Development of a Dielectric Heating System for Selective Thermal Targeting of Liver Fluke Regions in Cirrhinus microlepis

Abstract

Liver fluke infections, especially those induced by Opisthorchis viverrini, pose considerable health and economic difficulties in aquaculture, particularly in Southeast Asia. Traditional approaches for parasite elimination, including chemical treatments and freezing, exhibit constraints regarding efficacy, environmental sustainability, and practicality. This research investigates an improved dielectric heating system utilizing a 2.45 GHz horn antenna for the selective thermal targeting of parasite-associated regions in Cirrhinus microlepis (small-scale mud carp). The dielectric characteristics of fish tissues, encompassing scales, skin, and muscle, were analyzed utilizing an open-ended coaxial probe technique. Simulation and experimental evaluations were performed to improve energy absorption, heating uniformity, and a particular absorption rate to enable precise thermal localization while preserving the integrity of fish tissue. The findings demonstrate that dielectric heating can specifically elevate the temperature of fish scales, where parasites predominantly inhabit, to levels beyond 70 degrees Celsius, while reducing thermal impact on the underlying muscle tissue. The application of a salt coating on fish scales markedly increased their dielectric loss, exceeding that of muscle tissue, thus enhancing selective heating efficiency and supporting targeted thermal treatment. The ideal distance from the antenna to the sample was established as ranging from 6 to 9 cm, ensuring a balance between energy efficiency and homogeneous heating. This work illustrates the efficacy of dielectric heating as a novel and non-chemical approach for thermal management of parasite-prone tissues in aquaculture, providing a sustainable and viable substitute for traditional treatments.

1. Introduction

Liver fluke infections caused by trematodes such as Fasciola hepatica and F. gigantica pose significant health risks to humans and animals, especially in regions reliant on livestock and aquaculture [1,2]. These parasites can cause liver damage, secondary infections, and economic losses due to decreased productivity and treatment expenses. Considerable research has focused on mitigating fish-borne infections, particularly involving the liver fluke Opisthorchis viverrini and its complex life cycle [3,4].

Adult O. viverrini inhabit the bile ducts of definitive hosts, releasing eggs into freshwater via feces [5]. These eggs are consumed by Bithynia snails, where they develop through larval stages into cercariae [6]. The cercariae then infect freshwater fish, forming metacercariae, which can infect humans through the consumption of raw or undercooked fish. Once ingested, the metacercariae migrate to the bile ducts and mature into adult flukes, persisting in humans for up to 26 years [7].

Effective eradication of metacercariae relies on thermal or freezing methods, with heating at 70 °C for 5 min or higher temperatures being most efficient. Freezing between −20 °C and −60 °C for two hours yields similar results, although different freezing temperatures cause varying degrees of cyst wall damage [8,9]. However, conventional heating methods like boiling and steaming may compromise fish texture, prompting exploration of alternative dielectric heating techniques. Dielectric methods—including RF, microwave, capacitive, dielectric barrier discharge (DBD), and terahertz (THz) heating—use electromagnetic radiation to deliver uniform internal heat. These approaches aim to eliminate parasites while preserving fish quality.

Table 1. Comparison of dielectric heating methods for fish.

Method	Primary Applications	Advantages	Challenges
Radio Frequency (RF) Heating	Parasite removal, drying	Uniform heating, deep penetration	Slower than microwave heating
Microwave Heating	Parasite removal, drying, sterilization, cooking	Fast heating, efficient for wet tissues	Risk of non-uniform heating
Capacitive Heating	Deep tissue heating, disinfection	Controlled heating, suitable for large fish	Limited industrial adoption
Dielectric Barrier Discharge (DBD) Heating	Surface sterilization, freshness enhancement	Chemical-free, non-thermal	Limited penetration depth
Terahertz (THz)	Surface parasite removal, non-invasive tests	Selective heating, preserves texture	Expensive, still experimental

summarizes their key applications and trade-offs in fish processing.

Studies on fish from Thailand’s Pasak Cholasid Reservoir show clear localization patterns of metacercariae: Haplorchis pumilio was found predominantly in fish scales (95.29%), while Centrocestus formosanus was mainly present in gills (72.28%) [10]. Similarly, cod fish exhibited high parasite density on the skin and outer muscle layers, with minimal presence in deeper tissues [9]. These parasitic diseases pose compounded socio-economic burdens, particularly in Southeast Asia, where Opisthorchis viverrini is endemic. Infections are linked to traditional dietary practices and poorly managed aquaculture, especially in Northeast Thailand (Isaan), a region with the highest prevalence of liver fluke infection and high rates of cholangiocarcinoma [11,12,13,14]. Aquaculture in this area suffers reduced productivity and marketability due to these parasites [12].

Cirrhinus microlepis, a key freshwater species in the Mekong region, plays crucial ecological and economic roles by supporting biodiversity and small-scale fisheries [15,16,17]. However, it faces multiple threats—including parasite infection, overfishing, and habitat degradation—necessitating conservation and improved farming practices. Current liver fluke control methods face limitations. Chemical anthelmintics like triclabendazole are losing efficacy due to resistance and rising environmental concerns [18]. Management approaches and vaccination strategies also face challenges in scalability and effectiveness [19,20]. These limitations highlight the need for innovative technologies that are effective, sustainable, and scalable.

Technological innovations, including nanotechnology, genetic selection, and dielectric heating, are reshaping parasite control in aquaculture [21,22,23]. Dielectric heating offers targeted energy delivery without chemical residues. It enables uniform internal heating while preserving tissue quality [24,25] and can be tailored to fish tissue properties for optimized energy application [26,27]. Integration with real-time spectroscopic monitoring enhances processing control and safety [28]. The method has also proven effective for microbial control and fish preservation [29,30].

Horn antennas play a critical role in modern microwave systems due to their high directivity, efficiency, and structural simplicity. Their ability to focus electromagnetic energy makes them essential in applications such as communication, imaging, and material processing [31]. In aquaculture and biomedical contexts, they enable precise microwave energy delivery. For example, dual-polarized and wide-band horn antennas enhance imaging resolution and penetration in diagnostics [32], while 3D-printed designs improve energy distribution in heating and deicing processes [33]. Recent innovations, including quad-ridged structures and dielectric lens integration, have expanded their use to ultra-wideband, terahertz, and millimeter-wave applications [34,35,36]. Horn antennas are also vital in systems requiring low cross-polarization, like satellite communications and radio astronomy [37]. Their scalability and adaptability make them ideal for industrial uses. In aquaculture, horn antennas are increasingly integrated into dielectric heating systems to focus energy on parasite-rich regions, such as those affected by liver flukes. Their precision improves thermal efficiency and minimizes collateral damage. Advanced designs, including 3D-printed and lens-enhanced antennas, support large-scale deployments and can be tailored to specific environments [33,34]. Combined with real-time temperature monitoring, they enhance process control and support sustainable, energy-efficient aquaculture operations [26].

This study focuses on designing a horn antenna operating at 2.45 GHz to develop an advanced dielectric heating system for the selective thermal targeting of liver fluke-infected regions (Opisthorchis viverrini) predominantly found in the scales of Cirrhinus microlepis. The specific dielectric properties of the fish measured across its scales, skin, and muscle tissues are integral to optimizing the antenna’s performance and energy application. Prior research indicates that liver flukes exhibit a high affinity for the scale regions, necessitating localized heating interventions to thermally impact the parasite zones while preserving the structural and biological integrity of the fish. The dielectric heating system aims to deliver controlled microwave energy, selectively raising temperatures in the scales to levels potentially harmful to liver flukes, without adversely affecting the underlying skin or muscle tissues. Infrared thermal imaging cameras are employed to monitor real-time temperature distribution, ensuring that the targeted regions achieve sufficient thermal exposure while maintaining the overall health and quality of the fish. This integrated approach combines innovative antenna design with precise thermal monitoring to support target parasite management in aquaculture. The use of a horn antenna at 2.45 GHz enhances the uniformity and directivity of microwave energy, making it a suitable solution for large-scale applications. Additionally, the incorporation of infrared imaging provides a non-invasive method to validate heating patterns and safeguard against overexposure, aligning with the goals of environmental sustainability and aquaculture efficiency. Unlike conventional boiling or steaming methods, this approach enables rapid, localized heating of parasite-rich regions within seconds, minimizing total energy usage and preserving surrounding tissues. This highlights the potential of dielectric heating as a scalable, high-efficiency alternative for parasite control in aquaculture systems.

2. Materials and Methods

2.1. Sample Preparation

Specimens of Cirrhinus microlepis (small-scale mud carp) were obtained as deceased samples from Siam Makro Public Company Limited, Nakhon Ratchasima Province, Thailand, as shown in Figure 1a. The fish were freshly prepared, stored on ice during transport, and delivered to the laboratory within one hour of purchase to preserve tissue quality and integrity. All samples were sourced in compliance with local regulations for aquaculture products. In the laboratory, the fish were processed under aseptic conditions. Scales were carefully removed from the lateral region of the fish, an area with a higher prevalence of Opisthorchis viverrini infections, as depicted in Figure 1b. Skin samples were subsequently extracted from adjacent areas, as shown in Figure 1c, and muscle tissues were obtained from the dorsal fillet to ensure representation of different tissue types, as illustrated in Figure 1d. Each tissue type was rinsed with sterile physiological saline solution to remove debris and surface contaminants. Samples were then segmented into two subsets: one designated for dielectric property measurements and the other for heating experiments. For dielectric analysis, which is detailed in Section 2.2, muscle and skin tissue sections were trimmed to uniform dimensions (approximately 2 × 2 × 0.5 cm) and stored at 4 °C in sterile containers to maintain their natural properties. For dielectric heating experiments, the samples were arranged on a non-conductive surface, mimicking their anatomical arrangement to ensure experimental relevance. All sample preparation steps were conducted within 4 h of procurement to ensure optimal freshness and reliability for experimental analysis.

2.2. Measurement of Dielectric Properties

The dielectric properties of Cirrhinus microlepis tissues, specifically, scales, skin, and muscle, were measured using an open-ended coaxial probe method in conjunction with a vector network analyzer (VNA). This approach is widely recognized for its accuracy in characterizing the dielectric properties of biological tissues, as it enables precise measurements over a broad frequency range. The experimental setup consisted of a VNA (Keysight Technologies Inc., Santa Rosa, CA, USA) and an open-ended coaxial probe connected via a Keysight N1501A Dielectric Probe Kit, while data acquisition and analysis were conducted using Keysight N1500A software prior to conducting the measurements, such as the measurement of the properties of fertilizers, jasmine rice (Oryza sativa), soil, and fungi [38,39,40,41,42]. The system was calibrated using air, distilled water, and a standard dielectric reference block. Calibration ensured the accuracy of the measurements across the frequency range of 0.5–3 GHz, with a particular focus on 2.45 GHz, the frequency used for dielectric heating optimization. The calibration steps included verifying the reflection coefficient (S11) for each reference material to minimize systematic errors. Freshly prepared tissue samples from the scales, skin, and muscle of Cirrhinus microlepis, as described in the sample preparation section, were used to obtain region-specific dielectric data. Each sample was placed on a non-conductive surface to maintain structural integrity and avoid external interference [43,44,45]. The coaxial probe was gently positioned to ensure firm contact with the sample surface without deforming the tissue, as illustrated in Figure 2. Measurements were conducted at room temperature (25 ± 1 °C) to simulate experimental conditions, and each sample was measured five times to ensure consistency and reproducibility of the data.

The dielectric constant (${\epsilon }^{\prime }$) and loss factor (${\epsilon }^{″}$) of biological tissues, such as the scales, skin, and muscle of Cirrhinus microlepis, are derived from the reflection coefficient (S₁₁) measured using an open-ended coaxial probe connected to a vector network analyzer (VNA). The reflection coefficient, which represents the ratio of reflected to incident electromagnetic waves at the interface between the probe and the sample, is a complex value expressed as follows:

$$\Gamma ={\displaystyle \frac{Z_s-Z_o}{Z_s+Z_o}}$$

(1)

where $Z_s$ is the complex impedance of the sample, and $Z_o$ is the characteristic impedance of the coaxial system, typically 50 Ω. The dielectric properties of the sample are inferred from the impedance relationship and the calibration of the measurement system using standard materials such as air, distilled water, and a dielectric block. The complex permittivity $\left(\epsilon \right)$, which describes how the material interacts with the electromagnetic field, is expressed as $\epsilon ={\epsilon }^{\prime }+j{\epsilon }^{″}$, where ${\epsilon }^{\prime }$ represents the ability of the material to store electric energy, and ${\epsilon }^{″}$ represents the energy dissipated as heat. These properties are calculated from the impedance of the sample, given by $Z_s=1/j\omega \epsilon$, where $\omega =2\pi f$ is the angular frequency of the electromagnetic wave. In the case of Cirrhinus microlepis, high ${\epsilon }^{″}$ values were consistently observed in the scale region, indicating enhanced dielectric loss and thereby a higher microwave energy absorption potential compared to the muscle region. This aligns with the selective heating observed during system testing. To accurately measure the reflection coefficient, the VNA system is calibrated using standard materials such as air, distilled water, and a dielectric block, ensuring precise computation of Γ. Once the reflection coefficient is measured, the complex permittivity is derived using numerical inversion of the reflection coefficient model for the coaxial probe. Dielectric constant and dielectric loss are obtained as the real and imaginary components of $\epsilon$, respectively. These values are critical for understanding the material’s response to electromagnetic fields, particularly in applications involving dielectric heating. The penetration depth (PD) is another key parameter in dielectric heating, representing the depth at which the wave intensity decreases to approximately 36.8% of its original value. This is calculated using the following:

$$PD={\displaystyle \frac{{\lambda }_0}{2\pi {\epsilon }^{″}\sqrt{{\epsilon }^{\prime }}}}$$

(2)

where ${\lambda }_0$ is the wavelength in a vacuum. In this study, the PD values indicated that electromagnetic energy predominantly penetrated shallow layers (e.g., scales and skin), with minimal diffusion into muscle tissue. This supported the design choice of using a 2.45 GHz horn antenna and a 9 cm antenna-to-sample distance to maximize energy targeting of parasite-prone tissues while protecting edible muscle layers.

2.3. Simulation Setup for Dielectric Heating

The computational model is developed to analyze the interaction between electromagnetic waves and biological tissue, represented by a fish model. A horn antenna is used to emit microwave energy at 900 W feed power, operating at a frequency of 2.45 GHz, and is strategically positioned to optimize energy delivery to the fish sample. As illustrated in Figure 3, the distance between the antenna and the fish is a key parameter that significantly affects electromagnetic field distribution and the resulting dielectric heating patterns. The model considers four distances, 3 cm, 6 cm, 9 cm, and 12 cm, to evaluate the effect of energy absorption at varying proximities.

In CST Studio Suite, the simulation setup involves defining the horn antenna geometry and the fish tissue model. The antenna is designed with precise aperture size and flare angle to ensure focused microwave energy delivery. The fish model consists of three distinct layers—scales, skin, and muscle—each assigned specific dielectric properties, including the dielectric constant and loss factor, which are crucial for simulating realistic electromagnetic interactions. The model accounts for the different thicknesses of each layer, with the scales at 1 mm, the skin at 0.5 mm, and the muscle at 18.5 mm, as shown in the figure. To simulate an open environment, Perfectly Matched Layers (PML) are applied as boundary conditions to eliminate reflections at the domain edges. The horn antenna is excited using a waveguide feed in the TE10 mode, ensuring a well-controlled electromagnetic wave emission. A fine computational mesh is generated around both the fish model and the antenna, especially in regions with high field intensity variations, to achieve accurate results.

2.4. Experiment Setup for Dielectric Heating

The experimental setup was designed to evaluate the performance of the dielectric heating system utilizing a 2.45 GHz horn antenna for delivering controlled microwave energy to fish tissue samples containing liver flukes. The experiment was conducted within an anechoic chamber to minimize electromagnetic interference and ensure precise energy distribution, as shown in Figure 4a. The fish tissue sample was placed on a porcelain plate and positioned precisely beneath the horn antenna. The distance between the antenna and the sample was carefully measured using a tape measure to maintain consistency in energy delivery and optimize heating efficiency, as depicted in Figure 4b. Based on prior computational simulations, the optimal antenna-to-sample distance was determined to be approximately 6–9 cm to achieve efficient microwave absorption while preserving tissue integrity. Figure 4c presents a side view of the setup, providing a clear perspective on the alignment between the horn antenna and the fish tissue sample.

During the experiment, the horn antenna was activated to emit electromagnetic waves at 2.45 GHz, targeting the dielectric properties of the fish scales and underlying tissues. The energy distribution was designed to ensure uniform heating while preventing localized overheating. Infrared thermal imaging technology (Keysight U5857A, Keysight Technologies Inc., Santa Rosa, CA, USA) was employed for real-time monitoring of the temperature distribution across the sample, as illustrated in Figure 4d. This setup facilitated precise tracking of energy absorption and temperature variations in the fish tissue during microwave exposure.

This experimental setup provided a controlled and repeatable framework for assessing the feasibility of dielectric heating as a non-chemical approach for liver fluke eradication in aquaculture applications.

2.5. Energy Absorption and Heat Generation Measurements

The process of heat generation in biological tissues, such as fish muscle and scales, during microwave exposure is primarily governed by how well the tissue absorbs and converts electromagnetic energy into heat. This is quantified by a parameter known as the Specific Absorption Rate (SAR), which measures how much energy is absorbed per unit mass of tissue. The SAR is calculated using the following equation:

$$\mathrm{SAR}={\displaystyle \frac{1}{V}}{\displaystyle {\int }_{sample}\frac{\sigma \left(r\right)\left|E\left(r\right)\right|}{\rho \left(r\right)}}dr={\displaystyle \frac{\sigma {\left|E\right|}^2}{\rho }}$$

(3)

where $\sigma$ is the tissue’s electrical conductivity (S/m), ${\left|E\right|}^2$ is the root mean square (RMS) of the electric field strength (V/m), and $\rho$ is the tissue density (kg/m³). This equation shows that tissues with higher conductivity and stronger field exposure absorb more energy and therefore generate more heat. In the case of Cirrhinus microlepis, the fish scales exhibited higher conductivity than muscle, contributing to their faster heating during treatment. SAR directly correlates the tissue’s ability to absorb energy to its electrical and material properties. According to internationally recognized standards, such as those set by the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the maximum allowable SAR value for localized exposure in humans is 2 W/kg averaged over 10 g of tissue in the context of public exposure to electromagnetic fields [46]. Although these standards are not directly applicable to fish tissue, they provide a general framework for assessing safe exposure levels during dielectric heating.

Another important parameter is power density (P), which represents how much energy is dissipated per unit volume of tissue. It is calculated as follows:

$$P=\sigma \left|E^2\right|=2\pi f{\epsilon }_0{\epsilon }^{″}\left|E^2\right|$$

(4)

Here, f is the frequency of the applied field, ${\epsilon }_0$ is the permittivity of free space, and ${\epsilon }^{″}$ is the dielectric loss of the tissue. This expression emphasizes the role of tissue composition, particularly ${\epsilon }^{″}$, in determining how efficiently energy is absorbed and converted into heat. Tissues with high water content and ionic conductivity, such as fish muscle, typically have higher ε″ values, making them responsive to microwave energy.

To estimate how tissue temperature increases during heating, the following simplified bioheat equation is used:

$$\Delta T={\displaystyle \frac{P\Delta t}{\rho C}}={\displaystyle \frac{SAR\times \Delta t}{C}}$$

(5)

where C is the tissue’s specific heat capacity (J/kg·°C), ΔT is the change in temperature (°C), and Δt is the exposure time. This relationship links the absorbed energy to the tissue’s thermal properties and mass.

This relationship connects absorbed energy to the tissue’s thermal response over time. For instance, in this study, the scale regions reached 70 °C within approximately 75 s, while the muscle remained below 50 °C, demonstrating thermal selectivity.

In addition, applying salt to the fish scales increased their ionic conductivity, further enhancing energy absorption in these parasite-prone regions. This approach proved effective in improving heating efficiency and minimizing unwanted thermal effects on edible tissue layers.

2.6. Statistical Analysis

All experiments were performed in duplicate. Statistical analyses were conducted using Microsoft Excel, applying one-way analysis of variance (ANOVA) to ascertain significant differences among the means of several sample groups. A significance level of 0.05 (p < 0.05) was consistently utilized in the analysis. A factorial test was employed for comparisons among many groups. Results are presented as mean values with their respective standard deviations (mean ± SD).

3. Results and Discussion

3.1. Dielectric Properties of Cirrhinus microlepis Tissues

The dielectric properties of Cirrhinus microlepis tissues, including scales, skin, and muscle, were analyzed in terms of their dielectric constant (${\epsilon }^{\prime }$) and dielectric loss (${\epsilon }^{″}$) over a frequency range of 1.0–3.0 GHz. The results, as depicted in Figure 5a,b, reveal distinct frequency-dependent trends and highlight the differences among tissue types as well as the effect of salt addition on these properties. The dielectric constant of all tissue types, as shown in Figure 5a, decreases with increasing frequency, a characteristic behavior of biological tissues. This trend arises from the reduced ability of dipolar molecules in the tissues to align with an alternating electromagnetic field at higher frequencies [3], as described by Equation (1). The decline is more pronounced at lower frequencies, while, beyond 2 GHz, the values tend to stabilize. Among the three tissue types, fish scales (represented by the red line with upward-facing triangles) exhibit the highest dielectric constant across the frequency range. This behavior is likely due to the unique structure and composition of scales, which may include mineral content that enhances their dielectric response under electromagnetic fields. In contrast, fish skin (represented by the black line with upward-facing triangles) displays slightly lower dielectric constant values than scales. This suggests that the skin tissue has a lower polarization capacity, potentially due to differences in its water content or structural rigidity. Fish muscle (represented by the blue line with upward-facing triangles) has the lowest dielectric constant across the frequency range, likely because of its lower water content or reduced ionic mobility, which limits its capacity for polarization when exposed to an alternating electromagnetic field.

Similarly, the dielectric loss of all tissue types, as depicted in Figure 5b, decreases with increasing frequency. This trend is typical of biological tissues, as ionic conduction and dipolar relaxation mechanisms diminish at higher frequencies. The most significant decline occurs within the lower frequency range (1.0–2.0 GHz), with the values stabilizing beyond 2 GHz. Among the tissues, fish muscle exhibits the highest dielectric loss across the entire frequency range (blue line with upward-facing triangles), indicating its higher water content and ionic conductivity, which contribute to greater energy dissipation under electromagnetic fields. This aligns with the specific absorption rate (SAR) in Equation (3), which explains the relationship between tissue conductivity and energy absorption efficiency.

For dielectric heating applications, dielectric loss (${\epsilon }^{″}$) is the primary parameter as it directly determines the energy dissipated as heat within the material when exposed to an electromagnetic field. A higher dielectric loss indicates greater heating efficiency, making it critical for evaluating a material’s suitability. In this study, fish muscle, with the highest dielectric loss, is the most effective tissue for dielectric heating due to its ability to absorb and convert electromagnetic energy into heat. While the dielectric constant (${\epsilon }^{\prime }$) reflects a material’s capacity to store electromagnetic energy, it does not contribute directly to heat generation, making dielectric loss the focus for ensuring effective and efficient heating. Based on these findings, fish muscle will heat and cook the fastest among the tissues, followed by fish scales and fish skin, as it exhibits the highest capacity for energy dissipation. Based on the measured results, dielectric heating may not effectively target liver flukes, which are concentrated around the fish scales. Since muscle exhibits the highest dielectric loss, it heats and cooks faster than the scales, potentially leaving parasites untreated. To overcome this limitation, fish samples were coated with salt and left for 10 min to enhance the dielectric properties of the scales by increasing ionic conductivity. This adjustment aimed to promote more uniform heating, ensuring sufficient energy dissipation in the scales to eliminate parasites before the fish muscle is fully cooked. The increase in dielectric loss of fish scales led to a decrease in penetration depth (PD), as described in Equation (2), allowing for improved energy concentration in the scales.

After the salt coating, Figure 5b illustrates the dielectric loss (${\epsilon }^{″}$) of fish tissues, including muscle, skin, and scales, across the frequency range of 1.0–3.0 GHz, represented by downward-facing triangles. The results reveal significant changes in the dielectric loss of scales and skin following the salt treatment, while the muscle tissue shows relatively minor variation. These findings highlight the impact of salt on enhancing the ionic conductivity of the tissues and their capacity for energy dissipation under electromagnetic fields. Fish scales exhibit the highest dielectric loss, as shown by the red line with downward-facing triangles. Before the salt treatment, the scales had the lowest dielectric loss among the three tissue types, reflecting their relatively low ionic conductivity and limited capacity for energy dissipation. The significant increase in dielectric loss after the salt coating is attributed to the penetration of salt ions, which enhances the ionic conductivity of the scales, enabling them to dissipate energy more efficiently under electromagnetic fields. At 2.45 GHz, as presented in

Table 2. Dielectric properties of Cirrhinus microlepis tissues at 2.45 GHz.

Components of Fish	Dielectric Constant $\left({\mathit{\epsilon }}^{\prime }\right)$		Dielectric Loss $\left({\mathit{\epsilon }}^{″}\right)$
	Fish	Fish After Salt Coating	Fish	Fish After Salt Coating
Fish scale	32.77	56.87	9.59	17.64
Fish skin	32.62	34.31	10.18	15.92
Fish muscle	25.87	40.88	14.22	16.20

, the industrially relevant frequency for dielectric heating, the salt-induced increases in dielectric loss for fish scales and skin significantly improve the energy dissipation balance across all tissue types. Specifically, the dielectric loss of fish scales increases from 9.59 to 17.64 after the salt application, surpassing the muscle tissue’s initial value of 14.22 and becoming comparable to its post-salt value of 16.20. This substantial enhancement ensures that the scales, which previously lagged in energy absorption, now receive sufficient thermal energy to target parasites such as liver flukes effectively. Similarly, the dielectric loss of fish skin increases from 10.18 to 15.92, promoting more efficient energy dissipation.

The dielectric constant also increases across all tissues following salt treatment, with fish scales showing the most notable rise from 32.77 to 56.87. This enhancement reflects the improved polarization capacity of the scales, enabling them to absorb and store more electromagnetic energy. Fish muscle and skin also demonstrate increases in dielectric constant, from 25.87 to 40.88 and from 32.62 to 34.31, respectively, further supporting improved heating performance. These improvements in both dielectric loss and dielectric constant contribute to more uniform heating across all tissue types. The salt coating ensures that scales and skin, which previously absorbed less energy, now achieve sufficient thermal energy for effective parasite elimination. At the same time, the risk of overcooking the muscle is minimized, as the dielectric loss of the muscle remains within a balanced range. This balanced energy dissipation ensures a more efficient and effective dielectric heating process for Cirrhinus microlepis tissues.

3.2. Electromagnetic Simulation and Analysis for Dielectric Heating Applications

This section explores the electromagnetic simulation and analysis critical to optimizing dielectric heating applications for biological tissues. The investigations include the frequency-dependent behavior of the horn antenna, the distribution of power intensity across fish tissues, and the evaluation of Specific Absorption Rate (SAR) to assess energy absorption and safety. Each analysis aims to ensure effective energy delivery to target tissues while maintaining compliance with safety standards.

The Frequency Response of Horn Antenna subsection examines the antenna’s performance across different frequencies, focusing on its radiation pattern, reflection coefficient, and directivity. Following this, the Frequency Response of Power Intensity delves into the simulated power distribution across fish tissues at varying distances, emphasizing its impact on heating uniformity. Finally, the Simulated Specific Absorption Rate (SAR) analysis evaluates SAR values under ICNIRP safety guidelines to identify configurations that balance efficient energy transfer and safety. Together, these analyses provide a comprehensive understanding of the parameters influencing the dielectric heating process.

3.2.1. Frequency Response of Horn Antenna

The frequency response and performance characteristics of the horn antenna, as shown in Figure 6, highlight its suitability for dielectric heating applications. The 3D radiation pattern with horn antenna structure shown in Figure 6a demonstrates a highly directional radiation pattern, with concentrated electromagnetic energy along the z-axis. This efficient focusing of the electric field ensures effective energy delivery to the target dielectric material while minimizing energy losses with a 12 dB antenna gain. The uniformity of the field distribution suggests that the antenna design is optimized for directing electromagnetic waves, making it particularly effective for biological tissue heating at the specific frequency of 2.45 GHz. The S-parameter analysis in Figure 6b further supports the antenna’s performance by evaluating its return loss (S11 parameter). The graph identifies three resonant frequencies: 1.88 GHz, 2.45 GHz, and 2.97 GHz. At 2.45 GHz, the return loss reaches a minimum value of −34.98 dB, indicating optimal impedance matching and minimal power reflection. This signifies that the antenna is highly efficient at transmitting electromagnetic energy at 2.45 GHz, the primary frequency for dielectric heating. This behavior follows the reflection coefficient Equation (1), where a lower reflection coefficient (Γ) results in improved energy transmission into the target material. In comparison, the return losses at 1.88 GHz and 2.97 GHz are −10 dB, reflecting acceptable performance but reduced efficiency relative to 2.45 GHz. These variations in power reflection influence the power density (P), as described by Equation (4), which determines how effectively the electromagnetic energy is delivered to the target tissues. The 2D radiation pattern of the horn antenna, depicted in Figure 6c, provides additional insights into its performance. The E-plane exhibits a strong and highly directional main lobe, while the H-plane shows a broader energy distribution. This anisotropic radiation pattern highlights the antenna’s ability to concentrate electromagnetic waves in the desired direction, ensuring focused energy delivery to the target. The well-defined main lobe and reduced side lobes minimize energy dispersion and interference, enhancing the overall efficiency of the antenna in dielectric heating applications.

Since energy absorption is a function of the tissue’s dielectric properties and the intensity of the electric field, the power density Equation (4) and SAR Equation (3) provide a quantitative basis for evaluating how the antenna’s design impacts energy distribution within the biological tissues. In summary, the frequency response analysis of the horn antenna demonstrates its superior performance at 2.45 GHz, characterized by minimal return loss, a highly directional electric field distribution, and a focused radiation pattern. These attributes make the horn antenna an excellent choice for dielectric heating applications, ensuring efficient energy transfer, uniform heating, and effective performance for biological tissue treatment.

3.2.2. Frequency Response of Power Intensity

The frequency response of power intensity was analyzed through simulations comparing the cross-sectional power distributions in fish scales and fish muscle under a power density of 900 W/m³, as shown in Figure 7. The simulations were conducted at varying distances (3 cm, 6 cm, 9 cm, and 12 cm) between the horn antenna and the target tissues to evaluate how distance impacts the energy distribution within the tissues.

At 3 cm, fish scales exhibit a highly uniform power distribution with minimal localized variation. The red regions indicate consistent energy absorption, suggesting that shorter distances enable even energy distribution across the scales’ surface. In contrast, fish muscle at the same distance shows a more heterogeneous power distribution, characterized by distinct high-intensity zones. This localized energy absorption aligns with the power density Equation (4), which describes the relationship between electromagnetic energy distribution and tissue properties. This selective energy absorption in muscle is likely attributed to its higher water content and dielectric properties, which enhance localized heating. As the distance increases to 6 cm, the uniformity of power distribution in both fish scales and muscle begins to decline. In fish scales, energy absorption becomes concentrated in the central regions, while the peripheral areas show reduced intensity. Similarly, in fish muscle, the high-intensity zones diffuse, leading to a more distributed yet less focused energy pattern. This pattern is consistent with Equation (4), as increased distance leads to reduced energy density due to spatial dispersion of the electromagnetic field. At 9 cm, energy absorption in both fish scales and muscle continues to centralize, with peripheral regions experiencing a noticeable decrease in intensity. Fish scales display distinct localized heating zones, indicating a reduction in overall uniformity compared to shorter distances. Meanwhile, fish muscle retains concentrated regions but exhibits a more dispersed energy absorption pattern overall. The energy absorption trend at this distance suggests an optimal balance between power density and uniform distribution, preventing excessive heating in one region. At the farthest distance of 12 cm, power intensity in both fish scales and muscle significantly diminishes. The energy distribution in fish scales becomes fragmented, with irregular and less centralized intensity. In fish muscle, the power distribution becomes increasingly uniform, though at a lower intensity, as the energy dissipates more broadly with greater distance. This behavior can be explained using Equation (2), where PD is inversely related to the square root of the dielectric properties and distance. The further the energy source, the weaker the field strength and energy absorption.

These results emphasize the diminishing effectiveness of energy delivery as the distance between the horn antenna and the target tissues increases. Maintaining an optimal distance is critical to ensuring efficient energy transfer and effective heating. While fish scales achieve more uniform energy absorption at shorter distances, fish muscle localizes energy more effectively due to its dielectric properties. Based on the simulated power intensity distribution, 6 cm and 9 cm appear to be the most promising for practical applications. At 6 cm, energy absorption in fish scales remains highly concentrated in the central regions, providing sufficient intensity to target parasites such as liver flukes, while avoiding excessive energy delivery to the muscle, as predicted by Equation (4). At 9 cm, although the energy intensity reduces slightly, it remains sufficient for effective heating, with the added benefit of more evenly distributed energy in both scales and muscle. In contrast, distances of 3 cm and 12 cm are less suitable for dielectric heating applications. At 3 cm, although the energy absorption in fish scales is highly uniform, the overall intensity is excessively high. This increases the risk of overheating muscle tissue, which could lead to structural damage or reduced quality of the fish. Additionally, the extremely focused energy at such a short distance may not adequately reach parasites embedded deeper in the scales. This limitation aligns with the SAR Equation (3), which suggests that excessive energy concentration can lead to non-uniform tissue heating and potential thermal damage. At 12 cm, the energy intensity in both fish scales and muscle is significantly reduced and becomes fragmented in its distribution. This lower intensity compromises the efficiency of energy delivery, making it difficult to generate sufficient thermal energy to effectively target parasites or achieve uniform heating. This is consistent with Equation (2), as the greater distance reduces effective penetration and power absorption, making heating less effective. These findings underscore the importance of precise antenna positioning to balance energy intensity and distribution. Specifically, maintaining a distance within the range of 6–9 cm ensures optimal energy transfer, effective parasite elimination, and uniform heating, making it ideal for dielectric heating applications.

3.2.3. Simulated Specific Absorption Rate (SAR) Analysis

The simulated Specific Absorption Rate (SAR) values were analyzed using CST Studio Suite version 2024, with the highest SAR values recorded for fish scales, skin, and muscle at varying distances from the horn antenna. The analysis adheres to ICNIRP guidelines, ensuring compliance with electromagnetic exposure safety limits. Figure 8 illustrates the SAR distribution across tissue layers at distances of 3 cm, 6 cm, 9 cm, and 12 cm, respectively.

At 3 cm (Figure 8a), the SAR values peaked across all tissue types, with the maximum recorded SAR reaching 0.9473 W/kg. Fish scales experienced the highest SAR, closely followed by skin and muscle. The intense energy absorption at this distance indicates a high concentration of electromagnetic exposure, which, while effective for parasite eradication, also poses a risk of localized overheating, particularly in the scales and skin. This high SAR value is consistent with Equation (3), which describes how energy absorption in biological tissues depends on electrical conductivity, electric field intensity, and tissue density. The concentration of electromagnetic energy at short distances results in a higher SAR, increasing the likelihood of excessive heating. At 6 cm (Figure 8b), the SAR values moderated to 0.7454 W/kg, demonstrating a more uniform energy distribution across tissue layers. The reduced SAR suggests a safer and more controlled heating effect, ensuring sufficient energy delivery to the scales for parasite elimination while minimizing the risk of excessive heat buildup. This balance aligns with Equation (4), where optimal power density distribution contributes to controlled heating without excessive energy localization. This balance makes 6 cm an optimal configuration for dielectric heating applications. At 9 cm (Figure 8c), SAR values further decreased to 0.3819 W/kg, with scales still absorbing slightly more energy than skin and muscle. Although overheating risks are minimized at this distance, the lower SAR values indicate that energy absorption may no longer be sufficient for effective parasite inactivation, particularly for embedded parasites. The thermal distribution remains distinguishable, suggesting selective heating remains possible but with reduced intensity. At this distance, the decreasing SAR can be explained by Equation (2), which states that (PD) decreases as the dielectric properties of the tissue influence electromagnetic energy absorption. The lower SAR at this distance suggests that energy is dissipated more widely, reducing its concentration in target tissues. At 12 cm (Figure 8d), SAR values dropped significantly to 0.3459 W/kg, with energy distribution becoming less concentrated. While this distance remains within ICNIRP safety limits, the energy intensity may be too low for effective heating of fish scales, potentially limiting its application for deeply embedded parasites. The lower SAR at this distance indicates reduced microwave penetration efficiency, aligning with Equation (2), which describes how increased distance weakens energy concentration, leading to suboptimal heating.

Under ICNIRP guidelines, which allow a maximum SAR of 2 W/kg averaged over 10 g of tissue for public exposure, all SAR values at 6 cm, 9 cm, and 12 cm remain within safe limits, ensuring non-hazardous conditions. In contrast, SAR values at 3 cm approach levels that pose a risk of localized overheating, raising safety concerns. Based on these findings, maintaining a distance of at least 6 cm is recommended to ensure safe and effective dielectric heating while adhering to ICNIRP safety regulations. This recommendation aligns with the relationship between SAR and tissue properties described in Equation (3), ensuring sufficient energy absorption without surpassing safety thresholds.

3.3. Dielectric Heating Experiment

The dielectric heating experiment examined temperature profiles of fish scales and muscle over a 120 s period at various distances from the horn antenna. As shown in Figure 9, data from three independent trials (n = 3) reveal temperature differences influenced by tissue type and antenna distance, with standard deviation represented by error bars. The heating behavior corresponds with the power density Equation (4), indicating that energy absorption depends on electromagnetic field strength and tissue dielectric properties.

At shorter distances (3 cm and 6 cm), both scales and muscle tissues rapidly exceed 70 °C. While this facilitates parasite inactivation, it also leads to muscle heating above 60 °C, as evidenced in Figure 10, where dehydration and protein denaturation are observed. This reduces the thermal selectivity between tissue layers.

At 9 cm, however, selective heating is achieved. Scales reach 70 °C at approximately 75 s, while muscle temperatures remain around 44.0 °C. This temperature gap supports Equation (2), as electromagnetic energy is primarily absorbed in the outer scale layer, with limited penetration into deeper tissues. Thermal imaging in Figure 10 confirms the visible separation in thermal exposure between the two tissues. These findings illustrate the principle of selective heating, where distinct dielectric properties result in differential energy absorption and temperature profiles, aligning with Equation (4).

The experimental results are further illustrated in Figure 10, which visualizes the spatial thermal distribution and physical changes after heating. This figure provides visual validation of the system’s selectivity, demonstrating clear thermal gradients between fish scales and muscle tissues at different antenna distances. At 9 cm, the scales reached sufficient temperatures for parasite inactivation, while the muscle remained below the denaturation threshold. These visual observations reinforce the numerical findings from Figure 9 and support the practical application of dielectric heating for targeted treatment. At 12 cm, scales only reach 69.1 °C after 90 s, and muscle remains below 33 °C, indicating insufficient heating for reliable parasite inactivation. This diminished heating efficiency supports the prediction of reduced power density at longer distances from the antenna, as described by Equation (2).

Despite minor variations in tissue response—possibly due to differences in sample composition or slight antenna misalignments—the overall pattern remains clear. The 9 cm distance achieves optimal results: sufficient heating of parasite-laden scales while maintaining muscle temperature below thermal damage thresholds. In contrast, 12 cm proves ineffective, and 3 cm risks overheating the muscle. Thus, 6 cm and 9 cm represent promising configurations, with 9 cm providing the clearest thermal separation, making it particularly suitable for selective dielectric heating in fish tissue processing.

At 12 cm, the difference in heating behavior between tissues becomes even more pronounced. The fish scales reach only 69.1 °C after approximately 90 s, while the muscle tissue remains below 32.7 °C throughout the entire heating period. This suggests that electromagnetic energy penetration at this distance is significantly reduced, resulting in insufficient heating for effective parasite inactivation, particularly in deeper or more embedded regions. This observation aligns with Equation (2), which predicts a drop in power intensity and heating efficiency with increased distance from the antenna.

Thermal imaging data from Figure 10 further support this conclusion, showing uneven energy absorption and weaker thermal separation, thereby confirming that 12 cm is not suitable for reliable parasite control.

Despite minor variations in tissue temperature, attributed to natural differences in dielectric properties and slight inconsistencies in antenna placement, the overall trend is consistent across all tests. Shorter distances lead to rapid heating in both types, while longer distances delay or reduce heating, especially in muscle tissue.

From a practical perspective, the results indicate that 9 cm offers the best balance. At this range, the fish scales reliably reach 70 °C—sufficient for parasite inactivation—while the underlying muscle remains below 50 °C, preserving its raw texture. While 3 cm and 6 cm also achieve high scale temperatures, they cause excessive heating in the muscle. Conversely, 12 cm results in insufficient heating altogether. Therefore, 6 cm and 9 cm are the most effective distances for dielectric heating, with 9 cm providing the greatest selectivity between parasite-rich scales and heat-sensitive muscle tissue, making it ideal for targeted aquaculture applications.

3.4. Discussions

Traditional thermal methods, such as boiling, steaming, or freezing, can effectively eliminate fish-borne parasites, although they frequently compromise product quality. Sri-pan et al. [8] eliminated Opisthorchis viverrini by exposing fillets to 70 °C for 5 min or freezing at –20 °C for 2 h, but observed the consequent toughened meat and heightened energy expenditure. The study [9] achieved the instantaneous elimination of Cryptocotyle lingua by heating to 100 °C; however, overcooking rendered the fish unfit for consumption.

Dielectric-based techniques have been explored to decrease treatment times while preserving texture. Reference [28] indicated that an industrial microwave system raised muscle temperature to 50 °C after 10 min, facilitating roughly 90% parasite elimination; yet, uneven energy distribution led to overcooking in specific regions. Refs. [21,22] proposed the amalgamation of microwave and RF sources with real-time sensors to achieve uniform, adaptive heating; however, the associated complexity and expense hinder extensive application in aquaculture. These studies collectively reveal three persistent shortcomings: inadequate tissue-specific heating, limited energy efficiency, and operational complexity.

This study addresses these deficiencies by employing a horn-antenna dielectric heating system tuned for the dielectric properties of Cirrhinus microlepis. Positioning the antenna 9 cm from the sample enabled parasite-infested scales to reach 70 °C in approximately 75 s, although the underlying muscle stabilized at roughly 50 °C—a notable reduction from the 4–5 min boiling benchmark cited by [8]. The exact energy absorption decreased heat damage to muscle, preserving its unrefined texture and appearance. The approach is more energy efficient as it eliminates the need to heat a significant volume of water, aligning with the efficiency enhancements shown in [24,25]. In conclusion, our advanced dielectric-heating system offers rapid, tissue-targeted parasite inactivation with diminished energy usage and simplified hardware relative to earlier dielectric methods, thereby providing a scalable and sustainable solution for liver-fluke control in aquaculture.

4. Conclusions

This study presents an advanced dielectric heating system as a novel approach for the selective thermal targeting of parasite-associated regions in Cirrhinus microlepis using a 2.45 GHz horn antenna. The results demonstrate that dielectric heating can effectively elevate the temperature of fish scales, where parasites predominantly reside, to a lethal threshold while minimizing thermal effects on underlying tissues. The dielectric properties of fish tissues were characterized to optimize energy absorption and distribution, ensuring efficient and targeted heating. Through simulation and experimental validation, the optimal antenna-to-sample distance was determined to be between six and nine centimeters, allowing for uniform energy distribution and effective thermal localization. The findings indicate that dielectric heating provides a promising alternative to conventional parasite control methods, which often rely on chemical treatments or freezing. This technology offers a non-chemical, environmentally sustainable, and energy-efficient solution for aquaculture applications.

Future research should focus on scaling up this technology for commercial aquaculture operations and refining the system to enhance efficiency and adaptability across different fish species. Additionally, integrating real-time monitoring systems could further improve precision and ensure the safety and effectiveness of dielectric heating in targeted thermal applications for parasite management. The insights from this study contribute to the development of innovative and sustainable solutions for aquaculture health management, aligning with the increasing demand for safer and more efficient food production technologies.