Design, fabrication and characterization of a magnesium-based Split Ring Resonator for pH and strain detection

Bone substitution is the second most frequent tissue transplantation worldwide. However, some drawbacks induced by the surgical operations may arise. An inflammation of the tissues surrounding the fracture may take place. This increase in the acidity level in the environment may provoke cells death and subsequent patient pain. Moreover, there is a relation between the biomechanics and biological environment. Depending on the stability of the implant, a faster or slower bone healing will occur, with a trade-off on the mechanical properties of the formed bone. Hence a study on a biocompatible, biodegradable and wireless pH and strain sensor has been performed in this project. A first focus on the biodegradable materials that may be employed as bone substitutes has been discussed. The choice of magnesium as working material was unavoidable due to its mechanical properties closer to the natural bone ones and its bone growth ability thanks to the release of Mg2+ ions. Then a focus on the wireless sensing mechanisms and an overview of antenna’s properties, have been performed. The Additive Manufacturing (AM) of a magnesium-4%zinc alloyed Split Ring Resonator (SRR) for pH and strain sensing was performed, making the sample porous and improving its biological and biomechanical properties. Two different pH sensing devices were fabricated. The first defined as "ion resistive" sensor, is based on a double concentric split ring resonator coated by hydroxyapatite (HAp), and laminated. In this way only the inner ring will be in contact with the liquid environment, degrading once a pH below the 5.5 threshold is reached, thanks to HAp. The second, or "ion sensitive" sensor is based on zinc oxide nanoparticles, that will be deposited in the gap or on the metal lines of a split ring resonator. Depending on the pH level, more or less H+ ions will tend to bind on this layer, varying the resonance frequency of the device. For the strain sensing, instead, a novel interdigitated structure has been designed and fabricated. Here, depending on the strain applied on the device’s substrate, a displacement of the structure will be induced, generating a shift in resonance frequency. The fabricated devices have been studied and analysed through scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), scanning laser microscopy (SLM), atomic force microscopy (AFM), and X-ray diffraction analysis (XRD) to characterize their surface and material properties. It was possible to detect a shift of the resonance frequency peak of around 2 GHz for the ion resistive pH sensor, and of around 60 to 90 MHz proportional to the H ions for the pH sensitive sensor. Finally, it was also possible to detect a shift of around 0.5 GHz of the resonance frequency once a displacement on the strain device, was applied