Flexible sensors for biological applications
The Sekitani Lab is developing flexible electronic devices that make full use of organic materials. Prof. Araki and other lab members are seeking to create new forms of electronics by interdisciplinary collaboration across multiple fields, including materials development, physical properties evaluation, and circuit design. The lab is developing flexible sensor materials for detecting biological signals, such as those coming from parts of the human body.
Sensors are expanding rapidly from measuring objects to measuring people. To measure physiological information from the human body, it is important to design and build devices that do not bother you when you have them on, i.e., that are comfortable to wear. Medical sensors to date have been made from rigid electronic materials that have poor compatibility with the human body and cause irritation, inflammation, or minor injury. Research and development aimed at using flexible materials to create forms of electronics that are significantly more compatible with the human body is progressing fast.
The potential unleashed by flexible electronics
The electronics appliance industry and other areas of electronics have used to date “inorganics”, such as silicon semiconductors. These inorganic materials can break easily during use, which has been an impediment to making them thinner and lighter.
At the moment, there is continual progress in the development of organic electroluminescent (EL) displays and organic solar cells that are thin and bendable. At the same time, more biocompatible conductive and semiconductive materials are being developed by exploiting the flexibility of organic materials. Materials that show better compatibility with the human body, i.e., do not irritate or inflame it, will help expand applications in healthcare and medicine. They offer numerous possibilities including:
- devices that measure physiological signals, such as the heart beat and rate, when worn on the body;
- organ-embedded devices for measuring progress during and after surgery;
- contact lenses fitted with chemical sensors; and
- brain machine interfaces (BMIs) that measure brain waves.
From “bendable” to “stretchable”
Electronic devices work when electricity flows through an electronic circuit. To operate such devices, we control them by connecting the active elements with wiring. Some elements are active and some passive. The active ones have the ability to affect electrical signal features, such as waveform and frequency, and ultimately influence the signal properties.
Transistors are among those active elements for which developing flexible electronics is essential. On the other hand, the wiring and other components like that are passive elements. One critical problem is that the performance of conventional passive and active elements often deteriorates due to mechanical deformation. Thus, devices can be highly biocompatible only if their electronic circuits are able to change shape by expanding and contracting, allowing the organism to move freely and unhindered, without loss of performance.
Transparent, elastic wiring with networks formed by silver nanowires
Silver nanowires are very small wires with dimensions on the microscopic and nanoscopic scales. They are about 40 µm in length and 90 nm in diameter. The physical properties of silver itself mean that the wires have a high degree of transparency, conductivity, flexibility, and elasticity. They are coming into use for mass-producible, transparent, conductive films and are expected to spread to such applications as touch panels on devices like smart phones and tablets. Prof. Araki described how optical microscopes allow them to observe the extent of network formation by the silver nanowires. “The silver nanowires tend to form a random network, but it is possible to orient them by exerting certain controls. These controls enable us to make highly transparent conductors and electrodes with smooth surfaces. Higher levels of light permeability, of conductivity, and even greater elasticity can be obtained by increasing the aspect ratio of the nanowires.”
Brainwave sensor patches that are comfortable to wear
Biological signal sensors need to measure tiny voltages from brainwaves and the electrical activity of the eyes, muscles, and heart. Brainwaves in particular are lower in electric potential than other forms of biologically generated signals with a value around 100 µV. The electroencephalograms (EEGs) that have been used to date are highly accurate, but the tight fit of rigid electrodes on the head makes them uncomfortable to wear. The biological sensors developed in the Sekitani Lab, on the other hand, have enabled them to measure electric potential with the same accuracy as an EEG simply by attaching them to the forehead. As measuring brainwaves becomes simpler and easier, early detection and diagnosis of health problems like sleep disorders and dementia will also increase. Currently one in five people has a potential sleep disorder and the economic cost is estimated to be approximately JPY 3 trillion (USD 26.5 billion). It is very similar with dementia. Prof. Araki stated, “I do not think that we are far away from a time when people will be able to check themselves easily.”
In another experiment, the Sekitani Lab has been developing a technology for mapping brainwaves from the surface of the brain and communicating them to a robot. This technology allows a person’s mental state to be visualized, such as whether they are feeling relaxed, via the robot’s expression.
Making full use of different microscopes
Prof. Araki mentioned, “Organics and microscopes may not be things that you would associate with one another, but we make full use of a range of optical microscopes, from metallurgical to digital to stereo microscopes for lab operational purposes. We use metallurgical microscopes for checking the alignment of the silver nanowires and measuring their length, because their conductivity will vary depending on it. We carry out granularity analysis using polarization to view the appearance of the crystals. We also carry out differential interference contrast (DIC) and darkfield observation in order to ascertain the light scattering of the fine particles that are formed during the process and determine roughness. The brightness and contrast of the Leica metallurgical microscope remain constant and do not require adjustment, even if we change the method of observation, which makes it very easy to use.”
Imaging silver nanowires
The Sekitani Lab uses a stereo microscope for performing tasks, such as patterning on the order of several hundred µm and forming electrical contacts with dimensions of a few mm, which are required for biological device fabrication. “These jobs are ones that you cannot do without a microscope and which also require a delicate touch, but the Leica stereo microscope has great focal depth (depth of field), which makes it very easy to work with,” according to Prof. Araki.
He commented further, “we have samples of organic semiconductors that are thinner than Saran Wrap, just a few μm thick, and they are so soft that they form a number of wrinkles when made into free-standing films. We need to have a microscope with a good depth of field to obtain clear micrographic images of these films with the correct focus. A digital microscope allows us to achieve a good depth of field simply and cleanly, which makes life easy.”
Figures 1 and 2 show a 2D and 3D image of a network of silver nanowires. Both images were recorded with a Leica digital microscope.
In addition, Prof. Araki added, “I used Leica microscopes when I was away studying in Europe. You can see even fine details clearly in high definition and the depth of focus is good, so they became my favorite. I just had to have one when I got back to Japan, so I immediately called to make enquiries. A microscope may be a routine tool, but for our research it is also a really important partner.”
Photos of Prof. Araki with his colleagues Prof. Yoshimoto and Prof. Uemura in the Sekitani Laboratory are seen in figures 3 and 4. Among the microscopes in the lab is a DMS1000 digital microscope.
A final word
Most people tend to think of medical devices as being hard and inorganic. This visit to the Sekitani Laboratory has given the impression that flexible, stretchable electronic devices is something that we are going to be seeing a lot more of. Especially as they demonstrate their value for self-care and monitoring the health of patients and elderly people. With goals such as the quantification of emotions, its platforms may even be extending beyond health and medical care and into the worlds of artistic culture and forecasting markets.
- Original version in Japanese: http://xlab.leica-microsystems.com/database/id11
- T. Araki, R. Mandamparambil, D.M.P. van Bragt, J. Jiu, H. Koga, J. van den Brand, T. Sekitani, J.M.J. den Toonder, K. Suganuma, Stretchable and transparent electrodes based on patterned silver nanowires by laser-induced forward transfer for non-contacted printing techniques, Nanotechnology (2016) vol. 27, iss. 45, LTO2, DOI: 10.1088/0957-4484/27/45/45LT02.
- S. Ding J. Jiu, Y. Gao, Y. Tian, T. Araki, T. Sugahara, S. Nagao, M. Nogi, H. Koga, K. Suganuma, H. Uchida, One-Step Fabrication of Stretchable Copper Nanowire Conductors by a Fast Photonic Sintering Technique and Its Application in Wearable Devices, ACS Appl. Mater. Interfaces (2016) vol. 8, iss. 9, pp. 6190–6199, DOI: 10.1021/acsami.5b10802.
- Y. Yang, S. Ding, T. Araki, J. Jiu, T. Sugahara, J. Wang, J. Vanfleteren, T. Sekitani, K. Suganuma, Facile fabrication of stretchable Ag nanowire/polyurethane electrodes using high intensity pulsed light, Nano Res. (2016) vol. 9, iss. 2, pp. 401-414, DOI: 10.1007/s12274-015-0921-9.
- J. Wang, J. Jiu, T. Arak, M. Nogi, T. Sugahara, S. Nagao, H. Koga, P. He, K. Suganuma, Silver Nanowire Electrodes: Conductivity Improvement Without Post-treatment and Application in Capacitive Pressure Sensors, Nano-Micro Letters (2015) vol. 7, iss. 1, pp. 51–58, DOI: 10.1007/s40820-014-0018-0
- Digital Microscopes, Product Pages, Leica Microsystems
- Stereo Microscopes, Product Pages, Leica Microsystems