Nowdays, pressure ulcers are a major health problem. Several people suffer from them every year and many patients die from complications. Pressure ulcers are skin lesions of ischemic origin due to excessive or prolonged pressure between a hard surface and a bony prominence. The prevalence rate in hospitals is generally around 26% and reaches more than 66% in patients at risk. It is a common problem observed in spinal cord injuries, diabetics and people with reduced mobility in general. However, current methods of pressure ulcer prevention are mainly based on passive repositioning of patients according to a predetermined schedule. The annual cost of treatment is estimated at more than 11 billion US dollars. Faced with this crucial burden on health systems, the development of inexpensive active methods through new, robust devices that can withstand repeated washing cycles required in hospitals and that will allow precise, real-time and continuous monitoring of risk conditions before the onset of pressure ulcers is essential. Thus, the conception of highly sensitive devices, allowing not only to detect multiple pressure points simultaneously but also to obtain spatial information on the exact location where a critical pressure is applied at a given instant t, is necessary to effectively prevent pressure injuries.
In recent years, the ever-increasing need for monitoring the compression status of individuals in hospitals, homes and sports has motivated more research work. Various new devices such as pressure sensors using electronic sensing technologies, integrated into clothing, wheelchairs, mattresses, shoes etc, have been developed to respond the demand. Among others, we can mention : piezoresistive, capacitive, piezoelectric, triboelectric, electrochemical pressure sensors. However, it should be noted that the manufacturing of these types of sensors often requires the use of special materials to be able to generate the desired detection effect and also requires complex production processes, hence the high cost (especially for 2D mapping applications) of these devices on the market and therefore less accessible to the public. In addition, they are very fragile sensors due to the materials they are made of and they are generally susceptible to electromagnetic interference, requiring frequent calibrations to ensure their operation is optimal.
Optical technology which using light-matter interactions to detect entities based on the principles of absorption, scattering, transmission, reflection, emission, fluorescence etc., has motivated much recent research work, leading to the development of innovative devices. This technology has been deployed in biomedical and environmental sensing applications such as humidity, pressure, temperature, distance, material identification and composition measurements etc. Unlike electronic technologies, devices using optical sensing mechanism are accurate, faster, have high resolution and long life. Recent studies demonstrate that the development of modern smart wearable pressure sensors using optical sensing technology requires the use of soft, flexible and biocompatible materials. Elastomeric materials are a novel choice for the fabrication of these devices. These materials enable the fabrication of, among other things, elastomeric fibers capable of guiding light at short wavelengths, which is ideal especially for biomedical application. Polydimethylsiloxane (PDMS) which is part of the silicone family (polymerized siloxanes) is a better candidate due to its excellent thermomechanical and optical qualities. It is an easy to use and deform material, can be shaped by molding or by simple techniques such as soft lithography or UV photolithography.
In this thesis, we study the fabrication through a simple, inexpensive molding procedure and the characterization of PDMS optical waveguides with rectangular cross-section. The main objective is to develop a new quasi-distributed optical pressure sensor, by exploiting the phenomenon of microbubble diffusion of light come from the solid-core when a pressure is applied along the waveguide. We propose in this work, different designs of novel optical waveguides made from PDMS and functioning as pressure sensors. In the first phase of this thesis, we demonstrated the realization of a flexible, highly sensitive pressure sensor capable of detecting pressures below the blood capillary pressure estimated at 32 mmHg using PDMS and a photosensitive agent namely benzophenone. Then, in the second part of this thesis, we propose a new waveguide containing a porosity in its structure completely fabricated from PDMS. This porosity comes from the microbubbles constituting diffusion points and which are incorporated into the cladding of the waveguide during the manufacturing process. The characterization of this waveguide allowed a better understanding of the guiding of light scattered in the the porous cladding. We demonstrated the potential of this device to monitor the critical threshold pressure level beyond which an individual is at risk of developing pressure injuries if no pressure relief is provided in a timely manner. Finally, in the last part of this work, we propose a pressure location-aware sensor based on the structure of the waveguide studied in the second part. Using dyes (red and green), the top porous cladding of the new waveguide was colored locally. We demonstrated the operation of this waveguide as a pressure location-aware sensor through frustrated total internal reflection light guiding phenomenon. By applying pressure along the waveguide, the sensor response allows to know exactly the location where the pressure was applied. We believe these results are relevant to the development of the next generation of biomedical optical pressure sensing devices.
| Date | 17 Aug 2025 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Bora Ung (Supervisor) |
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