Constitutive modelling of brain tissue: experiment and theory

Recent developments in computer-integrated and robot-aided surgery--in particular, the emergence of automatic surgical tools and robots--as well as advances in virtual reality techniques, call for closer examination of the mechanical properties of very soft tissues (such as brain, liver, kidney, etc.). The ultimate goal of our research into the biomechanics of these tissues is the development of corresponding, realistic mathematical models. This paper contains experimental results of in vitro, uniaxial, unconfined compression of swine brain tissue and discusses a single-phase, non-linear, viscoelastic tissue model. The experimental results obtained for three loading velocities, ranging over five orders of magnitude, are presented. The applied strain rates have been much lower than those applied in previous studies, focused on injury modelling. The stress-strain curves are concave upward for all compression rates containing no linear portion from which a meaningful elastic modulus might be determined. The tissue response stiffened as the loading speed increased, indicating a strong stress-strain rate dependence. The use of the single-phase model is recommended for applications in registration, surgical operation planning and training systems as well as a control system of an image-guided surgical robot. The material constants for the brain tissue are evaluated. Agreement between the proposed theoretical model and experiment is good for compression levels reaching 30% and for loading velocities varying over five orders of magnitude.     

Psychogenic stress: theory, experiment, practice

Experiments on animals (rodents, beasts, primates) and observation in man have ascertained that psychogenic stress is the most important defensive state of living organisms, which is constantly involved in order to maintain their interaction with external psychogenic factors. Three types of stress should be distinguished, these include normostress, hypostress, and hyperstress. Normostress has definite limits within which optimal responses, which are typical for an individual are realized. Hyperstress develops as a defensive response to extremely strong (extraordinary) stimuli. Hypostress can be observed in the retarded development of self-regulation mechanisms. All three types of stress are provided by a fundamental property of living organisms--the self-regulation mechanism, which realizes the keeping back of normostress constancy, or return of hyper- and hypostress to the normostress status. And only if an extreme deficit of self-regulation mechanisms takes place, hyper(hypo)stress could become a condition for the development of pathology.     

Equity theory and career pay: A computer simulation approach.

Used computer simulation to convert E. Jaques's 1961 theory of equitable payment into the composites model utilized by the M. Haire et al's (see record 1967-14292-001) study of career pay. Salaries of 100 Ss were stochastically allocated for 25 time periods. A Markovian process model produced a set of pay parameters that more closely replicated past empirical findings than the parameters produced by an independent process model. Distributing pay increases according to differentially developing work capacity curves yielded pay increases distributed at random with respect to past salaries. Thus, Jaques's theory of equitable payment provides one explanation for the empirical findings generated by previous studies of career pay curves. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Adiabatic electron transfer: comparison of modified theory with experiment

The radical cations of properly designed bishydrazines allow comparison of observed and calculated electron transfer rate constants. These compounds have rate constants small enough to be measured by dynamic electron spin resonance spectroscopy and show charge transfer bands corresponding to vertical excitation from the energy well for the charge occurring upon one hydrazine unit to that for the electron-transferred species. Analysis of the data for all six compounds studied indicates that the shape of the adiabatic surface on which electron transfer occurs can be obtained from the charge transfer band accurately enough to successfully predict the electron transfer rate constant and that explicit tunneling corrections are not required for these compounds.     

Competitive immunosorbent assays using ligand-enzyme conjugates bifunctional liposomes: theory and experiment

Two models of immunoadsorbent assays are developed that describe the competitive adsorption of labeled antigen and unlabeled analyte to antibody binding sites immobilized on a solid surface. In the first model, a small labeled antigen and a small unlabeled analyte compete with only binding site limitations and no steric limitations. A multicomponent langmuir isotherm results that is sufficient to quantify competitive adsorption. This model can describe, with no adjustable parameters, the data of competitive assays for biotin using biotinylated horseradish peroxidase (B-HRP) over a wide range of anti-biotin antibody (ABA) surface densities. In the second model, the small unlabeled analyte competes with a large colloidal particle containing many antigens and enzyme labels attached to its surface. This model quantifies the steric interference that large particles can experience upon binding (large ligand effect) due to the lower probability of finding an available area of the right size to accommodate the larger adsorbent. This large ligand model also takes into account the increased probability of binding a large particle due to the larger number of antibody binding sites covered per collision. The resulting model is used to analyze the competitive assay data of biotin competing with liposomes to which many biotin and HRP molecules have been conjugated. This analysis is of interest because previous work has shown that these bifunctional liposomes can reduce the detection limit for antigens in bulk solution relative to assays performed with conventional small labeled antigens.