We are working on development of small biosensors which will be used in healthcare. The sensors can monitor different fluids from the body. The information we receive can help us improve or even save lives. One element we detect is pH. So what is pH? Everything around us is made up of elements and atoms and molecules. For example in water (H2O) we have two hydrogen atoms and one oxygen atom bound together. If a water molecule breaks into parts we will have one OH molecule and one H element. If a liquid produces more OH (hydroxide) we call it a base, while if it produces more hydrogen we call it an acid. The pH of a solution can be measured using a scale ranging from 1-14. Acidic solutions have a low pH value between 1 and 7, while basic solution have a high pH in the range 7-14. A solution with a pH of 7 is actually referred to as "neutral", which means that these solutions are neither basic nor acidic. Liquids with pH values of 1 or below or 13 or above are are highly acidic/basic and are very dangerous.
We can also monitor the frequency-dependent electrical resistance (impedance) using sinusoidal current excitations of the tissue and also changes in the concentration of ions such as potassium (K+), sodium (Na+) and pH, which all change depending on local condition of the tissue (e.g. during ischemia). Tissue ischemia induces biochemical and physiological changes. Cells don’t produce energy to “feed” their membrane ion pumps; extracellular water goes into the cell; the cell grows and extracellular space is reduced. This changes low and high frequency tissue impedance and there is a build-up of hydrogen ions and, therefore, tissue pH decreases and the concentrations of potassium (K+), sodium (Na+) and other ions change.
The main challenge in developing wireless integrated biosensors is the development of integrated sensors and circuits on the same chip. For example, the micro-fluidic device we have developed can measure pH, Sodium (Na+) and lactate simultaneously. It responds reliably, sensitively, and rapidly to changes of these biomarkers during physical activity such as cycling. Sweat lactate is a function of the eccrine gland energy metabolism. Increased exercise intensity leads to increased production of sweat lactate. During intense physical activity, the aerobic metabolism is incapable of satisfying the energy demands. In place comes the anaerobic process wherein the stored glycogen is consumed to produce energy and lactate by muscle cells. This process is called “glycolysis” or “lactate acidosis” and involves increased lactate levels in blood. Blood plasma and sweat electrolyte concentrations are related: low plasma water concentration (dehydration) is linked to low sodium concentrations (hyponatremia). Dehydration can be seen as an increase in the sodium concentration in sweat during exercise. Real-time pH measurements in sweat may provide a non-invasive method of detecting the build-up of acid in muscle cells during exercise. In addition to the wearable sensing platform, we have designed implantable multi-modal sensing platforms using the above sensors combined with electrical bio-impedance sensors. These platforms are intended to detect tissue ischemia in the vicinity of surgical sites in the gastrointestinal tract. More regarding this is described in the implant section of the website.
What are we using it for?
We are using biosensors for a range of wearable and implantable applications. In the case of sweat sensing, perspiration can be used for analysis of physical performance in an individual without the need for invasive blood sampling. Sweat lactate is also a sensitive marker for pressure ischemia and tissue viability related to insufficient oxidative metabolism. Measuring ion concentrations directly in bodily fluids such as sweat gives information on both electrolyte volume and concentration in plasma. The information we can obtain with our platform is important not only for the general well-being of a person, but also for sport science applications, as athletes and coaches can use these to optimise their performance and training.