As transplantation waiting lists lengthen because of the shortage of donor organs, the death rates of patients continue to rise. Xenotransplantation offers the potential to solve the problem of organ shortage br providing an unlimited supply of healthy donor organs. However, there are several barriers to xenotransplantation, including graft rejection, potential xenozoonosis, physiologic incompatibilities and ethical concerns. Experimental xenotransplantation studies continue in several areas, ranging from tissue to whole- organ grafting. Clinical studies continue in the area of tissue xenotransplantation. Trials with extracorporeal xenografts in an acute setting to support fulminant organ failure are likely to begin in the near future. The reintroduction of whole-organ xenotransplantation must be based on sound scientific analysis with broad societal input so as to offer the maximal benefit to transplant recipients and their families. (+info)
Long-term results of artificial anal sphincter implantation for severe anal incontinence.
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OBJECTIVE: To evaluate the long-term results of implantation of an artificial anal sphincter (AAS) for severe anal incontinence. SUMMARY BACKGROUND DATA: Implantation of an AAS is one of the options for treatment of anal incontinence when standard operations have failed. It is the only surgical option for treatment of anal incontinence in patients with neurologic disease that affects the pelvic floor and the muscles of the lower limb. METHODS: Seventeen patients underwent implantation of an AAS before 1993. These patients have been followed and their continence status evaluated. RESULTS: Two patients died of unrelated causes within the first 3 years after surgery, and in three patients the AAS was explanted because of infection. During the follow-up period, four patients had the AAS removed because of malfunction, and eight patients had a functioning AAS > or =5 years after the primary implantation. Five of these patients had revisional procedures, mainly because of technical problems in the early part of the study, when a urinary sphincter or slightly modified urinary sphincter was used. Continence at follow-up was good in four patients and acceptable in three, whereas one patient still had occasional leakage of solid stool. One patient had rectal emptying problems, which she managed by enema. CONCLUSIONS: An AAS based on the same principles as the artificial urinary sphincter seems to be a valid alternative in selected patients when standard surgical procedures have failed or are unsuitable. Approximately half of the patients have an adequate long-term result. Infectious complications still present a problem, whereas mechanical problems are less frequent with the modification of the device now available. (+info)
A cell-based constitutive relation for bio-artificial tissues.
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By using a combination of continuum and statistical mechanics we derive an integral constitutive relation for bio-artificial tissue models consisting of a monodisperse population of cells in a uniform collagenous matrix. This constitutive relation quantitatively models the dependence of tissue stress on deformation history, and makes explicit the separate contribution of cells and matrix to the mechanical behavior of the composite tissue. Thus microscopic cell mechanical properties can be deduced via this theory from measurements of macroscopic tissue properties. A central feature of the constitutive relation is the appearance of "anisotropy tensors" that embody the effects of cell orientation on tissue mechanics. The theory assumes that the tissues are stable over the observation time, and does not in its present form allow for cell migration, reorientation, or internal remodeling. We have compared the predictions of the theory to uniaxial relaxation tests on fibroblast-populated collagen matrices (FPMs) and find that the experimental results generally support the theory and yield values of fibroblast contractile force and stiffness roughly an order of magnitude smaller than, and viscosity comparable to, the corresponding properties of active skeletal muscle. The method used here to derive the tissue constitutive equation permits more sophisticated cell models to be used in developing more accurate representations of tissue properties. (+info)
Tissue engineering a blood vessel substitute: the role of biomechanics.
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The engineering of a functional blood vessel substitute has for a quarter of a century been a "holy grail" within the cardiovascular research community. Such a substitute must exhibit long term patency, and the critical issues in this area in many ways are influenced by biomechanics. One of the requirements is that it must be non-thrombogenic, which requires an "endothelial-like" inner lining. It also must have mechanical strength, i.e. a burst pressure, sufficient to operate at arterial pressures. Ideally, however, it must be more than this. It also must have viscoelastic properties that match those of the native vessel being replaced. Finally, if it is to be able to adapt to changing blood flow conditions, it must exhibit vasoactivity, a function which in and of itself can be viewed as biomechanical in nature. To achieve this requires having, as part of the construct, vascular smooth muscle cells, which are contractile in nature and oriented in a circumferential direction. Only if an engineered blood vessel substitute possesses all of these functional characteristics, can one say that the functionality exhibited by a native vessel is being mimicked. (+info)
Artificial cornea: towards a synthetic onlay for correction of refractive error.
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Synthetic onlays that are implanted onto the surface of the cornea have the potential to become an alternative to spectacles and contact lenses for the correction of refractive error. A successful corneal onlay is dependent on development of a biocompatible polymer material that will maintain a healthy cornea after implantation and that will promote growth of corneal epithelial cells over the onlay, and development of a method for attachment of the onlay with minimal surgical invasiveness. The ideal onlay should be made of a material that is highly permeable yet has sufficient surface characteristics to stimulate stable and firm attachment of the corneal epithelium over the onlay. Recent research indicates that collagen I coated polymer materials that mimic the basement membrane of the corneal epithelium promote the most favorable growth of epithelial cells in vivo in comparison to wholly biological or synthetic materials. (+info)
Chemical sensor based on nonlinearity: principle and application.
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Novel chemical sensors based on a time-dependent nonlinear response are reviewed. The strategy is to artificially mimic information transduction in living organisms. In taste and olfaction, information of chemical structure and concentration is transformed into nervous impulses in the nervous cell, i.e., time-dependent multi-dimensional information. Because the excitation and pulse generation in the nervous cell are typically nonlinear phenomena, it may be worthwhile to utilize the nonlinearity as the multi-dimensional information for molecular recognition. The principle of a "nonlinear" sensor is that a sinusoidal modulation is applied to a system, and the output signal is analyzed. The output signal of the sensor is characteristically deformed from the sinusoidal input depending on the chemical structure and concentration of the chemical stimuli. The characteristic nonlinear responses to chemical stimuli are discussed in relation to the kinetics of chemical compounds on the sensor surface. As a practical application, we introduced electrochemical sensors based on the differential capacitance, semiconductor gas sensors under the application of sinusoidal temperature or diffusion change, and a chemical sensor based on the spatio-temporal information. We demonstrated that mutli-dimensional information based on nonlinearity can provide quite useful information for the analysis of chemical species, even in the presence of another analyte or an interference with a single detector. (+info)
Midkine, a heparin-binding growth factor, promotes growth and glycosaminoglycan synthesis of endothelial cells through its action on smooth muscle cells in an artificial blood vessel model.
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To study the interactions between smooth muscle cells and endothelial cells in vitro, we developed an artificial blood vessel model, which consisted of collagen gel containing human aortic smooth muscle cells and human umbilical vein endothelial cells grown on the gel. The blood vessel model was utilized to investigate the role of midkine, a heparin-binding growth factor, in the intercellular interactions that are important in angiogenesis. In the blood vessel model, midkine induced stratification of the endothelial cells and increased their proliferation and glycosaminoglycan synthesis. However, midkine had no effect on the smooth muscle cells or endothelial cells when they were cultured separately. Increased proliferation of the endothelial cells was also attained by coculturing them with smooth muscle cells in the presence of midkine or culturing endothelial cells with the conditioned medium of the smooth muscle cells, which had been treated with midkine. These experiments indicate that the target of midkine was smooth muscle cells, which secreted factor(s) acting on the endothelial cells. We identified interleukin-8 as one such factor; the synthesis of interleukin-8 by the smooth muscle cells was increased by exposure to midkine, and anti-interleukin-8 inhibited the midkine action. Furthermore, interleukin-8 caused stratification of the endothelial cells in the blood vessel model. These results provided evidence that midkine is one of the factors involved in epithelial-mesenchymal interactions. (+info)