Controlled Measurement Setup for Ultra-Wideband Dielectric Modeling of Muscle Tissue in 20–45 °C Temperature Range

In order to design electromagnetic applicators for diagnostic and therapeutic applications, an adequate dielectric tissue model is required. In addition, tissue temperature will heavily influence the dielectric properties and the dielectric model should, thus, be extended to incorporate this temperature dependence. Thus, this work has a dual purpose. Given the influence of temperature, dehydration, and probe-to-tissue contact pressure on dielectric measurements, this work will initially present the first setup to actively control and monitor the temperature of the sample, the dehydration rate of the investigated sample, and the applied probe-to-tissue contact pressure. Secondly, this work measured the dielectric properties of porcine muscle in the 0.5–40 GHz frequency range for temperatures from 20 °C to 45 °C. Following measurements, a single-pole Cole–Cole model is presented, in which the five Cole–Cole parameters (ϵ∞, σs, Δϵ, τ, and α) are given by a first order polynomial as function of tissue temperature. The dielectric model closely agrees with the limited dielectric models known in literature for muscle tissue at 37 °C, which makes it suited for the design of in vivo applicators. Furthermore, the dielectric data at 41–45 °C is of great importance for the design of hyperthermia applicators

Controlled Measurement Setup for Ultra-Wideband Dielectric Modeling of Muscle Tissue in 20–45 °C Temperature Range

In order to design electromagnetic applicators for diagnostic and therapeutic applications, an adequate dielectric tissue model is required. In addition, tissue temperature will heavily influence the dielectric properties and the dielectric model should, thus, be extended to incorporate this temperature dependence. Thus, this work has a dual purpose. Given the influence of temperature, dehydration, and probe-to-tissue contact pressure on dielectric measurements, this work will initially present the first setup to actively control and monitor the temperature of the sample, the dehydration rate of the investigated sample, and the applied probe-to-tissue contact pressure. Secondly, this work measured the dielectric properties of porcine muscle in the 0.5–40 GHz frequency range for temperatures from 20 °C to 45 °C. Following measurements, a single-pole Cole–Cole model is presented, in which the five Cole–Cole parameters (ϵ∞, σs, Δϵ, τ, and α) are given by a first order polynomial as function of tissue temperature. The dielectric model closely agrees with the limited dielectric models known in literature for muscle tissue at 37 °C, which makes it suited for the design of in vivo applicators. Furthermore, the dielectric data at 41–45 °C is of great importance for the design of hyperthermia applicators

Complementary Split-Ring Resonator with Improved Dielectric Spatial Resolution

status: accepte

Effect of Open-Ended Coaxial Probe-to-Tissue Contact Pressure on Dielectric Measurements

Open-ended coaxial probes are widely used to gather dielectric properties of biological tissues. Due to the lack of an agreed data acquisition protocol, several environmental conditions can cause inaccuracies when comparing dielectric data. In this work, the effect of a different measurement probe-to-tissue contact pressure was monitored in the frequency range from 0.5 to 20 GHz. Therefore, we constructed a controlled lifting platform with an integrated pressure sensor to exert a constant pressure on the tissue sample during the dielectric measurement. In the pressure range from 7.74 kPa to 77.4 kPa, we observed a linear correlation of − 0.31 ± 0.09 % and − 0.32 ± 0.14 % per kPa for, respectively, the relative real and imaginary complex permittivity. These values are statistically significant compared with the reported measurement uncertainty. Following the literature in different biology-related disciplines regarding pressure-induced variability in measurements, we hypothesize that these changes originate from squeezing out the interstitial and extracellular fluid. This process locally increases the concentration of membranes, cellular organelles, and proteins in the sensed volume. Finally, we suggest moving towards a standardized probe-to-tissue contact pressure, since the literature has already demonstrated that reprobing at the same pressure can produce repeatable data within a 1% uncertainty interval

Effect of Open-Ended Coaxial Probe-to-Tissue Contact Pressure on Dielectric Measurements

Open-ended coaxial probes are widely used to gather dielectric properties of biological tissues. Due to the lack of an agreed data acquisition protocol, several environmental conditions can cause inaccuracies when comparing dielectric data. In this work, the effect of a different measurement probe-to-tissue contact pressure was monitored in the frequency range from 0.5 to 20 GHz. Therefore, we constructed a controlled lifting platform with an integrated pressure sensor to exert a constant pressure on the tissue sample during the dielectric measurement. In the pressure range from 7.74 kPa to 77.4 kPa, we observed a linear correlation of −0.31±0.09 % and −0.32±0.14 % per kPa for, respectively, the relative real and imaginary complex permittivity. These values are statistically significant compared with the reported measurement uncertainty. Following the literature in different biology-related disciplines regarding pressure-induced variability in measurements, we hypothesize that these changes originate from squeezing out the interstitial and extracellular fluid. This process locally increases the concentration of membranes, cellular organelles, and proteins in the sensed volume. Finally, we suggest moving towards a standardized probe-to-tissue contact pressure, since the literature has already demonstrated that reprobing at the same pressure can produce repeatable data within a 1% uncertainty interval.status: publishe

Complementary Split-Ring Resonator for Microwave Heating of µL Volumes in Microwells in Continuous Microfluidics

This work presents the design and evaluation of a planar device for microwave heating of liquids in continuous microfluidics (CMF) made in polydimethylsiloxane (PDMS). It deals with volumes in the µL range, which are of high interest and relevance to biologists and chemists. The planar heater in this work is conceived around a complementary split-ring resonator (CSRR) topology that offers a desired electric field direction to—and interaction with—liquids in a microwell. The designed device on a 0.25 mm thick Rogers RO4350B substrate operates at around 2.5 GHz, while a CMF channel and a 2.45 µL microwell are manufactured in PDMS using the casting process. The evaluation of the performance of the designed heater is conducted using a fluorescent dye, Rhodamine B, dissolved in deionized water. Heating measurements are carried out using 1 W of power and the designed device achieves a temperature of 47 °C on a sample volume of 2.45 µL after 20 s of heating. Based on the achieved results, the CSRR topology has a large potential in microwave heating, in addition to the already demonstrated potential in microwave dielectric sensing, all proving the multifunctionality and reusability of single planar microwave-microfluidic devices

An Interdigital Capacitor for Microwave Heating at 25 GHz and Wideband Dielectric Sensing of nL Volumes in Continuous Microfluidics

This paper proposes a miniature microwave-microfluidic chip based on continuous microfluidics and a miniature interdigital capacitor (IDC). The novel chip consists of three individually accessible heaters, three platinum temperature sensors and two liquid cooling and mixing zones. The IDC is designed to achieve localized, fast and uniform heating of nanoliter volumes flowing through the microfluidic channel. The heating performance of the IDC located on the novel chip was evaluated using a fluorescent dye (Rhodamine B) diluted in demineralized water on a novel microwave-optical-fluidic (MOF) measurement setup. The MOF setup allows simultaneous microwave excitation of the IDC by means of a custom-made printed circuit board (connected to microwave equipment) placed in a top stage of a microscope, manipulation of liquid flowing through the channel located over the IDC with a pump and optical inspection of the same liquid flowing over the IDC using a fast camera, a light source and the microscope. The designed IDC brings a liquid volume of around 1.2 nL from room temperature to 100 °C in 21 ms with 1.58 W at 25 GHz. Next to the heating capability, the designed IDC can dielectrically sense the flowing liquid. Liquid sensing was evaluated on different concentration of water-isopropanol mixtures, and a reflection coefficient magnitude change of 6 dB was recorded around 8.1 GHz, while the minimum of the reflection coefficient magnitude shifted in the same frequency range for 60 MHz.status: publishe

A Microwave Platform for Reliable and Instant Interconnecting Combined with Microwave-Microfluidic Interdigital Capacitor Chips for Sensing Applications

This work presents a novel platform conceived as an interconnect box (ICB) that brings high-frequency signals from microwave instruments to consumable lab-on-a-chip devices. The ICB can be connected to instruments with a standard coaxial connector and to consumable chips by introducing a spring-levered interface with elastomer conductive pins. With the spring-system, microwave-microfluidic chips can be mounted reliably on the setup in a couple of seconds. The high-frequency interface within the ICB is protected from the environment by an enclosure having a single slit for mounting the chip. The stability and repeatability of the contact between the ICB and inserted consumable chips are investigated to prove the reliability of the proposed ICB. Given the rapid interconnecting of chips using the proposed ICB, five different interdigital capacitor (IDC) designs having the same sensing area were investigated for dielectric permittivity extraction of liquids. The designed IDCs, embedded in a polydimethylsiloxane (PDMS) channel, were fabricated with a lift-off gold patterning technology on a quartz substrate. Water-Isopropanol (IPA) mixtures with different volume fractions were flushed through the channel over IDCs and sensed based on the measured reflection coefficients. Dielectric permittivity was extracted using permittivity extraction techniques, and fitted permittivity data shows good agreement with literature from 100 to 25 GHz.status: publishe

An Interdigital Capacitor for Microwave Heating at 25 GHz and Wideband Dielectric Sensing of nL Volumes in Continuous Microfluidics

This paper proposes a miniature microwave-microfluidic chip based on continuous microfluidics and a miniature interdigital capacitor (IDC). The novel chip consists of three individually accessible heaters, three platinum temperature sensors and two liquid cooling and mixing zones. The IDC is designed to achieve localized, fast and uniform heating of nanoliter volumes flowing through the microfluidic channel. The heating performance of the IDC located on the novel chip was evaluated using a fluorescent dye (Rhodamine B) diluted in demineralized water on a novel microwave-optical-fluidic (MOF) measurement setup. The MOF setup allows simultaneous microwave excitation of the IDC by means of a custom-made printed circuit board (connected to microwave equipment) placed in a top stage of a microscope, manipulation of liquid flowing through the channel located over the IDC with a pump and optical inspection of the same liquid flowing over the IDC using a fast camera, a light source and the microscope. The designed IDC brings a liquid volume of around 1.2 nL from room temperature to 100 °C in 21 ms with 1.58 W at 25 GHz. Next to the heating capability, the designed IDC can dielectrically sense the flowing liquid. Liquid sensing was evaluated on different concentration of water-isopropanol mixtures, and a reflection coefficient magnitude change of 6 dB was recorded around 8.1 GHz, while the minimum of the reflection coefficient magnitude shifted in the same frequency range for 60 MHz