Stronger and more flexible than graphene, a single-atom layer of boron could revolutionize sensors, batteries, and catalytic chemistry.
From MIT Technology Review March 2019
From MIT Technology Review March 2019
Considering that most wearable systems, healthcare electronics, and laboratory-on-a-chip testing tools can be expected to come into contact with arbitrarily curved interfaces, the flexibility of sensors is essential for improving their interactions with target systems and improving the reliability and stability of the tests. Over the past decade, the development of flexible and stretchable sensors for various functions has been accelerated by rapid advances in materials, processing methods, and platforms. For practical applications, new expectations are arising in the pursuit of highly economical, multifunctional, biocompatible flexible sensors.
Schematic illustration of flexible electronics in development today across a broad range of applications. (Reprinted with permission by Wiley-VCH Verlag)
A new review article in Advanced Materials (“An Overview of the Development of Flexible Sensors”) aims to illustrate various types of flexible sensors. Rather than summarizing the huge body of relevant previous work, the authors select noteworthy work that may suggest crucial future trends of flexible sensors.
They summarize the recent state-of-the-art flexible electronics currently employed as flexible light sensors, flexible pH sensors, flexible ion sensors, and flexible biosensors.The selections of materials and the fabrications of devices are included in every part. The authors also provide a detailed description of engineering technologies with an emphasis on flexible sensor fabrication.They then present market analysis on the world sensor market, printed sensors, and wearable sensors.
Relative market size by wearable sensor type by 2020. (© IDTechEx)
Numerous opportunities and challenges remain in the future research and development of high-performance flexible sensors.For flexible sensors attached to the human body or its organs, the biocompatibility of the active materials and flexible substrates, including long-term toxicity analyses, is a crucial research area, especially for invasive applications.Innovative utilization of device designs, materials, assembly methods, and surface engineering, as well as interface engineering, can address these challenges.
The development of new materials for active layers, substrates, and conductive layers can give rise to soft, stretchable sensors; this emerging paradigm can extend the scope of current technologies for different sensing functions.Improvement of both flexibility and sensitivity is another challenge for state-of-the-art flexible sensors. The development of novel elastic materials for flexible and stretchable substrates, geometrical electrode designs, combinations of molecular designs in organic materials, and utilization of conceptually novel materials could optimize the trade-off between sensitivity and flexibility.
High-density sensor arrays with multiple functions must be integrated for enabling a high spatiotemporal resolution to realize fully functional flexible electronics. Take for example human micromotion sensor (read more here: “Nanocurve-based sensor reads facial expressions“):
Schematic illustration of a nanoparticle curve array printed to flexible electronic devices and adopted to multi analysis for skin micromotion sensing. Nanoparticle curve arrays were printed on the PDMS substrate by pillar-patterned template induced printing with sliver nanoparticle ink. These flexible electronic sensors can be directly attached to human facial skin, and perform real-time multi analysis for skin micromotion monitoring. (Reprinted with permission by Wiley-VCH Verlag)
Nevertheless, raising the density of sensors leads to increased crosstalk. Reducing the size of sensors diminishes the amplitude of signals. These problems can be addressed by connecting each sensor with active devices such as transistors to enable local signal amplification and transduction.The development of sensors is the enabling technology for Internet of Things (IoT). A surge in IoT provides plentiful opportunities for spreading out of flexible sensors with reconfigurable shape and size.Thanks to their light weight, thinness, and robustness, flexible sensors can be seamlessly integrated onto any surface to provide users more improved avenues, which is difficult to realize in conventional electromechanical sensors.
With the development of polymers, oxides, printing technologies, and CMOS technologies, flexible sensors will unlock a completely novel set of IoT products. The achievement of these fantastic sensing applications will bring us closer to the new electronic era promised by flexible sensors.
Applications for Nano and Sensors
Nanoengineers at the University of California San Diego have developed the first printed battery that is flexible, stretchable and rechargeable. The zinc batteries could be used to power everything from wearable sensors to solar cells and other kinds of electronics.
The work appears in the April 19, 2017 issue of Advanced Energy Materials.
The researchers made the printed batteries flexible and stretchable by incorporating a hyper-elastic polymer material made from isoprene, one of the main ingredients in rubber, and polystyrene, a resin-like component. The substance, known as SIS, allows the batteries to stretch to twice their size, in any direction, without suffering damage.
The ink used to print the batteries is made of zinc silver oxide mixed with SIS. While zinc batteries have been in use for a long time, they are typically non-rechargeable. The researchers added bismuth oxide to the batteries to make them rechargeable.
“This is a significant step toward self-powered stretchable electronics,” said Joseph Wang, one of the paper’s senior authors and a nanoengineering professor at the Jacobs School of Engineering at UC San Diego, where he directs the school’s Center for Wearable Sensors. “We expect this technology to pave the way to enhance other forms of energy storage and printable, stretchable electronics, not just for zinc-based batteries but also for Lithium-ion batteries, as well as supercapacitors and photovoltaic cells.”
The prototype battery the researchers developed has about 1/5 the capacity of a rechargeable hearing aid battery. But it is 1/10 as thick, cheaper and uses commercially available materials. It takes two of these batteries to power a 3 Volt LED. The researchers are still working to improve the battery’s performance. Next steps include expanding the use of the technology to different applications, such as solar and fuel cells; and using the battery to power different kinds of electronic devices.
Researchers used standard screen printing techniques to make the batteries–a method that dramatically drives down the costs of the technology. Typical materials for one battery cost only $0.50. A comparable commercially available rechargeable battery costs $5.00 Batteries can be printed directly on fabric or on materials that allow wearables to adhere to the skin. They also can be printed as a strip, to power a device that needs more energy. They are stable and can be worn for a long period of time.
The key ingredient that makes the batteries rechargeable is a molecule called bismuth oxide which, when mixed into the batteries’ zinc electrodes, prolongs the life of devices and allows them to recharge. Adding bismuth oxide to zinc batteries is standard practice in industry to improve performance, but until recently, there hasn’t been a thorough scientific explanation for why.
Last year, UC San Diego nanoengineers led by Professor Y. Shirley Meng published a detailed molecular study addressing this question (download PDF here). When zinc batteries discharge, their electrodes react with the liquid electrolyte inside the battery, producing zinc salts that dissolve into a solution. This eventually short circuits the battery. Adding bismuth oxide keeps the electrode from losing zinc to the electrolyte. This ensures that the batteries continue to work and can be recharged.
The work shows that it is possible to use small amounts of additives, such as bismuth oxide, to change the properties of materials. “Understanding the scientific mechanism to do this will allow us to turn non-rechargeable batteries into rechargeable batteries—not just zinc batteries but also for other electro-chemistries, such as Lithium-oxygen,” said Meng, who directs the Sustainable Power and Energy Center at the UC San Diego Jacobs School of Engineering
Rajan Kumar, a co-first author on this Advanced Energy Materials paper, is a nanoengineering Ph.D. student at the Jacobs School of Engineering. He and nanoengineering professor Wang are leading a team focused on commercializing aspects of this work. The team is one of five to be selected to join a new technology accelerator at UC San Diego. The technology accelerator is run by the UC San Diego Institute for the Global Entrepreneur, which is a collaboration between the Jacobs School of Engineering and Rady School of Management.
Kumar is excited at the prospect of taking advantage of all that the IGE Technology Accelerator has to offer.
“For us, it’s strategically perfect,” said Kumar, referring to the $50,000 funding for prototype improvements, the focus on prototype testing with a strategic partner, and the entrepreneurship mentoring.
Kumar is confident in the team’s innovations, which includes the ability to replace coin batteries with thin, stretchable batteries. Making the right strategic moves now is critical for commercialization success.
“It’s now about making sure our energies are focused in the right direction,” said Kumar.
In addition to the IGE Technology Accelerator, the team was also recently selected to participate in the NSF Innovation-Corps (I-Corps) program at UC San Diego, also administered by the Institute for the Global Entrepreneur. One of the key tenets of the I-Corps program is helping startup teams validate their target markets and business models early in the commercialization process. Through NSF I-Corps, for example, Kumar has already started interviewing potential customers which has helped the team better focus their commercialization strategy.
Through these programs, Kumar is focused on leading the team through a series of milestones in order to best position their innovations to refine “both what to build and who to build it for,” he said.
“All-Printed, Stretchable Zn-Ag2O Rechargeable Battery via Hyperelastic Binder for Self-Powering Wearable Electronics” in the journal Advanced Energy Materials.http://onlinelibrary.wiley.com/doi/10.1002/aenm.201602096/full
Authors: Rajan Kumar, Jaewook Shin, Lu Yin, Jung-Min You, Prof. Shirley Meng and Prof. Joseph Wang, Department of Nanoengineering, Jacobs School of Engineering, University of California San Diego.
Joseph Wang is a distinguished professor, holds the SAIC endowed chair, and serves as chair of the Department of NanoEngineering at the UC San Diego Jacobs School of Engineering where he directs the Center for Wearable Sensors.
Shirley Meng is a professor in the Department of NanoEngineering and Director of the Sustainable Power and Energy Center at the UC San Diego Jacobs School of Engineering.
Research funders include: Advanced Research Projects Agency-Energy (DE-AR0000535); Rajan Kumar acknowledges the U.S. National Science Foundation (NSF) Graduate Research Fellowship under Grant No (DGE-1144086).
This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI), a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the U.S. National Science Foundation (NSF).
Researchers from the Graphene Flagship, working at the University of Cambridge (UK), Emberion (UK), the Institute of Photonic Sciences (ICFO; Spain), Nokia UK, and the University of Ioannina (Greece) have developed a novel graphene-based pyroelectric bolometer – an infrared (IR) detector with record high sensitivity for thermal detection, capable of resolving temperature changes down to a few tens of µK. This work may open the door to high-performance IR imaging and spectroscopy.
Cambridge team develops sensitive IR bolometer
The technology is focused on the detection of the radiation generated by the human body and its conversion into a measurable signal. The key point is that using graphene, the conversion reaches performance more than 250 times better than the best sensor already available. But the high sensitivity of the detector could be of use for spectroscopic applications beyond thermal imaging.
With a high-performance graphene-based IR detector that gives a strong signal with less incident radiation, it is possible to isolate different parts of the IR spectrum. This is of key importance in security applications, where different materials – explosives, for instance – can be distinguished by their characteristic IR absorption or transmission spectra.
The graphene-based devices consist of a pyroelectric substrate, with a conductive channel of single-layer graphene and a floating gate electrode placed on top. In pyroelectric materials, changes in temperature lead to a spontaneous electric field inside the material.
The floating gate electrode concentrates this field on the graphene, and the field causes changes in the electrical resistance of the graphene, which are measured as the device output. Typical IR photodetectors operate either via the pyroelectric effect, or as bolometers, which measure changes in resistance due to heating. The graphene-based pyroelectric bolometers combine both effects for excellent performance and could be used as pixels in a high resolution thermal imaging camera.
The team explains that graphene acts as a built-in amplifier for the pyroelectric signal, without needing external transistor amplifiers like in typical pyroelectric thermal detectors.
This direct integration means that there are no losses and no additional noise from connections to external amplifying circuits. “We can build the amplifier directly on the pyroelectric material. So, all the charge that it develops goes to the amplifier. There is nothing lost along the way,” said the co-author of the work.
The use of graphene also offers benefits for further integrating the detector pixels with the external readout integrated circuit (ROIC) used to interface with the detector pixels and the recording device. “To match the input impedance of the ROIC, you need something that is as conductive as possible. The intrinsic conductivity of graphene helps the further integration with silicon,” said the scientists. Impedance matching is essential to ensure that the signal is transmitted as efficiently as possible. This benefit is unique to graphene due to its combination of high conductivity and strong field effect.
In August 2016, researchers from the University of Cambridge developed a high performance room temperature graphene-based mid-infrared photodetectors. Such graphene-based “bolometers” enabled the researchers to achieve ultra high performance – a TCR as high as 900%/K. The researchers hope that this device could in the future be used in astronomy, medicine imaging, automotive and even smartphone infra-red cameras.
Researchers at the University of Texas have developed a graphene-based health sensor that attaches to the skin like a temporary tattoo and takes measurements with the same precision as bulky medical equipment. The graphene tattoos are said to be the thinnest epidermal electronics ever made. They can measure electrical signals from the heart, muscles, and brain, as well as skin temperature and hydration.
The research team hopes to integrate these sensors applications like consumer cosmetics, in addition to providing a more convenient replacement for existing medical equipment. The sensor takes advantage of graphene’s mechanical invisibility – when the sensor goes on the skin, it doesn’t just stay flat—it conforms to the microscale ridges and roughness of the epidermis.
The Texas researchers started by growing single-layer graphene on a sheet of copper. The 2D carbon sheet is then coated with a stretchy support polymer, and the copper is etched off. Next, the polymer-graphene sheet is placed on temporary tattoo paper, the graphene is carved to make electrodes with stretchy spiral-shaped connections between them, and the excess graphene is removed. Then the sensor is ready to be applied by placing it on the skin and wetting the back of the paper.
In their proof-of-concept work, the researchers used the graphene tattoos to take five kinds of measurements, and compared the data with results from conventional sensors. The graphene electrodes were able to pick up changes in electrical resistance caused by electrical activity in the tissue underneath. When worn on the chest, the graphene sensor detected faint fluctuations that were not visible on an EKG taken by an adjacent, conventional electrode. The sensor readouts for electroencephalography (EEG) and electromyography (EMG, which can be used to register electrical signals from muscles and is being incorporated into next-generation prosthetic arms and legs) were also of good quality. The sensors could also measure skin temperature and hydration, which could be useful for cosmetics companies.
Graphene’s conformity to the skin might be what enables the high-quality measurements. Air gaps between the skin and the relatively large, rigid electrodes used in conventional medical devices degrade these instruments’ signal quality. Newer sensors that stick to the skin and stretch and wrinkle with it have fewer airgaps, but because they’re still a few micrometers thick, and use gold electrodes hundreds of nanometers thick, they can lose contact with the skin when it wrinkles. The graphene in the Texas researchers’ device is 0.3-nm thick. Most of the tattoo’s bulk comes from the 463-nm-thick polymer support.
The next step is to add an antenna to the design so that signals can be transmitted from the device to a phone or computer.