_In Vivo_ Evaluation of a Subcutaneously Injectable Implant with a Low-Power Photoplethysmography ASIC for Animal Monitoring  
[PERSON]  
Department of Electrical and Computer Engineering, NC State University, Raleigh, NC 27695-7911, USA; [EMAIL_ADDRESS] (J.M.V.-S.); [EMAIL_ADDRESS] (P.A.) 1  
[PERSON]  
Department of Electrical and Computer Engineering, NC State University, Raleigh, NC 27695-7911, USA; [EMAIL_ADDRESS] (J.M.V.-S.); [EMAIL_ADDRESS] (P.A.) 1  
[PERSON]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +1-919-515-7349  
2  
Footnote 2: email: {j.s.j.v.m. The deployment of PPG sensors in veterinary medicine presents a challenge due to the animal's fur blocking most of the incident light where only a small fraction penetrates the tissue and reaches the photodetector [5]. Traditional clinical PPG and pulse oximetry measurements on animals require shaving hair or placing the sensors on the furless regions such as ear or tongue while constraining animal mobility due to required wiring. For that reason, these methods are only used in clinical settings by connecting clips to the ear or tongue of anesthetized animals [6]. One possible solution to overcome this barrier while maintaining a wearable form factor is to attach light pipes or optical fibers bundles to the light source to guide the light through the layer of fur and to increase the coupling with the skin [7], although this approach still requires higher current for the light source and is still relatively sensitive to motion artifacts. Another method to obtain good-quality optical signals is to enter the subcutaneous region and circumvent the layer of skin and fur in order to get a direct reading from the inner tissue. This has been studied and proposed in different ways, including reflective pulse oximetry systems surgically implanted in the neck area [8] or wrapped around an arterial blood vessel [2; 9]. These prior works showed the potential of performing PPG measurements inside the body of an animal, although these still require an elaborate surgical operation. We propose the use of an \"injectable\" system as a less invasive alternative to gain access to the subcutaneous tissue and improve the signal-to-noise ratio (SNR) of reflective PPG measurements. This would avoid the need for any surgical implantation procedure and free the animal from being tethered to a device. A similar injectable approach has been previously used for other applications in the literature, such as functional electric stimulation to reanimate paralyzed limbs in humans [10; 11] or monitoring electrocardiogram-based heart rate, core body temperature, and gross motor activity in laboratory rats [12]. Towards that end, we had previously shown a preliminary feasibility analysis of the use of inductive coupling in powering a light-emitting diode (LED) that had been packaged in a glass tube with an inner diameter of less than 4 mm, and the capability of employing this small form-factor device to obtain PPG readings when held between the fingers of a human subject [13]. We now present the _in vivo_ evaluation of an injectable capsule intended for the wireless monitoring of PPG in small animals. The capsule system includes a custom low-power analog front end (AFE) integrated into an application-specific integrated circuit (ASIC) for amplifying and reading the photocurrent generated from an off-chip photodiode (PD). This amplified photocurrent modulated by the hemodynamics of the blood vessels is used to extract the heart rate information. A microcontroller connected to the AFE generates the related control signals, serializes the output stream, and creates the data frame. Finally, an on-chip transmitter creates a binary frequency-modulated (FM) signal to wirelessly send the digital data stream outside the body. We employ a software-defined radio (SDR) to receive the data in a laptop and perform post-processing.