In this work, a planar microwave sensor based on a flexible polyimide substrate has been developed to distinguish if a skin lesion is malignant or benign. The sensor is a tuned loop resonator operating in the industrial, scientific, and medical (ISM) band at 2.465 GHz, providing a localized high-intensity electric field that penetrates into tissues with sufficient spatial and spectral resolutions. The loop resonator with a radius of 5.4 mm was tuned by a concentric metal pad to the desired resonant frequency with a sufficiently high quality factor of 98.7 and a reflection coefficient of $-$ 63.98 dB. The sensor is based on the detection of electromagnetic resonance change and sequential frequency shift that is susceptible to the dielectric property difference between cancerous and benign tissues. Basal Cell Carcinoma (BCC) and Seborrheic Keratosis (SK), the most commonly found malignant and benign skin lesions with close visual similarities, were selected to demonstrate the sensing concept. Tissue-mimicking materials were fabricated to have similar dielectric properties to those of healthy skin, SK, and BCC tissues in the literature. Simulations and measurements were conducted. Significant frequency shifts of 759 MHz and 415 MHz were observed between BCC and SK phantoms in simulations and measurements, respectively, when the size of the tumor phantom was a cuboid of 12 mm × 12 mm × 4 mm underneath and among healthy skin. Simulations were conducted for different cuboid side lengths from 2 to 16 mm while the thickness remained at 4 mm. Malignant lesions could be distinguished with a cuboid side length as small as 2 mm. Corresponding measurements for cuboid side lengths of 6, 8, 10, and 12 mm were conducted and matched the trend well with the simulation results. The promising results in simulations and measurements validate the sensing principle, showing great potential for skin cancer detection in a noninvasive, efficient, and lower-cost way.

In this work, a planar radio-frequency sensor based on a flexible polyimide substrate has been developed to monitor the water content changes noninvasively and efficiently in a human body or tissues. The sensor is based on the detection of electromagnetic resonance that is susceptible to dielectric property changes by water content variations. The planar loop resonator tuned with a metal pad features improved resonance, compact size, and flexibility to conform to a curved surface. Designs and experiments to continuously monitor human hydration processes in vivo have been demonstrated. The recorded data shows distinct trends when a person becomes hydrated from a dehydrated state. Discrete and continuous measurements are compared with the simulations conducted by using documented, generalized human skin permittivity properties. Discrepancies between the measurements and simulations are investigated and verified with directly measured skin permittivities. Measurements variations are also investigated. With the advantages of being compact, flexible, and planar, the sensor can be integrated into a wearable on the human forearm. Additionally, the sensor has been used to demonstrate its ability to detect water content changes in a phantom made of ground pork. The ex vivo results indicate the sensitivity and consequential variations in practical scenarios. The promising results show great potential for human body monitoring and also general applications in agriculture and the food processing industry from the demonstration of the sensing principle in the pork phantom.

A flexible pH sensor based on using iridium oxide (IrO_x) as the sensing film was developed by the roll-to-roll (R2R) process. The inert and biocompatible properties of IrO_x make it a desired metal oxide for pH-sensing applications. The flexible substrates being continuously processed by the R2R technique provides the advantages of scalability, reconfigurability, resiliency, on-demand manufacturing, and high throughput, without the need for vacuum systems. Potential sweeps by cyclic voltammetry across the IrO_x film against commercial and planar Ag/AgCl electrodes validated the reversible electrochemical mechanisms. Multiple IrO_x electrodes showed similar output potentials when continuously tested in the pH range of 2–13, indicating good fabrication uniformity. For practical applications, planar IrO_x/Ag-AgCl pairs developed on polyimide substrates were tested, with a good linear fit within pH 2–13, achieving Nernstian responses of around −60.6 mV/pH. The pH sensors showed good repeatability when analyzed with hysteresis, drift, fluctuation, and deviation as the stability factors. The selectivity of the interference ions and the effect of temperature were studied and compared with the reported values. The electrodes were further laminated in a process compatible with the R2R technique for packaging. The flexible sensors were tested under flat and curved surface conditions. Tests in artificial sweat and viscous solutions were analyzed in the Clarke error grid, showing reliable pH-sensing performance. The materials used during the manufacturing processes were sustainable, as the active materials were in small amounts and there was no waste during processing. No toxic chemicals were needed in the fabrication processes. The cost-effective and efficient materials and the fabrication process allow for rapid production that is necessary for disposable and point-of-care devices. Flexible electronics provide a platform for device and sensor integration and packaging, which enables Internet-of-things (IoT) network applications.

In this article, the significance, clinical needs, methods and principles for continuous human body hydration level monitoring are reviewed. Maintaining hydrated is critical for physical and mental health. A long-term, continuous, convenient and accurate wearable device that can monitor the whole-body water content to provide real-time feedback is needed, especially for infants, the elderly, and people who have special needs in their tasks and occupations. Noninvasive bioimpedance measurements and microwave sensing techniques suitable for conformable wearables are summarized. Microwave probing with deeper field penetrations into the dermis layer of the skin can provide a better assessment of whole-body water content. Frequency choices for the probing signals, the need for realistic phantoms, and evaluation standards suitable for clinical uses are discussed.

Loop structure has been used as a single resonator and in meta-materials. Variations from the loop structures such as split-ring resonators have been utilized as sensing elements in integrated devices for wearable applications or in array configurations for free-space resonance. Previously, impedance formula and equivalent circuit models have been developed for a single loop made of a conductor wire with a negligible wire diameter in the free space. Despite the features of being planar and small, however, the quality factors of single-loop resonators or antennas have not been sufficiently high to use them efficiently for sensing or power transfer. To investigate the limitation, we first experimentally examined the formula and equivalent circuits for a single loop made of planar metal sheets, along with finite element simulations. The loop performance factor was varied to validate the formula and equivalent circuits. Then a tuning element was utilized in the planar loop to improve resonance by providing distributed impedance-matching to the loop. The proposed tuning method was demonstrated with simulations and measurements. A new equivalent circuit model for the tuned loop resonator was established. Quality factors at resonance show significant improvement and the tuning can be done for a specific resonance order without changing the loop radius. It was also shown that the tuning method provided more robust performance for the resonator. The tuning mechanism is suitable for miniature planar device architectures in sensing applications, particularly for implants and wearables that have constraints in dimensions and form factors. The equivalent circuit model can also be applied for meta-materials in arrayed configurations.