Characterization of skin diseases via radiofrequency dielectric spectroscopy: complementarity of reflection and transmission measurements for a robust approach

1. Context
1.1 Problem statement and challenges
Early detection and diagnosis of skin diseases represent a major challenge in dermatology. Traditional diagnostic methods rely either on visual examination—limited to the superficial properties of the skin—or on more in-depth but invasive and costly procedures. Dielectric spectroscopy at RF and microwave frequencies is emerging as an innovative technique for skin tissue analysis.
Unlike visual examination, this approach provides access to the properties of internal tissue layers. It offers a non-invasive alternative for analyzing skin tissue properties at adjustable depths, which could be exploited for early disease detection (before visible marks appear) or to refine a diagnosis.
This approach is based on the interaction of electromagnetic waves with biological tissues, providing precise information on their electrical properties (via conductivity σ) and dielectric properties (via complex permittivity ϵ), and by extension, their composition and structure. The RF and/or microwave probes to be developed must be specifically adapted to the characteristics of the targeted pathologies to ensure accurate and reliable measurements. They must also be optimized for reproducible results in real-world conditions. Furthermore, these probes must be easily integrated into portable devices to ensure they are practical for healthcare professionals in both hospital and private practice settings.

1.2 ESYCOM activities related to the subject

The research presented here follows several projects conducted within the ESYCOM laboratory by the supervising team:

Houssein Mariam’s Thesis (2020): Focused on developing a wideband microwave microfluidic sensor (Fig. 1). Excellent results were obtained for characterizing small liquid volumes, showing sensitivity to micro-beads similar in size to biological cells.

Resonant Structures: Explored for their higher sensitivity. Work by Joséphine Pichereau (2025) utilized planar differential resonators to analyze liquids or solids within the opening of a rectangular resonator. Using two resonators allows one to serve as a reference, making the structure robust against experimental variations. The difference in permittivity between samples is reflected in the shift of resonance frequencies and bandwidths. This sensor was later made frequency-tunable using varactor diodes by Houssem Rouached.

This differential approach was also adopted in the doctoral work of Zied Fritiss, which focused on the design, simulation, and fabrication of a probe intended for the early detection of skin cancer. To this end, we designed a probe that transmits the electromagnetic field from the VNA (Vector Network Analyzer) to the skin or tissue under analysis as efficiently as possible. This is a wideband probe operating within a frequency band of 1 GHz to 5 GHz. 

The analysis frequency band was chosen based on the significant variation in the relative permittivity between healthy skin (around 30) and melanoma (around 50-60) in this band, causing a change in the reflection of the electromagnetic signal at the tip of the probe, at the point of contact between the probe and the skin being examined. This effect was exploited in a differential structure comprising two identical probes connected to a coupler, so as to directly measure an indicator characteristic of a difference in skin properties at the two probed points. This original approach is particularly relevant for the intended application because it overcomes the difficulty inherent in in-vivo measurements related to the variability of individuals' physiological parameters: thus, by choosing an area of healthy skin on the patient as the reference medium, the reliability of the diagnosis is increased by adapting to the properties of the patient's skin in the area of interest and to his physiological state at the time of the test. However, the coupler used in the differential measurement device limits its operating frequency band, so that in our future work we will retain the principle of comparison with the patient's skin while moving away from the differential structure.

The thesis topic presented here is a continuation of the latter thesis and follows on from real-world tests that revealed necessary changes to the device developed.

​​​​​​​1.3 Hospital testing and limitations of the current measurement device

Interest has been identified in monitoring the development of a rare disease that causes tumors on the skin, known as neurofibromatosis. It would be particularly useful to offer a device capable of detecting the development of a neurofibroma at an early stage, i.e., before a visible lump appears on the skin.

Experimental tests carried out on a patient and discussions with dermatologists at Henri Mondor Hospital in Créteil have highlighted desirable improvements to the sensor to make it easier to use and ensure that the device is a reliable aid to medical diagnosis. These various aspects will be examined in this thesis.

2. Thesis Objectives

The proposed work will build on the knowledge acquired in Zied Fritiss' thesis in order to optimize the sensor according to the thickness of the skin to be probed and the desired spatial resolution (area to be examined). To this end, the influence of the measurement frequency band will be examined, as well as the geometry of the sensor. This work will require an analysis of the measurement results by comparing them with electromagnetic simulations.

In addition, two approaches are possible for probing the skin, consisting of measuring a reflection coefficient using a single probe or a transmission coefficient between two probes placed close to each other on the skin. To test this second configuration, a probe will need to be developed that can take transmission measurements between several points on the skin that are a few millimeters apart. These two approaches yield different results, and a study will be conducted to evaluate their respective accuracy in extracting parameters useful for diagnosis (such as tissue properties, tumor width and depth), depending on the geometry of the sensor. This work will require an understanding of the biological phenomena examined (such as the evolution of the shape and depth of a tumor over time), which will involve discussions with dermatologists.

In addition, to ensure satisfactory contact between the probe and the skin, the addition of a contact layer will be considered. Based on the properties of this layer, solutions (relating to different material choices) will be proposed and tested.

Finally, we would like to design a more compact system to make it easier to handle and more ergonomic to use, and propose an easy-to-follow measurement protocol that provides easily interpretable results. The measurement configuration proposed in Zied Fritiss' thesis, namely a differential device, has shown several weaknesses (complex use, reduced operating frequency band, high sensitivity to connectivity), making it necessary to use an alternative approach. Based on the experience gained during the experimental tests, the aim will therefore be to propose a more ergonomic device and to test the robustness of the measurement protocol, bearing in mind that skin properties vary depending on the individual and the area of the body being considered.

Once the measurement protocol has been established, the device will be made portable through the use of programmable circuits and a nano-VNA. 

3. Proposed work plan

The objective of this thesis is to design and optimize an RF sensor for biomedical applications, particularly the detection and monitoring of skin abnormalities. The device developed should provide reliable and robust diagnostic support, despite the variability of skin properties depending on the individual, his physiological condition, and the area of the body being examined, by enabling the determination of relevant biological parameters to be defined in collaboration with partner dermatologists.

Several sensor topologies will first be tested, both for reflection measurements from a single excitation source and for transmission measurements between several closely spaced sensors. In the latter case, the issue of coupling between probes will need to be studied and taken into account in the design and optimization of the device. This work will be based on measurements taken with different probes and on electromagnetic simulations to determine the area of skin being probed (extent and depth) and optimize the measurement configuration.

Following this initial study, which will enable geometric parameters to be set with regard to the skin abnormalities being investigated, optimized probes for reflection or transmission measurements will be designed and tested. It is conceivable that several probe topologies will be selected to address several types of pathologies or to access different quantities. Work will then be carried out on a measurement protocol that is both easy to apply and ensures reliable results. In particular, the robustness of the diagnosis will be tested on skin with different properties to take into account biological variability in real conditions. These tests will initially be carried out on samples that mimic the skin properties (called “phantom tissues”) and which will need to be manufactured using common, non-toxic products, before considering tests on humans.

The results obtained will highlight the limitations of the first designed probes and allow them to be modified accordingly. Other aspects will be examined in a second phase:

- First, attention will be paid to the contact between the probe and the skin to avoid the presence of an uncontrolled layer of air and to ensure good contact with different parts of the body. This will require studying the properties of different flexible materials and testing different possibilities through simulation before manufacturing a prototype.

- The ergonomics of the measuring device will then be optimized to allow precise positioning by facilitating the practitioner's movements. This work will be based on the experience gained from the tests carried out and on feedback from doctors.

- Finally, the measuring device will be made compact and portable for greater ease of use and reduced cost (by avoiding the use of an expensive VNA). This part of the work will require the selection and programming of circuits, in particular with the use of a nano-VNA.

Le laboratoire ESYCOM s’inscrit dans les domaines de l’ingénierie des systèmes de communication, des capteurs et des microsystèmes pour la ville, l’environnement et la personne.

Les thèmes abordés sont plus spécifiquement :

-   les antennes et propagation en milieux complexes, les composants photoniques - micro-ondes  ;

-   les microsystèmes pour l'analyse de l'environnement et la dépollution, pour la santé et l'interface avec le vivant ;

-   les micro-dispositifs de récupération d'énergie ambiante mécanique, thermique ou électromagnétique.

Ideally, the candidate will have completed a master's degree or engineering training in the field of electronics and radio frequencies. He or she will be proficient in using electromagnetic simulation software and performing measurements using a VNA. A strong interest in applications related to living organisms (biological environment) would be an advantage.