A miniature frequency domain diffuse optical optode for quantitative wearable oximetry

Current wearable optical technologies generally utilize superficial tissue continuous-wave measurements for biological metrics such as heart rate monitoring. There has been limited prior work in wearables that extract quantitative information including tissue optical properties and hemoglobin concentrations. These parameters may assist in tracking physiological status for cardio-pulmonary conditions and cancer. Next-generation optical wearables must meet substantial technical requirements, including small footprint, high sensitivity, and thermal stability. Here we investigate the use of a new multi-wavelength optical laser and compact avalanche photodiode for use in a miniaturized diffuse optical frequency-domain optode (miniOptode). These components represent the most compact fiberless optode for frequency-domain measurements to date. The miniOptode had high SNR (53.5dB at 50 MHz), and achieved high accuracy and precision in optical property extractions (accuracy: μ_a 0.0018 mm^-1 and μ_s' 0.0547 mm^-1; precision: μ_a 0.00008mm^-1 and μ_s' 0.0015 mm^-1). It provided high SNR for test measurements taken on nine different anatomic locations and was capable of tracking hemodynamics during a cuff occlusion test. Active thermoelectric cooling was required for thermal stability during longer tissue measurements. This work demonstrates that frequency domain diffuse optical measurements can be achieved in a highly portable format, providing new opportunities for long-term monitoring with quantitative oximetry.