Contact-free sensing systems have gained interest in recent years for various medical, environmental, agricultural, petrochemical, and food safety applications. The major limitation associated with the existing approaches, is the need to include a local power source, which affects the price, durability, and safety of the system. Herein, we present a novel, low-cost, flexible, and passive resonant sensors for contact-less measurement of physical, chemical, and biological parameters. The primary method used for fabricating resonant sensors was designing Archimedean spirals with varying dimensions adjusted based on the application, and chemical etching copper-coated polyimide masked with the desired senor design. The significant feature of this development was the elimination of on-board power and contact-less data acquisition, for which an external reader antenna was designed and 3D-printed. To do so, a two-port vector network analyzer was used to excite the external reader to generate a local electromagnetic field for sensor interrogation. The principle of the sensor response was based on variations in the dielectric properties (e.g. relative permittivity and conductivity) of the sensor surrounding environment, which led to changes in the resonant frequency and peak-to-peak amplitude of the sensor scattering parameter signal. The sensor response was automatically recorded and then further analyzed via custom MATLAB scripts.

First, we demonstrated the application of a 3D resonant sensor, which was engineered into an extension sensor with the aid of Kirigami-inspired patterns, for wireless deformation monitoring. This approach improved the reportable extension and retraction distances from previously reported millimeter ranges to as high as 16 cm. Moreover, the PDMS-coated resonant sensor was applied in aqueous solution for wireless measurement of water flow rate in closed systems. This class of sensor has potential applications in wearable as well as soft robotics industry.

Next, we used these uncoated resonant sensors to measure the ion concentration in aqueous solutions. The novelty of this research was eliminating the sensitive coating layer for direct characterization of solvated ions for applications in which direct access to the sensor is not possible. Potassium chloride (KCl) solutions having concentration in the range of 100 nM to 1 M were tested on resonant sensors with varying geometries, all of which demonstrated a concentration-specific response. The sensor response to changes in the ionic concentration was also supported by lumped-element circuit modeling. Moreover, the ability of resonant sensors to have specific response for mixture of ions was studies using ternary mixtures. Finally, for a real-world use case, the resonant sensors were applied to agricultural field runoff water to monitor the nitrate level, which can lead to downstream eutrophication and algae blooms. The advantage of this type of sensing system over the previously introduced ion sensors is its ability for contact-free signal transduction from an opaque closed system without using communicational ICs, which are typically powered. Such system is essential for applications in which there are many measurement nodes, such as a farm field, as well as applications in which a disposable price point is required, such as single-use bioreactors.

Another purpose of this research was to investigate the feasibility of contact-free wound monitoring using resonant sensors embedded into commercial wound dressings. The sensor transduced the magnitude of forward scattering parameter (S21) to an external reader antenna connected to a vector network analyzer. We then extracted the resonant frequency and peak-to-peak amplitude, which were a function of relative permittivity and conductivity of surrounding tissue, respectively. The sensors were initially tested on gelatin-based tissue mimicking phantoms (TMPs) representing various stages of wound during the healing process. Moreover, the sensor response was confirmed using first-principle finite element simulations. Additionally, the sensors were tested on a canine model as well as a cohort of 12 rats. This work aimed to fill the gap in the previous studies by applying resonant sensors on living, moving subjects and demonstrating their applicability for in vivo wound monitoring. This sensing system can eliminate the need for unnecessary bandage changes which improves wound management.

Lastly, the effect of various fabrication methods and materials on the sensor performance was explored. For this purpose, resonant sensors were made using screen printing and wound metal wires, and compared to the copper etching method presented earlier. Moreover, several screen printing parameters such as screen width (250 – 500 μm), silver content and viscosity of the conductive paste, curing temperature, and type of substrate the sensors were printed on were studied as well. In addition to measuring the width and height profile of screen-printed sensors via a 3D digital microscope, they were electrically characterized to extract their resistance and inductance via a custom characterization board. The sensor response in terms of peak-to-peak amplitude was monitored at 0 to 5 cm sensor-reader step-off distances to determine the maximum gap at which the sensor can have detectable response. The sensor sensitivity to changes in the surrounding relative permittivity was also explored for resonators made with different fabrication criteria. Finally, the fabrication cost and the sensors performance were compared for these three different fabrication technologies, which demonstrated the added advantages of screen-printing technique for applications in which low-cost, direct printing on flexible substrate is required (e.g. direct print on fabric and wound dressing). Using this comprehensive study, the appropriate materials and methods for fabricating resonant sensors can be selected based on the requirements of the desired application (e.g. resonator feature size, cost per unit, and sensor-reader step-off distance).

In summary this thesis advances the current field of passive LC sensors by introducing resonant sensors consisting of a single-layer spiral inductor without incorporating any interdigital capacitor. In other words, the previously studied passive LC sensors have a multilayer design in which an inductor coil is connected to an interdigitated or parallel-plate capacitor, which results in a relatively low quality factor. Herein, the tuning capacitor is eliminated from the sensor design and the parasitic capacitance of the inductor coil (self-capacitance between windings) is used as the sensing element. Using this simplified planar design, the sensor has a larger interrogation area (the entire coil) and thus is well-suited for transducing bulk properties. Moreover, the fabrication cost of the sensor is significantly lower when compared to other passive LC sensors that require multiple prints and/or pick and placement of integrated circuit elements (e.g. rectifier and tuning capacitor). The low sensor unit cost enables applying this type of sensor for monitoring many locations in a heterogeneous environment (e.g. agricultural farm). In addition, sensor fabrication materials and methods were compared in this thesis, which helps further reduce cost and expand opportunities for integrated use. For instance, single-step screen printing of the planar LC sensor can create single-use, disposable sensors that can be directly printed on wound dressings, fabrics, petri-dishes, and food packaging.

This thesis also presents three unique advances to the field of LC sensors in novel applications in closed environments. We first demonstrated coupling Kirigami patterns and spiral inductors to create a LC sensor for deformation monitoring. We were able to improve the previously reportable extension range of millimeters to over 16 cm by using this approach. Second, we demonstrate progress towards using an array of uncoated resonant sensors for fingerprinting ions in a solution. Third, we are first in the field to perform an in vivo longitudinal study to determine the applicability of such sensors for wound management. In each of these we optimize the resonator geometry to working frequency regions with low absorption coefficient for an improved signal penetration depth while keeping the size of the transmitter and receiver in a practical range.