Optical Fibre-Based Pulse Oximetry Sensor with Contact Force Detection

Abstract

A novel optical sensor probe combining monitoring of blood oxygen saturation (SpO2) with contact pressure is presented. This is beneficial as contact pressure is known to affect SpO2 measurement. The sensor consists of three plastic optical fibres (POF) used to deliver and collect light for pulse oximetry, and a fibre Bragg grating (FBG) sensor to measure contact pressure. All optical fibres are housed in a biocompatible epoxy patch which serves two purposes: (i) to reduce motion artefacts in the photoplethysmogram (PPG), and (ii) to transduce transverse loading into an axial strain in the FBG. Test results show that using a combination of pressure measuring FBG with a reference FBG, reliable results are possible with low hysteresis which are relatively immune to the effects of temperature. The sensor is used to measure the SpO2 of ten volunteers under different contact pressures with perfusion and skewness indices applied to assess the quality of the PPG. The study revealed that the contact force ranging from 5 to 15 kPa provides errors of <2%. The combined probe has the potential to improve the reliability of reflectance oximeters. In particular, in wearable technology, the probe should find use in optimising the fitting of garments incorporating this technology.

Smart textiles and wearable technologies – opportunities offered in the fight against pandemics in relation to current COVID-19 state

“…Rothmaier et al [200] have demonstrated such a system, using a cotton glove with woven POFs positioned at a fingertip of the glove. To improve the reliability of reflectance oximeters Liu et al [201] demonstrated a novel optical fiber sensor probe for simultaneously monitoring the photoplethysmogram and contact pressure to provide reliable blood oxygen saturation monitoring. This probe combined a reflectance pulse oximeter with a fiber Bragg grating contact pressure sensor and enables more comfort of the user because it can be used in loosely fitting garments and the measurements can only be recorded when appropriate pressure is applied.…”

Optical Fibre Sensor for Simultaneous Measurement of Capillary Refill Time and Contact Pressure

“…It has a Young's modulus (137.9 MPa) lower than that of the optical fibre, which increases the strain induced on the FBG under a transverse load [33,34]. FBGs are also sensitive to temperature, and so a second FBG (FBG2) is used for temperature compensation [25]. The FBG2 is located inside a stainless-steel tube (internal diameter = 0.31 mm) in order to isolate it from the effects of pressure [35,36].…”

“…The pressure sensor consists of two FBGs which are fabricated in the core of the same silica fibre (PS1250, Fibercore Ltd., Hampshire, UK). One FBG (FBG1) was covered by epoxy in order to increase the pressure sensitivity, whilst the other FBG (FBG2) was applied to compensate for the temperature effect on FBG1 [25].…”

“…Figure 5 shows the experimental setup. Each volunteer was asked to locate their index finger on the sensor probe, and to press the probe manually by applying an external pressure which was high enough (over 15 kPa) to evacuate blood from the capillary bed, as no pulsatile PPG component from the reflected light could be observed [25]. Figure 6a shows the pressure response of the integrated sensor with temperature compensation, which follows that of the applied pressure.…”

“…Therefore, simultaneous recording of the applied pressure and PPG can be beneficial in classifying normal and abnormal CRT. In a previous study, we proposed the incorporation of polymer optical fibre (POF) pulse oximetry with a fibre Bragg grating (FBG) pressure sensor to simultaneously monitor oxygen saturation and the contact force of the subject [25]. Compared to other optical fibre-based pressure sensors, such as the extrinsic Fabry-Perot interferometry, the FBG-based pressure sensor is more suitable for skin contact measurements as no additional diaphragm is required, making it more robust [26][27][28].…”