A T Yokobori

Tohoku University, Sendai-shi, Miyagi-ken, Japan

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Publications (25)26.62 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The proposal of noninvasive diagnostic method of mechanical degradation of vascular wall is clinically useful and it will be correlated with noninvasive diagnostic method of atherosclerosis. Supersonic Doppler effect sensor has been used to measure blood flow velocity as a noninvasive measuring method. However, it is remain problem whether the output from the Doppler effect sensor really detects the pure blood flow velocity. Theoretically, when the Doppler effect sensor is set perpendicular to the blood flow direction, that is, perpendicular to the blood vessel, the output will correspond to the expansion velocity of blood vessel wall, because it detect the frequency of Doppler shifted supersonic scattered from vascular wall. Previously, on the basis of this concept, using Doppler effect sensor, we showed this method can really detect the deformation velocity of blood vessel wall and it correlates the degradation of elastic property of blood vessel. Furthermore, using this proposed measuring method, atherosclerosis is found to progress correspondingly with the visco-elastic degradation of vascular wall. In this paper, on the basis of our proposed method, the quantitative noninvasive estimation method of the degradation of vascular wall and the progressive degree of atherosclerosis by unique parameter has been proposed. Using this method, the degradation of vascular wall is correlated to the oxygen metabolic function of blood vessel corresponding to the function of oxygen transportation and progression of atherosclerosis. Furthermore, the organ correlation on the atherosclerosis between lower limb and carotid is investigated by this proposed method.
    Bio-medical materials and engineering 02/2004; 14(3):241-9.
  • C Y Jian, A T Yokobori
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    ABSTRACT: It is necessary to maintain mechanical compatibility between a blood vessel and a vascular substitute to promote encapsulation around the anastomosed part. From this point of view, using linear elastic theory, we had previously performed stress analyses at the part anastomosed by tissue adhesion, in order to propose some methods of preventing stress concentration at this junction. In this study, based on the previous analyses, we have attempted to develop a concept that can be applied under the conditions of operation. That is, the initial diameter of a vascular substitute with high rigidity is chosen larger than that of a blood vessel. This will reduce the stress concentration around the anastomosed part, on average, during expansion of the blood vessel. We analysed the optimum diameter ratio between the vascular substitute and the blood vessel which causes the least stress concentration, on average, during this process, using linear elastic theory. Furthermore, numerical analyses of blood vessel deformation were performed using various nonlinear stress-strain laws. These results were compared to the analytical solution based on linear elastic theory.
    Bio-medical materials and engineering 02/1999; 9(4):219-31.
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    ABSTRACT: In order to investigate the significance of hydroxyapatite based microporous composite (HA/mica composite) surfaces and a macroporous synthetic hydroxyapatite, rat marrow cell culture, which shows osteogenic differentiation, was carried out on six different culture substrata (two control culture dishes, two identical HA/mica composites, and two identical macroporous synthetic hydroxyapatites). A culture period of two weeks in the presence of beta-glycerophosphate (BGP), ascorbic acid, and dexamethasone resulted in abundant mineralized nodule formations that were positive for alkaline phosphatase (ALP) stain. The stain on the macroporous synthetic hydroxyapatite and the HA/mica composites were intense, the enzyme activity being about double that of control culture dishes. These data indicate that the synthetic macroporous hydroxyapatite surface and the HA/mica composite surface promotes osteoblastic differentiation.
    Bio-medical materials and engineering 02/1999; 9(1):21-6.
  • J. C. Ha, A. T. Yokobori, H. Takeda
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    ABSTRACT: Fatigue fracture of fiber-reinforced polymer composites (FRP) occurs when microcracks are induced by debonding, pull-out and delamination at the interface between the matrix and fiber. This microcrack area increases with increase in fatigue cycles and a damage region is formed. In our previous paper, fatigue life of a short fiber-reinforced polymer composite consisting of glass fibers and polycarbonate matrix was found to be related not to the main crack growth behavior but to the progression behavior of the damage region. In this paper, using our proposed real time observational system, we performed detailed observations on the behavior of fatigue damage and clarified the mechanism of damage progression. Furthermore, mechanical considerations were performed by finite-element elastic-plastic stress analysis. The results mentioned above indicate that control of short fiber alignment makes it possible to release the stress concentration caused in the matrix, and disperse fatigue damage. This results in an enormous improvement in fracture toughness.
    Journal of Materials Science 01/1999; 34(9):2103-2111. DOI:10.1023/A:1004567911562
  • M. Tabuchi, K. Kubo, K. Yagi, A.T. Yokobori, A. Fuji
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    ABSTRACT: Creep crack growth (CCG) tests on Ni-base superalloys were carried out in the Japanese VAMAS group as part of a round-robin program in order to assist the standardization of the CCG test method for creep-brittle alloys. The effect of temperature, load, specimen thickness and material microstructure on CCG behavior was investigated. The applicable range of fracture mechanical parameters to evaluate the CCG rate was evaluated. The CCG rate was characterized by the C∗ parameter independent of testing conditions in the range where the CCG rate accelerated. However, the acceleration stage occupied only a small portion of life time for the creep-brittle superalloys. In the range where the CCG rate was constant, the CCG rate and the fracture life could be predicted approximately by the Q∗ method based on the thermally activated process.
    Engineering Fracture Mechanics 01/1999; DOI:10.1016/S0013-7944(98)00082-4
  • A T Yokobori
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    ABSTRACT: In our earlier studies, we considered osteogenesis to be a nucleation mechanism accompanied by preliminary diffusion and showed the optimum mechanical conditions for promoting this ionic diffusion. In this study, we performed an analysis of the nucleation mechanism of hydroxyapatite (HA) in areas with a high concentration of transported Ca and PO4 ions in the pore region along the collagen alignment. We derived the equation of the HA nucleation rate as a function of hydrostatic tensile stress, sigma P and surface energy change (gamma-lambda), where gamma is the surface energy of the HA particle and lambda is the decreasing factor of gamma during HA nucleation. lambda is thus related to the effect of chemical and electric stimulations. Finally, we formulated a unified equation of the nucleation rate of HA, which consists of ionic diffusion and HA nucleation processes.
    Bio-medical materials and engineering 02/1998; 8(5-6):253-7.
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    ABSTRACT: By using the fracture toughness estimation method based on two-dimensional map, it was found that the ductility of the high porosity hydroxyapatite/mice composite was comparable with silicon carbide. It was measured to be higher than that of packed hydroxyapatite. Alumina ceramics with more than 96% aluminium oxide showed a higher fracture toughness than the composite material. When bending strength was compared, the strength of the composite was two or three times lower than that of packed hydroxyapatite and much lower than the other studied materials. The composite material showed high porosity, which in turn gives it a lower bending strength. However, the high porosity is more favourable for biocompatibility.
    Bio-medical materials and engineering 02/1998; 8(1):37-43.
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    ABSTRACT: A fine grained hydroxyapatite/mica composite material was studied by the atomic force microscopy method and the results were compared with results of atomic force microscopy studies of very five grained hydroxyapatite. In the investigation it was found that the fractal dimension diagram from the atomic force microscopy studies is a tool by which mechanical properties on the surface of the material can be predicted. The two investigated materials were found to show self-similarity properties, i.e., they are identical on the surface. The information given by the fractal dimension is important, and the fractal dimension analysis is an important tool in future designing and engineering of, especially, bioceramics and composites.
    Bio-medical materials and engineering 02/1997; 7(5):285-9.
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    ABSTRACT: The biomechanical properties of a porous hydroxyapatite (HA) intervertebral graft with or without anterior cervical plating were evaluated in cadaveric porcine cervical spine model using C3-4 discectomy and dissection of the posterior longitudinal ligament to cause instability. The experimental groups were intact (n = 11), instability (n = 11), autogenous bone implant (n = 6), HA graft implant (n = 5), autogenous bone with plating (n = 6), and HA graft with plating (n = 5). Porous HA with 40% porosity and scapular bone were used as grafts. The displacement rates of the cervical spine by compressive forces in the flexural, extensional, and lateral bending directions were evaluated using video-recording followed by computer-assisted analysis. The stiffness to compressive load was calculated from the load-displacement rate curve. The linear and non-linear coefficients of the Fung's equation were obtained based on the plot of Young's modulus against load. There were no statistical differences in the stiffness between the HA and autogenous bone graft in all directions. The two plating groups showed significantly increased stiffness in all directions. The non-linear coefficient value in Fung's equation was far larger in both HA and HA graft with plating groups than in the other groups in flexural compression. Porous HA graft has a compressive strength similar to autogenous graft in vitro, and anterior plating provides additional stiffness to the cervical spine. The larger non-linear coefficient value of the HA groups may represent the characteristic biomechanical brittleness of HA graft, but this is manifest only in flexural compression.
    Bio-medical materials and engineering 02/1997; 7(2):121-7.
  • Engineering Fracture Mechanics 01/1996; 55(3).
  • A. T. Yokobori, T. Yokobori, M. Tabuchi
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    ABSTRACT: It has been shown experimentally that the master curve for creep deformation versus the ratio of time to fracture time, can be obtained for smooth, notched and precracked specimens of Cr-Mo-V steel, a high-temperature ductile material. A simple unified constitutive equation, i.e. a master curve equation, has been proposed. It is suggested that there is some correlation between the creep deformation fracture curve and the creep damage size master curve. Although the range of the applicability of methodology might be rather limited, the development of this concept is needed for improved long-term creep lives and for other creep ductile materials.
    Journal of Materials Science 12/1995; 31(18):4767-4773. DOI:10.1007/BF00355859
  • A T Yokobori, Y Miyasaka, M Sakurai
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    ABSTRACT: Osteogenesis is completed by the nucleation mechanism on crystal nuclei formation and growth after amorphous calcium phosphate accretion to collagen fibers. For nuclei formation, it is necessary to have preliminary ionic diffusion such as that of Ca2+ and PO4(3-) ions to this part. Therefore, promotion of ionic diffusion to this part is the first essential condition for osteogenesis. We have considered this phenomenon as the nucleation mechanism accompanied by preliminary diffusion behavior and shown the optimum mechanical condition on promoting this ionic diffusion. Furthermore, we noticed callus by callotasis as the typical example which fits this mechanical condition and investigated its histology. However, necking occurs at the site of callus sandwiched by cortical bones due to tensile stress, three axial tensile stress in radial and tangential directions, in addition to the tensile stress occurs at the netsection of the callus due to the constraint of cortical bones. Therefore, the size of pore along the collagen lamination and hole zone becomes large under this mechanical condition and ionic diffusion such as Ca2+, PO4(3-) is liable to occur at this part. By solving the partial differential equation of the stress-induced diffusion, diffusive particles are shown to concentrate at the high three axial tensile stress region. Therefore, a high concentrated region of Ca, PO4 is achieved and selective osteogenesis due to hydroxyapatite nucleation is considered to progress in this region. Histological investigation of the callus of rabbit showed that mineralization occurs at this site and supports the theory of stress-induced diffusion on osteogenesis behavior.
    Bio-medical materials and engineering 02/1995; 5(4):209-17.
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    ABSTRACT: Cases with loss of articular cartilage or lowering of articular lubricant performance are frequent among patients afflicted by various gonarthropathies or those fitted with prosthetic joints. The management of such cases has necessitated evaluation studies on the clinical efficacy of using high molecular weight hyaluronic acid as an artificial synovial fluid with a view to improving articular lubricant performance. The proper implementation of such evaluation studies on lubrication performance requires testing methods capable of assessing basic lubricant performance not only with respect to articular cartilage alone but also with respect to various other combinations of materials, i.e., lubrication between articular bone and artificial joints, etc. In the present study, first, a simple standard lubricant performance test using several small readily procurable test specimens was devised. Then, this test was employed to evaluate the basic lubricant properties of hyaluronic acid intended for use as artificial synovial fluid.
    Bio-medical materials and engineering 02/1995; 5(2):117-24.
  • S Sasaki, A T Yokobori, T Ohkuma
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    ABSTRACT: Previously, we developed the noninvasive estimation method on the viscoelastic deterioration of the blood vessel and related materials by using Ultrasonic Doppler Effect Sensor and we presented its theoretical foundation. Furthermore, we applied this method to discriminate the human blood vessels with atherosclerosis from normal blood vessels, as the viscoelastic deterioration under in vitro condition and proposed the algorithm for its diagnosis. In the present paper, we propose the software for personal computer (AORTA) for clinical diagnosis on the atherosclerosis within a few minutes by using Ultrasonic Doppler Effect Sensor based on this algorithm. Using this software, we clinically applied this method to patients with atherosclerosis and normal people. It enables us to discriminate the atherosclerotic blood vessels from those of the normal blood vessel within several minutes. By using this system, it is found that the normal human blood vessel has elastic property. When the blood vessel becomes atherosclerotic, it deteriorates in elasticity and shows the viscoelastic property. This system evaluates noninvasively the viscoelastic property of the blood vessel and the degree of the atherosclerosis.
    Bio-medical materials and engineering 02/1994; 4(2):97-103.
  • H Yoshinari, A T Yokobori
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    ABSTRACT: It is important to estimate the mechanical characteristics and strength of biomedical membrane. For this purpose, previously we have proposed several mechanical test methods for biomedical membranes. To establish the safety design for biomedical membrane, such as cellophane membrane for hemodialysis, it is important to estimate the viscoelastic characteristics of these materials. On the other hand, artificial biomedical membrane such as tympanic membrane, are subjected to noncontact internal air pressure under the membranous state. To estimate mechanical characteristics of such membrane, it is necessary to develop a well-simulated test under the gaseous pressure and the other mechanical test under the membranous state. In this paper, several test methods for these purposes were shown. Furthermore, results obtained by these methods were shown and related to clinical problems. These proposed test methods are quite different from the axial tensile test. But they are also important to estimate the mechanical property of biomedical membrane. Each result obtained by these test methods has its own significance. By selection of the most suitable test method for each purpose and revealing these mutual relations, safety design of artificial organs can be performed from the viewpoint of the strength.
    Bio-medical materials and engineering 02/1994; 4(4):333-40.
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    ABSTRACT: Previously we proposed Acoustical Imaging and Processing Method to measure the viscoelastic property of the blood vessels of a canine and the related materials using Ultrasonic Doppler Effect Measurement. Furthermore, its theoretical foundation was presented. In this paper, this method is applied to measure the viscoelastic mechanical property, that is, the mechanical degradation of human blood vessels by a percutaneous noninvasive method. Based on these results, we proposed the algorithm of the noninvasive estimation method on the viscoelastic mechanical property of the blood vessel by using Doppler Effect Sensor. This method makes it possible to discriminate the blood vessel with arteriosclerosis from a normal blood vessel. Clinical applications were successfully performed using our developed computer software based on our proposed algorithm.
    Bio-medical materials and engineering 02/1994; 4(2):87-96.
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    ABSTRACT: Experimental callotasis was made in rabbit femurs. In order to compare mechanical behaviors and morphological structure of callus in callotasis, mechanical tests and histological examination were performed. Twenty Japanese white male rabbits were used. The right femurs were osteotomized at the level of the midshaft with a rotary cutter and fixed with a mini-model external fixator. After a 5-day waiting period, bone lengthening was started at the rate of 0.25 mm two times a day. Animals were divided into four groups. Group 1 and Group 2 were continual distraction group, Group 3 and Group 4 discontinued distraction group. The viscoelastic property was demonstrated in continual distraction group irrespective of distraction period, we thought the central undifferentiated connective tissue in callus was mainly responsible for the mechanical behavior. On the other hand, in discontinued distraction group, viscoelastic property shifted to elastic property corresponding to the rest period, we thought this change of mechanical behavior was histologically owing to the replacement of undifferentiated connective tissue by cartilage and the partial union of bone in callus.
    Bio-medical materials and engineering 02/1994; 4(4):273-81.
  • H Yoshinari, A T Yokobori, T Ohkuma
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    ABSTRACT: The method used in this paper is developed to estimate the degree of viscoelasticity of the blood vessel wall in a noninvasive mechanical method. In this proposed method, we can obtain an output waveform connected with mechanical behavior of blood vessels by using Ultrasonic Doppler Effect Sensory (UDES). Characterizing parameters on this waveform are established to estimate the mechanical properties of blood vessels. By analyzing the characteristics of these parameters, we attempt to estimate the viscoelastic property of blood vessels. Previously, it was found that these parameters are related to viscoelastic mechanical properties of materials. In this paper, we carried out computer analysis on the mechanical viscoelastic model and compared the theoretical behavior of blood vessels with the experimental output waveform by UDES. From these results, it was proved that values of these parameters connect with the change of the viscoelastic mechanical property of blood vessels. Therefore, the characterizing parameters of experimental output waveforms can be used to estimate the viscoelastic property of blood vessels in a noninvasive method.
    Bio-medical materials and engineering 02/1994; 4(2):77-86.
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    ABSTRACT: A mechanical test on small caliber blood vessel is difficult because the strength is very low and the dimension is very small. In the present paper a multifunctional axial test apparatus design is proposed. This has the characteristics such that the load cell and the environmental container for the specimen are built up in terms of cassette, respectively, and thus both are easy to equip and take off, that is, easy to replace. Also, the test methodology by internal pressure for small caliber blood vessels has been proposed. By using both of these methodologies, the characteristics of the mechanical behavior of small caliber blood vessels, such as compliance, stress relaxation, and viscoelastic property have been clarified. An attempt has been made on the application of this method to a clinical case.
    Bio-medical materials and engineering 02/1993; 3(4):175-83.
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    ABSTRACT: The aim of this study is to elucidate the relation of the inelastic expansive deformation of natural blood vessel and the degradation of elasticity to the time pattern of circulating pulsatile pressure flow. When pulsatile pressure amplitude (delta P = Pmax - Pmin) becomes smaller due to Pmin increasing, the blood vessel is subject to creep deformation. In this sense, pulsating pressure will play a role in avoiding creep effect. Fluctuation of maximum pressure Pmax will induce the increase of inelastic deformation and the decrease of rigidity of the blood vessel. The inelastic deformation and the decrease of rigidity in blood vessel is induced at the stage of pressure amplitude rising from a lower one to a higher, but not during pressure amplitude (delta P) kept constant nor at the pressure amplitude decreasing stage. In order to reduce the degradation of mechanical properties of blood vessel, it may be effective to avoid the increase of Pmin and the variability of Pmax.
    Bio-medical materials and engineering 02/1991; 1(4):223-41.