Optimizing Power and Security in Bluetooth Low Energy Medical Devices
September 18, 2023 3:00 p.m.
Nicola Wrachien, Staff Solutions Architect, and Brian Blum, Senior Product Marketing Manager – Silicon Labs, outline some practical approaches to optimizing power and security in Bluetooth Low Energy medical devices.
Introduction
For the healthcare industry, What’s more to improving hygiene, reducing bacterial spread and simplifying logistics, the concept of connected disposable medical devices will allow safer and more cost-effective medical devices to be offered to a broader population, as exemplified by the continuous monitoring of glucose (GGM). However, disposable medical devices must be replaced periodically, which means that the bill of materials (BOM) must be kept as low as possible to make it economically attractive. This article discusses two main (and interrelated) challenges (safety and power consumption) for creating practical disposable medical devices powered by coin cell batteries and using the Bluetooth Low Energy (BLE) communications protocol. It also considers some practical solutions that Silicon Labs proposes to address them.
Balancing privacy and security with crypto overhead
Patients and doctors use data from remote medical devices to inform their treatment decision-making, which means corrupted data can have serious consequences for patient health. They can occur deliberately (unwanted intrusion by a third party) or accidentally (due to environmental interference) when data travels from a device to the point of analysis. Privacy is also crucial for medical devices. Even detecting the presence of a device can be considered a privacy leak. Another privacy-related security issue is that intruders could use an unsecured device to track the user’s movements. Therefore, they must not contain information that could allow their detection by an unauthorized third party.
Device Microcontroller Security Download
Cryptographic techniques such as authentication and encryption can help mitigate security and privacy concerns, but implementing them is compute-intensive, requires increased power consumption, and negatively impacts battery life. One way to overcome this, which also increases the security of the device, is to store the root encryption keys in a separate security core that communicates with the main device’s microcontroller via a mailbox system (rather than shared memory). The security core receives encrypted keys, along with the data to be decrypted/encrypted from the main microcontroller, decrypts the encryption key using the root key and then communicates the encrypted/decrypted data to the main microcontroller using the same mailbox system.
Separate hardware accelerators
Differential power analysis (DPA) is often used by intruders to bypass cryptographic techniques. This approach involves analyzing the power consumption of the device while performing encryption operations, which can be used to determine encryption keys. For this reason, many device manufacturers implement DPA countermeasures, but they increase computing time, power consumption, and circuit complexity. Furthermore, the use of a separate secure element could result in higher power consumption due to the additional overhead due to communication between the secure element and the main core. A possible solution is to use two hardware accelerators: one in the main microcontroller that manages wireless communications protocol (prioritizing speed and low power) and another in a secure core. While this means that encryption/decryption operations may be slightly slower, it offers greater robustness and security.
Adapt energy consumption to useful life of product
A device’s battery must strike an optimal balance between having a practical form factor and providing sufficient power over its life. Some factors affecting wearable medical devices’ lifespan include chemical degradation and hygiene. Minimizing energy consumption allows for lower capacity batteries (and therefore smaller, lighter, lower cost), reducing bill of materials size and weight making disposable devices more comfortable to wear over a long period.
Optimization of radio operation
The radio subsystem is among the most power-consuming peripherals in an RF system-on-a-chip (SoC). Transmit and receive operations can dramatically affect battery life. One way to reduce power consumption is by reducing transmit power. Another optimization technique involves adjusting the advertising and connection intervals, reducing the duration of transmit/receive events relative to silent periods (the RF duty cycle).
Reduce power consumption in rack mode
Logistically, a device could spend much of its useful life in storage (shelf mode). Without proper design, a device’s battery could continue to drain or completely drain before the customer can use it. To prevent this, one solution is to place a device in a low-power state from which it wakes up sporadically, announces its presence, and waits for a host device to connect before pairing or returning to a low-power state. Another option is to provide an electrical means to activate a device from a very low-power state.
Conclusion
This article analyzes two interrelated challenges (safety and energy consumption) in creating practical disposable medical devices. It also presents some practical solutions proposed by Silicon Labs to enable the healthcare industry to offer more hygienic, safe, and cost-effective disposable medical devices to a broader population.
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