Physical Variables Underlying Tactile Stickiness During Fingerpad Detachment (original) (raw)
Related papers
Journal of neurophysiology, 2015
It is not known how changes in skin mechanics affect the responses of cutaneous mechanoreceptors in the finger pads to compression forces. We used venous occlusion to change the stiffness of the fingers and investigated whether this influenced the firing of low-threshold mechanoreceptors to surfaces of differing stiffness. Unitary recordings were made from 10 SAI, 10 FAI and 9 SAII units via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, 4 N) at a constant loading and unloading rate (2 N/s) via a flat 2.5 cm-diameter silicone disc over the centre of the finger pad. Nine silicone discs (objects), varying in compliance, were used. Venous occlusion, produced by inflating a sphygmomanometer cuff around the upper arm to 40 ± 5 mmHg, was used to induce swelling of the fingers and increase the compliance of the finger pulp. Venous occlusion had no effect on the firing rates of the SAI afferents, nor on...
Finger pad friction and its role in grip and touch
Journal of The Royal Society Interface, 2012
Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick -slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function.
Effect of skin hydration on the dynamics of fingertip gripping contact
Journal of The Royal Society Interface, 2011
The dynamics of fingertip contact manifest themselves in the complex skin movements observed during the transition from a stuck state to a fully developed slip. While investigating this transition, we found that it depended on skin hydration. To quantify this dependency, we asked subjects to slide their index fingertip on a glass surface while keeping the normal component of the interaction force constant with the help of visual feedback. Skin deformation inside the contact region was imaged with an optical apparatus that allowed us to quantify the relative sizes of the slipping and sticking regions. The ratio of the stuck skin area to the total contact area decreased linearly from 1 to 0 when the tangential force component increased from 0 to a maximum. The slope of this relationship was inversely correlated to the normal force component. The skin hydration level dramatically affected the dynamics of the contact encapsulated in the course of evolution from sticking to slipping. The specific effect was to reduce the tendency of a contact to slip, regardless of the variations of the coefficient of friction. Since grips were more unstable under dry skin conditions, our results suggest that the nervous system responds to dry skin by exaggerated grip forces that cannot be simply explained by a change in the coefficient of friction.
Tribology International, 2013
Measuring the tribological attributes of human skin is of limited value when it comes to haptic perception. Although recent studies discuss coefficients of friction of skin against various materials, no hints are given on how humans perceive these moments of touch. The authors have analysed in how far tribological measurements correlate to the subjective perception of grip and slipperiness by using a particularly designed haptic test device. The results show the poor correlation between the coefficient of dynamic friction and the perceived gliding quality of a surface.
Contact evolution of dry and hydrated fingertips at initial touch
PLOS One, 2022
Pressing the fingertips into surfaces causes skin deformations that enable humans to grip objects and sense their physical properties. This process involves intricate finger geometry, non-uniform tissue properties, and moisture, complicating the underlying contact mechanics. Here we explore the initial contact evolution of dry and hydrated fingers to isolate the roles of governing physical factors. Two participants gradually pressed an index finger on a glass surface under three moisture conditions: dry, water-hydrated, and glycerin-hydrated. Gross and real contact area were optically measured over time, revealing that glycerin hydration produced strikingly higher real contact area, while gross contact area was similar for all conditions. To elucidate the causes for this phenomenon, we investigated the combined effects of tissue elasticity, skin-surface friction, and fingerprint ridges on contact area using simulation. Our analyses show the dominant influence of elastic modulus over friction and an unusual contact phenomenon, which we call friction-induced hinging.
The contributions of skin structural properties to the friction of human finger-pads
Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology
This paper describes three series of tests that were designed to investigate how skin mechanical and structural properties, measured using a "Cutometer" and Optical Coherence Tomography, affect the frictional behaviour of human finger-pads. Firstly, the skin mechanical properties across all fingers and the palm in participants' dominant hands were assessed. Results showed that the distensibility of skin (total deformation in a suction test) is associated with stratum corneum thickness and that this in turn affects friction (thicker stratuem corneum leads to higher friction), giving a link between distensibility and friction. Tape stripping to remove the superficial layer of the skin led to increased moisture (and/or electric charge on the skin surface) led to higher friction. No accompanying changes were seen in structural properties so it was concluded that moisture was the main cause of the adhesion increase. More work is required to isolate moisture and possible changes in electric charge using alternative measurement techniques. When rubbing with sand paper, the stratum corneum thinned considerably and friction reduced. Moisture was ruled out as a cause of friction changes in this instance. Skin normal stiffness also did not change, but lateral stiffness changes have been seen in previous work when the stratum corneum thickness has been reduced so this is likely to be the cause of the reduced friction. This will be investigated further in future work using dynamic OCT measurements.
We investigated the relative effectiveness of tangential versus normal displacements of skin for producing tactile sensations. Subjects adjusted the magnitude of slow tangential and oblique displacements of a flat-ended, cylindrical, 1 mm diameter probe glued to the skin in order to match the perceived intensity of a reference displacement that indented the skin normal to the surface. At both the forearm and fingerpad, subjects chose tangential displacements only 0.3 to 0.6 times as large as the reference normal displacement, indicating a significantly higher sensitivity to tangential displacement. Based on measurements of the mechanical impedance of the skin to normal and tangential displacements, these results were also expressed in terms of forces. At the forearm, subjects were more sensitive to tangential forces than normal force. However, at the fingerpad, sensitivity to tangential forces was lower than sensitivity to normal force, due to the approximately five-fold greater stiffness of the fingerpad to tangential traction. These results provide guidance for development of tactile displays: (1) When an actuator is limited primarily in terms of peak displacement (e.g., the maximum strain of a ceramic peizoelectric actuator) then tangential stimulation is a superior choice for both body sites we tested. (2) When an actuator is limited primarily in terms of peak force (e.g., the stall torque of a DC micromotor) tangential stimulation is the superior choice for the hairy skin, but normal stimulation is the better choice on the fingerpad.
The frictional properties of human finger pads
Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 2009
It is generally assumed that fingerprints improve the grip of primates, but the efficiency of their ridging will depend on the type of frictional behaviour the skin exhibits. Ridges would be effective at increasing friction for hard materials, but in a rubbery material they would reduce friction because they would reduce contact area. In this study we investigated the frictional performance of human fingertips on dry acrylic glass using a modified universal mechanical testing machine, measuring friction at a range of normal loads while also measuring the contact area. Tests were carried out on different fingers, fingers at different angles and against different widths of acrylic sheet to separate the effects of normal force and contact area. The results showed that fingertips behaved more like rubbers than hard solids; their coefficients of friction fell at higher normal forces and friction was higher when fingers were held flatter against wider sheets and hence when contact area was greater. The shear stress was greater at higher pressures, suggesting the presence of a biofilm between the skin and the surface. Fingerprints reduced contact area by a factor of one-third compared with flat skin, however, which would have reduced the friction; this casts severe doubt on their supposed frictional function.
With the growing expectations of consumer in high value products from functionality and usability to emotional features, consumer's emotional preferences become an important aspect in the successful design of innovative and competitive products. As touch perception is stimulated when the human skin contacts or strokes over object surfaces, during active touch, friction is generated. This paper systematically reviewed the relationships between subjective tactile perception and measurable skin friction, based on the understanding of the friction mechanisms of human skin, the contribution of skin friction to tactile perception, the relationship between skin friction and other physical parameters (physical level) and the correlation between skin friction and tactile perception in both sensorial level and affective level, which offers insights on how to change the sensation experience of household and personal care products by changing skin friction.
Journal of Neurophysiology, 2015
Adaptation of fingertip forces to friction at the grasping surface is necessary to prevent use of inadequate or excessive grip forces. In the current study we investigated the effect of blocking tactile information from the fingertips noninvasively on the adaptation and efficiency of grip forces to surface friction during precision grasp. Ten neurologically intact subjects grasped and lifted an instrumented grip device with 18 different frictional surfaces under three conditions: with bare hands or with a thin layer of plastic (Tegaderm) or an additional layer of foam affixed to the fingertips. The coefficient of friction at the finger-object interface of each surface was obtained for each subject with bare hands and Tegaderm by measuring the slip ratio (grip force/load force) at the moment of slip. We found that the foam layer reduced sensibility for two-point discrimination and pressure sensitivity at the fingertips, but Tegaderm did not. However, Tegaderm reduced static, but not ...