Design and Analysis of Metaresonator-Based Tri-Band Antenna for Biosensing Applications

A metamaterial resonating antenna design having a hexagonal patch is proposed for biosensing applications to work in triband resonating frequencies at 5.2GHz, 9.6GHz, and 18.5GHz. The hexagonal patch is built on a dielectric constant of 4.4 FR4 epoxy substrate with a loss tangent of 0.030. Two different antennas are designed and tested with conducting metallic patches bounded by split ring resonator (SRR) slots and closed ring resonator slots which are also hexagon-shaped. Each antenna is capable of resonating at different frequency bands with good wide band characteristics due to the presence of partial ground plane. The hex-SRR resonates at 1.9GHz and 3.1GHz and hexagonal-CRR resonates at 5.2GHz, 9.6GHz, and 18.5GHz frequency bands resulting in multiband resonance characteristics. The proposed antenna is also validated with the study of metamaterial property of hexagonal SRR at the resonant frequency of 5.2GHz hence resulting in a compact (20 × 20 × 1.6) mm3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$mm^3$$\end{document} and more dominant design. Furthermore, the specific absorption rate (SAR) for the three-layer phantom model is confirmed, resulting in superior performance that is more appropriate for biosensing and on-body wearable devices. The testing is performed using an E5063A ENA vector network analyzer provided by Keysight Technologies. The measured and simulated results are very close, resulting in a better validation of the proposed work.


Introduction
The metamaterial antennas or metasurface antennas are the subwavelength periodic artificially defined structures which are capable of operating in a Ghz or higher regime with miniaturised structure.The metamaterial antennas possessing the properties of relative permittivity and relative permeability to be concurrently negative are also referred to as the double negative materials.These artificially fabricated structures [1] are preferred over complex lab equipment owing to their non-invasive characteristics, quick response, and microlevel sample requirements for biosensing applications.It is useful in various areas like monitoring of chronic illnesses, studying the environment, pharmacology, forensics, biomedical research, and food control.A biosensor [2] typically works based on the principle of signal transduction.The transducer in a biosensor converts the signal generated by the bioreceptor into a measurable optical or electrical signal through a process called signalisation [3].The metamaterial-based chemical sensors and the biological sensors are developed with the least possible wavelengths B.Sharmila, P. Jeyakumar and P. Muthuchidambaranathan contributed equally to this work.like t ∕12 and ∕15 respectively [4].The biosensing capa- bility of an antenna is improvised by the γ-dispersion principle which overcomes the capacitive barriers of each and every cell membrane when the frequency is increased [5].The unique atomic configurations of materials are directly proportional to the leading parameters namely permeability and permittivity which in turn affects the electromagnetic characteristics of the material [6].
Antenna biosensors [7] are preferred as they do not require any physical intrusion, chemical alteration, or labelling.The development in the field of antenna biosensing technology is mostly aimed at miniaturisation, noninvasiveness, and the capacity to integrate with various analytes.The biosensing-inspired metamaterial antennas can be categorised under few methods based on the sensing methodologies like molecular sensing, hyperthermal sensing, and plasmonic sensing.
Biorecognition initially aims in immobilisation of the ligand at the surface of the sensing element over which the enzyme or protein to be investigated is allowed to flow through it [8].This results in the permissible interaction with the biomolecules which are immobilised well in advance.The analyte and the preconditioned biomolecules are made to react at the sensitive areas of SRR resulting in the frequency shifts for different concentrations of the specimen analyte.The antigens like PSA and cortisol were realised with the label-free testing procedures using this principle of molecular sensing with the designed single SRR and coupled SRR respectively [9,10].
The failed thermoregulation principles leading to hyperthermia in human body can be diagonised by the zerothorder resonance of metamaterials.This absorption of heat by the tissues directly affects the polarisation effects of the antenna [11] resulting in hyperthermal sensing.This sensing aids in treating the cancerous cells in a non-invasive manner.
Plasmonic biosensing [12] is the operation of MMinspired sensors that is based on changes in the permittivity, permeability, or refractive index of the MM-based resonator, which results in changes in the reflection/transmission coefficients (scattering parameters-S 11 , S 21 ).This kind of detection always employs a specific bioreceptor surface to analyse viral proteins, and diverse methods of plasmonic biosensing are shown in Fig. 1.
A 4 × 4 × 4 mm 3 millimetre scale antenna [14] that is more suited for WSN applications was proposed and evaluated for sensitivity and EIRP.It met the substrate's conformal requirements, but lagged in terms of SAR performance.The biosensing antenna array [15], which is made of a silicon substrate and glass coated water, runs at multiple wavelengths of 850 nm, 1310 nm, and 1550 nm, and performs well with the best possible bulk sensitivity, but the SAR parameters were not verified.With the aid of 3D printing technology, an RFID antenna [16,17] created for biosensing applications was constructed with polypropylene (PP)  [13] and acrylonitrile butadiene styrene (ABS) substrate.This antenna can function as a perpetual biological sensing system with blood or oil samples and has validated the salinity and conductivity, but lags in the SAR simulation.Direct measurements of analytes, which are normally thought to be unfeasible in real-time, are made using RFID antennas created for microfluid biosensing that are resonant at 915 MHz with the polyethylene substrate [18].The sensitivity enhancement technique at 2.4 GHz led to the proposal of a dipole antenna [19], which demonstrated improved performance for heartbeat monitoring as a result of the variance in reflection coefficient.
This work puts forward a novel compact metaresonator antenna.In order to create a multiresonance characteristic and miniaturisation, a simple yet efficient approach like SRR (split ring resonator) and CRR (closed ring resonator) is proposed.The CRR provides appreciable return loss of −30dB, −24dB, and −26dB at the resonant frequencies 5.2GHz, 9.6GHz, and 18.5GHz respectively.SRR, in addition to the CRR, gives an enhanced return loss of −22dB and −37dB at 1.9GHz and 3.1GHz, accordingly.The design also offers an analysis in such a way that it can be best suited for the biosensing and body sensor network (BSN).The application-specific properties like surface current distribution, parametric study of metamaterials, and the specific absorption rate (SAR) are also verified along with the conventional properties of antenna such as S 11 and VSWR.This paper stood unique with minimal SAR for all resonant frequencies which is less than 0.4W/kg.This paper is organised in such a way that in the "Proposed Metaresonator Antenna" section the design specifications of the proposed antenna and mathematical theory of split ring resonators are discussed.Simulated results and its inference are discussed in the "Results and Discussion" section.Following this, the "Fabricated Results" section contains the comparative analysis of simulated and fabricated design.The work concludes with a comparative table showing the proposed design with other relevant works in multiple parameters and future scope of the approach.

Configurations of Metamaterial Structures
The metamaterials when added to the antennas, it behaves as though it is of much bigger size, as it stores the radiated energy and re-radiates it.Split ring resonators of hexagonal shape are used to bound the patch.They are used to achieve wave selectivity and to couple only the resonating frequency to the output [20].The key word metamaterial is accountable to many artificially fabricated structures like CRLH-TL-based metamaterials, MTM loading, metaresonators, and metasurfaces [21].Each category has their own unique different behavioral characteristics like antennas working on the principle of engineered dispersion curves; antennas with metamaterial loadings like epsilon-negative and mu-negative, antennas inspired by metaresonators like SRR, CSRR, and Hex-CRR; and antennas with metasurfaces like reactive impedance surface (RIS), artificial magnetic conductor (AMC), and electromagnetic band gap (EBG) structures.The proposed methodology comes under the third category of metaresonators which can also prove that even with a small antenna profile inspired by metamaterials, good radiation properties may be retained.

Split Ring Resonators (SRRs)
Split ring resonators are extremely compact and therefore have minimum radiation losses.Another major advantage of split ring resonators is that they have high magnetic coupling.The characteristics and measurements of a split ring resonator have a significant impact on its performance.The gap between two consecutive split rings influences the resonating frequencywider the gap, greater the resonating frequency.Similarly, the splits in the rings also influence the resonating frequency.When the gap width increases, the capacitance decreases, thereby increasing the frequency at which the circuit resonates.When additional capacitance is added externally, then the resonating frequency decreases.
A split ring resonator circuit usually behaves like a parallel LC resonating circuit.The current flows across the rings of the split ring resonator and brings about a small capacitive difference that enables the circuit to resonate.
The formula for the calculating resonant frequency of hexagonal disconnected split ring resonator is given as The formula for calculating the hexagonal closed ring resonators can be analysed using the LC tank equivalent circuit model as shown below in Fig. 3 (1) where C signifies the speed of light, S stands for each hexagonal sides, d represents the slot distance in between individual hexagonal closed ring resonator, W S represents the width of the slot, and K represents Legendre elliptic integrals of first-order modes and the 0 and 0 represent the magnetic permeability in vacuum and electric permittivity in vacuum respectively.

Proposed Work
In this paper, the metamaterial antenna designed with a hexagonal patch is proposed to resonate in tri-bands such as 5.2GHz, 9.6GHz, and 18.5GHz.The polygonal patch does indeed have a side length of 4 mm and is built on a FR4 epoxy substrate.The patch is enclosed by split ring resonators and closed ring resonators of hexagonal shape.The outer ring resonator of the both configuration is coupled to the microstrip line feed, resulting in intensified concentration of energies around the sensing element.The proposed structure shown in Fig. 2 represents the top view and the bottom ground plane of antenna with 2 disconnected hexagonal CRR.This structure is a variant of the 2-linked split ring structure shown in Fig. 5a and it greatly improves the S 11 parameter to − 57.8 dB at a resonance frequency of 5.2GHz.
The metaresonator antenna with hexagonal CRR has a substrate made of FR4 epoxy with a dielectric constant ( ) of 4.4 and a thickness of 1.6 mm.On top of the substrate is a hexagonal patch of side 4 mm.This patch serves as a sensing material on which the analyte can be deposited when the metaresonator is focused for biosensing application.The hexagonal patch is bounded by a couple of closed loop split ring resonators of the same hexagonal shape with sides of length 8 mm and 7 mm as it progresses inwards respective width of the split ring resonators are fixed at 0.5 mm.A microstrip line feed is used to connect the antenna to the port.Table 1 depicts the dimensions of the envisaged antenna design.Below the substrate is a partial ground plane of dimensions G L × W sub .It also has a square-shaped slot of dimensions 2.5 mm × 2.5 mm to bring the input impedance closer to the characteristic impedance.This way, the ground plane behaves as a load and improves the S 11 parameter of the proposed structure.Ansoft's high-frequency structural simulator is being employed to design and simulate the antenna (HFSS).

Results and Discussion
The performance metric of the antenna is evaluated by the parameters like return loss, VSWR, bandwidth, and radiation pattern characteristics.These aforementioned parameters are compared and studies for both the hexagonal-SRR and the proposed hexagonal-CRR.In addition to these, the CRR element with its surface current distribution at its resonance is also computed using the simulation studies so that the same design can be loaded with the biomaterials which can perform better if used for biosensing application listed under the plasmonic sensing category of biorecognition process.Since the antenna is working on the principle of metaresonator for biosensing applications, it is also essential to carry out the study of real and imaginary negative permeability, negative permittivity, refractive index, and impedance characteristics along with the specific absorption rate (SAR) so that it can be best suited for biosensor based and also as a wearable antenna.

Return Loss and VSWR
The amount of signal returned by a transmission line discontinuity is measured as return loss.By varying the number of split ring resonators in the metamaterial antenna, two different comparative analyses were performed.When simulated in the same frequency band, the resonating frequencies for Fig. 5a are 1.9GHz and 3.1GHz.The return loss is greatest at 3.1 GHz, which corresponds to −37dB.When the interconnections between the split rings and the patch were removed, as shown in Fig. 5b, multiple higher resonating frequency bands of 5.2GHz, 9.6GHz, and 18.5GHz were observed, with corresponding return losses of −30dB, −24dB, and −26dB as represented in Fig. 6.
One of the significant criteria describing the antenna performance is the voltage standing wave ratio (VSWR).It determines the efficiency of an antenna's RF power transmission.Figure 7 depicts the VSWR of both metaresonator antennas with two connected split rings and disconnected split ring.The connected SRR has a VSWR of 1.5 at 1.9GHz and 0.5 at 3.1GHz, whereas the disconnected SRR has a VSWR of 0.53, 1.0, and 0.7 for 5.2GHz, 9.6GHz, and 18.5GHz, respectively.An antenna's radiation pattern indicates how an electromagnetic wave interacting with an antenna is transmitted.In other words, it is a graphical representation of the antenna's far field.In both antenna configurations, the partial ground plane enabled by a square shaped slot is responsible for inducing the additional reactance element, which results in a wide band frequency response characteristics.
The antenna design has been modified to get the required resonant frequency.The split rings are predominantly used to get multiband resonant frequencies.The ring structures in the split ring resonator antenna has an average loop length of L n , where n represents the num- ber of split rings available in the structure of metamaterial antenna.The same is calculated using the following expressions where n corresponds to value 2.  The resonant frequency of each loop is calculated at half wavelengths g 2 .The principle of band stop filter has been used here.The band stop or band reject filter has the characteristic of allowing frequencies that are higher or lower than the specified bandwidth of frequencies.This way the required resonant frequency can be obtained by rejecting other frequencies.By removing the connections, the metamaterial antenna was given the properties of a band-stop filter and multiresonance is achieved at the frequencies of 5.2GHz, 9.6GHz, and 18.5GHz over wide bandwidths of 4.8 to 5.7 GHz, 9.2 to 10 GHz, and 17.5 to 19 GHz respectively.

Parametric Study of Metamaterial Using Nicolson-Ross-Weir Method
The importance of parametric study comes into role when most of the materials are analysed merely with the permittivity and permeability.They deal with the electric and ( 8) magnetic stimulus correspondingly.The advancements in the artificially structured man made metamaterials have attracted this recent need for the electromagnetic study.
The equation for calculating the permittivity and permeability [22] of the proposed closed ring resonator antenna by the Nicolson-Ross-Weir (NRW) method is given in Eqs. ( 9) and ( 10) where V 1 and V 2 correspond to the voltage maxima and volt- age minima value of frequency (unit:radian) C speed of light d thickness of substrate.
The negative permeability and permittivity characteristics of metaresonator Hex-CRR get altered due to the accumulation of artificially induced structure.This change in the addition of inductive and capacitive components of the basic design holds responsibility for the ELC structures which gives rise to the negative permeability and permittivity component at the different bandwidths of the resonating metamaterial antenna.Figure 8 illustrates parameters such as real and imaginary negative permeability, negative permittivity, refractive index, and impedance characteristics across the closed ring resonator's first resonating band.The permeability value obtained from the NZW method has a peak value of −15dB at 5.2GHz, similar to the fact that permittivity has its highest negative peak point across 5.2GHz, as shown in Fig. 8.
Because the refractive index is a product of permeability and permittivity, it also has its first maximum at 5.2GHz.The duality theory [23] proposed states that the permeability and permittivity can most of the times be equal to each other.

SAR Evaluation
Having analysed the performance of reflection coefficient, VSWR negative permeability, and permittivity characteristics of the proposed antenna, the effect of calculating the specific absorption rate (SAR) on the human body can be investigated.
A three-layer human phantom model in the Ansys HFSS was proposed to reduce the simulation time and to perform the SAR calculations as shown in the Figs. 9 and 10.The phantom model is composed of a three-layer fat, muscle, and skin having a relative permittivity of 5.93, 52.729, and 39.2 and loss tangent value of 0.145, 0.241, and 0.282 respectively as listed in Table 2.
Although the Federal Communications Commission (FCC) in the USA has set a maximum SAR limit of 1.6 W/ kg for cell phones, while the European Union has set a limit of 2 W/kg, based on ANSI C95.1-1982 guidelines, radio frequency protection standards are designed to ensure that the energy accumulation in body tissues does not exceed an average of 144 J/kg during any 0.1-h period of unrestricted exposure.This equates to a specific absorption rate (SAR) of approximately 0.40 W/kg or less when averaged across the entire body weight.When the optimal distance for the on body communication is 10 mm, SAR values obtained at an input power of 100mW comply much better with the aforementioned standards.The SAR of the 1 g and 10 g of tissue for the frequencies 5.2, 9.6, and 18.5GHz is given in Table 3.

Fabricated Results
In this section, the results of the antenna design after fabrication and measurement using a vector network analyser (VNA) E5063A series are discussed in this section.Two antennae designed based on the concepts of hexagonal split ring resonator and hexagonal closed ring resonator in this paper have been fabricated using FR4 epoxy whose dielectric constant is 4.4 and loss tangent is 0.030.The E5063A ENA vector network analyzer is a cost-effective device that has the potential to deal with passive components like filters, cables, connectors, and antennas.The frequency of operation is from 100KHz to 18 GHz.The

Conclusion
In this paper, a multiband metaresonator-inspired patch antenna with a partial ground plane operating on improved bandwidth is designed and proposed to work for biosensing applications falling in the wide range of frequency spectrum from 1 to 30 GHz of macro capacity with mid-band use cases predominantly focussing on health care industry.The measured hexagonal SRR and CRR work well on 3.5 GHz, 5.7 GHz, and 6 GHz frequencies which belong to FR1 band in 5G communication.The hexagonal rings in the antennas can be loaded with biosensing materials if they are more application specific and behave like a plasmonic sensing element.The medical monitoring and biosensing require unique features like great penetration capabilities, high precision with multipath effects, lower energy consumption, and low electromagnetic radiation to avoid the near-field proximity of the human body.Hence, the proposed SRRand CRR-based antennas will act as the best-suited ones for biosensing applications.Additionally, the hexagonal closed ring resonator is more compact in design and has demonstrated minimal specific absorption rate for multibands, representing 0.378 W/kg, 0.298 W/kg, and 0.372 W/ kg for the 5.2GHz, 9.6GHz, and 18.5GHz frequency bands, respectively.This value is significantly lower than the FCC standards and earlier works.In order to further support the wearable application, flexibility qualities to guarantee the conformal nature of the antenna might be further verified for the suggested design with a changeable substrate.

Author Contribution
The authors contributed equally in this work.

Fig. 1
Fig. 1 Diagrams of various virus-sensing methodologies which depend on the plasmonic-and metamaterial-based structures (a) Planar Structure based sensing (b) Opti-fludic sensing (c) Nano particle

Figures 3 and 4
show the schematic diagram of the Split ring resonators and the equivalent circuit of the microstrip line feed at the radiating edge.The coupling between the microstrip and the patch is in the form of edge coupling as shown in the figure.The excitation of the patch by the edge

Fig. 2
Fig. 2 Proposed metamaterial antenna design.a Top view of the 2 disconnected hexagonal CRR.b Bottom ground plane of 2 disconnected hexagonal CRR

Fig. 9 Fig. 10
Fig.9 Top view of the metaresonator antenna with its specific absorption rate (SAR ) in W/kg on a three-layer phantom model evaluated over 1 g of tissue for different resonant frequencies: case 1, 5.2 GHz; case 2, 9.6 GHz; and case 3, 18.5 GHz

Fig. 12 Fig. 13
Fig.12 Comparative results of antenna with 2 hexagonal connected split rings

Table 1
Design specifications

Table 2
Dielectric properties of three-layer phantom model

Table 3
Specific absorption rate over 1 g and 10 g of tissue in W/kg

Table 4 A
comparison of the proposed metaresonator antenna with other relevant work connected split rings and 2 disconnected split rings.The comparative study with the counterparts of recent works in similar area is listed in Table4which shows the proposed work is more liable for biosensing and WBAN application.