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In this work, surface engineering is applied to polyimide (PI) films to fabricate low-cost Ag/PI wireless humidity sensors with a resonant frequency of 2.45 GHz. The sensors were obtained by in situ metallization technique coupled with inkjet printing, where PI plays triple roles as a flexible substrate, ion-exchange surface, and sensing material to moisture. Moreover, the humidity sensitivity can be enhanced by the improvement of hydrophilicity via loading with different ions on the PI surface, which has been demonstrated by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The wireless humidity sensor loaded with K+ ions has the maximum sensitivity of 97.7 kHz/% RH at a low relative humidity range of 20–65% and 359.7 kHz/% RH at a high relative humidity of 65–90%, respectively. Accordingly, a sensing mechanism of the fabricated humidity sensor has been discussed in detail. On the other hand, the characteristics of the humidity sensor such as response and recovery speed and stability are analyzed. The mechanical performance tests show that the humidity sensor displays excellent flexibility and good mechanical stability. A strong adhesion between the Ag antenna and PI substrate can be found as well. The passive wireless humidity sensor described in this work has the advantages of having a simple structure, low cost, high sensitivity, long-term stability, and good mechanical properties, which has potential applications in automated industry and healthcare with real-time humidity monitoring.

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In this work, the silver films with tuned morphologies have been fabricated on flexible polyimide substrate by in situ direct-ion-exchange technique. The morphology of Ag films with loose nanoparticles, dense polyhedrons, aggregated nanoparticle clouds, and dendrite structure can be obtained by a controlled reduced process as illustrated by scanning electron microscopy (SEM) and optical microscopy, respectively. All of the Ag films show good crystalline and high conductivity, which is confirmed by X-ray diffraction (XRD) and four-point probe resistance measurements. Infrared (IR) spectra demonstrate the occurrence of the polyimide surface metallization, which favors good adhesion between the Ag films and the flexible substrate. The adhesion test proves the strong adhesion of these Ag films, especially for the Ag films with the dendritic structure. Moreover, the mechanical properties of these Ag/PI films have been investigated as well. It can be found that all of the Ag/PI films exhibit low sensitivity to the bending test. However, the strain sensitivity strongly depends on the morphology of the Ag films, which can be applied for diverse flexible electronics.

This article presents a cost-efficient flexible chipless radio frequency identification (RFID) tag with wireless humidity sensing, which is fabricated by in situ metallization and inkjet printing techniques. The inkjet printing technique is applied to print the mask for RFID antenna, which is designed with frequency-encoding simulation by a high-frequency structure simulator (HFSS). A high-quality patterned Ag antenna is realized by the in situ metallization of a polyimide (PI) film, leading to strong adhesion between the Ag antenna and PI substrate. The patterned Ag antenna of the chipless RFID tag consists three parallel dipole resonators, one of which is sensitive to humidity, while the other two are utilized to encode and store data. As a result, a 2-bit chipless RFID with high humidity sensitivity based on a Ag/PI film is developed, which displays excellent flexibility and good mechanical stability. The performance of the fabricated tag shows good agreement with the simulation results. Moreover, the tag is applied to detect the water source, where the resonance frequency shows good linearity versus the distance to the water source. These results demonstrate that the proposed chipless RFID tag with humidity sensing has a 2-bit storage capacity, high humidity sensitivity, excellent mechanical properties, and long-term stability, confirming a cost-efficient preparation process for flexible electronics.