Leukocyte deformability: finite element modeling of large viscoelastic deformation.

An axisymmetric deformation of a viscoelastic sphere bounded by a prestressed elastic thin shell in response to external pressure is studied by a finite element method. The research is motivated by the need for understanding the passive behavior of human leukocytes (white blood cells) and interpreting extensive experimental data in terms of the mechanical properties. The cell at rest is modeled as a sphere consisting of a cortical prestressed shell with incompressible Maxwell fluid interior. A large-strain deformation theory is developed based on the proposed model. General non-linear, large strain constitutive relations for the cortical shell are derived by neglecting the bending stiffness. A representation of the constitutive equations in the form of an integral of strain history for the incompressible Maxwell interior is used in the formulation of numerical scheme. A finite element program is developed, in which a sliding boundary condition is imposed on all contact surfaces. The mathematical model developed is applied to evaluate experimental data of pipette tests and observations of blood flow.     

Head kinematics in mini-sled tests of foam padding: relevance of linear responses from free motion headform (FMH) testing to head angular responses

The revised Federal Motor Vehicle Safety Standard (FMVSS) No. 201 specifies that the safety performance of vehicle upper interiors is determined from the resultant linear acceleration response of a free motion headform (FMH) impacting the interior at 6.7 m/s. This study addresses whether linear output data from the FMH test can be used to select an upper interior padding that decreases the likelihood of rotationally induced brain injuries. Using an experimental setup consisting of a Hybrid III head-neck structure mounted on a mini-sled platform, sagittal plane linear and angular head accelerations were measured in frontal head impacts into foam samples of various stiffness and density with a constant thickness (51 mm) at low (approximately 5.0 m/s), intermediate (approximately 7.0 m/s), and high (approximately 9.6 m/s) impact speeds. Provided that the foam samples did not bottom out, recorded peak values of angular acceleration and change in angular velocity increased approximately linearly with increasing peak resultant linear acceleration and value of the Head Injury Criterion (HIC36). The results indicate that the padding that produces the lowest possible peak angular acceleration and peak change in angular velocity without causing high peak forces is the one that produces the lowest possible HIC36 without bottoming out in the FMH test.     

Experimental cavitation studies in a model head-neck system

The response of two different fluid-filled head-neck models to impact was studied experimentally to provide information concerning the validity of the widely prevalent cavitation hypothesis of brain damage. The structures consisted of an acrylic spherical shell with an outside diameter of about 188 mm and a human calvarium with a clear polyester resin occiput, representing the head, each coupled to an articulated artificial viscoelastic neck. Transient phenomena were initiated by the impact of either cylindrical projectiles fired from a pneumatic gun or by the pendulum drop of an aluminum spherical shell onto a small truncated aluminum cone attached to the head models. A short strain-gaged aluminum cylinder served to measure the input force history, while the pressure in the brain-simulating fluid was ascertained by means of Z-cut tourmaline crystals located along the impact axis at the coup, center and contrecoup positions. The occipital regions of the models were photographed at framing rates of 4000–8000 s⁻¹ to visually examine the cavitation phenomena.

Coup, contrecoup and resonating cavitation were detected and found to coincide temporally with negative pressure transients in both head-neck models. These results lend some support to the cavitation theory as a possible mechanism for brain damage.     

Effect of bending on the axisymmetric vibrations of a spheroidal model of the head

The head is modelled as an elastic prolate spheroidal shell filled with an inviscid, compressible fluid. Bending effects are included, and the free vibration frequency spectrum obtained is compared with that of an earlier spheroidal model using extensional (membrane) shell theory and with a spherical model including bending. The differences between the present results and those reported previously are significant.     

Human shoulder response to side impacts: a finite element study

This study aimed at developing a shoulder finite element (FE) model able to simulate the dynamic behaviour and to predict injuries in case of side impacts. This model is an updated version of the initial Human Model for Safety (HUMOS) FE model of the human body. Simulations performed with the model have been compared to experimental results of side impact tests conducted previously at INRETS. The shoulder model response under lateral impact appears to be in good agreement with experimental data such as impact force and shoulder deflections for different impact speeds and impact directions. These results seem promising for future applications such as shoulder injury prediction in simulated car crashes.     

Brain tissue analysis of impacts to American football helmets

Concussion in American football is a prevalent concern. Research has been conducted examining frequencies, location, and thresholds for concussion from impacts. Little work has been done examining how impact location may affect risk of concussive injury. The purpose of this research was to examine how impact site on the helmet and type of impact, affects the risk of concussive injury as quantified using finite element modelling of the human head and brain. A linear impactor was used to impact a helmeted Hybrid III headform in several locations and using centric and non-centric impact vectors. The resulting dynamic response was used as input for the Wayne State Brain Injury Model to determine the risk of concussive injury by utilizing maximum principal strain as the predictive variable. The results demonstrated that impacts that occur primarily to the side of the head resulted in higher magnitudes of strain in the grey and white matter, as well as the brain stem. Finally, commonly worn American football helmets were used in this research and significant risk of injury was incurred for all impacts. These results suggest that improvements in American football helmets are warranted, in particular for impacts to the side of the helmet.     

An analytical model of traumatic diffuse brain injury

Diffuse axonal injury (DAI) with prolonged coma has been produced in the primate using an impulsive, rotational acceleration of the head without impact. This pathophysiological entity has been studied subsequently from a biomechanics perspective using physical models of the skull-brain structure. Subjected to identical loading conditions as the primate, these physical models permit one to measure the deformation within the surrogate brain tissue as a function of the forces applied to the head. An analytical model designed to approximate these experiments has been developed in order to facilitate an analysis of the parameters influencing brain deformation. These three models together are directed toward the development of injury tolerance criteria based upon the shear strain magnitude experienced by the deep white matter of the brain. The analytical model geometry consists of a rigid, right-circular cylindrical shell filled with a Kelvin-Voigt viscoelastic material. Allowing no slip on the boundary, the shell is subjected to a sudden, distributed, axisymmetric, rotational load. A Fourier series representation of the load allows unrestricted load-time histories. The exact solution for the relative angular displacement (V) and the infinitesimal shear strain (epsilon) at any radial location in the viscoelastic material with respect to the shell was determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dynamic response of intraocular pressure and biomechanical effects of the eye considering fluid-structure interaction

The vibration characteristics of shell structures such as eyes have been shown to vary with intraocular pressure (IOP). Therefore, vibration characteristics of the eye have the potential to provide improved correlation to IOP over traditional IOP measurements. As background to examine an improved IOP correlation, this paper develops a finite element model of an eye subject to vibration. The eye is modeled as a shell structure filled with inviscid pressurized fluid in which there is no mean flow. This model solves a problem of a fluid with coupled structural interactions of a generally spherically shaped shell system. The model is verified by comparing its vibrational characteristics with an experimental modal analysis of an elastic spherical shell filled with water. The structural dynamic effects due to change in pressure of the fluid are examined. It is shown that the frequency response of this fluid-solid coupled system has a clear increase in natural frequency as the fluid pressure rises. The fluid and structure interaction is important for accurate prediction of system dynamics. This model is then extended to improve its accuracy in modeling the eye by including the effect of the lens to study corneal vibration. The effect of biomechanical parameters such as the thicknesses of different parts of the eye and eye dimensions in altering measured natural frequencies is investigated and compared to the influence of biomechanical parameters in Goldmann applanation tonometry models. The dynamic response of the eye is found to be less sensitive to biomechanical parameters than the applanation tonometry model.     

Finite element analysis for the evaluation of protective functions of helmets against ballistic impact

The ballistic impact of a human head model protected by a Personnel Armor System Ground Troops Kevlar^® helmet is analysed using the finite element method. The emphasis is to examine the effect of the interior cushioning system as a shock absorber in mitigating ballistic impact to the head. The simulations of the frontal and side impacts of the full metal jacket (FMJ) and fragment-simulating projectile (FSP) were carried out using LS-DYNA. It was found that the Kevlar^® helmet with its interior nylon and leather strap was able to defeat both the FMJ and FSP without the projectiles penetrating the helmet. However, the head injuries caused by the FMJ impact can be fatal due to the high stiffness of the interior strap. The bulge section at the side of the Kevlar^® helmet had more room for deformation that resulted in less serious head injuries.     

Using a gel/plastic surrogate to study the biomechanical response of the head under air shock loading: a combined experimental and numerical investigation

A combined experimental and numerical study was conducted to determine a method to elucidate the biomechanical response of a head surrogate physical model under air shock loading. In the physical experiments, a gel-filled egg-shaped skull/brain surrogate was exposed to blast overpressure in a shock tube environment, and static pressures within the shock tube and the surrogate were recorded throughout the event. A numerical model of the shock tube was developed using the Eulerian approach and validated against experimental data. An arbitrary Lagrangian-Eulerian (ALE) fluid–structure coupling algorithm was then utilized to simulate the interaction of the shock wave and the head surrogate. After model validation, a comprehensive series of parametric studies was carried out on the egg-shaped surrogate FE model to assess the effect of several key factors, such as the elastic modulus of the shell, bulk modulus of the core, head orientation, and internal sensor location, on pressure and strain responses. Results indicate that increasing the elastic modulus of the shell within the range simulated in this study led to considerable rise of the overpressures. Varying the bulk modulus of the core from 0.5 to 2.0 GPa, the overpressure had an increase of 7.2%. The curvature of the surface facing the shock wave significantly affected both the peak positive and negative pressures. Simulations of the head surrogate with the blunt end facing the advancing shock front had a higher pressure compared to the simulations with the pointed end facing the shock front. The influence of an opening (possibly mimicking anatomical apertures) on the peak pressures was evaluated using a surrogate head with a hole on the shell of the blunt end. It was revealed that the presence of the opening had little influence on the positive pressures but could affect the negative pressure evidently.     

Inflation waves induced by axial acceleration of the aorta

The phenomenon of high-amplitude inflation waves resulting from a sharp axial acceleration of the aorta, as may occur in road accidents, is investigated theoretically. The aorta is modeled as an axisymmetric tapered membranic shell (tube) made of an incompressible, nonlinear viscoelastic material with cylindrical orthotropy. It is filled with an inviscid, incompressible fluid whose flow is considered as quasi-one dimensional along the tube axis. The equations of motion of the tube and of the fluid are solved numerically, by using a two-step explicit scheme, for several axial acceleration profiles. The solutions shows that an inflation wave is generated and it propagates in opposite direction to that of the acceleration. The wall stresses, deformations and their time derivatives as well as fluid velocity and pressure are determined along the tube at different time intervals. Peak axial and circumferential stresses are high, with the latter far exceeding the former. These stresses may cause rupture of the aorta.     

Structural models for human spinal motion segments based on a poroelastic view of the intervertebral disk

Analytical and finite element models (FEMs) were used to quantify poroelastic material properties for a human intervertebral disk. An axisymmetric FEM based on a poroelastic view of disk constituents was developed for a representative human spinal motion segment (SMS). Creep and steady-state response predicted by FEMs agreed with experimental observations, i.e., long-time creep occurs with flow in the SMS, whereas for rapid steady-state loading an "undrained," nearly incompressible response is evident. A relatively low value was determined for discal permeability. Transient and long-term creep FE analyses included the study of deformation, pore fluid flow, stress, and pore fluid pressure. Relative fluid motion associated with transient creep is related to nuclear nutrition and the overall mechanical response in the normal disk. Degeneration of the disk may be associated with an increase in permeability.     

Different phosphorylated forms of an insulin-sensitive glycosylphosphatidylinositol from rat hepatocytes

Labeling with [3H]galactose was employed to isolate a glycosylphosphatidylinositol from rat hepatocytes which might be involved in the action of insulin. The polar head group of this glycosylphosphatidylinositol was generated by phosphodiesterase hydrolysis with a phosphatidylinositol-specific phospholipase C from Bacillus cereus. By Dowex AG1 x 8 chromatography the polar head group could be separated into three radioactive peaks eluting at 100 mM (peak I), 200 mM (peak II) and 500 mM (peak III) ammonium formate, respectively. Peak III was the most active as an inhibitor of the cAMP-dependent protein kinase. Treatment of peak III with alkaline phosphatase markedly reduced its activity on cAMP-dependent protein kinase. When peaks I, II or III were treated with alkaline phosphatase and analyzed again by Dowex AG1 x 8 chromatography, the radioactivity eluted with the aqueous fraction. The above results indicate that the polar head group of the insulin-sensitive glycosylphosphatidylinositol from rat hepatocytes exists in three different phosphorylated forms and that the biological activity of this molecule depends on its phosphorylation state.     

An electrocatalytic transducer for l-cysteine detection based on cobalt hexacyanoferrate nanoparticles with a core–shell structure

The electrocatalytic oxidation of l-cysteine (CySH) was studied on cobalt hexacyanoferrate nanoparticles with a core–shell structure (iron(III) oxide core–cobalt hexacyanoferrate shell) using cyclic voltammetry and chronoamperometry. Voltammetric studies represented two quasi-reversible redox transitions for the nanoparticles in phosphate buffer solution (pH 7.4). In the presence of CySH, the anodic peak current of the Fe(II)/Fe(III) transition was increased, followed by a decrease in the corresponding cathodic peak current, whereas the peak currents related to the Co(II)/Co(III) transition almost remained unchanged. The results indicated that the nanoparticles oxidized CySH via a surface mediation electrocatalytic mechanism. The catalytic rate constant, the electron transfer coefficient, and the diffusion coefficient involved in the electrooxidation process of CySH are reported here. Ultrasensitive and time-saving determination procedures were developed for the analysis of the CySH, and the corresponding analytical parameters are reported. According to the proposed methods, CySH was determined with detection limits of 40 and 20 nm in batch and flow systems, respectively. The proposed amperometric method was also applied to the analysis of CySH in human urine and serum blood samples.     

On selection of functional response models: Holling’s models and more

Model selection is a common and established research method. Statistically rigorous model selection methods are used in a variety of research fields. In contrast, studies that characterize functional response models commonly use a model selection method that is specific to functional response studies. The specific method aims to distinguish between Holling’s type II and type III functional response models. This paper discusses problems associated with the specific method and suggests that it would be better to use general model selection methods that allow to consider a variety of models.     

Improved model for myocardial diffuse reflectance spectra by including mitochondrial cytochrome aa3, methemoglobin,and inhomogenously distributed RBC

The aim of this study was to compare a previously used light transport model (I) comprising the chromophores hemo‐ and myoglobin, fat, and water, with two extended models, where the chromophores of cytochrome aa3, methemo‐ and metmyoglobin are added (model II), and in addition, accounting for an inhomogenous hemoglobin distribution (model III). The models were evaluated using calibrated diffuse reflectance spectroscopy measurements on the human myocardium. Model II proved a significantly better spectral fitting, especially in the wavelength ranges corresponding to prominent absorption characteristics for the added chromophores. Model III was significantly better than model II and displayed a markedly higher tissue fraction and saturation of hemo‐ and myoglobin. The estimated tissue chromophore fractions, saturation and oxidation levels, were in agreement with other studies, demonstrating the potential of diffuse reflectance spectroscopy measurements for evaluating open heart surgery. However, the choice of chromophores and vessel packaging effects in the light transport model has a major effect on the results. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Protein expression during the embryonic development of a gastropod

Despite the potential use of gastropod embryos in basic and applied research, little is known about their protein expression. We examined, for the first time, changes in proteomic profile during embryonic development of Pomacea canaliculata from an embryo without a shell (stage II) to an embryo with a fully formed shell (stage III) to understand the roles that proteins play in critical developmental events, such as the formation of shell, operculum and heart, and the differentiation of head and foot. To analyze protein expression during development, we used 2‐DE to detect, MS to analyze, and de novo peptide sequencing followed by MS‐BLAST to identify the proteins. The de novo cross‐species protein identification method was adopted because of a lack of genomic and proteomic data in the whole class of Gastropoda. 2‐DE detected approximately 700 protein spots. Among the 125 spots that were abundant, 52% were identified, a marked improvement over the conventional direct MS‐BLAST method. These proteins function in perivitelline fluid utilization, shell formation, protein synthesis and folding, and cell cycle and cell fate determination, providing evidence to support that this embryonic period is a period of dynamic protein synthesis and metabolism. The data shall provide a basis for further studies of how gastropod embryos respond to natural and human‐induced changes in the environment.     

A biomechanical evaluation of whiplash using a multi-body dynamic model

This paper presents a biomechanical evaluation of whiplash injury potential during the initial extension motion of the head in a rear-end collision. A four-segment dynamic model is developed in the sagittal plane for the analysis. The model response is validated using the existing experimental data and is shown to simulate the "S-shape" kinematics of the cervical spine and the resulting dynamics observed in human and cadaver experiments. The model is then used to evaluate the effects of parameters such as collision severity, head/headrest separation, and the initial head orientation in the sagittal plane on the "S-shape" kinematics of the cervical spine and the resulting neck loads. It is shown, for example, that the cervical spine forms an "S-shape" for a range of change in speeds and that at lower and higher speeds changes the spine does not form the "S-shape." Furthermore, it is shown that the "S-shape" formation also depends on the head to headrest separation distance.     

A six degree of freedom head acceleration measurement device for use in football

The high incidence rate of concussions in football provides a unique opportunity to collect biomechanical data to characterize mild traumatic brain injury. The goal of this study was to validate a six degree of freedom (6DOF) measurement device with 12 single-axis accelerometers that uses a novel algorithm to compute linear and angular head accelerations for each axis of the head. The 6DOF device can be integrated into existing football helmets and is capable of wireless data transmission. A football helmet equipped with the 6DOF device was fitted to a Hybrid III head instrumented with a 9 accelerometer array. The helmet was impacted using a pneumatic linear impactor. Hybrid III head accelerations were compared with that of the 6DOF device. For all impacts, peak Hybrid III head accelerations ranged from 24 g to 176 g and 1,506 rad/s(2) to 14,431 rad/s(2). Average errors for peak linear and angular head acceleration were 1% ± 18% and 3% ± 24%, respectively. The average RMS error of the temporal response for each impact was 12.5 g and 907 rad/s(2).     

Surface friction in near-vertex head and neck impact increases risk of injury

A computational head-neck model was developed to test the hypothesis that increases in friction between the head and impact surface will increase head and neck injury risk during near-axial impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. For frictionless impact surfaces, the model reproduced the kinematics and kinetics observed in near-axial impacts to cadaveric head-neck specimens. Increases in the coefficient of friction between the head and impact surface over a range from 0.0 to 1.0 resulted in increases of up to 40, 113, 9.8, and 43% in peak post-buckled resultant neck forces, peak moment at the occiput-C1 joint, peak resultant head accelerations, and HIC values, respectively. The most dramatic increases in injury-predicting quantities occurred for COF increases from 0.0 to 0.2, while further COF increases above 0.5 generally produced only nominal changes. These data suggest that safety equipment and impact environments which minimize the friction between the head and impact surface may reduce the risk of head and neck injury in near-vertex head impact.