One of the toughest problems facing portable medical devices today is power consumption. Bluetooth low energy technology could help solve it.
Wireless technology has been replacing cables and allowing increased patient mobility for decades through ambulatory patient monitors. Portable patient monitors, infusion pumps, surgical foot switches, and dozens of other medical devices currently use wireless connectivity to maintain a connection to monitoring and information systems.
One of the toughest issues facing portable medical devices today is power consumption. Power requirements for wireless connections constrain architecture and limit applications.
|Figure 1. Bluetooth low energy chip sets are available in two versions.|
That could all change, thanks to the introduction of Bluetooth low energy technology specified in Bluetooth v4.0, which is making its way to designers and consumers today. Recent announcements by Microsoft and Apple supporting this new technology show its wide-ranging deployment in standard computing and communications platforms. The stage is set to deploy truly low-power wireless medical applications that have relied on custom components and platforms until now.
Bluetooth low energy technology is different from other wireless technologies because it combines a standardized technology designed from the very beginning for ultra-low-power batteries and a new sensor-based data collection framework. Bluetooth low energy technology will also be integrated in most handheld devices.
Bluetooth Special Interest Group (SIG) working groups have already released several profiles for health, fitness, and medical device use, and others are in progress. While these devices are not directly compatible with IEEE 11073, a whitepaper detailing the data conversion and compatibility mapping is also being developed to make data available to systems using the IEEE 11073 standard.
The Bluetooth SIG forecasts that Bluetooth low energy technology will be implemented in billions of products within just a few years:
Bluetooth low energy technology is the key feature of the Bluetooth Core Specification 4.0 (Bluetooth v4.0) and has inherited several technical features from Classic Bluetooth technology that provide for robust, reliable connections. New features allow for event-driven data acquisition, proximity sensing, and time synchronization. But in many ways, Bluetooth low energy technology is a very new wireless technology. Bluetooth v4.0 is fundamentally different in that it is designed for transmission of small amounts of data instead of periodic data streaming connections featured in Classic Bluetooth technology. For example, Classic Bluetooth technology provides support for headset and streaming audio data, a feature fundamentally absent from the Bluetooth low energy technology model. The technology features efficient discovery and connection set-up, short packages, and asymmetric design for small devices.
|Figure 2. Bluetooth low energy technology uses 40 instead of the Classic Bluetooth technology’s 79 channels.|
The Lowest Possible Power Consumption. Everything from physical design to use models is designed to keep the power consumption at a minimum. In order to reduce the power consumption, a Bluetooth low energy device is kept in sleep mode for most of the time. When an event occurs, the device wakes up and a short message is transferred to a gateway, PC or a smart phone. Maximum/peak power consumption is less than 15mA and the average power consumption is about 1μA. The active power consumption is reduced to a tenth of the energy consumption of Classic Bluetooth technology. In low duty cycle applications, a button cell battery CR2032 could last for 5–10 years of operation.
Cost Efficient and Compatible. In order to offer compatibility with Classic Bluetooth technology and cost efficiency for small battery-operated devices, there are two chipset types:
|Figure 3. This illustration demonstrates AFH operation in the presence of three channels of 802.11b/g (Wireless LAN). The three advertising channels as well as nine data channels out of the 37 data channels are located in-between the channels that are used by Wireless LAN.|
Robustness, Security and Reliability. Bluetooth low energy technology uses the same adaptive frequency hopping (AFH) technology as Classic Bluetooth technology to achieve a robust transmission in noisy RF environments found in the home, industrial, and medical applications. To minimize the cost and energy consumption, Bluetooth low energy technology has reduced the number of channels to 40 2 MHz wide channels instead of the 79 1 MHz wide channels in Classic Bluetooth technology.
Wireless Co-existence. Bluetooth technology, Wireless LAN, IEEE 802.15.4/ZigBee, and several proprietary radios use the license-free 2.4 GHz industrial, scientific, and medical (ISM) bands. With so many technologies in the same radio space, interference can decrease the wireless performance (latency and throughput) due to the need for error correction and retransmission. In demanding applications, interference can be reduced through frequency planning and special antenna solutions. Both Classic Bluetooth technology and Bluetooth low energy technology utilize AFH, making the Bluetooth transmission robust and reliable. AFH also minimizes interference from Bluetooth technology to other wireless ISM band radio technologies.
|Figure 4. An advertiser periodically sends messages and will always be a slave once the connection is established. A scanner is ready to receive an advertisement message and a connection request and will always be a master once the connection is established.|
Connection Range. Bluetooth low energy technology has a slightly different modulation than Classic Bluetooth technology. This modulation differentiation offers a range of up to 300 meters with a 10dBm radio chipset (Bluetooth low energy maximum).
A Bluetooth low energy solution is typically based on a master connected to a number of slaves. A device is either a master or a slave, but never both. There is no scatternet topology for Bluetooth low energy technology. The master controls how often the slaves are allowed to communicate, and the slave only communicates by request from the master.
A new Bluetooth low energy feature is the “advertising” functionality (Figure 4). A device (acting as a slave) can in this way announce that it has something to transmit to the master. An advertisement message can also include an event or a measurement value. This feature could be used, for example, to implement a fall detector that would advertise its need for help while transmitting the location of the person who fell—all without completing a connection.
Software Structure. All parameters in Bluetooth low energy technology have a state that is accessed using what is called the Attribute Protocol. All attributes are represented as characteristics that describe signal value, presentation format, client configuration, etc. A battery, for example, could have the following characteristics:
|Figure 5. Example home medical Bluetooth low energy use model.|
In the Generic Attribute Profile (GATT) service groups, features, and declarations are brought together to specify the standard set of features available in all devices. In the Generic Access Profile (GAP) connections, discoverability, connectability, and bonding are described. Through these attributes, numerous basic services and profiles can be built. Some examples of basic services and profiles include the following:
Example Use Models. The reduced power and cost of Bluetooth low energy technology enable many medical use models from the home to the hospital. The example model in the home could be used to log data and establish trends for a congestive heart failure (CHF) patient using several types of sensors and the patient’s mobile phone.
A similar architecture is shown (Figure 6) for use as an in-hospital vital signs monitor, using Bluetooth low energy and Classic Bluetooth technology where appropriate based on the technologies’ strengths.
|Figure 6. Example of a hospital medical Bluetooth low energy use model.|
Bluetooth low energy technology is a powerful application enabler that will change the way we experience wireless applications. Its low power and cost, as well as its robust communication technology, make it ideal for low-power medical applications in the home and hospital.
As the technology is being implemented in almost all mobile devices, there is an opportunity to utilize off-the-shelf handheld devices and enable new use models that leverage the rich user interface and communication features of those platforms.
Rolf Nilsson is the CEO and founder of connectBlue. Bill Saltzstein is the company's president and medical business development manager.