What’s in store for optical biosensors? Part 1
Optical sensors represent the most common type of biosensor. This two-part series provides a technical background on how optical techniques are used for bioanalytical applications. This article provides an overview on using reflectometry for a pulse plethysmograph (PPG) waveform and describes the physical and physiological principals at work. Part 2 looks at common noise and error sources affecting optical sensors in mobile and wearable applications, including the effects of confounders from the environment captured in the measurement and the physiological variations among the user population. It also provides a summary of current capabilities of wearable biosensors and the future direction of optical biosensing applications.
Overview: optical biosensing
Optical methods are among the most common approaches to biosensing for plants and animals. For example, remote sensing equipment from satellites routinely use reflectance quality to determine the greenness and stresses of vegetation. Nurses clamp pulse oximeters on their patients’ fingertips as a regular part of taking vital signs prior to doctor visits.
This wide range of applications reflect the versatility of optical sensing, Light, coherent or non-coherent, interacts with matter it passes through and becomes absorbed, reflected, scattered, dispersed, or otherwise altered. Scientists can examine the magnitude and shape of light pulses, their spectral contents, and polarization to derive information about the analytes in the media the light pulses traversed.
This article focuses on the second of our aforementioned examples, using a photoplethysmogram (PPG) to monitor blood flow in real time.
Real-time blood-flow monitoring via PPG
A plethysmogram is a volumetric measurement. As blood flows, a cardiovascular pulse wave goes from the heart and propagates through the body, periodically distending the arteries and arterioles in the subcutaneous tissue. PPG uses a light to interrogate tissue. Because blood in the tissue absorbs more light than the surrounding tissue, a reduction in the amount of blood results in an increase in the intensity of the light reflected back, or backscattered.
Depending on the relative positions of the light source and the photodetector, two configurations are possible for PPG: transmissive absorption and reflection. In a transmissive configuration, the light source and the sensor are on directly opposite sides of the tissue. In a reflective arrangement, they could be on the same side. Reflective configurations take advantage of the light-scattering effect of body tissue, which the following section discusses.
Although PPG is normally performed using a finger clip in clinics and hospitals, it is possible to obtain a valid PPG signal in many other body locations, so long as there is easy access to tissue rich with blood vessels, especially when using a reflective configuration. Examples include the forehead, the outer ear canal, the areas around the bicep or calve muscles, and even the area around a wrist. Some of these alternate locations allow sports equipment and wearable devices to incorporate PPG sensors with varying successes, as the second part of this series will explain.