Thin-film electronic devices can be integrated with skin for health monitoring and/or for interfacing with machines. Minimal invasiveness is highly desirable when applying wearable electronics directly onto human skin. However, manufacturing such on-skin electronics on planar substrates results in limited gas permeability. Therefore, it is necessary to systematically investigate their long-term physiological and psychological effects. As a demonstration of substrate-free electronics, here we show the successful fabrication of inflammation-free, highly gas-permeable, ultrathin, lightweight and stretchable sensors that can be directly laminated onto human skin for long periods of time, realized with a conductive nanomesh structure. A one-week skin patch test revealed that the risk of inflammation caused by on-skin sensors can be significantly suppressed by using the nanomesh sensors. Furthermore, a wireless system that can detect touch, temperature and pressure is successfully demonstrated using a nanomesh with excellent mechanical durability. In addition, electromyogram recordings were successfully taken with minimal discomfort to the user.
Next-generation biomedical devices will need to be self-powered and conformable to human skin or other tissue. Such devices would enable the accurate and continuous detection of physiological signals without the need for an external power supply or bulky connecting wires. Self-powering functionality could be provided by flexible photovoltaics that can adhere to moveable and complex three-dimensional biological tissues and skin. Ultra-flexible organic power sources that can be wrapped around an object have proven mechanical and thermal stability in long-term operation, making them potentially useful in human-compatible electronics. However, the integration of these power sources with functional electric devices including sensors has not yet been demonstrated because of their unstable output power under mechanical deformation and angular change. Also, it will be necessary to minimize high-temperature and energy-intensive processes when fabricating an integrated power source and sensor, because such processes can damage the active material of the functional device and deform the few-micrometre-thick polymeric substrates. Here we realize self-powered ultra-flexible electronic devices that can measure biometric signals with very high signal-to-noise ratios when applied to skin or other tissue. We integrated organic electrochemical transistors used as sensors with organic photovoltaic power sources on a one-micrometre-thick ultra-flexible substrate. A high-throughput room-temperature moulding process was used to form nano-grating morphologies (with a periodicity of 760 nanometres) on the charge transporting layers. This substantially increased the efficiency of the organophotovoltaics, giving a high power-conversion efficiency that reached 10.5 per cent and resulted in a high power-per-weight value of 11.46 watts per gram. The organic electrochemical transistors exhibited a transconductance of 0.8 millisiemens and fast responsivity above one kilohertz under physiological conditions, which resulted in a maximum signal-to-noise ratio of 40.02 decibels for cardiac signal detection. Our findings offer a general platform for next-generation self-powered electronics.
We report a fabrication method for flexible and printable thermal sensors based on composites of semicrystalline acrylate polymers and graphite with a high sensitivity of 20 mK and a high-speed response time of less than 100 ms. These devices exhibit large resistance changes near body temperature under physiological conditions with high repeatability (1,800 times). Device performance is largely unaffected by bending to radii below 700 μm, which allows for conformal application to the surface of living tissue. The sensing temperature can be tuned between 25°C and 50°C, which covers all relevant physiological temperatures. Furthermore, we demonstrate flexible active-matrix thermal sensors which can resolve spatial temperature gradients over a large area. With this flexible ultrasensitive temperature sensor we succeeded in the in vivo measurement of cyclic temperatures changes of 0.1°C in a rat lung during breathing, without interference from constant tissue motion. This result conclusively shows that the lung of a warm-blooded animal maintains surprising temperature stability despite the large difference between core temperature and inhaled air temperature. T emperature control plays a very important role in homeostasis, and body temperature varies both spatially and temporally in an effort to transfer heat between the living body and the environment via skin and respiratory organs. Accurate measurement of localized temperature changes in soft tissue regardless of large-scale motion is important in understanding thermal phenomena of homeostasis and realizing future sophisticated health diagnostics (1-3). Therefore, flexible temperature sensors which softly interface with tissue have been investigated frequently for applications in the medical field. However, these applications require the combination of sensitivity, fast response time, stability in physiological environments, and multipoint measurement. Before this work, to our knowledge, no experiment has simultaneously demonstrated orders-of-magnitude changes in electrical properties (sensitivity) repeatedly at varying physiological temperatures and conditions (stability) in a robust, easy-to-fabricate, flexible temperature sensor (processability).When sensors and electronics are directly attached to the surface of an animal body, the use of soft and flexible electronic devices is expected to reduce mechanical stress induced on the body. From this viewpoint, the field of flexible electronics has attracted much attention recently. The ability to gather information such as pressure and temperature from curvilinear and dynamic surfaces without impairing the movement or usability of the users is unmatched by conventional silicon electronics. There have been reports of the potential application of flexible electrodes on ultrathin substrates (4), flexible sensors that measure biological signals, electrocardiograms, temperature, pressure (5, 6), organic amplifier systems (7), high-sensitivity pressure sensors (8), and ultrathin and imperceptible devices (9, 10).To meas...
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