The understanding and control of human skin contact against technological substrates is the key aspect behind the design of several electromechanical devices. Among these, surface haptic displays that modulate the friction between the human finger and touch surface are emerging as user interfaces. One such modulation can be achieved by applying an alternating voltage to the conducting layer of a capacitive touchscreen to control electroadhesion between its surface and the finger pad. However, the nature of the contact interactions between the fingertip and the touchscreen under electroadhesion and the effects of confined material properties, such as layering and inelastic deformation of the stratum corneum, on the friction force are not completely understood yet. Here, we use a mean field theory based on multiscale contact mechanics to investigate the effect of electroadhesion on sliding friction and the dependency of the finger–touchscreen interaction on the applied voltage and other physical parameters. We present experimental results on how the friction between a finger and a touchscreen depends on the electrostatic attraction between them. The proposed model is successfully validated against full-scale (but computationally demanding) contact mechanics simulations and the experimental data. Our study shows that electroadhesion causes an increase in the real contact area at the microscopic level, leading to an increase in the electrovibrating tangential frictional force. We find that it should be possible to further augment the friction force, and thus the human tactile sensing, by using a thinner insulating film on the touchscreen than used in current devices.

There is growing interest in touchscreens displaying tactile feedback due to their tremendous potential in consumer electronics.

While the effect of normal compression on the measured shear material properties of viscoelastic solids has been already acknowledged in rheological studies in the literature, to our knowledge, no systematic study has been conducted to investigate this effect in detail to date. In this study, we perform two sets of experiments to investigate the effect of normal strain and strain rate on the dynamic shear moduli of bovine liver. First, we apply normal compressive strain to the cylindrical bovine samples up to 20% at loading rates of v=0.000625, 0.00625, 0.0625, 0.315, 0.625 mm/s. Second, we perform torsional shear loading experiments, in the frequency range of ω=0.1-10 Hz, under varying amounts of compressive pre-strain (ε=1%, 2.5%, 5%, 7.5%, 10%, 12.5%, 15%, 17.5% and 20%) applied at the quasi-static loading rate of v=0.000625 mm/s. The results of the experiments show that the shear moduli of bovine liver increase with compressive pre-strain. A hyper-viscoelastic constitutive model is developed and fit to the experimental data to estimate the true shear moduli of bovine liver for zero pre-compression. With respect to this reference value, the mean relative error in the measurement of shear moduli of bovine liver varies between 0.2% and 243.1% for the compressive pre-strain varying from ε=1% to 20%. The dynamic shear modulus of bovine liver for compressive pre-strain values higher than ε>2.5% are found to be statistically different than the true shear moduli estimated for zero compressive strain (p<0.05).

In order to gain further insight into the mechanisms of tissue damage during the progression of liver diseases as well as the liver preservation for transplantation, an improved understanding of the relation between the mechanical and histological properties of liver is necessary. We suggest that this relation can only be established truly if the changes in the states of those properties are investigated dynamically as a function of post mortem time. In this regard, we first perform mechanical characterization experiments on three bovine livers to investigate the changes in gross mechanical properties (stiffness, viscosity, and fracture toughness) for the preservation periods of 5, 11, 17, 29, 41 and 53h after harvesting. Then, the histological examination is performed on the samples taken from the same livers to investigate the changes in apoptotic cell count, collagen accumulation, sinusoidal dilatation, and glycogen deposition as a function of the same preservation periods. Finally, the correlation between the mechanical and histological properties is investigated via the Spearman's Rank-Order Correlation method. The results of our study show that stiffness, viscosity, and fracture toughness of bovine liver increase as the preservation period is increased. These macroscopic changes are very strongly correlated with the increase in collagen accumulation and decrease in deposited glycogen level at the microscopic level. Also, we observe that the largest changes in mechanical and histological properties occur after the first 11-17h of preservation.

BACKGROUND:In liver transplantation, the donor and recipient are in different locations most of the time, and longer preservation periods are inevitable. Hence, the choice of the preservation solution and the duration of the preservation period are critical for the success of the transplant surgery. OBJECTIVE: In this study, we examine the mechanical and histological properties of the bovine liver tissue stored in Lactated Ringer's (control), HTK and UW solutions as a function of preservation period. METHODS:The mechanical experiments are conducted with a shear rheometer on cylindrical tissue samples extracted from 3 bovine livers and the change in viscoelastic material properties of the bovine liver is characterized using the fractional derivative Kelvin-Voigt Model. Also, the histological examinations are performed on the same liver samples under a light microscope. RESULTS:The results show that the preservation solution and period have a significant effect on the mechanical and histological properties of the liver tissue. The storage and loss shear moduli, the number of the apoptotic cells, the collagen accumulation, and the sinusoidal dilatation increase, and the glycogen deposition decreases as the preservation period is longer. CONCLUSIONS: Based on the statistical analyses, we observe that the liver tissue is preserved well in all three solutions for up to 11 h. After then, UW solution provides a better preservation up to 29 h. However, for preservation periods longer than 29 h, HTK is a more effective preservation solution based on the least amount of change in mechanical properties. On the other hand, the highest correlation between the mechanical and histological properties is observed for the liver samples preserved in UW solution.

We developed a compact tactile imaging (TI) system to guide the clinician or the self-user for noninvasive detection of breast tumors. Our system measures the force distribution based on the difference in stiffness between a palpated object and an abnormality within. The average force resolution, force range, and the spatial resolution of the device are 0.02 N, 0-4 N, and 2.8 mm, respectively. To evaluate the performance of the proposed TI system, compression experiments were performed to measure the sensitivity and specificity of the system in detecting tumor-like inclusions embedded in tissue-like cylindrical silicon samples. Based on the experiments performed with 11 inclusions, having two different sizes and two different stiffnesses located at three different depths, our TI system showed an average sensitivity of 90.8 ± 8.1 percent and an average specificity of 89.8 ± 12.7 percent. Finally, manual palpation experiments were performed with 12 human subjects on the same silicon samples and the results were compared to that of the TI system. The performance of the TI system was significantly better than that of the human subjects in detecting deep inclusions while the human subjects performed slightly better in detecting shallow inclusions close to the contact surface.

Abstract. We developed a compact tactile sensor in order to guide the clinician or the self-user for non-invasive detection of lumps. The new design has an advantage over the existing discrete tactile sensors and detection methods by efficiently sensing force distribution over an area without any side effects. The sensor consists of 10×10 infrared emitter-detector pairs, a silicon-rubber elastic pad, and a contoured tactile interface (25x21 moving pins) for palpating threedimensional objects. To demonstrate the practical use of the sensor, first a cylindrical tissue-like silicon phantom was prepared, then a 13 mm diameter rigid spherical object was placed at varying depths of 0-20 mm to simulate cancerous lumps in breast tissue, and finally the tactile sensor was systematically pressed on the phantom to successfully detect the lumps for compression depths of 10-24 mm. The location and the estimated radius of each lump were calculated from the recorded tactile images.

Surface haptics technologies offer an augmented user experience by providing a unique and distinctive interaction between the finger and touchscreen. In this study, we focus on a touch screen design to display vibrotactile tactile feedback to the user through piezo patches located on its surface. We investigated the effects of boundary conditions, piezo configurations, and materials of the touch surface and piezo patches that will achieve the highest deformation on the touch surface, considering the most sensible human tactile perception frequency using the ANSYS FEM software package. In our analysis, we used three different touch surface and piezo patch materials, three different boundary conditions, four different piezo patch locations, and three different touch surface thicknesses. The results showed that the boundary conditions and thickness of the glass have a significant effect on the first natural frequency of the touch surface, and the results leading to best human tactile perception were obtained by fixing four piezo patches at four sides of the touch surface. Based on the determined configuration in the modal analyses, we performed a response surface optimization study to estimate the geometry of the touch surface (width, height, thickness), which will result in maximum deformation on the touch surface. We achieved the best configuration (max total deformation at about 250 Hz first modal frequency) with 160 × 90 × 0.28 mm and 190 × 110 × 0.4 mm dimensions. In the future, we will develop models to render localized tactile feedback on a touchscreen-based on piezo patches operating at various combinations (i.e., sequence, amplitude, frequency), which will be predicted based on the FEM simulations.