## 2 System Architecture  
The various blocks that constitute the proposed injectable system are presented in Figure 1, where the solid blocks are parts of the ASICs fabricated using a 0.5-\(\upmu\)m CMOS process (ON Semiconductor, Denver, CO, USA), and the dashed blocks are implemented with commercial off-the-shelf (COTS) components. For the current proof-of-concept prototype, we created a custom and miniaturized printed circuit board (PCB) to fit into an injectable glass capsule where the two different ASICs containing the PPG analog front end and the wireless transmitter are placed adjacent to each other on the same PCB. Figure 2a shows the photo of the die that contains the AFE within an active area of 0.9 mm\({}^{2}\), while Figure 2b shows the photo of the second die with the transmitter occupyingan active area of 0.6 mm\({}^{2}\). The remaining building blocks use external COTS components with small footprints without compromising the target capsule size (Figure 2c). For injection, the largest size COTS needle we had access to is 2 gauge which allows for a capsule with an outer diameter of 6 mm. Considering the glass wall thickness, we targeted a PCB width of 3.2 mm. In terms of the length of the capsule, we were able to fit all the necessary components into a 19-mm long PCB layout which is a reasonable size for a injectable device intended for animal monitoring. In this current version, we implemented the control logic using an ultra-small COTS microcontroller (Atmel\({}^{\text{\textregistered}}\)ATtiny10, 2.0 \(\times\) 2.0 \(\times\) 0.6 mm\({}^{3}\), Microchip Technology, Inc., Chandler, AZ, USA) which can actually fit into the limited volume of our implantable capsule. This is a low-power CMOS 8-bit microcontroller based on the AVR\({}^{\text{\textregistered}}\)enhanced RISC architecture with 1024 Bytes of  
Figure 1: Block diagram of the subcutaneous PPG system where the blocks in solid boxes have been integrated using the ON Semiconductor 0.5-m CMOS process and the blocks in dashed boxes have been implemented with smaller COTS components. Figure 2: Micrographs of the two ASICs (before being wire bonded to the PCB) and a photo of the overall system. (**a**) The delimited die area measures approximately 1.3 \(\times\) 0.7 mm\({}^{2}\) and represents the transimpedance amplifier, current digital-to-analog converter, and switched integrator. (**b**) The delimited die area measures approximately 0.7 \(\times\) 0.9 mm\({}^{2}\) and represents the FSK transmitter. (**c**) Picture of the injectable optical probe with an OSRAM BioMon sensor in the glass encapsulation (top) and the two sides of the fully assembled injectable PPG system. programmable flash program memory and internal 8-bit ADC [14]. The clock frequency can be scaled based on the supply voltage. In our case, we ran the system at 4 MHz with a supply of 2.5 V. The output of the AFE is directly connected to one of the IO lines of the microcontroller, which digitizes the signal using its internal 8-bit ADC, creates the data frame, and outputs a serial stream ready to be transmitted, thereby making the recorded PPG waveform available for visualization and post-processing in the receiver. On the top side of the PCB, we soldered the ATtiny10 microcontroller along with the required passive components and an ultra-small bilateral analog switch (SN74 LVC1G66, 1.0 \(\times\) 1.0 \(\times\) 0.4 mm\({}^{3}\), Texas Instruments, Dallas, TX, USA) that briefly switches off the LED between sampling pulses and reduces the power consumption significantly. The bottom side of the PCB contains a 2.5 V capacitor-free low-dropout regulator (LDO) available in a small X2 SON package (TLV733P, 1.0 \(\times\) 1.0 \(\times\) 0.4 mm\({}^{3}\), Texas Instruments, Dallas, TX, USA). We included another IC containing a pair of N-channel MOSFETs in a leadless ultra-small and ultra-thin SMD plastic package (PMDXB600 UNE, 1.1 \(\times\) 1.0 \(\times\) 0.37 mm\({}^{3}\), Nexperia, Nijmegen, The Netherlands) to serve as current references for the AFE circuit.