## 4 _In Vivo_ Experimental Results  
For proof of concept, we performed two _in vivo_ experiments as shown in Figure 11 to test the functionality of the integrated system and the feasibility of using the injectable probe in obtaining PPG signals with adequate quality (Figure 12). In the first part of the study, we tested the functionality of the completely assembled system along with the battery and used the finger of a human subject to generate a representative PPG pulse for a quick assessment. Although the system was designed for subcutaneous applications and on animals, this setup provided a simpler access to a real PPG signal for initial testing. The system depicted in Figure 2c was placed inside a glass tube that the subject held during the measurement with the index finger touching the surface of the glass and facing the photodetector and thumb pressing on the opposite site for adequate contact. We used the maximum gain of the readout circuit and an LED peak current of \(7.5\) mA. An excerpt of the received signal after being demodulated and low-pass filtered in the SDR is shown in Figure 13a, and the equivalent analog representation of the received samples after the post-processing is shown in Figure 13b. This plot clearly demonstrates the PPG waveform and identifies the heart rate being approximately 66 bpm as also verified by a gold standard COTS PPG system (CMS50D+ Pulse Oximeter, Contec Medical Systems Co., Ltd., Qinhuangdao, Hebei, China). In the second experiment, we used an anesthetized rat (_Rattus norvegicus_) to evaluate the performance of the subcutaneous PPG sensor. All animal procedures took place in the College of Veterinary Medicine at NC State University and were approved by the Institutional Animal Care and Use Committee (IACUC) of NC State University. The rat was first anesthetized using a controlled flow of isoflurane. For experimental flexibility, we subcutaneously injected the glass capsule containing only the OSRAM BioMon sensor (the top capsule in Figure 2c) at several locations of the rat's body using a COTS metal needle attached to a 3D-printed injector with Acrylonitrile Butadiene Styrene (ABS) plastic (Figure 12a). For benchmarking, we connected it alternatively with our test board and a  
Figure 10: **(a)** Histograms of received binary signal levels showing the actual distribution for the symbols ‘0’ and ‘1’. **(b)** Gaussian fit for probability density functions (\(P_{0}\) and \(P_{1}\)) of the received signal levels for the two symbols. commercially available evaluation module for pulse oximetry (AFE4490 SPO2 EVM, Texas Instruments, Dallas, TX, USA). The test board included the ASIC with the signal conditioning circuit, the LED driver, and the ATtiny10 microcontroller. In this experiment, we directly connected the output of the SI to an oscilloscope (DSOX3014A, Keysight Technologies, Santa Rosa, CA, USA) to get rapid feedback on the signal quality. The whole experiment lasted for about an hour (10-15 min of data collection from each of the body locations). Figure 14 shows the result that we were able to obtain from the thoracic area on the back of the rat (Figure 12b) using the maximum gain setup in our readout circuit, with \(R_{f}=1\) M\(\Omega\), \(C_{f}=1.5\) pF in the TIA, and \(C_{int}=25\) pF in the integrator, accounting for a total gain of 8 M\(\Omega\). The respiration pattern can be clearly seen whereas the external noise has visually masked the light modulation from the blood volume changes caused by the heart contraction and made the heart rate estimation challenging. Figure 11: Visual description of the two experimental setups. (**a**) Wireless transmission was tested with the handheld capsule transmitting PPG data to the laptop through the dipole antenna and the Airspy R2 receiver. (**b**) _In vivo_ performance was tested on an anesthetized rat with the optical probe injected in the thoracic area. The probe was connected to the ASIC test board and the AFE4490 evaluation board alternatively and the data was displayed on an oscilloscope.