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In everyday life, people interact with numerous products through touch. A massive amount of information is generated through such tactile contact with a product surface. Surfaces thus influence the perception of the product physical properties, grip performance and forces applied during object handling and manipulation. The main aim of the current work is to control and enhance tactile perception through modification of the frictional behaviour by surface topography design. Following a systematic approach, an asperity and texture design was selected for this research. A contact model was developed to predict finger pad contact area on the micro- and macro- scales. The model was applied to understand the role of component parts in a tactile tribological system – namely to estimate skin elastic modulus, characterise skin deformation and determine the role of varying surface texture dimensions. A significant influence of the skin contact state on tactile friction was shown experimentally. Deterministic surfaces, which remained in asperity contact, showed a considerable reduction of friction coefficient. A bidirectional frictional behaviour was achieved with ellipsoidal texture design and was correlated to the feature geometry and material properties. The friction measurements performed with a group of volunteers, with the aim of normalisation, show that the reference sample can be used to normalise and compare values between individuals. A foundation for a texture design map is developed, towards establishing a connection between the texture dimensions and effects attributed to tactile friction. The design map can be used as a reference for the geometrical boundaries in surface texture design with the aim to control and predict frictional behaviour and to enhance tactile perception.

Finger pad friction varies significantly between individuals due to personal differences and environmental effects. For this reason, comparison of the absolute friction values becomes non-informative and is rarely reported in literature. This work investigates whether friction data can be normalised to correlate between individuals. A set of textured samples were tested by 10 volunteers in dry sliding conditions with a range of normal loads up to 4 N. A reference smooth sample was used for each participant to normalise the dataset. The proposed approach allows to eliminate one of the unknown parameters, e.g., finger pad ridge contact area function, and discuss the data with respect to other variables.

This study focused on active light touch with predefined textures specially-designed for tactile perception. The counter-body material is stainless steel sheet. Three geometric structures (grid, crater and groove) were fabricated by pulsed laser surface texturing. A total number of twenty volunteers participated in the research which contains two parts: perception tests and skin friction measurements. The perception tests focused mainly on the participants׳ perceptual attributes: perceived roughness and perceived stickiness. For the skin friction measurements, a multi-axis force/torque transducer was used to measure the normal force and friction force between skin and counter-surface along with the fingertip position. The results of the predefined textures showed the ability to reduce skin friction due to the reduction of contact area. Moreover, the participants׳ perceptual attributes were greatly influenced by the predefined micro-structures in the light touch regime.

Research on human skin interactions with healthcare and lifestyle products is a topic continuously attracting scientific studies over the past years. It is possible to evaluate skin mechanical properties based on human or animal experimentation, yet in addition to possible ethical issues, these samples are hard to obtain, expensive and give rise to highly variable results. Therefore, the design of a skin equivalent is essential. This paper describes the design and characterization of a new Epidermal Skin Equivalent (ESE). The material resembles the properties of epidermis and is a first approach to mimic the mechanical properties of the human skin structure, variable with the length scale. The ESE is based on a mixture of Polydimethyl Siloxane (PDMS) and Polyvinyl Alcohol (PVA) hydrogel cross-linked with Glutaraldehyde (GA). It was chemically characterized by XPS and FTIR measurements and its cross section was observed by macroscopy and cryoSEM. Confocal Microscope analysis on the surface of the ESE showed an arithmetic roughness (Ra) between 14-16 μm and contact angle (CA) values between 50-60°, both of which are close to the values of in vivo human skins reported in the literature. The Equilibrium Water Content (ECW) was around 33.8% and Thermo Gravimetric Analysis (TGA) confirmed the composition of the ESE samples. Moreover, the mechanical performance was determined by indentation tests and Dynamo Thermo Mechanical Analysis (DTMA) shear measurements. The indentation results were in good agreement with that of the target epidermis reported in the literature with an elastic modulus between 0.1-1.5 MPa and it showed dependency on the water content. According to the DTMA measurements, the ESE exhibits a viscoelastic behavior, with a shear modulus between 1-2.5MPa variable with temperature, frequency and the hydration of the samples. Copyright © 2015 Elsevier Ltd. All rights reserved. Chemicals/CAS: dimeticone, 32028-95-8, 68248-27-1, 9004-73-3, 9006-65-9; polyvinyl alcohol, 37380-95-3, 9002-89-5; water, 7732-18-5; baysilon; Dimethylpolysiloxanes; Polyvinyl Alcohol; polyvinyl alcohol hydrogel; Water

Surface topography significantly influences tactile friction and perception. While friction forces can be reduced by surface texturing, selection of pattern dimensions is challenging due to the highly variable elastic modulus of the skin. This work proposes an empirical approach for the evaluation of the skin elasticity through surface transition from asperity to full contact state. To highlight the contact transition, two textures with evenly distributed identical micro asperities, but varying density, were moulded with several grades of silicone rubber. Dynamic friction coefficient measurements were performed during finger pad sliding against the textured samples with a range of normal loads up to 5 N. A combination of analytical and numerical contact models is used to explain the observed friction behaviour, estimate the development of contact area and calculate the effective elastic modulus of the skin at the micro-scale. Low density textures clearly indicate the transition to the full contact state, which is reflected in friction coefficient development, while high density textures remain in an asperity contact state, with significantly lower friction values. The effective Young's modulus is hereby estimated in the range of 0.2–0.5 MPa. Observed frictional behaviour is explained by the change in the apparent and real contact areas. The presented approach allows to study the influence of individual surface parameters on effective skin elastic modulus, which is essential for the development of functional surfaces with improved tactile perception.

The friction behaviour of the human fingerpad as a function of asperity geometry was investigated experimentally. Surface textures consisting of evenly distributed spherically tipped asperities were used for in vivo testing. Using analytical expressions, a multi-scale model was developed to explain the observed friction behaviour as a function of texture geometry, load and skin properties. Friction is found to increase with increasing tip radius. A minimum value for the coefficient of friction seems to exist as a function of asperity density. A maximum value for tip spacing exists above which the contact is not determined by the texture properties only. According to the model normal adhesion plays an important role in the observed friction behaviour. © 2012 Elsevier Ltd. All rights reserved.

To control tactile friction, that is the friction between fingertip and counter-body, the role of surface texture is required to be unveiled and defined. In this research, an experimental approach is used based on measuring tactile friction for directional texture (grooved channel) with varying depths. For a reference surface, in this current case a polished surface from the same tool steel is compared. The experimental results are analyzed to explain the observed skin friction behavior as a function of surface texture parameters, sliding direction and applied normal load. Sliding parallel to the groove length shows greater values in COF than sliding perpendicular to the groove direction. Furthermore, parallel sliding reveals a higher dependency of COF on the depth of the grooved channel texture than perpendicular sliding. Application of the two term friction model suggests that the adhesion component of friction has greater impact on parallel than perpendicular sliding direction. According to the observations, grooved channels are well suited to control skin friction in direction dependent sliding, for moderately loaded contact situations. This experimental research contributes to the haptic perception related research, and to the development of other direction-dependent surface structures for touch.

The surface topography of the human wrist skin is studied by using optical and atomic force microscopy (AFM) methods. By using these techniques the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for the dry and wet skin. The measured friction coefficient between a glass ball and dry and wet skin can be explained assuming that a frictional shear stress σf ≈ 13 MPa and σf ≈ 5 MPa, respectively, act in the area of real contact during sliding. These frictional shear stresses are typical for sliding on surfaces of elastic bodies. The big increase in friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as result of the increase in the contact area arising from the attraction of capillary bridges. Finally, we demonstrated that the real contact area can be properly defined only when a combination of both AFM and optical methods is used for power spectrum calculation.

The contact and friction behavior of a medical compression stocking (MCS) under different strains was investigated in friction and compression experiments against a mechanical skin model. In addition, the 3D topography of the MCS surfaces was analyzed in order to study the relationship between macroscopic friction and microscopic surface properties. The load dependence of friction coefficients was found to be in accordance with the adhesion friction model. The surface structure of MCS samples was considerably changed when varying the strain state, while friction coefficients remained comparable, indicating real contact areas independent of strain on the microscopic level. The experimental findings could be confirmed and explained on the basis of the microscopic surface analyses, when interpreting the fabric surfaces to be composed of numerous individual round asperities obeying the Hertz contact model., Tribology Letters, 56 (3), ISSN:1023-8883, ISSN:1573-2711

The aim of this thesis is to interconnect human tactile perception with various physical properties of materials. Tactile perception necessitates contact and relative motion between the skin and the surfaces of interest. This implies that properties such as friction and surface roughness ought to be important physical properties for tactile sensing. In this work, a method to measure friction between human fingers and surfaces is presented. This method is believed to best represent friction in tactile perception. This study is focused on the tactile perception of printing papers. However, the methodology of finger friction measurements, as well as the methodology to link physical properties with human perception data, can be applied to almost whichever material or surfaces. This thesis is based on three articles. In Article I, one participant performed finger friction measurements, using a piezoelectric force sensor, on 21 printing papers of different paper grades and grammage (weight of the papers). Friction coefficients were calculated as the ratio of the frictional force and the normal force, shown to have a linear relationship. The values were recorded while stroking the index finger over the surface. The results show that measurements with the device can be used to discriminate a set of similar surfaces in terms of finger friction. When comparing the friction coefficients, the papers group according to paper surface treatment and an emerging trend is that the rougher (uncoated) papers have a lower friction coefficient than the smoother (coated) papers. In the latter case, this is interpreted in terms of a larger contact between the finger and paper surface. In addition, a decrease in friction coefficient is noted for all papers on repeated stroking, where the coated papers display a larger decrease. XPS (X-ray Photoelectron Spectroscopy) reveals that skin lipids are transferred from the finger to the paper surface, acting as a lubricant and hence decrease friction. Nevertheless, there is evidence that mechanical changes of the surface cannot be completely ruled out. The reproducibility of the finger friction measurements is elaborated in Article II, by using many participants on a selection of eight printing papers out of the 21. The trends in friction are the same; once again, the coated papers display the highest friction. There are notably large variations in the exact value of the friction coefficient, which are tentatively attributed to different skin hydration and stroking modes. These same participants also took part in a tactile study of perceived paper coarseness (“strävhet” in Swedish). The results reveal that the participants can distinguish a set of printing papers in terms of perceived coarseness. Not unexpectedly, surface roughness appears to be an important property related to perceived coarseness, where group data display that perceived coarseness increases with increasing surface roughness. Interestingly, friction also appears to be a discriminatory property for some subjects. A few participants showed opposite trends, which is evidence for that what is considered coarse is subjective and that different participants “weigh” the importance of the properties differently. This is a good example of a challenge when measuring one-dimensional perceptions in psychophysics. In Article III, a multidimensional approach was used to explore the tactile perception of printing papers. To do this, the participants scaled similarity among all possible pairs of the papers, and this similarity data are best presented by a three-dimensional space solution. This means that there are three underlying dimensions or properties that the participants use to discriminate the surface feel. Also, there is a distinct perceptual difference between the rougher (uncoated) and smoother (coated) papers. The surface roughness appears to be the dominant physical property when discriminating between a real rough paper and a smooth paper, whereas friction, thermal conductivity and grammage are more important when discriminating among the smooth coated papers.