Search

Although evaporimetry (the measurement of water vapour flux density from the skin) has often been used to study the impact on skin hydration of using products such as baby diapers and incontinence pads, it is difficult to interpret results and to compare data from different studies because of the diversity of unvalidated methodologies used. The aim of this work was to develop a robust methodology for measuring the excess water in over-hydrated skin and test it on volar forearm and hip skin which had been occluded with saline soaked patches. Three repeat measurements were made on the volar forearm and the hip of five young (31-44 years) and six older (67-85 years) women and moderately good within-subject repeatability was found for both skin sites for both subject groups. Measurements taken from the hip were significantly higher (P = 0.001) than those from the arm and had larger coefficients of variation (3.5-22.1%) compared to arms (3.0-14.0%). There were no significant differences between young and older skin, implying that women for future studies could be recruited without regard to age. This is the first time that a robust evaporimetric methodology for quantifying excess water in over-hydrated skin has been described and validated, and it will form a solid basis for future work.

A new method for measuring the coefficient of friction between nonwoven materials and the curved surface of the volar forearm has been developed and validated. The method was used to measure the coefficient of static friction for three different nonwoven materials on the normal (dry) and over-hydrated volar forearms of five female volunteers (ages 18–44). The method proved simple to run and had good repeatability: the coefficient of variation (standard deviation expressed as a percentage of the mean) for triplets of repeat measurements was usually (80 per cent of the time) less than 10 per cent. Measurements involving the geometrically simpler configuration of pulling a weighted fabric sample horizontally across a quasi-planar area of volar forearm skin proved experimentally more difficult and had poorer repeatability. However, correlations between values of coefficient of static friction derived using the two methods were good ( R = 0.81 for normal (dry) skin, and 0.91 for over-hydrated skin). Measurements of the coefficient of static friction for the three nonwovens for normal (dry) and for over-hydrated skin varied in the ranges of about 0.3–0.5 and 0.9–1.3, respectively. In agreement with Amontons' law, coefficients of friction were invariant with normal pressure over the entire experimental range (0.1–8.2 kPa).

An analytical mathematical model for friction between a fabric strip and the volar forearm has been developed and validated experimentally. The model generalizes the common assumption of a cylindrical arm to any convex prism, and makes predictions for pressure and tension based on Amontons' law. This includes a relationship between the coefficient of static friction ( μ) and forces on either end of a fabric strip in contact with part of the surface of the arm and perpendicular to its axis. Coefficients of friction were determined from experiments between arm phantoms of circular and elliptical cross-section (made from Plaster of Paris covered in Neoprene) and a nonwoven fabric. As predicted by the model, all values of μ calculated from experimental results agreed within ±8 per cent, and showed very little systematic variation with the deadweight, geometry, or arc of contact used. With an appropriate choice of coordinates the relationship predicted by this model for forces on either end of a fabric strip reduces to the prediction from the common model for circular arms. This helps to explain the surprisingly accurate values of μ obtained by applying the cylindrical model to experimental data on real arms.