Contact between a medical device and tissue accounts for approximately a third of damage to soft tissue in hospitals. The use of sensors at the interface between the device and tissue can be used to maintain the optimum pressure and reduce such injuries. A sensorised endotracheal tube has been developed and is proposed as a method of reducing soft tissue damage and improving design of future devices. Optical fibre sensing is used to monitor both cuff contact pressure (via a fibre Bragg grating) and tissue perfusion (via reflectance photoplethysmography). The monitoring system developed has been used in an animal study and demonstrates reliable measurement of contact pressure and perfusion.
Optical fibre sensing is a platform technology for applications in biomedical and environmental monitoring. Fibre Bragg Gratings can be used to monitor parameters such as pressure and temperature. Alternatively, coating the fibres with functional layers, either at the tip of the fibre, on a U-shaped fibre, or a long period grating enables sensing of analytes in liquids and gases. This paper describes the application of optical fibre sensing techniques during mechanical ventilation via an endotracheal tube in critical care. Functional coatings on the fibres are used to monitor humidity of inspired air and can be used to monitor other analytes.
A method for measuring the contact pressure between an endotracheal tube cuff and the trachea was designed and
developed by using a fibre Bragg grating (FBG) based optical fibre sensor. The FBG sensor is encased in an epoxy based
UV-cured cuboid patch and transduces the transversely loaded pressure into an axial strain that induces wavelength shift
of the Bragg reflection. The polymer patch was created by using a PTFE based mould and increases tensile strength and
sensitivity of the bare fibre FBG to pressure to 2.10×10-2 nm/kPa. The characteristics of the FBG patch allow for
continuous measurement of contact pressure. The measurement of contact pressure was demonstrated by the use of a 3D
printed model of a human trachea. The influence of temperature on the measurements is reduced significantly by the use
of a second FBG sensor patch that is not in contact with the trachea. Intracuff pressure measurements performed using a
commercial manometer agreed well with the FBG contact pressure measurements.
An optical fibre sensor for simultaneous temperature and humidity measurements consisting of one fibre Bragg grating (FBG) to measure temperature and a mesoporous film of bilayers of Poly(allylamine hydrochloride)(PAH) and silica (SiO2) nanoparticles deposited onto the tip of the same fibre to measure humidity is reported. The hygroscopic film was created using the layer-by-layer (LbL) method and the optical reflection spectra were measured up to a maximum of 23 bilayers. The temperature sensitivity of the FBG was 10 pm/°C while the sensitivity to humidity was (-1.4x10-12 W / %RH) using 23 bilayers. The developed sensor was tested in the mechanical ventilator and temperature and humidity of the delivered artificial air was simultaneously measured. Once calibrated, the optical fibre sensor has the potential to control the absolute humidity as an essential part of critical respiratory care.
An optical fibre Bragg grating (FBG) was used to measure local strain (due to contact pressure) at the interface of a cuffed endotracheal tube (ETT) tested in a tracheal model. The tracheal model consisted of a corrugated tube. Two FBG sensors written in a single optical fibre were attached to the outside wall of the cuff of the ETT. Intracuff endotracheal pressure was measured using a digital manometer, while the contact pressure between the model trachea and the ETT was measured using Flexiforce sensors. Changes in the Bragg wavelengths in response to the inflation of the cuff of the ETT, and concomitant pressure increase, were observed to be dependent on the location of the FBGs at the corrugations, i.e., the annular peaks and troughs of the corrugated tube. The performance of both contact pressure sensors FBG and Flexiforce suggests that FBG technology is better suited to this application as it allows the measurement of contact pressures on non-uniform surfaces such as in the tracheal model.
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