Ultraflexible, large-area, physiological temperature sensors for multipoint measurements

Abstract

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...

Pursuing prosthetic electronic skin

“…The thermal sensitivity of p-n junctions is based on the thermal activation of charge carriers 58,59 . Although these devices have improved sensitivity compared with TCR sensors, they are also sensitive to light 60 . Highly sensitive devices can be made from composites based on a polymer matrix and a conducting filler 49 .…”

“…As the temperature changes, thermal expansion causes the fillers to move apart, increasing the resistance 60,61 by up to six orders of magnitude with a resolution of 0.1 °C (ref. 60). These sensors often have large electrical hysteresis 61 , but they can be engineered to have minimal strain dependence 49 .…”

A Low‐Operating‐Power and Flexible Active‐Matrix Organic‐Transistor Temperature‐Sensor Array

“…[ 17,18 ] One possible approach to enhance the sensitivity of the temperature sensor is utilizing the percolation effect, which can signifi cantly reduce the resistance of the thermistor by several orders via fi lling a conductive material into an insulative matrix, such as poly(dimethylsiloxane) (PDMS), [ 19 ] acrylate copolymers, [ 20 ] or poly(ethylene oxide). [ 21 ] Although percolation-type thermistors typically offer a very high Δ R / R value, as listed in Table 1 , such resistance changes typically occur over a narrow temperature range (within 10 °C), [ 20,21 ] which limits their applications for wide-range temperature sensing. Instead of focusing on a narrow operating temperature, our fl exible active-matrix temperature sensor focuses on a broader sensing range, from 20 to 100 °C.…”

Efficient Skin Temperature Sensor and Stable Gel‐Less Sticky ECG Sensor for a Wearable Flexible Healthcare Patch

“…In fact, many studies have examined flexible and stretchable sensors to monitor health data [1][2][3][4][5][6][7][8] such as skin temperature, [9][10][11] electrocardiogram (ECG), [12][13][14][15][16][17] chemical contents in the body through sweat, [18][19][20][21][22][23] and activity. [24,25] These demonstrations are steps toward future wearable multifunctional flexible electronic devices by further developing into a system with improved reliability.…”