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SKIN ELASTICITY MEASUREMENT IN-VIVO - THE GOODYER LINEAR SKIN RHEOMETER

1 Overview Extracted from linked document 'How Does it Work'

In-vivo measurement of the elastic and viscous properties of skin is an essential requirement when assessing the effectiveness of moisturisers and other cosmetic skin treatments. By their very nature such measurements are difficult to perform, not just because they are in-vivo, but also because the forces and displacements that are required to be measured are extremely small - typically a peak lateral force of around 3 gf will produce displacements of less than 1 mm.

It is now possible to manufacture such an instrument which employs micromechanical moving components, and computer feedback control with full graphical display and analysis of the data in real time. Such an Instrument is the GSI Linear Skin Rheometer, which is detailed in this paper.

When measuring the elastic and viscous properties of a material we are seeking to determine how far the material moves when a lateral force is applied to it. If we apply a sinusoidal force, then we expect to see a resultant displacement that also changes sinusoidally. The phase shift between the force and displacement curves is also of great interest.

To carry out this measurement a probe is attached to the surface of the skin, and a sinusoidal force is then applied along its' axis and thereby onto the skin. Typically the peak force applied will be in the region of 3g. If we simultaneously measure the displacement of the skin caused by the force then we will obtain a pair of readings as shown in figure 1. These are ideal readings, shown for the purpose of this discussion only, some real readings are shown in figure 3, and it can be seen that they do correspond reasonably well with the predicted behaviour.

Three parameters can be obtained from the curves -

F Max which is the peak force that is applied to the skin surface

P Max which is the peak displacement that occurs as a result of that force

T which is the phase shift between the two signals

The elastic component of the skin is given simply by the formulae F max / P max, and is usually expressed in units of grams force per millimetre.

The more usual way of presenting this data is to plot force directly against displacement. In which case an ellipse will be formed, as the component parts are two sine waves with an identical period, but shifted in time. Such a picture, as taken from the Goodyer Skin Rheometer, is shown in figure 2 below.

The phase shift is due to the viscous properties of the skin, and is represented in figure 2 by the area of the enclosed ellipse in units of gram force metres. In effect the area of the ellipse represents the energy that is lost in moving the probe over one complete cycle. Had the force and displacement sine waves been in phase then the resultant plot (a straight line) would have represented a perfectly elastic material.

HOW DOES THE SKIN RHEOMETER WORK?

THE LINEAR SKIN RHEOMETER USERS MANUAL

PUBLISHED RESULTS FROM PROCTER & GAMBLE

PUBLISHED RESULTS FROM DeMontfort University and Harvard Medical School at MEEI

NEW NEW NEW SKIN FRICTION MEASUREMENT ATTACHMENT

Other Instrumentation Projects 

SKIN ELASTICITY MEASUREMENT - SKIN ELASTICTY MEASUREMENT - SKIN ELASTICITY MEASUREMENT

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SKIN ELASTICITY MEASUREMENT - SKIN ELASTICTY MEASUREMENT - SKIN ELASTICITY MEASUREMENT