Conor Slater

A wearable electrochemical sensor for the real-time measurement of sweat sodium concentration

We report a new method for the real-time quantitative analysis of sodium in human sweat, consolidating sweat collection and analysis in a single, integrated, wearable platform. This temporal data opens up new possibilities in the study of human physiology, broadly applicable from assessing high performance athletes to monitoring Cystic Fibrosis (CF) sufferers. Our compact Sodium Sensor Belt (SSB) consists of a sodium selective Ion Selective Electrode (ISE) integrated into a platform that can be interfaced with the human body during exercise. No skin cleaning regime or sweat storage technology is required as the sweat is continually wicked from the skin to a sensing surface and from there to a storage area via a fabric pump. Our results suggest that after an initial equilibration period, a steady-state sodium plateau concentration was reached. Atomic Absorption Spectroscopy (AAS) was used as a reference method, and this has confirmed the accuracy of the new continuous monitoring approach. The steady-state concentrations observed were found to fall within ranges previously found in the literature, which further validates the approach. Daily calibration repeatability (n = 4) was ±3.0% RSD and over a three month period reproducibility was ±12.1% RSD (n = 56). As a further application, we attempted to monitor the sweat of Cystic Fibrosis (CF) sufferers using the same device. We observed high sodium concentrations symptomatic of CF (∼60 mM Na+) for two CF patients, with no conclusive results for the remaining patients due to their limited exercising capability, and high viscosity/low volume of sweat produced.

Autonomous microfluidic system for phosphate detection.

Miniaturization of analytical devices through the advent of microfluidics and micro total analysis systems is an important step forward for applications such as medical diagnostics and environmental monitoring. The development of field-deployable instruments requires that the entire system, including all necessary peripheral components, be miniaturized and packaged in a portable device. A sensor for long-term monitoring of phosphate levels has been developed that incorporates sampling, reagent and waste storage, detection, and wireless communication into a complete, miniaturized system. The device employs a low-power detection and communication system, so the entire instrument can operate autonomously for 7 days on a single rechargeable, 12V battery. In addition, integration of a wireless communication device allows the instrument to be controlled and results to be downloaded remotely. This autonomous system has a limit of detection of 0.3mg/L and a linear dynamic range between 0 and 20mg/L.

An Autonomous Microfluidic Sensor for Phosphate: On-Site Analysis of Treated Wastewater

A microfluidic sensor for long-term monitoring of phosphate levels has been developed that incorporates sampling, reagent and waste storage, detection, and wireless communication into a compact and portable device. The sensor is based on the yellow method for phosphate determination, a simple colorimetric technique involving the formation of vanadomolybdophosphoric acid when a phosphate-containing sample is mixed with an acidic reagent containing ammonium molybdate and ammonium meta-vanadate. This paper describes the application of the phosphate sensor to the on-site analysis of effluent from a wastewater treatment plant. The data was validated by comparison with the plants existing online monitor, and a good correlation between the two sets of data was achieved, showing that the phosphate sensor is capable of operating satisfactorily at low mg L-1 levels over extended periods of time.

Polystyrene bead-based system for optical sensing using spiropyran photoswitches

Spiropyran derivatives have been immobilised on the surface of polystyrene microbeads using different immobilisation strategies. These functionalised polymeric beads can be reversibly switched between the colourless inactive spiropyran (SP) and highly coloured (purple) active merocyanine (MC) forms using low power light sources, such as light-emitting diodes (LEDs). A UV LED (375 nm) is used for the SP → MC conversion, and a white LED (430–760 nm) for the reverse MC → SP conversion. The photochromic behaviour of the coated beads has been characterised using different LEDs and reflection spectroscopy, employing optic fibres and an in-house-designed holder. Investigations into the metal-ion binding behaviour of the spiropyran-modified microbeads have shown that Cu2+ ions cause an appreciable colour and spectral change when brought into contact with the beads in the MC form, suggesting that a significant interaction is occurring. However, the Cu2+ ions can be completely expelled by photonic-conversion of the beads into the inactive SP form using a white LED. This sequence has been successfully repeated six times, suggesting that it is possible to cycle through activation of the functionalised beads from a non-binding to a binding form (SP → MC) using a UV LED, allow binding with Cu2+ ions to occur, and subsequently, expel the bound ions and regenerate the passive SP surface using a white LED. Other metals, such as calcium, do not cause any appreciable colour or spectral change over the same concentration range and in the presence of the same anion (final concentration 7.1 × 10−4 M nitrate salt in ethanol). The system is therefore self-indicating in terms of whether the active MC or inactive SP forms are present, and whether Cu2+ ions are bound to the MC form. In principle, therefore, these functionalised beads could form the basis of a photoswitchable stationary phase for metal ion binding and detection: irradiation of the stationary phase with UV LEDs causes retention of guest species due to the presence of the MC form, while subsequent exposure to white LEDs causes release of guest species into the mobile phase.

Validation of a fully autonomous phosphate analyser based on a microfluidic lab-on-a-chip

This work describes the design of a phosphate analyser that utilises a microfluidic lab-on-a-chip. The analyser contains all the required chemical storage, pumping and electronic components to carry out a complete phosphate assay. The system is self-calibrating and self-cleaning, thus capable of long-term operation. This was proven by a bench top calibration of the analyser using standard solutions and also by comparing the analysers performance to a commercially available phosphate monitor installed at a waste water treatment plant. The output of the microfluidic lab-on-a-chip analyser was shown to have sensitivity and linear range equivalent to the commercially available monitor and also the ability to operate over an extended period of time.

Wireless Ion-Selective Electrode Autonomous Sensing System

A paradigm shift in sensing methods and principles is required to meet the legislative demands for detecting hazardous substances in the molecular world. This will encompass the development of new sensing technologies capable of performing very selective and sensitive measurements at an acceptable cost, developed by multidisciplinary teams of chemists, engineers and computer scientists to harvest information from a multitude of molecular targets in health, food and within the environment. In this study we present the successful implementation of a low-cost, wireless chemical sensing system that employs a minimum set of components for effective operation. Specifically, our efforts resulted in a wireless, tri-electrode, ISE pH sensor for use in environmental monitoring. Sensor calibration and validated in situ field trials have been carried out and are presented in this paper.

Wearable sensors for monitoring sports performance and training

Textile based devices for biochemical analysis of body fluids represent a new development in the area of wearable sensors. This paper outlines the development of a fluid handling system and wireless sensors for the real-time analysis of sweat pH and sodium levels during exercise. Liquid is drawn into the system using a moisture wicking material and passive pump. The sensor then displays pH induced colorimetric changes, which are recorded using an optical detection system. The device has been tested under laboratory conditions and can easily detect increments of 0.2 pH units. At present, changes in sodium content are determined using a specially constructed classic ion selective electrode and reference electrode, combined to form a single probe. This is placed in contact with the fabric of the fluid handling system in order to obtain a real-time potentiometric sodium measurement. Both devices have successfully been used for the investigation of sweat composition during on-body trials.

Autonomous field-deployable device for the measurement of phosphate in natural water

This work describes the ongoing development of an autonomous platform for the measurement of phosphate levels in river water. This device is designed to operate unassisted for one year, taking a measurement every hour and relaying the result to a laptop computer. A first generation prototype has already been developed and successfully field tested. The system contains the sampling, chemical storage, fluid handling, colorimetric data acquisition and waste storage capabilities necessary to perform the phosphate measurement. In addition to this, the device has the embedded control, GSM communications system and power supply to allow independent operation. The entire system is placed inside a compact and rugged enclosure. Further work discussed here builds on the successes of the prototype design to deliver a system capable of one full year of operation. The second generation system has been built from the ground up. Although identical in operation to the prototype its design has a greater emphasis on power efficient components and power management to allow for a longer lifetime. Other improvements include an automated two-point calibration to compensate for drift and a more rugged design to further increase the lifetime of the device.

In situ monitoring of environmental water quality using an autonomous microfluidic sensor

An autonomous microfluidic sensor for phosphate in environmental waters has been developed and assessed in laboratory and field trials. The sensor is based on the molybdenum yellow method for phosphate detection in which a phosphate-containing sample is mixed with a reagent containing ammonium molybdate and ammonium metavanadate in an acidic medium. The yellow-colored compound which is formed absorbs strongly below 400nm and its absorbance is proportional to the concentration of phosphate in the original sample. The sensor utilizes a microfluidic manifold where mixing, reaction and detection take place. Optical detection is performed using a LED (light emitting diode) light source and a photodiode detector. The sensor also combines pumping system, power supply, reagent and waste storage, and wireless communications into a compact and portable device. Here we report the successful use of the sensor to monitor phosphate levels in an estuarine environment.

Wearable technology for the real-time analysis of sweat during exercise

Textile based sensors which can be used to measure the chemical composition of bodily fluids represents a major advancement in the area of wearable technology. BIOTEX is an EU funded project aiming to develop such sensors with a particular interest in monitoring perspiration. A textile based fluid handling system has been developed for sample collection and transport. Sodium, conductivity and pH sensors have also been developed. This paper details the integration and testing of these sensors. Results show that the developed system can collect and analyze sweat in real time during exercise and transmit this data wirelessly to a remote receiver.