) cultures, and the peptides self-assembled with a growing elastic matrix. After grafting the peptides onto the surface of PEG-DA hydrogels, EM-23 increased SMC adhesion by 6000% over PEG-RGDS hydrogels, which have been the gold standard of cell adhesive PEG scaffolds. Moreover, EM-23 grafted surfaces were able to promote elastin deposition that was comparable to tissue cultured polystyrene (TCPS) surface even though TCPS had roughly 4.5 times more SMCs adhered. Once translated to a 3D model, EM-23 also stimulated increased elastin deposition and improved the mechanical strength of the scaffold over time. Moreover, degradation studies suggested that EM-23 may serve as a template that not only promotes ECM deposition, but also allows ECM remodeling over time. The characterization studies in this thesis suggest that this peptide is an extremely promising candidate for improving vascular ECM deposition within a synthetic substrate, and that it may be beneficial to incorporate EM-23 within polymeric scaffolds to engineer compliant vascular grafts.

No description available.

To be successful a tissue engineered small diameter blood vessel must be non-immunogenic, non-thrombogenic, have mechanical properties similar to native vessel and be vasoactive. The vascular media is responsible for the mechanical properties and the vasoactivity of the vessel. The collagen hydrogel approach has been long used and has many advantages, but has not yet achieved the mechanical integrity needed for implantation. No collagen-based tissue engineered vascular media has been shown to be vasoactive using culture techniques required to achieve the cell numbers needed to make a vascular graft. To study collagen synthesis, two model systems were used. Cells were seeded on top of an adsorbed collagen I or fibrin layer. Alternatively the cells were encapsulated in a collagen or fibrin hydrogel. Collagen I, decorin and biglycan synthesis was affected by both matrix type and presentation. After two weeks in culture the smooth muscle cells produce more type I collagen in the collagen based hydrogels then in the fibrin hydrogels and was used for further studies. The collagen based tissue engineered vascular media produced a consistent vasoactive response between two and eight weeks of culture. The smooth muscle cells have functional endothelin, kinin, adrenergic, serotonergic and purinergic receptors. The application of cyclic strain improves both the tissue strength and the contractile response. Use of transforming growth factor-β improved tissue strength, but reduced the contractile response. Transforming growth factor- β actually promoted a more contractile cell phenotype, but a stronger contractile force was required to overcome the thick compact collagen hydrogel and elicit a measurable contraction. This work adds to what is known about collagen-based tissue engineered vascular medias by identifying means of improving not only strength but vasoactivity. The trade-offs found between these two important characteristics are relevant to all tissue engineered medias.

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on July 9, 2009. Major professor: Kristen O'Halloran Cardinal, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Engineering, with a Specialization in Biomedical Engineering." "April 2009." Includes bibliographical references (p. 114-121). Also available on microfiche.

Thesis (doctoral)--Lund University, 1993. / Added t.p. with thesis statement inserted.