Over the last 15 years, the Bluetooth low-energy standard has enabled the development of wireless products where space is at a premium. But implantable continuous glucose monitors (CGMs) present particular challenges. This article examines the unique complexities of this application and demonstrates a systems-based approach to design and test two candidate antennas for implantable CGMs.

Launched in 2011, the Bluetooth low-energy (BLE) wireless communications standard has become widespread in settings from asset tracking in industrial manufacturing to data collection in healthcare, including smart watches.

One advantage of BLE is low energy consumption, which means battery lifetime is now less of an obstacle to the development of implantable wireless sensors than it used to be. That’s spurred the development of implantable devices for several applications, but implantable continuous glucose monitors (CGMs) pose particular challenges—even when the need is clearly there.

The lifetime of glucose sensors (typically around six to 12 months for implantables) means implanting CGMs by means of invasive surgical procedures every so often is not an option. Instead, implantable CGM devices need to be easily injectable during a simple outpatient procedure, but this constrains the form factor.

TTP’s Biosensing team recently embarked on a project to overcome the challenges of designing an implantable continuous glucose monitor (CGM) that maintains reliable communication links—for example, with a smartphone.

While this work focuses on CGM, many of the same wireless design considerations apply to other implantable devices, such as neurostimulators or drug delivery implants, where the balance of form factor, battery life, and connectivity introduces its own unique set of challenges

Accounting for Component Size Constraints and Case-to-Case Variability

The first and most obvious design constraint is the implant shape and size. The only injectable shape with enough volume to contain the necessary components will likely be a cylinder of about 40 mm long and 4.0 mm wide to maintain user comfort and an acceptable appearance.

The second constraint comes from the battery. Even with the best existing battery technology and BLE, any battery would nevertheless be expected to occupy a considerable fraction of the volume of such an implant.

These two constraints greatly limit the space available for the antenna. This is troublesome, in part, because antenna performance drops off rapidly at smaller sizes. A further constraint is the longitudinal axis of the implant must lie parallel to the skin in order to be easily implantable, and in some body locations, the implant must also be parallel with a body part, such as the arm.

Another constraint for the designer is that a reliable signal needs to be maintained in a wide range of use scenarios.

Some CGM products may be implanted in several body locations, or they may move after implantation. Then, there are the types, thicknesses, and anatomies of the various tissues in different people. But if the implant is going to communicate to a smartphone or other monitor, it needs to work regardless of the user’s posture.

This means the signal can’t just be good when the user is standing upright and holding the device in front of them. It also needs to maintain that connection when, for example, the device is in the user’s back pocket on the other side of their body, or when they’re sleeping, and the device is on a bedside table.

In such scenarios, the device may well be over a meter away, with various materials in between, leading to sub-optimal impedance matching and consequent loss of signal.

Systems-Level Design and Body-Phantom Testing

Together, the form factor constraints and use scenarios can make it difficult to know where to start with antenna design for this application.

The first step was to take a systems approach to identify suitable candidate antenna designs, before devising a tissue model to test them. Two designs were then validated using a simplified “body phantom,” which showed promising results—even under the tight size constraints imposed by an injectable CGM.

Overall, the results show there should be no insurmountable barriers in the way of developing a truly self-contained injectable CGM, reducing risk for manufacturers interested in grasping this exciting opportunity.

Importantly, the principles demonstrated are also relevant beyond CGM, with the potential to guide the development of other implantable technologies, from neuro to drug delivery systems, where dependable wireless communication is equally mission critical.

Simon Calcutt is an experienced technical program lead, specializing in medical devices and biosensors. With a background in electronics engineering and system design, he has a strong interest in low-power and analogue systems. Since 2010, he has managed and technically led developments from early-stage concepts and technology exploration through to device verification and manufacturing setup. His expertise spans implants, wearable sensors, surgical systems, and drug delivery devices. Calcutt excels at leveraging multidisciplinary teams to help clients manage complex device developments, capturing commercial and product needs, identifying the right solutions to technical trade-offs, navigating risks, and guiding projects through a full ISO 13485 development process to verification.