Influence of the mechanical properties of a manipulandum on human operator dynamics

An active servo-system was used to change the stiffness of a manipulandum used in a positioncontrol pursuit-tracking task. The elastic stiffness of the manipulandum connected to the forearm was set by a computer at one of five levels ranging from 0 N/m to 2000 N/m. Subjects were required to track, either by moving their forearm or by generating a force isometrically, a visually presented target whose position changed randomly every second for 100 s. Nonparametric and parametric impulse response functions were calculated between the input (target) and output (force or position) in each tracking condition, and revealed that for all subjects force control was faster than position control when the stiffness of the manipulandum was set at 0 N/m. Subjects were also consistently faster in reaching the target when the stiffness was greater than zero, and were more accurate (steadystate response) when the stiffness of the manipulandum was set at lower rather than higher amplitudes. The parametric impulse response functions revealed that the human operator system was underdamped (0.7) with a natural frequency of approximately 8 rad/s. These findings were interpreted in terms of the responses of the various subsystems (visual, cognitive, contractile, limb mechanics) that comprise the human operator's response.     

Influence of thickness on the mechanical properties for starch films

The aim of the present study was to investigate some mechanical properties of starch films. Starch is a natural common polymer in nature and the use of natural materials is increasing in the industries. In this study, the mechanical properties of starch plasticized with 30 parts by weight, of glycerol, are investigated. For the mechanical testing films of different thickness were used, the thickness varied between 0.5 and 2.5 mm. T_g was measured with a differential scanning calorimeter and with a dynamical mechanical analysis. The starch films were tested in tension and characterised in terms of stiffness, strength and failure strain. Fracture toughness was measured by single edge notch tests. Both stiffness and strength showed a strong dependence on film thickness, stronger then expected from linear fracture mechanics. This can be due to the different molecule orientation in the films, and due to the crystallinity of the films.     

Targeting drug delivery to the lungs by inhalation

Most drugs targeted to the respiratory tract are used for their local action. For example, ephidrine for nasal decongestion, beta-2 agonists for bronchodilatation, and inhaled steroids to suppress the inflammation seen in asthmatic airways. Since the drug is delivered directly to its required site, only a small quantity is needed for an adequate therapeutic response, and consequently there is a low incidence of systemic side effects compared with oral or intravenous administration. More recently, it has become apparent that the lining of the respiratory tract, from nasal mucosa to airways and alveoli, may be used for the absorption of a drug for its systemic effect. This route of administration may be particularly attractive if it avoids the metabolic destruction encountered when some drugs are administered by alternative routes (for instance, peptides and proteins are rapidly destroyed by peptidases when Oven by the oral route). If there is a lack ofclinical response to an aerosolized drug, it is important to question whether the drug has failed or whether delivery to the site of action is inadequate. To deliver therapeutic agents by inhalation to the lower respiratory tract, inhaled drug particles must have appropriate aerodynamic characteristics. In addition, the anatomy and pathophysiology of the patient's respiratory tract, mode of inhalation through the inhaler, and the characteristics of the inhalational device itself, may significantly affect drug deposition.     

Influence of hydrocortisone on the mechanical properties of the cerebral endothelium in vitro

Cerebral endothelial cells accomplish the barrier functions between blood and brain interstitium. Structural features are the tight junctions between adjacent endothelial cells and the formation of marginal folds at the cell-cell contacts. The glucocorticoid hydrocortisone (HC) has been reported to enforce the blood-brain-barrier in vitro measurable by an increase of the transendothelial electrical resistance. This study shows the impact of HC on the mechanical and morphological properties of confluent cell layers of brain microvascular endothelial cells. HC induces an increase in height of these marginal folds and a reduction of the intercellular contact surface. These morphological changes are accompanied by changes in cell elasticity. Staining of fibrous actin indicates that HC induces a reorganization of the actin cortex. The quantitative determination of the local elastic properties of cells reveals for the first time an HC-induced increase of the representative Young's modulus according to cytoskeletal rearrangements. For this study, cells of two different species, porcine brain capillary endothelial cells and murine brain capillary endothelial cells, were used yielding similar results, which clearly demonstrates that the HC effect on the cell elasticity is species independent.     

Influence of solvent composition on the mechanical properties of arterial elastin

We have investigated the effects of changes in solution composition on the mechanical properties of rings of arterial elastin. The time course of force equilibration at constant strain following a change in the composition of the bathing solution was measured. Both the force developed during slow extension and force relaxation following rapid straining were also measured in each of the test solutions. The results are difficult to summarize because all of the primitive quantities measured--sample dimension, slope of the force-extension curve, force overshoot and time of relaxation--as well as the derived quantities such as elastic modulus changed in different and apparently uncorrelated ways. Changes in pH and ionic composition of the bathing solution had small effects consistent with the low fixed charge density of elastin. Solutions of glucose, sucrose, and ethylene glycol had larger effects consistent with changes in hydrophobic interactions. The viscosity of the solution that penetrated the intrafibrillar space of the elastin appeared to be a major determinant of the dynamic response.     

Influence of domain orientation on the mechanical properties of regenerated cellulose fibers

The determination of the crystal orientation of regenerated cellulose fibers produced under different drawing regimes is presented. Orientation is determined by using wide-angle X-ray diffraction from a synchrotron source and by measuring the azimuthal width of equatorial reflections. The orientation parameter theta is then determined to compare fiber samples. By using a 500 nm beam size, clear differences between the crystal orientations of the skin and the core of the fibers are reported for a range of differently processed fibers for the first time. These results are shown to have implications for the mechanical properties of regenerated cellulose fibers. By applying tensile deformation to fiber bundles it is shown that the most misoriented samples undergo rapid decreases in the orientation parameter, which is an indication of crystal reorientation. However, the more highly oriented fibers undergo little reorientation. An average shear modulus for these fibers is determined by placing the data on a master curve and fitting with a model equation. By using another model for the fibers of low orientation and the shear modulus from the master curve analysis, it is shown that the deformation of less oriented fibers is dominated by shear between crystals, whereas the more oriented filaments are likely to undergo more significant chain deformation. By using a new model for fibers of low orientation, a parameter ksigma is introduced that gives the proportion of the fiber stress that is due to crystal shear. Systematic differences between this parameter for fibers of increasing initial orientation are reported. Moreover it is shown that the fibers of initially lower average orientation are governed by uniform strain, in agreement with the new model, whereas more highly oriented fibers deform under uniform stress. Furthermore, the model that we propose for misoriented domains and the use of a new factor dictating the proportion of shear stress may have general applications in materials engineering.     

Influence of network topology on mechanical properties of network polymers

We studied theoretically how the network topology influences the mechanical properties of polymers. We used conclusions of thermofluctuation theory of fracture and graph theory. The long-term strengths of monofunctional and polyfunctional networks were compared. The cross-link functionality distribution of the polyfunctional networks is a power function. All other conditions being equal, the long-term strengths of the polyfunctional polymer networks are some three to four times the long-term strengths of the monofunctional networks.