# A Split Ring Resonator-Based Metamaterial for Microwave Impedance Matching with Biological Tissue
## 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-pharmaceu industry [3; 4], drying, sterilization and disinfection in agri-food [5; 6] and microwave diathermy, hyperthermia cancer treatment and thermo-ablation therapy in medicine [7; 8; 9; 10; 11; 12; 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; 8; 9; 10; 11; 12; 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; 11; 12]. Different shapes--rectangular, circular, and horseshoe--have been considered for the radiating slot of microstrip antennas for microwave hyperthermia treatment of cancer [13]. 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; 15; 16; 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; 23; 24; 25; 26; 27; 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.