Tribology Goes Soft

Think friction, wear, and lubrication, and race car engines may leap to mind. The field of tribology, which encompasses those topics, has long focused on the surface properties of pistons, gears, and other metal machine parts that maintain contact as they slide past one another, often at high speed. But these days, some friction specialists have turned their attention from engine performance and other traditional topics to a “softer” side of tribology.

“In recent years, there has been a growing recognition that much of our interaction with the world around us—especially the way we perceive the things we touch—is governed by the rules of tribology,” says Philippa Cann, a tribologist and principal research fellow at Imperial College London. Those rules, or scientific principles, influence everyday tactile experiences, such as the ones caused by interactions between textiles and skin, contact lenses and eyes, fingers and hair, and food and drink and the inside of a mouth.

Those subjects and others took center stage last month in Toronto at a conference on biotribology. Sponsored by Elsevier, the meeting, which was only the second of its kind, brought together researchers who are trying to quantify, understand, and ultimately control interfacial phenomena central to these familiar experiences. They aim to lay the scientific groundwork needed for improving commercial products and the health and comfort of people who use them.

“Making these kinds of advances means measuring tribology effects on the human body, which can be very challenging,” says Imperial College’s Marc A. Masen, who coorganized the conference with Cann. Masen explains that in traditional tribology studies, for example, ones designed to optimize engine oil, sample reproducibility and other experimental conditions are easily controlled.

But the results of friction experiments on human skin, for example, may differ strongly from subject to subject because of age, skin condition, and other factors. And for a single subject, the results can differ from day to day because of the use of soaps and other variables. These challenges force biotribologists to identify suitable reference standards and devise novel experimental methods to probe ordinary activities that until now have rarely been investigated in detail.

Lying in bed is one such activity. For healthy people, that topic is a snooze. But for bedridden patients, frictional forces between skin and bedding, coupled with constant pressure on select body areas and underlying tissues, lead to painful and potentially dangerous pressure ulcers or bedsores, a widespread health problem. So researchers at Empa, the Swiss Federal Institute of Technology’s materials science institute in St. Gallen, teamed up with a Swiss paraplegic care center to compare tribological properties of standard hospital bedsheets and prototype textiles.

The research team, which includes Siegfried Derler, Gelu-Marius Rotaru, and coworkers, used a mechanical skin-fabric interface simulator to measure frictional forces under dry and moist conditions between a polyurethane-coated polyamide fleece, a material that mimics human skin topography and texture, and various textiles. The fabric samples included commercial woven and knitted materials (cotton and cotton-polyester blends) and proprietary fabrics made from blends of synthetic fibers. The group also measured frictional forces between these fabrics and a region of healthy volunteers’ forearms with properties similar to skin on the lower back, an area prone to bedsores.

In simulations and human skin tests alike, the researchers found that standard hospital bedsheets consistently lead to friction that is up to three times as high as one of the synthetic prototypes. The findings, which suggest a simple way to improve conditions for bedridden patients, are highlighted in a pair of studies in Tribology International (2012, DOI: 10.1016/j.triboint.2011.03.011 and 2013, DOI: 10.1016/j.triboint.2013.02.005).

The thought of friction between eyelids and eyeballs may make your eyes water. That response is normal and thought to be a protection mechanism that keeps those body surfaces lubricated. But key details of that lubrication process, for example the chemical identity of naturally occurring eye lubricants and their tribological effects on contact lenses, have remained unknown. To better understand these issues, which could lead to increased comfort for millions of contact lens wearers and people who suffer from debilitating dry eye conditions, Michael L. Samsom, Tannin A. Schmidt, and coworkers at the University of Calgary, in Alberta, studied various properties of eye surfaces.

One of their key findings is that epithelial cells on eye tissues produce an amphiphilic glycoprotein known as lubricin or PRG4, which functions as a lubricant in knees and other bone joints. By using a force measurement device to study frictional properties of human eyelids and corneas (from body donors) and animal eye tissues, including ones from mice modified to inhibit their ability to produce PRG4, the team demonstrated that PRG4 significantly reduces friction between those tissues and protects them from shear stresses caused by blinking (JAMA Ophthalmol. 2013, DOI:10.1001/jamaophthalmol.2013.2385).

In addition, the team measured forces induced by sliding eye tissues against commercial silicone hydrogel soft contact lenses and model hydrogels. They found that lubricating the sliding contacts with PRG4, which just recently became available from Framingham, Mass.-based Lubris, led to a lower coefficient of friction than using other lubricants, including saline solution and commercial eye drops containing hyaluronic acid.

Another pair of common experiences that researchers are trying to understand through friction analysis is eating and drinking. “In-mouth lubrication or oral tribology is believed to be a major contributor to people’s perception of surface-related mouthfeel attributes such as roughness, creaminess, and astringency,” says Jason R. Stokes, a chemical engineer at the University of Queensland, in Australia.

Those properties, which are critical to consumer choice and acceptability, can be altered substantially when food formulations are changed to reduce fat, salt, and sugar, says Stefan K. Baier, a research scientist at food giant PepsiCo, in Hawthorne, N.Y. Ultimately, scientists would like to rely on an understanding of fundamental food properties to design new foods and beverages that are healthy and acceptable to consumers. But Stokes, Baier, and other specialists in this area acknowledge that they are not yet able to predict texture or mouthfeel based on friction coefficients and other basic properties.

Part of the difficulty arises from the complexity of the eating process. For solids, it involves a multiscale breakdown starting with the first bite and continuing through chewing, granulation, bolus formation, and swallowing. Unique mouth sensations may be associated with each of those steps and with the residue that remains after swallowing. That complexity makes it difficult to mimic eating and drinking in tribology experiments.

In addition, “saliva clearly plays an important role in oral processing,” Baier stresses, “but capturing its role analytically has been especially challenging.” Baier and Stokes discuss many of these challenges in a recently published paper (Curr. Opin. Colloid Interface Sci. 2013, DOI: 10.1016/j.cocis.2013.04.010).

To alleviate some of the experimental difficulties, Gleb E. Yakubov, also at University of Queensland, is working to develop a saliva mimic. He notes that saliva contains a large number of proteins, including mucins, proline-rich compounds, amylases, histatins, cystatins, and others. But which ones endow saliva with the lubricating properties that are essential for eating, drinking, and speaking normally?

By separating saliva into protein fractions and probing their friction properties under various conditions, Yakubov found that some proteins resemble saliva under one set of conditions, for example, a low value of applied force (load) but not others. But combining a load-supporting mucin-rich fraction with an adhesive proline-rich fraction yields a saliva-like lubricant, he says.

Running fingers through a head of hair is another tactile experience familiar to most people but challenging to quantify. Like other soft tribology processes, it, too, is yielding to the rigors of laboratory analysis. Gustavo S. Luengo, a research scientist at cosmetics manufacturer L’Oréal, in Aulnay-sous-Bois, France, has teamed up repeatedly to design novel techniques to analyze hair properties.

A few years ago, for example, Luengo and scientists at the Royal Institute of Technology, in Stockholm, demonstrated an atomic force microscopy (AFM) method that used a cantilever-mounted hair to measure nanoscale friction and other forces between two human hairs. The team showed that the method can compare properties of untreated hair and hair subjected to bleaching, cationic surfactants, and other treatments (Langmuir 2010, DOI: 10.1021/la103001s). They also showed that the AFM method readily resolved friction differences between healthy and damaged hair (Langmuir 2013, DOI: 10.1021/la400468f).

And just recently, Luengo teamed up with researchers at Anasys Instruments, in Santa Barbara, Calif., to develop a combined AFM-infrared method capable of providing IR spectra and absorption images with submicrometer spatial resolution. The technique was used to pinpoint the location of structural lipids in the outer (cuticle) and inner (cortex) layers of hair fibers (Appl. Spectrosc. 2014, DOI: 10.1366/13-07328).

Biotribology tends to involve complex natural materials and ordinary conditions that can be especially difficult to control and reproduce in a laboratory. Yet the field is exciting and growing quickly, says Cann of Imperial College, because it is likely to reveal simple ways to improve human health and comfort.