Engineers Reveal Fabrication Process For Revolutionary Transparent Sensors

In 2014, when University of Wisconsin–Madison engineers announced in the journalNature Communications that they had developed transparent sensors for use in imaging the brain, researchers around the world took notice.

Then the requests came flooding in. “So many research groups started asking us for these devices that we couldn’t keep up,” says Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW–Madison.

Ma’s group is a world leader in developing revolutionary flexible electronic devices. The see-through, implantable micro-electrode arrays were light years beyond anything ever created.

Although he and collaborator Justin Williams, the Vilas Distinguished Achievement Professor in biomedical engineering and neurological surgery at UW–Madison, patented the technology through the Wisconsin Alumni Research Foundation, they saw its potential for advancements in research. “That little step has already resulted in an explosion of research in this field,” says Williams. “We didn’t want to keep this technology in our lab. We wanted to share it and expand the boundaries of its applications.”

As a result, in a paper published Thursday (Oct. 13, 2016) in the journal NatureProtocols, the researchers have described in great detail how to fabricate and use transparent graphene neural electrode arrays in applications in electrophysiology, fluorescent microscopy, optical coherence tomography, and optogenetics. “We described how to do these things so we can start working on the next generation,” says Ma.

Now, not only are the UW–Madison researchers looking at ways to improve and build upon the technology, they also are seeking to expand its applications from neuroscience into areas such as research of stroke, epilepsy, Parkinson’s disease, cardiac conditions, and many others. And they hope other researchers do the same.

“We didn’t want to keep this technology in our lab. We wanted to share it and expand the boundaries of its applications.”

Justin Williams

“This paper is a gateway for other groups to explore the huge potential from here,” says Ma. “Our technology demonstrates one of the key in vivo applications of graphene. We expect more revolutionary research will follow in this interdisciplinary field.”

Funding for the initial research came from the Reliable Neural-Interface Technology program at the U.S. Defense Advanced Research Projects Agency. Other authors on the Nature Protocols paper include Dong-Wook Park, Sarah Brodnick, Jared Ness, Lisa Krugner-Higby, Solomon Mikael, Joseph Novello, Hyungsoo Kim, Dong-Hyun Baek, Jihye Bong, Kyle Swanson and Wendell Lake of UW–Madison; Farid Atry, Seth Frye and Ramin Pashaie of the University of Wisconsin-Milwaukee; Amelia Sandberg of Medtronic PLC Neuromodulation; Thomas Richner of the University of Washington; and Sanitta Thongpang of Mahidol University in Bangkok, Thailand.

Engineers reveal fabrication process for revolutionary transparent sensors

In 2014, when University of Wisconsin–Madison engineers announced in the journal Nature Communications that they had developed transparent sensors for use in imaging the brain, researchers around the world took notice.

A blue light shines through a clear, implantable medical sensor onto a brain model. See-through sensors, which have been developed by a team of UW–Madison engineers, should help neural researchers better view brain activity. Image credit: Justin Williams Research Group

Then the requests came flooding in. “So many research groups started asking us for these devices that we couldn’t keep up,” says Zhenqiang (Jack) Ma, the Lynn H. Matthias Professor and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW–Madison.

Ma’s group is a world leader in developing revolutionary flexible electronic devices. The see-through, implantable micro-electrode arrays were light years beyond anything ever created.

Although he and collaborator Justin Williams, the Vilas Distinguished Achievement Professor in biomedical engineering and neurological surgery at UW–Madison, patented the technology through the Wisconsin Alumni Research Foundation, they saw its potential for advancements in research. “That little step has already resulted in an explosion of research in this field,” says Williams. “We didn’t want to keep this technology in our lab. We wanted to share it and expand the boundaries of its applications.”

As a result, in a paper published Thursday (Oct. 13, 2016) in the journal Nature Protocols, the researchers have described in great detail how to fabricate and use transparent graphene neural electrode arrays in applications in electrophysiology, fluorescent microscopy, optical coherence tomography, and optogenetics. “We described how to do these things so we can start working on the next generation,” says Ma.

Now, not only are the UW–Madison researchers looking at ways to improve and build upon the technology, they also are seeking to expand its applications from neuroscience into areas such as research of stroke, epilepsy, Parkinson’s disease, cardiac conditions, and many others. And they hope other researchers do the same.

“This paper is a gateway for other groups to explore the huge potential from here,” says Ma. “Our technology demonstrates one of the key in vivo applications of graphene. We expect more revolutionary research will follow in this interdisciplinary field.”

Funding for the initial research came from the Reliable Neural-Interface Technology program at the U.S. Defense Advanced Research Projects Agency. Other authors on the NatureProtocols paper include Dong-Wook Park, Sarah Brodnick, Jared Ness, Lisa Krugner-Higby, Solomon Mikael, Joseph Novello, Hyungsoo Kim, Dong-Hyun Baek, Jihye Bong, Kyle Swanson and Wendell Lake of UW–Madison; Farid Atry, Seth Frye and Ramin Pashaie of the University of Wisconsin-Milwaukee; Amelia Sandberg of Medtronic PLC Neuromodulation; Thomas Richner of the University of Washington; and Sanitta Thongpang of Mahidol University in Bangkok, Thailand.

Transparent, Gel-Based Robots Can Catch And Release Live Fish

Engineers at MIT have fabricated transparent, gel-based robots that move when water is pumped in and out of them. The bots can perform a number of fast, forceful tasks, including kicking a ball underwater, and grabbing and releasing a live fish.
The robots are made entirely of hydrogel — a tough, rubbery, nearly transparent material that’s composed mostly of water. Each robot is an assemblage of hollow, precisely designed hydrogel structures, connected to rubbery tubes. When the researchers pump water into the hydrogel robots, the structures quickly inflate in orientations that enable the bots to curl up or stretch out.

The team fashioned several hydrogel robots, including a finlike structure that flaps back and forth, an articulated appendage that makes kicking motions, and a soft, hand-shaped robot that can squeeze and relax.

Because the robots are both powered by and made almost entirely of water, they have similar visual and acoustic properties to water. The researchers propose that these robots, if designed for underwater applications, may be virtually invisible.

Engineers at MIT have fabricated transparent gel robots that can perform a number of fast, forceful tasks, including kicking a ball underwater, and grabbing and releasing a live fish

Video: Melanie Gonick/MIT

The group, led by Xuanhe Zhao, associate professor of mechanical engineering and civil and environmental engineering at MIT, and graduate student Hyunwoo Yuk, is currently looking to adapt hydrogel robots for medical applications.

“Hydrogels are soft, wet, biocompatible, and can form more friendly interfaces with human organs,” Zhao says. “We are actively collaborating with medical groups to translate this system into soft manipulators such as hydrogel ‘hands,’ which could potentially apply more gentle manipulations to tissues and organs in surgical operations.”

Zhao and Yuk have published their results this week in the journal Nature Communications. Their co-authors include MIT graduate students Shaoting Lin and Chu Ma, postdoc Mahdi Takaffoli, and associate professor of mechanical engineering Nicholas X. Fang.

Robot recipe

For the past five years, Zhao’s group has been developing “recipes” for hydrogels, mixing solutions of polymers and water, and using techniques they invented to fabricate tough yet highly stretchable materials. They have also developed ways to glue these hydrogels to various surfaces such as glass, metal, ceramic, and rubber, creating extremely strong bonds that resist peeling.

The team realized that such durable, flexible, strongly bondable hydrogels might be ideal materials for use in soft robotics. Many groups have designed soft robots from rubbers like silicones, but Zhao points out that such materials are not as biocompatible as hydrogels. As hydrogels are mostly composed of water, he says, they are naturally safer to use in a biomedical setting. And while others have attempted to fashion robots out of hydrogels, their solutions have resulted in brittle, relatively inflexible materials that crack or burst with repeated use.

In contrast, Zhao’s group found its formulations leant themselves well to soft robotics.

“We didn’t think of this kind of [soft robotics] project initially, but realized maybe our expertise can be crucial to translating these jellies as robust actuators and robotic structures,” Yuk says.

Fast and forceful

To apply their hydrogel materials to soft robotics, the researchers first looked to the animal world. They concentrated in particular on leptocephali, or glass eels — tiny, transparent, hydrogel-like eel larvae that hatch in the ocean and eventually migrate to their natural river habitats.

“It is extremely long travel, and there is no means of protection,” Yuk says. “It seems they tried to evolve into a transparent form as an efficient camouflage tactic. And we wanted to achieve a similar level of transparency, force, and speed.”

To do so, Yuk and Zhao used 3-D printing and laser cutting techniques to print their hydrogel recipes into robotic structures and other hollow units, which they bonded to small, rubbery tubes that are connected to external pumps.

To actuate, or move, the structures, the team used syringe pumps to inject water through the hollow structures, enabling them to quickly curl or stretch, depending on the overall configuration of the robots.

Yuk and Zhao found that by pumping water in, they could produce fast, forceful reactions, enabling a hydrogel robot to generate a few newtons of force in one second. For perspective, other researchers have activated similar hydrogel robots by simple osmosis, letting water naturally seep into structures — a slow process that creates millinewton forces over several minutes or hours.

Catch and release

In experiments using several hydrogel robot designs, the team found the structures were able to withstand repeated use of up to 1,000 cycles without rupturing or tearing. They also found that each design, placed underwater against colored backgrounds, appeared almost entirely camouflaged. The group measured the acoustic and optical properties of the hydrogel robots, and found them to be nearly equal to that of water, unlike rubber and other commonly used materials in soft robotics.

In a striking demonstration of the technology, the team fabricated a hand-like robotic gripper and pumped water in and out of its “fingers” to make the hand open and close. The researchers submerged the gripper in a tank with a goldfish and showed that as the fish swam past, the gripper was strong and fast enough to close around the fish.

“[The robot] is almost transparent, very hard to see,” Zhao says. “When you release the fish, it’s quite happy because [the robot] is soft and doesn’t damage the fish. Imagine a hard robotic hand would probably squash the fish.”

Next, the researchers plan to identify specific applications for hydrogel robotics, as well as tailor their recipes to particular uses. For example, medical applications might not require completely transparent structures, while other applications may need certain parts of a robot to be stiffer than others.

“We want to pinpoint a realistic application and optimize the material to achieve something impactful,” Yuk says. “To our best knowledge, this is the first demonstration of hydrogel pressure-based acutuation. We are now tossing this concept out as an open question, to say, ‘Let’s play with this.’”

This research was supported, in part, by the Office of Naval Research, the MIT Institute for Soldier Nanotechnologies, and the National Science Foundation.

Biological Experiments Become Transparent — Anywhere, Any Time

Biological experiments are generating increasingly large and complex sets of data. This has made it difficult to reproduce experiments at other research laboratories in order to confirm – or refute – the results. The difficulty lies not only in the complexity of the data, but also in the elaborate computer programmes and systems needed to analyse them. Scientists from the Luxembourg Centre for Systems Biomedicine (LCSB) of the University of Luxembourg have now developed a new bioinformatics tool that will make the analysis of biological and biomedical experiments more transparent and reproducible.

The tool was developed under the direction of Prof. Paul Wilmes, head of the LCSB group Eco-Systems Biology, in close cooperation with the LCSB Bioinformatics Core. A paper describing the tool has been published in the highly ranked open access journal Genome Biology. The new bioinformatics tool, called IMP, is also available to researchers online.

Biological and biomedical research is being inundated with a flood of data as new studies delve into increasingly complex subjects – like the entire microbiome of the gut – using faster automated techniques allowing so-called high-throughput experiments. Experiments that not long ago had to be carried out laboriously by hand can now be repeated swiftly and systematically almost as often as needed. Analytical methods for interpreting this data have yet to catch up with the trend. “Each time you use a different method to analyse these complex systems, something different comes out of it,” says Paul Wilmes. Every laboratory uses its own computational programs, and these are often kept secret. The computational methods also frequently change, sometimes simply due to a new operating system. “So it is extremely difficult, and often even impossible, to reproduce certain results at a different lab,” Wilmes explains. “Yet, that is the very foundation of science: an experiment must be reproducible anywhere, any time, and must lead to the same results. Otherwise, we couldn’t draw any meaningful conclusions from it.”

The scientists at LCSB are now helping to rectify this situation. An initiative has been launched at the LCSB Bioinformatics Core, called “R3 – Reproducible Research Results”. “With R3, we want to enable scientists around the world to increase the reproducibility and transparency of their research – through systematic training, through the development of methods and tools, and through establishing the necessary infrastructure,” says Dr. Reinhard Schneider, head of the Bioinformatics Core.

The insights from the R3 initiative are then used in projects such as IMP. “IMP is a reproducible pipeline for the analysis of highly complex data,” says Dr. Shaman Narayanasamy. As co-author of the study, he has just completed his doctor’s degree on this subject in Paul Wilmes’ group. “We preserve computer programs in the very state in which they delivered certain experimental data. From this quasi frozen state, we can later thaw the programs out again if the data ever need reprocessing, or if new data need to be analysed in the same way.” The scientists also aggregate different components of the analytical software into so-called containers. These can be combined in different ways without risking interference between different program parts.

“The subprograms in the containers can be stringed together in series as needed,” says the first author of the study, Yohan Jarosz of the Bioinformatics Core. This creates a pipeline for the data to flow through. Because the computational operators are frozen in containers, one does not need reference data to know the conditions – e.g. type of operating system or computer processor – under which to perform the analysis. “The whole process remains entirely open and transparent,” says Jarosz. Every scientist can thus modify any step of the program – of course diligently recording every part of the process in a logbook to ensure full traceability.

Paul Wilmes is especially interested in using this method to analyse metagenomic and metatranscriptomic data. Such data are produced, for example, when researching entire bacterial communities in the human gut or in wastewater treatment plants. By knowing the full complement of DNA in the sample and all the gene products, they can determine what bacterial species are present and active in the gut or treatment plant. What is more, the scientists can also tell how big the population of each bacterial species is, what substances they produce at a given point in time, and what influences the organisms have on one another.

The catch, until recently, was that researchers at other laboratories have had a hard time reproducing the experimental results. With IMP, that has now changed, Wilmes continues: “We have already put data from other laboratories through the first tests with IMP. The results are clear: We can reproduce them – and our computations in IMP bring far more details to light than came forth in the original study, for example identifying genes that play a crucial role in the metabolism of bacterial communities.”

“Thanks to IMP, only standardised and reproducible methods are now used in microbiome research at LCSB – from the wet lab, where the experiments are done, to the dry lab, where above all computer simulations and models are run. We have an internationally pioneering role in this,” says Wilmes. “Thanks to R3, IMP also sets standards which other institutes, not only LCSB, will surely be interested to apply,” adds Reinhard Schneider of the Bioinformatics Core. “We therefore make the technology of other researchers openly available – the standard ought to be quickly adopted. Only reproducible analyses of results will advance biomedicine in the long term.”

Transparent, Flexible Solar Cells

Imagine a future in which solar cells are all around us — on windows and walls, cell phones, laptops, and more. A new flexible, transparent solar cell developed at MIT is bringing that future one step closer.

The device combines low-cost organic (carbon-containing) materials with electrodes of graphene, a flexible, transparent material made from inexpensive and abundant carbon sources. This advance in solar technology was enabled by a novel method of depositing a one-atom-thick layer of graphene onto the solar cell — without damaging nearby sensitive organic materials. Until now, developers of transparent solar cells have typically relied on expensive, brittle electrodes that tend to crack when the device is flexed. The ability to use graphene instead is making possible truly flexible, low-cost, transparent solar cells that can turn virtually any surface into a source of electric power.

Photovoltaic solar cells made of organic compounds would offer a variety of advantages over today’s inorganic silicon solar cells. They would be cheaper and easier to manufacture. They would be lightweight and flexible rather than heavy, rigid, and fragile, and so would be easier to transport, including to remote regions with no central power grid. And they could be transparent. Many organic materials absorb the ultraviolet and infrared components of sunlight but transmit the visible part that our eyes can detect. Organic solar cells could therefore be mounted on surfaces all around us and harvest energy without our noticing them.

Researchers have made significant advances over the past decade toward developing transparent organic solar cells. But they’ve encountered one persistent stumbling block: finding suitable materials for the electrodes that carry current out of the cell.

“It’s rare to find materials in nature that are both electrically conductive and optically transparent,” says Professor Jing Kong of the Department of Electrical Engineering and Computer Science (EECS).

The most widely used current option is indium tin oxide (ITO). ITO is conductive and transparent, but it’s also stiff and brittle, so when the organic solar cell bends, the ITO electrode tends to crack and lift off. In addition, indium is expensive and relatively rare.

A promising alternative to ITO is graphene, a form of carbon that occurs in one-atom-thick sheets and has remarkable characteristics. It’s highly conductive, flexible, robust, and transparent; and it’s made from inexpensive and ubiquitous carbon. In addition, a graphene electrode can be just 1 nanometer thick — a fraction as thick as an ITO electrode and a far better match for the thin organic solar cell itself.

Graphene challenges

Two key problems have slowed the wholesale adoption of graphene electrodes. The first problem is depositing the graphene electrodes onto the solar cell. Most solar cells are built on substrates such as glass or plastic. The bottom graphene electrode is deposited directly on that substrate — a task that can be achieved by processes involving water, solvents, and heat. The other layers are then added, ending with the top graphene electrode. But putting that top electrode onto the surface of the so-called hole transport layer (HTL) is tricky.

“The HTL dissolves in water, and the organic materials just below it are sensitive to pretty much anything, including water, solvents, and heat,” says EECS graduate student Yi Song, a 2016-2017 Eni-MIT Energy Fellow and a member of Kong’s Nanomaterials and Electronics Group. As a result, researchers have typically persisted in using an ITO electrode on the top.

The second problem with using graphene is that the two electrodes need to play different roles. The ease with which a given material lets go of electrons is a set property called its work function. But in the solar cell, just one of the electrodes should let electrons flow out easily. As a result, having both electrodes made out of graphene would require changing the work function of one of them so the electrons would know which way to go — and changing the work function of any material is not straightforward.

A smooth graphene transfer

For the past three years, Kong and Song have been working to solve these problems. They first developed and optimized a process for laying down the bottom electrode on their substrate.

In that process, they grow a sheet of graphene on copper foil. They then transfer it onto the substrate using a technique demonstrated by Kong and her colleagues in 2008. They deposit a layer of polymer on top of the graphene sheet to support it and then use an acidic solution to etch the copper foil off the back, ending up with a graphene-polymer stack that they transfer to water for rinsing. They then simply scoop up the floating graphene-polymer stack with the substrate and remove the polymer layer using heat or an acetone rinse. The result: a graphene electrode resting on the substrate.

But scooping the top electrode out of water isn’t feasible. So they instead turn the floating graphene-polymer stack into a kind of stamp, by pressing a half-millimeter-thick frame of silicon rubber onto it. Grasping the frame with tweezers, they lift the stack out, dry it off, and set it down on top of the HTL. Then, with minimal warming, they can peel off the silicon rubber stamp and the polymer support layer, leaving the graphene deposited on the HTL.

Initially, the electrodes that Song and Kong fabricated using this process didn’t perform well. Tests showed that the graphene layer didn’t adhere tightly to the HTL, so current couldn’t flow out efficiently. The obvious solutions to this problem wouldn’t work. Heating the structure enough to make the graphene adhere would damage the sensitive organics. And putting some kind of glue on the bottom of the graphene before laying it down on the HTL would stick the two layers together, but would end up as an added layer between them, decreasing rather than increasing the interfacial contact.

Song decided that adding glue to the stamp might be the way to go — but not as a layer under the graphene.

“We thought, what happens if we spray this very soft, sticky polymer on top of the graphene?” he says. “It would not be in direct contact with the hole transport layer, but because graphene is so thin, perhaps its adhesive properties might remain intact through the graphene.”

To test the idea, the researchers incorporated a layer of ethylene-vinyl acetate, or EVA, into their stamp, right on top of the graphene. The EVA layer is very flexible and thin — sort of like food wrap — and can easily rip apart. But they found that the polymer layer that comes next holds it together, and the arrangement worked just as Song had hoped: The EVA film adheres tightly to the HTL, conforming to any microscopic rough features on the surface and forcing the fine layer of graphene beneath it to do the same.

The process not only improved performance but also brought an unexpected side benefit. The researchers thought their next task would be to find a way to change the work function of the top graphene electrode so it would differ from that of the bottom one, ensuring smooth electron flow. But that step wasn’t necessary. Their technique for laying down the graphene on the HTL actually changes the work function of the electrode to exactly what they need it to be.

“We got lucky,” says Song. “Our top and bottom electrodes just happen to have the correct work functions as a result of the processes we use to make them.”

Putting the electrodes to the test

To see how well their graphene electrodes would perform in practice, the researchers needed to incorporate them into functioning organic solar cells. For that task, they turned to the solar cell fabrication and testing facilities of their colleague Vladimir Bulović, the Fariborz Maseeh (1990) Professor of Emerging Technology and Associate Dean for Innovation for the School of Engineering.

For comparison, they built a series of solar cells on rigid glass substrates with electrodes made of graphene, ITO, and aluminum (a standard electrode material). The current densities (or CDs, the amount of current flowing per unit area) and power conversion efficiencies (or PCEs, the fraction of incoming solar power converted to electricity) for the new flexible graphene/graphene devices and the standard rigid ITO/graphene devices were comparable. They were lower than those of the devices with one aluminum electrode, but that was a finding they expected.

“An aluminum electrode on the bottom will reflect some of the incoming light back into the solar cell, so the device overall can absorb more of the sun’s energy than a transparent device can,” says Kong.

The PCEs for all their graphene/graphene devices — on rigid glass substrates as well as flexible substrates — ranged from 2.8 percent to 4.1 percent. While those values are well below the PCEs of existing commercial solar panels, they’re a significant improvement over PCEs achieved in prior work involving semitransparent devices with all-graphene electrodes, the researchers say.

Measurements of the transparency of their graphene/graphene devices yielded further encouraging results. The human eye can detect light at wavelengths between about 400 nanometers and 700 nanometers. The all-graphene devices showed optical transmittance of 61 percent across the whole visible regime and up to 69 percent at 550 nanometers. “Those values [for transmittance] are among the highest for transparent solar cells with comparable power conversion efficiencies in the literature,” says Kong.

Flexible substrates, bending behavior

The researchers note that their organic solar cell can be deposited on any kind of surface, rigid or flexible, transparent or not. “If you want to put it on the surface of your car, for instance, it won’t look bad,” says Kong. “You’ll be able to see through to what was originally there.”

To demonstrate that versatility, they deposited their graphene-graphene devices onto flexible substrates including plastic, opaque paper, and translucent Kapton tape. Measurements show that the performance of the devices is roughly equal on the three flexible substrates — and only slightly lower than those made on glass, likely because the surfaces are rougher so there’s a greater potential for poor contact.

The ability to deposit the solar cell on any surface makes it promising for use on consumer electronics — a field that’s growing rapidly worldwide. For example, solar cells could be fabricated directly on cell phones and laptops rather than made separately and then installed, a change that would significantly reduce manufacturing costs.

They would also be well-suited for future devices such as peel-and-stick solar cells and paper electronics. Since those devices would inevitably be bent and folded, the researchers subjected their samples to the same treatment. While all of their devices — including those with ITO electrodes — could be folded repeatedly, those with graphene electrodes could be bent far more tightly before their output started to decline.

Future goals

The researchers are now working to improve the efficiency of their graphene-based organic solar cells without sacrificing transparency. (Increasing the amount of active area would push up the PCE, but transparency would drop.) According to their calculations, the maximum theoretical PCE achievable at their current level of transparency is 10 percent.

“Our best PCE is about 4 percent, so we still have some way to go,” says Song.

They’re also now considering how best to scale up their solar cells into the large-area devices needed to cover entire windows and walls, where they could efficiently generate power while remaining virtually invisible to the human eye.

This research was supported by the Italian energy company Eni S.p.A. as part of the Eni-MIT Alliance Solar Frontiers Center. Eni is a Founding Member of the MIT Energy Initiative.