I ‘heart’ my smartwatch

Personal fitness monitoring is a multibillion pound industry. It is now widely accepted that your resting heart rate (RHR) is a decent indicator of changes to your cardiovascular fitness. For instance, a decline in RHR over several weeks of exercise indicates that your heart has become more efficient at pumping blood and oxygen around your body.

Modern wearable devices such as smartwatches allow you to monitor this progression over time, as well as other metrics related to your general health. The devices work by taking advantage of several core biological and chemical concepts common to everyone.

The green light

Modern smartwatches use a flashing green light to measure your heart rate from your wrist. Spectroscopy tells us that blood absorbs green light because red and green are opposite each other on the colour wheel. The rear of the smartwatch contains an optical sensor to detect the reflected light. The main difference from spectrometers is that the light source and detector are positioned on the same side in smartwatches, while they are opposite each other in spectrometers. The layout used in smartwatches can result in lower accuracy compared to spectrometers because the detector is entirely dependent on the amount of light reflected from the sample, ie your blood.

Measuring heart rate using light is called photoplethysmography. The device measures the change in concentration of red blood cells as the blood vessels expand and contract – ie expanded blood vessels absorb more green light, contracted blood vessels absorb less green light. The detector measures the reflected light and a software algorithm converts the changes in light intensity into your pulse rate.

The next generation

Recent advances in battery technology have allowed for more sensors and a higher degree of accuracy in wearables. Valeria Nicolosi, expert in nanomaterials and energy storage and a professor at Trinity College Dublin, says: ‘there are lots of very cool things the battery and materials science fields are working on which will revolutionise this technology. The next generation of batteries are far more efficient and will support monitoring on a much finer scale.’

In the near future, there will be no reason to wear a smartwatch as we will be able to have clothes with seamlessly embedded sensors

Oxygen saturation (SpO₂) – the percentage of haemoglobin bound to oxygen in your blood – is the latest metric to arrive in wearables thanks to more efficient power use. It has risen in prominence during the Covid-19 pandemic because one of the many puzzling aspects of the disease is that is causes silent hypoxia, a condition where oxygen levels in the body drop abnormally low without patients showing signs of respiratory distress. The most accurate way to measure SpO₂ is to test a blood sample, but pulse oximetry is the more common method used by health professionals. It involves placing an optical device on a finger or ear lobe and, like optical heart rate monitoring, this device forces light through the skin towards a detector.

Unlike optical heart rate monitoring though, pulse oximetry uses two light sources: red and infrared (IR). This is because oxygenated haemoglobin absorbs IR light and allows red light to pass through, whereas deoxygenated haemoglobin allows IR light to pass through and absorbs red light. The newest generation of smartwatches measure SpO₂ using pulse oximetry by calculating the difference between IR and red light absorption taken from your wrist. Pulse oximetry has become particularly relevant for detecting sleep apnoea, a disorder that if left untreated could lead to high blood pressure and other health conditions.

The future’s bright

The use of spectroscopy in personal fitness monitoring has progressed rapidly over the past decade. Technological advancements and miniaturisation have even put optical heart rate monitors into a ring that you can wear on your finger. Based on recent advances in materials science, Valeria predicts the next generation of fitness wearables may even take the form of smart clothing: ‘In the near future, there will be no reason to wear a smartwatch as we will be able to have clothes with seamlessly embedded sensors.’

Fitness wearables have evolved far beyond their origins of measuring your heart rate at a specific moment. In addition to new sensors and battery technology for measuring health metrics, software algorithms have also advanced significantly to interpret all the data that your wearable is measuring. As a result, accuracy is continually improving and wearable technology could eventually become hugely important in the early detection of health abnormalities.