# Feasibility Analysis on the Use of Ultrasonic Communications for Body Sensor Networks
## 1 Introduction  
With the increasing attention being paid to healthcare, wireless sensor networks for physiological monitoring have been widely applied in daily life. The rapid development of biosensors provides a means for real-time monitoring for healthcare and can help realize ubiquitous healthcare. Patients could be monitored by doctors from a personal monitoring device in any environment and without restricting activity. Nanoscale sensors can detect physiological parameters inside the body and then transmit signals to a sensor attached on the skin. Thereafter, the data can be wirelessly transmitted by radio frequency (RF) communication networks. Various body sensors, such as accelerometers and monitors for ECG, temperature, glucose, and SpO\({}_{2}\), have been developed for pervasive healthcare monitoring systems [1]. These sensors are placed inside the body or attached to the skin to monitor the health status of a patient. For example, diabetics use an implanted glucose sensor and a wearable insulin actuator to adjust their glucose level. When the glucose level increases, the glucose sensor transmits a signal to the actuator, which has a microneedle attached to the skin that triggers the injection of insulin. The system monitors the patient in real-time, and does not require patient hospitalization. Physiological data can be wirelessly transmitted through RF to doctors for analysis [2]. This can reduce the patient healthcare costs and the frequency of hospital visits. With recent developments in nanotechnology, materials science, and integrated circuits, researchers are focusing more attention on the demands of wearable healthcare systems. However, there are still few standards or effective communication methods for body-area networks, especially for implanted sensors. Therefore, communication among sensor nodes is an important role for current and future device development. RF waves have been successfully used in wireless communication systems. Conventionally, in body sensor networks (BSNs), an RF microchip is embedded in the sensor board for communication [3; 4]. To reduce the size of the sensor module, it is necessary to reduce the size of the RF antenna chips [5; 6; 7]. The RF modules in portable devices also consume battery power quickly [8; 9]. This is a concern for implantable sensors, because it is difficult to change the batteries frequently. Depending on the specific environment, RF electromagnetic waves have poor propagation in water. This is a particular problem for in-body communication, because 60% to 70% of the human body is made of water. The RF signals transmitted through tissues suffer high attenuation. To enhance the signal, high-power RF devices are used, which can result in heating of soft tissue because of absorption. Furthermore, RF signals are easily affected when other electronic devices operate at the same frequency. The RF bandwidth is limited, which inherently limits the communication rates [10]. Physical monitoring data are considered confidential personal medical information, which requires the wireless system to be secure to prevent eavesdropping or interference. A lack of security could be a risk for the patient, especially in the case of a monitoring board with a therapy actuator. Prolonged RF radiation can overheat tissue, which can lead to bioeffects [11; 12]. Health concerns therefore impose restrictions on RF signal levels. To address all of these issues, we propose using ultrasonic waves as the communication medium for healthcare systems. Ultrasound has been pervasively applied in medicine [13]. Therefore, it can be an alternative method for short-distance digital communication. Ultrasound has advantages over electromagnetic waves in the application of implantable medical devices: The human body is composed mostly of water, and RF waves are subject to high absorption. For communication purposes, an RF module requires high transmission power. That requirement increases the size and weight of an implantable sensor.