Performance Optimization of Wearable Printed Human Body Temperature Sensor Based on Silver Interdigitated Electrode and Carbon-Sensing Film

Abstract

The human body's temperature is one of the most important vital markers due to its ability to detect various diseases early. Accurate measurement of this parameter has received considerable interest in the healthcare sector. We present a novel study on the optimization of a temperature sensor based on silver interdigitated electrodes (IDEs) and carbon-sensing film. The sensor was developed on a flexible Kapton thin film first by inkjet printing the silver IDEs, followed by screen printing a sensing film made of carbon black. The IDE finger spacing and width of the carbon film were both optimized, which considerably improved the sensor's sensitivity throughout a wide temperature range that fully covers the temperature of human skin. The optimized sensor demonstrated an acceptable temperature coefficient of resistance (TCR) of 3.93 × 10^-3 °C^-1 for temperature sensing between 25 °C and 50 °C. The proposed sensor was tested on the human body to measure the temperature of various body parts, such as the forehead, neck, and palm. The sensor showed a consistent and reproducible temperature reading with a quick response and recovery time, exhibiting adequate capability to sense skin temperatures. This wearable sensor has the potential to be employed in a variety of applications, such as soft robotics, epidermal electronics, and soft human-machine interfaces.

Keywords: carbon black; flexible biosensor; human body temperature sensor; inkjet material printer; printed IDE; silver nanoparticles.

Figure 1

(a) Interdigitated electrode design with finger spacing from 0.1 to 1 mm, and (b) geometric parameters of the optimized sensor.

Figure 2

(a) The photograph and optical microscopic image of the fabricated silver IDEs on Kapton film, and (b) Schematic illustration of continuous line development caused by individual ink droplet overlap.

Figure 3

Photograph of the screen printer (Aurel Automation 2015A) used for deposition of the sensing films. The insets show the schematic diagram (top right) of the tested sensing films with various film widths (0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.5 mm, and 2.0 mm) and a photograph (bottom right) of the final optimized device.

Figure 4

Photograph of the optimized temperature sensor (top) and the corresponding SEM images of the carbon film (bottom left) and silver IDEs (bottom right).

Figure 5

Electrical characterization of the devices. (a) Resistance variation with the temperature. (b) Resistance variation with sensing film width and an optimum finger spacing of 0.4 mm.

Figure 6

Electrical characterization of the temperature sensor. (a) Resistance variation of the sensor against temperature, ranging from 25 to 50 °C. (b) Corresponding sensitivity graph.

Figure 7

Sensor endurance test on human skin: (a) photographs of the sensors on different parts of the body (foreheads, neck, palm); (b) the corresponding resistance variations against temperature.

Figure 8

The cyclic response and recovery time of the temperature sensor.