System Bits: Sept. 10

Enabling flexible touchscreens
While transparent conductors make touchscreens possible, the cost and the physical limitations of the material these conductors are usually made of are hampering progress toward flexible touchscreen devices but a research collaboration between the University of Pennsylvania and Duke University has shown a new a way to design transparent conductors using metal nanowires that could enable less expensive — and flexible — touchscreens.

The current industry-standard material for making transparent conductors is indium tin oxide (ITO), which is deposited as two thin layers on either side of a separator film. Contact, in the form of a fingertip or a stylus, changes the electrical resistance between the two ITO layers enough so that the device can register where the user is touching. While this material performs well, its drawbacks have led industrial and academic researchers to look for alternatives.

The two problems with ITO are that indium is relatively rare making its cost and availability erratic, and — more importantly for flexible devices — it’s brittle. Industry would like to make touchscreens that use a network of thin, flexible nanowires, but predicting and optimizing the properties of these nanoscale networks has been a challenge.

Metal nanowires are increasingly inexpensive to make and deposit; they are suspended in a liquid and can easily be painted or sprayed onto a flexible or rigid substrate, rather than grown in vacuum as is the case for ITO. The challenge stems from the fact that this process forms a random network, rather than a uniform layer like ITO.

The Penn group previously worked on simulating nanowire networks in 3D nanocomposites, particularly the number of nanowires it takes to ensure there is a connected path from one end of the system to the other. The Duke took note of this work and contacted the Penn researchers to ask if they would develop 2D simulations that could be applied to data from silver nanowire networks that the Duke team had fabricated.

With the Duke group able to provide the nanowire length, diameter and area fraction of their networks, the Penn team was able to use the simulation to work backward from the network’s overall electrical resistance to uncover the elusive contact resistance. Alternative methods for finding the contact resistance are laborious and incompatible with typical network processing methods.

Researchers simulate electrical resistances (lines) to match experimental data (points) and extract the contact resistance. (Source: University of Pennsylvania)

In its next modeling studies, the Penn team will consider several additional parameters that factor into the performance of nanowire networks for transparent conductors, including nanowire orientation, to mimic nanowire networks produced by various continuous deposition methods, as well as the degree to which individual nanowires vary in length or diameter.

Heart rhythms for identity authentication
Levering the user’s unique heartbeat, a wearable authentication device from a start-up founded by University of Toronto engineering graduates recently hit the market.

The Nymi from Bionym enables secure authentication through an embedded electrocardiogram (ECG) sensor. When the Nymi recognizes the user’s personal ECG, it communicates their identity to devices. The user remains authenticated until the Nymi is removed. The activated Nymi can then be used to gain access to all registered devices, completely bypassing passwords and PINs for seamless and secure access. Passwords, PINs and even keys and cards will become a thing of the past.

~Ann Steffora Mutschler