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When a finger actively slides over a surface, contact conditions including the contact area, sliding speed, and finger moisture naturally fluctuate. These random fluctuations lead to an apparent change of frictional properties and influence tactile pleasantness. Nonetheless, this probabilistic behavior has not been explicitly analyzed in previous studies on human fingertips. This study investigates the dependence of the coefficients of kinetic friction on the normal force produced by sliding a bare finger over different artificial skins with seven levels of hardness. The coefficient of friction was modeled as a power function of the normal force. An experimental study that involved sliding a finger over artificial skin surfaces was carried out under two conditions: the fingertip being wiped by a dry cloth or a cloth soaked in ethanol. Although the exponential term was assumed to be nearly constant for identical tribological conditions, we observed that the exponent varied randomly and could be negative, zero, or positive. This can be attributed to the variation of gross finger deformation that was not controlled during the observation. The probability density function of the exponent depended on the moisture content of the finger and object hardness. The variability of the exponent was higher for a soft material than it was for a harder material. In other words, for the softer materials, the exponent appears more random. Furthermore, the exponent tended to be positive and the coefficient of friction increased with the normal force when the finger was wiped with ethanol. These findings play an important role in understanding the frictional forces produced during skin–skin contact in terms of determining the root cause of random variations in the dependence of the coefficient of friction on the normal force. [ABSTRACT FROM AUTHOR]

Understanding the tribological phenomena when fingers slide over soft surfaces such as skin is important for many practical applications. Therefore, this article analyzed the coefficients of friction for a bare finger sliding over artificial skin with different hardness under dry and lubricated surface conditions. This article contrasts with previous research that predominantly analyzed the contact between skin and hard surfaces or probes. Under dry conditions, the coefficient of friction was constant for artificial skins that were harder than the finger pad, irrespective of the normal force of the finger. However, the coefficient of friction decreased with increasing normal force for softer artificial skins. When the surface of the artificial skin model was lubricated with mica, the coefficient of friction exhibited normal-force dependence only for soft artificial skins, similar to the observations under dry conditions. This effect was due to the deformation friction; thus, the coefficient of friction increased as the normal force increased. Conversely, when the model was lubricated with ${\mathrm{ TiO}}_2$ , the coefficient of friction depended on the normal force for all hardness levels. These findings provide insights into the friction experienced during skin-skin or skin-soft material contact under dry and lubricated conditions that can easily occur in daily life. Thus, the results of this study can be useful for the development of skin care products or assistive robots involving human-robot contact. [ABSTRACT FROM AUTHOR]

This study is mainly concerned with the potential hazards of interactive friction force on a human–robot contact surface, which may result in various skin traumas, such as friction blisters. In this study, a novel safety verification method is developed for helping physical assistant robot users to avoid this hazardous condition. First, based on the microscopic structure of the epidermis–dermis layer and the characteristics of blisters on porcine skin, we prove that the porcine skin, instead of the human skin, is feasible as an innovative experimental material for generating friction blisters. Second, the conditions for generating blisters on porcine skin are confirmed using a series of adaptive time-gradient tests. Considering the properties of the censored results, we clarify an inherently safe region in the tangential traction–time relationship, which is based on a nonparametric estimation of the survival function on the subjects. After a condition-matching calculation, this region for the porcine skin is further verified to be consistent with that for the human skin. Third, the safety of the ‘stand-up and sit-down’ motion of a physical assistant robot user is successfully validated by applying this verification method, which finally proves its practical usability with regard to the possible physical stress hazard. [ABSTRACT FROM AUTHOR]

When a human rubs a contactor that is vibrating laterally at 3-10 Hz with an amplitude of ~1 mm, the friction is perceived as being greater than that sensed when touching a stationary contactor. This phenomenon could be exploited for a new vibrotactile approach to friction perception; however, the principles behind it have yet to be explained. In this study, we hypothesized that the perceived friction increases because of the Stribeck characteristic and stick-slip phenomenon of friction, whereby the friction varies depending on the relative velocity between a subject's fingertip and contactor. The relative velocity instantaneously decreases because of the lateral vibration of the contactor, while the friction rises during the same period. To test this hypothesis, we simulated the friction forces between a fingertip and a vibrating contactor. Furthermore, we performed psychophysical experiments using five participants, in which each explored a vibrating contactor and subjectively compared the perceived friction under several stimulus conditions. The simulated friction forces, the results of the psychophysical experiments, and physical observation suggested that the transient increases in friction caused by lateral vibration of the contactor influenced friction perception for a laterally vibrating contactor. [ABSTRACT FROM AUTHOR]

Humans perceive a force applied to their fingertips by integrating skin and proprioceptive sensations. In this study, we investigated the relative contribution ratios of these sensations using two approaches. Decoupled forces were applied to the finger pad and proximal interphalangeal joint of the index finger of the participants. First, we calculated the ratios from the point of subjective equality between the skin and the proprioceptive perceptions. Second, we obtained discrimination limens of the two perceptions to compute their contribution ratios. The results of these two approaches showed good agreement. Additionally, we investigated how the magnitudes of forces, which were 1.0 and 0.3 N, applied to a fingertip affect the relative contribution ratios of the two sensory channels. When humans perceived the force of 1.0 N, the relative contribution ratios of skin and proprioceptive sensations were 16-28 and 72-84 percent, respectively. In contrast, when humans perceived the force of 0.3 N, the relative contribution ratios were 37-55 and 45-63 percent, respectively. These relative contribution ratios can be utilized for the design of efficient haptic interfaces. [ABSTRACT FROM PUBLISHER]