Epidermal electronics: Skin health monitoring.

Our skin reveals many aspects of our overall health, ethnicity, age, and social and physical environment. Its mechanical properties are often tested by practitioners as a first gauge of our status; for instance, applying light pressure with a finger on the skin can be used to give an indication of circulatory function, and a gentle pinch can provide information on the skin stiffness and hydration. But these procedures and their outcome strongly depend on the practitioner and their experience1,2. Also, cutaneous pathologies may be recognized by the changes they induce in the skin elasticity, but this metric is challenging to measure reliably without invasive procedures such as a biopsy. A potential solution to these issues is now proposed by John Rogers and colleagues, who report in Nature Materials a bioelectronic adhesive bandage that performs on-skin monitoring and mapping of the viscoelasticity of the skin with a spatial resolution unmatched so far3. The smart bandage emanates from the long experience of the authors in designing and manufacturing ultrathin and conformal electronic transducers, a technology referred to as epidermal electronics4,5. Briefly, inorganic thin-film devices are turned from rigid and brittle to flexible and conformable to soft biological tissues by making them as thin as possible. These devices are then embedded in micrometre-thick plastic films, which are further patterned in a meandering twodimensional mesh and supported by a thin elastomer adhesive (Fig. 1a). The epidermal electronic circuit sticks to the biological tissue by van der Waals forces alone, and may be removed and reapplied many times. The conformal modulus sensor demonstrated here hosts a linear array of alternating piezoelectric sensors and actuators prepared with nanoribbons of lead zirconate titanate (PZT). Small oscillations generated by a given PZT actuator are mechanically coupled through the biological tissue and the supporting adhesive to the neighbouring PZT sensors. Output voltages of the sensor are directly proportional to the elastic modulus of the biological tissue, offering a straightforward and non-invasive method to quantify the mechanical metric. Rogers and colleagues successfully implemented the bioelectronic bandage to monitor skin stiffness in vivo across a diverse cohort of patients, and revealed specific changes in skin properties as a function of gender, age, body location, skin treatment and lesions. Applied to the skin of a patient with cancer, the device mapped with millimetre resolution the regional stiffness of the skin and revealed localized changes in elasticity near lesion sites (Fig. 1b,c). Importantly, the smart wearable sensor was readily accepted by the patients, highlighting the comfort and imperceptible nature of the non-invasive bioelectronic device. The possibility to perform such systematic clinical trials can help in deciphering the mechanics of skin and developing new therapies in dermatology and, in particular, in skin cancer research. In fact, the incidence of melanomas has been increasing over the past decades, with an alarming number of one in three cancers diagnosed being a skin cancer6. Hence, there is a great need to bring accurate monitoring devices to dermatologists and ultimately to patients to evaluate skin lesions and to monitor the benefits of a specific treatment. The researchers also demonstrated the relevance of these devices in cosmetic research. Taking advantage of the ultrathin piezoelectric transducers, they directly monitored the effects of moisturizing agents on ex vivo samples of skin. The results highlighted the relative increase in elasticity and hydration of the upper layer of the skin along with the temporal effect of the moisturizing agents. Accurate measure of the levels of skin hydration and elasticity promises the design of new cosmetic products better adapted to the ageing effects on skin. 1 cm b