Wireless communication for implanted wireless sensors

AntennaWare set out to answer the ultimate question for wireless wearable technology, which has been a challenge in the Antennas and Propagation research community for almost two decades:

Can all the challenging propagation requirements of wearable communication devices be achieved efficiently and robustly using a single wearable antenna?

The overall objective of the research, conducted in the Centre of Wireless Innovation, at Queens University Belfast, UK, since 2005, was to create a step change in wearable sensor systems for medical applications by addressing significant challenges with wearable antennas and advanced electromagnetic wave propagation requirements. The vision of this research [1] was to realise a unique antenna concept, that meets all the propagation requirements of any wearable application yet delivers high performance regardless of the hostile and diverse properties of its host platform, in this case, The Human Body. The goal was to design and prototype a single advanced antenna structure to adapt to medical propagation requirements and the diverse physiological and morphological parameters of the human host.

AntennaWare has now been granted the European Patent for the antenna, and this article describes the challenges and the motivation behind the innovative technology.

The Need

The adoption of wireless communications in body-centric applications could potentially soon impact everybody’s life, particularly, as a higher portion of the population gets older. Such technology would enable better quality of life, early detection, and faster intervention for the patient, which translates into better patient outcomes and the potential for more optimised patient-specific care.

Government and private healthcare facilities are currently trying to apply wireless technology in everyday medical practices. For example, continuous glucose monitoring (CGM) devices, which are worn continuously (24/7) by the user, are one of the fastest-growing applications. Wearable computing technology, as it is continually interacting with the user, requires synergy between the user and the wearable device.

Wireless wearable systems offer considerable advantages over hand-held devices, allowing the user to continue what they are doing while sensing or executing system commands. The enabling component which makes such wireless Body Area Networks (BANs) efficient and useable, is the Wearable Antenna. Efficient performance, not only for reduced power and thus battery size, but for extended range performance and/or wireless link reliability is highly desirable.

For medical healthcare technology, reliability and trust in the system are critical for widespread adoption of deployable solutions. For example, a network of sensors monitoring multiple patients in a hospital (or any application where multiple nodes are present) could be transformed by extended range, more complete reliable wireless coverage, and more robust link performance. The number of patients which can be monitored by one master node (base station) and sensor network, is currently not limited by bandwidth, but rather, the operation area of the sensor.

In wearable systems, it is primarily the wearable node that defines the network operation range, thus, the number of base stations needed. In a clinical patient monitoring network, the wearable sensors are ideally of disposable cost, and the base stations are premium in comparison. Therefore, any advancement in the performance of the wearable system efficiency, increases the operational range of each clinical sensor network, reducing the number of base stations.

Reduced systems costs are beneficial to both commercial potential and the end user in society. Reduced costs will greatly impact hospitals, meaning more patients can be wirelessly monitored, especially non-critical illnesses, reducing the financial burden of healthcare costs. Lower-cost systems can mature faster in consumer markets and benefit economies of scale.

Antennas for Healthcare

The current state-of-the-art in wireless sensing consists of sensor systems that are either too large, have high energy requirements or have insufficient performance in the challenging environment of the human body to meet the demands of emerging therapeutic and monitoring applications. While there have been steady improvements in antenna performance over the past two decades, overall link budgets remain marginal and therefore unreliable for more challenging applications.

Current research practice in wearable antenna design is to ignore the complex variability between users in analysing the antenna performance. Although acceptable for antenna solutions that utilised a significant ground plane to minimise antenna-tissue coupling, these bulky antennas, however, only support one propagation mode. Consequently, sub-optimal printed antenna technology is generally adopted by industry leaders as there is a lack of compact, high-performing adaptive antennas, which meet realistic wearable requirements [2].

Emerging and next-generation wireless communication research is increasingly searching for new more efficient techniques of modulation, communication protocols, coding, power management, security, and encryption, etc. In modern wearable applications, where the dynamic body is an extremely hostile environment, degrading the communication channel, performance can be extremely marginal. However, significant changes in performance and robustness of these marginal links could be made at the physical layer through better antenna design [3].

• Off-body mode - Antennas designed to maximise wave propagation in the Off-body direction.
• On-body mode - Antennas designed to enhance surface waves polarized normal to the body surface, maximising surface wave propagatInto-Body mode - Antennas designed to maximise coupling to implanted antennas with potentially unknown polarization and location (multiple nodes)
• Into-body mode - Antennas designed to maximise coupling to implanted antennas with potentially unknown polarization and location (multiple nodes).
• In-body mode -Essentially active implanted devices, with implantable antennas designed to maximise communication to other implanted nodes within surrounding tissue.

An example of the Into- body channel which involves communicating to an implanted active wireless node from outside the body (or vice-versa), would be: a surface worn wireless system communicating into an implanted intra-tumoral pH biosensor or nerve controller on a prosthetic limb (Into-Body), synchronising with other surface worn sensors within a network (On-Body), and relaying the data to a remote base station for monitoring (Off-body), alerts and medical action. Future applications may also be advanced by direct communication between active implanted nodes (In-body).

The Solution

Increasing the transmission power of an implanted device [4] is generally not a feasible option due to Specific Absorption Rate (SAR) regulation restrictions and power consumption considerations of the frequently battery-powered devices. Therefore, innovative advances for the surface worn “repeater” devices become a viable solution to the challenge [5]. The repeater device facilitates efficient communication between the body worn and implanted devices at as low a transmission power as possible (thereby decreasing power consumption and SAR) and increasing the lifetime of the implanted device and the communication range in the off-body link.

The AntennaWare Multiple Mode Antenna technology replaces conventional single-purpose sub-optimal antenna design for dynamic, complex applications. The antenna achieves all three propagating modes using a single antenna with optimal performance, where at least two or more antennas with sub-optimal characteristics and performance would normally be required.

The multiple-mode antenna technology is low-profile on-body antenna solution for robust and efficient communication with a deep implant antenna in an unknown location and unknown orientation. The solution exploits the electromagnetic fields as they propagate from an implanted source to the body’s surface.

The antenna also overcomes the issue of implant orientation and polarisation diversity, by incorporating a Circularly Polarized (CP) into-body mode in the antenna structure to mitigate cross-polarization losses with the implanted antenna with an off-body mode introduced for communication with an off-body node. In conjunction with the on-body mode for non-aligned implant communication, this produced a triple-mode antenna.

Summary

The Multiple Mode Antenna technology takes away the uncertainty in deploying systems on dynamic, variable platforms, giving confidence and a new edge in system performance. It makes the communication link where others cannot. The AntennaWare multi-mode antenna technology has also broader applications, as it could also be used in industrial applications to relay information from sensors embedded deep inside structures to the outside world.

References

• UKRI Grant EP/P000983/1, G.A. Conway, “Adaptive Multiple Propagating Mode Wearable Antennas” https://gow.epsrc.ukri.org/NGB...
• G.A. Conway, & W.G. Scanlon, “Wearable antennas for medical monitoring systems”. in Proceedings of 2015 International Workshop on Antenna Technology (iWAT). Institute of Electrical and Electronics Engineers Inc., pp. 19-21, 2015 International Workshop on Antenna Technology (iWAT), Seoul, Korea, Republic of, 04/03/2015. https://doi.org/10.1109/IWAT.2...
• M.K. Magill, G.A. Conway, & W.G. Scanlon, “Challenges in Multiple Propagating Mode Wearable Antennas for Medical Applications”. in 2018 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting: Proceedings. Institute of Electrical and Electronics Engineers Inc., 2018 IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting - APSURSI2018, Boston, Massachusetts, United States, 08/07/2018.
• M. K. Magill, G. A. Conway, and W. G. Scanlon. “Tissue independent implantable antenna for in-body communications at 2.36 - 2.5 GHz”. IEEE Transaction on Antennas and Propagation, 65(9):4406 – 4417, Nov 2017.
• W.G Scanlon. “Rethinking antenna requirements for medical implant systems”. In European Conference on Antennas and Propagation, 2011.

An edited version of this article was published in Electronic Product Design & Test