Minimally invasive muscle-based recording of photoplethysmogram toward chronic implantation

This paper reports on the feasibility of recording the photoplethysmogram (PPG) signal subcutaneously from muscle groups in minimally invasive fashion as a step toward chronic implantation. A system has been developed that records the PPG signals in the reflective mode using a portable sensor board interfaced with a battery-powered main board for control and data processing. Three different sensor boards with a miniaturized footprint of 15mm ×15mm are developed, with each containing a pair of IR (850nm wavelength), red (660nm wavelength), and green (570nm wavelength) light-emitting diodes (LEDs) along with their corresponding photodiodes and transimpedance amplifiers (TIAs). Initial experiments with a human subject volunteer show the basic functionality of the system in wearable PPG recording from the fingertip and wrist area of the subject, as well as from the thenar eminence muscle group at the base of the thumb. Furthermore, Parylene-C-coated sensor boards are implanted subcutaneously on the quadriceps muscle group of an anesthetized laboratory rat, yielding PPG recordings that are of high quality for extracting the heart rate and respiration rate of the subject.