applied sciences
Article A Split Ring Resonator-Based Metamaterial for Microwave Impedance Matching with Biological Tissue
Vincenza Portosi , Antonella Maria Loconsole and Francesco Prudenzano *
Department of Electrical and Information Engineering, Politecnico di Bari Via Orabona 4, 70125 Bari, Italy; [email protected] (V.P.); [email protected] (A.M.L.) * Correspondence: [email protected]
Received: 9 September 2020; Accepted: 24 September 2020; Published: 26 September 2020
Abstract: A metamaterial lens based on a split ring resonator (SRR) array has been designed and optimized to improve the focusing and the penetration depth in human biological tissue of a microwave beam irradiated by a substrate integrated waveguide (SIW) cavity backed patch antenna. The impedance matching of the antenna loaded with human tissue is strongly improved. The simulations have been performed by using CST Microwave Studio®. A prototype of the device has been fabricated with the printed board circuits (PCB) process and has been characterized using a Network Analyzer and an antenna measurement system in anechoic chamber. A novel microwave applicator for hyperthermia therapy of skin cancer could be developed. The performances of the proposed applicator have been evaluated in terms of measured S11 scattering parameter modulus and simulated power loss density. The obtained results indicate that an SRR-based metamaterial is a promising solution for external microwave applicators to employ in dermatology.
Keywords: split ring resonator (SRR); metamaterials; substrate integrated waveguide (SIW); planar antennas; microwave devices
1. Introduction Microwave applicators have been proposed for applications in various areas, such as food cooking in domestic and industrial field, microwave-assisted extraction in environmental and chemical field [1,2], microwave-assisted synthesis in industrial material processing, bio-technology and chemical-pharmaceutics industry [3,4], drying, sterilization and disinfestation in agri-food [5,6] and microwave diathermy, hyperthermia cancer treatment and thermo-ablation therapy in medicine [7–13]. Microwave applicators for hyperthermia treatment in medical field are suitable antennas irradiating into the tissue to be treated in order to produce a sufficient heating of the entire tumor volume without damages of the surrounding healthy tissue. Microwave applicators can be divided into external and internal applicators depending on their peculiar use. External microwave applicators for superficial cancer, have been investigated and described in literature [7–13]. They typically consist of a single radiating element or of an antenna array placed in contact with the skin surface. A water bolus is usually placed between the applicator and the body in order to improve the matching of the antenna and at same time maintain the temperature of the skin at normal human body level. Among the various types of antennas studied for the external applicators there are waveguide and horn antennas [8], phased array antennas [9], and patch antennas [10–12]. Different shapes—rectangular, circular, and horseshoe—have been considered for the radiating slot of microstrip antennas for microwave hyperthermia treatment of cancer [13].
Appl. Sci. 2020, 10, 6740; doi:10.3390/app10196740 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 6740 2 of 14
Patch antennas are considered very interesting radiating elements for the treatment of skin cancers because of their intrinsic advantages such as small size, low profile, light weight, high integration and low-cost fabrication. The design of the patch antenna and the choice of the operating frequency must take into account the size and depth of the tumoral mass. At low frequencies, a large penetration depth of the electromagnetic field can be obtained, but also the radiating system employed in order to produce an adequate localized heating becomes larger. Microstrip applicators, at 915 MHz, 433 MHz and 190 MHz, have been designed with low loss, high dielectric constant, substrate to treat tumors at 2 cm, 3 cm and 4 cm depth [7]. Among the principal drawbacks of patch antennas are the narrow bandwidth and low penetration depth capability. Several studies in the literature proved that metamaterial lens can improve the antenna directivity and reduce the beam width and side lobe ratio [14–17]. Left-handed metamaterial lenses have been numerically investigated in order to improve the focusing in superficial tumor [18,19]. Metamaterial zeroth-order mode applicators have been proposed in order to produce a more homogeneous specific absorption ratio (SAR) distribution, similar to that of an ideal plane wave, and larger penetration capability [20,21]. In particular, the metamaterials based on split ring resonator (SRR) are largely investigated since they offer several advantages, among which lightness, compact size and easy and low-cost fabrication. Substrate integrated waveguide (SIW) technology allows to obtain an efficient and low-cost three-dimensional bounding of electromagnetic power via the planar printed board circuits (PCB) technology [22–28]. The metallic cavity improves the radiation performance of the patch antenna, increases the bandwidth and reduces the losses due to surface waves reduction [29]. In this paper, for the first time, to the best of our knowledge, a split ring resonator-based metamaterial is designed for the microwave therapy of cancer at the frequency f = 10.7 GHz. This frequency allows a good trade-off between the following needs: (a) good microwave absorption by biological tissue [30]; (b) small applicator size; and (c) localized/reduced ablation zone. Both (b) and (c) require the exploitation of high frequency. Generally, the SRR metamaterials are optimized for sensing/imaging applications at different operating frequencies. The design is focused (i) to improve the impedance matching of the antenna loaded with human tissue, thus avoiding the impedance mismatch and the formation of stationary waves which are deleterious for an efficient operation of the microwave applicator; (ii) to obtain a better focusing of the electromagnetic power into human tissue. This feasibility investigation constitutes a preliminary proof of concept for the development of a compact and low-cost microwave applicator for dermatology. The heating investigation is neglected in this paper, while the matching properties of metamaterial are experimentally demonstrated by employing a pre-prototype applicator which consists of a suitable SIW cavity-backed patch antenna with the optimized metamaterial lens based on SRR array. The antenna cavity has been designed by considering SIW technology. Higher gain, efficiency and directivity have been obtained by putting a SRR metamaterial over a patch antenna applicator. The impact of this research lies in the novelty of the proposed device and in its practical potential due to the possibility to refine a set of different SRR geometries. These can be optimized by considering the kind of organ, age and gender of the patients, for the same source and applicator. The obtained results pave the way towards the construction of an external microwave applicator for skin cancer thermal treatment.
2. Recall of Theory In this section, the analytical model and physics of the split ring resonators is recalled since it has been employed in order to roughly obtain preliminary geometric parameters, before the actual design, which is numerically refined via CST Microwave Studio® as described in Section3. The metamaterial based on SRR exhibits an effective magnetic permeability described by the Lorentz model approximation [31,32]:
Fω2 µ (ω) = 1 (1) reff 2 2 − ω ω m + jγω − 0 Appl. Sci. 2020, 10, 6740 3 of 14
where ω = 2π f is the angular frequency; ω0m is the resonant frequency; F is the fractional of the unit cell occupied by interior ring; γ is the damping factor due to metal losses. Starting from (1), a frequency range in which Re(µreff) is negative can be identified:
ω0m ω0m < ω < = ωpm (2) √1 F − where ωpm is the plasma magnetic frequency. Both ωpm and ω0m are finely tunable by optimizing the geometry of the unit cell. When an electromagnetic wave propagates orthogonally to the SRR plane, the SRR behavior can be approximatively described by an equivalent LC circuit, where the inductance and the capacitance are related to the currents induced in the metal rings and to the capacitive phenomena between the split terminations (capacitive gap), respectively [32]. If the dimension of unit cell is very small with respect to wavelength λ, the metamaterial layer exhibits an effective capacitance and an effective inductance at the macroscopic/average level, according to the effective medium theory, which leads to the calculation of an effective permeability [31]. Considering a single broadside coupled SRR with square form rings in the xy plane, the effective magnetic permeability is described by (3) [14]:
j ω Leff S µr = 1 (3) eff − j ∆x∆y Reff + j ω Leff − ω Ceff
S = lxly (4) where ∆x and ∆y are the dimensions of the SRR unit cell in xy plane; lx and ly are the lengths of the metallic ring in the x and y directions, respectively; Reff, Ceff and Leff are the effective resistance, effective capacitance and effective inductance of the metamaterial, respectively. The effective capacitance is approximatively obtained from the formula for the capacitance per unit length of a strip line [33].
εr ε0 l K(k) Ceff = (5) 4 K0(k) π w k = tan h (6) 2 t l = 2 lx + ly s (7) − where K(k) and K0(k) are elliptic integrals [34], w, s are the width and the split gap of the SRR; t is the distance between the two SRR in the z direction. The effective inductance is [33]:
µ0 S L = (8) eff t
The effective resistance Reff includes the radiation resistance and the loss resistance [33]. In other words, the control of the effective magnetic permeability, by varying both the inductive and capacitive properties of metamaterial, via a proper SRR optimization, is the basic physics principle of the electromagnetic field focusing. A planar metamaterial lens based on SRR, formed by an array of the unit cell in the rings plane, exhibits an anisotropic negative permeability; if the wave propagates in the z direction, the permeability tensor is [14]: 1 0 0 = µ µ0 0 1 0 (9) 0 0 µreff Appl. Sci. 2020, 10, 6740 4 of 14
When the antenna radiation frequency is within the range for which the permeability element µreff(ω) < 0, the x-axis wave vector component is imaginary kx = ω √εyµz, and therefore the component Appl.of the Sci. electromagnetic 2020, 10, x FOR PEER field REVIEW parallel to SRRs plane is evanescent and the sideward radiation forbidden.4 of 15 Consequently, the main lobe of antenna radiation pattern will be improved while the side lobe is attenuated,The metamaterial and there is lens an overallplaced improvementon the antenna, in in terms addition of directivity to the electromagnetic and gain [15]. field focusing properties,The metamaterial allowing the lens gain placed increasement, on the antenna, can be inused addition as impedance to the electromagnetic matching layer. field When focusing the radiatedproperties, electromagnetic allowing the gain waves increasement, propagate can thro beugh used media as impedance with different matching permittivity layer. When and the permeabilityradiated electromagnetic values an impedance waves propagate mismatch through occurs media. The with scheme different of the permittivity normal wave and propagation permeability throughvalues an the impedance two layers mismatchsandwiched occurs. between The two scheme semi-infinite of the normal media waveis illustrated propagation in Figure through 1, where the thetwo SRR layers metamaterial sandwiched layer between is placed two at semi-infinite distance from media the is biological illustrated layer in Figure to be 1matched., where the SRR metamaterial layer is placed at distance t2 from the biological layer to be matched.
Figure 1. Scheme of the normal wave propagation through the two layers sandwiched between two
Figuresemi-infinite 1. Scheme media. of the The normal split ring wave resonator propagation (SRR) metamaterialthrough the two layer layers is placed sandwiched to distance betweent2 from two the semi-infinitemedium to be media. matched. The split ring resonator (SRR) metamaterial layer is placed to distance from the medium to be matched. The metamaterial lens, if properly designed, can minimize the mismatch and maximize the energy transmission.The metamaterial With reference lens, if to properly Figure1, thedesigned, general can form minimize of wave matricesthe mismatch is given and by maximize (10). the energy transmission. With reference to Figure 1, the general form of wave matrices is given by (10). " # 3 !" # !" # i Y iγntn iγntn t t E 1 e Γn 1e− E a11 a12 E = 1 − = (10) r = i γntn iγntn = E Tn Γn 1e e− 0 a21 a22 0 (10) = 0 0 n 1 − r E a21 Γin = = (11) = i = E a 11 (11) where Tn, γn and tn are the transmission coefficient, propagation constant and thickness of the layer n, where , and are the transmission coefficient, propagation constant and thickness of the and Γn 1 is the reflection coefficient at the (n 1) n layers interface. The total reflection coefficient layer −, and is the reflection coefficient −at the− ( − 1) − layers interface. The total reflection of the multiple layers Γin, given by (11), is zero for a ‘perfect’ impedance match. By considering (11), coefficientthe total reflection of the multiple coefficient layers is a function , given ofby frequency (11), is zero and for can a be‘perfect’ minimized impedance by appropriately match. By consideringplanning the (11), effective the total magnetic reflection permeability coefficient of is the a function metamaterial of frequency and by adjusting and can be the minimized distance from by appropriatelythe medium to planning be matched. the effective magnetic permeability of the metamaterial and by adjusting the distance from the medium to be matched. 3. Design 3. Design The considered microwave applicator, depicted in Figure2, is an SIW cavity-backed patch antenna,The considered designed and microwave optimized applicator, for Ku-Band depicted applications in Figure (10.7 2,– 12.7is anGHz SIW). cavity-backed It consists of apatch stack antenna,structure: designed a microstrip and patch optimized antenna for coupled Ku-Band with applications a circular resonant (10.7– 12.7 cavity GHz based). It consists on SIW technology.of a stack structure:The substrate a microstrip Rogers Duroid patch 5880 antenna with lowcoupled dielectric with constant a circularεr = resonant2.2 and low cavity losses basedtan δ =on0.0009 SIW technology.has been considered The substrate for bothRogers layers. Duroid This 5880 antenna with low combines dielectric the constant attractive =2.2 features and of low both losses SIW tancavity-backed = 0.0009 and has patchbeen considered antennas with for truncatedboth layers. corners. This an Thetenna design combines has been the performed attractive features by varying of boththe sizeSIW of cavity-backed the cavity, the and hole patch diameters, antennas thewith hole truncated pitch, andcorners. the patchThe design geometry. has been All performed the details by varying the size of the cavity, the hole diameters, the hole pitch, and the patch geometry. All the details pertaining to designs of this kind of antenna are reported in [22]; they are not reported for shortness. The two cuts on the sides of the rectangular patch were designed to provide a wider frequency band and a frequency downshift. This allows a more compact size. The metallized cavity further broadens the bandwidth and improves the gain of the antenna; furthermore, the SIW technology offers the typical advantages of the standard PCB processes, such as easiness of Appl.Appl. Sci. Sci. 2020 2020, 10, 10, x, FORx FOR PEER PEER REVIEW REVIEW 5 of5 15 of 15 fabrication,fabrication, low low cost, cost, compact compact size size andand highhigh integration withwith the the planar planar circuits. circuits. Figure Figure 3 shows3 shows the the Appl.layout Sci. 2020 of an, 10 ,SIW 6740 cavity-backed patch antenna, the geometric parameters of which are listed in Table5 of 14 layout of an SIW cavity-backed patch antenna, the geometric parameters of which are listed in Table 1. 1. The metamaterial theory reported in Section 2 has been applied to roughly identify the geometric pertainingThe metamaterial to designs of theory this kind reported of antenna in Section are reported2 has been in applied [22]; they to roughly are not reportedidentify the for geometric shortness. parameters of the metamaterial operating at the Ku-Band applications. The analytical model for Theparameters two cuts of on the the metamaterial sides of the rectangular operating at patch the wereKu-Band designed applications. to provide Th ae wider analytical frequency model band for simple split ring resonators has been employed in order to obtain simple and preliminary line guides simpleandfor a the frequency split actual ring design, downshift.resonators which has This is been numerically allows employed a more performed in compact order via to size. obtainCST The Microwave simple metallized and Studio preliminary cavity®. furtherThe employed line broadens guides forthe the bandwidth actual design, and improves which is thenumerically gain of the performed antenna; via furthermore, CST Microwave the SIW Studio technology®. The employed offers the dielectric substrate is Rogers RO4350B with dielectric constant =3.48 and dissipation factor of dielectrictypicaltan advantages = 0.004substrate. ofis theRogers standard RO4350B PCB with processes, dielectric such constant as easiness =3.48 of fabrication, and dissipation low cost, factor compact of tansize and = 0.004 high. integration with the planar circuits. Figure3 shows the layout of an SIW cavity-backed patch antenna, the geometric parameters of which are listed in Table1.
Figure 2. Layout 3D view of the microwave applicator with the split ring resonator-based Figuremetamaterial 2. Layout superstrate 3D view of placed the microwave on the substrat applicatore integrated with the waveguide split ring resonator-based(SIW) cavity-backed metamaterial patch Figure 2. Layout 3D view of the microwave applicator with the split ring resonator-based superstrateantenna at placeda distance on the . substrate integrated waveguide (SIW) cavity-backed patch antenna at a metamaterialdistance d. superstrate placed on the substrate integrated waveguide (SIW) cavity-backed patch antenna at a distance .
(a) (b)
FigureFigure 3. 3.Layout Layout of of SIW SIW cavity-backed cavity-backed patch antenna. (a (a) )Top Top view view of of the the patch patch antenna antenna and and (b) ( btop) top viewview of of the the SIW SIW circular circular(a) resonant resonant cavity. cavity. (b) Figure 3. Layout of SIW cavity-backed patch antenna. (a) Top view of the patch antenna and (b) top TableTable 1. 1.List List of of the the geometric geometric parametersparameters of the the SIW SIW cavity-backed cavity-backed patch patch antenna. antenna. view of the SIW circular resonant cavity. ParameterParameter Dimension Dimension Description Description Table a1. List of the8.00 geometric 8.00mm mm parameters of the Length SIW Length cavity-backedof the of patch the patch patch antenna. b 2.86 2.86mm mm Length Length of the of lateral the lateral cuts cuts Parameter Dimension Description c 1.50 1.50mm mm Width Width of the of lateral the lateral cuts cuts r 8.006.72 mm 6.72mm mm Radius Length Radius of the of of theSIW the patch SIWcavity cavity v 2.861.24 mm 1.24mm mm Length Diameter Diameterof the of thelateral of vias the cuts vias α 15 Angular distance between the consecutive vias 1.5015° mm ◦ Angular distance Width betw of theeen lateral the consecutive cuts vias 6.72 mm Radius of the SIW cavity The metamaterial theory1.24 reported mm in Section2 has Diameter been applied of the to vias roughly identify the geometric parameters of the metamaterial 15° operating Angular at the Ku-Band distance applications. between the The consecutive analytical vias model for simple split ring resonators has been employed in order to obtain simple and preliminary line guides for the actual design, which is numerically performed via CST Microwave Studio®. The employed dielectric substrate is Rogers RO4350B with dielectric constant εr = 3.48 and dissipation factor of tan δ = 0.004. Appl. Sci. 2020, 10, 6740 6 of 14
The geometry of the SRR unit cell has been suitably modified and scaled with respect to the literature [14] in order to operate at the resonance frequency of the SIW microwave applicator. Moreover, with respect to [14], the unit cell of the proposed SRR presents a further double ring with splits placed on the opposite side and operates at a different frequency. A complementary configuration, i.e., a couples of coplanar rings written symmetrically with respect to the common center, has been considered to obtain a resonance frequency close to that of a single ring with the same dimensions, but with a larger magnetic moment due to higher current density. Since they are written on both sides of a dielectric substrate, they are broadside coupled. The analytical model of a single (not complementary) broadsideAppl. Sci. coupled 2020, 10, x SRR FOR PEER satisfactorily REVIEW approximates the complementary configuration when the6 of 15 couple of rings are very close each other. The metamaterial considered in this paper consists of an array of complementaryThe geometry broadside of the coupled SRR unit split cell square-rings.has been suitably The modified lattice of and this scaled inhomogeneous with respect structureto the is literature [14] in order to operate at the resonance frequency of the SIW microwave applicator. shorter than the guided wavelength of antenna radiation, so the composite behaves as an effective Moreover, with respect to [14], the unit cell of the proposed SRR presents a further double ring with homogeneoussplits placed medium. on the opposite side and operates at a different frequency. A complementary Theconfiguration, design and i.e., optimization a couples of coplanar of the metamaterial rings written lenssymmetrically has been numericallywith respect to performed the common via CST ® Microwavecenter, has Studio been considered. After several to obtain simulations, a resonance in whichfrequency the close SRR to geometry that of a andsingle cell ring size with have the been parametricallysame dimensions, changed, but the with optimized a larger magnetic geometric moment parameters due to arehigher listed current in Table density.2. The Since optimized they are SRR metamaterialwritten on exhibits both sides resonant of a dielectric frequency substrate, range th overlappingey are broadside the operatingcoupled. The frequency analytical range model of of the a SIW cavity-backedsingle (not patchcomplementary) antenna. Figurebroadside4 shows coupled the SRR 3D satisfactorily view (a) and approximates plan front view the complementary (b) of the unit cell of theconfiguration SRR, respectively. when the The couple metamaterial of rings are very lens clos usede each as superstrate other. The metamaterial of the SIW considered cavity-backed in this patch antennapaper is consists shown inof an Figure array5 .of complementary broadside coupled split square-rings. The lattice of this inhomogeneous structure is shorter than the guided wavelength of antenna radiation, so the composite behaves as an effective homogeneous medium. Table 2. List of the geometric parameters of the SRR Unit Cell of Figures2,4 and5. The design and optimization of the metamaterial lens has been numerically performed via CST MicrowaveParameter Studio Dimension®. After several (mm) simulations, in which the SRR Description geometry and cell size have been parametrically∆x changed, the3.20 optimized geometric parameters Length of the are unit list celled in in the Tablex direction 2. The optimized SRR metamaterial∆y exhibits resonant3.20 frequency range overlapping Length of the unitthe celloperating in the y frequencydirection range of the SIW cavity-backedlx1 patch2.45 antenna. Figure 4 shows Length the 3D of the view external (a) and SRR plan in the frontx direction view (b) of the unit cell of thely1 SRR, respectively.2.45 The metamaterial Length lens used of the as external superstrate SRR in theof they direction SIW cavity-backed l x patch antennax2 is shown in2.00 Figure 5. Length of the internal SRR in the direction ly2 2.00 Length of the internal SRR in the y direction The s parameter of0.11 the microwave applicator placed Split at gap distance of the SRR from a model of human tissue is simulatedw to evaluate0.11 the lens metamaterial behavior Width as impedance of the SRR matching layer. Figure 6 shows theg sideview of the0.11 layout of SIW Distance cavity-backed between patch the complementary antenna covered SRRs by in thethexy metamaterialplane lens and att distance from3.04 the biological Distance tissue. between The biologic the broadsideal tissue coupled considered SRRs in in the thez direction simulation d 6.90 Distance between the SIW antenna and the metamaterial lens is a slab of skin-fat-muscle, whose electromagnetic parameters at the = 11 GHz frequency are listed Lx 22.40 Length of the metamaterial lens in the x direction in Table 3 [35]. Ly 36.00 Length of the metamaterial lens in the y direction
(a) (b)
FigureFigure 4. 3D 4. model3D model of complementaryof complementary broadside broadside coupled SRR SRR unit unit cell cell designed designed in CST in CST Microwave Microwave ® StudioStudio®.(a). ( 3Da) 3D view view and and (b ()b top) top view. view. Appl.Appl. Sci. 2020,, 10,, 6740x FOR PEER REVIEW 7 of 1415
Figure 5. Layout of the SRR metamaterial placed as the patch antenna superstrate. Figure 5. Layout of the SRR metamaterial placed as the patch antenna superstrate.
The S11 parameter of the microwave applicator placed at distance D from a model of human tissue is simulatedTable 2. to List evaluate of the geometric the lens metamaterial parameters of behaviorthe SRR Unit as impedanceCell of Figures matching 2, 4 and layer.5. Figure6 shows the sideviewDimension of the layout of SIW cavity-backed patch antenna covered by the metamaterial Parameter Description lens and at distance D( from) the biological tissue. The biological tissue considered in the simulation is a slabAppl. Sci. ofΔ skin-fat-muscle,2020 , 10, x FOR PEER3.20 whose REVIEW electromagnetic Length parameters of the at unit the cellf = in11 theGHz frequencydirection are listed8 of in15 Table3Δ [35 ]. 3.20 Length of the unit cell in the direction