Abstract
Photoplethysmography is an extensively-used, portable, and noninvasive technique for measuring vital parameters such as heart rate, respiration rate, and blood pressure. The deployment of this technology in veterinary medicine has been hindered by the challenges in effective transmission of light presented by the thick layer of skin and fur of the animal. We propose an injectable capsule system to circumvent these limitations by accessing the subcutaneous tissue to enable reliable signal acquisition even with lower light brightness. In addition to the reduction of power usage, the injection of the capsule offers a less invasive alternative to surgical implantation. Our current prototype combines two application-specific integrated circuits (ASICs) with a microcontroller and interfaces with a commercial light emitting diode (LED) and photodetector pair. These ASICs implement a signal-conditioning analog front end circuit and a frequency-shift keying (FSK) transmitter respectively. The small footprint of the ASICs is the key in the integration of the complete system inside a 40-mm long glass tube with an inner diameter of 4 mm, which enables its injection using a custom syringe similar to the ones used with microchip implants for animal identification. The recorded data is transferred wirelessly to a computer for post-processing by means of the integrated FSK transmitter and a software-defined radio. Our optimized LED duty cycle of 0.4% at a sampling rate of 200 Hz minimizes the contribution of the LED driver (only 0.8 mW including the front-end circuitry) to the total power consumption of the system. This will allow longer recording periods between the charging cycles of the batteries, which is critical given the very limited space inside the capsule. In this work, we demonstrate the wireless operation of the injectable system with a human subject holding the sensor between the fingers and the in vivo functionality of the subcutaneous sensing on a pilot study performed on anesthetized rat subjects.
Highlights
Photoplethysmography (PPG) is an optical measurement of blood volume that is obtained by shining light at a specific wavelength into the body and detecting the amount of light transmitted through, or backscattered from, the tissue [1]
We used various time scopes and fast Fourier transform (FFT) sinks in the system to check the signal at different stages of the receiver, and a file sink to save the incoming stream of data so we can post-process it in MATLAB®
The use of traditional clinical photoplethysmography or pulse oximetry in veterinary medicine has been limited to the ear and tongue regions of anesthetized animals
Summary
Photoplethysmography (PPG) is an optical measurement of blood volume that is obtained by shining light at a specific wavelength into the body and detecting the amount of light transmitted through, or backscattered from, the tissue [1]. 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], 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].
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