Wirelessly networked systems of intra-body sensors and actuators could enable revolutionary applications at the intersection between biomedical science, networking, and control with a strong potential to advance medical treatment of major diseases of our times. Yet, most research to date has focused on communications among devices interconnected through traditional electromagnetic radio-frequency (RF) carrier waves. The main obstacle to enabling this vision of networked implantable devices is posed by the physical nature of propagation in the human body; composed primarily (65%) of water, through which RF electromagnetic waves notoriously hardly propagate, even at relatively low frequencies.  

We take a different perspective and propose to investigate and study the use of ultrasonic waves to wirelessly internetwork intra-body devices.

Ultrasonic vs. Electromagnetic RF Communication

Potential advantages of using ultrasonic communications (UC) as compared to RF communications (RFC) can be summarized as follows:

• Propagation: Ultrasonic waves are subject to lower absorption as compared to electromagnetic waves, mainly because of the significant water content in human tissues.

• Health Concerns: UC have been successfully used for therapeutic and diagnostic purposes inside the human body since the 1960s with no detrimental effects. On the contrary, the medical community is still divided on the risks caused by exposure of human tissues to RF radiations.

• Interference Management: RFC may raise significant electromagnetic compatibility concerns, since environmental or malicious interference may potentially jam intra-body RF devices.

Ultrasoninc Intra-body Channel Modeling

The first step towards the design of high-performance ultrasonic networked systems is to characterize in detail the ultrasonic propagation channel in tissues. We use a deterministic propagation model, based a propagation simulator suited for ultrasonic intra-body communications.

A realistic channel model of ultrasonic signal propagation in human tissues that incorporates attenuation, scattering and multipath needs to satisfy three coupled first-order equations simultaneously, which represent relationships among acoustic pressure, acoustic particle velocity, and medium density. The solution represents the acoustic field behavior in time, evaluated at each spatial coordinate of the propagation medium.

We solve the partial differential equations using a computationally more efficient approach based on the pseudo-spectral and k-space methods implemented in Matlab (k-wave).

An appropriate physical layer scheme needs to be designed to achieve low-complexity and reliable communications against the effect of multipath reflections and scattering within the human body, but also limit the thermal effect of radiations, which may be detrimental to human health. Broadly used carrier-modulated waveforms are strongly affected by multipath replicas that destructively overlap in time, and continuous waveform can also produce undesired bio-effects. Based on this, we designed and proposed a new ultrasonic transmission and multiple access technique, which we refer to as Ultrasonic WideBand (UsWB).

Ultrasonic WideBand is based on the idea of transmitting very short ultrasonic pulses with an adaptively controllable duty cycle following a pseudo-random adaptive time-hopping pattern, and with a spreading code of adaptive length superimposed. Impulsive transmission and spread-spectrum encoding combat the effects of multipath and scattering and introduce waveform diversity among interfering nodes.

Medium Access Control (MAC) Layer Design

The design of a distributed MAC can enable multiple access and combat the effect of interference, thus allowing users to share the communication medium fairly. Unfortunately, the characteristics of the ultrasonic channel make the design of an efficient MAC protocol for in-body communications a challenging task.

• Multipath: caused by the inhomogeneity of the medium, strongly affects narrowband signals.
• Delay: caused by the low speed of sound in the medium, makes implementation of efficient time-division and carrier-sense schemes challenging.

Ultrasonic WideBand, using a low duty cycle impulsive transmission together with a superimposed spreading code allows different users to coexist in the same channel. Dynamically adapting the time-hopping frame length and spreading code length allows to implement a MAC protocol that does not require mutual temporal exclusion between different transmitters, and optimally, and distributively regulate tradeoffs among resilience to interference, achievable information rate, and energy efficiency.

Software Defined Ultrasonic Testbed

The development of tools and frameworks for validating the designed system is a key feature ultrasonic IBAN. Experimental platforms and testbeds can offer sophisticated engineering approaches to provide realistic testing results. Unfortunately, no off-the-shelf solutions are available for ultrasonic BANs.

We designed a software defined experimental platform and testbed architecture for ultrasonic intra-body area networks, based on Universal Software Radio Peripheral (USRP) N210 software-defined radios. The devices communicate using ultrasonic transducers and physical phantoms (i.e., tissue-mimicking materials with acoustically accurate representations of anatomy) are interposed between transmitters and receivers to emulate propagation through tissues with high fidelity.