Friction and shear forces, as well as moisture between the human skin and textiles are critical factors in the formation of skin injuries such as blisters, abrasions and decubitus. This study investigated how epidermal hydration affects the friction between skin and textiles. The friction between the inner forearm and a hospital fabric was measured in the natural skin condition and in different hydration states using a force plate. Eleven males and eleven females rubbed their forearm against the textile on the force plate using defined normal loads and friction movements. Skin hydration and viscoelasticity were assessed by corneometry and the suction chamber method, respectively. In each individual, a highly positive linear correlation was found between skin moisture and friction coefficient (COF). No correlation was observed between moisture and elasticity, as well as between elasticity and friction. Skin viscoelasticity was comparable for women and men. The friction of female skin showed significantly higher moisture sensitivity. COFs increased typically by 43% (women) and 26% (men) when skin hydration varied between very dry and normally moist skin. The COFs between skin and completely wet fabric were more than twofold higher than the values for natural skin rubbed on a dry textile surface. Increasing skin hydration seems to cause gender-specific changes in the mechanical properties and/or surface topography of human skin, leading to skin softening and increased real contact area and adhesion.

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Influence of epidermal hydration on the friction of human skin against textiles.

Bioengineering skin using mechanisms of regeneration and repair.

Inverse gas chromatography applications: a review

The nano-scale mechanical properties of the extracellular matrix regulate dermal fibroblast function.

Background/aims: The study of changes in skin structure with age is becoming all the more important with the increase in life. The atrophy that occurs during aging is accompanied by more profound changes, with a loss of organization within the elastic collagen network and alterations in the basal elements. The aim of this study is to present a method to determine the mechanical properties of total human skin in vivo compared with dermal equivalents (DEs) using indentation and static friction tests.



Methods: A new bio-tribometer working at a low contact pressure for the characterization the mechanical properties of the skin has been developed. This device, based on indentation and static friction tests, also allows to characterize the skin in vivo and reconstructed DEs in a wide range of light contact forces, stress and strain.



Results: This original bio-tribometer shows the ability to assess the skin elasticity and friction force in a wide range of light normal load (0.5–2 g) and low contact pressure (0.5–2 kPa). The results obtained by this approach show identical values of the Young's modulus E* and the shear modulus G* of six DEs obtained from a 62-year-old subject (E*=8.5±1.74 kPa and G*=3.3±0.46 kPa) and in vivo total skin of 20 subjects aged 55 to 70 years (E*=8.3±2.1 kPa, G*=2.8±0.8 kpa).

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0846.2008.00329.x

Characterization of the mechanical properties of a dermal equivalent compared with human skin in vivo by indentation and static friction tests

Background/purpose: The mechanical properties of the skin have been previously analyzed by a number of different techniques including torsional analysis, cutometery, gas-bearing electrodynamometry, etc. The objective of this work is to present quantitative analysis of skin rheology by a technique termed indentometry.



Methods: The instrument used was a texture analyzer, which is a mechanical tensiometer simulating the process of touch. The experiments were carried out on human subjects as well as on artificial skin models. They included indentometry tests performed by using spherical probes with various geometrical dimensions as well as stress relaxation and creep experiments. The experimental data were interpreted by using the Hertz theory of contact mechanics and by calculation of fundamental parameters such as the modulus of elasticity.



Results: The calculated Young's modulae for skin models ranged from 5.5 × 104 to 17.7 × 104 N/m2, while the corresponding values for forearm and facial skin of ten panelists were found to be in the range of 0.7 × 104 –3.3 × 104 N/m2. In addition, stress relaxation and creep experiments were conducted, which permitted the assessment of the viscoelastic properties of skin. The results of these measurements were interpreted within the framework of the Kelvin–Voigt model of delayed elasticity leading to the calculation of viscosities and relaxation times. Indentomeric data, obtained by varying the diameter of the indentor and the indentation depth, are also discussed.



Conclusion: The indentometric analysis for both in vivo skin and artificial skin models could be interpreted by using the Hertz theory of contact mechanics. The loading and unloading indentometric curves could be used to assess the viscoelasticity of the investigated materials while creep and stress relaxation processes were analyzed quantitatively by the Kelvin–Voigt model with one relaxation time.

Indentometric analysis of in vivo skin and comparison with artificial skin models.

Background/purpose: High molecular weight polymers such as proteins and polysaccharides have been commonly employed in cosmetic practice to induce skin tightness. The effect is perceived by users as an increase in skin tightness (firmness) accompanied by skin smoothing and elimination of wrinkles and lines. The aim of the study was to assess whether high molecular weight synthetic polymers, in the form of simple skin treatment formulations, could modify mechanical properties of natural skin as well as artificial skin models.



Methods: In experiments involving natural skin, the formulations were employed to treat the inner forearm of five panelists, who ranged in age from mid-20s to mid-40s. Indentometric analysis also included rubber skin models, with various viscoelastic characteristics, modified by deposition of thin polymer films on the surface. Polymers such as polyvinylpyrrolidone, polyquaternium-55, polyimide-1, VP/acrylates/lauryl methacrylate copolymer, and sodium alginate were investigated. The effects of the products were quantified by mechanical skin indentation performed at small deformations. Data analysis was performed using the Hertz theory of contact mechanics, which included the calculation of fundamental parameters such as the modulus of elasticity.



Results: The indentometric analysis revealed an increase in the Young's modulae for both artificial and natural skin as a result of treatment with the polymers. The effect was dependent on the amount of deposited product, which was varied in the range from 0.053 to 1.06 mg/cm2 of polymer. The observed increases in Young's modulae were typically from about 0.7 to 1.4 × 104 N/m2 for untreated skin to 1.4–2.0 × 104 N/m2 for polymer-modified skin, depending on the polymer structure (molecular weight) and the amount of deposition. By the analysis of artificial skin models, it was found that the magnitude of stiffening depends on the mechanical characteristics of the base material. Softer skin models displayed larger increases in Young's modulus after polymer treatment than stiffer skin models. Also, the analysis of skin models suggested that polymeric treatments can lower the viscoelasticity of skin as demonstrated by decreased values of hysteresis loss factors calculated from indentation force vs. penetration depth plots. The performance of the polymers was also shown to be dependent on ambient humidity with the most hygroscopic materials loosing their stiffening effect at high humidity.



Conclusions: The data indicate that high molecular weight polymers can be useful as skin stiffening, firming, or tightening agents. This was demonstrated by indentometric, mechanical analysis of natural skin and artificial skin models, and by determining Young's modulae and viscoelastic parameters.

Alteration of skin mechanics by thin polymer films.

Various types of hair, including white (unpigmented), Piedmont (yellow-colored), blonde, brown, curly black of African origin, straight black of Chinese origin, and chemically processed (bleached) hair, were studied by using fluorescence spectroscopy. Fluorescence measurements were obtained by using a single- or double-grating fluorescence spectrophotometer and a bifurcated fiber optics accessory to measure the spectra directly from the surface of the hair. The results have shown that all types of hair share similar fluorescence characteristics, as recorded by excitation at 290 nm, 320 nm, 350 nm, and 380 nm, with emissions that could be ascribed to chromophores such as tryptophan (Trp), N-formylkynurenine, kynurenine, and 3-hydroxykynurenine. The relative intensities of fluorophore emissions were found to be dependent on factors such as melanin content and the history of UV light or thermal exposure. Trp fluorescence was also found to be dependent upon the state of the hair matrix. Softening of hair keratin by chemical reduction (breakage) of disulfide bonds or by hydration, leads to a 50%-100% increase in Trp emission intensity. Conversely, stiffening of the hair matrix by re-oxidation of reduced hair with hydrogen peroxide, or drying of wet hair, produces a decrease in fluorescence intensity. The results were interpreted by invoking the behavior of certain enzymes, which demonstrate Trp quenching by neighboring disulfide bonds or by hydrogen bonding with alanine residues, or with certain side-chain amino acids.

Tryptophan fluorescence in hair-examination of contributing factors.

Using various physicochemical methods of analysis, we examined human hair in its virgin and delipidized state. Free lipids were removed by a solvent extraction technique (covalently bound lipids were not removed) using a series of solvents with varying polarity. We analyzed the surface properties of hair by conducting mechanical combing and dynamic contact angle analysis. In addition, we used inverse gas chromatography surface energy analysis to explore the chemical composition of the hair surface based on interactions of various nonpolar and polar probes with biological molecules residing on the hair surface. Further, we investigated the importance that free lipids play in the internal structural properties of hair using dynamic scanning calorimetry and tensile strength measurements. The microstructure of the hair surface was probed by atomic force microscopy, whereas the lipid content of hair's morphological components was determined by infrared spectroscopic imaging. We also monitored the water management properties of virgin and delipidized hair by dynamic vapor sorption, which yielded unique water sorption isotherms for each hair type. Using all these techniques, differences were found in the chemical composition and physical behavior of virgin and delipidized hair. To better understand the influence of hair lipid composition on hair styling treatments, we conducted mechanical analyses of hair shaped into omega loops to determine the stiffness, elasticity, and flexibility of hair-polymer assemblies. Although there were no discernible differences between untreated virgin and delipidized hair, in terms of stiffness and elasticity, we found that treatment with hair styling agents produced different effects depending on the hair type used. Likewise, streaming potential measurements were carried out to monitor the binding capacity of rinse-off treatments on virgin and delipidized hair. Using this technique, we monitored the surface potential of hair and found significant differences in the binding behavior of cationic polymers and surfactants (polyquaternium-55 and quaternium-26) on both hair types.

Determination of physicochemical properties of delipidized hair

What is described herein is a hair care composition including a water soluble or water dispersible styling polymer characterized by a glass transition temperature below room temperature, and a gel-forming polymer which has a glass transition temperature higher than or lower than room temperature.

Roger L McMullen

Papers

Indentometric analysis of in vivo skin and comparison with artificial skin models.

Alteration of skin mechanics by thin polymer films.

Tryptophan fluorescence in hair-examination of contributing factors.

Determination of physicochemical properties of delipidized hair

Hair gels based on polymers having a low glass transition temperature