KEY CONCEPTS
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While traditional tribological models are good, they are not adequate to describe complex skin tribology and tactile phenomena.
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Frustrated by the limitations of available tribometers, touch researchers are creating their own.
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Practitioners on the front lines, such as braille teachers, have been an excellent resource for the development of studies.
Until recently, the relationships between the friction of skin (especially the fingertips) against rough surfaces and the corresponding perception of touch were too complex to be studied. This is due to the fact that skin is a multi-layered, soft material unsuitable for study with conventional tribometers.
The multi-direction surface structures and mechanical properties of skin vary from one part of the body to another and from person to person; they may change with age and external parameters. Realistic countersurfaces may themselves be soft and/or have roughness on many different length scales and need to be characterized accordingly.
Ultimately the perception of surface feel comes from nerve signals, a research area outside the experience of most traditional tribologists.
However, in the past decade, significant progress has been made toward defining and understanding the tribology of touch through inventive, interdisciplinary research. This progress has been made possible through:
1.
The development of specialized instrumentation for investigations of the tribological responses of soft surfaces at different length scales
2.
Collaborations with researchers studying the neurological functions involved in sensory perception.
Today, this emerging field is on a sound scientific footing, with the most recent research beginning to answer questions as to why some surfaces feel the way they do and also providing design guidelines.
Current investigations focus on identifying and characterizing tribological parameters of human skin in contact with different types of well-characterized model patterned surfaces and also rough surfaces of practical interest (paper, touch screens)
(see Real Skin versus Alternatives).
Real skin versus alternatives
So far all of M. Cynthia Hipwell’s (Oscar S. Wyatt, Jr. ’45 Chair II Professor, Texas A&M University) studies have used real skin. “This is because of extremely interesting and useful qualities of our human skin that enable us to regulate our grip well,” she observes. “First, human sweat is the primary source of the capillary bridges formed at the finger-material interface, so the electrowetting and capillary force effects can only be properly considered with a human finger that sweats. Moreover, the elastic modulus of real skin changes with varying humidity and sweat pore occlusion during extended contact. Currently available artificial fingers can’t secrete sweat, and the modulus of the outer layer material doesn’t change during extended contact and with humidity.”
Mark Rutland, professor, KTH Royal Institute of Technology, Stockholm, Sweden, also is a proponent of using real skin. “Biotribology is measured on skin. Skin mimics can provide insight on tribological interactions, e.g., with additives such as moisturizer. Touch is a sense. Thus, measurements on humans (skin friction, biomechanical properties, mechanoreceptor density, etc.) and by humans (sensorial and psychophysical analysis of materials stimuli) are a prerequisite. I would argue that research on touch can only be performed with human participants—biotribology is not touch, though it is a necessary input.”
Roland Bennewitz, professor, INM – Leibniz Institute for New Materials in Saarbrücken, Germany, works only with human participants.
Dr. Cris Schwartz, professor of mechanical engineering, Iowa State University, says that while the majority of their research relies on human subjects interacting with tactile features in a realistic encounter, he sees this as a stepping off point for incorporating other experiments that use equipment and materials that simulate aspects of skin.
Several research groups are applying their understanding of existing, fundamental friction mechanisms to systems of higher and higher complexity, where natural systems provide both inspiration and an ever-present challenge. Other research groups have started with a particular application in mind—for example, the process of reading braille, the prevention of high textile friction against skin or the potential use of cosmetic textures on skin to change its feel to the touch. These discoveries are starting to enable predictive models.
Mark Rutland, professor, KTH Royal Institute of Technology, Stockholm, Sweden, reports, “At the inaugural 2011 International Conference on BioTribology in London, a presenter declared boldly that tribology had no role in tactile perception. At the same meeting, we provided compelling evidence that the reverse is true—for fine textures I would say that a tribological approach is necessary and sufficient. However, for coarse textures on identical/similar materials, the friction coefficient between the finger and the substrate can be essentially invariant. This makes it tempting to write tribology off for coarse textures—but even there, the vibrations and deformations that are detected by our mechanoreceptors are generated by tribological contacts. So, I would argue that tribology is determining for touch at all scales.”
The current direction
Roland Bennewitz, professor at the INM– Leibniz Institute for New Materials in Saarbrücken, Germany, was intrigued by the finding that a positive touch experience is related to skin friction (as studied by Anne Klöcker in the group of Jean-Louis Thonnard).
1 “It is somewhat intuitive that low-friction surfaces are pleasant to touch, but the group showed that the friction fluctuations should be distributed over the whole spectral range to be perceived as pleasant.”
Another interesting discovery he mentions was the demonstration that human tactile perception is sensitive to nanometer scale structures, by way of friction modulation.
2 “On the technological side, I was impressed by how effectively fingertip friction can be switched by ultrasonic devices,” he says.
3
Dr. Cris Schwartz, professor of mechanical engineering, Iowa State University, Ames, Iowa, observes that, as a whole, the field of tribology and touch is beginning to incorporate some aspects of cognitive research to try to understand how the brain processes situations where tactile tribology plays a role. “The challenge is that these two fields—tribology and neural/cognitive science—have not had much of a reason to talk to each other before and so the paradigms are different,” he says.
Schwartz continues, “There is not much, if any, literature in established tribology journals about cognitive aspects of skin tribology. However, there are some classic studies in the neuroscience literature that incorporate tribological stimuli, but not in a direct way, nor do they use the term ‘tribology.’ It will be interesting to see if the boundaries between these two very different disciplines will evolve.”
Rutland believes that the field has been progressing rather slowly, with the two most important recent developments being:
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Surface/material engineering to produce stimuli sets varying in only one parameter (wavelength, chemistry, elastic modulus, amplitude, etc.)
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The combination of psychology/psychophysics with tribology to identify how the tribological parameters—of both the biomaterial of the finger and the stimulus itself—contribute to the ability to perceive and discriminate.
M. Cynthia Hipwell, Oscar S. Wyatt, Jr. ’45 Chair II Professor, Texas A&M University, College Station, Texas, says, “Understanding of the finger-device interface is incredibly important both for the design of devices and products that we touch as well as for applications such as design of dexterous robots. There have been important developments in understanding the tribology of human grip and exploratory capability, including the understanding of interface sweat and changing skin properties with environmental and contact conditions.”
4,5
Hipwell adds, “The tribology community has an important role to play because the texture geometry, surface properties and changing skin properties person to person and with environment greatly impact the performance of these devices. Fundamental understanding, models and characterization tools are critical to invention and eventual reliable commercialization.”
Exciting discoveries
Bennewitz and his team have been researching the perception of similarity between different rough surfaces. “We designed randomly rough surfaces by an algorithm, where topographic resemblance and fine roughness could be varied independently,” he says. “We found in psychophysical experiments that perceived similarity is dominated by fine roughness, not by topographic resemblance. Fine roughness was correlated with fingertip friction. By recording friction data for each participant in each similarity decision, we were able to show that the measured differences in fingertip friction were good predictors for perceived dissimilarity.”
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Hipwell and colleagues were the first to consider and analyze the impact of electrowetting on the electroadhesion-based haptic devices through experiments and modeling
(see Electroadhesion).7 This finding was made when they measured friction force with electroadhesion effect of different nanotextured samples at different relative humidity levels. They ultimately found that some samples were very sensitive to the humidity change while others were not. “This can’t simply be explained by the existing theories such as the skin modulus change,” Hipwell notes.
Electroadhesion
M. Cynthia Hipwell, Oscar S. Wyatt, Jr. ’45 Chair II Professor, Texas A&M University, defines electroadhesion as an electrostatic attraction between two surfaces under an electric field. In this context, when a high voltage is applied between a finger and a conductive surface covered with a thin dielectric layer, the attractive forces (electrostatic and electrowetting) pull down the finger, increasing the real contact area that forms between nanoscale asperities of the finger and the surface and, thus, increasing the friction force.
“When we apply the voltage with different on-off ratios, it modulates the friction force on our finger in ways that mimic the forces on our fingers when we touch different surfaces,” she explains. “Since electroadhesion technology has many advantages such as no moving parts, low energy consumption and fast response/high bandwidth, it is currently the most promising technology for producing tactile effects for touch-based human-machine interfaces.”
Examples of important applications could be controls that can be identified by feel for applications such as an automobile control panel, where drivers don’t want to take their eyes off the road, or being able to feel virtual products, such as a fabric while shopping online. “The latter requires advances in technology beyond where we are today,” Hipwell says.
Hipwell then modeled capillary force with and without electrowetting effect for these samples and found that the variations in the friction forces result from the capillary. Based on modeling results, they also proposed novel textures to optimize the performance of the electroadhesive devices with stronger effects and reduced variability.
8
“Also, we found that increased surface temperature can soften the outermost layer of a human finger, the stratum corneum, and increase friction force,”
9 Hipwell says. “Interestingly, this response is fast enough that humans can actually perceive the changed friction of the heated region (42 C, 10 mm wide) as a physical bump without feeling the heat when the finger slides over the region. This is because it takes longer for the heat to propagate to thermoreceptors than it does for mechanical stimuli (vibrations) from interface friction changes to reach mechanoreceptors.”
Much of Schwartz’s recent work has involved focusing on end-users—specifically those with visual impairments—the media used for communication, instruction and standardized testing of this group and the role tribology plays in the usefulness of these materials.
“There is a great deal of empirical knowledge of what tactile communication approaches work well in different settings (teaching math, transcribing a high school textbook, creating an aural version of the screen of a graphing calculator, etc.), and the vast majority of this knowledge has come from practitioners on the front lines, such as math teachers, braille transcribers or the people who craft tactile instructional materials,” Schwartz says.
Schwartz focuses on user-facing aspects in his research because he believes this is the ultimate test of whether research discoveries have any relevance. “In this light, my group has made a couple of exciting discoveries,” he explains. “Recently, we published the results of a study of the perceptive ability of people to differentiate between simple tactile graphics textures, both to see how different the textures must be in order to be recognized as different, as well as to see if the frictional experience when exploring these textures helps or hinders the ability to tell them apart. Secondly, we are getting ready to publish results that show the areas of the brain that react when test subjects run their fingers across samples of varying textures—essentially using the brain as a type of tribometer.”
For Schwartz, even more intriguing is that this work seems to show how long it takes the brain not only to detect a change in texture but also the time taken to recognize a new texture. “This ability to measure the time of recognition could have use for assessing the usefulness of different braille coding methods for math and science formulas in the classroom and instructional materials,” he says.
Rutland and his colleagues have been very active, too. In an effort to understand how different tribological properties affect both the measured friction and also how people distinguish and relate this to their sensing function—the mechanoreceptors—it was necessary to analyze data in terms of individual parameters, at a buried interface.
“We developed a methodology that permitted a stimuli set to be made where a sinusoidal topography could be systematically varied from the nm/μm level to the μm/100μm level—without changing any other parameters that are important: thermal capacity, elastic modulus and surface chemistry/polarity,” he explains. “The second crucial aspect was to collaborate with psychologists to perform measurements on and with humans. It took at least two years of intimate collaboration to be able to develop a common language to the point where we could unify physical data with perceptual data to quantify tactile perception.”
While there have been many important discoveries regarding tribology and touch, per Rutland, the most exciting discoveries his group has made are:
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Surfaces with topographic features on the level of 10 nm can be distinguished from unpatterned surfaces both by humans as measuring instruments and by biotribometers with a finger. Previously the cutoff for perception was considered to be around a micrometer.
10
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Simple patterned polymer materials are detected using only two psychological dimensions—which can be directly related to measured physical parameters and the corresponding mechanoreceptors. 10 More complex surfaces, such as printing papers, require three or more perceptual dimensions.
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The friction coefficient is one of these parameters; they have shown that people unconsciously regulate how hard they press the surface in response to the friction. Thus, while friction is the physical parameter that is measurable, it seems that the internal measure is related to how hard people need to press to achieve a satisfactory friction force.
11
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The team has developed a methodology for measuring and diagnosing dynamic tactile acuity of fine textures, which, prior to this, was difficult.
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Based on this methodology, they have demonstrated how tactile acuity declines among the elderly and that, while it is possible to correlate this decline directly to biomechanical properties of the finger, it is necessary to consider the reduction in mechanoreceptor density for a complete description. Acuity can be restored temporarily by addressing the biomechanical properties of the finger through rehydration, for example.
12
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At the tribological level, the friction coefficient is related to the deformation of the finger. Rutland and his team demonstrated that it is the stratum corneum (SC or top layer of skin) that deformed—typically very locally. They also were able to model this deformation the modulus of the using the SC—not the apparent modulus that can be measured for a finger using macroscopic approaches.
13
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They also discovered how surface chemistry/polarity affects perception and biotribology.
14
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In view of this and given the dependence of the SC modulus on moisture, they were able to show that it is possible to understand the role of humidity/moisture on touch and biotribology. This is different and complementary to other aspects such as occlusion of liquid water into the contact.
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They showed that the tactile profile of furniture coatings can be systematically controlled to tune the tactile perception of the coating using existing coating technology.
15
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Another finding was why wood is such a difficult material to describe in terms of touch—and without the tribology skills of Rutland’s colleagues, he says his group would still be at square one. The tribology of touch is truly a multidisciplinary challenge.
16
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They also determined that touch and finger tribology is a parameter one can use to determine that their hair doesn’t feel right.
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Limitations of existing tribology models According to Schwartz, the most exciting and most challenging aspect of this field is that traditional models tend to not describe these phenomena well. “This is not because the models are not good; it is generally because skin tribology and tactile phenomena are so incredibly complex,” he clarifies. “We have done work in the past looking at applying very fundamental models to tactile elements in isolation, such as a small number of isolated braille dots. It turns out the classic adhesion and hysteresis models of friction describe these interactions relatively well. However, as we get closer to something resembling a realistic end-user scenario, we must rely more and more on deducing what mechanisms play a role in the very complicated results observed.”
Much of this work involves taking a well-established practice that was developed empirically—for example, the use of particular patterns for tactile graphics—and performing some forensic work to see if a fundamental tribological model can explain why that practice has merit. “If we find that there is a good model to explain things, then we think about using the model to suggest ways to improve the established way of doing things,” Schwartz says.
Bennewitz points out that skin is a complex material. The stiff bone, the viscoelastic dermis and the elastic but stiffer stratum corneum efficiently provide a combination of rigidity, local deformability and low friction. “There is no ‘traditional model,’ and one of the challenges of modeling skin friction is the lack of availability of parameters describing the mechanical properties of skin, namely the elastic and viscous mechanical response across relevant time scale and also across relevant length scales,” he says.
While existing theories can be applied very successfully, the challenge is the complex biomaterials that need to be described, Rutland says. To understand the transmission of vibrations to mechanoreceptors, new models and approaches need to be developed.
According to Hipwell, the current state-of-the-art model in contact mechanics is a multiscale magnification theory developed by Persson in 2001.
18 “This powerful theory describes the real contact area and interfacial separation between two surfaces,” she says. “Many parts of the theory require the assumption of a self-affine surface—a surface that looks similar when magnified to a smaller scale but distorted in one direction. We have found that some of the substructures of the finger are not self-affine due to cell substructure and wear of the outer skin surface, and there are times when the actual geometries of the two surfaces are important.
19 Moreover, the complex multi-physical forces at the finger-device interface, such as capillary force, electrostatic force, van der Waals forces, etc., are not simply additive. They’re interacting with each other and affecting the friction in different ways. We developed a multi-physics model for a better prediction of the capillary force and the friction force, accounting for the exact geometries.”
So, while traditional tribological measurements and measuring devices have limitations, innovations are occurring.
Innovative tribometers
Hipwell and her team created a tribometer with a carefully designed finger holder that can constrain the finger movement in three directions as well as rotation. “Artificial fingers do not replicate the performance of human skin, so a tribometer that enabled consistent motion and contact conditions from a live human finger was important,” she observes. In addition, Hipwell’s tribometer can keep a constant, normal load applied to the fingertip while measuring the dynamic friction force. This has been achieved by integrating a proportional integral derivative (PID) control loop to move the vertical stage to compensate for the changes in normal load as the horizontal stage moves back and forth, replicating a fingertip swipe on a touchscreen. “Constant normal force allows us to achieve much more consistent results,” Hipwell says.
Bennewitz and colleagues developed tribometer innovations derived from their studies with human participants. They needed tribometers that hold several samples for direct comparison in touch, and these samples must be exchanged quickly during a study.
“This requirement made us build our own tribometers,” he explains. “Another advantage of implementing one’s own solution is the flexibility in developing dedicated software for study designs.”
Finally, Bennewitz synchronized the fingertip tribometer with a motion capture system so that they could simultaneously record position, velocity and force data during free tactile exploration by participants.
Differences in the properties of skin Hipwell points out that there are many differences in skin properties between people—for example, the finger pad size (gross contact area), the Young’s modulus, the thickness of the stratum corneum layer, the sweat rate, the hydration, sebum level, etc. “We did a lot of work to account for these differences in our research,” Hipwell says. “For example, we used Teflon finger masks with a punctured hole to keep the same gross contact area for everyone. We control humidity and temperature conditions subject to subject. We also cleaned fingertips with isopropyl alcohol (IPA) each time before measuring the friction to remove any previous contaminations, lipids and sweat. We also measure properties such as skin hydration for each user so that we can understand and explain differences in results
(see The Effect of Finger Hydration).”
The effect of finger hydration
Per M. Cynthia Hipwell, Oscar S. Wyatt, Jr. ’45 Chair II Professor, Texas A&M University, the hydration level of a human skin is of particular importance, since it is affected by humidity as well as user variation, temperature and contact conditions. It can influence the Young’s modulus
A by two to three orders of magnitude, which leads to the change in the real contact area, friction and, thus, how humans feel the surface. “When one designs a haptic surface, a complete understanding of contact mechanics between a finger and surfaces at multiple scales is required so that the haptic surface is less sensitive to the hydration level or has a predictable effect of the property variation on the friction to compensate for the variation effect,” she says.
According to Mark Rutland, professor, KTH Royal Institute of Technology, Stockholm, Sweden, a more hydrated skin means better elastic properties.
B “Skin moisture and skin elasticity (cosmetic rather than physical definition) are basically the same parameter,” he says. “This leads to the ability of the stratum corneum (SC) to deform in response to local asperities and also to a larger overall contact between skin and surface,” he explains. “This leads to a broader range of friction coefficients with different surfaces/topographies, making it easier to distinguish.
C It also is the mechanism by which the reduced tactile acuity of the elderly can be restored. Stiff dry skin can be made temporarily more supple and responsive with humectants.”
D
A. The Young’s modulus is a property of material that defines how easily it can stretch and deform.
B. Skedung, L., El Rawadi, C., Arvidsson, M., Farcet, C., Luengo, G. S., Breton, L. and Rutland, M. W. (2018), “Mechanisms of tactile sensory deterioration amongst the elderly,” Scientific Reports, 8 (1), pp. 1-10.
C. Arvidsson, M., Ringstad, L., Skedung, L., Duvefelt, K. and Rutland, M. W. (2017), “Feeling fine-the effect of topography and friction on perceived roughness and slipperiness,” Biotribology, 11, pp. 92-101.
D. Skedung, L., El Rawadi, C., Arvidsson, M., Farcet, C., Luengo, G. S., Breton, L. and Rutland, M. W. (2018), “Mechanisms of tactile sensory deterioration amongst the elderly,” Scientific Reports, 8 (1), pp. 1-10.
Hipwell adds that they often pair data and look for relative changes on the same person and also take samples of many users to get a full picture of variation and sensitivity. “We also use appropriate statistical approaches (i.e., analysis of variance [ANOVA]) and data processing to make meaningful conclusions,” she says. “During perception tests, we will measure friction and applied normal load so that we can connect the physical phenomena to what is being perceived by the user. The level of the user variation depends on what signal we used to stimulate the user and what results we are interested in.”
Bennewitz observes that friction coefficients vary by up to a factor of two between participants. “When comparing friction between materials, we often normalize friction data by dividing each friction coefficient by the average friction coefficient of that participant,” he explains. “We have not analyzed differences between individual abilities to discriminate surface features but prefer to determine statistical significance levels for findings which are based on the whole set of subjective judgements by all participants.”
Schwartz cautions that, if not well controlled, there can be a huge difference from person to person. “I am aware that some labs even go to the extent of monitoring the amount of salt in the diet of test participants for a few weeks before the study, just to better control for the condition of the skin,” he says. “However, end-users are going to encounter tactile information in an endless number of ways and venues.”
Schwartz’s approach is to create moderate standardization of subjects, such as following a particular handwashing protocol just before the experiment and using a large and diverse group of test subjects. “In this way, we address some of these challenges,” he says. “This approach works very well for tactile perception, but it is still not enough to eliminate much of the variation that we see when we are doing friction measurement of test subjects’ skin against various media. This suggests that there are likely many non-tribological cues that people are using when they perceive tactile media.”
According to Schwartz, the steps taken to make the tribology studies cleaner also tend to make the experiment less similar to the real-world application. He adds, “This is a big deal when we realize how much of skin tribology, and especially tactile communication, involves cognitive aspects.”
Since both biomechanical properties and tactile perception vary greatly, looking at the averages and statistics in isolation can be incredibly misleading, Rutland cautions, adding, “One also needs to consider the data at the individual level. For example, there is a statistical correlation at group level between biomechanical properties and reduced tactile ability. But the biomechanical properties are only a secondary effect and not primarily responsible for the reduction in perception. Mechanoreceptor density is the primary reason.
20 I would say that biotribology is useful for understanding ‘mechanisms of touch,’ but it is of limited use in characterizing touch at the individual level.”
Most important surface properties
While there is agreement among biotribology researchers that there are many skin properties that need to be taken into consideration, the conundrum is how to prioritize. Surface roughness is a start.
“Really smooth surfaces lead to huge frictional forces because of the large area of contact, which, in turn, leads to stick-slip and paradoxically to the surface often being perceived as rough,” Rutland says. “Large asperities can be sensed in static touch; small features require movement and variation in applied pressure. Hydrophobic surfaces provide lower friction due to the smaller adhesive friction component to friction (and as a result are generally perceived as both warmer and nicer). The difficulty of the perceptual task basically increases as the feature size decreases, particularly for random roughness.”
Bennewitz says, “For hard surfaces, a long history of research indicates that roughness on a scale smaller than the finger ridge structure is key to tactile perception. The translation of roughness into friction forces, both average and fluctuating, is the reason why tribologists should be involved in perception research. For compliant surfaces, the field is still developing. We recently found for fibrillar elastomer surfaces that bending of fibrils is a key physical parameter in tactile perception.”
21
Hipwell goes further. “Surface topography (roughness and texture shapes), wetting behavior (surface energy), compliance and thermal properties create the feelings of sticky/slippery, texture (vibration), compliance and warm/cool,” she says. “The surface topography, in general, affects the real contact area between the finger and the surface, and our recent works showed that people can easily tell surfaces with different capillary forces due to different nanotexture shape and surface energy.”
She adds, “In addition, humans perceive objects based on thermal cues—the reason that metal feels colder than wood is that metal takes away more heat from the finger than wood and, thus, decreases the skin temperature faster. This is due to copper’s higher thermal conductivity and lower specific heat capacity, which the human can detect.”
Per Schwartz, it is hard to generalize all of the different tactile interactions into a reasonable number of categories. “This mirrors the challenge of tribology in general, in that each case is unique,” he says. “In my work with tactile graphics printed on materials and with methods common to educational institutions, we have found that spacing between features is often important. And the complexity of the pattern elements (dots versus ridges versus more complicated three-dimensional features) looks to be important. I also think that the vibratory frequencies produced during fingertip exploration likely play a role.”
An interesting discovery that Schwartz made is that there is a change in test subjects’ ability to tell two different textures apart based on the order in which they encounter each member of the pair. If the sequence of the textures is changed, the percentage of subjects who detect them as the same or different can change.
Tribology and braille
One of the many areas where this study of touch has an important application is braille, which Schwartz has been researching. It hasn’t been easy. “Skin deformation plays a role over the full area of contact of the fingertip against the printed media,” he explains. “However, it is the length scale of the other frictional mechanisms and their competition with deformation that makes this research so challenging. With very simple isolated tactile elements such as braille dots or raised ridges, we can do a reasonable job of separating out the components of adhesion, hysteresis and deformation of the skin against distinct features. However, once the spacing between individual pattern elements gets closer than about 1 mm, we begin to see these mechanisms get superimposed in a way that is more complex than just summing up individual contributions.”
Schwartz believes that much of this depends on the highly non-elastic behavior of the skin (especially the fingertip) and how it deforms around, and penetrates between, pattern elements that are closely spaced.
“There are different media used for braille such as paper, polymer film and metal, and all of these can have different friction,” Schwartz says. “I think a very interesting research question would be to see if changing friction would help on the local scale, i.e., recognition of individual braille characters, but would make things worse at the large scale in terms of tactile fatigue of scanning a page full of higher/lower friction characters.”
One intriguing finding that Schwartz published several years ago is that it is not so much the magnitude of friction that may be important, but it might be the detection of individual friction events that is key.
When two braille dots are separated by more than about 2 mm, they produce two distinct friction events that are roughly comparable in magnitude. As they get closer to each other, they start to merge into one single prolonged event. Friction will hit a first plateau, then will climb to a second plateau, then fall back to a third plateau. When the dots get even closer, the multi-tiered phenomenon disappears and goes back to a single plateau but with longer duration than a single dot.
Schwartz explains, “There does seem to be empirical support for dot separation and legibility, because there are braille materials designed for people who lose their vision later in life and often have a very difficult time learning braille. These materials use larger dots and/or greater spacing between dots to enhance character recognition. I took an introductory braille course several years ago, and though I’m sure many younger braille users would laugh at this, I had a very hard time detecting the difference between the ‘p’ and the ‘q’ characters because they feel so similar to me.”
Design guidelines and next challenges According to Rutland, the next challenge (and the best route toward general design guidelines) will be interpreting how people make tactile decisions. This will involve the integration of neuroscience and psychotribology.
“However now that many of the important questions are answered, design can progress through combining current psychotribological understanding with physical characterization of surfaces—for example, artificial fingers do little to advance our understanding of touch but will provide a useful tool (if applied with understanding) for assisting in specification of tactile attributes of materials,” he explains.
Bennewitz believes that a new challenge is to design tribological systems that provide users with a designated tactile experience and account for individual differences. “Can we take, for example, an individual’s finger ridge structure into account when producing a personalized device?” he asks, adding, “Another aspect which is to be addressed is the durability of surfaces under touch. Wear on the one hand and transfer of skin material (sebum, salts and lipids) on the other hand may influence the tactile experience when optics or other functions are not affected yet.”
Schwartz notes that there is not a large budget for groundbreaking innovation in the methods or materials used for tactile communication. Given this, the practices and materials used in this area are often the product of using available materials in the limited time that teachers and facilitators have. “In some ways, that is what makes the current practices so amazing,” he says. “The teachers and instructional designers, who produce textbooks, scientific diagrams, tactile representation of famous artwork, etc., are truly artists and crafts people.”
Schwartz observes that many of the accepted practices are communicated by groups like the Braille Authority of North America (BANA) or provided by American Printing House for the Blind (APH), as a result of things that have developed in the field. “The challenge for us in the research world is to be able to catch up with them and figure out why these evolved approaches work, and what can be tweaked to push through some of the long-standing obstacles.”
Schwartz notes that topics for further tribological research include:
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How much information can be packed into a finite page space and still be useful?
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How do we make the production of these materials more accessible to teachers and students who use them daily?
Hipwell would like to see contact mechanics play a bigger role. “Current approaches have been mostly trial and error,” she says. “To find the optimum design and to accelerate the process, we need to have a comprehensive understanding of the contact mechanics of the human finger, especially the real contact area down to the nanoscale, as it impacts the friction force, skin deformation and, thus, human perception of the surfaces. Estimation of the real contact area is still limited due to the optical diffraction limit. Experimental measurement of the real contact area at the nanoscale, including differentiation between water and dry contact of skin structure, would be a breakthrough that could increase understanding of the physics and contact mechanics. In addition, multiphysics models that can
predict mechano and thermoreceptor responses and the resulting human perception to be used as a design tool would be extremely powerful.”
Bennewitz concludes, “Research into touch offers tribologists great opportunities for interdisciplinary work beyond the more traditional engineering disciplines, for example, in collaboration with robotics, kinesiology, psychology, physiology, neurophysiology or cosmetics.”
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Klöcker, M., Wiertlewski, V., Theate, V., Hayward, V. and Thonnard, J.L. (2013), “Physical factors influencing pleasant touch during tactile exploration,”
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2.
Skedung, L., Arvidsson, M., Chung, J.Y., Stafford, C.M., Berglund, B. and Rutland, M.W. (2013), “Feeling small: Exploring the tactile perception limits,”
Scientific Reports, 3, 2617.
3.
Wiertlewski, M., Friesen, R.F., Colgate, J.E. (2016), “Partial squeeze film levitation modulates fingertip friction,”
Proc. Natl. Acad. Sci. USA, 113, pp. 9210-9215.
4.
André, T., Lévesque, V., Hayward, V., Lefèvre, P. and Thonnard, J-L. (2011), “Effect of skin hydration on the dynamics of fingertip gripping contact,”
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