| Human skin is an important tactile organ through which people communicate directly with the external environment.The skin of fingertip undergoes compressive and tensile deformation when sliding on the surface of objects,which stimulates the mechanoreceptors and subsequently produces electrical signals transmitting to the cerebral cortex by neurons.In daily life,people perceive the object surface trough touching,grasping,rubbing,lifting,holding and so on.Mechanical and physical signals will be produced during fingertip sliding against object surface,such as vibration,friction,noise etc.These signals are influenced by a variety of factors,such as contact force,sliding direction,contact angle,sliding speed,as well as skin temperature and humidity.In addition,human skin is an active soft tissue with complex mechanical and physiological characteristics,the research on the investigation into the biotribological mechanism involved in fingertip perception is a very challenging problem.From the tribological perspective,by combining the estimation method of subjective sensation and brain function visualization technology,this study systematically investigated the effect of different motion conditions and mechanical parameters on fingertip frictional perception behaviour,and try to build up the relationship between tactile perception and fingertip friction.The main work and summary were as follows:(1)Based on a previous device for friction measurement of finger touch,a novel custom setup was developed.This device enabled finger sliding and rotating movement with a modular design.Different mechanical signals and skin deformation were recorded accurately and simultaneously.The device was also characterized with an ergonomic design,increasing the comfort level of subjects,and provided a better experimental platform for analyzing the mechanism of fingertip frictional perception behaviour.(2)The effect of different friction parameters on fingertip frictional perception was analyzed,such as sliding speed,contact angle,sliding direction etc.The perception test and the friction test on sandpapers were both conducted.It was found that the continuous movement benefited the tactile perception due to the occurrence of local collisions between the fingerprint ridges and roughness asperities which enhanced the stimulation of mechanoreceptors.The occurrence of stick-slip phenomenon interrupted the local collision process,and interfered human’s perception by distracting their attention,thus reducing tactile perception.In addition,the higher frequency of the stick-slip occurred,the greater interference was caused.(3)The effect of liquid environments on finger frictional perception behavior was examined.First,participants performed both a perception test and a tribological test in air,deionized water(DW)and thickened water(TW)to examine the effect of different liquid environments.Subsequently,a study was conducted using thickened water with three concentrations to investigate different viscosities of lubricants and their effect on tactile perception and fingertip friction.It was found that there was a significant main effect of different liquid environments,the surface roughness,as well as the relevant interaction on roughness perception.There was no significant difference on roughness perception in air and deionized water.Roughness perception decreased in thickened water compared to air and deionized water,and with increasing viscosity respectively.A boundary lubrication occurred between finger-sample surface in water.In thickened water,a relatively thick film was formed between the finger and the sample surface.This changed the contact of the asperities with the skin,thus changing subjective judgment and friction.(4)Functional near-Infrared Spectroscopy(f NIRS)was used to investigate the difference between active and passive frictional perception behaviour in a proof-of-concept study.The change of hemoglobin concentration on the prefrontal cortex was analyzed during fingertip active and passive sliding against two roughness samples,with its frictional properties,to understand the mechanism of tactile perception.It was found that different sliding modes and surface roughness activated different brain regions,and the level of activation was also varied.Different roughnesses did not have an obvious effect on brain response signals,while a significant effect existed for sliding modes.A higher level of activation of brain regions was found for fingertip passive sliding.There was no main effect of sliding modes on fingertip friction properties.The investigation into fingertip tactile perception behaviour will deepen the understanding of human tactile perception mechanism,also be helpful for a range of important applications,as follows:(1)Fingertip skin is an active soft tissue with complicated physiological structure,influenced by various factors,such as gender,ethnics,environment etc.Thus,the tribological properties of fingertip skin are still unclear.This work has important significance for enriching and developing the theoretical system of Biotribology.(2)From fingertip friction to perception,there are still some problems about how to transform mechanical stimulus signals into brain signals,to determine the factors influencing the perception ability and identification sensitivity.This study has important scientific exploration significance for understanding the unknown world.(3)This work provides reference for how to increase the sensitivity of frictional perception behaviour(especially for blind people),and further improve the ergonomic design for daily utensils,medical and sport devices,as well as the comfort feel of product packaging.(4)For robot hands or artificial fingers,the intelligence and control of perception is still a problem difficult to overcome.This study has a positive effect on the application of tactile sensing technology,which realizes the intelligent,dexterous and quick grasp design. |
