The Experts below are selected from a list of 66 Experts worldwide ranked by ideXlab platform

Donald J Lyman - One of the best experts on this subject based on the ideXlab platform.

  • effects of a vascular graft Natural Artery compliance mismatch on pulsatile flow
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J Lyman
    Abstract:

    Abstract Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host Artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or stagnant zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

  • Effects of a vascular graft/Natural Artery compliance mismatch on pulsatile flow.
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J Lyman
    Abstract:

    Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host Artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or stagnant zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

Sandy F C Stewart - One of the best experts on this subject based on the ideXlab platform.

  • effects of a vascular graft Natural Artery compliance mismatch on pulsatile flow
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J Lyman
    Abstract:

    Abstract Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host Artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or stagnant zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

  • Effects of a vascular graft/Natural Artery compliance mismatch on pulsatile flow.
    Journal of Biomechanics, 1992
    Co-Authors: Sandy F C Stewart, Donald J Lyman
    Abstract:

    Attempts have been made to correlate small-diameter vascular graft patency with compliance matching between the graft and the host Artery. Without knowledge about the mechanisms of failure by compliance mismatch, however, such correlations remain empirical. We have developed a flow system which mimics the flow in peripheral arteries and techniques to model a compliance mismatch in a straight elastic tube, as might occur with vascular repair. Our goal was to investigate one proposed mechanism of graft failure by compliance mismatch, that of a blood flow disturbance. Flow visualization experiments showed that, under pulsatile flow, a compliance mismatch caused trapping of 40 μm microspheres at the wall near the distal or downstream anastomosis. This suggests that the presence of a microscopic flow separation or stagnant zone in vivo may contribute to the intimal hyperplasia and thrombosis seen in failed grafts.

Takehisa Matsuda - One of the best experts on this subject based on the ideXlab platform.

  • A hybrid vascular model biomimicking the hierarchic structure of arterial wall: neointimal stability and neoarterial regeneration process under arterial circulation.
    The Journal of Thoracic and Cardiovascular Surgery, 1995
    Co-Authors: Takehisa Matsuda, Hiromichi Miwa
    Abstract:

    A layered-structure hybrid vascular graft, mimicking the hierarchic structure of the intima and media of a Natural Artery, is expected to exhibit antithrombogenicity and to accelerate neoarterial tissue formation. Two models of hybrid vascular grafts were prepared on knitted Dacron fabric grafts (inner diameter 4 mm, length 6 cm). Model I grafts consisted of an endothelial cell monolayer formed on collagenous matrix, and model II grafts consisted of an endothelial cell monolayer that formed on hybrid collagenous medial tissue in which smooth muscle cells were incorporated. Both models ( n = 17 for each model) were implanted bilaterally in carotid arteries and left in place for up to 26 weeks. Although all of the implanted grafts were patent, the two models significantly differed in the degree of maturity of the regenerated neoarterial wall especially at earlier implantation periods. At 2 weeks, model II grafts showed a much higher degree of neointimal integrity than model I grafts: a smooth and organized neointimal layer was formed, which was free from leukocyte adhesion. On further increase of implantation time, the formation of neomedia (subendothelial smooth muscle cell layers) and circumferential orientation of both smooth muscle cells and collagenous extracellular matrix were much more advanced in model II grafts than in model I grafts. At 26 weeks after implantation, layered elastic laminae regenerated along circumferentially oriented smooth muscle cells in neomedia were observed only in model II grafts. Irrespective of model, little excessive smooth muscle cell proliferaion occurred. A hierarchically structured hybrid graft eventually provided a more integrated neointimal layer and accelerated neoarterial tissue formation much more than model I grafts. The significance of the incorporation of smooth muscle cells into hybrid grafts is discussed.

  • An integrated approach to the design and engineering of hybrid arterial prostheses
    Journal of Vascular Surgery, 1994
    Co-Authors: Hiromichi Miwa, Takehisa Matsuda
    Abstract:

    Abstract Purpose: A newly devised hybrid small-caliber graft was developed. The graft consisted of three components: a microporous polyurethane graft (inside diameter 3 mm; length 5 cm) with compliance close to that of a Natural Artery; an artificial basement membrane composed of a complex gel of type I collagen and dermatan sulfate, which showed enhanced adhesion and growth of endothelial cells (ECs) and reduced adhesion of platelets in vitro; and an autogenous EC monolayer with high degrees of cell-substrate and cell-cell interactions, which was performed before implantation. Methods: Twenty EC-seeded grafts were implanted bilaterally into carotid arteries of dogs without anticoagulant. The implantation period was up to 26 weeks. Results: The overall patency rate for seeded grafts was 75%. The percentage of endothelial coverage of seeded grafts was 98% as implanted, 92% at 2 weeks, and 100% after 12 weeks. The mean intimal thickness of grafts was around 80 μm at 12 weeks. Little additional increase was observed at 26 weeks. Conclusions: It appears that the complete endothelialization as implanted, high cell-to-substrate adhesive strength that resists hydrodynamic shear stress, and biomechanical compatibility of the polyurethane graft functioned cooperatively to provide a vascular graft with high antithrombogenicity and minimal hyperplasia. The integrated approach of combining biomechanical and cellular engineering designs leading to an important functional smaller-caliber graft is discussed. (J VASC SURG 1994;19:658-67.)

  • Development of a hierarchically structured hybrid vascular graft biomimicking Natural arteries.
    Asaio Journal, 1993
    Co-Authors: Hlromichi Miwa, Takehisa Matsuda, Futoshi Iida
    Abstract:

    : A hierarchically structured hybrid vascular graft, resembling the layered structure of the intima and the media of a Natural Artery, is expected to exhibit structural stability and function similar to those of an Artery. Two models of hybrid vascular grafts were constructed on knitted Dacron grafts (internal diameter, 4 mm; length, 6 cm; Golaski Laboratories, Inc., Philadelphia, PA) in vitro. The model I graft consisted of only an endothelial cell (EC) monolayer, and the model II graft was hierarchically structured with an EC monolayer and smooth muscle cell (SMC) multilayers embedded in a mixed gel of type I collagen and dermatan sulfate. Both models were implanted bilaterally in carotid arteries of 11 dogs for up to 12 weeks without anticoagulant. All grafts were patent after the implantation. There was a marked difference in the formation of the neomedia between the two models at 2 weeks: only a collagenous layer was observed beneath the EC monolayer in model I grafts, whereas the reconstructed neomedia consisting of proliferating SMCs and extracellular matrix was observed in model II grafts. In model I grafts at 2 weeks, the luminal surface with a wavelike form, along with crimps of the vascular graft, was almost endothelialized, but EC orientation was disorganized at the troughs of the crimps. Leukocyte adhesion on ECs and migration into their interstices were observed in some areas. Such events were infrequent in model II grafts at 2 weeks. Model II grafts at 12 weeks had an integrated neomedia comparable to that of a Natural Artery: SMCs exhibited dense accumulation and circumferential orientation.(ABSTRACT TRUNCATED AT 250 WORDS)

Hiromichi Miwa - One of the best experts on this subject based on the ideXlab platform.

  • A hybrid vascular model biomimicking the hierarchic structure of arterial wall: neointimal stability and neoarterial regeneration process under arterial circulation.
    The Journal of Thoracic and Cardiovascular Surgery, 1995
    Co-Authors: Takehisa Matsuda, Hiromichi Miwa
    Abstract:

    A layered-structure hybrid vascular graft, mimicking the hierarchic structure of the intima and media of a Natural Artery, is expected to exhibit antithrombogenicity and to accelerate neoarterial tissue formation. Two models of hybrid vascular grafts were prepared on knitted Dacron fabric grafts (inner diameter 4 mm, length 6 cm). Model I grafts consisted of an endothelial cell monolayer formed on collagenous matrix, and model II grafts consisted of an endothelial cell monolayer that formed on hybrid collagenous medial tissue in which smooth muscle cells were incorporated. Both models ( n = 17 for each model) were implanted bilaterally in carotid arteries and left in place for up to 26 weeks. Although all of the implanted grafts were patent, the two models significantly differed in the degree of maturity of the regenerated neoarterial wall especially at earlier implantation periods. At 2 weeks, model II grafts showed a much higher degree of neointimal integrity than model I grafts: a smooth and organized neointimal layer was formed, which was free from leukocyte adhesion. On further increase of implantation time, the formation of neomedia (subendothelial smooth muscle cell layers) and circumferential orientation of both smooth muscle cells and collagenous extracellular matrix were much more advanced in model II grafts than in model I grafts. At 26 weeks after implantation, layered elastic laminae regenerated along circumferentially oriented smooth muscle cells in neomedia were observed only in model II grafts. Irrespective of model, little excessive smooth muscle cell proliferaion occurred. A hierarchically structured hybrid graft eventually provided a more integrated neointimal layer and accelerated neoarterial tissue formation much more than model I grafts. The significance of the incorporation of smooth muscle cells into hybrid grafts is discussed.

  • An integrated approach to the design and engineering of hybrid arterial prostheses
    Journal of Vascular Surgery, 1994
    Co-Authors: Hiromichi Miwa, Takehisa Matsuda
    Abstract:

    Abstract Purpose: A newly devised hybrid small-caliber graft was developed. The graft consisted of three components: a microporous polyurethane graft (inside diameter 3 mm; length 5 cm) with compliance close to that of a Natural Artery; an artificial basement membrane composed of a complex gel of type I collagen and dermatan sulfate, which showed enhanced adhesion and growth of endothelial cells (ECs) and reduced adhesion of platelets in vitro; and an autogenous EC monolayer with high degrees of cell-substrate and cell-cell interactions, which was performed before implantation. Methods: Twenty EC-seeded grafts were implanted bilaterally into carotid arteries of dogs without anticoagulant. The implantation period was up to 26 weeks. Results: The overall patency rate for seeded grafts was 75%. The percentage of endothelial coverage of seeded grafts was 98% as implanted, 92% at 2 weeks, and 100% after 12 weeks. The mean intimal thickness of grafts was around 80 μm at 12 weeks. Little additional increase was observed at 26 weeks. Conclusions: It appears that the complete endothelialization as implanted, high cell-to-substrate adhesive strength that resists hydrodynamic shear stress, and biomechanical compatibility of the polyurethane graft functioned cooperatively to provide a vascular graft with high antithrombogenicity and minimal hyperplasia. The integrated approach of combining biomechanical and cellular engineering designs leading to an important functional smaller-caliber graft is discussed. (J VASC SURG 1994;19:658-67.)

V.g. Hart - One of the best experts on this subject based on the ideXlab platform.

  • EFFECTS OF INITIAL DEFORMATION IN THIN TUBES ON WAVE REFLECTION AND TRANSMISSION AT MECHANICAL MISMATCH
    Journal of Sound and Vibration, 1995
    Co-Authors: V.g. Hart
    Abstract:

    The aim of this paper is to investigate the reflection and transmission of deformations and stress waves in two initially deformed thin dissimilar orthotropic elastic tubes joined longitudinally and containing pulsatile fluid flow. In the undeformed state the tubes have the same radius. In the initially deformed state, the area of the cross-section around the joint varies with axial distance, but far upstream and downstream of the joint the cross-sections are essentially uniform. The segment of joined tube with varying cross-section is approximated as a discontinuous change of the cross-sectional area when the wavelength is long enough compared to the diameter of the tubes. Then superposition of two Womersley solutions for a single frequency is used in each tube, and continuity conditions for both the fluid and the tubes are used. The effects of the initial stretch ratios on reflection and transmission are illustrated for a blood pulse in a Natural Artery joined with an artificial graft. Numerical calculations for the amplitudes of reflected and transmitted waves at the joint are made, and also for the corresponding energy flow rates. It appears that only the faster of the two incident waves considered leads to a significant mixing of wave types at the joint. The theoretical approach is justified since it would be extremely difficult to find the stresses at the joint experimentally.

  • Wave propagation in joined thin dissimilar elastic tubes containing viscous fluid
    International Journal of Engineering Science, 1994
    Co-Authors: V.g. Hart
    Abstract:

    The aim of the present paper is to investigate theoretically the reflection and transmission of the deformations and stress waves in thin dissimilar eleastic tubes joined longitudinally and containing pulsatile fluid flow. With the long wavelength approximation, there exist two waves (with different velocities) corresponding to a single frequency. Superimposition of the two waves for each tube produces two reflected and two transmitted waves each with constant amplitudes which are determined from four dominant conditions of the six conditions at the joint cross-section. Rate of energy transfer in the system is also formulated. Although the application of the theory is two-fold: to engineering structures where a segment of high strength pipe may be inserted in a weaker tube for reasons of corrosion resistance for example, and the arterial system when an artificial implant is interposed or a contraction or stenose forms, the numerical examples are given for the latter. Effects of frequency on the reflection and transmission are illustrated for blood pulse in a Natural Artery joined with an artificial graft. Our numerical results correspond here to zero initial stresses. It is found that there is no discontinuity in the circumferential tension at a joint. The theoretical approach is justified since it would be extremely difficult to find the stresses at the joint experimentally.