Transparent sensors are flexible and ultrathin

Japanese researchers have fabricated ultrathin, flexible, transparent sensors with cross-aligned silver nanowire microelectrodes, fabricated using a print technique that is scalable and requires minimal raw materials.

Transparent electronics — such as head-up displays that allow pilots to read flight data while keeping their eyes ahead of them — are designed to improve safety and to allow users to access data while in transit. For healthcare applications, such electronics should be cheap and straightforward to fabricate, but also sufficiently flexible to conform to skin. Silver nanowire networks meet these criteria; however, current methods of development create random nanowire alignment that is insufficient for advanced applications.

Looking to solve this problem, researchers from Osaka University used high-resolution printing to fabricate centimetre-scale cross-aligned silver nanowire arrays, with reproducible feature sizes from 20 to 250 µm. As a proof of concept for functionality, they used their arrays to detect electrophysiological signals from plants.

The researchers first created a patterned polymer surface to define the subsequent nanowire feature size. Using a glass rod to sweep silver nanowires across the pattern led to either parallel or cross-aligned nanowire networks, depending on the direction of the sweep. Nanowire cross-alignment, alignment within the pattern and electro-optical properties were impressive, according to results published in the journal Advanced Intelligent Systems.

“The sheet resistance of patterns less than 100 µm ranged from 25 to 170Ω per square, and the visible light transmittance at 550 nm was 96% to 99%,” said Teppei Araki, co-senior author on the study. “These values are well suited for transparent electronics.”

The researchers showed off the utility of their technology by monitoring the electric potential of Brazilian waterweed leaves. Because the nanowire arrays are transparent, the researchers were able to keep the leaf under visual observation while acquiring data over long periods of time. A 2 to 3 µm-thick device conformed to the surface of a leaf without causing damage.

“Our microelectrodes-based organic field-effect transistors exhibited excellent multifunctionality,” said Tsuyoshi Sekitani, co-senior author on the study. “For example, transparency of 90%, the on–off ratio was ~106, and the leakage current remained stable upon bending at a radius of 8 mm.”

Transparent electronics is an emerging technology that must be simple and inexpensive to mass-produce for biomedicine, civil engineering, agriculture and other applications that require underlying visual observation. The advance described here is an important step in that direction, helping to minimise the raw material input of electronics and exceed the functionality of conventional non-transparent electronics. The Osaka researchers plan on making further technical improvements, such as incorporating graphene onto the nanowire’s surface; this will improve the uniformity of the microelectrodes’ sheet resistance.

Image caption: The researchers say they have developed the world’s thinnest and most transparent potential sensor sheet. Image credit: Osaka University.

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