Gertjan Maenhout

Cancer therapies are constantly evolving. Currently, heating tumor tissue is becoming more accessible as a stand-alone method or in combination with other therapies. Due to its multiple advantages over other heating mechanisms, microwave hyperthermia has recently gained a lot of traction. In this work, we present a complementary split-ring resonator that is simultaneously excited in two independent frequency bands. With a high-power signal, the applicator is excited and heats the tissue-under-test up to 50°C with an average heating rate of 0.72°C per second. Furthermore, we present a dielectric temperature control system using the same applicator for microwave hyperthermia applications, which currently still requires an additional thermometry system. By exciting the applicator with a low-power signal, we can constantly monitor its resonant frequency. This resonant frequency depends on the tissue properties, which in turn are temperature-dependent. In the temperature range from 20–50°C, a positive correlation between the temperature and resonant frequency was established.Clinical relevance - Exploiting the dual-band behavior of the complementary split-ring resonator to heat the tissue-under-test while dielectrically monitoring its temperature, creates new possibilities towards a theranostic, non-invasive microwave hyperthermia applicator.

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.

Due to the worldwide increasing incidence of colorectal cancer, more and more focus is shifting towards methods for the early detection of polyps in the colon. In this work, we present a technique and a design to use dielectric spectroscopy with microwaves to assist in the early detection of colorectal cancer. A flexible, tubular design with six different segments was manufactured on a thin polyimide substrate. This design can be slid over an endoscope and used as a cap-assisted endoscopic attachment. Three different complementary split ring resonators were implemented into this design. When the design is deployed, the segments expand such that the sensors directly contact the colon tissue, even for varying colon diameters. The sensor behavior proved to be stable at different dilation levels and a sensitivity of up to 20.6 MHz per a.u. of permittivity was achieved over a broad permittivity range. Additionally, frequency multiplexing of different sensors was implemented to increase the sensor count while restricting the number of interconnections. Furthermore, several biological tissue samples, with a close resemblance to colorectal tissue, were measured with this design and were correctly identified. At last, a low-cost measurement implementation, that tracks the insertion loss of the sensors, was proposed to replace the expensive microwave lab equipment and a strong correlation was achieved between both measurement setups.

A nondestructive complementary split ring resonator (CSRR), working in the 0.5 to 6 GHz frequency range, is presented in this work. Three CSRR designs, on the same 5 mm by 5 mm footprint, with different internal dimensions were designed, fabricated, and measured. The results demonstrate a large shift in resonant frequency, up to 2.544 GHz, over a broad relative permittivity range from 1 to 80, and a large penetration depth, up to 7 mm, into the material under test. Secondly, this work presents a deconvolution-based method to increase the spatial resolution of the presented sensors compared to other sensing structures and algorithms. A conformal mapping function relating the resonant frequency to the relative permittivity was fitted through measurements and the point spread function (PSF) was derived from a known dielectric permittivity distribution. Experimental results demonstrated that the extracted PSF was able to reconstruct the dielectric distribution in six different scenarios. Additionally, a spatial resolution of 2 mm has been established according to the Rayleigh criterion, and objects smaller than the size of the sensor have been reconstructed as well. Objects with a different permittivity values can be distinguished as well. The reconstructed images of the objects have a good signal-to-clutter ratio up to 12.0%, which results from a smooth recovered background, and no spatial offset was present between the original and reconstructed objects.

Microfluidic devices present new opportunities for a wide range of applicationsin which the temperature control of nanoliter droplets is required. Due to the inherent size of the employed miniature volumes, several challenges exist, related to temperature measurements. In this work, we present a non-contact and label-free dielectric-based temperature measurement technique. A capacitive sensor transduces the temperature-dependent dielectric properties into a capacitive value. With a post-processing tuned matching network, the reflection coefficient of the capacitive sensor is minimized at a specific frequency, directly related to the sample temperature. The system is calibrated via a water-only calibration method, using two calibration values. A measurement accuracy of ± 0.24 °C is achieved, with an offset of −0.19 °C, over a temperature range from 20 °C to 55 °C. The potential for simultaneous use of the proposed sensor as a microwave heater, gives us a dual-purpose device that minimizes the footprint of the chip, and thus its cost, while also reducing the number of electrical connections to the chip.

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.

The lack of a standardized operating protocol for dielectric measurements of biological tissues makes it difficult to objectively compare reported dielectric properties since these properties are affected by multiple environmental confounders. This work investigates the effect of dehydration on dielectric properties as a function of time at different temperatures. Additionally, it proposes a dehydration mitigating technique. The dielectric properties of porcine liver samples were measured during a 35 minute measurement interval at different temperatures. A first set of six experiments was conducted in standard laboratory conditions. A second set of two experiments was conducted with a modified measurement setup with reduced air flow. Under standard conditions, relative changes in the dielectric properties up to 9% were observed. With the modified setup, the relative change was halved compared to standard conditions. This reduction indicates that the environmental conditions have a considerable influence and that modifying these conditions extends the stability of the dielectric properties over time. For several applications relying on the difference in dielectric properties between healthy and malignant tissue, an error of 9% in measured data is on the same order as the effects of interest. The expected difference in dielectric contrast could be obscured due to dehydration.