Fully Printed Flexible Polystyrene/Graphite-Based Temperature Sensor with Excellent Properties for Potential Smart Applications

Abstract

This study presents an innovative temperature sensor based on a thermistor nanocomposite of graphite (Gt) and polystyrene (PS). The sensor exhibited notable thermal stability and film integrity, offering two distinct linear response regions within the tested temperature range of -10 to 60 °C. It demonstrated a sensitivity of 0.125% °C^-1 between -10 and 10 °C, followed by another linear response with a sensitivity of 0.41% °C^-1 from 20 to 60 °C. Furthermore, it exhibited a response/recovery time of 0.97/1.3 min at a heating/cooling rate of 60 °C min^-1. The sensor maintained minimal baseline drift even when subjected to varying humidity levels. We assessed its mechanical flexibility and stability for hundreds of bending cycles at a bending angle of 30°, adapting to dynamic environmental conditions. The sensor's thermomechanical test (response to mechanical stress under temperature fluctuations) underscored its adaptability over a temperature range of -10 to 60 °C. Notably, it displayed excellent chemical stability, maintaining consistent performance when subjected to harsh environmental conditions like exposure to corrosive gases and prolonged immersion in tap water. Real-world tests demonstrated its practical utility, including precise temperature measurements in solid objects and breath temperature monitoring. These findings suggest promising applications in healthcare, environmental monitoring, and various IoT applications.

Figure 1

Schematic illustration of the process for fabricating the wearable temperature sensor. (A) Preparation of PS solution in xylene solvent. (B) Ultrasonication of Gt powder in xylene for 10 min. (C) Add 1 mL of PS solution (250 mg mL^–1) to the Gt precipitate after centrifugation at 1000 rpm for 10 min. (D) Photograph of the Gt/PS nanocomposite ink after mixing on a magnetic stirrer for 1 h. (E) Prepare a thin Gt/PS nanocomposite film using the doctor blade method on prescreen printed carbon electrodes on a PET substrate. (F) Curing the thin film at 90 and 180 °C for 15 min each. (G) Photograph of the fabricated temperature sensor.

Figure 2

Temperature-dependent resistance changes of the Gt/PS nanocomposite-based temperature sensor: (A) Three cycles covering temperature ranges from −10 to 100, −10 to 80, and −10 to 60 °C. (B) Repeated heating–cooling cycles within the temperature range of −10 to 60 °C, and the inset zooms on the sensor’s performance during the repeated heating–cooling cycles. These tests were conducted under RH ≈ 10%.

Figure 3

Temperature-dependent resistance changes of the Gt/PS nanocomposite-based temperature sensor: (A) step-like response to temperature from −10 to 60 °C at RH 10 and 80%, and sensor response analysis at (B) RH ≈ 10% and (C) RH ≈ 80%. (D) Dynamic response curve of the temperature sensor in uncontrolled RH conditions (RH 40–90%), and (E) analysis of dynamic response curves for the temperature sensor. Experiments used three independently fabricated sensors, each tested in triplicate, yielding nine measurements per data point. Reported values represent the average of these measurements.

Figure 4

(A) Schematic representation of the Gt and PS nanocomposite coating applied over prescreen printed carbon electrodes on a PET substrate. (B) Schematic representation of the sensor’s mechanism, showing how it responds to both increasing and decreasing temperatures.

Figure 5

(A) Sensor response and recovery times under different heating/cooling rates of 20, 40, and 60 °C min^–1. (B) Sensor sensitivity to ±3 °C fluctuations at 0, 20, 40, and 60 °C temperatures.

Figure 6

(A) Changes of the normalized resistance at different bending angles and (insets) for the temperature sensor. (B) Normal resistance changes during the repetitive bending test at bending angles of 30°. (C) DMA results showing tan(δ), storage, and loss modulus.

Figure 7

Evaluation of the sensing thin film’s chemical and physical stabilities. (A) Response of the sensor’s electrical resistance to corrosive gases at 4 ppm concentration. (B) Sensor sensing performance was measured between −10 and 60 °C at RH 10% after a month of immersion in tap water and (inset) a photo of the immersed sensor.

Figure 8

Real-world evaluation of temperature sensor performance. (A) Sensor installation on the beaker’s surface for measuring its response to varying water temperatures. (B) Sensor’s response to hot and cold water. (C) Sensor integration into an inhaler for breath temperature monitoring. (D) Sensor’s response to breath temperature.