Electrochemical systems for wearable applications

Non-invasive detection of biomarkers in sweat is of great interest for assessing the body’s response to physical activity, as well as for clinical diagnostics. Moreover, if fabricated in a form factor of a wearable device, sweat sensors offer advantages of continuous data acquisition, high performance and low cost. Despite significant progress up to date, major developments in fabrication of reliable sensing components still have to be realized in order to implement the vision of using sweat for continuous health monitoring. Commercially viable sweat sensors should demonstrate fast response, sensitivity to physiologically relevant concentrations of analyte, batch to batch reproducibility, stable performance under continuous operation, specificity, i.e. insensitivity to other analytes present in sweat, long-term storage, and ideally, no calibration requirement. These sensors are designed to be compliant and nearly imperceptible to the wearer. The need to power these devices while retaining their mechanical properties has been driving active innovation in the field of wearable compliant batteries. Unfortunately, incorporating additional functionalities commonly compromises the energy storage capacity of the wearable devices: they usually show lower gravimetric or volumetric capacity than those of commercially available systems. It remains important to maximize the energy density of wearable batteries. Additionally, despite numerous innovative design strategies for the compliant batteries, there are few reports of batteries that exhibit fatigue resistance satisfactory enough for applications in wearable systems which are likely to undergo thousands of deformation cycles throughout their lifetime. Not to mention, the disposal of batteries becomes a concern with increased worldwide use of these devices. At the same time, the bulk of the demonstrations are based on Li-ion ion battery chemistry, which often relies on toxic and/or flammable components. Designing non-toxic systems comprising more abundant and disposable materials is of great importance. This thesis focuses on addressing some of the existing shortcomings in the development of the wearable electrochemical devices: both electrochemical sensors for sweat monitoring and batteries. We demonstrate several unique design strategies to achieve high-performance flexible and stretchable batteries for wearable applications. For example, we develop an electrode composite with interpenetrated binder network to achieve a flexible battery with energy density matching that of the commercial AA alkaline battery. We also show wire-shaped batteries based on helical band springs that are resilient to fatigue and retain electrochemical performance over 17,000 flexure cycles. Both of these innovations address important limitations of existing flexible battery systems. Moreover, we use intrinsically safe, non-toxic silver-zinc (Ag/Zn) and zinc-manganese dioxide (Zn/MnO2) battery chemistries, which are highly desirable for wearable applications. For electrochemical sensors, we outline optimization steps to achieve printed flexible sensors for continuous monitoring of the lactate, Na+ or NH4+ ions and show how different optimization parameters affect the sensor performance. Last but not the least, we point out aspects that remain to be studied and/or addressed before such system can be claimed to be commercially viable