Friction mechanisms and abrasion of the human finger pad in contact with rough surfaces

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.

Download Full-text

Effectiveness of Liquid Soap and Hand Sanitizer against Norwalk Virus on Contaminated Hands

Applied and Environmental Microbiology ◽

10.1128/aem.01729-09 ◽

2009 ◽

Vol 76 (2) ◽

pp. 394-399 ◽

Cited By ~ 100

Author(s):

Pengbo Liu ◽

Yvonne Yuen ◽

Hui-Mien Hsiao ◽

Lee-Ann Jaykus ◽

Christine Moe

Keyword(s):

Sodium Hypochlorite ◽

Pathogen Transmission ◽

Norwalk Virus ◽

Liquid Soap ◽

Hand Sanitizer ◽

Reverse Transcription Quantitative Pcr ◽

Rnase Treatment ◽

American Society ◽

Human Finger ◽

Finger Pad

ABSTRACT Disinfection is an essential measure for interrupting human norovirus (HuNoV) transmission, but it is difficult to evaluate the efficacy of disinfectants due to the absence of a practicable cell culture system for these viruses. The purpose of this study was to screen sodium hypochlorite and ethanol for efficacy against Norwalk virus (NV) and expand the studies to evaluate the efficacy of antibacterial liquid soap and alcohol-based hand sanitizer for the inactivation of NV on human finger pads. Samples were tested by real-time reverse transcription-quantitative PCR (RT-qPCR) both with and without a prior RNase treatment. In suspension assay, sodium hypochlorite concentrations of ≥160 ppm effectively eliminated RT-qPCR detection signal, while ethanol, regardless of concentration, was relatively ineffective, giving at most a 0.5 log10 reduction in genomic copies of NV cDNA. Using the American Society for Testing and Materials (ASTM) standard finger pad method and a modification thereof (with rubbing), we observed the greatest reduction in genomic copies of NV cDNA with the antibacterial liquid soap treatment (0.67 to 1.20 log10 reduction) and water rinse only (0.58 to 1.58 log10 reduction). The alcohol-based hand sanitizer was relatively ineffective, reducing the genomic copies of NV cDNA by only 0.14 to 0.34 log10 compared to baseline. Although the concentrations of genomic copies of NV cDNA were consistently lower on finger pad eluates pretreated with RNase compared to those without prior RNase treatment, these differences were not statistically significant. Despite the promise of alcohol-based sanitizers for the control of pathogen transmission, they may be relatively ineffective against the HuNoV, reinforcing the need to develop and evaluate new products against this important group of viruses.

Download Full-text

Differential sensitivity to surface compliance by tactile afferents in the human finger pad

Journal of Neurophysiology ◽

10.1152/jn.00589.2013 ◽

2014 ◽

Vol 111 (6) ◽

pp. 1308-1317 ◽

Cited By ~ 12

Author(s):

Melia Condon ◽

Ingvars Birznieks ◽

Kathryn Hudson ◽

David K. Chelvanayagam ◽

David Mahns ◽

...

Keyword(s):

Firing Rate ◽

Linear Increase ◽

Differential Sensitivity ◽

Minor Role ◽

Type I ◽

Unloading Rate ◽

Low Threshold ◽

Human Finger ◽

A Minor ◽

Finger Pad

We undertook a neurophysiological investigation of the responses of low-threshold mechanoreceptors in the human finger pad to surfaces of differing softness. Unitary recordings were made from 26 slowly adapting type I (SAI), 17 fast-adapting type I (FAI), and 9 slowly adapting type II (SAII) afferents 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 silicone disc over the center of the finger pad. Nine discs were used, which linearly increased in stiffness across the range. Population responses of the SAI afferents showed the greatest sensitivity to compliance, with a steep monotonic increase in mean firing rate with increasing stiffness (decreasing compliance) of the surface during the loading and plateau (but not unloading) phases. FAI afferents also showed a linear increase in firing during the loading but not unloading phase, although the slope was significantly lower than that of the SAI afferents at all amplitudes. Conversely, SAII afferents were influenced by object compliance only in certain conditions. Given their high density in the finger pads and their linear relationship between firing rate and object compliance during the loading and plateau phases, SAI afferents (together with FAI afferents during the loading phase) are ideally suited to contributing information on surface compliance to the overall estimation of softness, but the SAII afferents appear to play only a minor role.

Download Full-text

Friction of the Human Finger Pad: Influence of Moisture, Occlusion and Velocity

Tribology Letters ◽

10.1007/s11249-011-9828-0 ◽

2011 ◽

Vol 44 (2) ◽

pp. 117-137 ◽

Cited By ~ 91

Author(s):

Subrahmanyam M. Pasumarty ◽

Simon A. Johnson ◽

Simon A. Watson ◽

Michael J. Adams

Keyword(s):

Human Finger ◽

Finger Pad

Download Full-text

Effects of changing skin mechanics on the differential sensitivity to surface compliance by tactile afferents in the human finger pad

Journal of Neurophysiology ◽

10.1152/jn.00176.2014 ◽

2015 ◽

Vol 114 (4) ◽

pp. 2249-2257 ◽

Cited By ~ 4

Author(s):

Kathryn M. Hudson ◽

Melia Condon ◽

Rochelle Ackerley ◽

Francis McGlone ◽

Håkan Olausson ◽

...

Keyword(s):

Venous Occlusion ◽

Differential Sensitivity ◽

Type I ◽

Firing Rates ◽

Skin Mechanics ◽

Unloading Rate ◽

Low Threshold ◽

Firing Properties ◽

Human Finger ◽

Finger Pad

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 slowly adapting type I (SAI), 10 fast adapting type I (FAI) and 9 slowly adapting type II (SAII) units via tungsten microelectrodes inserted into the median nerve at the wrist. A servo-controlled stimulator applied ramp-and-hold forces (1, 2, and 4 N) at a constant loading and unloading rate (2 N/s) via a flat 2.5-cm-diameter silicone disk over the center of the finger pad. Nine silicone disks (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 the slopes of the relationship between mean firing rate and object compliance at each amplitude, but did significantly reduce the slopes for the FAI afferents. Although the SAII afferents possess a poor capacity to encode changes in object compliance, mean firing rates were significantly lower during venous occlusion. The finding that venous occlusion had no effect on the firing properties of SAI afferents indicates that these afferents preserve their capacity to encode changes in object compliance, despite changes in skin mechanics.

Download Full-text