Wireless Communication for Implanted Wireless Sensors

06 June 2023

There has been an issue relating to wireless wearable technology, which has been bothering the antenna/propagation research community for almost two decades. Dr. Gareth A. Conway, Dr. Matthew K. Magill & the AntennaWare team ask - Can all the propagation requirements of wearable communication devices be achieved efficiently and robustly using a single antenna?

The overall objective of research being conducted at the Centre of Wireless Innovation, in Queen’s University Belfast since 2005, has been 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 researchers involved in this work is to realise a unique antenna concept, that meets all the propagation requirements of any wearable application yet still delivers high performance regardless of the hostile and diverse properties of its host platform - in this case, the human body. They were set the goal of designing/prototyping a single advanced antenna structure to adapt to medical propagation requirements plus the various physiological and morphological parameters of the human host. 

The need
Adoption of wireless communications in body-centric applications could soon have a beneficial impact on everybody, particularly given the aging population demographic. Via such technology, better quality of life could be provided to the elderly and infirmed. Early detection and faster intervention for patients would be possible, translating into better patient outcomes and scope 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, worn by users 24/7, are one of the fastest-growing applications. Wearable computing technology, as it is continually interacting with the wearer, requires synergy between the wearer and the wearable device.  

Wireless wearable systems offer considerable advantages over handheld devices, allowing the user to continue what they are doing while sensing or system commands are being executed. The enabling component which makes such wireless body area networks (BANs) efficient and useable is the wearable antenna. Efficient performance is not only important in terms of reduced power budget and battery size, but also for extended range and/or wireless link reliability. In healthcare technology, reliability and trust in the system are critical if widespread adoption is to be secured. 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, and thus the number of base stations needed. In a clinical patient monitoring network, wearable sensors are ideally of disposable cost and the base stations are premium in comparison. Therefore, any advancement in wearable system efficiency will increase the operational range of each clinical sensor network - thereby lowering the number of base stations required. Reduced systems costs will allow more patients to be wirelessly monitored, for especially non-critical illnesses.

Antennas for healthcare
The state-of-the-art in wireless sensing currently consists of sensor systems that are either too large, have high energy requirements or have insufficient performance in a human body context for them to meet the demands of emerging therapeutic and monitoring applications. While there have been steady improvements in antenna performance over recent years, overall link budgets remain marginal and therefore unreliable for more challenging scenarios. Current research practice in wearable antenna design activities 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 can only support one propagation mode. Consequently, sub-optimal printed antenna technology is generally adopted, as there is a lack of compact, high-performing adaptive antennas that meet realistic wearable requirements.

Next-generation wireless communication research is exploring more efficient modulation techniques, communication protocols, coding, power management, security and encryption. In modern wearable applications, where the dynamic body presents an extremely difficult environment that degrades communication channels, 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.

Antenna and propagation requirements 
The perfect antenna would be one which supports the propagating modes for the required communication link, but maintains this performance regardless of the variable characteristics of the human body host. The target application for this technology is for wireless sensing and communication for medical body area networks (MBAN), where there are four core propagating modes required for each channel. Three of these can be addressed by the surface worn antenna, with the fourth concerning implanted devices only. The four propagation modes are as follows:

•Off-body mode - Antennas are designed to maximise wave propagation in the off-body direction.
•On-body mode - Antennas are designed to enhance surface waves polarised normal to the body surface, maximising surface wave propagation.
•Into-body mode - Antennas are designed to maximise coupling to implanted antennas with potentially unknown polarisation 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, synchronising with other surface worn sensors within a network (on-body) and relaying 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 implanted device transmission power is generally not a practical option, due to specific absorption rate (SAR) regulation restrictions and power consumption constraints of battery-run devices. Therefore, innovative advances for the surface worn ‘repeater’ devices become a viable approach. 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, as well as the communication range in the off-body link. 

Leveraging the work done at Queen’s University Belfast, AntennaWare’s multiple-mode antenna technology replaces conventional single-purpose sub-optimal antenna design for complex dynamic applications. All three propagating modes can be addressed 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 enables low-profile implementations through which robust and efficient communication with a deep implant antenna (in an unknown location and unknown orientation) can be derived. It exploits the electromagnetic fields as they propagate from an implanted source to the body’s surface. The issue of implant orientation and polarisation diversity is also overcome, by incorporating a circularly polarised (CP) into-body mode in the antenna structure to mitigate cross-polarisation losses in 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, a triple-mode antenna is produced.

Multiple-mode antenna technology takes away the uncertainty in deploying systems on dynamic, variable platforms - giving assured confidence in system performance and attaining communication links where others could not. AntennaWare has been granted the European patent for this technology and is now marketing products featuring it. There are broader applications that should be considered for multiple-mode antennas too – with opportunities in industrial settings to relay information from sensors that are embedded deep inside structures to the outside world.

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