- Photonic Sensing: Principles and Applications for Safety and Security Monitoring
- Microwave Noncontact Motion Sensing and Analysis
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In distortion polarization electronic and atomic , the electric field tries to change the distance between the charges involved and in response a restoring force acts. In classical terms [ 5 ] it behaves as a resonator and is characterized by a resonance frequency. As mentioned above, five types of polarization mechanisms are observed when a dielectric is subjected to an alternating field of increasing frequency as shown in Figure 3 b. On subjecting the dielectric with a field of increasing frequency, first polarization of ions is observed, followed by polarization of molecules and finally atomic polarization is observed at very high frequencies.
Their detailed description is provided next.
Their motion is impeded at an interface and in case of tissues, it occurs at the cell membrane. This type of polarization is referred to as space-charge or interfacial or Maxwell-Wagner polarization.
Photonic Sensing: Principles and Applications for Safety and Security Monitoring
This is known as dipolar or oriented polarization. Let us now look at the mathematical representation of EM-wave interaction with dielectrics. We know that application of electric field leads to displacement of charges in a dielectric, inducing dipoles. As discussed later in Section 1. As shown in Figure 3 , ionic polarization is the first one to be dropped due to its inability to keep up with the increase in frequency of the applied alternating field, followed by interfacial and dipolar polarizations.
Permittivity also depends on the number of dipoles in the given volume. Hence there is an opportunity to utilize this phenomenon to measure the concentration of biomolecules by observing the changes in permittivity, as done in glucose [ 5 ], heparin [ 7 ] and melanoma [ 8 ] biosensors. Conduction of charges in a dielectric gives rise to conduction currents, while displacement of charges due to polarization leads to displacement currents.
All EM wave sensors rely on detecting dielectric properties, i. As mentioned earlier in Section 1. As the field direction is reversed, the alignment of the dipole changes. The time taken by the charges to catch up with the changing field which is known as relaxation time. Thus, as the frequency increases, the charges get lesser time to align and eventually fail to keep up.
This frequency is known as relaxation frequency. Alternatively, we can say that polarization lags applied field, as established by Debye [ 6 ]. Equation 16 accounts for just one type of polarization mechanism e. Equation 18 indicates that the dielectric loss is maximum when frequency of the applied field coincides with the relaxation frequency for a given polarization i.
For instance, the f r of water lies in the microwave region and this concept is exploited in ovens, where the applied frequency matches f r and loss in electromagnetic energy at relaxation frequency is used for heating. As the frequency increases, the dipoles are unable to fully restore their original positions during field reversals and P lags E , leading to loss in permittivity. When applied frequency coincides with the relaxation frequency, polarization fails to keep up with the fast changing electric field and drops out, and stops contributing, seen as a fall in permittivity.
This frequency dependence of permittivity is known as dispersion [ 9 ]. Figure 3 b shows the general trend of dispersions with increase in applied frequency on biomatter [ 10 ]. In this region, dipolar polarization stops contributing towards polarizability, and at dipolar relaxation frequency, dielectric loss peaks up while the real permittivity falls.
In summary, the variation of permittivity as a function of frequency is known as dispersion.
Microwave Noncontact Motion Sensing and Analysis
A sharp drop in permittivty occurs at relaxation frequency marked by a loss in electromagnetic energy which occurs when a particular polarization fails to keep up with the fast changing electric field. Furthermore, this permittivity spectrum provides a unique dielctric signature and can be utilized for sensing purpose.
The biological target can be biomolecules, like proteins, DNA, biomarkers, pathogenic organisms, hormones, or other medically relevant analytes like glucose, medical parameters like pulse, heart-beat, etc. Biosensing applications include identification of these biological targets such as cancer cells hepatoma liver cancer cells, skin cancer melanoma cells, blood cancer lymphoma cells , detection of biomarkers [ 11 ] prostate specific antigen PSA for prostate cancer, Human epididymis protein 4 HE4 for ovarian cancer , detection of vital signs respiration and heart rate and detection of biomolecules glucose, cortisol, heparin.
For instance, in glucose biosensors, as shown in Figure 4 a, change in concentration of glucose on the detection surface leads to change in permittivity and shift in relaxation frequency which is detected. In case of biomarker detection, biomarkers are captured at the detection surface leading to a change in permittivity which is sensed.
In general, permittivity is different for different biological tissues, providing unique dielectric signatures for all. Dead [ 12 ] and tumor [ 13 ] cells differ in their cytoplasmic contents from living and healthy cells due to the presence of pores and blisters [ 14 ], providing a permittivity contrast. By directly measuring the unique permittivity curves of cells and tissues or by indirectly measuring the changes in permittivity, we can perform the aforementioned biosensing applications.
There are two prominent ways of observing changes in permittivity, either over a wide range of frequencies broadband or over a small frequency band narrowband. Figure 4 a [ 15 ] shows the broadband permittivity spectrum of glucose Section 4. As shown later in Section 4. The electrical model of cell in suspension medium helps in explaining the variation of permittivity with frequency, in terms of capacitance and conductivity. The impedance of the medium is represented by resistor R m and capacitor C m in parallel.
The membrane of a viable cell has very low conductivity and can be modeled as a capacitor C m e m , preventing current flow through the cell at low frequencies. The cell cytoplasm is represented by a resistor R i , impeding current flow through it and the electrode-electrolyte interface is modeled as a capacitor C D L , known as electrical double layer EDL.
Hence the impedance measurements in this range are of little use in sensing as they are dominated by EDL and are unable to provide information about cell properties both inside and outside the cell membrane. Since currents cannot pass through the cell, intracellular information is still not accessible. When frequency is increased 1— MHz , the capacitive reactance C m e m of cell reduces, increasing the intracellular currents and consequently conductivity.
At these frequencies, the signal becomes sensitive to changes in the intracellular contents achieving maximum conductivity. This has been utilized in characterizing the different types of WBCs e. This range is useful for detection of biomarkers, glucose and DNA etc. Table 1.
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Based on the application, the corresponding frequency band can be chosen. For instance, the dielectric response of live cells is fundamentally different from that of dead cells [ 20 ]. Live cells contain a large number of negatively charged molecules which attract positive charges sodium and potassium ions from suspension medium.
The accumulation of mobile charges on the surface of the membrane gives rise to a membrane potential. In case of dead cells, pores are present, permitting exchange of ions with suspension medium.
contsikinglighsi.tk: Lin Li - Geography / Sciences, Technology & Medicine: Books
Thus, capacitive contrast of cell with respect to suspending medium is less compared to live cells [ 12 ]. Figure 7 summarizes the effects on permittivity with varying cell thickness, cell potential, charge mobility and concentration of cells [ 20 ]. Thus, by choosing frequency bands accordingly, biosensing applications can be performed, i. Next we discuss the commercially available medical biosensors and their categorizations. The first medical biosensor , for sensing oxygen during cardiovascular surgery, was reported in [ 23 ].
Since then there has been tremendous research in this field towards detection of cancer, glucose, genetic disorders and pathogens aiming for better reliability, sensitivity and selectivity while reducing time of acquisition of results. Point of care testing POCT [ 14 ] biosensors are becoming widely popular in the market due to their ability to perform a quick diagnostic test near the patient without the need for laboratory analysis Figure 8.
The commercially available sensors for glucose [ 24 ], HIV, tuberculosis, cholesterol, hCG pregnancy , malaria and cancer are listed in Refs. These biosensors available on the market Table 2 are based on electrochemical or optical methods. To the best of our knowledge, the only commercially available RF sensor is the vital sign sensor based on Doppler and a blood glucose monitoring device named Glucowise MediWise, London, UK, Section 4.
However, there is ongoing research in trying to develop microwave sensors with good repeatability and accuracy comparable with the state of the art commercial electrochemical and optical biosensors. Notable efforts have been made in Ref. Another microwave glucose monitoring system, proposed in [ 28 ], based on split ring resonators was evaluated for its performance using clinical trials and shows promise in terms of integration into wearable devices.
RF biosensors for detection of biomolecules have undergone a large amount of research and have potential for commercialization as POCT devices. Detection of biomolecules require biorecognition elements [ 29 ], which recognize the target biomolecules and include antibodies, nucleic acid probes, bacteriophages and proteins.
Antibodies can be monoclonal which recognize single epitope of target molecule, or polyclonal; capable of recognizing multiple epitopes of same target. However, they are expensive and require proper storage conditions to keep them from denaturing. Nucleic acid probes are attractive due to their associaition with genetic disorders, cancer etc. Phages are viruses that attack bacteria and use their replication system for phage duplication, hence can be employed for detection of clinically important bacteria.