bluuebunny/crossref_metadata_2025 huggingface dataset:
Combined Solution of the Inverse Stefan Problem for Successive Freezing/Thawing in Nonideal Biological Tissues
Authors: Rabin Y. , Shitzer A.
Published: February 2008

A new combined solution of the one-dimensional inverse Stefan problem in biological tissues is presented. The tissue is assumed to be a nonideal material in which phase transition occurs over a temperature range. The solution includes the thermal effects of blood perfusion and metabolic heat generation. The analysis combines a heat balance integral solution in the frozen region and a numerical enthalpy-based solution approach in the unfrozen region. The subregion of phase transition is included in the unfrozen region. Thermal effects of blood perfusion and metabolic heat generation are assumed to be temperature dependent and present in the unfrozen region only. An arbitrary initial condition is assumed that renders the solution useful for cryosurgical applications employing repeated freezing/thawing cycles. Very good agreement is obtained between the combined and an exact solution of a similar problem with constant thermophysical properties and a uniform initial condition. The solution indicated that blood perfusion does not appreciably affect either the shape of the temperature forcing function on the cryoprobe or the location and depth of penetration of the freezing front in peripheral tissues. It does, however, have a major influence on the freezing/thawing cycle duration, which is most pronounced during the thawing stage. The cooling rate imposed at the freezing front also has a major inverse effect on the duration of the freezing/thawing.
Temperature Distributions During Thermoradiotherapy: A Sensitivity Study With a Transient Numerical Model of the Rabbit Eye
Authors: Tompkins D. T. , Klein S. A. , Steeves R. A.
Published: February 2008

An approach to the treatment of medium-sized choroidal melanomas combines radiation with ferromagnetic hyperthermia. The study herein discusses results with a numerical thermal model of a choroidal melanoma in the rabbit eye as treated with episcleral, thermoradiotherapy plaques. The sensitivity of a temperature–dependent blood perfusion model is investigated.
Numerical Analysis of an Extremity in a Cold Environment Including Countercurrent Arterio-Venous Heat Exchange
Authors: Shitzer A. , Stroschein L. A. , Vital P. , Gonzalez R. R. , Pandolf K. B.
Published: February 2008

A model of the thermal behavior of an extremity, e.g., a finger, is presented. The model includes the effects of heat conduction, metabolic heat generation, heat transport by blood perfusion, heat exchange between the tissue and the large blood vessels, and arterio-venous heat exchange. Heat exchange with the environment through a layer of thermal insulation, depicting thermal handwear, is also considered. The tissue is subdivided into four concentric layers simulating, from the center outward, core, muscle, fat, and skin. Differential heat balance equations are formulated for the tissue and for the major artery and the major vein traversing the finger. These coupled equations are solved numerically by a finite-difference, alternating direction method employing a Thomas algorithm. The numerical scheme was extensively tested for its stability and convergence. This paper presents the model equations and results of the convergence tests, and shows plots of blood and tissue temperatures along the axis of the model for combinations of parameters including the effect of countercurrent heat exchange between the artery and the vein.
Measurements of Velocity and Wall Shear Stress Inside a PTFE Vascular Graft Model Under Steady Flow Conditions
Authors: Loth F. , Jones S. A. , Giddens D. P. , Bassiouny H. S. , Glagov S. , Zarins C. K.
Published: February 2008

The flow field inside a model of a polytetrafluoroethylene (PTFE) canine artery end-to-side bypass graft was studied under steady flow conditions using laser-Doppler anemometry. The anatomically realistic in vitro model was constructed to incorporate the major geometric features of the in vivo canine anastomosis geometry, most notably a larger graft than host artery diameter. The velocity measurements at Reynolds number 208, based on the host artery diameter, show the flow field to be three dimensional in nature. The wall shear stress distribution, computed from the near-wall velocity gradients, reveals a relatively low wall shear stress region on the wall opposite to the graft near the stagnation point approximately one artery diameter in axial length at the midplane. This low wall shear stress region extends to the sidewalls, suture lines, and along the PTFE graft where its axial length at the midplane is more than two artery diameters. The velocity distribution inside the graft model presented here provides a data set well suited for validation of numerical solutions on a model of this type.
Predicting the Impact Response of a Nonlinear Single-Degree-of-Freedom Shock-Absorbing System From the Measured Step Response
Authors: Robinovitch S. N. , Hayes W. C. , McMahon T. A.
Published: February 2008

We measured the step response of a surrogate human pelvis/impact pendulum system at force levels between 50 and 350 N. We then fit measured response curves with four different single-degree-of-freedom models, each possessing a single mass, and supports of the following types: standard linear solid, Voigt, Maxwell, and spring. We then compared model predictions of impact force during high-energy collisions (pendulum impact velocity ranging from 1.16 to 2.58 m/s) to force traces from actual impacts to the surrogate pelvis. We found that measured peak impact forces, which ranged from 1700 to 5600 N, were best predicted by the mass-spring, Maxwell, and standard linear solid models, each of which had average errors less than 3 percent. Reduced accuracy was observed for the commonly used Voigt model, which exhibited an average error of 10 percent. Considering that the surrogate pelvis system used in this study exhibited nonlinear stiffness and damping similar to that observed in simulated fall impact experiments with human volunteers, our results suggest that these simple models allow impact forces in potentially traumatic falls to be predicted to within reasonable accuracy from the measured response of the body in safe, simulated collisions.
A Dynamic System Model of an Off-Road Cyclist
Authors: Wang E. L. , Hull M. L.
Published: February 2008

To optimize the performance of off-road bicycle suspension systems, a dynamic model of the bicycle/rider system would be useful. This paper takes a major step toward this goal by developing a dynamic system model of the cyclist. To develop the cyclist model, a series of four vibrational tests utilizing random inputs was conducted on seven experienced off-road cyclists. This allowed the transfer functions for the arms and legs to be determined. To reproduce the essential features (i.e., resonance peaks) of the experimental transfer functions, the system model included elements representing the visceral mass along with the arms and legs. Through simulations, the frequency responses of the system model of the rider in each of the four tests were computed. Optimal stiffness and damping parameter values for each subject were determined by minimizing the difference between the experimental and simulation results. Good agreement between experimental and simulation results indicates that modeling the rider as a lumped parameter system with linear springs and dampers is possible.
A New Fundamental Bioheat Equation for Muscle Tissue: Part I—Blood Perfusion Term
Authors: Weinbaum S. , Xu L. X. , Zhu L. , Ekpene A.
Published: February 2008

A new model for muscle tissue heat transfer has been developed using Myrhage and Eriksson’s [23] description of a muscle tissue cylinder surrounding secondary (s) vessels as the basic heat transfer unit. This model provides a rational theory for the venous return temperature for the perfusion source term in a modified Pennes bioheat equation, and greatly simplifies the anatomical description of the microvascular architecture required in the Weinbaum-Jiji bioheat equation. An easy-to-use closed-form analytic expression has been derived for the difference between the inlet artery and venous return temperatures using a model for the countercurrent heat exchange in the individual muscle tissue cylinders. The perfusion source term calculated from this model is found to be similar in form to the Pennes’s source term except that there is a correction factor or efficiency coefficient multiplying the Pennes term, which rigorously accounts for the thermal equilibration of the returning vein. This coefficient is a function of the vascular cross-sectional geometry of the muscle tissue cylinder, but independent of the Peclet number in contrast to the recent results in Brinck and Werner [8]. The value of this coefficient varies between 0.6 and 0.7 for most muscle tissues. In part II of this study a theory will be presented for determining the local arterial supply temperature at the inlet to the muscle tissue cylinder.
Evaluation of the Proximal Flow Field to Circular and Noncircular Orifices of Different Aspect Ratios
Authors: Myers J. G. , Fox J. F. , Elmahdi A. M. , Perry G. J. , Anayiotos A. S.
Published: February 2008

Investigations of valvular regurgitation attempt to specify flow field characteristics and apply them to the proximal isovelocity surface area (PISA) method for quantifying regurgitant flow. Most investigators assume a hemispherical shape to these equivelocity shells proximal to an axisymmetric (circular) orifice. However, in vivo flow fields are viscous and regurgitant openings vary in shape and size. By using centerline profiles and isovelocity surfaces, this investigation describes the flow field proximal to circular and elliptical orifices. Steady, proximal flow fields are obtained with two- and three-dimensional computational fluid dynamic (CFD) simulations. These simulations are verified by in vitro, laser-Doppler velocimetry (LDV) experiments. The data show that a unique, normalized proximal flow field results for each orifice shape independent of orifice flow or size. The distinct differences in flow field characteristics with orifice shape may provide a mechanism for evaluating orifice characteristics and regurgitant flows. Instead of the hemispherical approximation technique, this study attempts to show the potential to define a universal flow evaluation method based on the details of the flowfield according to orifice shape. Preliminary results indicate that Magnetic Resonance (MR) and Color Doppler (CD) may reproduce these flow details and allow such a procedure in vivo.
A Protocol to Evaluate Semi-Rigid Pedicle Screw Systems
Authors: Clausen J. D. , Goel V. K. , Sairyo K. , Pfeiffer M.
Published: February 2008

The objective of the current study was to develop an in vitro testing protocol to evaluate semi-rigid pedicle screw devices. A corpectomy model protocol exists to evaluate rigid spinal implants; however, semi-rigid devices are contraindicated for this condition. This paper describes a technique that simulates more closely the conditions a semi-rigid device would see in vivo. Finally, the new testing protocol is used to evaluate the DDS® pedicle screw-cable system. Benefits and shortcomings of the new protocol are discussed.
Uniform Stress State in Bone Structure With Residual Stress
Authors: Adachi T. , Tanaka M. , Tomita Y.
Published: February 2008

Residual stress and strain in living tissues have been investigated from the viewpoint of mechanical optimality maintained by adaptive remodeling. In this study, the residual stresses in the cortical-cancellous bone complex of bovine coccygeal vertebrae were examined. Biaxial strain gages were bonded onto the cortical surface, so that the gage axes were aligned in the cephalocaudal and circumferential directions. Strains induced by removal of the end-plate and the cancellous bone were recorded sequentially. The results revealed the existence of compressive residual stress in the cortical bone and tensile residual stress in the cancellous bone in both the cephalocaudal and the circumferential direction. The observed strains were examined on the basis of the uniform stress hypothesis using a three-bar model for the cephalocaudal direction and a three-layered cylinder model for the circumferential direction. In this model study, the magnitude of effective stresses, which is defined as the macroscopic stress divided by the area fraction of bone material, was found not to differ significantly between cephalocaudal and circumferential directions, although they were evaluated using independent models. These results demonstrate that the uniform stress state of the cortical-cancellous bone structure is consistent with results obtained in the cutting experiment when the existence of residual stress is taken into account.
A Model for Aortic Growth Based on Fluid Shear and Fiber Stresses
Authors: Taber L. A.
Published: February 2008

Stress-modulated growth in the aorta is studied using a theoretical model. The model is a thick-walled tube composed of two pseudoelastic, orthotropic layers representing the intima/media and the adventitia. Both layers are assumed to follow a growth law in which the time rates of change of the growth stretch ratios depend linearly on the local smooth muscle fiber stress and on the shear stress due to blood flow on the endothelium. Using finite elasticity theory modified to include volumetric growth, we computed temporal changes in stress, geometry, and opening angle (residual strain) during development and following the onset of sudden hypertension. For appropriate values of the coefficients in the growth law, the model yields results in reasonable agreement with published data for global and local growth of the rat aorta.
Swelling and Curling Behaviors of Articular Cartilage
Authors: Setton L. A. , Tohyama H. , Mow V. C.
Published: February 2008

A new experimental method was developed to quantify parameters of swelling-induced shape change in articular cartilage. Full-thickness strips of cartilage were studied in free-swelling tests and the swelling-induced stretch, curvature, and areal change were measured. In general, swelling-induced stretch and curvature were found to increase in cartilage with decreasing ion concentration, reflecting an increasing tendency to swell and “curl” at higher swelling pressures. An exception was observed at the articular surface, which was inextensible for all ionic conditions. The swelling-induced residual strain at physiological ionic conditions was estimated from the swelling-induced stretch and found to be tensile and from 3–15 percent. Parameters of swelling were found to vary with sample orientation, reflecting a role for matrix anisotropy in controlling the swelling-induced residual strains. In addition, the surface zone was found to be a structurally important element, which greatly limits swelling of the entire cartilage layer. The findings of this study provide the first quantitative measures of swelling-induced residual strain in cartilage ex situ, and may be readily adapted to studies of cartilage swelling in situ.
A Generic Tissue Convective Energy Balance Equation: Part I—Theory and Derivation
Authors: Roemer R. B. , Dutton A. W.
Published: February 2008

A new equation for calculating temperatures in living tissues, the tissue convective energy balance equation (TCEBE), is derived using only a few assumptions. The resulting equation is basic, general and applicable to any tissue. The (unsolved) TCEBE is used: (a) to relate both Pennes’ BHTE perfusion-related parameter (W) and the effective thermal conductivity equation’s perfusion-related parameter (keff) to the true capillary perfusion P˙cap, and (b) to show that both W and keff are defined, nonphysiological variables, which are only related to P˙cap in a problem-dependent manner. Finally, the derivation of the relationship between W and P˙cap provides a complete derivation of Pennes’ BHTE, something that has not been previously done.
Augmentation of Axial Dispersion by Intermittent Oscillatory Flow
Authors: Tanaka G. , Ueda Y. , Tanishita K.
Published: February 2008

The efficiency of axial gas dispersion during ventilation with high-frequency oscillation (HFO) is improved by manipulating the oscillatory flow waveform such that intermittent oscillatory flow occurs. We therefore measured the velocity profiles and effective axial gas diffusivity during intermittent oscillatory flow in a straight tube to verify the intermittency augmentation effect on axial gas transfer. The effective diffusivity was dependent on the flow patterns and significantly increased with an increase in the duration of the stationary phase. It was also found that the ratio of effective diffusivity to molecular diffusivity is two times greater than that in sinusoidal oscillatory flow. Moreover, turbulence during deceleration or at the beginning of the stationary phase further augments axial dispersion, with the effective diffusivity being over three times as large, thereby proving that the use of intermittent oscillatory flow effectively augments axial dispersion for ventilation with HFO.
Modeling of Airflow in the Pharynx With Application to Sleep Apnea
Authors: Shome B. , Wang L.-P. , Santare M. H. , Prasad A. K. , Szeri A. Z. , Roberts D.
Published: February 2008

A three-dimensional numerical modeling of airflow in the human pharynx using an anatomically accurate model was conducted. The pharynx walls were assumed to be passive and rigid. The results showed that the pressure drop in the pharynx lies in the range 200-500 Pa. The onset of turbulence was found to increase the pressure drop by 40 percent. A wide range of pharynx geometries covering three sleep apnea treatment therapies (CPAP, mandibular repositioning devices, and surgery) were modeled and the resulting flow characteristics were investigated and compared. The results confirmed that the airflow in the pharynx lies in the laminar-to-turbulence transitional flow regime and thus, a subtle change in the morphology caused by these treatment therapies can significantly affect the airflow characteristics.
Computational Analysis of Confined Jet Flow and Mass Transport in a Blind Tube
Authors: Cavanagh D. P. , Eckmann D. M.
Published: February 2008

A computational analysis of confined nonimpinging jet flow in a blind tube is performed as an initial investigation of the underlying fluid and mass transport mechanics of tracheal gas insufflation. A two-dimensional axisymmetric model of a laminar steady jet flow into a concentric blind-end tube is put forth and the governing continuity, momentum, and convection-diffusion equations are solved with a finite element code. The effects of the jet diameter based Reynolds number (Rej), the ratio of the jet-to-outer tube diameters (ε), and the Schmidt number (Sc) are evaluated with the determined velocity and contaminant concentration fields. The normalized penetration depth of the jet is found to increase linearly with increasing Rej for ε = O(0.I). For a given ε, a ring vortex that develops is observed to be displaced downstream and radially outward from the jet tip for increasing Rej. The axial shear stress profile along the inside wall of the outer tube possesses regions of fixed shear stress in addition to a local minimum and maximum in the vicinity of the jet tip. Corresponding regions of axial shear stress gradients exist between the fixed shear stress regions and the local extrema. Contaminant concentration gradients develop across the ring vortex indicating the inward diffusion of contaminant into the jet flow. For fixed ε and Sc and Rej ~ 900, normalized contaminant flow rate is observed to be approximately twice that of simple diffusion. This model predicts modest net axial contaminant transport enhancement due to convection-diffusion interaction in the region of the ring vortex.
Simulation of Particle-Hemodynamics in a Partially Occluded Artery Segment With Implications to the Initiation of Microemboli and Secondary Stenoses
Authors: Buchanan J. R. , Kleinstreuer C.
Published: February 2008

Computational results of laminar incompressible blood-particle flow analyses in an axisymmetric artery segment with a smooth local area constriction of 75 percent have been presented. The flow input waveform was sinusoidal with a nonzero average. The non-Newtonian behavior of blood was simulated with a modified Quemada model, platelet concentrations were calculated with a drift-flux model, and monocyte trajectories were described and compared for both Newtonian and Quemada rheologies. Indicators of “disturbed flow” included the time-averaged wall shear stress (WSS), the oscillatory shear index (OSI), and the wall shear stress gradient (WSSG). Implications of the vortical flow patterns behind the primary stenosis to the formation of microemboli and downstream stenoses are as follows. Elevated platelet concentrations due to accumulation in recirculation zones mixed with thrombin and ADP complexes assumed to be released upstream in high wall shear stress regions, could form microemboli, which are convected downstream. Distinct near-wall vortices causing a local increase in the WSSG and OSI as well as blood-particle entrainment with possible wall deposition, indicate sites susceptible to the onset of an additional stenosis proximal to the initial geometric disturbance.
Simulation of Pressure Drop and Energy Dissipation for Blood Flow in a Human Fetal Bifurcation
Authors: Hellevik L. R. , Kiserud T. , Irgens F. , Ytrehus T. , Eik-Nes S. H.
Published: February 2008

The pressure drop from the umbilical vein to the heart plays a vital part in human fetal circulation. The bulk of the pressure drop is believed to take place at the inlet of the ductus venosus, a short narrow branch of the umbilical vein. In this study a generalized Bernoulli formulation was deduced to estimate this pressure drop. The model contains an energy dissipation term and flow-scaled velocities and pressures. The flow-scaled variables are related to their corresponding spatial mean velocities and pressures by certain shape factors. Further, based on physiological measurements, we established a simplified, rigid-walled, three-dimensional computational model of the umbilical vein and ductus venosus bifurcation for stationary flow conditions. Simulations were carried out for Reynolds numbers and umbilical vein curvature ratios in their respective physiological ranges. The shape factors in the Bernoulli formulation were then estimated for our computational models. They showed no significant Reynolds number or curvature ratio dependency. Further, the energy dissipation in our models was estimated to constitute 24 to 31 percent of the pressure drop, depending on the Reynolds number and the curvature ratio. The energy dissipation should therefore be taken into account in pressure drop estimates.
Pendelluft Flow in Symmetric Airway Bifurcations
Authors: Feng Z. C. , Poon C.-S.
Published: February 2008

We propose a mathematical model for pendelluft flow in a single airway bifurcation. The model is motivated by an apparatus used in an experimental study of the pendelluft by Ultman et al. (1988). We derive differential equations governing the fluid flow, which directly connect physiological parameters to the variables determining the pendelluft; this approach allows us to include nonlinearity in the model. If nonlinearity is neglected, our model is identical to the R-I-C circuits used by previous investigators. If nonlinearity is retained, we show that pendelluft can occur even in perfectly symmetric airway bifurcations. For the specific apparatus used in the experiments of High et al. (1991), we demonstrate that two qualitatively different pendelluft flows can occur in the system.
Numerical Analysis for Stability and Self-Excited Oscillation in Collapsible Tube Flow
Authors: Hayashi S. , Hayase T. , Kawamura H.
Published: February 2008

This paper describes numerical analysis of collapsible tube flow based on the one-dimensional distributed parameter model of Hayashi. In the present model the effect of flow separation at the collapsed part is replaced with simple viscous friction along the tube, so no ad-hoc modeling for flow separation in former studies is required. A stable semi-implicit numerical procedure based on the SIMPLE method is developed for the problem of flow and tube interaction. The numerical result for a characteristic self-excited oscillation agrees qualitatively with the experimental result. Nonlinear stability of the steady state dependent on the amplitude of the disturbance is numerically investigated and the result is compared with the linear stability analysis based on the former lumped parameter model. Finally, initiation of the self-excited oscillation is examined by applying the initial disturbance at the upstream end of the tube. The disturbance propagates in the downstream direction and is amplifled to the self-excited oscillation.
Strain Inhomogeneity in the Anterior Cruciate Ligament Under Application of External and Muscular Loads
Authors: Bach J. M. , Hull M. L.
Published: February 2008

To determine whether mathematical relations between strains in different bundles and loads would be needed to predict injury of the anterior cruciate ligament (ACL), this work tested the hypothesis that strains developed in two bundles of the ACL were significantly different under the application of a number of loads important to injury etiology of the ACL. To provide the data for testing this hypothesis, liquid mercury strain gages were installed on both the anteromedial (AMB) and posterolateral (PLB) bundles of the ACL of ten specimens, which were then subjected to passive flexion/extension, hyperextension moment, anterior force, internal and external axial moments, quadriceps, and hamstrings forces. Various combinations of these loads were also applied. Flexion angles ranged from 8 deg of hyperextension through 120 deg of flexion. The data were analyzed using a repeated measures analysis of variance. The analyses indicated that significant strain differences existed between the two bundles only for passive flexion/extension. However, the analyses did not support the hypothesis that AMB and PLB strains are significantly different from each other under the application of external and muscular loads. Because noticeable differences (>3 percent) between bundle strains did exist in some load cases for limited ranges of flexion and the PLB strain was consistently higher than the AMB strain, it may be sufficient to consider strain in only the PLB when predicting ligament damage based on strain-load relations.
Local Mechanical Anisotropy in Human Cranial Dura Mater Allografts
Authors: Sacks M. S. , Jimenez Hamann M. C. , Otan˜no-Lata S. E. , Malinin T. I.
Published: February 2008

Human cranial dura mater (CDM) allograft’s success as a repair biomaterial is partly due to its high mechanical strength, which facilitates its ability to form water-tight barriers and resist high in-vivo mechanical loads. Previous studies on CDM allograft mechanical behavior used large test specimens and concluded that the allograft was mechanically isotropic. However, we have quantified CDM microstructure using small angle light scattering (SALS) and found regions of well-aligned fibers displaying structural symmetry between the right and left halves (Jimenez et al., 1998). The high degree of fiber alignment in these regions suggests that they are mechanically anisotropic. However, identification of these regions using SALS requires irreversible tissue dehydration, which may affect mechanical properties. Instead, we utilized CDM structural symmetry to estimate the fiber architecture of one half of the CDM using computer graphics to flip the SALS fiber architecture map of the corresponding half about the plane of symmetry. Test specimens (20 mm × 4 mm) were selected parallel and perpendicular to the preferred fiber directions and subjected to uniaxial mechanical failure testing. CDM allografts were found to be locally anisotropic, having an ultimate tensile strength (UTS) parallel to the fibers of 12.76 ± 1.65 MPa, and perpendicular to the fibers of 5.21 ± 1.01 MPa (mean ± sem). These results indicate that uniaxial mechanical tests on large samples used in previous studies tended to mask the local anisotropic nature of the smaller constituent sections. The testing methods established in this study can be used in the evaluation of new CDM processing methods and post-implant allograft mechanical integrity.
An Asymptotic Model of Viscous Flow Limitation in a Highly Collapsed Channel
Authors: Jensen O. E.
Published: February 2008

A viscous flow through a long two-dimensional channel, one wall of which is formed by a finite-length membrane, experiences flow limitation when the channel is highly collapsed over a narrow region under high external pressure. Simple approximate relations between flow rate and pressure drop are obtained for this configuration by the use of matched asymptotic expansions. Weak inertial effects are also considered.
Finite Element Analysis of Anterior Tooth Root Stresses Developed During Endodontic Treatment
Authors: Silver-Thorn M. B. , Joyce T. P.
Published: February 2008

Vertical tooth root fractures are diagnostically challenging, frustrating, and an increasingly common cause of failure of tooth restoration. These vertical root fractures have been associated with many causes, including the endodontic process itself. To investigate these endodontic causes, various phases of crown replacement for an anterior tooth were modeled using nonlinear, plane strain finite element (FE) analysis. Stresses developed during obturation, post positioning, crown placement, and masticatory and occlusal loading of the restored tooth were estimated using this analysis method. The minimum (compressive) principal stress was greatest during obturation of cones 1 and 2, and during mastication of the restored tooth. Although these stresses were significant (−150 to −280 MPa), they did not exceed the compressive strength of dentin. The maximum (tensile) principal stresses, 160 to 300 MPa, were also observed during obturation of cones 1 and 2. These peak stresses exceed the dentin tensile strength.
Viscoelastic Characterization of Mesenchymal Gap Tissue and Consequences for Tension Accumulation During Distraction
Authors: Richards M. , Wineman A. S. , Alsberg E. , Goulet J. A. , Goldstein S. A.
Published: February 2008

Nonlinear viscoelastic analysis was used to characterize the time-dependent behavior of mesenchymal gap tissue generated during distraction osteogenesis. Six (n = 6) lengthened tibiae were harvested from New Zealand white rabbits at 18 days. This gap tissue was subjected to a series of step displacement tests of increasing magnitude, and force relaxation behavior was monitored. Isochrones in stress-strain space were fit to odd cubic functions of strain. An analytic expression, linear in both e and e3, was developed to predict stress accumulation within the gap tissue as a function of time during distraction. Stress relaxation functions were described well by two-term Prony series. The two time constants determined from mechanical testing results were consistent, suggesting the presence of two fundamental physiologic relaxation processes. Gap tissue stresses were predicted to rise considerably during early stages of lengthening when distraction magnitudes exceeded the clinical norm of 0.25 mm. These differences in tension accumulation were less pronounced by the time lengthening was completed. Specifically, these results may in part explain clinical observations of decreased bone regeneration and altered tissue proliferation and differentiation at higher distraction rates. More generally, this work provides a framework for the rigorous characterization of the viscoelastic properties of biologic tissues ordinarily exposed to step strains.
Mechanical Properties of Collagen Fascicles From the Rabbit Patellar Tendon
Authors: Yamamoto E. , Hayashi K. , Yamamoto N.
Published: February 2008

Tensile and viscoelastic properties of collagen fascicles of approximately 300 μm in diameter, which were obtained from rabbit patellar tendons, were studied using a newly designed micro-tensile tester. Their cross-sectional areas were determined with a video dimension analyzer combined with a CCD camera and a low magnification microscope. There were no statistically significant differences in tensile properties among the fascicles obtained from six medial-to-lateral locations of the patellar tendon. Tangent modulus, tensile strength, and strain at failure of the fascicles determined at about 1.5 percent/s strain rate were 216 ± 68 MPa, 17.2 ± 4.1 MPa, and 10.9 ± 1.6 percent (mean ± S.D.), respectively. These properties were much different from those of bulk patellar tendons; for example, the tensile strength and strain at failure of these fascicles were 42 percent and 179 percent of those of bulk tendons, respectively. Tangent modulus and tensile strength of collagen fascicles determined at 1 percent/s strain rate were 35 percent larger than those at 0.01 percent/s. The strain at failure was independent of strain rate. Relaxation tests showed that the reduction of stress was approximately 25 percent at 300 seconds. These stress relaxation behavior and strain rate effects of collagen fascicles differed greatly from those of bulk tendons. The differences in tensile and viscoelastic properties between fascicles and bulk tendons may be attributable to ground substances, mechanical interaction between fascicles, and the difference of crimp structure of collagen fibrils.
Micromachining in Plastics Using X-Ray Lithography for the Fabrication of Micro-Electrophoresis Devices
Authors: Ford S. M. , Davies J. , Kar B. , Qi S. D. , McWhorter S. , Soper S. A. , Malek C. K.
Published: February 2008

Micromachining was performed in polymethylmethacrylate (PMMA) using X-ray lithography for the fabrication of miniaturized devices (microchips) for potential applications in chemical and genetic analyses. The devices were fabricated using two different techniques: transfer mask technology and a Kapton® mask. For both processes, the channel topography was transferred (1:1) to the appropriate substrate via the use of an optical mask. In the case of the transfer mask technique, the PMMA substrate was coated with a positive photoresist and a thin Au/Cr plating base. Following UV exposure, the resist was developed and a thick overlayer (∼3 μm) of Au electroplated onto the PMMA substrate only where the resist was removed, which acted as an absorber of the X-rays. In the other technique, a Kapton® film was used as the X-ray mask. In this case, the Kapton® film was UV exposed using the optical mask to define the channel topography and following development of the resist, a thick Au overlayer (8 μm) was electrodeposited onto the Kapton® sheet. The PMMA wafer during X-ray exposure was situated directly underneath the Kapton® mask. In both cases, the PMMA wafer was exposed to soft X-rays and developed to remove the exposed PMMA. The resulting channels were found to be 20 μm in width (determined by optical mask) with channel depths of ∼50 μm (determined by x-ray exposure time). In order to demonstrate the utility of this micromachining process, several components were fabricated in PMMA including capillary/chip connectors, injectors for fixed-volume sample introduction, separation channels for electrophoresis and integrated fiber optic fluorescence detectors. These components could be integrated into a single device to assemble a system appropriate for the rapid analysis of various targets.
Rapid, Automated Nucleic Acid Probe Assays Using Silicon Microstructures for Nucleic Acid Concentration
Authors: Christel L. A. , Petersen K. , McMillan W. , Northrup M. A.
Published: February 2008

A system for rapid point-of-use nucleic acid (NA) analysis based on PCR techniques is described. The extraction and concentration of DNA from test samples has been accomplished utilizing silicon fluidic microchips with high surface-area-to-volume ratios. Short (500 bp) and medium size (48,000 bp) DNA have been captured, washed, and eluted using the silicon dioxide surfaces of these chips. Chaotropic (GuHCl) salt solutions were used as binding agents. Wash and elution agents consisted of ethanol-based solutions and water, respectively. DNA quantities approaching 40 ng/cm2 of binding area were captured from input solutions in the 100–1000 ng/mL concentration range. For dilute samples of interest for pathogen detection, PCR and gel electrophoresis were used to demonstrate extraction efficiencies of about 50 percent, and concentration factors of about 10× using bacteriophage lambda DNA as the target. Rapid, multichannel PCR thermal cycling modules with integrated solid-state detection components have also been demonstrated. These results confirm the viability of utilizing these components as elements of a compact, disposable cartridge system for the detection of NA in applications such as clinical diagnostics, biowarfare agent detection, food quality control, and environmental monitoring.
A Tissue-Like Culture System Using Microstructures: Influence of Extracellular Matrix Material on Cell Adhesion and Aggregation
Authors: Knedlitschek G. , Schneider F. , Gottwald E. , Schaller Th. , Eschbach E. , Weibezahn K. F.
Published: February 2008

Special microenvironmental conditions are required to induce and/or maintain specific qualities of differentiated cells. An important parameter is the three-dimensional tissue architecture that cannot be reproduced in conventional monolayer systems. Advanced tissue culture systems will meet many of these demands, but may reach their limits, especially when gradients of specific substances over distinct tissue layers must be established for long-term culture. These limitations may be overcome by incorporating microstructures into tissue-like culture systems. The microstructured cell support presented consists of a flat array of 625 cubic microcontainers with porous bottoms, in which cells can be supplied with specific media from both sides of the tissue layer. Permanent cell lines and primary rat hepatocytes have been used to test the culture system. In order to define reproducible conditions for tissue formation and for cell adherence to the structure, several ECM (extracellular matrix) components were tested for coating of microstructured substrata. The described tissue culture system offers great flexibility in adapting the cell support to specific needs.
Numerical Model of Fluid Flow and Oxygen Transport in a Radial-Flow Microchannel Containing Hepatocytes
Authors: Ledezma G. A. , Folch A. , Bhatia S. N. , Balis U. J. , Yarmush M. L. , Toner M.
Published: February 2008

The incorporation of monolayers of cultured hepatocytes into an extracorporeal perfusion system has become a promising approach for the development of a temporary bioartificial liver (BAL) support system. In this paper we present a numerical investigation of the oxygen tension, shear stress, and pressure drop in a bioreactor for a BAL composed of plasma-perfused chambers containing monolayers of porcine hepatocytes. The chambers consist of microfabricated parallel disks with center-to-edge radial flow. The oxygen uptake rate (OUR), measured in vitro for porcine hepatocytes, was curve-fitted using Michaelis–Menten kinetics for simulation of the oxygen concentration profile. The effect of different parameters that may influence the oxygen transport inside the chambers, such as the plasma flow rate, the chamber height, the initial oxygen tension in the perfused plasma, the OUR, and Km was investigated. We found that both the plasma flow rate and the initial oxygen tension may have an important effect upon oxygen transport. Increasing the flow rate and/or the inlet oxygen tension resulted in improved oxygen transport to cells in the radial-flow microchannels, and allowed significantly greater diameter reactor without oxygen limitation to the hepatocytes. In the range investigated in this paper (10 μm < H < 100 μm), and for a constant plasma flow rate, the chamber height, H, had a negligible effect on the oxygen transport to hepatocytes. On the contrary, it strongly affected the mechanical stress on the cells that is also crucial for the successful design of the BAL reactors. A twofold decrease in chamber height from 50 to 25 μm produced approximately a fivefold increase in maximal shear stress at the inlet of the reactor from 2 to 10 dyn/cm2. Further decrease in chamber height resulted in shear stress values that are physiologically unrealistic. Therefore, the channel height needs to be carefully chosen in a BAL design to avoid deleterious hydrodynamic effects on hepatocytes.
Electrical, Chemical, and Topological Addressing of Mammalian Cells With Microfabricated Systems
Authors: Kapur R. , Calvert J. M. , Rudolph A. S.
Published: February 2008

This communication describes our work in electrical, topological, and chemical micromodification of surfaces to modulate cellular form and function. We have addressed the surface physico-chemico-mechano properties of cell culture substrates that play a role in modulating cellular behavior. Single factorial model systems have been built using techniques adapted from microlithography. The tools and techniques of microfabrication, if harnessed and used correctly, can be enabling in elucidating the underlying principles and fundamental forces driving the cell–substrate interface. Additionally, the long-term practical applications of microfabrication in medicine and biomaterial/tissue engineering lie in enabling “communication” with living cells/tissues at the cellular and subcellular levels.
Microcontact Printing for Precise Control of Nerve Cell Growth in Culture
Authors: Wheeler B. C. , Corey J. M. , Brewer G. J. , Branch D. W.
Published: February 2008

Microcontact printing, facilitated by silane linker chemistry and high-relief stamps, creates precise patterns of proteins, which in turn control growth of hippocampal neurons in culture. This additive, multi-mask technique permits several different molecules to be patterned on the same substrate. The covalent linker technology permits relatively long-term (two-week) compliance of neurons to the stamped pattern against a polyethylene glycol background. When polylysine was stamped adjacent to a laminin/polylysine mixture, neural somata and dendrites preferred the polylysine while axons prefer the mixture or the border between the two.
Surfactant-Spreading and Surface-Compression Disturbance on a Thin Viscous Film
Authors: Bull J. L. , Nelson L. K. , Walsh J. T. , Glucksberg M. R. , Schu¨rch S. , Grotberg J. B.
Published: February 2008

Spreading of a new surfactant in the presence of a pre-existing surfactant distribution is investigated both experimentally and theoretically for a thin viscous substrate. The experiments are designed to provide a better understanding of the fundamental interfacial and fluid dynamics for spreading of surfactants instilled into the lung. Quantitative measurements of spreading rates were conducted using a fluorescent new surfactant that was excited by argon laser light as it spread on an air–glycerin interface in a petri dish. It is found that pre-existing surfactant impedes surfactant spreading. However, fluorescent microspheres used as surface markers show that pre-existing surfactant facilitates the propagation of a surface-compression disturbance, which travels faster than the leading edge of the new surfactant. The experimental results compare well with the theory developed using lubrication approximations. An effective diffusivity of the thin film system is found to be Deff = (E*Γ¯)/(μ/H¯), which indicates that the surface-compression disturbance propagates faster for larger background surfactant concentration, Γ¯, larger constant slope of the σ*−Γ* relation, −E*, and smaller viscous resistance, μ/H¯. Note that σ* and Γ* are the dimensional surface tension and concentration, respectively, μ, is fluid viscosity, and H¯ is the unperturbed film thickness.
A Poroelastic Model That Predicts Some Phenomenological Responses of Ligaments and Tendons
Authors: Atkinson T. S. , Haut R. C. , Altiero N. J.
Published: February 2008

Experimental evidence suggests that the tensile behavior of tendons and ligaments is in part a function of tissue hydration. The models currently available do not offer a means by which the hydration effects might be explicitly explored. To study these effects, a finite element model of a collagen sub-fascicle, a substructure of tendon and ligament, was formulated. The model was microstructurally based, and simulated oriented collagen fibrils with elastic-orthotropic continuum elements. Poroelastic elements were used to model the interfibrillar matrix. The collagen fiber morphology reflected in the model interacted with the interfibrillar matrix to produce behaviors similar to those seen in tendon and ligament during tensile, cyclic, and relaxation experiments conducted by others. Various states of hydration and permeability were parametrically investigated, demonstrating their influence on the tensile response of the model.
Optimal Seat Suspension Design Based on Minimum “Simulated Subjective Response”
Authors: Wan Y. , Schimmels J. M.
Published: February 2008

This work addresses a method for improving vertical whole body vibration isolation through optimal seat suspension design. The primary thrusts of this investigation are: (1) the development of a simple model that captures the essential dynamics of a seated human exposed to vertical vibration, (2) the selection and evaluation of several standards for assessing human sensitivity to vertical vibration, and (3) the determination of the seat suspension parameters that minimize these standards to yield optimal vibration isolation. Results show that the optimal seat and cushion damping coefficients depend very much on the selection of the vibration sensitivity standard and on the lower bound of the stiffnesses used in the constrained optimization procedure. In all cases, however, the optimal seat damping obtained here is significantly larger (by than a factor of 10) than that obtained using existing seat suspension design methods or from previous optimal suspension studies. This research also indicates that the existing means of assessing vibration in suspension design (ISO 7096) requires modification.
Shear Properties of Human Brain Tissue
Authors: Donnelly B. R. , Medige J.
Published: February 2008

The objective of this study was to determine a relationship between shear stress and strain for human brain tissue by performing transient, single-pulse, high-rate, shear displacement tests. A constant velocity, parallel plate shear test device was designed and fabricated. This equipment generated constant rate shear strains in cylindrical tissue samples mounted between the shear plates. The transverse reaction force at the upper end of the sample was measured during the event with a sensitive quartz piezoelectric force transducer, thus obtaining the force associated with the displacement versus time ramp. Shear tests were performed on 125 tissue samples taken from twelve fresh cadaver brain specimens. The average true shear stress and finite strain were calculated. A nonlinear, viscoelastic, standard solid model was fit to the constant rate test data and the material constants were determined.