Development of a conformal woven fabric antenna for wearable breast hyperthermia

Breast hyperthermia is a non-invasive cancer treatment, where breast temperature is mildly elevated by a localized electromagnetic (EM) irradiation to deactivate and damage cancer cells. The emerging needs associated with this medical modality include the development of a highly wearable microwave applicator with a low power requirement to enable a more patient-friendly and continuous hyperthermia therapy. As a potential solution, we propose a textile antenna that consists of a copper-plated woven polyester fabric as a radiating patch and a ground plane and a woven polyester fabric as a dielectric substrate and a padding layer. The porous nature of these textile materials enables construction of a lightweight and flexible antenna with a low dielectric loss for a more comfortable hyperthermia treatment. By incorporating a synthetic breast tissue for a model study, the temperature rises were measured to be 3.3 °C and 1.9 °C at 5 mm and 15 mm depths, respectively, after 15 min of heating (input power of 1 W). This suggests that the textile-based approach could be an effective solution for comfortable and long-term applications of breast hyperthermia therapy.

Current microwave hyperthermia research focuses primarily on aperture antennas (waveguides) (Van Der Zee et al. 2010). Aperture antennas are relatively easy to operate, but because they are rigid and bulky, patient discomfort is a significant challenge for a continuous treatment (Curto et al. 2018;Mukai 2016). Thus, development of a more comfortable, personal microwave device is highly encouraged for widespread acceptance of hyperthermia treatment (Curto et al. 2018;Mukai 2016).
In order to reduce the antenna size for a wearable device, various compact antenna structures have been proposed and tested in literature. For instance, (Montecchia 1992); (Singh 2015) and  examined the performance of planar antennas built on printed circuit boards (PCBs). Although these antennas were small and designed specifically for on-body hyperthermia applications, they would not conform to the body contour owing to the highly rigid nature of PCB materials.
More wearable forms of hyperthermia devices were discussed by ,  and (Curto et al. 2018) with patch antennas embedded in water bolus systems. The proposed antennas had water boluses that ease antenna impedance matching, reduce the applicator size and cool both the antenna and the skin surface . It was also claimed that a flared ground plane improved the radiation efficiency and the conformability of the applicator, which led to satisfactory heating .
There are, however, notable drawbacks to have a water bolus (Curto et al. 2018. Water is a highly lossy dielectric at microwave frequencies (Skaar 1988) and thus brings about low efficiency. Consequently, the input power requirement for the antenna was as high as 50 W (Curto et al. 2018). Moreover, an additional power is required to circulate water for cooling. Because the power source needs to be physically carried by patients to run the wearable systems, this high-power requirement is a critical disadvantage. These large, heavy and non-breathable applicators with water boluses would deteriorate the wearability.
The objective of this research was to present a wearable conformal patch antenna fabricated with textile materials for a more comfortable breast hyperthermia therapy. The proposed textile antenna followed a hemispheric contour and consists of copper-plated woven polyester for a radiating patch and a ground plane and a woven polyester fabric for a substrate and a padding. The textile antenna is lightweight, breathable and flexible, and therefore, can be inherently embedded into clothing such as a brassiere or a slip (Ghahremani Honarvar and Latifi 2017;Ramasamy et al. 2018). Moreover, due to the porous nature of textiles, the proposed antenna benefits from a dielectric permittivity close to air, and that results in a high gain and a high efficiency (Mukai et al. 2018aSalvado et al. 2012). Thanks to this, the power requirement could stay considerably low.

Methods
As a model study, a hemispheric breast phantom was incorporated in this research, and the dimensions of the textile antenna were optimized using a 3D full-wave EM simulation software. The reflection coefficient of the antenna and the distribution of specific absorption rate (SAR) in the breast phantom were also calculated by using the EM software for theoretical analysis. A transient thermal simulator examined the heat distribution under thermal diffusion. After prototyping, the performance of the textile antenna was experimentally evaluated to verify its feasibility as a wearable hyperthermia applicator.

Breast model
In microwave breast hyperthermia, the expected treatment effect varies depending on biophysical (perfusion and dielectric) and dimensional (breast size and tumour location) characteristics of treating breasts (Curto et al. 2018). However, these breast characteristics differ significantly from individual to individual, and therefore, a microwave applicator will need to be customized for each patient in clinics (Curto et al. 2018;Pang and Lee 2016). As a model study, a simple homogeneous breast phantom was employed in this work. The breast phantom (density of 900 kg·m −3 (Ito et al. 2001)) had a hemispheric shape with a radius of 50 mm and its thermal and dielectric properties are given in Table 1.

Antenna design
The patch antenna was designed with a conductive patch and a ground plane mounted on a dielectric substrate (Fig. 1). The patch and ground plane were made of a 0.08 mm-thick copper-plated woven polyester fabric (Fig. 2a). Being in a plain weave, this fabric had a basis weight of 80 g/m 2 and a highly dense (360-by-280 threads-per-inch) structure. This helped to attain a low sheet resistance (sheet resistance of 0.03 Ω/sq., Less EMF Inc.) required for the microwave application. The

Properties Values References
Specific heat (J g −1 K −1 ) 3.63 Ito et al. (2001) Thermal conductivity (W m −1 K −1 ) 0.55 Ito et al. (2001) Density ( dielectric substrate was a 0.32 mm-thick woven polyester fabric with a basis weight of 192 g/m 2 (Fig. 2b). Woven with 64-tex warp and 57-tex weft yarns in a relatively loose plain-weave construction (45 ends per inch and 33 picks per inch), this fabric had a high porosity (56%) and hence a low complex permittivity (1.55 − 0.014 j). This would be suitable to produce an antenna with a high gain and a wide bandwidth (Mukai et al. 2018a, b). A padding layer made of the same dielectric polyester fabric was also included in the antenna design to protect the conductive patch from corrosive environments such as skin secretions, and a 50 Ω SMA connector was incorporated to feed a 1 W input power. In this antenna geometry, the microwave radiation is achieved by the fringing fields that are generated by the non-uniform distribution of electric fields between the patch and the ground plane (Balanis 2012(Balanis , 2016. The conformal ground plane serves as a back reflector, and the maximum radiation is directed in the normal direction from the center of the patch (Bancroft 2009). This could be ideal for hyperthermia applications since the focused microwave beam would deposit a concentrated microwave (thermal) energy in the target tumor .
The dimensions of the antenna (Fig. 1) were optimized by using a 3D EM simulator with a finite element solver (Ansys HFSS ® ). The patch sheet was designed to be 44 mm by 28 mm, and the optimized thicknesses of the substrate and padding were 3.20 mm and 1.92 mm, respectively.

Theoretical evaluation
In order to theoretically evaluate the EM performance of the proposed hyperthermia applicator, 3D full-wave EM simulations were performed by using Ansys HFSS ® . The reflection coefficient of the designed antenna and the SAR distribution within the tissue were computed with an input power of 1 W. Since the phantom had a high thermal conductivity (0.55 W m −1 K −1 ), 3D transient thermal analysis was also performed by using the Ansys Transient Thermal simulator. By establishing an EMthermal link (Sabbagh 2011) in the Ansys Workbench software (Fig. 3), the SAR distribution obtained in the EM simulation was fed into the thermal analysis as a heat source. The temperature increments in the breast tissue were monitored for 15 min of continuous microwave irradiation.

Preparation of breast phantom
The breast phantom was prepared as follows (Ito et al. 2001). 10.46 g of agar (LIVING JIN Co., Ltd.), 3.76 g of sodium chloride (Sigma-Aldrich Corporation), 0.20 g of sodium azide (Sigma-Aldrich Corporation) were completely dissolved in 337.50 g of deionized water, followed by heating on a stove. Once the solution reached the boiling point, heat was immediately removed and 8.44 g of TX151 (Oil Center Research Inc.) and 33.75 g of polyethylene powder (Sigma-Aldrich Corporation) were sprinkled and mixed uniformly. The liquid was then poured into a 3D-printed breast mold and then cooled at room temperature for 24 h to form the hemispheric shape (Fig. 4). The phantom was maintained sealed by a plastic wrap at room condition for preservation.

Antenna fabrication
In order to fabricate the textile antenna in the hemispheric shape (Fig. 2), the conventional flat-pattern method was employed. By following (Thyssen 1997), the optimized  antenna patterns for the patch, ground plane, substrate and padding layers were created (Fig. 5). For the substrate and the padding, 10 and 6 layers of the polyester were respectively stitched together by using a lockstitch machine (ISO 301) to attain the designed thicknesses of 3.20 mm and 1.92 mm, respectively. The patch, ground plane, substrate and padding layers were cut by a laser cutter (Universal Laser Systems VLS 6.60) into the intended dimensions (Fig. 5) after immersed in water to prevent burn marks (Haagenson et al. 2015;Mukai et al. 2018b). Next, the patch and ground plane were mounted on the substrate with a polyamide fusible web (Bostik Inc.). A 50 Ω panel-mount SMA connector (Amphenol Corp.) was soldered to the patch and ground plane, and the connection was reinforced with a silver conductive epoxy for better electrical conduction (MG Chemicals Ltd.). The antenna prototype is shown in Fig. 6a.

Experimental evaluation
A calibrated VNA (Agilent E5071C ENA Series Network Analyzer) was used to validate the antenna design. Based on the two-port scattering (S)-parameter network (Huang and Boyle 2008), the reflection coefficient (S 11 ) of the antenna prototype was measured in the frequency range of 1-6 GHz to evaluate the impedance matching.  For the evaluation of heating performance, the antenna prototype was placed over the breast phantom and housed inside the plastic template (Fig. 6b) to ensure dimensional stability throughout the measurements. This holder was incorporated merely for the purpose of accurate characterization and was not a part of the actual antenna system. A 20-dB gain block (Sireen 2.4 GHz 1 W High Gain Amplifier Module) was connected to a computer-controlled 10 dBm signal generator (Windfreak Technologies SynthNV RF Signal Generator) and supplied a power of 30 dBm (1 W) at 2.45 GHz to the antenna (Fig. 7). The temperature increment in the breast phantom was monitored by a thermometer (Fluke 52 II Dual Probe Digital Thermometer) at the two tissue locations (Fig. 1), in 5 mm and 15 mm depths for 15 min at the room temperature (~ 22 °C). Figure 8 shows the simulated and measured reflection coefficients of the designed antenna. The impedance of the fabricated antenna was well-matched to the source impedance of 50 Ω at the operating frequency of 2.45 GHz. The measured S 11 was − 14.7 dB which was comparable to the simulated value of − 17.3 dB. Also, accounting of the electromagnetically permeable (and hence low dielectric loss) nature of the porous textile fabric, the antenna showed a wide fractional bandwidth of 13.8% (measured) and 20.4% (calculated) (|S 11 |≤ -10 dB). This successful impedance matching and the wide bandwidth at the target frequency (2.45 GHz) confirms that the proposed design of the wearable antenna is valid.

SAR distribution
The SAR distribution in the breast phantom was successfully calculated and is shown in Fig. 9. It was found that there was a major microwave absorption (> 100 W/kg) near the center of the patch. The SAR as a function of the tissue depth along the normal direction of the center of the patch is given in Fig. 10. This plot illustrates that there was no significant energy absorbed in the deeper (> 30 mm) tissue, which indicates that the proposed system would be only effective to superficial tumors.

Temperature increment
The simulated temperature distribution after 0, 300 and 900 s are given in Fig. 11. A significant thermal diffusion was observed in the tissue due to its high thermal conductivity (0.55 W m −1 K −1 ), which contributed to a wider spread of the heat. Figure 12 shows the calculated and measured temperature rises at the 5 mm and 15 mm depths. According to the calculation, the temperature was elevated by 6.7 °C and 3.5 °C, respectively, after 900 s of heating (Table 2). However, the measured temperature increments were significantly less than the simulated values-the measured temperature rises in the 5 mm and 15 mm locations were 3.3 °C and 1.9 °C, respectively (Table 2).
There are several possible reasons for these differences between the simulated and measured temperature rises. The first reason would be the heat leakage into the ambient air from the tissue, which was not considered in the calculations. Because the air flow promotes thermal dissipation, this might have contributed to the difference. Another attribute could be the limited fabrication accuracy of the antenna sample. In the simulation model, the antenna was assumed to perfectly conform to the hemispheric contour of the breast phantom. In prototyping, however, the textile antenna did not perfectly conform to the hemispheric surface since it was constructed from flat patterns of the multi-layered woven fabric. As the radiation performance of patch antennas is reported to be highly sensitive to its geometry (Balanis 2016;Mukai et al. 2018b;Xu et al. 2017), this imperfection could have lowered the radiation efficiency. Also, the cold solder joint of the conductive fabric and the SMA connector could have led to a radiofrequency (RF) power loss at the insertion. Although the silver epoxy adhesive was applied over the

Conclusions
A textile conformal antenna was designed and fabricated with woven polyester fabrics, and the EM and heating characteristics were simulated and measured using a tissueequivalent phantom. From the simulations, it was found that the effective heating is limited to a superficial region (< 30 mm) due to the high EM dissipation of the tissue. According to the measurements with the antenna sample, up to 3.3 °C and 1.9 °C temperature elevation was observed at 5 mm and 15 mm depths, respectively, after 15 min of heating. Considering the normal body temperature of 37° and the target range of 39-45 °C in hyperthermia treatment (Pang and Lee 2016), the reachable temperatures by the proposed textile antenna would be in the lower side. Although this was not experimentally elaborated in the current work, a higher temperature is achievable by increasing the input power. It is in theory that the temperature rise is nearly linear to the increase of the input power (Biagi et al. 2011).
Based on these observations, this paper concludes that satisfactory heating would be possible for a hyperthermia treatment of superficial tumors with a polyester-based textile antenna. Being flexible, lightweight and low dielectric loss owing to the porous  textile structure, the presented textile-based approach sheds light on a more patientfriendly and a long-term solution in cancer treatment. It needs to be noted, however, that there are a couple of limitations in this work. Firstly, the presented textile antenna was produced from flat patterns and hence they had limited conformity with curved surfaces. As a potential alternative, advanced fabrication methods, such as 3D knitting (Narayanan et al. 2018), could be incorporated. Enabling an intrinsic construction of curved surfaces in a single step, the 3D knitting option would further improve the wearability while reducing the production-related workload.
Secondly, the results obtained in this research are not intended to directly apply to the case of human tissues. The actual tissues have a number of internal components such as fat and blood vessels with various dielectric properties, and the energy deposition and the temperature elevation could vary depending on such biophysical and dimensional factors. In this context, future work is also recommended with a more realistic phantom to further investigate the potential use of the textile-based wearable microwave applicator.