Energy-efficient sensing with physically secure communication for biosensors on, around, and within the human body is a major area of research for the development of low-cost health care devices, enabling continuous monitoring and/or secure perpetual operation. When used as a network of nodes, these devices form the Internet of Bodies, which poses challenges including stringent resource constraints, simultaneous sensing and communication, and security vulnerabilities. Another major challenge is to find an efficient on-body energy-harvesting method to support the sensing, communication, and security submodules. Due to limitations in the amount of energy harvested, we require a reduction in energy consumed per unit information, making the use of in-sensor analytics and processing imperative. In this article, we review the challenges and opportunities of low-power sensing, processing, and communication with possible powering modalities for future biosensor nodes. Specifically, we analyze, compare, and contrast ( a) different sensing mechanisms such as voltage/current domain versus time domain, ( b) low-power, secure communication modalities including wireless techniques and human body communication, and ( c) different powering techniques for wearable devices and implants. Expected final online publication date for the Annual Review of Biomedical Engineering, Volume 25 is June 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Continuous long-term sensing of biopotential signals is vital to facilitate accurate diagnosis. The current state of the art in wearable health monitoring relies on radiative technology for communication. Due to their radiative nature, these systems result in lossy and inefficient transmission, limiting the device's life span. Human Body Communication has emerged as an energy-efficient secure communication modality, and literature has shown body communication to transmit biopotential signals at 100x lower power than traditional radiative technologies. Unlike radiative communication that uses airwaves, HBC, specifically Capacitive Electro-Quasistatic HBC (EQS-HBC), couple signals and confine them within the human body. In Capacitive EQS-HBC, the transmitter uses an electrode to modulate the body potential to transmit data. The modulation of body potential by HBC raises the following concerns. Will HBC transmissions affect the quality of biopotential signals sensed from the body? Additionally, since biopotential sensing systems commonly use Right Leg Drive (RLD) to bias body potential, there is also a concern if RLD can affect the quality of HBC transmissions. For the first time, our work studies the interactions between EQS-HBC and biopotential sensing. Our work is important since understanding HBC-RLD interactions is integral to developing EQS-HBC-based biosensors for Body Area Networks (BANs). For the studies, we conducted lab experiments and developed circuit theoretic models to back the experimental outcomes. We show that due to their higher frequency content and common-mode nature, HBC transmissions do not affect the differential sensing of low-frequency biopotential signals. We show that biopotential sensing using RLD affects HBC. RLD deteriorates the signal strength of HBC transmissions. We thus propose not to use RLD with HBC. We demonstrate our proposed solution by transmitting ECG signals using HBC with 96% correlation compared to the traditional wireless system at a fraction of the power.
Body Area Network (BAN) demands a secure and low-power technology. Traditionally, RF signals were used, which suffer from high losses and lack of security. Recently, electro-quasistatic human body communication has emerged, enabling low-power communication, but somewhat susceptible to leakage. Ultrasound is presented as a promising alternative owing to its ability of propagating in water-dominated media, like the human body, besides being safe. Existing studies employ ultrasound devices for powering or communicating across tissues; either through transmitter-receiver aligned configurations (not suitable for non-line-of-sight communication) or using omnidirectional transducers, hence limiting the communication distance to only a few centimeters. This paper presents a theoretical study on confining ultrasound waves inside the human tissues (muscle, fat and skin). Through oblique incidence on the bone/muscle interface, the waves can bypass the bone (highly attenuative tissue) through total internal reflection, and be guided through water-rich body tissues. In addition, the high reflection at the skin/air interface (∼99.9%) confines most of the signal inside the body, ensuring secure communication. Simulations are performed on a simplified cylindrical-shaped human body model at 100 kHz, demonstrating the possibility of transmitting a signal over a distance of ∼1 m with losses <50 dB and leaked signal attenuation of additional >20 dB.
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