Researchers from Carnegie Mellon University&rsquo;s College of Engineering and the Institute of Systems and Robotics of the University of Coimbra, Portugal have developed a simple, efficient method to make robust, highly flexible, tattoo-like circuits for use in wearable computing. The low-cost process adds trace amounts of an electrically-conductive, liquid metal alloy to tattoo paper that adheres to human skin. These ultrathin tattoos can be applied easily with water, the same way one would apply a child&rsquo;s decorative tattoo with a damp sponge.Other tattoo-like electronics either require complex fabrication techniques inside a cleanroom or lack the material performance required for stretchable digital circuit functionality on skin.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1573729030390-1011-em-carmel-tattoo-1.png" style="width: 1114px;" class="fr-fic fr-dib">The tattoos can conform and adhere to highly curved 3D surfaces, like a model of the human brain.Carmel Majidi, an associate professor of mechanical engineering and the director of the&nbsp;<a href="http://sml.me.cmu.edu/" rel="noopener" target="_blank">Soft Machines Lab</a> at Carnegie Mellon, and Mahmoud Tavakoli, the director of the&nbsp;<a href="https://sites.google.com/site/fsrlabisr/" rel="noopener" target="_blank">Soft and Printed Microelectronics Laboratory</a> <a href="https://sites.google.com/site/fsrlabisr/" target="_blank"></a>at the University of Coimbra, teamed up to develop methods for direct printing of stretchable electronic circuits.Their collaboration succeeded.&nbsp;&ldquo;This is a breakthrough in the printed electronics area,&rdquo; said Tavakoli. &ldquo;We showed for the first time that inkjet-printed patterns of silver nano particles can be sintered at room temperature using the gallium indium alloy. Removing the need for high temperature sintering makes our technique compatible with thin-film and heat sensitive substrates.&rdquo;<blockquote>The technique is simple. The tattoos are ultrathin, very stretchable, and inexpensive to produce.</blockquote>In addition to low-cost processing, these tattoos provide other advantages. Because they have mechanical properties similar tolightweight fabrics, they remain functional under bending, folding, twisting, and strains above 30% (which is the typical stretchability of human skin). They can conform and adhere to highly curved 3D surfaces, like a model of a human brain or a lemon.Applications for ultrathin, compliant tattoos include epidermal biomonitoring, soft robotics, flexible displays, and 3D-transferable printed electronics.The findings were published in Advanced Materials&nbsp;in a paper titled &ldquo;<a href="https://doi.org/10.1002/adma.201801852" rel="noopener" target="_blank">EGaIn‐Assisted Room‐Temperature Sintering of Silver Nanoparticles for Stretchable, Inkjet‐Printed, Thin‐Film Electronics</a>.&rdquo; Carmel Majidi explains the technology behind the electronic tattoos:<iframe src="https://www.youtube.com/embed/OaITzyMptFQ?&wmode=opaque" frameborder="0" allowfullscreen="" class="fr-draggable" style="width: 1115px; height: 619px;"></iframe>Video credit: Institute of Systems and Robotics of the University of Coimbra, Portugal and College of Engineering, Carnegie Mellon University

The ultrathin tattoos can be applied and removed easily with water, similar to deorative, temporary tattoos.

These stretchable, inexpensive, tattoo-like circuits are made with an inkjet printer.

Electronic tattoos for wearable computing

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The first programmable memristor computer&mdash;not just a memristor array operated through an external computer&mdash;has been developed at the University of Michigan. It could lead to AI processing directly on small, energy-constrained devices such as smartphones and sensors.A smartphone AI processor would mean that voice commands would no longer have to be sent to the cloud for interpretation, speeding up response time.&ldquo;Everyone wants to put an AI processor on smartphones, but you don&rsquo;t want your cell phone battery to drain very quickly,&rdquo; said <a href="http://www-personal.umich.edu/~wluee/" target="_blank">Wei Lu</a>, a professor of electrical and computer engineering at U-M and senior author of the paper in Nature Electronics.In medical devices, the ability to run AI algorithms without the cloud would enable better security and privacy.<h4>Why memristors are good for machine learning</h4>The key to making this possible could be an advanced computer component called the memristor. This circuit element, an electrical resistor with a memory, has a variable resistance that can serve as a form of information storage. Because memristors store and process information in the same location, they can get around the biggest bottleneck for computing speed and power: the connection between memory and processor.This is especially important for machine-learning algorithms that deal with lots of data &nbsp;to do things like identify objects in photos and videos&mdash;or predict which hospital patients are at higher risk of infection. Already, programmers prefer to run these algorithms on graphical processing units (GPUs) rather than a computer&rsquo;s main processor, the central processing unit (CPU).&ldquo;GPUs and very customized and optimized digital circuits are considered to be about 10-100 times better than CPUs in terms of power and throughput. Memristor AI processors could be another 10-100 times better,&rdquo; said Lu.GPUs perform better at machine learning tasks because they have thousands of small cores for running calculations all at once, as opposed to the string of calculations waiting their turn on one of the few powerful cores in a CPU. A memristor array takes this even further. Each memristor is able to do its own calculation, allowing thousands of operations within a core to be performed at once. In this experimental-scale computer, there were more than 5,800 memristors. A commercial design could include millions of them.Memristor arrays are especially suited to machine learning problems. The reason for this is the way that machine learning algorithms turn data into vectors&mdash;essentially, lists of data points. In predicting a patient&rsquo;s risk of infection in a hospital, for instance, this vector might list numerical representations of a patient&rsquo;s risk factors. Then, machine learning algorithms compare these &ldquo;input&rdquo; vectors with &ldquo;feature&rdquo; vectors stored in memory.&nbsp;These feature vectors represent certain traits of the data (such as the presence of an underlying disease). If matched, the system knows that the input data has that trait. The vectors are stored in matrices, which are like the spreadsheets of mathematics, and these matrices can be mapped directly onto the memristor arrays.&nbsp;What&rsquo;s more, as data is fed through the array, the bulk of the mathematical processing occurs through the natural resistances in the memristors, eliminating the need to move feature vectors in and out of the memory to perform the computations. This makes the &nbsp;arrays highly efficient at complicated matrix calculations.&nbsp;Earlier studies demonstrated the potential of memristor arrays for speeding up machine learning, but they needed external computing elements to function.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569831908719-memristors-9content.jpg" style="width: 594px;" class="fr-fic fr-dib">Wei Lu stands with Seung Hwan Lee, an electrical engineering PhD student and co-author, who holds the memristor array. Photo: Robert Coelius, Michigan Engineering Communications &amp; Marketing&nbsp;<h4>Building a programmable memristor computer</h4>To build the first programmable memristor computer, Lu&rsquo;s team worked with <a href="https://zhang.engin.umich.edu/" target="_blank">Zhengya Zhang</a> and <a href="https://www.mpflynngroup.com/michael-flynn.html" target="_blank">Michael Flynn</a>, an associate professor and a professor of electrical and computer engineering at U-M, to design a chip that could integrate the memristor array with all the other elements needed to program and run it. Those components included a conventional digital processor and communication channels, as well as digital/analog converters to serve as interpreters between the analog memristor array and the rest of the computer.&nbsp;Lu&rsquo;s team then integrated the memristor array directly on the chip at U-M&rsquo;s Lurie Nanofabrication Facility. They also developed software to map machine learning algorithms onto the matrix-like structure of the memristor array.The team demonstrated the device with three bread-and-butter machine learning algorithms:<ul><li>Perceptron, which is used to classify information. They were able to identify imperfect Greek letters with 100% accuracy.</li><li>Sparse coding, which compresses and categorizes data, particularly images. The computer was able to find the most efficient way to reconstruct images in a set and identified patterns with 100% accuracy.</li><li>Two-layer neural network, designed to find patterns in complex data. This two-layer network found commonalities and differentiating factors in breast cancer screening data and then classified each case as malignant or benign with 94.6% accuracy.</li></ul>There are challenges in scaling up for commercial use: Memristors can&rsquo;t yet be made as identical as they need to be, and the information stored in the array isn&rsquo;t entirely reliable because it runs on analog&rsquo;s continuum rather than the digital either/or. These are future directions of Lu&rsquo;s group.

The memristor array chip plugs into the custom computer chip, forming the first programmable memristor computer. The team demonstrated that it could run three standard types of machine learning algorithms. Photo: Robert Coelius, Michigan Engineering Communications & Marketing

Circuit elements that store information in their electrical resistances enable a brain-like form of computing, storing and processing information in the same place.

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