Measuring the Salt Content of Sweat inside a Sweat-Absorbing Skin Adhesive.

Biofluids contain a wealth of different biomarkers, and their concentrations are indicative of the state of the body. As one of those biofluids, sweat is easily accessible, and its composition can, for example, be related to particular diseases or sports performance. Due to the relatively low sweat flow rates, however, adequate sampling is paramount. Here, we aim to explore the potential use of sweat-absorbing skin adhesives as a sweat sampling system for wearable sensors with a simple construction. Upon absorption of sweat, the electrochemical properties of the skin adhesive are determined by the composition of sweat and the amount of sweat within the skin adhesive (i.e., hydration). Through the incorporation of two polarizable electrodes within the skin adhesive, its electrical properties can be monitored using impedance spectroscopy. Here, the double layer capacitance is used as an indicator of hydration, while the conductance depends on both the ion concentration and hydration (the mobility of ions). By evaluating the conductance as a function of hydration, the ion concentration within an electrolyte solution can be estimated. We demonstrate the concept based on a simple model sensor patch, which is exposed to electrolyte solutions containing various concentrations of NaCl and an artificial sweat solution. Finally, we show that ion concentrations in human sweat can be estimated when the model sensor patch is worn during exercise.

Skin and Artificial Skin Models in Electrical Sensing Applications.

Skin electrical properties play a significant role in recording biopotentials by using electrophysiological sensors. To test and evaluate sensor systems, it is commonly accepted to employ artificial skin models due to complications associated with testing on living tissues. The first goal of this Review is to provide a systematic understanding of the relation between skin structure and skin electrochemical behavior at an appropriate depth for electrophysiological sensing applications through a focus on skin structure, electrochemical properties of skin, and theoretical models (equivalent circuits) representing skin electrochemical behavior. The second goal is to review artificial skin models mimicking the electrochemical properties of skin and to give suggestions for future studies on relevant skin models based on a comparison between the behavior of skin and that of artificial skin models. The Review aims to help the reader to analyze the relation between the structure, elements of the equivalent circuits, and the resulting impedance data for both skin and artificial skin models.

Water transport in polymer composites through swelling-induced networks of hydrogel particles.

Water diffusion in polymer composites is not only affected by the chemical nature of the materials but also by their internal structures. To enable the design of polymer composites with controlled diffusion kinetics, we investigate the effect of hydrogel particle networks on the water transport. The composites in this study comprise hydrogel particles based on sodium poly(acrylic acid), which are incorporated at different concentrations into a soft and sticky polymer matrix. Through the use of X-ray micro computed tomography, the internal structure of the polymer composites is examined and the interparticle distances are calculated. The structure of the composites is then related to the water diffusion kinetics upon exposure to saline solution as well as humid air. Even though the hydrogel particles are isolated and the interparticle distances are in the order of several micrometers, a sudden increase in the water diffusion kinetics is observed above a critical concentration. Due to the low water permeability of the matrix, such a change in the water diffusion kinetics is indicative of network formation. During hydration, swelling enables the hydrogels to overcome the interparticle distances and form a network for water transport.

Dynamics of Water Absorption in Polymer Skin Adhesives.

Skin adhesives are used to attach medical devices to the body and are required to adhere to the skin also during challenging conditions such as exposure to moisture and sweat. In order to maintain good skin-adhesive contact in these situations, the adhesives should be able to absorb water and remove fluids from the skin-adhesive interface. Consequently, the water absorption properties and distribution of water in the adhesives influence the overall adhesive properties. Understanding how the material composition impacts the absorption and distribution of water in these adhesives is required for rational formulation of skin adhesives. Here, we probe the dynamic water absorption in different skin adhesives using time-resolved impedance measurements and gravimetric analysis. Skin adhesive formulations consisting of soft, hydrophobic polymers providing stickiness, rigid block copolymers enhancing the structure, and hydrogel particles allowing for water uptake were designed. The hydrogel particle content and polymer matrix rigidity were varied, and the adhesives were systematically investigated to link the material composition and water absorption functionality.