j. Cosmet. Sci., 49, 321-333 (September/October 1998) A new instrument to measure the mechanical properties of human stratum corneum in vivo PAUL J. MATTS and ERIC GOODYER, Procter & Gamble Health & Beauty Care, Rusham Park Technical Centre, Whitehall Lane, Egham, Surrey TW20 9NW, U.K. Accepted for publication August 31, 1998. Synopsis The gas-bearing electrodynamometer (GBE) (1) has been used for the last 20 years to obtain sensitive measurements of the stratum corneum. A new instrument for measuring the mechanical properties of the stratum corneum incorporates all of the measurement principles of the GBE but none of its components. A force-controlled miniature d.c. servo, gearing, and leadscrew replace the magnet/solenoid arrangement of the GBE. Error resulting from conversion of an electrical signal to a mechanical force is automatically com- pensated. Consequently, this control renders the need for a friction-free bearing redundant. The orginal linear variable difi•rential transformer (LVDT) has been replaced with a unit with a sensitivity of 0.01%, and force is now measured by a calibrated 50-g load beam. The function generator, signal conditioner, and storage oscilloscope have been replaced by user-friendly software run by a small portable computer. The new design offers greater inherent accuracy than the GBE and requires minimal servicing. The new instrument (linear skin rheometer, "LSR") has been shown to provide sensitive measurements of stratum corneum mechanics and was used to measure the mechanical responses of the stratum corneum to two topical moisturizing treatments of difi•ring relative hydration performance (as determined by impedance measure- ments using the Nova TM DPM9003). The relative performance of the two products as measured by the LSR compared favorably with corresponding impedance data, indicating the ability of the LSR to differentiate varying degrees of stratum corneum plasticization in response to hydradon. INTRODUCTION There is a wide variety of methods available to the dermatological researcher to deter- mine changes in the mechanical properties of human skin in vivo. However, to measure sensitive changes in the mechanical properties of the stratum corneum, there is only a small number of instruments and methods that may be used with confidence. This is principally because the majority of available instruments and methods involve relatively large displacements of the stratum corneum, either parallel or perpendicular to the skin surface (1). Consequently, the tissue underneath the stratum corneum will have an unacceptably large effect on the measurement. Eric Goodyer's present address is E & C Consultancy, 20 Gladstone Street, Hathern, Leicestershire, U.K. 321

322 JOURNAL OF COSMETIC SCIENCE The instrument that appears to have been most widely used over the last 20 years to obtain sensitive measurements of the stratum corneum is the gas-bearing electrodyna- toometer (GBE) (1-5). It is able to apply a sinusoidal loading stress of less than 5 g parallel to the skin surface, with a resulting displacement of less than 1 mm in each direction. This is achieved by suspending an armature in a gas bearing to create near friction-free movement. Changes in the magnetic field generated by a surrounding coil cause the armature to oscillate at a known frequency and amplitude. The coil is activated by a sinusoidal signal from a low-frequency function generator or from a suitable software trigger. The armature of the instrument is typically attached to the skin surface by a stiff wire probe bent to 90 ø at its free end. A small plastic stub is usually cemented to the free end of the probe and used to attach the probe to the skin surface with a circular piece of double-sided sticky tape. Displacement of the armature is measured by a sensitive LVDT, mounted coaxially with the coil. Coil and LVDT outputs (force and displacement) are amplified and then supplied for analysis to either a storage oscilloscope or a computer equipped with suitable software. Equipment used in a "classic" GBE workstation is shown in Figure 1. Results of force and displacement measurements of skin are typically displayed as a hysteresis loop (Figure 2). Analysis of the gradient of the loop (force/displacement or displacement/force) yields derivatives of the dynamic spring rate (DSR), usually ex- pressed as g/ram (a measure of the force required to stretch or compress the skin per unit Figure 1. Equipment used in a GBE workstation: a, GBE probe b, storage oscilloscope c, function generator d, signal conditioner e, air compressor.