## 4 _In Vivo_ Experimental Results
Figure 12: (**a**) Subcutaneous injection procedure demonstrated with a cat cadaver. (The picture has been republished from [13] with authors’ permission ©2014 IEEE.) (**b**) An anesthetized rat with the subcutaneously injected optical probe temporarily wire-connected to the commercial AFE4490 system for benchmarking and validation. In order to clean up the noisy PPG signal, we have used a moving-average filter with a 3-dB cutoff frequency at 6 Hz. As it is fundamentally difficult to record a simultaneous \"ground-truth\" signal for such optical measurements, we calculated the SNR of the PPG signal using this filtered signal (the red waveform in Figure 14) as the ground-truth to get more insight on the noise. We found the SNR to be 5.14 dB which is very low. Furthermore, the large respiratory artifacts made it difficult to detect the periodicity of the heart pulses even from the filtered signal. Hence, we applied another moving-average filter with a 3-Hz cutoff frequency on the raw PPG signal to obtain the baseline for the respiratory signal. Subtraction of these two signals partially removed the respiratory artifacts. Finally, after these post-processing steps, we performed peak detection on the cleaner PPG signal (Figure 15). The short-term average of the heart rate values were calculated using the peaks in between each pair of respiration events. The mean \(\pm\) SD of these averaged values was found to be 241 \(\pm\) 18 ppm. Figure 14: The raw signal from the output of the switched integrator and the filtered signal after performing a moving average when using the presented PPG readout circuit during the rat experiment demonstrating a more pronounced respiratory pattern and buried heart beats. Figure 13: (a) An excerpt of the received data stream after being demodulated and low-pass filtered in the SDR. (b) Received PPG waveform collected from the human finger after extracting the 8-bit samples from the serial data stream. For validation and comparison purposes, we connected the same optical probe to the AFE4490 evaluation board and obtained the representative results shown in Figure 16 with a gain setup of 10 k\(\Omega\). We observe the same respiration pattern that we had previously obtained, but in this case, the heart-beat generated variations were more visible. The mean \(\pm\) SD of the averaged heart rate values in between each pair of respiration events was calculated to be 225 \(\pm\) 6 bpm. Although the standard deviation of the heart rate values extracted from our system is higher than that of the AFE4490, the interquartile ranges are similar in both cases excluding a few outliers (Figure 17). Figure 16: Signal from the AFE4490 evaluation board connected to the probe injected in the anesthetized rat. The respiratory pattern and heart contraction related modulations can be clearly seen. The mean \(\pm\) SD of the averaged heart rate values in between each pair of respiratory events is 225 \(\pm\) 6 bpm and that of respiration rate is 27 \(\pm\) 1.20 breaths per minute. Figure 15: Digital post-processing of the PPG signal from the ASIC output demonstrates discernible heart beats. The mean \(\pm\) SD of the averaged heart rate values in between each pair of respiratory events is 241 \(\pm\) 18 bpm and that of respiration rate is 35 \(\pm\) 1.45 breaths per minute.