Ell, James
(2025)
Optical fibre interface pressure sensors for compression therapy.
PhD thesis, University of Nottingham.
Abstract
Compression therapy is a cornerstone treatment for venous leg ulcers (VLUs): a variant of chronic wound with a growing burden on healthcare institutions around the world. The therapy involves applying various types of compression devices: stockings, bandages, or hosiery—applying a therapeutic pressure gradient to the afflicted limb. Medical compression of this type helps to improve venous return, reduce swelling, and promote wound healing in venous disorders of the lower limbs. For optimal treatment outcomes, it is important to ensure a correct pressure ‘dose’, known as the interface pressure or sub-bandage pressure. Empirical levels of interface pressure for optimal treatment outcomes remain clinically uncertain. As such, there has been recent interest in refining the accuracy and reliability of medical pressure sensors to inform compression therapy practice.
The landscape of available pressure transducers is diverse and extensive. Recent advances in flexible electronics have resulted in renewed interest in the potential for pressure measurement in developing medical and healthcare wearables. With electronic and pneumatic approaches dominating the market. Notable examples, such as the Microlab PicoPress, are considered ‘gold standard’ reference devices. More recently, there has been interest in developing optical fibre-based instrumentation to inform and supplement compression therapy treatments. Fibre optic sensing offers distinct advantages over traditional electronic transducers in the field of healthcare wearables: chemical stability, biocompatibility, small size and operating footprint, immunity to electromagnetic interference, distributed sensing capabilities, and increased bandwidth for multi-parameter measurements.
This thesis presents two interface pressure sensor configurations that incorporate fibre Bragg grating (FBG) technology. FBGs are a commonly reported optical fibre sensing technique for physical measurements across a variety of engineering disciplines and show promise for sensing applications in the medical sector. The proposed techniques: encapsulated fibre optic pressure sensors (EFOPSs) and cantilever fibre optic pressure sensors (CFOPSs), have been investigated in-silico through finite element analysis (FEA), and through experimental work in the laboratory. The simulations were used to inform and supplement the design of each sensor and involved the refinement, parameterisation, and extension of a previously reported FEA framework using ABAQUS CAE.
Both sensors were designed with reference to a clinical consensus statement on the ‘ideal’ properties for a sub-bandage pressure monitor. Techniques for encapsulation and mechanical protection of the sensors were optimized to enhance performance and reliability in the compression therapy environment, improving upon previous examples in the literature. Laboratory investigations established the pressure-sensing capabilities of both techniques, validating their sensitivity, hysteresis, and repeatability, over the clinically anticipated pressure range of compression therapy, in both normal and abnormal scenarios: 0-100 mmHg.
For the EFOPS—where the FBG was encapsulated in a host material—two sensors were designed and evaluated, fabricated using Norland Optical Adhesive 68 (NOA68) as the encapsulant media. The maximum average sensitivity of this configuration was 2.36 pm/mmHg, a value that was commensurate with the highest in-class sensitivity of competing EFOPS designs from the wider literature. A minimum hysteresis error (as a percentage of the full-span) of less than 15 %, and a minimum repeatability error of 1.68 % were determined for these sensors.
The CFOPSs—where the FBGs were mechanically bonded to a cantilever structure—were fabricated using a modular housing design and additive manufacturing techniques. Stereolithography (SLA), using a biocompatible light-cure resin (Formlabs BioMed® Black), was used for the cantilever and housing structure. The highest average sensitivity of this arrangement was 36.64 pm/mmHg, with a minimum hysteresis error of 6.16 % and a minimum repeatability error of less than 1 %. This increased sensitivity facilitates a move towards low cost, small size, and portable interrogation devices, supporting a transition towards a fully wearable platform.
The EFOPS and the CFOPS were evaluated through a bandaging study, where a two-component compression system was applied to the lower limbs of ten healthy adult volunteers by an experienced wound care nurse. Two optical fibre sensors and a reference transducer, the Microlab PicoPress, were interrogated and compared using Bland-Altman analysis over repeated bandaging sessions. Although absolute agreement with the PicoPress® was not achieved, the fibre optic sensors were able to resolve more granular pressure fluctuations, detecting changes in bandage relaxation and pressure during various static positions and dynamic movements.
The results of this work highlight the potential for optical fibre interface pressure monitors in the context of healthcare wearables. With proposals to advance the designs into portable devices with commercial potential. The proposed sensors have pressure sensitivities comparable to—and in some cases exceeding—the sensitivities reported by the nearest competitors. The CFOPSs show the highest-pressure sensitivity of any reported fibre optic sub-bandage pressure sensor for this application to date. Despite their merits, further improvements to the packaging of the sensors, plug-and-play accessibility, miniaturisation, and a move towards a full or partial fabric integration are recommended. Otherwise, practical suitability for the clinical environment and the adoption of these sensors will be limited. Avenues for implementing these recommendations have been suggested, such as a transition towards multicore fibre Bragg gratings (MFBGs).
Despite these limitations, several potential applications for the proposed sensors have been identified: a training aid for wound care clinicians on phantom or surrogate limbs; a tool for ensuring manufacturing compliance with developing regulatory standards, and a method of quantitatively evaluating compression devices in a laboratory context, thereby informing routine clinical practice. Accounting for the highlighted recommendations for design improvements and potential for further miniaturization and refinement, optical fibre sub-bandage pressure sensors of this type (EFOPSs and CFOPSs) have the potential to support continuing efforts to improve wound healing outcomes for patients and benefit the activities of wound care professionals in the healthcare sector.
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