Skin viscoelasticity effects on the periodic mechanical stimuli propagation between skin layers.

Our daily lives are constantly surrounded by dynamic stimuli, and our skin is deformed in a time-dependent manner. Although skin plays an important role in transmitting stimuli received at the surface to mechanoreceptors, few studies have investigated how differences in skin viscoelasticity affect the mechanical stimuli propagation in the skin. Therefore, using a finite element model, we evaluated the effects and trends of changes in the stiffness and viscoelasticity of the skin on the propagation of mechanical quantities between skin layers where mechanoreceptors are present when subjected to periodic stimuli. First, we constructed a new, sophisticated mathematical model of skin viscoelasticity based on the history-dependent deformation behavior of human skin obtained experimentally. We were able to construct a skin model that thoroughly reproduced the actual human skin deformation behavior at oscillations as fast as 10 Hz by setting viscoelastic parameters with a short time constant (0.001-0.006 s). Then, we calculated how skin material parameters affect the propagation of the mechanical quantities in the skin during the history-dependent skin deformation response to periodic stimuli. The finite element analysis showed that not only stiffness but also viscoelasticity markedly affected the mechanical stimuli propagation in the skin, and the effect differed depending on the layer. In particular, greater immediate responsiveness of the dermis contributed to greater propagation of the mechanical stimulus. Our results indicate that more attention needs to be given to the differences in the time-dependent intradermal mechanical stimuli propagation caused by individual's skin viscoelasticity.

Representing Fine Texture of Pencil Hardness by High-Frequency Vibrotactile Equivalence Conversion Using Ultra-Thin PZT-MEMS Vibrators.

This study aims to represent fine texture differences in pencil hardness using intensity segment modulation (ISM), a sensory equivalent conversion method of vibration from high to low frequencies. This method enables the presentation of delicate tactile sensations even with small transducers. We integrated this approach in the world's thinnest ultra-thin PZT-MEMS vibrator with a stylus-type device. The vibration waveforms of four types of pencil hardness were captured under the same conditions, and the differences in the frequency components were confirmed. We compared the fine texture feelings under raw signal, ISM, and ISM below 1 kHz conditions by conducting discrimination tests and subjective similarity evaluations. The results showed that ISM could reproduce similar feelings of the pencil hardness.

Lever Mechanism for Diaphragm-Type Vibrators to Enhance Vibrotactile Intensity.

Thin and light vibrators that leverage the inverse piezoelectric effect with a diaphragm mechanism are promising vibrotactile actuators owing to their form factors and high temporal and frequency response. However, generating perceptually sufficient displacement in the low-frequency domain is challenging. This study presents a lever mechanism mounted on a diaphragm vibrator to enhance the vibrotactile intensity of low-frequency vibrotactile stimuli. The lever mechanism is inspired by the tactile contact lens consisting of an array of cylinders held against the skin on a sheet that enhances micro-bump tactile detection. We built an experimental apparatus including our previously developed thin-film diaphragm-type vibrator, which reproduced the common characteristic of piezoelectric vibrators: near-threshold displacement (10 to 20 µm) at low frequency. Experiments demonstrated enhanced vibrotactile intensity at frequencies less than 100 Hz with the lever mechanism. Although the arrangement and material of the mechanism can be improved, our findings can help improve the expressiveness of diaphragm-type vibrators.

Stereohaptic Vibration: Out-of-Body Localization of Virtual Vibration Source Through Multiple Vibrotactile Stimuli on the Forearms.

This paper proposes a novel concept of "stereohaptic vibration," which employs distributed vibration to localize vibration sources outside the body. Inspired by amplitude panning, a stereophonic sound display technique, we developed a method to localize a virtual vibration source (VVS) by polarizing the perceived intensity of multiple vibration stimuli to a specific orientation. Considering the perceptual characteristics of high-frequency vibration, the perceived intensity of the VVS was allocated to multiple vibrators according to the distance and direction of the target. The velocity discrimination performance was confirmed by utilizing four stimuli around the arm and one vibration stimulus to the palm to localize the movement of a VVS throughout the arm. Discrimination experiments of the trajectory of outgoing objects with a single arm and dual arms revealed that our approach could localize in three dimensions, even outside the body. The proposed technology for localizing external virtual vibration sources is expected to enhance the virtual reality experience.

Stratum corneum compliance enhances tactile sensitivity through increasing skin deformation: A study protocol for a randomized controlled trial.

BACKGROUND: Tactile sensation plays a crucial role in object manipulation, communication, and even emotional well-being. It has been reported that the deformability of skin (also described as skin compliance) that shows a large mechanical response to stimuli is associated with high tactile sensitivity. However, although the compliance of the stratum corneum, the outermost layer of skin, can change daily due to skin care and environmental factors, few studies have quantified the effect of the stratum corneum on tactile sensation. AIMS: We investigated the changes in tactile sensitivity resulting from skin hydration and identified corresponding alterations in the compliance of the stratum corneum. METHODS: A randomized controlled trial was conducted. Participants were randomly assigned to an intervention group (n = 20) that had a moisturizing cream applied to their cheeks or a control group (n = 19) that had Milli-Q water applied to their cheeks. Tactile discrimination performance was assessed using psychophysical techniques before and after application. The water content, mechanical response characteristics, and penetration of PEG/PPG-17/4 dimethyl ether from the cream in the stratum corneum were evaluated to identify hydration effects. Skin deformations occurring during tactile sensation were measured concurrently using a suction device employed for tactile stimulation. RESULTS: Tactile sensitivity was increased in participants who had cream applied to the skin surface, while no significant change was observed in participants who received Milli-Q water. The improved discrimination of tactile stimulus intensity was directly related to the magnitude of skin displacement. The higher water content of the stratum corneum due to cream application decreased the dynamic modulus of elasticity of the stratum corneum and increased the skin's extensibility in response to tactile stimuli. CONCLUSIONS: Hydrating the stratum corneum significantly enhances tactile sensitivity and is accompanied by an increase in skin extensibility, a factor in tactile intensity perception. The compliance of the thin stratum corneum layer plays a crucial role in tactile experiences that involve skin stretching.

Wearable High-Resolution Haptic Display Using Suction Stimuli to Represent Cutaneous Contact Information on Finger Pad.

A high-resolution haptic display that reproduces tactile distribution information on the contact surface between a finger and an object realizes the presentation of the softness of the object and the magnitude and direction of the applied force. In this article, we developed a 32-channel suction haptic display that can reproduce tactile distribution on fingertips with high resolution. The device is wearable, compact, and lightweight, thanks to the absence of actuators on the finger. A FE analysis of the skin deformation confirmed that the suction stimulus interfered less with adjacent stimuli in the skin than when pressing with positive pressure, thus allowing more precise control of local tactile stimuli. The optimal layout with the least error was selected from three configurations dividing 62 suction holes into 32 ports.The suction pressures were determined by calculating the pressure distribution by a real-time finite element simulation of the contact between the elastic object and the rigid finger. A discrimination experiment of softness with different Young's modulus and its JND investigation suggested that the higher resolution of the suction display improved the performance of the softness presentation compared to a 16-channel suction display previously developed by the authors.

On the Analysis of Tactile Sensation Based on Time Measurement: An Experimental Case Study on the Interaction Between Skin and Lotion.

This paper presents a novel experimental case study in which tactile sensation is analyzed as 4-dimensional subjective data consisting of stimulus, participant, evaluation term, and temporal components, using a temporal measurement approach. Specifically, the skin and lotion interaction was evaluated using the Temporal Check-All-That-Apply (TCATA) method. Two practical analysis examples were conducted to experimentally demonstrate the potential use cases of time-series subjective tactile data. In the first example, stimulus classification accuracy was compared between different sampling periods, including the whole and late periods, with the latter being akin to the conventional Semantic Differential (SD) method condition. The results indicate that the whole and early periods exhibit higher accuracy compared to the latest period, implying that temporal measurements may capture more stimulus characteristics than the conventional approach. In the second example, cluster analysis based on the time-series subjective data was conducted. The results revealed that the participants were classified into two distinct clusters, with the trends of time-series changes being significantly different between the clusters.

The dynamic behavior of skin in response to vibrating touch stimuli affects tactile perception.

BACKGROUND: The tactile perceptions arising on the skin mediate representations of the body and perceptions of the external physical world. Thus, these tactile sensations greatly impact our lives. Although tactile perception is caused by skin deformation, few studies have investigated the contribution of skin physical properties to tactile perception because the skin deformation in response to mechanical stimuli is difficult to measure in real time. In this study, we investigated how the skin deforms in response to externally applied mechanical stimuli and the effect of skin deformation on tactile perception. MATERIALS AND METHODS: Tactile perception was assessed using psychophysical methods. A suction device was used to measure skin deformation in response to mechanical stimuli while assessing tactile perception. The relationship between skin deformation and tactile perception was investigated. RESULTS: Individuals show different skin deformation behavior in response to stimuli of the same intensity, and the amount of skin deformation affects the perceived pressure induced by suction stimulation. Furthermore, when the amount of skin deformation is small, tactile perception becomes more difficult, and the ease of tactile perception varies. CONCLUSION: We argue that dynamic skin behavior is an important factor in tactile perception. Focusing on skin physical characteristics from a constructivist perspective of complex tactile perception may lead to improved tactile communication perception through the control of skin physical properties and realistic tactile presentation in remote environments.

Development of a remotely controllable 4 m long aerial-hose-type firefighting robot.

In a fire outbreak, firefighters are expected to rapidly extinguish fires to stop the spread of damage and prevent secondary disasters. We proposed the concept of a dragon firefighter (DFF), which is a flying-hose-type firefighting robot. We developed a 3.6 m long DFF equipped with two nozzle units and achieved stable flight. However, the system was not yet completed because the root of the robot, which should have been operated remotely, was operated manually. In addition, the system's reliability was insufficient to successfully repeat the demonstration several times. The development of a robot demonstration system is crucial for the practical application of such a firefighting robot. In this study, we developed a demonstration system for a remotely controllable 4 m flying firehose robot for demonstration at the World Robot Summit 2020 (WRS 2020) opening ceremony in Fukushima as a milestone. This paper focuses on the following issues: 1): installation of the remotely controllable mobile base, 2): redesign of the water channels (the sizes of nozzle outlets) to get enough thrusts to fly with a fire engine, 3): development of nozzle units with a larger movable range (1.5 times larger than the conventional nozzle) in addition to waterproofing technique to improve system reliability, and 4): redesign of a passive damping mechanism to ensure better stability. Thus, a firefighting demonstration was successfully conducted at the opening ceremony of the World Robot Summit 2020 in Fukushima, Japan, and we discuss the lessons learned through the demonstration. We found that the developed DFF system incorporating a mobile base could achieve remote fire extinguishing.

Haptic Exploration During Fast Video Playback: Vibrotactile Support for Event Search in Robot Operation Videos.

Fast playback allows quick video exploration, but it also decreases the saliency of short events. We propose a haptic exploration for detection of short events during fast video playback, considering that event-related information in vibrotactile feedback can be preserved during fast playback using Time Scale Modification (TSM) methods developed for audio. We evaluate our proposal in two collision detection experiments using first-person view (FPV) videos. In the first experiment, viewers watched at a fixed playback speed, i.e., 1× or 2×, videos recorded with a camera mounted on a platform cart. In this experiment, event-related vibrations were measured at the back of the camera. In the second experiment, viewers used a media controller to adjust the playback speed in videos simulating an exploration with a mobile robot. In this experiment, event-related vibrations were generated from the measurements used in the first experiment. We show that a haptic exploration improves collision awareness under either constant or adjustable playback speed. In both experiments, the number of collisions reported without vibrotactile feedback deviated the greatest from the actual number of collisions in a video. Moreover, collision detection performance with vibrations time-scaled without Time Scale Modification (TSM) methods was not significantly different from performance without vibrations.

Introducing Whole Finger Effects in Surface Haptics: An Extended Stick-Slip Model Incorporating Finger Stiffness.

The kinematic serial chain configuration of a finger modulates the frictional properties during tactile exploration tasks. This paper analyzes and subsequently models the effects of the entire finger during sliding operations on a surface. Qualitative and quantitative study of finger movement patterns with postures, sliding directions, and contact angles first indicate the effect of finger stiffness on contact mechanics. A "stiffness ellipse" is subsequently modeled to incorporate finger pose effects, and then coupled with the lumped mass-spring-damper model of the finger pad to estimate resultant contact forces. The performance of the proposed model is verified by comparing with experimental results obtained from ten subjects. The proposed model could estimate the general tendencies of contact forces with change in postures (Extended and Flexed), sliding directions (proximal and distal), and contact angles (20°, 40° and 60°). The experimental results indicate that finger stiffness significantly modulates the contact forces, stick-slip frequency, preloading duration and initial spike during sliding. Introduction of finger posture effects could explain the change in finger normal force during tactile exploration tasks. The proposed haptic rendering model can be used to give a more natural user feedback in virtual fingertip-surface interactions.

Hardness Perception Based on Dynamic Stiffness in Tapping.

A human can judge the hardness of an object based on the damped natural vibration caused by tapping the surface of the object using a fingertip. In this study, we investigated the influence of the dynamic characteristics of vibrations on the hardness perceived by tapping. Subjectively reported hardness values were related to the dynamic stiffness of several objects. The dynamic stiffness, which characterizes the impulsive response of an object, was acquired across the 40-1,000 Hz frequency range for cuboids of 14 types of materials by administering a hammering test. We performed two psychophysical experiments-a ranking task and a magnitude-estimation tasks-wherein participants rated the perceived hardness of each block by tapping it with a finger. We found that the perceptual effect of dynamic stiffness depends on the frequency. Its effect displayed a peak around 300 Hz and decreased or disappeared at higher frequencies, at which human perceptual capabilities are limited. The acquired results help design hardness experienced by products.

Sharp Tactile Line Presentation Using Edge Stimulation Method.

We provide supplemental data to a vibrator array tactile display, as well as additional data for application of the edge stimulation (ES) method proposed in our previous study. By vibrating two surfaces in different phases and touching their boundary, a strong continuous line sensation, not on the vibrators themselves, but along the boundary, is obtained. This vibrotactile edge is suitable for presenting virtual lines, areas, and shapes on a rigid flat surface. We investigated the fundamental performance of the ES method through psychophysical experiments. The effects on the vibrotactile detection thresholds were investigated for three mechanical parameters, i.e., the vibratory frequency, the phase difference between the vibrations, and the gap distance between adjoining vibratory surfaces. Two-line discrimination thresholds for lines presented by the ES method were also determined. We found that the detection thresholds under the ES method was lower than 10 um even at the low frequencies (lower than 50 Hz), which is significantly lower than that under simply touching to a single vibratory surface. A comparison of the perceived widths revealed that the ES method provides a more localized tactile image than a single-pin vibrator or a flat-top vibrator. A 3 X 3 vibrator array display was developed using the ES method based on the properties obtained from the experiments. Seven categories of display patterns were presented with the ES array display and the participants' responses matched at 95 percent.