Pulse oximeter bench tests under different simulated skin tones.

Pulse oximeters' (POs) varying performance based on skin tones has been highly publicised. Compared to arterial blood gas analysis, POs tend to overestimate oxygen saturation (SpO2) values for people with darker skin (occult hypoxemia). The objective is to develop a test bench for assessing commercial home and hospital-based POs in controlled laboratory conditions. A laboratory simulator was used to mimic different SpO2 values (~ 70 to 100%). Different neutral density and synthetic melanin filters were used to reproduce low signal and varying melanin attenuation levels. Six devices consisting of commercial home (Biolight, N = 13; ChoiceMMed, N = 18; MedLinket, N = 9) and hospital-based (Masimo Radical 7 with Neo L, N = 1; GE B450 Masimo SET with LNCS Neo L, N = 1; Nonin 9550 Onyx II™, N = 1) POs were reviewed and their response documented. Significant variations were observed in the recorded SpO2 values among different POs when exposed to identical simulated signals. Differences were greatest for lower SpO2 (< 80%) where empirical data is limited. All PO responses under low signal and melanin attenuation did not change across various simulated SpO2 values. The bench tests do not provide conclusive evidence that melanin does not affect in vivo SpO2 measurements. Research in the areas of instrument calibration, theory and design needs to be further developed.

Long Period Grating Mach-Zehnder Interferometer Based Immunosensor with Temperature and Bulk Refractive Index Compensation.

A long period grating Mach-Zehnder interferometer (LPGMZI) that consists of two identical long period gratings (LPGs) in a single fibre was developed to measure immunoglobulin M (IgM). The measured spectrum has fringes due to the interference between the core mode and cladding mode. This immunosensor inherits the advantages of an LPG and has the potential to compensate for unwanted signal changes due to bulk refractive index (RI) and temperature fluctuations by analysing interference fringes and their envelope. The external RI was measured from 1.3384 to 1.3670 in two different cases: (i) only the connecting section between the two LPGs is immersed or (ii) the whole LPGMZI is immersed. The fringes shift with an external RI in both scenarios, whereas the envelope stays still in case (i) or shifts at the same rate as the fringes in case (ii). The LPGMZI was also characterised at different temperatures between 25 °C and 30 °C by placing the whole LPGMZI in a water bath. The fringes and envelope shift at the same rate with temperature. The LPGMZI platform was then used to create an IgM immunosensor. The connecting section between the two LPGs was functionalised with anti-IgM and immersed into solutions with IgM concentrations from 20 µg/mL to 320 µg/mL. The fringes shift with IgM concentration and the envelope remains static. The results from this work show that LPGMZI has the potential to compensate for the temperature and bulk RI fluctuations and perform as a portable biosensor platform.

Fibre Bragg Grating Based Interface Pressure Sensor for Compression Therapy.

Compression therapy is widely used as the gold standard for management of chronic venous insufficiency and venous leg ulcers, and the amount of pressure applied during the compression therapy is crucial in supporting healing. A fibre optic pressure sensor using Fibre Bragg Gratings (FBGs) is developed in this paper to measure sub-bandage pressure whilst removing cross-sensitivity due to strain in the fibre and temperature. The interface pressure is measured by an FBG encapsulated in a polymer and housed in a textile to minimise discomfort for the patient. The repeatability of a manual fabrication process is investigated by fabricating and calibrating ten sensors. A customized calibration setup consisting of a programmable translation stage and a weighing scale gives sensitivities in the range 0.4-1.5 pm/mmHg (2.6-11.3 pm/kPa). An alternative calibration method using a rigid plastic cylinder and a blood pressure cuff is also demonstrated. Investigations are performed with the sensor under a compression bandage on a phantom leg to test the response of the sensor to changing pressures in static situations. Measurements are taken on a human subject to demonstrate changes in interface pressure under a compression bandage during motion to mimic a clinical application. These results are compared to the current gold standard medical sensor using a Bland-Altman analysis, with a median bias ranging from -4.6 to -20.4 mmHg, upper limit of agreement (LOA) from -13.5 to 2.7 mmHg and lower LOA from -32.4 to -7.7 mmHg. The sensor has the potential to be used as a training tool for nurses and can be left in situ to monitor bandage pressure during compression therapy.

A single-film fiber optical sensor for simultaneous measurement of carbon dioxide and relative humidity.

Colorimetric measurement is a versatile, low-cost method for bio-/chemical sensing and that has importance in biomedical applications. General carbon dioxide (CO2) sensors based on colorimetric change of a pH indicator report only one parameter at a time and are cross-sensitive to relative humidity (RH). This work describes a novel optical fiber sensor with a thin film on the distal end of the fiber, combining colorimetric measurement and a white light Fabry-Pérot interferometer (FPI) for the simultaneous measurement of CO2 and RH. The CO2 sensitive dye ion-pair: thymol blue and tetramethylammonium hydroxide are encapsulated inside organically modified silica forming an extrinsic FPI cavity (refractive index of 1.501 ± 0.02 and thickness of 5.83 ± 0.09 µm). The sensor reversibly responds to 0-6% CO2 and 0-90% RH with negligible cross-sensitivity and allows measurement of both parameters simultaneously. A sensitivity of ∼0.19 nm/%RH is obtained for RH measurement based on the wavelength shift of the FPI and there is a polynomial correlation between the average intensity of selected wavelengths and the concentration of CO2. The applicability of the sensor is demonstrated by measuring the CO2 and RH exhaled from human breath with a percent error of 3.1% and 2.2% respectively compared to a commercial datalogger. A simulation model is provided for the dye-encapsulated FPI sensor allowing simulation of spectra of sensors with different film thicknesses.

Intra-tracheal multiplexed sensing of contact pressure and perfusion.

Incorrect endotracheal tube (ETT) cuff inflation pressure causes significant problems for intubated patients. The technical development and first in vivo use of a smart ETT for measurements at the cuff-trachea interface during mechanical ventilation are described. The intra-tracheal multiplexed sensing (iTraXS) ETT contains integrated optical fibre sensors to measure contact pressure and blood perfusion. The device is tested during mechanical ventilation in a porcine model (N=6). For contact pressure, signals were obtained in all 30 measurements. For perfusion, data could be obtained in all 33 measurements. In the 3 cases where the cuff was inflated to an artificially high-level, blood occlusion is observed.

Localised plasmonic hybridisation mode optical fibre sensing of relative humidity.

This work reports an optical fibre probe functionalised with 'cotton-shaped' gold-silica nanostructures for relative humidity (RH) monitoring. The sensor response utilises the localised surface plasmon resonance (LSPR) of self-assembled nanostructures: gold nanospheres (40 nm) surrounded by one layer of poly (allylamine hydrochloride) and hydrophilic silica nanoparticles (10-20 nm) on the end-facet of an optical fibre via a wavelength shift of the reflected light. Sensor optimisation is investigated by varying the density of gold nanoparticles on the end-facet of an optical fibre. It is demonstrated that the plasmonic hybridisation mode appearing when the average gold interparticle distance is small (Median: 7.5 nm) is more sensitive to RH after functionalisation than the singular plasmonic mode. The plasmonic hybridisation mode sensor demonstrates a high linear regression to RH with a sensitivity of 0.63 nm/%RH and excellent reversibility. The response time (T10-90%) and recovery time (T90-10%) are calculated as 1.2 ± 0.4 s and 0.95 ± 0.18 s. The sensor shows no measurable cross-talk to temperature in the tested range between 25 °C to 40 °C and the 95% limit of agreement is 3.1%RH when compared to a commercial reference sensor. Simulation with finite element analysis reveals a polarisation-dependent plasmonic hybridisation with a redshift of plasmonic wavelength as a decrease of the interparticle distance and a higher refractive index sensitivity, which results in a high sensitivity to RH as observed in the experiment.

Optical Fibre Sensor for Capillary Refill Time and Contact Pressure Measurements under the Foot.

Capillary refill time (CRT) refers to the time taken for body tissue to regain its colour after an applied blanching pressure is released. Usually, pressure is manually applied and not measured. Upon release of pressure, simple mental counting is typically used to estimate how long it takes for the skin to regain its colour. However, this method is subjective and can provide inaccurate readings due to human error. CRT is often used to assess shock and hydration but also has the potential to assess peripheral arterial disease which can result in tissue breakdown, foot ulcers and ultimately amputation, especially in people with diabetes. The aim of this study was to design an optical fibre sensor to simultaneously detect blood volume changes and the contact pressure applied to the foot. The CRT probe combines two sensors: a plastic optical fibre (POF) based on photoplethysmography (PPG) to measure blood volume changes and a fibre Bragg grating to measure skin contact pressure. The results from 10 healthy volunteers demonstrate that the blanching pressure on the subject's first metatarsal head of the foot was 100.8 ± 4.8 kPa (mean and standard deviation), the average CRT was 1.37 ± 0.46 s and the time to achieve a stable blood volume was 4.77 ± 1.57 s. For individual volunteers, the fastest CRT measured was 0.82 ± 0.11 and the slowest 1.94 ± 0.49 s. The combined sensor and curve fitting process has the potential to provide increased reliability and accuracy for CRT measurement of the foot in diabetic foot ulcer clinics and in the community.

Volatile Organic Compound Vapour Measurements Using a Localised Surface Plasmon Resonance Optical Fibre Sensor Decorated with a Metal-Organic Framework.

A tip-based fibreoptic localised surface plasmon resonance (LSPR) sensor is reported for the sensing of volatile organic compounds (VOCs). The sensor is developed by coating the tip of a multi-mode optical fibre with gold nanoparticles (size: 40 nm) via a chemisorption process and further functionalisation with the HKUST-1 metal-organic framework (MOF) via a layer-by-layer process. Sensors coated with different cycles of MOFs (40, 80 and 120) corresponding to different crystallisation processes are reported. There is no measurable response to all tested volatile organic compounds (acetone, ethanol and methanol) in the sensor with 40 coating cycles. However, sensors with 80 and 120 coating cycles show a significant redshift of resonance wavelength (up to ~9 nm) to all tested volatile organic compounds as a result of an increase in the local refractive index induced by VOC capture into the HKUST-1 thin film. Sensors gradually saturate as VOC concentration increases (up to 3.41%, 4.30% and 6.18% in acetone, ethanol and methanol measurement, respectively) and show a fully reversible response when the concentration decreases. The sensor with the thickest film exhibits slightly higher sensitivity than the sensor with a thinner film. The sensitivity of the 120-cycle-coated MOF sensor is 13.7 nm/% (R2 = 0.951) with a limit of detection (LoD) of 0.005% in the measurement of acetone, 15.5 nm/% (R2 = 0.996) with an LoD of 0.003% in the measurement of ethanol and 6.7 nm/% (R2 = 0.998) with an LoD of 0.011% in the measurement of methanol. The response and recovery times were calculated as 9.35 and 3.85 min for acetone; 5.35 and 2.12 min for ethanol; and 2.39 and 1.44 min for methanol. The humidity and temperature crosstalk of 120-cycle-coated MOF was measured as 0.5 ± 0.2 nm and 0.5 ± 0.1 nm in the humidity range of 50-75% relative humidity (RH) and temperature range of 20-25 °C, respectively.

Non-invasive cuff-less blood pressure machine learning algorithm using photoplethysmography and prior physiological data.

Conventional blood pressure (BP) measurement methods have a number of drawbacks such as being invasive, cuff-based or requiring manual operation. Many studies are focussed on emerging methods of noninvasive, cuff-less and continuous BP measurement, and using only photoplethysmography to estimate BP has become popular. Although it is well known that physiological characteristics of the subject are important in BP estimation, this has not been widely explored. This article presents a novel method which adopts photoplethysmography and prior knowledge of a subject's physiological features to estimate DBP and SBP. Features extracted from a fingertip photoplethysmography signal and prior knowledge of a subject's physiological characteristics, such as gender, age, height, weight and BMI is used to estimate BP using three different machine learning models: artificial neural networks, support vector machine and least absolute shrinkage and selection operator regression. The accuracy of BP estimation obtained when prior knowledge of the physiological characteristics are incorporated into the model is superior to those which do not take the physiological characteristics into consideration. In this study, the best performing algorithm is an artificial neural network which obtains a mean absolute error and SD of 4.74 ± 5.55 mm Hg for DBP and 9.18 ± 12.57 mm Hg for SBP compared to 6.61 ± 8.04 mm Hg for DBP and 11.12 ± 14.20 mm Hg for SBP without prior knowledge. The inclusion of prior knowledge of the physiological characteristics can improve the accuracy of BP estimation using machine learning methods, and the incorporation of more physiological characteristics enhances the accuracy of the BP estimation.

Carboxyl-fentanyl detection using optical fibre grating-based sensors functionalised with molecularly imprinted nanoparticles.

Butyrylfentanyl is a new designer drug reported with growing use and related deaths. Routine toxicological analyses of this novel synthetic opioid drug have not been established yet. This work reports a fibre optic sensor that measures carboxyl-fentanyl which is the major metabolite of butyrylfentanyl presented in blood, providing a promising tool for detecting butyrylfentanyl intoxication. A long period fibre grating (LPG) sensor array operating at phase-matching condition is deployed in combination with a state-of-the-art molecular imprinting technique. Nano-sized molecularly imprinted polymers (nanoMIPs) are synthesised via a solid-phase approach and coated on the surface of an LPG array. An LPG array consists of two parts: a detection and a reference LPG. The former is functionalised with nanoMIPs prior to the measurements, whilst the latter is used to take into account the temperature response of the detection LPG. The developed sensor exhibits a gradual response over increasing concentrations of carboxyl-fentanyl from 0 to 1000 ng/mL with a minimal detected concentration of 50 ng/mL, that corresponds to a wavelength shift of 1.20 ± 0.2 nm. The Langmuir adsorption isotherm is applied to fit the analytical data which reveal a binding constant of 2.03 µM-1. The developed sensor shows high selectivity in detecting carboxyl-fentanyl among other drugs and potential interferents including morphine, cocaine, glucose and albumin. It shows a certain degree of cross-response to fentanyl which shares the same binding sites as carboxyl-fentanyl and therefore can be potentially used to detect fentanyl.

A Textile Sleeve for Monitoring Oxygen Saturation Using Multichannel Optical Fibre Photoplethysmography.

Textile-based systems are an attractive prospect for wearable technology as they can provide monitoring of key physiological parameters in a comfortable and unobtrusive form. A novel system based on multichannel optical fibre sensor probes integrated into a textile sleeve is described. The system measures the photoplethysmogram (PPG) at two wavelengths (660 and 830 nm), which is then used to calculate oxygen saturation (SpO2). In order to achieve reliable measurement without adjusting the position of the garment, four plastic optical fibre (POF) probes are utilised to increase the likelihood that a high-quality PPG is obtained due to at least one of the probes being positioned over a blood vessel. Each probe transmits and receives light into the skin to measure the PPG and SpO2. All POFs are integrated in a stretchable textile sleeve with a circumference of 15 cm to keep the sensor in contact with the subject's wrist and to minimise motion artefacts. Tests on healthy volunteers show that the multichannel PPG sensor faithfully provides an SpO2 reading in at least one of the four sensor channels in all cases with no need for adjusting the position of the sleeve. This could not be achieved using a single sensor alone. The multichannel sensor is used to monitor the SpO2 of 10 participants with an average wrist circumference of 16.0 ± 0.6 cm. Comparing the developed sensor's SpO2 readings to a reference commercial oximeter (reflectance Masimo Radical-7) illustrates that the mean difference between the two sensors' readings is -0.03%, the upper limit of agreement (LOA) is 0.52% and the lower LOA is -0.58%. This multichannel sensor has the potential to achieve reliable, unobtrusive and comfortable textile-based monitoring of both heart rate and SpO2 during everyday life.

Surface polymer imprinted optical fibre sensor for dose detection of dabrafenib.

Dabrafenib is one of the most widely used of the new generation of targeted anti-cancer drugs. However, its therapeutic window varies for different patients and so there is an unmet need for methods to monitor the dose of drug which the patient receives and at the specific site where it acts. In the case of cancers, it is critical to measure the concentration of drug not just in the bloodstream overall, but in or near tumours, as these will not be the same over multiple time periods. A novel sensor based on an optical fibre long period grating (LPG) modified with a molecular imprinted polymer (MIP) has been developed with the ultimate aim of achieving minimally invasive measurements of Dabrafenib at the tumour site. A molecularly imprinted polymer specific for Dabrafenib was coated on a methacryloylalkoxysilane-functionalised optical fibre long period grating. In vitro experimental results demonstrate that the Dabrafenib sensitivity is 15.2 pm (µg mL-1)-1 (R2 = 0.993) with a limit of detection (LoD) of 74.4 µg mL-1 in serum solution. Moreover, the proposed sensor shows selective response to Dabrafenib over structurally similar 2-Aminoquinoline.

Real-Time Humidity Measurement during Sports Activity using Optical Fibre Sensing.

An optical fibre sensor for monitoring relative humidity (RH) changes during exercise is demonstrated. The humidity sensor comprises a tip coating of poly (allylamine hydrochloride) (PAH)/silica nanoparticles (SiO2 NPs) deposited using the layer-by-layer technique. An uncoated fibre is employed to compensate for bending losses that are likely to occur during movement. A linear fit to the response of the sensing system to RH demonstrates a sensitivity of 3.02 mV/% (R2 = 0.96), hysteresis ± 1.17% RH when 11 bilayers of PAH/SiO2 NPs are coated on the tip of the fibre. The performance of two different textiles (100% cotton and 100% polyester) were tested in real-time relative humidity measurement for 10 healthy volunteers. The results demonstrate the moisture wicking properties of polyester in that the relative humidity dropped more rapidly after cessation of exercise compared to cotton. The approach has the potential to be used to monitor sports performance and by clothing developers for characterising different garment designs.

Real time monitoring of biofilm formation on coated medical devices for the reduction and interception of bacterial infections.

Real time monitoring of bacterial attachment to medical devices provides opportunities to detect early biofilm formation and instigate appropriate interventions before infection develops. This study utilises long period grating (LPG) optical fibre sensors, incorporated into the lumen of endotracheal tubes (ETTs), to monitor in real time, Pseudomonas aeruginosa surface colonisation and biofilm formation. The wavelength shift of LPG attenuation bands was monitored for 24 h and compared with biofilm biomass, quantified using confocal fluorescence microscopy imaging. Biofilm formation was compared on uncoated ETTs and optical fibres, and on a biofilm resistant acrylate polymer, after challenge in an artificial sputum or minimal growth medium (RPMI-1640). The LPG sensor was able to detect a biofilm biomass as low as 81 µg cm-2, by comparison with the confocal image quantification. An empirical exponential function was found to link the optical attenuation wavelength shift with the inverse of the biofilm biomass, allowing quantification of biofouling from the spectral response. Quantification from the sensor allows infection interception and early device removal, to reduce, for example, the risk of ventilator associated pneumonia.

Numerical Simulation of a Scanning Illumination System for Deep Tissue Fluorescence Imaging.

The spatial resolution and light detected in fluorescence imaging for small animals are limited by light scattering, absorption and autofluorescence. To address this, novel near-infrared fluorescent contrast agents and imaging configurations have been investigated. In this paper, the influence of the light wavelength and imaging configurations (full-field illumination system and scanning system) on fluorescence imaging are compared quantitatively. The surface radiance for both systems is calculated by modifying the simulation tool Near-Infrared Fluorescence and Spectral Tomography. Fluorescent targets are embedded within a scattering medium at different positions. The surface radiance and spatial resolution are obtained for emission wavelengths between 620 nm and 1000 nm. It was found that the spatial resolution of the scanning system is independent of the tissue optical properties, whereas for full-field illumination, the spatial resolution degrades at longer wavelength. The full width at half maximum obtained by the scanning system is 25% lower than that obtained by the full-field illumination system when the targets are located in the middle of the phantom. The results indicate that although imaging at near-infrared wavelength can achieve a higher surface radiance, it may produce worse spatial resolution.

Optical Fibre-Based Pulse Oximetry Sensor with Contact Force Detection.

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.

Ultrasound-mediation of self-illuminating reporters improves imaging resolution in optically scattering media.

In vivo imaging of self-illuminating bio-and chemiluminescent reporters is used to observe the physiology of small animals. However, strong light scattering by biological tissues results in poor spatial resolution of the optical imaging, which also degrades the quantitative accuracy. To overcome this challenging problem, focused ultrasound is used to modulate the light from the reporter at the ultrasound frequency. This produces an ultrasound switchable light 'beacon' that reduces the influence of light scattering in order to improve spatial resolution. The experimental results demonstrate that apart from light modulation at the ultrasound frequency (AC signal at 3.5 MHz), ultrasound also increases the DC intensity of the reporters. This is shown to be due to a temperature rise caused by insonification that was minimized to be within acceptable mammalian tissue safety thresholds by adjusting the duty cycle of the ultrasound. Line scans of bio-and chemiluminescent objects embedded within a scattering medium were obtained using ultrasound modulated (AC) and ultrasound enhanced (DC) signals. Lateral resolution is improved by a factor of 12 and 7 respectively, as compared to conventional CCD imaging. Two chemiluminescent sources separated by ~10 mm at ~20 mm deep inside a 50 mm thick chicken breast have been successfully resolved with an average signal-to-noise ratio of approximately 8-10 dB.

Incorporation of an ultrasound and model guided permissible region improves quantitative source recovery in bioluminescence tomography.

Bioluminescence imaging has shown great potential for studying and monitoring disease progression in small animal pre-clinical imaging. However, absolute bioluminescence source recovery through tomographic multi-wavelength measurements is often hindered through the lack of quantitative accuracy and suffers from both poor localisation and quantitative recovery. In this work a method to incorporate a permissible region strategy through not only a priori location (permissible region) but also based on a model of light propagation and hence light sensitivity is developed and tested using both simulations and experimental data. Reconstructions on two different numerical models (a simple slab, and the digital version of a heterogeneous mouse) show an improvement of localisation and recovery of intensity (around 25% for the slab model and around 10% for the digital mouse model). This strategy is also used with experimental data from a phantom gel, which demonstrated an improved recovered tomographic image.

Nanoscale Ultrasound-Switchable FRET-Based Liposomes for Near-Infrared Fluorescence Imaging in Optically Turbid Media.

A new approach for fluorescence imaging in optically turbid media centered on the use of nanoscale ultrasound-switchable FRET-based liposome contrast agents is reported. Liposomes containing lipophilic carbocyanine dyes as FRET pairs with emission wavelengths located in the near-infrared window are prepared. The efficacy of FRET and self-quenching for liposomes with a range of fluorophore concentrations is first calculated from measurement of the liposome emission spectra. Exposure of the liposomes to ultrasound results in changes in the detected fluorescent signal, the nature of which depends on the fluorophores used, detection wavelength, and the fluorophore concentration. Line scanning of a tube containing the contrast agents with 1 mm inner diameter buried at a depth of 1 cm in a heavily scattering tissue phantom demonstrates an improvement in image spatial resolution by a factor of 6.3 as compared with images obtained in the absence of ultrasound. Improvements are also seen in image contrast with the highest obtained being 9% for a liposome system containing FRET pairs. Overall the results obtained provide evidence of the potential the nanoscale ultrasound-switchable FRET-based liposomes studied here have for in vivo fluorescence imaging.

Ultrasound Induced Fluorescence of Nanoscale Liposome Contrast Agents.

A new imaging contrast agent is reported that provides an increased fluorescent signal upon application of ultrasound (US). Liposomes containing lipids labelled with pyrene were optically excited and the excimer fluorescence emission intensity was detected in the absence and presence of an ultrasound field using an acousto-fluorescence setup. The acousto-fluorescence dynamics of liposomes containing lipids with pyrene labelled on the fatty acid tail group (PyPC) and the head group (PyPE) were compared. An increase in excimer emission intensity following exposure to US was observed for both cases studied. The increased intensity and time constants were found to be different for the PyPC and PyPE systems, and dependent on the applied US pressure and exposure time. The greatest change in fluorescence intensity (130%) and smallest rise time constant (0.33 s) are achieved through the use of PyPC labelled liposomes. The mechanism underlying the observed increase of the excimer emission intensity in PyPC labelled liposomes is proposed to arise from the "wagging" of acyl chains which involves fast response and requires lower US pressure. This is accompanied by increased lipid lateral diffusivity at higher ultrasound pressures, a mechanism that is also active in the PyPE labelled liposomes.