Towards flexible and safe touchscreens

10-07-2019

One of the weaknesses of smartphones is their rigid screen, which can easily crack when the phone drops on the floor. An international, interdisciplinary team of scientist have shown that very thin silver nanowires may be the safe solution to manufacturing flexible screens without losing properties. They publish their results in PNAS.

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Silver nanowires are great candidates as flexible, transparent conductors for touch screens, as well as other applications like high-strength adhesives or papers incorporating LEDs, sensors or transistors. They entail, however, health risks when these fibres get in contact with skin: “Silver nanowires bear a striking resemblance to a notoriously toxic nanomaterial of the 20th century: asbestos,” explains Benjamin Gilbert, corresponding author of the study and scientist at the Lawrence Berkeley National Lab (LBNL) in the US. In a similar way to asbestos fibres, their interaction with skin cells can lead to cell dysfunction, inflammation and eventually cell death.

Silver nanowires, unlike indium tin oxide, the material currently used in touchscreens, do not break down with repeated bending and stretching. Because they will likely be used in smartphones, flexible electronics, and even clothes, an international scientific collaboration, funded by the U.S. Consumer Products Safety Commission, LabEx SERENADE of France, and the European Commission’s Safe Implementation of Innovative Nanoscience and Nanotechnology program, was formed to assess the risk and investigate whether nanowires could be designed to be safer.

With this goal, Gilbert joined forces with scientists from the Grenoble Alpes University, LBNL, the University of Lille, CEA-LITEN, the University of Florida and researchers from two beamlines at the ESRF, the European Synchrotron.

The researchers confirmed that silver nanowires could be toxic to skin cells.  Based on previous work, the team had expected nanowire length to be key, with shorter being less toxic, so initial research focused on nanowire length.  The scientists also explored how different diameters could change the nanowires’ properties.

Surprisingly, thinner nanowires were considerably less toxic to skin cells than thick ones. The researchers were puzzled, however, by standard microscopy studies that showed that thinner nanowires were just as likely to enter the cells as thicker nanowires, so why are thinner ones less toxic?  Nanowires, which are almost invisible, can be very hard to see, so in order to understand the mechanism of this diameter-dependent toxicity, the researchers needed even higher resolution imaging to better see what was going on in the cells.

Super high-resolution imaging at the ESRF beamlines ID16A and ID21 provided them with the answer. “Using coherent X-ray imaging we can directly see how the nanowires interact with the cell. Nano-tomography of frozen cells provides 3D snapshots showing the location and shape of the individual nanowires.  X-ray fluorescence and spectroscopy complete the chemical picture”, said Peter Cloetens, in charge of ID16A. In addition to Cloetens, five other scientists of the ESRF co-author the study.

Crumpling versus puncturing

The results showed that thinner nanowires were deformed and crumpled inside the cell within specific intracellular vesicles (endolysosomes), which seem to function in both taking up nanowires and keeping them separated from the rest of the cell.  By contrast, thicker nanowires were not significantly bent, and actually punctured through the vesicle membrane into the cell interior.  “It is remarkable that a cell vesicle composed of protein and lipid bilayers could mechanically deform metallic wires. The observations clearly implicate this deformation as being the critical mechanism for reducing toxicity of the thin wires versus the thick ones”, explains Chris Vulpe at the University of Florida.

Reduced toxicity does not come at the cost of performance, as thinner nanowires are equally or better suited for portable electronic displays than their thicker counterparts. This work illustrates how interdisciplinary collaboration can identify approaches to reduce the potential harm from advanced nanotechnologies early in the design stage. 

Reference:
Lehmann, S. et al, PNAS first published July 8, 2019. https://doi.org/10.1073/pnas.1820041116 

Text by Montserrat Capellas Espuny

 

Top image: Silver nanowires in the cells. The thicker ones don't bend, the thinner are crumpled. Credits: A. Pacureanu