This review provides an outlook on some of the significant research work done on printed and flexible sensors. Printed sensors fabricated on flexible platforms such as paper, plastic and textiles have been implemented for wearable applications in the biomedical, defense, food, and environmental industries. This review discusses the materials, characterization methods, and fabrication methods implemented for the development of the printed and flexible sensors. The applications, challenges faced and future opportunities for the printed and flexible sensors are also presented in this review.

A resistive flexible humidity sensor based on multi-walled carbon nanotubes (MWCNTs) was designed and fabricated. Screen and gravure printing processes were used for monolithically fabricating the humidity sensor containing interdigitated electrodes (IDE), a sensing layer and a meandering conductive heater. An average thickness and surface roughness of 0.99 μm and 0.23 μm, respectively, was registered for the printed MWCNTs sensing layer. The capability of the printed sensor, with heater, was investigated by subjecting it to relative humidity (RH) ranging from 10% to 90%. The response demonstrated an overall resistance change of 55% when the sensor was subjected to 90% RH, when compared to 10% RH. A maximum hysteresis of 5.1%, at 70% RH, was calculated for the resistive response of the sensor. The printed sensors can be bend with radius of curvature of 1.5 inch with literally no effect.

A printed strain gauge based on metal/non-metal composite was successfully fabricated and tested for micro-strain (μƐ) detection. A silver ink was blended with a carbon ink to achieve a silver-carbon (Ag/C) composite ink. The composite ink was then screen printed on a polyimide substrate in a meandering pattern to achieve a desired resistance of ~350 Ω. The printed strain gauge was bonded on to a flat aluminum beam. The capability of the printed strain gauge to detect linear strain was investigated by applying varying tensile and compressive loads on the aluminum beam, to simulate μƐ. Corresponding linear gauge factors for tensile and compressive loads were calculated as 2.26 and 1.47, respectively. In addition, transverse gauge factors for tensile and compressive loads were calculated as 1.18 and 0.43, respectively. The strain gauge has a temperature coefficient resistance of 0.26 %/°C. The screen-printed Ag/C composite strain gauge was also compared to a commercial strain gauge of similar gauge resistance. The results demonstrate that a strain gauge with screen-printed Ag/C and encapsulation ink on a polyimide substrate can be utilized for μƐ detection. The electromechanical response of the fabricated strain gauge as a function of resistance is investigated and presented in this paper.

A flexible capacitive force sensor was developed for force sensing applications. The force sensor consists of two electrodes and a dielectric layer. The electrodes were fabricated by depositing conductive silver (Ag) ink on a flexible polyimide platform using screen-printing process. The dielectric layer was prepared by mixing the contents of polydimethylsiloxane (PDMS) (pre-polymer and curing agent) in a 16:1 ratio. Then the PDMS dielectric layer was sandwiched between the screen-printed Ag electrodes. The capability of the fabricated force sensor was investigated by recording its capacitive response for varying applied forces of 100 N. It was observed that the capacitance increased from 5.83 pF to 6.36 pF as the force was increased from 0 N (no load) to 100 N. A sensitivity and correlation coefficient of 0.081%N ⁻¹ and 0.998 were calculated for the force sensor. In addition, a response time and recovery time of 3.7 seconds and 5.7 seconds was measured for the fabricated force sensor. The relative humidity (% RH) tests performed from 20% RH to 80% RH, in steps of 20% RH, revealed that there was a minimal effect of RH on the base capacitance of the force sensor at room temperature.

Wearable devices with integrated sensors for tracking human vitals are widely used for a variety of applications, including exercise, wellness, and health monitoring. Photoplethysmography (PPG) sensors use pulse oximetry to measure pulse rate, cardiac cycle, oxygen saturation, and blood flow by passing a light beam of variable wavelength through the skin and measuring its reflection. A multi-channel PPG wearable system was developed to include multiple nodes of pulse oximeters, each capable of using different wavelengths of light. The system uses sensor fusion along with machine learning model to perform feature extraction of relevant cardiovascular metrics across multiple pulse oximeters and predict saturated oxygen (SpO ₂ ). The developed model predicted SpO ₂ with a root mean square (RMSE) of 0.07 and accuracy of 99.5%. The wearable system was applied to the plant of the foot for vascular assessment. Wearable PPG systems capable of sensor fusion demonstrates a potential capability for continuous evaluation/monitoring of wounds and diseases associated with abnormal blood flow.