Printed microfluidic sweat sensing platform for cortisol and glucose detection.

Wearable sweat biosensors offer compelling opportunities for improved personal health monitoring and non-invasive measurements of key biomarkers. Inexpensive device fabrication methods are necessary for scalable manufacturing of portable, disposable, and flexible sweat sensors. Furthermore, real-time sweat assessment must be analyzed to validate measurement reliability at various sweating rates. Here, we demonstrate a "smart bandage" microfluidic platform for cortisol detection and continuous glucose monitoring integrated with a synthetic skin. The low-cost, laser-cut microfluidic device is composed of an adhesive-based microchannel and solution-processed electrochemical sensors fabricated from inkjet-printed graphene and silver solutions. An antibody-derived cortisol sensor achieved a limit of detection of 10 pM and included a low-voltage electrowetting valve, validating the microfluidic sensor design under typical physiological conditions. To understand effects of perspiration rate on sensor performance, a synthetic skin was developed using soft lithography to mimic human sweat pores and sweating rates. The enzymatic glucose sensor exhibited a range of 0.2 to 1.0 mM, a limit of detection of 10 µM, and reproducible response curves at flow rates of 2.0 µL min-1 and higher when integrated with the synthetic skin, validating its relevance for human health monitoring. These results demonstrate the potential of using printed microfluidic sweat sensors as a low-cost, real-time, multi-diagnostic device for human health monitoring.

Direct Imprinting of Scalable, High-Performance Woodpile Electrodes for Three-Dimensional Lithium-Ion Nanobatteries.

The trend of device downscaling drives a corresponding need for power source miniaturization. Though numerous microfabrication methods lead to successful creation of submillimeter-scale electrodes, scalable approaches that provide cost-effective nanoscale resolution for energy storage devices such as on-chip batteries remain elusive. Here, we report nanoimprint lithography (NIL) as a direct patterning technique to fabricate high-performance TiO2 nanoelectrode arrays for lithium-ion batteries (LIBs) over relatively large areas. The critical electrode dimension is below 200 nm, which enables the structure to possess favorable rate capability even under discharging current densities as high as 5000 mA g-1. In addition, by sequential imprinting, electrodes with three-dimensional (3D) woodpile architecture were readily made in a "stack-up" manner. The height of architecture can be easily controlled by the number of stacked layers while maintaining nearly constant surface-to-volume ratios. The result is a proportional increase of areal capacity with the number of layers. The structure-processing combination leads to efficient use of the material, and the resultant specific capacity (250.9 mAh g-1) is among the highest reported. This work provides a simple yet effective strategy to fabricate nanobatteries and can be potentially extended to other electroactive materials.

Direct Printing of Graphene Electrodes for High-Performance Organic Inverters.

Scalable fabrication of high-resolution electrodes and interconnects is necessary to enable advanced, high-performance, printed, and flexible electronics. Here, we demonstrate the direct printing of graphene patterns with feature widths from 300 µm to ∼310 nm by liquid-bridge-mediated nanotransfer molding. This solution-based technique enables residue-free printing of graphene patterns on a variety of substrates with surface energies between ∼43 and 73 mN m-1. Using printed graphene source and drain electrodes, high-performance organic field-effect transistors (OFETs) are fabricated with single-crystal rubrene (p-type) and fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN2) (n-type) semiconductors. Measured mobilities range from 2.1 to 0.2 cm2 V-1 s-1 for rubrene and from 0.6 to 0.1 cm2 V-1 s-1 for PDIF-CN2. Complementary inverter circuits are fabricated from these single-crystal OFETs with gains as high as ∼50. Finally, these high-resolution graphene patterns are compatible with scalable processing, offering compelling opportunities for inexpensive printed electronics with increased performance and integration density.