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.

LINEAR SKIN RHEOMETER 323 Position Force Figure 2. Typical position vs force hysteresis loop produced by a one GBE measurement cycle. extension), mm/N or Fm/g (measures of stretching or compression of the skin in response to a given applied force). Such analysis yields information about the elastic properties of the skin. Analysis of the phase lag between force and displacement responses yields information about the viscous properties of the skin. After 20 years of experience with the GBE within our laboratories, we believe that the principle of the GBE measurement is still the best available for measuring sensitive changes in the mechanical properties of the human stratum corneum in vivo. Subtle though important changes in skin elasticity [dubbed "softness" by Maes eta/. (3)] in response to the application of moisturizing formulae have been measured, as have changes in skin "tightness" due to surfactant damage. Our experience has, however, also highlighted the drawbacks of employing the original Hargens GBE instrument in a modern laboratory. These are as follows: 1. Importantly, the instrument employs an "open-loop" method of control, i.e., during calibration of the instrument, and subsequently in routine operation, one assumes that an applied current equals a given force. As the GBE is calibrated on one point only (3 g), linearity is not guaranteed over the whole measurement range of the GBE. In addition, calibration drift over time is certain. 2. The components needed to run the GBE are bulky and dated (function generator, signal conditioner, storage oscilloscope, compressed gas/air). 3. The probe components are fragile and, in our experience, break easily and require excessive servicing when used routinely (for example, the fine copper wires connect- ing the armature to the body of the probe). 4. The air-bearing employed in the probe design is inherently susceptible to misalign- ment, soiling, and malfunction. In recent years, the cost of precision has improved greatly. We can now achieve with conventional technology what was achieved previously through Hargens' (1) consider- able ingenuity. We have designed and built a new instrument that retains and builds on