Table 3.
Sensor types and related sensing principles for wearable, non-invasive, continuous molecular digital biomarkers.
| Sensor type | Sensing principlea |
|---|---|
| Selector-transducer | |
| Potentiometric sensors | An ionophore binding specific ions (e.g., Na+, K+, etc.) combined with a transducer that senses the voltage differences with a reference electrode |
| Amperometric sensors | An enzyme catalyzing the target metabolite (e.g., glucose, lactate, etc.) combined with a transducer that senses the change in the electrical current at an electrode |
| Conductometric sensors | An enzyme catalyzing the target analyte combined with a transducer that senses the changes in ionic conductance |
| Colorimetric sensors | A sensor that changes color upon binding with a specific metabolite (e.g., glucose, lactate) or an electrolyte (e.g., Na+, Cl−) |
| Fluorometric sensors | A sensor that changes fluorescent properties upon interaction with a specific metabolite (e.g., glucose, lactate, O2) or an electrolyte (e.g., Na+, Cl−) |
| Spectroscopy | |
| Transmission spectroscopy | Transmission of light at a specific wavelength (ultraviolet, visible, near-infrared, infrared) through a sample to measure the absorption, which is proportional to the number of molecules of interest. A well-known example is Fourier Transform Infrared (FTIR) spectroscopy |
| Reflectance spectroscopy | Reflectance of light at a specific wavelength (ultraviolet, visible, infrared, near-infrared) in a sample to measure the absorption, which is proportional to the number of molecules of interest |
| Photoplethysmography | A specific form of transmission or reflectance spectroscopy to detect volume changes in peripheral blood vessels as a measure of heart rate and other cardiovascular variables |
| Photoacoustic spectroscopy | Energy gained by light absorption is released as heat in a gas chamber or tissue to form a pressure wave measured as sound, mostly applied for breath analysis |
| Photoluminescence (fluorescence) spectroscopy | Energy gained by light absorption is released as light with longer wavelength due to energy loss to thermal energy |
| Raman spectroscopy | Energy gained by light absorption is released as light, with a slightly different energy because of interactions with vibrational modes in the molecules |