Medical monitoring gets personal, goes mobile
The trend towards technology "personalization" continues, and it is heading into new areas. It began with personal computers in the 1980s, went on to desktop publishing, and has extended to our ubiquitous voice and data communications via smartphones. We are now seeing this trend branch into personal medicine and medical devices which are no longer confined to formal clinical settings or occasional use.
Vital signs monitoring (VSM) of individuals includes both those who have medical issues (remote health monitoring), as well as those who do not yet but are nonetheless concerned about their health and well-being (personal health management). Those who have medical issues want point-of-care monitoring and don’t want to be "tied down", while those who are well are often on-the-go individuals, and expect their VSM to be a relatively transparent part of their daily lifestyle and routine.
We are entering an era where individuals want to monitor their vital signs at home, at work, in the car, at the gym, on the sports field, and more. With these new expectations come new challenges for system developers. The increasing level, diversity, and spread of VSM is due to a synergistic combination of improvements across multiple areas: sensors, analog front ends, signal processing, and multiple wireless connectivity options. Many of these advances are driven by Moore’s law and its corollaries, and are somewhat predictable, such as advances in highly capable, integrated analog front ends.
The appearance and impact of MEMS devices as compact, easy-to-use, low-cost, high-performance sensors was not in the crystal balls of many industry forecasters and pundits. Consider this: what began as an application-specific, tightly focused sensor technology for one application–namely, automotive airbags–has spawned an incredibly diverse array of sensors and resultant applications including health and safety monitoring at clinical levels using personal venues.
The challenge for designers goes well beyond the electronics alone. Having these devices available and useful in so many diverse settings puts new demands on product packaging, materials, sensor interface, user interface, ease of use, reliability, run time, regulatory certification (in some cases) and, of course, cost.
Note that the use of these sensors in mass-market, consumer products affects more than just the obvious electronic-design challenges. Packaging and appearance must shift from just functional to esthetically acceptable and even fashionable, while still meeting the technical requirements. The "soft" design mandate to also be stylish adds yet another dimension to the engineer’s list of product design challenges and constraints.
In addition, the in-use environment for these devices is less controlled, less respectful, and more varied than in static, clinical settings. Users may toss the devices, inadvertently abuse them, or get them dirty or wet, even when they are not designed for such handling. In addition, the devices often need to be designed to be used or worn 24/7 on the hip, wrist, upper arm, shoe, or elsewhere, and without battery-replacement or charging issues.
Common parameters yield to portable electronics
Let’s start with basic fitness monitoring for individuals who like to know how they are doing in daily activities. This is general-health monitoring as opposed to the type of monitoring performed during strenuous sporting activities.
We want to measure, track, and even alarm physiological factors such as temperature, heart rate, motion/activity level, and perhaps exertion and overheating. This information can be used, in part, to determine health-related characteristics such as calorie burn, sleep profile and number of steps taken over a defined time period.
In order to achieve a level of efficacy that keeps them out of the "gadget" classification, the measurement devices need to take on the role of lifestyle monitors that are personalized to our individual needs. They must be devices that fit in to our way of life non-intrusively and deliver on their promise of providing useful, accurate data.
To do this, they use strategically located sensors to monitor vital signs such as skin temperature, galvanic skin response, motion, heart rate, calories burned, activity level, and exertion. Whether the user is sitting, walking, driving, sleeping, working, or working out, these body-mounted monitors reveal calories burned and much more. When combined with information about food intake (calories consumed), a profile of lifestyle and fitness is developed.
Supporting the needs of the professional and amateur sports enthusiast provides a tougher challenge to system developers. Measurements taken during inactivity or carrying out moderate tasks is challenging enough, but achieving the same level of accuracy, consistency, and credibility while running, swimming or generally being pounded on, requires significant additional post-processing of the data using sophisticated algorithms, to address and eliminate the many artifacts which result from such motion.
It’s important to note that it is not just these extreme user situations which need insight into the nature of the sensor output and significant signal analysis and processing. Consider a "fall detector" aimed primarily at the elderly living alone. What makes such detectors possible, among other factors, are ICs such as the ADXL362 from Analog Devices, an ultra-low power, ±2/±4/±8g, 3-axis MEMS accelerometer that consumes less than 3 µA across its full range of output data rates, and just 300 nA in motion-triggered wake-up mode.
This device forms the sensing core of a continuous motion-and-fall detector, an application where portability and long battery life are critical features of the product. Its internal 12-bit ADC provides 1 mg/LSB resolution on the 2g range, and interfaces to a microcontroller via an SPI port.
But it takes more than a sensor alone for a successful end product. OEM designers must implement algorithms which interpret and act on the waveforms provided by the device, and these waveforms are not trivial: they occur along multiple axes and with uncertain timings (Figure 1).
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Common parameters yield to portable electronics
What about the gym or the car, which is where people increasingly spend their time, and which have increasingly become electronics-laden environments? For exercise equipment such as a treadmill or elliptical trainer, the user can grab the handles—and these can be a placement location for sensors. In the car, there is a built-in location for mounting VSM sensors: the steering wheel.
By attaching conducting electrodes, and an array of LEDs and photodiodes, to the steering wheel it’s as if the driver’s hands are being read by an old-fashioned palm reader, except in this case, the readings are not speculative. Of course, advances in the materials used and construction of the wheel will be needed to make this practical. (Figure 2)
Measuring values and changes in skin capacitance, conductivity, temperature, and position allows such a configuration to determine heart rate, sleep/drowsiness state, and even stress levels. More sophisticated algorithms can combine these various parameters to present a more comprehensive picture of the driver’s physical and even emotional well being.
Mixed-signal, low-cost ICs for this function make this application possible. For example, the AD8232 heart rate monitor (HRM) analog front end (AFE) is an integrated signal-conditioning block for single-lead electrocardiogram (ECG) and other bio-potential measurement applications. (Figure 3) It converts the tiny, noisy signals from body electrodes into large, filtered signals that can be easily converted by a medium-resolution ADC.
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Unlike clinical ECG units, which monitor up to 12 leads, designs based on the AD8232 are connected to just two or three electrodes. While this clearly simplifies the physical connection, here again we see the "conflict" among technical needs, product design, and user habits: The auto steering wheel (or gym machine) is certainly not a clinical setting. For viable ECG data for heart rate monitoring, the system needs both hands of the driver on the wheel or handlebars—which, in reality, is not often the case.
In addition, unlike the setting where the user is sitting still or lying down, the constant motion of the driver or a person on the machine affects the reading continuity. For this reason, the AD8232 includes a two-pole high-pass filter to eliminate these misleading signals. Once the signal has settled, the filter automatically switches to a lower cut-off frequency, to improve the overall noise behavior – this is the "fast restore” function of the AD8232.
The smartphone opens another avenue
The soon-to-be-ubiquitous smart phone is potentially rich with apps that use native sensors to determine vital parameters such as activity levels or sleep patterns, with the built-in accelerometer used to detect heart rate via the image sensor.
But there is additional potential beyond the smart phone becoming the communications link between vital-signs monitor and health service provider. It may also become the vital -signs monitor itself.
For example, a front-end loaded with sensors can be used in conjunction with the handset, directly reporting data to the phone. Alternatively, the handset can have embedded sensors which can be used to determine heart rate or, in a more advanced design, the level of oxygen in the blood (pulse oximetry). (Figure 4)
The pulse oximeter is a spin-off from the long-established technology for assessing patient’s blood-oxygen saturation level (SpO2—the ratio of oxyhemoglobin to the total concentration of hemoglobin present in the blood) and pulse rate non-invasively, in real time, by using photometric techniques. The method uses measurement of the transmission of both red and infrared wavelengths through the skin and blood vessels, and then uses an algorithm to calculate SpO2. (Figure 5)
It is even feasible for additional sensors to be embedded within the smartphone and on its case exterior. In addition to motion sensors in the phone, the periphery could have contact temperature sensors, as well as contact electrodes for measuring skin conductance and even bio-potential signals to capture heart rate and electrocardiogram (ECG/EKG) waveform data.
Here, as with the steering wheel or gym handles, engineers will need innovative thinking in enclosure design and materials, since the smartphone will need electrically conductive sensors on its periphery, and users will need to hold the phone with both hands for the HRM reading.
Home/out of hospital disease monitoring and management
It’s one thing to engage in VSM for people who have no apparent medical issues, and are trying to assess and improve their lifestyles through exercise, activity, and diet. But when the individual has a condition which needs close, detailed monitoring, the demands on the technology become more stringent.
For example, to provide detailed ECG waveform data, the traditional solution is to use a Holter monitor which records the biometric signals for one or just several days. However, for patients with arrhythmia symptoms (very sporadic), a longer recording time frame may be required, and the monitor should be as unobtrusive as possible.
The final challenge in a successful, viable personal VSM is getting the data "out". As with most engineering situations, there is no single, best solution that fits all applications. The VSM devices cited above use a variety of approaches.
A wireless link is preferred, of course, since it eliminates connectors and the problems they endure: dirt, intermittents, and physical abuse. Bluetooth and Bluetooth Low Energy (BLE), both in the 2.4 GHz band, have strong market positions, with the latter becoming increasingly dominant in VSM applications.
But don’t rule out network-based approaches, either. For users such as those in an assisted-living facility, ZigBee (also 2.4 GHz) is a viable option and does not require smartphones or similar nodes. Further, even proprietary approaches have their place: the sub-1 GHz ISM band is used in products with low-rate, intermittent data, such as fall detection alarms associated with seniors living alone.
About the author
Tony Zarola is a Strategic Marketing Manager, Vital Signs Monitoring, in the Healthcare Group at Analog Devices. A 24-year veteran with ADI from the UK, Tony has worked in various parts of the company around the world, including periods as a Characterization Engineer working on ADCs, a Marketing Manager in the broadband group, and a Product Line Manager for DSP products. Currently, Tony is focusing his attention on defining new products to support the next wave of medical innovation.
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