Epidermal Differential Impedance Sensor for Conformal Skin Hydration Monitoring

Abstract

We present the design and use of an ultrathin, stretchable sensor system capable of conformal lamination onto the skin, for precision measurement and spatial mapping of levels of hydration. This device, which we refer to as a class of ‘epidermal electronics’ due to its ‘skin-like’ construction and mode of intimate integration with the body, contains miniaturized arrays of impedance-measurement electrodes arranged in a differential configuration to compensate for common-mode disturbances. Experimental results obtained with different frequencies and sensor geometries demonstrate excellent precision and accuracy, as benchmarked against conventional, commercial devices. The reversible, non-invasive soft contact of this device with the skin makes its operation appealing for applications ranging from skin care, to athletic monitoring to health/wellness assessment.

Multimodal epidermal devices for hydration monitoring

“…Thermal measurements (1) are less susceptible to contact resistances due to the increased penetration depth of the thermal signal (as seen in the FEA shown in Figure 4; (2) do not require direct contact, allowing for a soft elastomeric layer to provide further insulation from leakage currents into the skin (in addition to the stiff, PI layer) 15 , in addition to imparting robustness to the device, unlike impedancebased sensors where the electrodes need be in direct contact with skin 19 ; and (3) offer advanced capabilities in temperature 16 and blood flow mapping 17 using similar resistive elements. Impedance-based measurements (1) build on an extensive library of knowledge on skin-based electrical measurements 25,44 ; (2) provide versatility in probing volumes derived from the ability to use a wide range of applied frequencies 45 ; (3) yield physical information about skin through both the capacitive reactance (permittivity) and the resistance (conductivity) 23 embedded in the measured impedance; and (4) enable comparisons with existing commercial measurements, almost all of which rely on electrical sensing 9 .…”

“…Here we describe recent advances in multimodal sensors of skin hydration, via measurements of intrinsic thermal and electrical properties, in which advanced materials, mechanics and concepts render devices that have soft, 'skin-like', or 'epidermal', formats [11][12][13][14][15][16][17][18][19][20] . The thin geometries and compliant mechanics lead to a mode of integration with the skin that does not require application of pressure, and instead relies on van der Waals adhesion forces alone.…”

Recent advances in flexible sensors for wearable and implantable devices

“…A reversible interconnection system between EES and data acquisition could be one possible solution. 79 …”

Stretchable Magnetoelectronics

“…The intimate contact with the epidermal surface throughout its complex shape is of great advantage for sensorics in particular 53,57,58 . Inspired by natures archetype, electronic skins are already able to perceive temperature changes 44,59 , mimic the sensation of touch 41,60,61 , monitor physiological conditions 53,58,62 and even release drug doses on demand for therapy 63 . Approaches that enable self-healing 64 will lead to durable and multifunctional artificial skin.…”