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This article looks at the challenging requirements of wireless monitoring in portable medical systems. From ultra-low-power transceivers and different protocols to highly efficient power management and sensor interfaces, there is a wide range of engineering tradeoffs to be considered for a high reliability medical design.
Developers and users of portable medical devices are demanding ever more connectivity and longer battery lifetimes, and as a result security is becoming a key issue.
This is being tackled in several ways, from adding security features into protocols such as Bluetooth Smart to making home monitoring more secure.
IMS Research projects that Bluetooth Smart used in medical devices powered by coin-cell batteries will become a significant factor in the next few years, with over 4.7 million systems shipping in 2016 alone. Bluetooth Smart will be providing the wireless connectivity for over a third of these, in part driven by the security of the connections.
This demonstrates the close connection between the security and reliability of the design, the microcontroller, the wireless transceiver, and the design tradeoffs between the different elements.
Bluetooth Smart has gained in popularity as it allows a smartphone to be used as the interface to a portable medical device. The data sent to the phone can either be processed locally or sent via the cellular network or a local Wi-Fi network to the cloud for storage and analysis. All of this significantly reduces the demands on the equipment designer but imposes stronger needs on ensuring that the data is secure throughout the transmission chain, which is where Bluetooth Smart is particularly popular.
One of the ways to do this is through the Health Device Profile (HDP) developed by the Bluetooth Special Interest group (SIG). This optimizes the performance of the wireless link for lower drain on the battery and adds a number of formats for particular applications, especially in medical equipment. This means a medical product designer can choose a profile to suit a specific application, reducing the memory and power overheads. The HDPs support medical and fitness applications such as body temperature measurement, blood pressure, weight scale, glucose, pulse oximetry, heart rate, pedometer, speed, distance, cycle cadence, simple remote control and battery status.
Figure 1: The data flow from a wireless medical device via Bluetooth to a smartphone and onto the cloud.
Alongside security of the data, Bluetooth Smart has also added privacy protection, especially when one is on the move and out and about. This limits the ability to track a transmitting device by using a random device address that is frequently changed.
As the trend toward electronic enabled medical records and telemedicine continues, medical practitioners have the same duty to safeguard a patient’s medical records and keep their treatments confidential as was required with traditional paper records. With electronic documents more easily being duplicated and transmitted, the storage of electronic files, images, audio and video needs to be done with an even higher level of security. As a result, digital security needs to be carefully considered in the development of any portable medical device.
TI’s MSP430FR59xx family of 16-bit microcontrollers uses ferroelectric memory (FRAM) for low-power storage and when combined with wireless products, can be used for blood pressure monitors, blood glucose meters, weight scales, pulse oximeters and more through the Bluetooth Smart personal healthcare device class (PHDC) protocol.
This uses the same AES encryption technology that is specified in classic Bluetooth, but implements it in a hardware accelerator on the FRAM microcontroller to keep the power as low as possible. The 256-bit AES engine can be used for 128-, 196- and 256-bit key sizes and there is DMA support for the different types of cipher modes such as ECB, CBC, OFB and CFB. This helps to reduce power consumption by transferring data directly to the AES engine without having to use the central core of the wireless transceiver.
The connection to the transceiver is also as low power as possible, with a 12-bit analog-to-digital converter (ADC) that consumes 75 µA at 200 ksps and supports up to eight differential inputs from channels. There is also a window comparator function on every input to make it simple to make comparisons of data so that only significant changes are captured, which further saves power in medical applications.
The FRAM memory in the controller saves 12-15% of the battery life by making wireless firmware updates easier and providing a faster response for time-sensitive data storage. The long cycle endurance of FRAM also eliminates the need for external EEPROMs, which makes the design more secure as the code is stored internally, reducing power, the bill of materials and manufacturing complexity. The device also includes IP protection, device ID, tamper detection and secure data logging to further protect the data running through the wireless system.
Ultra-low-power modes are also vital for this type of design and the platform supports seven low power modes with a fast wakeup of under 6 µs.
These microcontrollers are intended for use with a wireless transceiver such as Texas Instruments' CC2541 Bluetooth transceiver. This includes royalty-free firmware called BLE-Stack that enables over-the-air (OTA) downloads to a portable design to make it easy to upgrade. The controller, host and application processor are all integrated into one 6 mm x 6 mm package. The low power design of the transceiver with an 18.5 mA transmit current means it can operate for over a year on a single coin-cell battery.
Figure 2: The CC2541 development kit includes the Bluetooth Smart transceiver for secure wireless links.
A different approach has been taken by Dialog Semiconductor with the DA14580. This is a fully integrated radio transceiver and baseband processor for Bluetooth Smart. It can be used as a standalone application processor or as a data pump in hosted systems with a separate microcontroller.
The DA14580 supports a flexible memory architecture for storing Bluetooth profiles and custom application code, which can be updated OTA. The Bluetooth Smart protocol stack is stored in a dedicated ROM and runs on the integrated 16 MHz ARM Cortex-M0 processor via a simple scheduler.
The Bluetooth Smart firmware includes the L2CAP service layer protocols, Security Manager (SM), Attribute Protocol (ATT), the Generic Attribute Profile (GATT) and the Generic Access Profile (GAP) and these are supported by a 128-bit AES encryption processor to secure the data traffic.
To simplify the equipment design the transceiver has a direct connection to the antenna at one end and a dedicated accelerator for the Link Layer at the other end, providing a 93 dB link budget. All the RF blocks in the design are supplied by on-chip low-dropout regulators (LDOs), which are programmable per block and optimized for minimum power consumption.
The DA14580 comes with Dialog’s SmartSnippets Bluetooth software platform that includes a qualified Bluetooth Smart single-mode stack on-chip. This has a range of Bluetooth Smart profiles for consumer wellness, sport, fitness, security and proximity applications as standard, and additional customer profiles can be easily developed and added to the stack. The SmartSnippets software development environment is based on the uVision tools from Keil, which include example code for both embedded and hosted modes.
Not all the portable medical designs need be Bluetooth Smart though. Home health monitoring systems can make use of devices such as Silicon Labs’ Si106x/8x and connect to home networks rather than mobile phones and so make use of additional computing resources in the hub to implement the security. For systems that will not leave the home, this can provide smaller and lower cost implementations. The 106x/108x combine high-performance wireless connectivity and ultra-low power 8051 microcontroller processing into a small 5 mm x 6 mm form factor. Support for major frequency bands in the 142 to 1050 MHz range is provided, including an integrated advanced packet handling engine and the ability to realize a link budget of up to 146 dB. This allows the designer to trade off the link budget and range for lower power consumption and longer battery life.
Figure 4: The Si106x transceiver combines an 8051 controller core with a sub-GHz wireless link for low-cost monitors in the home.
The Silicon Labs devices have been optimized to minimize energy consumption for battery-backed applications by minimizing the transmit, receive, active, and sleep mode current as well as supporting fast wake-up times. The Si106x wireless MCUs are pin-compatible with the Si108x devices that can scale from 8 to 64 kB of Flash and provides a robust set of analog and digital peripherals including an ADC, dual comparators, timers, and GPIO.
Security is an increasingly important design requirement in wireless portable medical equipment. Using the latest profiles within Bluetooth Smart provides the ability to add secure links while at the same time optimizing the design for the particular medical applications to deliver the best possible battery life. This can be done through a combination of an ultra-low-power microcontroller and transceiver, or through a highly integrated system-on-chip design. It can also be achieved with a simpler transceiver linking to a home hub that has more processing power to ensure that data is kept safe through the transmission chain.