Human Fingerpad Research Articles

What the Mind Can Comprehend from a Single Touch

This paper investigates the versatility of force feedback (FF) technology in enhancing user interfaces across a spectrum of applications. We delve into the human finger pad’s sensitivity to FF stimuli, which is critical to the development of intuitive and responsive controls in sectors such as medicine, where precision is paramount, and entertainment, where immersive experiences are sought. The study presents a case study in the automotive domain, where FF technology was implemented to simulate mechanical button presses, reducing the JND FF levels that were between 0.04 N and 0.054 N to the JND levels of 0.254 and 0.298 when using a linear force feedback scale and those that were 0.028 N and 0.033 N to the JND levels of 0.074 and 0.164 when using a logarithmic force scale. The results demonstrate the technology’s efficacy and potential for widespread adoption in various industries, underscoring its significance in the evolution of haptic feedback systems.

How Tactile Afferents in the Human Fingerpad Encode Tangential Torques Associated with Manipulation: Are Monkeys Better than Us?

Dexterous object manipulation depends critically on information about forces normal and tangential to the fingerpads, and also on torque associated with object orientation at grip surfaces. We investigated how torque information is encoded by human tactile afferents in the fingerpads and compared them to 97 afferents recorded in monkeys (n = 3; 2 females) in our previous study. Human data included slowly-adapting Type-II (SA-II) afferents, which are absent in the glabrous skin of monkeys. Torques of different magnitudes (3.5-7.5 mNm) were applied in clockwise and anticlockwise directions to a standard central site on the fingerpads of 34 human subjects (19 females). Torques were superimposed on a 2, 3, or 4 N background normal force. Unitary recordings were made from fast-adapting Type-I (FA-I, n = 39), and slowly-adapting Type-I (SA-I, n = 31) and Type-II (SA-II, n = 13) afferents supplying the fingerpads via microelectrodes inserted into the median nerve. All three afferent types encoded torque magnitude and direction, with torque sensitivity being higher with smaller normal forces. SA-I afferent responses to static torque were inferior to dynamic stimuli in humans, while in monkeys the opposite was true. In humans this might be compensated by the addition of sustained SA-II afferent input, and their capacity to increase or decrease firing rates with direction of rotation. We conclude that the discrimination capacity of individual afferents of each type was inferior in humans than monkeys which could be because of differences in fingertip tissue compliance and skin friction.SIGNIFICANCE STATEMENT We investigated how individual human tactile nerve fibers encode rotational forces (torques) and compared them to their monkey counterparts. Human hands, but not monkey hands, are innervated by a tactile neuron type (SA-II afferents) specialized to encode directional skin strain yet, so far, torque encoding has only been studied in monkeys. We find that human SA-I afferents were generally less sensitive and less able to discriminate torque magnitude and direction than their monkey counterparts, especially during the static phase of torque loading. However, this shortfall in humans could be compensated by SA-II afferent input. This indicates that variation in afferent types might complement each other signaling different stimulus features possibly providing computational advantage to discriminate stimuli.

The role of friction on skin wetness perception during dynamic interactions between the human index finger pad and materials of varying moisture content.

Mechanosensory inputs arising from dynamic interactions between the skin and moisture, such as when sliding a finger over a wet substrate, contribute to the perception of skin wetness. Yet, the exact relationship between the mechanical properties of a wet substrate, such as friction, and the resulting wetness perception remains to be established under naturalistic haptic interactions. We modeled the relationship between mechanical and thermal properties of substrates varying in moisture levels (0.49 × 10-4; 1.10 × 10-4; and 2.67 × 10-4 mL·mm-2), coefficient of friction (0.783, 0.848, 1.033, 0.839, 0.876, and 0.763), and maximum thermal transfer rate (Qmax, ranging from 511 to 1,260 W·m-2·K-1), and wetness perception arising from the index finger pad's contact with such substrates. Forty young participants (20M/20F) performed dynamic interactions with 21 different stimuli using their index finger pad at a controlled angle, pressure, and speed. Participants rated their wetness perception using a 100-mm visual analog scale (very dry to very wet). Partial least squares regression analysis indicated that coefficient of friction explained only ∼11% of the variance in wetness perception, whereas Qmax and moisture content accounted for ∼22% and 18% of the variance, respectively. These parameters shared positive relationships with wetness perception, such that the greater the Qmax, moisture content, and coefficient of friction, the wetter the perception. We found no differences in wetness perception between males and females. Our findings indicate that although the friction of a wet substrate modulates wetness perception, it is still secondary to thermal parameters such as Qmax.NEW & NOTEWORTHY Our skin often interacts with wet materials, yet how their physical properties influence our experience of wetness remains poorly understood. We evaluated wetness perception following naturalistic haptic interactions with materials varying in moisture content, friction, optical profiles, and heat transfer rates. We show that although mechanical parameters can influence wetness perception, their role is secondary to that of thermal factors. These findings expand our understanding of multisensory integration and could guide innovation in healthcare product design.

Phi 6 recovery from inoculated fingerpads based on elution buffer and methodology

Phi 6 (Φ6) bacteriophage is a proposed surrogate to study pathogenic enveloped viruses including SARS-CoV-2—the causative agent of COVID-19—based on structural similarities, BSL-1 status, and ease of use. To determine the role of virus-contaminated hands in disease transmission, an enhanced understanding of buffer and method performance for Φ6 recovery needs to be determined. Four buffer types and three methodologies were investigated for the recovery of Φ6 from human fingerpads over a 30 min duration. Phosphate buffered saline (PBS), PBS + 0.1 % Tween, 0.1 M glycine + 3% beef extract, and viral transport medium were evaluated as buffers for recovery of Φ6 via a dish, modified glove juice, and vigorous swabbing method. Φ6 concentrations on fingerpads were determined at 0-, 5-, 10-, and 30-min post-inoculation. While there were observed differences in virus recovery across buffer and method types depending on the time point, log PFU recovery based on buffer type or methodology was not significantly different at any time point (P > 0.05). The results presented in this study will allow for future work on Φ6 persistence, transfer between hands and surfaces, and efficacy of hand hygiene methods to be performed using a well-characterized and validated recovery method.

Three-Axis Pneumatic Haptic Display for the Mechanical and Thermal Stimulation of a Human Finger Pad

Haptic displays have been developed to provide operators with rich tactile information using simple structures. In this study, a three-axis tactile actuator capable of thermal display was developed to deliver tactile senses more realistically and intuitively. The proposed haptic display uses pneumatic pressure to provide shear and normal tactile pressure through an inflation of the balloons inherent in the device. The device provides a lateral displacement of ±1.5 mm for shear haptic feedback and a vertical inflation of the balloon of up to 3.7 mm for normal haptic feedback. It is designed to deliver thermal feedback to the operator through the attachment of a heater to the finger stage of the device, in addition to mechanical haptic feedback. A custom-designed control module is employed to generate appropriate haptic feedback by computing signals from sensors or control computers. This control module has a manual gain control function to compensate for the force exerted on the device by the user’s fingers. Experimental results showed that it could improve the positional accuracy and linearity of the device and minimize hysteresis phenomena. The temperature of the device could be controlled by a pulse-width modulation signal from room temperature to 90 °C. Psychophysical experiments show that cognitive accuracy is affected by gain, and temperature is not significantly affected.

Understanding handling performance of rugby balls under wet conditions: analysis of finger-ball friction

ABSTRACT This paper presents work with the aim of investigating the effect of moisture on the skin frictional behaviour of human finger-pads in contact with rugby balls. During sports activities, human body experiences high volumes of thermoregulatory sweating as a result of achieving body’s temperature balance. Consequently, sweating alters human skin properties and contact conditions between hand skin and sports equipment, and may adversely affect exercise/sports performance. In this work, a rugby ball passing test under wet conditions was conducted to examine the influence of skin hydration on rugby ball handling performance. Then, a comprehensive study was carried out to assess skin structures, frictional properties and contact areas of the interface between human finger-pads and flat surfaces at different moist conditions. It was found that the handling performance of rugby balls is strongly associated with the skin moisture level. The experimental results of the skin friction study showed that the skin friction coefficient changes with hydration time following a “bell-shape” curve. It also showed that the corresponding thickness of the stratum corneum of the examined fingers increased due to the transmission of water in skin tissues. This leads to an increase in the contact area and friction force with hydration time.

Morphology of a human finger pad during sliding against a grooved plate: A pilot study

This pilot study shows that Optical Coherence Tomography can be used to capture the morphology of the finger pad during sliding interaction with a grooved plate. The videos show how the finger pad ridges deform to slide or compress closer to the edge of the grooves. This study has demonstrated the future possibilities of investigating and visualising the interaction of a finger pad with a plate specimen of controlled roughness to improve and optimise surface characteristics of consumer products.

A novel optical 3D force and displacement sensor – Towards instrumenting the PapillArray tactile sensor

As robots move into unstructured environments, tactile sensing will become an essential capability of any robotic gripper attempting to engage in dexterous object manipulation. Sensing the frictional properties of the sensor-object interface (along with contact forces and torques) is essential for determining the minimum grip force required to securely grasp an object; however, most existing artificial tactile sensors are incapable of sensing the property of friction. In previous work, a design was presented for a grip security sensor – the PapillArray – which consists of an array of independent silicone pillars with different uncompressed heights, inspired by the papillae in the skin of the human finger pad. It was shown that tracking the relative deflections of the pillar tips can be used to detect incipient slip; and when incipient slip is detected, measuring the 3D force on the pillar provides an estimate of the coefficient of static friction. In a continuation of this work, here we present a method of instrumenting the movement and forces experienced by the pillars of the PapillArray. A novel pinhole camera design is implemented whereby deformation of the pillar results in both movement and change in area of a spot of light, projected onto a quadrant photodiode. The four quadrant photodiode signals are mapped to true 3D displacement and force using multivariate regression. The accuracy of the mapping’s position and force estimates are validated using both video-tracking of the pillar tip and measurements made with a 3D force sensor, respectively, when stimulating the pillar with a spiral XY displacement pattern at various Z compressions. The overall displacement estimation error (mean ± SD) in the X, Y and Z axes was 0.012 ± 0.077 mm, 0.041 ± 0.074 mm and 0.010 ± 0.023 mm, respectively, for a full-scale deflection of 10.0 mm in X and Y, and 3.25 mm in Z. The overall force estimate error (mean ± SD) was 0.002 ± 0.045 N, -0.007 ± 0.049 N and 0.073 ± 0.118 N, for full-scale force of approximately 4 N in X and Y and 11 N in Z, respectively. The sensitivity of the pillar to vibration (between 100 and 1000 Hz) was also tested, as this may help to detect incipient slip events. The pillar can detect vibration up to 1000 Hz for a vibration displacement amplitude as small as 0.35 μm. This technique can be used to instrument all pillars in the PapillArray sensor, thereby facilitating incipient slip detection and friction measurement. Such a sensor would be of particular benefit in a robotic gripping application where it is important to apply only the grip force required to prevent slip of the object of interest, both to reduce the energy usage of the gripping system and to avoid damaging the gripped object.

Variable-Friction Finger Surfaces to Enable Within-Hand Manipulation via Gripping and Sliding

The human hand is able to achieve an unparalleled diversity of manipulation actions. One contributor to this capability is the structure of the human finger pad, where soft internal tissue is surrounded by a layer of more rigid skin. This permits conforming of the finger pad around object contours for firm grasping, while also permitting low-friction sliding over object surfaces with a light touch. These varying modes of manipulation contribute to the common ability for in-hand-manipulation, where an object (such as a car key) may repositioned relative to the palm. In this letter, we present a simple mechanical analogy to the human finger pad, via a robotic finger with both high- and low-friction surfaces. The low-friction surface is suspended on elastic elements and recesses into a cavity when a sufficient normal force is applied (∼1.2 to 2.5 N depending on contact location), exposing the high-friction surface. We implement one “variable friction” finger and one “constant friction” finger on a 2-DOF gripper with a simple torque controller. With this setup, we demonstrate how within-hand rolling and sliding of an object may be achieved without the need for tactile sensing, high-dexterity, dynamic finger/object modeling, or complex control methods. The addition of an actuator to the finger design allows controlled switching between variable-friction and constant-friction modes, enabling precise object translation and reorientation within a grasp, via simple motion sequences. The rolling and sliding behaviors are characterized with experimentally verified geometric models.

High-pressure endurable flexible tactile actuator based on microstructured dielectric elastomer

We demonstrate a robust flexible tactile actuator that is capable of working under high external pressures. The tactile actuator is based on a pyramidal microstructured dielectric elastomer layer inducing variation in both mechanical and dielectric properties. The vibrational performance of the actuator can be modulated by changing the geometric parameter of the microstructures. We evaluated the performance of the actuator under high-pressure loads up to 25 kPa, which is over the typical range of pressure applied when humans touch or manipulate objects. Due to the benefit of nonlinearity of the pyramidal structure, the actuator could maintain high mechanical output under various external pressures in the frequency range of 100–200 Hz, which is the most sensitive to vibration acceleration for human finger pads. The responses are not only fast, reversible, and highly durable under consecutive cyclic operations, but also large enough to impart perceivable vibrations for haptic feedback on practical wearable device applications.

Bioinspired flexible microfluidic shear force sensor skin

There is a need to gather rich, real-time tactile information to enhance robotic hand performance during haptic exploration and object manipulation. Measuring shear forces is useful for grasping and manipulating objects; however, there are limited effective shear sensing strategies that are compatible with existing end effectors. Here, we report a bioinspired and flexible, resistive microfluidic shear force sensor skin. The sensor skin is wrapped around a finger-shaped end effector and fixed at the location of the nail bed. When the skin is subjected to shear force, one side of the skin experiences tension while the other side experiences compression and bulges similar to a human fingerpad. The tension and compression are measured by liquid metal strain gauges, embedded in PDMS, that are strategically placed adjacent to the nail bed, away from regions of direct finger-object contact. We present the sensor design, a finite element analysis static mechanical characterization model, as well as static response experiments. The resistive shear sensing skin exhibits greater than 10-bit dynamic range (up to 5N) that is insensitive to the applied normal force. The resistive shear sensing skin is intrinsically flexible and immune to fatigue and other problems of solid-state sensors when subjected to repeated, large strains.

A nonlinear rheological model for the ultrasonically induced squeeze film effect in variable friction haptic displays

A squeeze film induced by ultrasonic vibration between two solid surfaces in contact can dramatically reduce the friction between them. This phenomenon, so-called the squeeze film effect, has been utilized in variable friction tactile displays for texture rendering purposes. Such tactile displays can provoke a haptic sensation to a finger pad in a controllable way. A real-time adjustment of the coefficient of lateral friction between the human finger pad and the tactile display can be accomplished by modulating the vibration amplitude of the tactile panel. Therefore, driving a reliable friction model is a key step towards designing and controlling tactile displays utilizing the squeeze film effect. This paper derives a modified Herschel- Bulkley rheological model to express the lateral friction exerted on a human fingertip via an air squeeze film as a function of the operating parameters such as the driving voltage amplitude, the finger sliding speed, and the contact pressure. In contrast to the conventional Coulomb friction model, such a rheology model can account for the sliding velocity dependence. This modeling work may contribute to the optimal control of the ultrasonic variable friction tactile displays.

A preliminary approach to study the behavior of human fingertip at contact via experimental test and numerical model

<p>How human fingertip deforms during the interaction with the environment represents a fundamental action that shapes our perception of external world. In this work, we present the <em>proof of concept</em> of an experimental <em>in vivo</em> set up that enables to characterize the mechanical behavior of human fingertip, in terms of contact area, force and a preliminary estimation of pressure contour, while it is put in contact against a flat rigid surface. Experimental outcomes are then compared with the output of a 3D Finite Element Model (FEM) of the human fingerpad, built upon existing validated models. The good agreement between numerical and experimental data suggests the correctness of our procedure for measurement acquisitions and finger modeling. Furthermore, we will also discuss how our experimental data can be profitably used to estimate strain limiting deformation models for tactile rendering, while the here reported 3D FE model has also been profitably employed to investigate hypotheses on human tactile perception.</p>

Measuring contact area in a sliding human finger-pad contact.

The work outlined in this paper was aimed at achieving further understanding of skin frictional behaviour by investigating the contact area between human finger-pads and flat surfaces. Both the static and the dynamic contact areas (in macro- and micro-scales) were measured using various techniques, including ink printing, optical coherence tomography (OCT) and Digital Image Correlation (DIC). In the studies of the static measurements using ink printing, the experimental results showed that the apparent and the real contact area increased with load following a piecewise linear correlation function for a finger-pad in contact with paper sheets. Comparisons indicated that the OCT method is a reliable and effective method to investigate the real contact area of a finger-pad and allow micro-scale analysis. The apparent contact area (from the DIC measurements) was found to reduce with time in the transition from the static phase to the dynamic phase while the real area of contact (from OCT) increased. The results from this study enable the interaction between finger-pads and contact object surface to be better analysed, and hence improve the understanding of skin friction.

Study of the temporal characteristics of friction and contact behavior encountered during braille reading

Beyond the sense of sight, the sense of touch is one of the primary ways that individuals experience their surrounding environment. Fundamentally understanding the relationship of skin-surface tribology and its elicited tactile attributes could provide a breakthrough in improving the ability to efficiently transmit tactile information to those who rely on the sense of touch to interact with their surroundings, such as the blind and visually impaired (BVI) community. The tactile language of braille has been adopted by the BVI community, employing configurations of raised dome-shape dots to convey what is ordinarily presented in text and image form. The coefficient of friction caused by skin sliding across a these dot features is hypothesized to affect the reader's tactile sensitivity, and skin-on-braille coefficient of friction has been investigated in previous work, where macro-scale deformation of the human fingerpad sliding over the dot contour was identified as the dominant friction mechanisms. This investigation succeeds that study by examining a simplified large-scale, two-dimensional representation of skin-on-braille sliding to characterize the underlying contact mechanisms in the loading behaviors that dictate the resulting coefficient of friction. This was accomplished by using a multi-axis tribometer to sliding a 25.4mm radius cylindrical polyurethane (representing a human fingerpad) rod over a lubricated 3.17 mm aluminum half rod (representing a braille dot) under displacement-displacement-controlled conditions. The results from the tribometer study indicate that the presence of the dot feature drastically affects the vertical and lateral loading behavior by vertically displacing the body's elastic bulk, generating rubber-like Poisson effect contributions. Most importantly, the Poisson effect rapidly increases the lateral load when the body contacts the dot's leading edge, and rapidly decreases when the body rests largely in contact with the dot's trailing edge. This rapid decrease is caused by a “propulsion” effect, where vertical compression expands the material laterally, and when situated on the trailing edge of the dot, propels it into the direction of sliding, virtually negating adhesive surface friction. Computational modeling of this system discovered that while normal contact pressures dominated the fluctuations seen in the vertical loading, effects due to both normal contact pressures and frictional shears nearly equally drove the lateral loading behavior.

Contact mechanics of the human finger pad under compressive loads.

The coefficient of friction of most solid objects is independent of the applied normal force because of surface roughness. This behaviour is observed for a finger pad except at long contact times (greater than 10 s) against smooth impermeable surfaces such as glass when the coefficient increases with decreasing normal force by about a factor of five for the load range investigated here. This is clearly an advantage for some precision manipulation and grip tasks. Such normal force dependence is characteristic of smooth curved elastic bodies. It has been argued that the occlusion of moisture in the form of sweat plasticises the surface topographical features and their increased compliance allows flattening under an applied normal force, so that the surfaces of the fingerprint ridges are effectively smooth. While the normal force dependence of the friction is consistent with the theory of elastic frictional contacts, the gross deformation behaviour is not and, for commonly reported values of the Young's modulus of stratum corneum, the deformation of the ridges should be negligible compared with the gross deformation of the finger pad even when fully occluded. This paper describes the development of a contact mechanics model that resolves these inconsistencies and is validated against experimental data.

ヒト指腹部の固着・滑り運動に起因する皮膚振動の計測

ヒト指腹部の固着・滑り運動に起因する皮膚振動の計測

Hand–tool–tissue interaction forces in neurosurgery for haptic rendering

Haptics provides sensory stimuli that represent the interaction with a virtual or tele-manipulated object, and it is considered a valuable navigation and manipulation tool during tele-operated surgical procedures. Haptic feedback can be provided to the user via cutaneous information and kinesthetic feedback. Sensory subtraction removes the kinesthetic component of the haptic feedback, having only the cutaneous component provided to the user. Such a technique guarantees a stable haptic feedback loop, while it keeps the transparency of the tele-operation system high, which means that the system faithfully replicates and render back the user's directives. This work focuses on checking whether the interaction forces during a bench model neurosurgery operation can lie in the solely cutaneous perception of the human finger pads. If this assumption is found true, it would be possible to exploit sensory subtraction techniques for providing surgeons with feedback from neurosurgery. We measured the forces exerted to surgical tools by three neurosurgeons performing typical actions on a brain phantom, using contact force sensors, while the forces exerted by the tools to the phantom tissue were recorded using a load cell placed under the brain phantom box. The measured surgeon-tool contact forces were 0.01-3.49N for the thumb and 0.01-6.6N for index and middle finger, whereas the measured tool-tissue interaction forces were from six to 11 times smaller than the contact forces, i.e., 0.01-0.59N. The measurements for the contact forces fit the range of the cutaneous sensitivity for the human finger pad; thus, we can say that, in a tele-operated robotic neurosurgery scenario, it would possible to render forces at the fingertip level by conveying haptic cues solely through the cutaneous channel of the surgeon's finger pads. This approach would allow high transparency and high stability of the haptic feedback loop in a tele-operation system.

3-D Fingertip Touch Force Prediction Using Fingernail Imaging With Automated Calibration

This paper presents an automated routine for calibrating a fingernail imaging system, with the intent of predicting fingerpad forces. The system uses a magnetic levitation haptic device to apply forces to the human fingerpad while recording images of the nail and surrounding skin. A novel force controller is implemented to interact stably with the human fingerpad. The data are used to calibrate a principal component regression model relating pixel intensity to 3-D force. Using data from this automated routine, this model simultaneously predicts 3-D force with an RMS error of 0.56 $\pm$ 0.03 N (7.7% of the full range of forces).

Investigation of friction mechanisms in finger pad sliding against surfaces of varying roughness

Human finger pad friction and the effects of surface features on the fingerprint ridges, shape of the finger and the material properties of the finger were investigated. The friction coefficients for three fine-grit abrasive papers were measured to assess these effects. Four probing surfaces were used: human index finger pad, silicone replicas of the finger with and without fingerprints, and a smooth silicone sphere. Friction tests were performed at a constant normal load of 0.5 N in two probe orientations: normal and perpendicular to the orientation of the fingerprint ridges. Scanning electron microscopy of the abrasive papers was performed to examine their surface topography. Based on the trends in coefficients of friction, topography of the abrasive papers, surface features of the finger pad, and the shape and elastic properties of the finger, possible friction mechanisms were discussed. It was inferred that the change in the shape of the probe (from a sphere to the finger shape) changes the adhesion and deformation components of friction, the presence of fingerprints adds an interlocking contribution to the friction and decrease the adhesion and deformation components of friction, and the elastic properties of the finger lead to an increase in all the three components of friction. • Effects of adhesion, deformation and interlocking friction mechanisms were all observed fingertips sliding against rough abrasive papers. • Surface roughness was found to be a primary factor in the control of friction coefficient during dry sliding of the fingertip • Silicone simulants with and without fingerprint ridges were found to model the tribological behavior fairly well against the surfaces of different roughness.