Metamalzeme tabanlı sensörler ve bu sensörlerin mikrodalga algılamada kullanılması

Abstract

Metamalzemeler doğada bulunmayan ve yapay olarak elde edilen malzemelerdir ve negatif kırılma indeksi, negatif faz hızı gibi özellikleri ile doğadaki malzemelerden ayrılmaktadırlar. Metamalzemelerin elektromanyetik dalga ile etkileşimi incelenmiş ve elektromanyetik dalga yayınımı üzerine çalışmalar yapılmıştır. Bu incelemeler ışığında Ayrık Halka Rezonatörleri (Split Ring Rezonators (SRRs)) ve Tamamlayıcı Ayrık Halka Rezonatörleri (Complimentary Split Ring Rezonator (CSRRs)) keşfedilmiştir. CSRR ve SRR yapılarının yakınlarına yerleştirilen dielektrik malzemelere duyarlı olduğu bilinmektedir. Bu yapılar rezonans frekansında ve elektrik alanının Q faktöründe değişiklikler üreten iyi kurulmuş bir elektrik alanı sergiler ve böylelikle algılama alanında kullanılabilirler. CSRR yapının olumlu taraflarının olması sebebiyle hem SRR hem de CSRR yapılarının iyi bir kararlı frekans yanıtı sağlamasına rağmen tasarımlarda genellikle CSSR tercih edilir. Diyabet günümüzde oldukça sık görülen bir hastalık haline gelmiştir ve diyabet hastası olan kişiler için kan şekeri seviyesinin izlenmesi oldukça önemlidir. Kan şekeri seviyesini ölçmek için kullanılan yöntemlerden biri parmak delmedir ve bu yöntemin ağrılı, invaziv ve enfeksiyon açısından riskli olduğu bilinmektedir. Minimal invaziv olarak bilinen Sürekli Glikoz İzleme (Continuous Glucose Monitoring (CGM)) ise kan şekerini ölçmek için deri altına yerleştirilen sensörleri kullanarak sürekli bir ölçüm sağlar. Ancak bu yöntemin de doğruluk be maliyet gibi bir çok olumsuz tarafı bulunmaktadır. Bu nedenle kan şekerinin takibi için İnvaziv Olmayan (Non- Invazive (NI)) yeni nesil mikrodalga algılama yöntemleri önem kazanmıştır. Bu çalışmada SRR ve CSRR yapıları kullanılarak elde edilen sensörler ile algılama yapılabileceği gösterilmiştir. Öncelikle bir çözeltideki su miktarını algılayan sensör ve kullanılan yöntem incelenmiş ve bu yöntemden yola çıkarak farklı sensörler üzerinde incelemeler yapılmıştır. Kompleks dielektrik geçirgenlikleri farklı olan glikoz karışımları kullanılan CSRR sensörü sayısal olarak modellenmiş ve S11 ve Q faktörü gibi rezonans parametrelerinin değişimi analiz edilmiştir. karmaşık dielektrik geçirgenlikteki değişikliği hesaplamak için MLS (En Küçük Kareler Yöntemi) algoritmasında kullanılmıştır. ayrıca MLS algoritmasında Q faktörlerini dahil etmek yerine iki farklı rezonans frekansında S11 büyüklükleri kullanılmıştır. Son olarak, diyabet durumuyla ilgili kan şekeri düzeylerini tespit etmek için karmaşık dielektrik geçirgenliği ve glikoz konsantrasyonları arasındaki ilişki kullanılır. Son olarak, daha düşük hata başarımı elde etmek için yeni bir sensör önerilmiştir. Benzer bir tasarım prosedürü kullanılmış ancak CSRR yapıları hem şekil hem de boyut olarak yenilenmiştir. Sonuçların gerçeğe yakın olması için sensörler incelenirken deri katmanı da kullanılmıştır.

The main characteristic parameters affecting electromagnetic wave propagation are dielectric permittivity 𝜀 and magnetic permeability 𝜇. This is because these parameters are the only material parameters in the dispersion equation. Materials are divided into four basic classes according to the signs of 𝜀 and 𝜇 values. Metamaterials are artificial materials with negative dielectric permeability and negative magnetic permeability. It is known that substances with negative 𝜀 and 𝜇 parameters have some different properties than substances with positive 𝜀 and 𝜇 parameters. Metamaterials were first proposed by Veselago. In addition, Veselago predicted that these materials could exhibit artificial properties such as negative refraction, reverse Doppler effect, and reverse Cherenkov radiation. As a result of these investigations, studies have been carried out on artificially obtained Split Ring Resonators (SRRs) and Complementary Split Ring Resonators (CSRRs). There are many metamaterial unit structure designs. Among them, the most studies have been done on the SRR structure. In these structures, excitation with the time-varying magnetic field component perpendicular to the plane of the rings leads to an induced resonance current in the loops, producing an equivalent dipole moment exhibiting negative electromagnetic permeability. Such a structure induced by the microstrip transmission line has great potential for biosensing. The CSRR structure is obtained by etching a conductive sheet to form rings on the ground plane. This arrangement causes the ring to be excited by an axially polarized electric field. In the SRR structure, there is a dielectric sheet under the substrate. The presence of this dielectric plate imposes an additional boundary condition at a distance h from the SRR plane. Therefore, the SRR is not strictly coupled to the CSRR structure. Since the two structures are considered to be approximately double, the behavior of the CSRR structure excited by an electric field in the axial direction will be similar to the behavior of the SRR structure excited by an axial magnetic field. In both cases, it is ensured that the particle has a rejected frequency band around its resonant frequency. The main part of metamaterial-based microwave sensors are SRRs or CSRRs. The SRR and CSRR structures exhibit a well-established electric field that is sensitive to dielectric materials placed nearby, producing changes in the resonant frequency and Q factor of the electric field. Although both resonators provide a good stable frequency response, CSSR structure is used instead of SRR in the designs. The CSSR sensor does not need extra circuit space, making the proposed sensor more compact. In addition, CSRR structures are more sensitive to electrical permeability changes in the sensing area of the resonator than SRR structures, due to the widely diffused electric field. Additionally, the sensor size is relatively small, thus allowing for a compact, miniaturized rendering in a wearable format. More importantly, the bound electric field is highly concentrated and localized in a relatively large sensing region. Thus, it provides enhanced detection that makes it possible to detect subtle changes in electromagnetic properties at different levels of the same substance. In this study, firstly, information about metamaterials will be given. Afterwards, the structural properties of metamaterial-based SRRs and CSRRs will be explained and equivalent circuit models will be examined. The metamaterial-based microwave sensor, which forms the basis of this work, consists of a microstrip line combined with a CSRR structure. The liquid material to be measured is filled into the glass capillary tube placed in the substrate. The operation of the sensor is based on the principle that when a microstrip transmission line is fed with an electromagnetic signal, it excites the metamaterial-based structure due to resonance. Thus, an electric field is generated between the capacitive plate of the CSRR and the circular resonator. This makes the region near and inside the CSRR sensitive to dielectric changes. Thus, the sensor can estimate the complex dielectric permittivity of the sample placed in the tube passing through the substrate and parallel to the CSRR sensor plane. The sensor uses the electric field between a circular CSRR structure and the transmission line. First of all, the geometric structure of the sensor was examined, then the equivalent circuit model was given. It has been shown that the investigated CSRR sensor detects the amount of water in water-ethanol and water-methanol solutions with the proposed method. Diabetes is one of the most common diseases that is increasingly common among people. Diabetes can be defined as the inability of the pancreas to produce a stable source of insulin hormone, and the inability of body cells to accept glucose in the bloodstream as caloric energy due to insulin deficiency and resistance. Diabetes is divided into two categories, type 1 and type 2. Type 1 patients should check their blood glucose levels (BGL) four to ten times a day based on their insulin intake, while type 2 patients should check it two to four times a day. Patients should also monitor their blood glucose levels before and after meals/snacks, before exercising, before all focus-based tasks such as driving, operating heavy machinery, before going to sleep, and especially when symptoms of hypoglycemia and hyperglycemia are observed. Patients who cannot manage their diabetes face many serious conditions such as heart disease, stroke, coma, kidney failure, blindness and premature death. Therefore, monitoring of blood sugar levels in people with diabetes has been developing rapidly in recent years. The leading methodology commercially available and reliably used by diabetics is the capillary blood sample-based invasive glucometer, in which a blood sample is taken from the fingertip and analyzed on a test paper strip. This finger piercing procedure is painful, invasive, and risky for infection. It is also costly for users who have to prick their fingers several times a day to draw blood and constantly have to purchase fresh control solutions, alcohol wipes and test papers to analyze samples. These invasive devices can only provide blood glucose level measurements at a specific time point during the test. Therefore, they do not reflect any long-term patterns or trends in glucose fluctuations due to certain lifestyles, dietary regimens, or medication intake. Continuous Glucose Monitoring (CGM) is a minimally invasive method used to measure blood glucose levels. In this method, glucose levels are continuously measured with the help of sensors placed under the skin. However, most of these sensors are problematic in terms of accuracy and cost. In recent years, the development of designs for non-invasive (NI) glucose monitoring has gained more importance. Microwave detection methods, which are among these NI methods, are useful for non-invasive monitoring of blood sugar, thanks to electromagnetic radiation that has no ionizing effect when it enters the human body. Recently, many approaches have been analyzed for microwave and millimeter wave sensing-based glucose monitoring, such as sensors designed using SRRs, antennas, and waveguides, as well as reflection, transmission methods, and radar techniques. The resonance perturbation method, which is a subset of reflection and transmission methods, uses a near-field sensor with a very high quality factor. The purpose of this method is to measure changes in both the resonant frequency and the quality factor and correlate them with changes in the dielectric properties of the medium under test. Therefore, in the resonance perturbation method, the relevant microwave sensors operate in a very narrow frequency range. One of the studies in the literature is an NI continuous blood glucose monitoring microwave sensor operating at approximately 1.4 GHz, based on the resonance perturbation method. The sensor is designed using microwave double split ring resonators, in which one of the rings is used as a reference to calibrate the effects from changes in temperature. This sensor is attached directly to the skin in the abdomen using an adhesive patch. Glucose levels are also measured using two commercially available sensors. Relationships between change in glucose concentration, change in bandwidth, and change in resonance over time are also plotted using linear and/or nonlinear regression. In another study, the performance of the proposed double split-ring resonator-based sensor was evaluated for accuracy and reproducibility in a clinical trial on 24 human subjects with and without diabetes. The results obtained are quite promising, but its use is rather restrictive as the system involves the use of a portable network analyzer and is not fully wearable/portable. In this study, the method applied for the measurement of liquid mixtures is proposed for monitoring the blood glucose level. First, a wearable microwave biosensor consisting of circular CSRRs placed on a FR4 dielectric substrate was investigated for use for non-invasive real-time monitoring of blood glucose level. The CSRR sensor loaded with glucose mixtures with different complex dielectric permeabilities was modeled numerically and the variation of resonance parameters such as S11 and Q factor were analyzed according to glucose concentrations. The resonance values of S11 magnitudes and/or Q factors for different glucose concentrations measured by the proposed sensor are used in the MLS (Least Squares Method) algorithm to calculate the change in complex dielectric permittivity in a similar approach for liquid dielectric spectroscopy. In this study, it has also been shown that it is possible to use S11 magnitudes at two different resonance frequencies instead of including the Q factors in the MLS algorithm. Finally, the complex relationship between dielectric permittivity and glucose concentrations is used to detect blood glucose levels related to the diabetes state. Using the same method, analyzes were made on a different type of honeycomb sensor. Finally, the investigated sensor was modified and a new sensor was proposed to achieve lower error performance. Despite using a similar design procedure, CSRR structures differ in both shape and size. For the most realistic analysis, the skin layer was also used when examining the sensors. Finally, the results are given.