Flexible Wireless Temperature Sensors Based on Ni Microparticle‐Filled Binary Polymer Composites

Abstract

Flexible and disposable sensors have great potential toward applications such as medical diagnostics, food safety and environmental monitoring. [ 1 , 2 ] Radio frequency identifi cation (RFID) tags were originally developed for identifi cation and tracking purposes. When RFID tags are combined with fl exible organic sensors, they could function as wireless sensors while possessing the merits of organic electronics, such as mechanical fl exibility, light weight and low cost. Remote temperature sensors would offer the possibility of monitoring temperature of human body, perishable food and medicine by simply attaching a fl exible tag. Several types of temperature sensors have been fabricated on fl exible substrates, such as thermocouples, [ 3 ] resistive temperature detectors (RTDs), [ 4 ] and organic diodes. [ 5 ] Thermocouples are based on the Seebeck effect and typically generate a voltage change between 40 to 70 μ V/ ° C. [ 3 , 6 ] As the resistance change of a standard 100 Ω RTD is only 0.4 Ω / ° C, [ 3 , 6 ] the readout of these temperature sensors requires hihgly accurate and complex electronic circuits, i.e. high gain amplifi ers and high precision analog to digital converters (ADCs), [ 7 ] making it diffi cult for integration and hence increases the overall cost in their fabrication process.In this work, we developed Ni microparticle-fi lled binary polymer composites as temperature sensors with greater improved reproducibility compared to single polymer composites. These materials showed a much higher sensitivity than other types of fl exible temperature sensors. In addition, the higher improved sensitivity enabled us to fabricate a wireless temperature sensor by integrating it with a passive RFID antenna. Our temperature sensor consists of a Ni microparticle-fi lled binary polymer composite with polyethylene (PE) and polyethylene oxide (PEO) as the matrix designed to be highly sensitive for monitoring human body temperature. Conductive particle-fi lled polymer composites have already been widely used commercially as antistatic and electromagnetic interference shielding materials. [ 8 ] In addition, it has also been shown that some polymer composites showed several orders of resistivity change with temperature. This phenomenon has been explored for self-limiting heating elements, over-current protectors and resettable fuses. [ 9 ] However, conductive particle-fi lled polymer composites have yet been used as temperature sensor. This is primarily due to the diffi culty in achieving a reproducible temperature sensing response. [10][11][12] Additionally, the sensitive ranges previously reported were > 70 ° C, [13][14][15] which is much higher than the temperature range needed for medical diagnostics, food safety and environmental monitoring.In our system, we chose a matrix polymer blend with both semi-crystalline PEO and PE. The resistivity versus temperature characteristic of a metal microparticle-fi lled polymer is determined largely by the thermal behavior of the polymer. As shown in Table 1 , amorphous ...

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

“…Many previously reported devices have used a vertical structure where the polymer PTC film is sandwiched between two electrodes (denoted as the vertical-type sensor). Here, we demonstrate a lateral-type sensor where the polymer PTC is printed across two neighboring electrodes (16,23). Both sensor types exhibit similar initial resistivity changes of six orders of magnitude over a temperature change of only 4°C.…”

“…Despite this large PDI the width of the melt transition spans less than 15°C and the peak temperature is highly repeatable over multiple syntheses, which is difficult to achieve with main chain crystallizing polymers. Thus, a simple method of tuning the sensing temperature is established which does not rely on changing the polymer molecular weight which is sensitive to the PDI, and is the method used in almost all previous PTC devices (16,30).…”

“…temperature sensors that exhibit orders-of-magnitude changes in resistivity over just a few degrees (16). Extraordinarily large resistivity changes can eliminate the need for per-pixel amplification circuitry, as the output signal of the sensor can be directly multiplexed and fed to external recording equipment, ultimately reducing device complexity and manufacturing costs.…”

“…Specifically, the need for perpixel amplification limits the proliferation of simple medical sensors that lend themselves to continued miniaturization, low power utilization, and wireless addressability. Competing organic devices suffer in terms of repeatability, commonly exhibiting less than 100 stable thermal cycles (16,19,20). This work presents an important step toward the realization of implantable and wearable devices that are sensitive, cyclically stable, and processable.…”

A skin-like stretchable colorimetric temperature sensor

“…Likewise, a wear-able sensor consisting of flexible circuits and materials was designed to measure the brain temperature [21]. In another report, a Ni microparticle-filled binary polymer composite was fabricated as a flexible wireless sensor to monitor human body temperature [22]. A similar methodology was also seen with the use of PDMS and polyimide (PI) to disperse graphite for flexible temperature sensor arrays [23].…”