IBM’s announcement that they had produced the world’s smallest computer back in March raised a few eyebrows at the University of Michigan, home of the previous champion of tiny computing.

Now, the Michigan team has gone even smaller, with a device that measures just 0.3 mm to a side—dwarfed by a grain of rice.

The reason for the curiosity is that IBM’s claim calls for a re-examination of what constitutes a computer. Previous systems, including the 2x2x4mm Michigan Micro Mote, retain their programming and data even when they are not externally powered.

Unplug a desktop computer, and its program and data are still there when it boots itself up once the power is back. These new microdevices, from IBM and now Michigan, lose all prior programming and data as soon as they lose power.

“We are not sure if they should be called computers or not. It’s more of a matter of opinion whether they have the minimum functionality required,” said David Blaauw, a professor of electrical and computer engineering, who led the development of the new system together with Dennis Sylvester, also a professor of ECE, and Jamie Phillips, an Arthur F. Thurnau Professor and professor of ECE.

In addition to the RAM and photovoltaics, the new computing devices have processors and wireless transmitters and receivers. Because they are too small to have conventional radio antennae, they receive and transmit data with visible light. A base station provides light for power and programming, and it receives the data.

One of the big challenges in making a computer about 1/10th the size of IBM’s was how to run at very low power when the system packaging had to be transparent. The light from the base station—and from the device’s own transmission LED—can induce currents in its tiny circuits.

“We basically had to invent new ways of approaching circuit design that would be equally low power but could also tolerate light,” Blaauw said.

For example, that meant exchanging diodes, which can act like tiny solar cells, for switched capacitors.

Another challenge was achieving high accuracy while running on low power, which makes many of the usual electrical signals (like charge, current and voltage) noisier.

Designed as a precision temperature sensor, the new device converts temperatures into time intervals, defined with electronic pulses. The intervals are measured on-chip against a steady time interval sent by the base station and then converted into a temperature. As a result, the computer can report temperatures in minuscule regions—such as a cluster of cells—with an error of about 0.1 degrees Celsius.

The system is very flexible and could be reimagined for a variety of purposes, but the team chose precision temperature measurements because of a need in oncology. Their longstanding collaborator, Gary Luker, a professor of radiology and biomedical engineering, wants to answer questions about temperature in tumors.

Some studies suggest that tumors run hotter than normal tissue, but the data isn’t solid enough for confidence on the issue. Temperature may also help in evaluating cancer treatments.

“Since the temperature sensor is small and biocompatible, we can implant it into a mouse and cancer cells grow around it,” Luker said. “We are using this temperature sensor to investigate variations in temperature within a tumor versus normal tissue and if we can use changes in temperature to determine success or failure of therapy.”

Even as Luker’s experiments run, Blaauw, Sylvester and Phillips look forward to what purposes others will find for their latest microcomputing device.

“When we first made our millimeter system, we actually didn’t know exactly all the things it would be useful for. But once we published it, we started receiving dozens and dozens and dozens of inquiries,” Blaauw said.

And that device, the Michigan Micro Mote, may turn out to be the world’s smallest computer even still—depending on what the community decides are a computer’s minimum requirements.

What good is a tiny computer? Applications of the Michigan Micro Mote:

• Pressure sensing inside the eye for glaucoma diagnosis

• Cancer studies

• Oil reservoir monitoring

• Biochemical process monitoring
• Surveillance: audio and visual

• Tiny snail studies

The study was presented June 21 at the 2018 Symposia on VLSI Technology and Circuits. The paper is titled “A 0.04mm3 16nW Wireless and Batteryless Sensor System with Integrated Cortex-M0+ Processor and Optical Communication for Cellular Temperature Measurement.”

The work was done in collaboration with Mie Fujitsu Semiconductor Ltd. Japan and Fujitsu Electronics America Inc.

Microelectrodes can be used for direct measurement of electrical signals in the brain or heart. These applications require soft materials, however. With existing methods, attaching electrodes to such materials poses significant challenges. A team at the Technical University of Munich (TUM) has now succeeded in printing electrodes directly onto several soft substrates.

Researchers from TUM and Forschungszentrum Jülich have successfully teamed up to perform inkjet printing onto a gummy bear. This might initially sound like scientists at play – but it may in fact point the way forward to major changes in medical diagnostics. For one thing, it was not an image or logo that Prof. Bernhard Wolfrum’s team deposited on the chewy candy, but rather a microelectrode array. These components, comprised of a large number of electrodes, can detect voltage changes resulting from activity in neurons or muscle cells, for example.

Researchers from the Technical University of Munich (TUM) have succeeded in printing microelectrode arrays directly onto several soft substrates. Soft materials are better suited for devices that directly measure electrical signals from organs like the brain or heart. Credit: N. Adly / TUM

Second, gummy bears have a property that is important when using microelectrode arrays in living cells: they are soft. Microelectrode arrays have been around for a long time. In their original form, they consist of hard materials such as silicon. This results in several disadvantages when they come into contact with living cells. In the laboratory, their hardness affects the shape and organization of the cells, for example. And inside the body, the hard materials can trigger inflammation or the loss of organ functionalities.

Rapid prototyping with inkjet printers

When electrode arrays are placed on soft materials, these problems are avoided. This has sparked intensive research into these solutions. Until now, most initiatives have used traditional methods, which are time-consuming and require access to expensive specialized laboratories. “If you instead print the electrodes, you can produce a prototype relatively quickly and cheaply. The same applies if you need to rework it,” says Bernhard Wolfrum, Professor of Neuroelectronics at TUM. “Rapid prototyping of this kind enables us to work in entirely new ways.”

Wolfrum and his team work with a high-tech version of an inkjet printer. The electrodes themselves are printed with carbon-based ink. To prevent the sensors from picking up stray signals, a neutral protective layer is then added to the carbon paths.

Materials for various applications

The researchers tested the process on various substrates, including PDMS (polydimethylsiloxane) – a soft form of silicon – agarose – a substance commonly used in biology experiments – and finally various forms of gelatin, including a gummy bear that was first melted and then allowed to harden. Each of these materials has properties suitable for certain applications. For example, gelatin-coated implants can reduce unwanted reactions in living tissue.

Through experiments with cell cultures, the team was able to confirm that the sensors provide reliable measurements. With an average width of 30 micrometers, they also permit measurements on a single cell or just a few cells. This is difficult to achieve with established printing methods.

“The difficulty is in fine-tuning all of the components – both the technical set-up of the printer and the composition of the ink,” says Nouran Adly, the first author of the study. “In the case of PDMS, for example, we had to use a pre-treatment we developed just to get the ink to adhere to the surface.”

Wide range of potential applications

Printed microelectrode arrays on soft materials could be used in many different areas. They are suitable not only for rapid prototyping in research, but could also change the way patients are treated. “In the future, similar soft structures could be used to monitor nerve or heart functions in the body, for example, or even serve as a pacemaker,” says Prof. Wolfrum. At present he is working with his team to print more complex three-dimensional microelectrode arrays. They are also studying printable sensors that react selectively to chemical substances, and not only to voltage fluctuations.

Infrared spectroscopy is the benchmark method for detecting and analyzing organic compounds. But it requires complicated procedures and large, expensive instruments, making device miniaturization challenging and hindering its use for some industrial and medical applications and for data collection out in the field, such as for measuring pollutant concentrations. Furthermore, it is fundamentally limited by low sensitivities and therefore requires large sample amounts.

However, scientists at EPFL’s School of Engineering and at Australian National University (ANU) have developed a compact and sensitive nanophotonic system that can identify a molecule’s absorption characteristics without using conventional spectrometry.

The authors show a pixelated sensor metasurface for molecular spectroscopy. It consists of metapixels designed to concentrate light into nanometer-sized volumes in order to amplify and detect the absorption fingerprint of analyte molecules at specific resonance wavelengths. Simultaneous imaging-based read-out of all metapixels provides a spatial map of the molecular absorption fingerprint sampled at the individual resonance wavelengths. This pixelated absorption map can be seen as a two-dimensional barcode of the molecular fingerprint, which encodes the characteristic absorption bands as distinct features of the resulting image. Credit: EPFL

Their system consists of an engineered surface covered with hundreds of tiny sensors called metapixels, which can generate a distinct bar code for every molecule that the surface comes into contact with. These bar codes can be massively analyzed and classified using advanced pattern recognition and sorting technology such as artificial neural networks. This research – which sits at the crossroads of physics, nanotechnology and big data – has been published in Science.

Translating molecules into bar codes

The chemical bonds in organic molecules each have a specific orientation and vibrational mode. That means every molecule has a set of characteristic energy levels, which are commonly located in the mid-infrared range – corresponding to wavelengths of around 4 to 10 microns. Therefore, each type of molecule absorbs light at different frequencies, giving each one a unique “signature.” Infrared spectroscopy detects whether a given molecule is present in a sample by seeing if the sample absorbs light rays at the molecule’s signature frequencies. However, such analyses require lab instruments with a hefty size and price tag.

The pioneering system developed by the EPFL scientists is both highly sensitive and capable of being miniaturized; it uses nanostructures that can trap light on the nanoscale and thereby provide very high detection levels for samples on the surface. “The molecules we want to detect are nanometric in scale, so bridging this size gap is an essential step,” says Hatice Altug, head of EPFL’s BioNanoPhotonic Systems Laboratory and a coauthor of the study.

The system’s nanostructures are grouped into what are called metapixels so that each one resonates at a different frequency. When a molecule comes into contact with the surface, the way the molecule absorbs light changes the behavior of all the metapixels it touches.

“Importantly, the metapixels are arranged in such a way that different vibrational frequencies are mapped to different areas on the surface,” says Andreas Tittl, lead author of the study.

This creates a pixelated map of light absorption that can be translated into a molecular bar code – all without using a spectrometer.

The scientists have already used their system to detect polymers, pesticides and organic compounds. What’s more, their system is compatible with CMOS technology.

“Thanks to our sensors’ unique optical properties, we can generate bar codes even with broadband light sources and detectors,” says Aleksandrs Leitis, a coauthor of the study.

There are a number of potential applications for this new system. “For instance, it could be used to make portable medical testing devices that generate bar codes for each of the biomarkers found in a blood sample,” says Dragomir Neshev, another coauthor of the study.

Artificial intelligence could be used in conjunction with this new technology to create and process a whole library of molecular bar codes for compounds ranging from protein and DNA to pesticides and polymers. That would give researchers a new tool for quickly and accurately spotting miniscule amounts of compounds present in complex samples.