Thrombosis and restenosis are common problems associated with intravascular procedures such as anastomoses, balloon angioplasty, and carotid endarterectomies. Application of a molecular barrier at the site of injury to inhibit platelet deposition would be advantageous. Additional therapeutic benefit could be achieved if the modified surface provided a target for delivery of pharmaceuticals, vectors, or cells. This dissertation focuses on the development of an intravascular modification and targeted delivery system that possesses numerous potential applications in the treatment of vascular injury. Polyethylene glycol is commonly used for modification of molecules and surfaces to increase biocompatibility, reduce immunogenicity, and provide stealth characteristics. Protein-reactive polyethylene glycols could be used to modify vascular surfaces forming a molecular barrier. In addition, the polymer could be used as a target for delivery of agents by applying a recognizable tag to the terminus. Agents could be targeted to modified vascular tissue using, for instance, the biotin/avidin recognition system. The ability to modify vascular surfaces with protein-reactive polyethylene glycols was confirmed using quantitative flow cytometry and epi-fluorescence microscopy. Furthermore, in vitro perfusion studies with cultured cells and scrape-damaged arteries demonstrated preferential delivery of microspheres and cells to polyethylene glycol-biotin modified vascular surfaces. An in vivo rabbit model provided a more rigorous assessment of the polymer modification and targeted delivery system. Polymer modification of balloon injured rabbit femoral arteries persisted for a minimum of 72 hours. Targeted microspheres preferentially adhered to healthy and injured arteries modified with the reactive polymer as opposed to untreated controls. Furthermore, the ability to target microspheres to the modified arteries persisted for a minimum of 72 hours. In conclusion, it was shown that it is possible to modify vascular tissue with a protein-reactive polyethylene glycol and that this modification with signaling molecules can also provide a target for the site-specific delivery of vascular-infused agents. An intravascular targeted delivery system such as this might find numerous applications in the treatment of intravascular injury that is associated with angioplasty, stenting, and endarterectomy procedures.

Falls precipitated by slipping are a major cause of injury, death and disability in the elderly. This research focused on muscle activation patterns generated in response to slipping and anticipation of slippery surfaces. The goal was to identify the muscle activation patterns of the stance leg in response to an unexpected slip (reactive strategies) and investigate muscle activity when anticipating slippery floors during gait on dry surfaces (proactive strategies). Additionally, age-related differences were examined. Electromyographic recordings were made from the Vastus Lateralis, Medial Hamstring, Tibialis Anterior and Medial Gastrocnemius of eleven young and nine older adults. Participants walked during the following conditions: (1) baseline dry (subjects knew the floor was dry); (2) unexpected slip (contaminant was applied to floor without subjects knowledge); (3) alert dry (subjects were uncertain of the floors condition). Reactive strategies, which were similar among young and older adults, consisted of activation of the Medial Hamstring at around 21% stance (~ 175 ms) followed by the Vastus Lateralis at around 29% stance (~ 240 ms). Corrective responses were scaled to slip severity with more severe slip reactions consisting of longer, higher magnitude responses. Delayed Vastus Lateralis latency and Medial Hamstring cessation were associated with an increased slip severity as quantified by peak slip velocity. Additionally, when experiencing a severe slip, young adults demonstrated a longer, more powerful response compared to older adults. Anticipation of a slippery surface resulted in increased magnitude of activation (48% increase) and ankle/knee co-contraction (30% increase), as well as earlier onsets and longer durations of posterior muscles. Young adults demonstrated earlier onsets (3% stance, 24 ms) and longer durations (10% stance, 83 ms) than older adults reducing their slip potential. Finally, adults with baseline gait on dry floors characterized by greater ankle co-contraction at heel contact and delayed Tibialis Anterior onset were predisposed to experience less severe slips when encountering an unexpected slippery floor. Older adults natural gait predisposes them to experience a less hazardous slip. However, once a slip occurs, older adults cannot react with the long, powerful response needed to prevent balance loss whereas young adults are capable of this response.

Abdominal aortic aneurysm (AAA), a localized dilation of the infrarenal aorta, represents a significant disease in the western population. There are approximately 200,000 patients in the US and 500,000 patients worldwide diagnosed with AAAs every year, and rupture of AAAs currently ranks as the 13th leading cause of death in the US. The formation of aneurysm within the abdominal aorta presents a unique clinical dilemma, requiring surgeons to offer intervention when the risks of rupture outweigh those associated with the repairing the AAA. The golden standard for quantitatively assessing a AAAs risk of rupture is the maximum transverse diameter with intervention typically recommended at a diameter of 5.5cm. This criterion, however, is not based on the sound physical properties governing the mechanical failure of the AAA wall the stresses acting on the wall and the ability to withstand those stresses (its strength). The current work describes the continued improvement of a rupture potential index (RPI) which is defined as the ratio of local wall stress and strength. The effect of mechanical anisotropy on the constitutive modeling and finite element analyses of AAA has been neglected in the literature. In order to address the assumption of isotropy, planar biaxial tensile testing was performed on AAA wall and intraluminal thrombus (ILT) tissue excised from patients undergoing elective open repair of their AAA. The peak stretch values and maximum tangential moduli for AAA versus nonaneurysmal tissue indicate a preferential circumferential stiffening of the abdominal aorta in the presence of aneurysm. It was concluded that aneurysmal degeneration of the abdominal aorta is associated with an increase in mechanical anisotropy, with preferential stiffening in the circumferential direction. This anisotropy was modeled using an exponential strain energy function which was able to minimize the covariance between model parameters. Implementation of the this relation into a commercially available finite element code (ABAQUS) resulted in a more realistic estimation of in-vivo wall stress. There was a significant increase in peak wall stress in AAAs utilizing the anisotropic constitutive relation versus those using the previously derived isotropic relation (38.30 ± 3.04, 36.06 ± 2.73, p<0.001). This result was not universal, however, indicating the presence of anisotropy on peak wall stress may be patient-specific. Previous work in our laboratory resulted in an initial statistical model for noninvasively estimating AAA wall strength. This model has currently been improved with several notable enhancements some of which include a larger construction data set and a CT-based method of local diameter measurement. This model contains four, non-invasively measurable predictors: the square root of local ILT thickness, normalized local diameter, patients sex, and the patients family history of AAA. The noninvasive statistical model for predicting AAA wall strength derived here predicted a statistically weaker wall for ruptured AAAs than for non-ruptured AAAs (119.41 ± 4.48 and 137.06 ± 1.49 N/cm2, p=0.02). In fact, the current model performed better than either the previously derived AAA wall strength model or the clinically utilized maximum cross sectional diameter in identifying ruptured AAAs. The currently developed rupture potential index resulted in an increased peak value of RPI for a set of electively repaired AAAs in comparison to the previously developed RPI technique (0.34 ± 0.03 vs. 0.22 ± 0.03, p < 0.001). In addition, comparisons of peak RPI values for ruptured and non-ruptured AAAs suggest an improvement in rupture prediction utilizing the current methodology (p=0.10) as opposed to the previously developed RPI (p=0.79) as well as the maximum diameter criterion (p=0.17). The locally acting AAA wall stress divided by the local AAA wall strength, termed the rupture potential index, has been introduced as an alternative to the maximum diameter criterion for AAA rupture assessment. The clinical relevance of this method for rupture assessment has yet to be validated, however its success will undoubtedly aid surgeons in clinical decision making and AAA patient management.

The National Safety Council listed falls as the third ranked cause (14.6%) of unintentional deaths in the general population of the US. It is postulated that an attempt to control the COM is employed to prevent falls during perturbed gait. The goal of this research was to gain an understanding of (1) the relationship between COM dynamics at slip initiation and slip severity, and (2) how individuals control their COM dynamics when warned about the possibility of slipping (anticipatory control). The dynamics of the bodys COM during slips may reveal insights into the biomechanical reasons behind the high prevalence of slip-precipitated falls in the elderly. The findings may also be helpful in differentiating between postural strategies that successfully recover balance and responses that result in falls. Sixteen healthy young (20-35 yrs) and 11 older (55-70 yrs) subjects were exposed to an unexpected slip (no prior knowledge of the floors contaminant condition), and alert slip (warned of the potential contamination), and known slip after two baseline walking trials. Body motion from 79 VICON markers attached to the body was sampled at 120 Hz. Segmental mass was generated using a segmental analysis. For an unexpected slip, maintaining the COM closer to the leading leg, an elevated COM position and fast medial-lateral COM transfers to the slipping leg at heel contact were associated with an increase in slip severity. For anticipation conditions (alert and known), COM placement and velocity was geared toward continuing the gait cycle. Age was significant in regards to COM position variables.

Clinicians have a limited amount of time for performing computer access and augmentative and alternative communication (AAC) assessments. In addition, they do not have access to all of the computer access devices that could potentially be useful for each client. An accurate modeling technique would help clinicians to identify the most appropriate kind of devices and device configurations for their clients. It would also be able to provide accurate prediction of performance, learning and fatigue. Investigators are using word prediction (WP) as a test-bed for user modeling techniques. The goal is to develop accurate models that will form the basis for clinical assessment tools. The focus of this research was to observe users interaction with WP in great detail, in preparation for future studies and developing a model.

Independent mobility is critical to individuals of any age. While the needs of many individuals with disabilities can be satisfied with power wheelchairs, some members of the disabled community find it difficult or impossible to operate a standard power wheelchair. This population includes, but is not limited to, individuals with low vision, visual field neglect, spasticity, tremors, or cognitive deficits. To meet the needs of this population, our group is involved in developing cost effective modularly designed Smart Wheelchairs. Our objective is to develop an assistive navigation system which will seamlessly integrate into the lifestyle of individual with disabilities and provide safe and independent mobility and navigation without imposing an excessive physical or cognitive load. The Smart Wheelchair Component System (SWCS) can be added to a variety of commercial power wheelchairs with minimal modification to provide navigation assistance. Previous versions of the SWCS used acoustic and infrared rangefinders to identify and avoid obstacles, but these sensors do not lend themselves to many desirable higher-level behaviors. To achieve these higher level behaviors we integrated a Continuously Adapted Mean Shift (CAMSHIFT) target tracking algorithm into the SWCS, along with the Minimal Vector Field Histogram (MVFH) obstacle avoidance algorithm. The target tracking algorithm provides the basis for two distinct operating modes: (1) a follow-the-leader mode, and (2) a move to stationary target mode. The ability to track a stationary or moving target will make smart wheelchairs more useful as a mobility aid, and is also expected to be useful for wheeled mobility training and evaluation. In addition to wheelchair users, the caregivers, clinicians, and transporters who provide assistance to wheelchair users will also realize beneficial effects of providing safe and independent mobility to wheelchair users which will reduce the level of assistance needed by wheelchair users.

Because current treatments have had limited success in reducing morbidity and mortality associated with heart failure, the transplantation of cells into the heart has emerged as a potential therapy to repair damaged myocardium and reverse end-stage heart failure. While an array of lineage-committed cell types has been evaluated for experimental cardiac cell transplantation, recent studies have focused on adult stem cells due to their capacity for self-renewal and potential for multilineage differentiation. Here we investigated the application of postnatal murine skeletal musclederived stem cells (MDSCs) for cardiac cell therapy. We initially tested the ability of MDSCs to regenerate cardiac muscle after intramyocardial injection into the hearts of dystrophin-deficient mdx mice, a model of cardiomyopathy and muscular dystrophy. After transplantation, we observed that MDSCs generated large persistent grafts consisting primarily of numerous skeletal muscle myocytes and, to a substantially lesser degree, donor-derived cardiomyocytes, which were primarily located at the grafthost myocardium border. Further experiments revealed that more than half of these donor-derived cardiomyocytes resulted from the fusion of transplanted MDSCs with host cardiomyocytes. Next, we investigated the therapeutic potential of MDSC transplantation for cardiac repair using a mouse model for acute myocardial infarction. We report that in comparison with committed skeletal myoblast and control saline-injected hearts, MDSCs implanted into infarcted hearts elicited significant improvements in cardiac performance. This beneficial effect was partially attributed to the ability of MDSCs to induce neovascularization of ischemic myocardium. In the final study, we investigated the mechanism by which transplanted MDSCs contribute to revascularization of ischemic myocardium. To address this issue, we employed a gain- and loss-of-function approach using MDSCs genetically engineered to express the potent angiogenic factor vascular endothelial growth factor (VEGF) or the anti-angiogenic factor soluble Flt1, a VEGF-specific antagonist. When we transplanted MDSCs expressing soluble Flt1, we observed significantly less neoangiogenesis and a significant decrease in cardiac function when compared to the transplantation of control MDSCs and VEGF-engineered MDSCs. These results suggest that the transplantation of MDSCs elicits improvements in cardiac performance by inducing neovascularization of ischemic myocardium through the secretion of VEGF. In conclusion, these results suggest that MDSCs represent a promising cell type for cardiac repair and further translational research is warranted.

Abdominal aortic aneurysm (AAA) is a localized dilation of the infrarenal aorta. Ruptured AAA has a mortality rate of 95% and is ranked as the 13th leading cause of death in the US. The ability to reliably evaluate the susceptibility of a particular AAA to rupture could vastly improve the clinical management of AAA patients. Currently, no such reliable evaluation technique exists. The purpose of this work was to develop a noninvasive technique to evaluate the rupture potential of individual AAA. To predict the wall strength distribution, experimentally determined wall strength data were used for construction of a mathematical model using multiple linear regression techniques. The developed model was then validated using data from a different group of specimens. The strength distributions for four different AAA were then generated using the validated model. The finite element method was used to estimate the wall stress distribution for all four AAA based on their realistic geometries (reconstructed from CT images) which included intraluminal thrombus (ILT). The measured systolic blood pressure was applied as the loading condition. Nonlinear hyperelastic constitutive models for AAA and ILT tissue were used, the latter being developed here based on uniaxial tensile testing data. For each patient, a local Rupture Potential Index (RPI) distribution was calculated as local (nodal) wall stress divided by local wall strength. The developed model contains four independent variable parameters: AAA size, patients age, family history, local ILT thickness, and normalized local AAA diameter (R Squared = 0.86, p = 0.001). The model predicted the actual (measured) strength very accurately (R Squared = 0.81 for model validation). The wall strength values predicted for the four AAA studied ranged from 130 to 306 N/(cm squared), whereas the measured wall strength values ranged from 39 to 324 N/(cm squared). The peak wall stress for the four AAA studied ranged from 19 to 37 N/(cm squared). The peak RPI values ranged from 0.15 to 0.55. This patient-specific, computer-based, noninvasive RPI estimation technique could become an import and reliable diagnostic tool for AAA patient management. However, further clinical studies are needed to validate this technique.

A diseased artery often becomes blocked, compromising blood flow to downstream tissues and organs. One common surgical intervention is to bypass this blocked region with a vascular graft. However, these grafts can fail due to an overhealing response, known as intimal hyperplasia (IH), which occurs at the graft/artery junction (i.e., anastomosis). The goal of this research was to determine if a quantitative correlation exists between the hemodynamic phenomena at the distal anastomosis of a vascular bypass graft and carefully selected, acute biological precursors of intimal hyperplasia. To accomplish this task, we perfused porcine, artery-artery, end-to-side and end-to-end anastomoses ex vivo and developed computational fluid dynamics (CFD) models incorporating the reconstructed geometry and perfusion conditions present in each experimental anastomosis. The perfusion experiments allowed us to assess the levels of immediate early gene (IEG) proteins and vascular cell apoptosis at various regions along the anastomoses. Since the pressure and flow rate in the ex vivo perfusion model were precisely known, the CFD models utilized this information along with the 3D reconstructed anastomotic geometries to accurately estimate wall shear stress (WSS) and WSS gradient (WSSG) in the same regions of interest. This process allowed for a distinct and unique coupling between the perfusion experiments and the computational simulations that has not been achieved previously. Through linear and nonlinear regression analyses on this directly-coupled data, we found that low levels of WSS and WSSG cause upregulation of IEG proteins. Because increased levels of IEG expression leads to IH formation, our results suggest that low levels of WSS and WSSG correlate with increased IH formation. Our coupled experimental and computational approach has allowed us to evaluate IEG protein expression by vascular cells in response to the hemodynamics present in vascular anastomoses. Based on the correlations we found between low levels of WSS and WSSG and the subsequent increase in IEG protein expression, treatments such as graft geometry optimization or targeted gene therapy may improve the clinical success rate of vascular bypass grafts. However, some issues need to be addressed before the results of this study can be applied clinically.

Skin, tendons, and other tissues can heal, but with formation of scar tissue, characterized by altered biochemical composition, distorted tissue architecture, and decreased mechanical properties compared to the normal tissues. Excessive cellular contraction in wounds can lead to formation of scar tissue, whereas insufficient cellular contraction may impede wound closure. In addition, although both TGF-b1 and TGF-b3 have been found to increase cellular contraction, only TGF-b3 has been shown to reduce formation of scar tissue in rat skin wounds. Therefore, the overall objective of this project is to reduce the formation of scar tissue by regulating cellular contraction. As part of this objective, this thesis project studies human fibroblast contractility and the differential effect of TGF-b1 and TGF-b3 on human fibroblast contraction and collagen synthesis using in vitro models. Either human skin or tendon fibroblasts were used in this project, depending on the nature of the specific study. Human tendon fibroblasts were found to contract in vitro and the degree of contraction was dependent on serum concentration. Further, a multi-station culture force monitor (CFM) system was developed to characterize cellular contraction. Using this system, human tendon fibroblasts were found to have a significantly lower maximum contraction force and a markedly different contraction pattern than human skin fibroblasts, illustrating the ability of this system to differentiate between cells from different tissues. In addition, the effect of TGF-b1 and TGF-b3 on cellular contraction and collagen synthesis of human skin fibroblasts was studied using the CFM system. Both TGF-b1 and TGF-b3 were found to increase human fibroblast contraction and collagen synthesis, but TGF-b3 increased cellular contraction and collagen synthesis to a lesser extent than TGF-b1. As there is great interest in improving the quality of healing tissue, these studies provide a foundation to further study the cellular and molecular mechanisms of tissue wound healing. In addition, these findings suggest that TGF-b3 instead of TGF-b1 may be applied to regulate tendon fibroblast contraction, which may reduce formation of scar tissue in healing tendons. Future studies will continue to elucidate the relationship between cellular contraction and collagen synthesis.