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By illuminating the skin, a photoplethysmograph can measurea patient’s heart rate and blood oxygenation. The optical signals measured (PPGs) are highly susceptible to motion, which can distort the PPG derived data. It is hypothesized that motion induced opticalartifacts in PPGs correlate to movement of the sensor with respect to the skin. To investigate this correlation, a displacement measuring method using the self-mixing interferometric effect of a laser diode has been designed and tested in a laboratory setup. It is shown that displacement between the laser diode and a skin phantom can be measured accurately using the proposed method. Therefore the proposedmethod can be applied to measure displacement between a PPG sensorand skin and used to determine whether sensor displacement correlates to optical motion artifacts in PPGs. Conclusion - It has been demonstrated that displacement between a laser diode and a Delrin skinphantom can be accurately measured using the SMI effect of a laser diode of which the driving current is amplitude modulated. Therefore,this method can be applied to measure displacement between a PPG sensor and skin, and used to determine whether sensor displacement correlates to optical motion artifacts in PPGs.

Currently, photoplethysmograms (PPGs) are mostly used to determine apatient's blood oxygenation and pulse rate. However, PPG morphologyconveys more information about the patient's cardiovascular status.Extracting this information requires measuring clean PPG waveformsthat are free of artifacts. PPGs are highly susceptible to motion, which can distort the PPG derived data. Part of the motion artifactsare considered to result from sensor-tissue motion and sensor deformation. It is hypothesized that these motion artifacts correlate withmovement of the sensor with respect to the skin. This hypothesis has been proven true in a laboratory setup. In-vitro PPGs have been measured in a skin perfusion phantom that is illuminated by a laser diode. Optical motion artifacts are generated in the PPG by translating the laser diode with respect to the PPG photodiode. The optical motion artifacts have been reduced significantly in-vitro, by using anormalized least-mean-square algorithm with only a single coefficient that uses the laser's displacement as a reference for the motion artifacts. Laser displacement has been measured accurately via self-mixing interferometry by a compact laser diode with a ball lens integrated into the package, which can be easily integrated into a commercial sensor.

Pulse oximeters measure a patient’s heart rate and blood oxygenationby illuminating the skin and measuring the intensity of the light that has propagated through it. The measured intensities, called photoplethysmograms (PPGs), are highly susceptible to motion, which candistort the PPG derived data. Part of the motion artifacts are considered to result from sensor deformation, leading to a change in emitter-detector distance. It is hypothesized that these motion artifacts correlate to movement of the emitter with respect to the skin. This has been investigated in a laboratory setup in which motion artifacts can be reproducibly generated by translating the emitter with respect to a flowcell that models skin perfusion. The top of the flowcell is a diffuse scattering Delrin skin phantom under which a cardiac induced blood pulse is modeled by a changing milk volume. By illuminating the flowcell, a PPG can be measured. The emitter’s translation has been accurately measured using self-mixing interferometry (SMI). The motion artifacts in the PPG as a result of emitter motion are shown to correlate with the emitter’s displacement. Moreover, it is shown that these artifacts are significantly reduced by a least-mean-square algorithm that uses the emitter’s displacement measured via SMI as artifact reference.