A Wireless Multimodal Physiological Monitoring ASIC for Animal Health Monitoring Injectable Devices

Utilizing injectable devices for monitoring animal health offers several advantages over traditional wearable devices, including improved signal-to-noise ratio (SNR) and enhanced immunity to motion artifacts. We present a wireless application-specific integrated circuit (ASIC) for injectable devices. The ASIC has multiple physiological sensing modalities including body temperature monitoring, electrocardiography (ECG), and photoplethysmography (PPG). The ASIC fabricated using the CMOS 180 nm process is sized to fit into an injectable microchip implant. The ASIC features a low-power design, drawing an average DC power of 155.3 µW, enabling the ASIC to be wirelessly powered through an inductive link. To capture the ECG signal, we designed the ECG analog frontend (AFE) with 0.3 Hz low cut-off frequency and 45-79 dB adjustable midband gain. To measure PPG, we employ an energy-efficient and safe switched-capacitor-based (SC) light emitting diode (LED) driver to illuminate an LED with milliampere-level current pulses. A SC integrator-based AFE converts the current of photodiode with a programmable transimpedance gain. A resistor-based Wheatstone Bridge (WhB) temperature sensor followed by an instrumentation amplifier (IA) provides 27–47 °C sensing range with 0.02 °C inaccuracy. Recorded physiological signals are sequentially sampled and quantized by a 10-bit analog-to-digital converter (ADC) with the successive approximation register (SAR) architecture. The SAR ADC features an energy-efficient switching scheme and achieves a 57.5 dB signal-to-noise-and-distortion ratio (SNDR) within 1 kHz bandwidth. Then, a back data telemetry transmits the baseband data via a backscatter scheme with intermediate-frequency assistance. The ASIC’s overall functionality and performance has been evaluated through an in vivo experiment.

I. Introduction

Monitoring animals’ health in farms and veterinary clinics requires measuring vital signals such as electrocardiography (ECG), photoplethysmography (PPG), and body temperature [1], [2], [3], [4], [5]. Wearable devices like smartwatches, chest patches, and wristbands have facilitated personalized health monitoring and health profile creation in humans [6], [7], [8], [9], [10]. However, when these are used on animals, wearables face several challenges, as shown in Fig. 1(a). First, animals typically have thicker skin, which leads to poor coupling between wearable devices and tissue, resulting in weak received signals and compromised signal-to-noise ratio (SNR) [11], [12]. Second, animals’ thicker skin which typically has fur or scales on it, necessitating hair shaving for placing the devices and electrodes before each routine assessment. Third, when animals move freely, wearable devices need to be stuck onto the subject’s body through tape or bandages. However, even with these securing measures, the weak attachment of the devices makes them susceptible to motion artifacts, impacting data accuracy [13]. Recently, injectable microchip implants, conventionally employed for animal identification, have shown its great potential as a promising alternative for monitoring animal health [14], [15], [16], [17]. These injectable devices operating subcutaneously can improve coupling with tissue and enhance SNR compared to conventional wearables. Furthermore, the injectable devices can anchor firmly inside the body, which improves the device's immunity to motion artifacts.