Described in a paper titled “A Fully Integrated and Self-Powered Smartwatch for Continuous Sweat Glucose Monitoring” published in the ACS Sensors journal, the device packs a thin flexible electrochemical sensor for sweat glucose monitoring, small control electronics, two flexible amorphous silicon (a-Si) photovoltaic cells forming the wristband and facing outwards for efficient energy harvesting, and three flexible Zn-MnO2 rechargeable batteries connected in series, that can be charged up to 6.0V. An electronic ink (E-ink) display completes the Glucose monitoring bracelet, giving the wearer simple to understand threshold-based readings for real-time monitoring.
Here, the two-electrode enzymatic sensor used for glucose monitoring (from sweat secretion) outputs amperometric signals which once amplified and digitized, can be processed into readable results. Current detection resolution for the sensor can reach 6nA, which corresponds to a glucose concentration of about 2μM. The sensor is fabricated on a thin flexible PET substrate and can be directly inserted into the PCB hosting the control electronics, for if it needs to be replaced. Hence, one could envisage a wrist-worn health monitoring platform capable of receiving multiple types of swappable sensors.
The feat which the authors seem to be the most proud about, is the use for this medical application of Zn-MnO2 batteries based on quasi-solid-state aqueous electrolytes. While they eliminate safety concerns (having no flammable organic electrolytes), the batteries they crafted reportedly exhibited a capacity of 301 mA h g–1 at 0.3 A g–1. They retained their capacities under bending and compression and withstood over 1000 charging/discharging cycles even at a high current density of 1.8 A g–1.
The authors observed that the flexible batteries integrated in the glucose monitoring smartwatch would be fully charged (up 6.0V) within one hour under outdoor sunlight, enabling the system to then operate up to 8h with a cut-off potential of 3V. Even under relatively low illuminance (in a lit room), the solar cells would still charge the batteries to 4.2V in under 2 hours, to support system functions for about an hour.
Although their prototype proved to be sufficiently self-powered to operate without external power sources both indoor and outdoor, the researchers are confident they could still optimize their design to lower the system’s overall power budget.
To provide clear signals to the wearer, the smartwatch display only gives out threshold-based alarm signals switching from “low” to “medium” and from “medium” to “high” according to the output currents of the sensors at glucose concentrations below 40μM, then over 40μM but under 120μM, and finally over 120μM, respectively. These simple alarm signals, instead of reading out and updating glucose concentration values, also contribute to limit the E-ink display power consumption.
University of California, Berkeley – www.berkeley.edu