A new device for assessing changes in skin viscoelasticity using indentation and optical measurement

Skin is a viscoelastic material, comprised of fluidic and fibrous components. Changes in viscoelasticity can arise due to a number of conditions including dehydration, swelling (associated with injury or disease), impaired heart function, rehydration therapy, ageing, scarring, sun exposure and genetic conditions affecting connective tissue. Quantification of changes in skin viscoelasticity due to these processes is of great clinical interest in the fields of therapy monitoring, wound healing and disease screening. However, devices currently available to measure aspects of the mechanical properties of skin have limitations in ease-of-use, accessibility, and depth of measurement. This paper describes a new technique to follow changes in the viscoelasticity of the skin, using a novel approach to an indentation manoeuvre. The device is portable, low-cost and easy to use while at the same time providing rich information on the mechanical response of the skin.

The method proposed optically tracks the skin's recovery from an initial strain, made with a novel linear indentor, using diffuse side-lighting and a CCD video camera. Upon indentation, the skin's elastin fibres are stretched and fluid is displaced from the compressed region. When the indentor is removed, the rate of recovery of the skin from this imprint is therefore principally dependent on its hydration and elasticity. Using the blue colour plane of the image and polarisation filtering, it is possible to examine the surface topography only, and track the decay of the imprint over time.

The decrease in size of the imprint over time (decay curve) recorded by the device is shown to agree with the theoretical predictions of an appropriate viscoelastic model of skin mechanical behaviour. The contributors to the response measured using the indentation device are fully characterised and evaluated using separate measurement techniques including high-frequency ultrasound, polarisation spectroscopy and optical coherence tomography.

The device developed is capable of tracking the viscoelastic response of skin to minimal indentation. The high precision achieved using low-cost materials means that the device could be a viable alternative to current technologies.