Invisible nanofilm enables on-screen fingerprint detection

A transparent, flexible smartphone sensor also measures skin temperature and tactile pressure, which could combat fingerprint forgery and make mobile devices even more compact.

Fingerprints have become a popular password alternative for mobile devices, but accommodating the hardware necessary for biometric-enabled privacy can be a headache for designers. Developers are keen to maximize screen size in relation to the rest of the device, which makes integrating fingerprint readers into small- er products, such as smart watches, a big challenge.

Researchers in Korea may have found a solution: an on-screen detector featuring an invisible network of nanofibres that can be placed over a portion of the display. Because the ultrathin fingerprint reader is sensitive to tactile pressure and skin temperature, the design could boost security by distinguishing between real and artificial fingerprints.

The sensor, called a transparent capacitive detector, was developed by a team of engineers led by Jang-Ung Park and Franklin Bien at Ulsan National Institute of Science and Technology (UNIST) in South Korea. To fabricate the detector, scientists deposited a small quantity of highly conductive, ultra-long, silver nanofibres and fine silver nanowires onto a flexible plastic film separated by thin insulating layers. The porous nano-networks allow light to easily pass through, which is essential for preserving the optical characteristics of the host display beneath them. The fibres are also mechanically strong, remaining intact when the plastic films are folded or curved.

Patterned to create a fingertip-sized matrix of 80 x 80 detection points, the crisscross of nanofibre strips registers a fingerprint (ridges and valleys in skin pressed up against the screen) based on the capacitance at each node in the sensor array. Additional active elements include a thin film of temperature-sensitive molecules and a layer of pressure-sensitive silicone elastomers, which provide further outputs from the unit.

The spatial resolution of the sensor array is sufficient to satisfy criteria set by the FBI for extracting fingerprint patterns, the team reports in the journal Nature Communications. The pressure sensitivity exhibited by the design could potentially eliminate the need for an activation but- ton on smartphones, leading to further device miniaturization.

Having demonstrated the feasibility of on-screen fingerprint detection, the team is now gearing up for mass production and hopes to see its ideas incorporated into devices in the near future, explains Park, who is now at Yonsei University.

For further information, contact:

Associate Professor Jang-Ung Park
School of Materials Science and Engineering
Ulsan National Institute of Science and Technology
E-mail: [email protected]

Associate Professor Franklin Bien
School of Electrical Engineering
Ulsan National Institute of Science and Technology
E-mail: [email protected]

Published: 26 Mar 2019

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