Accuracy of determining the point of force application with piezoelectric force plates

The accuracy of determining the point of force application with piezoelectric force plates, as specified by the manufacturer, is lower than needed for certain applications. The purpose of this study was to evaluate the accuracy of a commonly used plate (KISTLER type 9287) and to improve it by proposing a correction algorithm. Forces were applied to a wooden board, supported in one corner by a stylus that rested on the force plate. To determine the influence of position and magnitude of the force vector, the stylus was placed on 117 different locations, and calibrated masses were used to exert vertical forces between 0 and 2000 N. To determine the influence of loading rate, dynamic tests were performed in which a subject ran across the board. In static tests at a given stylus position with actual coordinates x (short axis) and y (long axis), it was found that the calculated coordinates x and y of the point of force application had virtually constant values at forces above 1000 N. In dynamic tests, oscillations could occur in x and y with an amplitude of more than 20 mm. When these were avoided or removed by filtering, static and dynamic tests at a given stylus position showed the same values for x and y at forces above 1000 N. Across stylus positions, the errors x-x and y-y (measured at 1600 N) ranged from -20 to +20 mm. The average over 117 points of the absolute errors magnitude of x-x and magnitude of y-y amounted to 3.5 and 6.3 mm, respectively (mean values of three plates).(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancements in the accuracy of the center of pressure (COP) determined with piezoelectric force plates are dependent on the load distribution

Errors up to +/- 30 mm in determining the COP with piezoelectric force plates have been reported in the literature. To compensate for these errors, correction formulas were proposed, based on measurements with single point loads. In this paper, it will be shown that the errors in the COP depend on the load distribution. Two examples are presented: (1) simulated balance study, and (2) different pressure patterns during walking. Accurate corrections can only be made for forces distributed over a small area. Errors are expected to be overcompensated if there are only a few pressure peaks separated by large distances. These errors can be as large as the statistical errors (5.8 +/- 3.7 mm) after compensation. For certain situations, it is probably better not to use correction formulas.     

Parameters influencing the accuracy and practical applicability of UV indicator cards.

In order to evaluate the potential hazard for the skin and the eye presented by solar ultraviolet radiation, and to take appropriate precautions, it is necessary to characterise the biological effective irradiance or dose. Recently, inexpensive UV indicator cards became available that in principle would provide an attractive means to roughly indicate the local level of erythemal solar UV irradiance. We have characterised the properties of a number of different types of UV indicator cards. Several parameters which may influence the colour of the cards were examined with both outdoor trials under solar UV as well as indoor trials using a filtered xenon arc lamp. Our findings show that the tested cards do not give an appropriate estimation of the effective irradiance due to their spectral sensitivity and their temperature dependence. The application area of the tested UV indicator cards is therefore limited to certain temperature ranges and to seasons where a certain ratio between solar UV-A and solar UV-B occurs.     

Consequences of forward translation of the point of force application for the mechanics of running

In running humans, the point of force application between the foot and the ground moves forwards during the stance phase. Our aim was to determine the mechanical consequences of this 'point of force translation' (POFT). We modified the planar spring-mass model of locomotion to incorporate POFT, and then compared spring-mass simulations with and without POFT. We found that, if leg stiffness is adjusted appropriately, it is possible to maintain very similar values of peak vertical ground reaction force (GRF), stance time, contact length and vertical centre of mass displacement, whether or not POFT occurs. The leg stiffness required to achieve this increased as the distance of POFT increased. Peak horizontal GRF and mechanical work per step were lower when POFT occurred. The results indicate that the lack of POFT in the traditional spring-mass model should not prevent it from providing good predictions of peak vertical GRF, stance time, contact length and vertical centre of mass displacement in running humans, if an appropriate spring stiffness is used. However, the model can be expected to overestimate peak horizontal GRF and mechanical work per step. When POFT occurs, the spring stiffness in the traditional spring-mass model is not equivalent to leg stiffness. Therefore, caution should be exercised when using spring stiffness to understand how the musculoskeletal system adapts to different running conditions. This can explain the contradictory results in the literature regarding the effect of running speed on leg stiffness.     

Factors influencing the anterior component of occlusal force

We hypothesized that the anterior component of occlusal force (ACF) generated by mandibular molars was a function of molar inclination, height of the transverse condylar axis above the occlusal plane, steepness of the occlusal plane, gape, molar root dimensions, interproximal tooth contact force when not biting, and bite force. Our research aim was to identify those biomechanical factors which determine ACF. Mandibular second molars were axially loaded with a 90 N force (10 mm second molar gape) in 15 subjects, and the resulting ACF was measured at the mandibular first molar-second premolar contact using a recording technique based on interproximal frictional forces. Morphologic measurements were obtained from lateral cephalometric radiographs of each subject and included: Frankfort mandibular plane angle, occlusal plane angle, angles formed by the longitudinal axis of the second molar and the occlusal and mandibular planes, perpendicular distance from the top of the condyle to the occlusal plane, and second molar root width and root length. For ten subjects, ACF resulting from axial loads of 50, 100, 150, and 200 N was measured. For ten subjects, ACF resulting from an axial load of 50 N and second molar gapes of 10 mm, 14 mm, 18 mm, and 22 mm were measured. ACF increased with increasing gape and increased proportionally to increasing bite force. Correlation and stepwise regression analyses revealed that ACF varies with interproximal tooth contact force when not biting (contact ‘tightness’) and molar root width (model R² = 0.71, p < 0.01). The hypothesis that ACF is a function of bite force, gape, molar root width, and interproximal contact tightness has been supported, and the hypothesis that ACF is a function of molar inclination, occlusal plane steepness, condylar axis height, and root length was rejected.     

Geometrical factors influencing muscle force development. II. Radial forces.

If the subfragment-2 (S2) portion of the myosin cross-bridge to actin does not lie parallel to the myofilament axes then when a muscle fiber contracts, there will be a radial component to the cross-bridge force. When the subfragment-1 (S1) portion of the cross-bridge attaches to actin with its long axis projecting through the filament axis, the magnitude of the radial force depends upon the azimuthal location of the actin site, but when the attachment of the S1 to actin is slewed, as in the reconstruction of Moore et al. (J. Mol. Biol., 1970, 50:279-294), then for a single cross-bridge the radial component of the cross-bridge force is not quite so sensitive to actin site location and is approximately 0.1 the axial component. In both cases, the ratio of the radial to axial force decreases with decreasing filament separation. If the radial-axial force ratio for each cross-bridge is approximately 0.1, then at full overlap in a frog skeletal muscle fiber the radial component of the cross-bridge force accompanying full activation will exert a compressive pressure of approximately 5 X 10(-3) atm. This would have little effect upon an intact muscle fiber where the volume constraints are likely osmotic, but it might produce a 1-2% change in filament spacing in a "skinned" muscle fiber from which the sarcolemma had been removed. These computations assume that the S2 link between the S1 head and the myosin filament does not support a bending moment of shear. If it does, then the radial component of the cross-bridge will be either greater or less, depending on the specific cross-bridge geometry.     

Factors influencing accuracy of screw displacement axis detection with a D.C.-based electromagnetic tracking system.

Recent technical improvements and cost reductions in electromagnetic motion tracking systems invite their application to motion axis determination in the surgical setting. After evaluation of the accuracy of a state-of-the-art D.C. electromagnetic tracking system, which generates complete three-dimensional kinematic outputs from just a single receiver, we calculated screw displacement axes (SDA's) from its source data. The accuracy of SDA determination from such source data was evaluated for various rotational increment sizes around a revolute joint. A novel smoothing procedure, customized for this type of source data, was developed, enabling SDA detection from incremental rotations of less than 1 deg, at an accuracy appropriate for intra-operative measurement of human joint motion. Examples of SDA determination are given for motion tracking of a ball joint and of the elbow articulation.     

Factors influencing the output of an implantable force transducer

The objective of this study was to evaluate the performance of the Arthroscopically Implantable Force Probe (AIFP; MicroStrain, Burlington VT) for measuring force in a patellar tendon graft. Transducer drift, reproducibility of output due to the number of loading cycles and device location, and sensitivity to the tendon cross-sectional area were investigated. The AIFP was initialized, and then implanted into five human patellar tendon grafts three times; twice within the same location and once in a different location. The tendons were cyclically loaded in uniaxial tension for 500 cycles in each insertion site. The AIFP was then removed from the tendon and the baseline output was remeasured. It was determined that transducer drift was negligible. The relationship between the tensile load applied to the graft and AIFP output was quadratic and specimen dependent. The cyclic load response of the tendon-AIFP interface demonstrated a 24.9% decrease over the first 20 loading cycles, and subsequent cycling yielded relatively reproducible output. The output of the transducer varied when it was removed from the tendon and then reimplanted in the same location (range 3.7-109. 4% error), as well as in the second location (range 1.5-202.8% error). No correlation was observed between the cross-sectional area of the tendon and transducer output. This study concludes that implantable force probes should be used with caution and calibrated without removing the transducer from the graft.     

Influence of sensor size on the accuracy of in-vivo ligament and tendon force measurements.

In-vivo tendon forces are commonly measured using transducers, which detect tension in the tendon fibers. A poorly understood source of measurement errors is the difference in stress distribution within the tendon between experimental and transducer calibration conditions. The objective of this study was to investigate this source of error, and to determine whether these errors could be minimized by proper selection of transducer size. The study was conducted using the infrapatellar ligament (patellar tendon) of New Zealand White rabbits. Tendon force was measured with two different size implantable force transducers (IFTs), one Wide and one Narrow, and by a strain gaged load cell in series with the tendon. Tests were conducted at five different loading conditions selected to produce five different stress distributions within the tendon. One loading condition corresponded to a typical post-experiment calibration, and the data from that condition were used to develop a calibration equation for the transducer. The errors that resulted from using this calibration were determined by comparing the tendon force measured by the in-series load cell with the force predicted from the IFT output using the calibration equation. Changes in stress distribution produced measurement errors up to 64 N with the Narrow IFT but only 24 N with the Wide IFT. We found the measurement error was dependent on sensor width. Our results support the hypothesis that measurement errors can be caused by differences in tendon stress distribution between calibration and experimental conditions. We further showed that these errors can be minimized by using an IFT, which samples the tension in a large percentage of the tendon fibers. Information from this study can be used for selection of an appropriately sized implantable force transducer for measuring tendon and ligament force.     

Steady-state force-velocity relation in human multi-joint movement determined with force clamp analysis

To study the force-velocity characteristics of human knee-hip extension movement, a dynamometer, in which force was controlled by a servo system, was developed. Seated subjects pressed either bilaterally or unilaterally a force plate, a horizontal position of which was servo-controlled so as to equalize the measured force and a force command generated by a computer at a time resolution of 2 ms (force clamp). The force command was based on the relation between maximum isometric force and foot position within the range between 70% and 90% of "leg length" (LL: longitudinal distance between the sole of the foot and the hip joint), so that the same force relative to the maximum isometric force was consistently applied regardless of the foot position. By regulating the force according to this function, the force-velocity relation was determined. The force-velocity relation obtained was described by a linear function (n=17, r=-0.986 for 80% LL, r=-0.968 for 85% LL) within a range of force between 0.1 and 0.8F(0) (maximum isometric force). The maximum force extrapolated from the linear regression (F(max)) coincided with F(0) (n=17, F(0)/F(max)=1.00+/-0.09 for 80% LL and 1.00+/-0.20 for 85% LL). Also, the velocity at zero force (V(max)) was obtained from the extrapolation. When compared to the bilateral movements, unilateral movements gave rise to a smaller F(max) but the same V(max), suggesting that V(max) is independent of force and therefore represents the proper unloaded velocity. It is suggested that some neural mechanisms may be involved in the force-velocity relation of the knee-hip extension movement, and make it exhibit a linear appearance rather than a hyperbola.     

Protein molecular dynamics with electrostatic force entirely determined by a single Poisson-Boltzmann calculation

The electrostatic force including the intramolecular Coulombic interactions and the electrostatic contribution of solvation effect were entirely calculated by using the finite difference Poisson-Boltzmann method (FDPB), which was incorporated into the GROMOS96 force field to complete a new finite difference stochastic dynamics procedure (FDSD). Simulations were performed on an insulin dimer. Different relative dielectric constants were successively assigned to the protein interior; a value of 17 was selected as optimal for our system. The simulation data were analyzed and compared with those obtained from 500-ps molecular dynamics (MD) simulation with explicit water and a 500-ps conventional stochastic dynamics (SD) simulation without the mean solvent force. The results indicate that the FDSD method with GROMOS96 force field is suitable to study the dynamics and structure of proteins in solution if used with the optimal protein dielectric constant.     

Cell stiffness determined by atomic force microscopy and its correlation with cell motility

BackgroundCell stiffness is a crucial mechanical property that is closely related to cell motility. AFM is the most prevalent method used to determine cell stiffness by the quantitative parameter designated as Young's modulus. Young's modulus is regarded as a biomarker of cell motility, especially in estimating the metastasis of cancer cells, because in recent years, it has been repeatedly shown that cancerous cells are softer than their benign counterparts. However, some conflicting evidence has shown that cells with higher motility are sometimes stiffer than their counterparts. Thus, the correlation between cell stiffness and motility remains a matter of debate.Scope of reviewIn this review, we first summarize the reports on correlations between cell motility and stiffness determined by AFM and then discuss the major determinants of AFM-determined cell stiffness with a focus on the cytoskeleton, nuclear stiffness and methodological issues. Last, we propose a possible correlation between cell stiffness and motility and the possible explanations for the conflicting evidence.Major conclusionsThe AFM-determined Young's modulus is greatly affected by the characteristics of the cytoskeleton, as well as the procedures and parameters used in detection. Young's modulus is a reliable biomarker for the characterization of metastasis; however, reliability is questioned in the evaluation of pharmacologically or genetically modified motility.General significanceThis review provides an overview of the current understanding of the correlation between AFM-determined cell stiffness and motility, the determinants of this detecting method, as well as clues to optimize detecting parameters.     

Suppression of UV-induced erythema by topical treatment with melatonin. Influence of the application time point

The UV-suppressive effect of topical melatonin was assessed at different application time points in a double-blind randomized clinical trial. The lower back of 20 healthy volunteers was treated with 0.6 mg/cm2 melatonin or vehicle either 15 min before or 1, 30 or 240 min after UV irradiation. The erythema was evaluated visually and measured by chromametry 24 h after irradiation. UV-absorbing effects of melatonin were measured at a concentration of 8 microg/ml with a spectrophotometer. Melatonin absorbs UV light at a wavelength of 225-275 nm which is clearly below the wavelength of UVA and UVB (290-390 nm). The visual score showed that application of melatonin 15 min before irradiation significantly suppressed erythema compared to treatment with vehicle alone (p < 0.001). Similar results were found by chromametry (p < 0.001). Treatment after irradiation showed no UV suppression. The erythema suppressive effect of melatonin might be explained by the radical-scavenging mechanism of quenching meanly hydroxyl radicals (.OH) which are known to be most present in sunburn reaction of the skin. The protective effect of the pre-irradiation treatment might be explained by penetration into the skin within 15 min and the presence in a local concentration at the irradiation time point.     

Extent of sperm chromatin hydration determined by atomic force microscopy

Volume measurements were performed on intact bull and mouse sperm heads and amembranous sperm nuclei, both in the fully hydrated (fluid cell) and dehydrated (air-dried on glass coverslips) states by atomic force microscopy (AFM). Data were obtained by analyzing a small population of cells/nuclei, as well as by performing repeated measurements on single cells imaged following the addition of increasing concentrations of propanol. Results show that the volume of fully hydrated, intact sperm heads and amembranous sperm chromatin particles are at least twice the volume of their air-dried counterparts. Dehydration occurs rapidly in air, and the reduction in volume of chromatin induced by water loss appears to be completely reversible. These studies demonstrate that both mouse and bull sperm chromatin are extensively hydrated in the native state, and are not as compact as previous studies have suggested. © 1996 Wiley-Liss, Inc.     

Parameters influencing the determination of liposome lamellarity by 31P-NMR

The lamellarity of liposomes influences to a great extent the encapsulation efficiency, the efflux rate of liposomally encapsulated material, and the fate of a drug after cellular uptake. 31P-NMR in combination with the use of chemical shift reagents has been described for the determination of lamellarity of liposomes and this study was performed to evaluate the applicability of 31P-NMR analysis as published in the past. To date, very few details about the required conditions throughout the measurements are known. In this study the influence of various parameters on the measurement, such as different buffers with changing ion concentrations, varying pH and different shift reagents at increasing concentrations was investigated. Results were discussed by using cryo-electronmicroscopy as a reference method. The data of this study show that 31P-NMR might not result in the correct determination of liposome lamellarity, depending on the experimental settings and the shape of the liposomes.     

Improving piezoelectric cell printing accuracy and reliability through neutral buoyancy of suspensions

The sedimentation and aggregation of cells within inkjet printing systems has been hypothesized to negatively impact printer performance. The purpose of this study was to investigate this influence through the use of neutral buoyancy. Ficoll PM400 was used to create neutrally buoyant MCF‐7 breast cancer cell suspensions, which were ejected using a piezoelectric drop‐on‐demand inkjet printing system. It was found that using a neutrally buoyant suspension greatly increased the reproducibility of consistent cell counts, and eliminated nozzle clogging. Moreover, the use of Ficoll PM400 was shown to not affect cellular viability. This is the first demonstration of such scale and accuracy achieved using a piezoelectric inkjet printing system for cellular dispensing. Biotechnol. Bioeng. 2012; 109: 2932–2940. © 2012 Wiley Periodicals, Inc.     

Detection of clinically relevant point mutations by a novel piezoelectric biosensor

Piezoelectric sensing is here applied to point mutation detection in human DNA. The mutation investigated is in the TP53 gene, which results inactivated in most cancer types. TP53 gene maps on chromosome 17 (17p13.1). It contains 11 exons and codifies for the relative protein, involved in cell proliferation. The TP53 gene has a wide mutation spectrum that is related to different tumours. In particular, those occurring in the structurally important L2 and L3 zinc-binding domains, have been linked to patient prognosis and more strongly to radiotherapy and chemotherapy resistance in several major cancers. For this reason, the identification of these mutations represents an important clinical target and biosensors could represent good candidate for fast mutation screening. In this paper, a DNA-based piezoelectric biosensor for the detection of the TP53 gene mutation at codon 248 is reported. A biotinylated probe was immobilised on the sensor surface via dextran-streptavidin modified surfaces. The sensor was optimised using synthetic oligonucleotides. Finally, the sensor system was successfully applied to polymerase chain reaction (PCR)-amplified real samples of DNA extracted from two cell lines, one normal (wild-type) and one mutated, carrying the mutation at codon 248 of the TP53 gene. The results obtained demonstrate that the DNA-based piezoelectric biosensor is able to detect the point mutations in PCR-amplified samples showing the potentialities of this approach for routine analysis.