Advances in semiconductor technology have recently enabled extremely low-power battery-powered embedded systems that are so light weight and small that they are wearable. These systems are typically characterized by powerful, very low power microcontrollers that interface to a set of highly sophisticated sensors while communicating information via low-power radio frequency links to external systems.
The combination of powerful, yet very low power digital microcontrollers, very low powered analog body signal sensors, and innovative power and battery management circuits is coenabling the development of the wearable healthcare market.
Body signals have been monitored for many years to provide physicians with useful health diagnoses information. The same monitoring equipment has also been used in high-performance sports applications to help optimize performance. Wearable body signal monitoring products can now provide the same type of information to consumers at much lower price points for both health and performance optimization markets.
For health sensing and monitoring, almost all the signals that are traditionally monitored in a clinical environment can be obtained by a wearable product. These traditional signals include:
For information about designing wearable health products, click the "Design Considerations" tab. For a list of featured Maxim ICs, click the "Circuits" tab. To view block diagrams of typical wearable products, click the "Block Diagrams" tab.
This market is being enabled by high performance and sophisticated ICs. These ICs have been power optimized so they can provide body signal monitoring functionality using small light-weight rechargeable lithium-ion batteries or replaceable nonrechargeable coin-cell type batteries.
While many of these products' main features are implemented via firmware algorithm, the physical design provides a platform to host these features. Once a platform is developed, it can be re-used for an array of different products.
Power is a very important aspect of any wearable healthcare platform. This product category must be small and nonintrusive, and so must have a very small, lightweight battery. The total charge available in the battery along with the power dissipation characteristics of the platform determine the product's usability. Typically, any wearable product would be expected to function over the period of at least a day before requiring recharging. Products with nonrechargeable batteries should have multimonth battery lifetimes.
For devices with rechargeable batteries, the battery management system must include a battery charger and a battery fuel gauge. The battery management system must allow the device to operate while the device is charging.
The power system must be able to regulate voltage from a battery—a voltage source with a declining voltage output. The regulators must be very efficient so as to maximize charge usage, and must also supply the number of rails required by the design. The usable voltage range of a rechargeable Li+ battery ranges from 4.2V to approximately 3.2V. Most wearable products use main power rails that are below the minimum charge of a single-cell Li+ battery, so the main rails within a wearable design are sourced from a step-down regulator. Some functions within a wearable product might require a higher voltage level than is provided by a single-cell battery. To provide these voltage levels the power management function must contain at least one step-up regulator. The number of rails required depends on the device functionality, but for optimum efficiency is it best to minimize the number of required rails.
The MAX14676 wearable charge management IC includes multiple voltage regulators, a battery charger, a power selector, and a fuel gauge. This device features Maxim's ModelGauge™ fuel gauging algorithm, and Smart Power Selector™ technology. This single IC takes care of all aspects of battery management for designs that are powered with a single-cell Li+ type battery.
This single IC provides output rails of 1.8V, 3.2V out (LDO), 6.6V out (via a charge pump), and 12Vout (via a boost switching regulator).
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Power usage and processing capabilities are the most important selection determinants for a microcontroller for this application. A system partitioning strategy should be used to decide which system functions are best integrated into the microcontroller and which can be handled externally. Because the wearable health devices read body signals, the capabilities of any on-chip data analog circuitry must also be taken into account to ensure they can accurately process low-level body signals.
For a microcontroller, two general low-power strategies are available:
If the lower cost microcontroller option is chosen, the precision signal conversion must take place on external signal processing chains that input sensor signals into the microcontroller digitally. Very small, high-precision and low-power analog circuits are available to support this option.
Many low-power microcontrollers have been recently introduced to the market. The most popular for wearable applications have ARM® architectures that are optimized for low power dissipation. Depending on the processing requirements of the device, the processor will range from 16 bits to 32 bits. The processor will incorporate multiple power consumption modes and the system software will have programmable shut-off and sensor-based wake-up capability.
For wearable applications, Maxim offers a precision low-power ARM-based microcontroller that has a high level of integrated precision analog circuitry. The MAX32600 combines a low-power ARM Cortex® M3 microcontroller with, among other features, a 16-bit ADC, and an integrated trust protection unit that feature on-board public key authentication, data encryption, and tamper detection to provide the highest level of security.
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Many sensors are available to monitor body signals in a wearable device. The sensor technology for obtaining body signals has been available for many years, but only recently have sensors become available that can provide good signals without consuming large amounts of power.
Sensors technology is available to measure the following:
The electrical outputs from these sensors are very small, in the millivolt range. However, many of these popular sensors have been combined with amplification and conversion circuits within a single package so that they output either a higher level analog signal or a serialized digital signal. The interface circuitry for these sensors is engineered for extremely low power operation.
In the case of electrocardiogram sensors, these are essentially physical skin contacts that capture the very small electric field around a skin area and transfer the signal to the EKG signal chain. Low-cost wearable electrocardiograms are limited to between 2 to 3 contact points and do not provide the resolution of higher cost professional ECG/EKG systems that employ from 9 to 11 sensors that are dispersed throughout the body and attached at strategic points.
Maxim's MAX30100 is an integrated pulse oximetry sensor solution. It combines two LEDs, a photodetector, optimized optics, and low-noise analog signal processing to detect pulse oximetery signals, which also provide heart-rate signals.
Body temperature sensors are usually resistance temperature detectors (RTDs) that require an analog signal chain to excite and amplify. The signal chain can consist of external analog signal chain or run directly into the system microcontroller if the microcontroller contains precision analog circuitry such as that found in the MAX32600 wellness measurement microcontroller.
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Modern wearable devices will all generally provide a Micro-USB port for mass data transfer, firmware updates, and battery charging. In addition, many wearable wellness products employ a low-power wireless transceiver to transmit and receive data in real time while the device is in use. Wireless transfer allows data transmission to larger display screens or to remote data collection facilities. Low-power Bluetooth is an emerging standard for this purpose. In addition, NFC (near field communication) provides limited-range wireless connectivity that is ideal for short content transfers such as configuration information and logged data retrieval.
Maxim's MAX66242 secure RFID tag can authenticate a user, thereby accepting communications only from authenticated sources via NFC.
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The user interface for a wearable product will vary based on needed functionality. Low-power design is paramount so the display size is minimized. Depending on the product, the UI will consist of a single-line LCD display with a few control buttons. Products that need to display more information will have a low-power TFT display and most likely include touch-screen capability.
Because processing power has become so cheap and powerful, it is possible that many wearable devices will eventually have a voice command interface.
Maxim's ultra-low power MAX98091 audio codec has microphone inputs as well as high quality audio output capability.
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