Sensor wear

Couch potatoes beware. You may be able to fool yourself about your unhealthy ways, but you won't fool the SenseWear Armband. Thanks to its array of sensors and embedded systems, this wearable medical monitor gathers raw physiological data and transforms it into detailed lifestyle pictures. It knows when you sleep. It knows how often and how hard you exercise. And it can capture information about eating, television watching, and other daily activities. So much personal information might sound like a narcissist's dream come true, but don't write the monitor off as a tool for the self-obsessed. The Armband, which just won a 2002 Industrial Design Excellence Award (IDEA) from the Industrial Design Society of America, instead helps health care professionals and patients fight diseases with a strong lifestyle component. Diabetes, obesity, and heart disease can all respond to a dose of healthy living, but managing these diseases requires accurate information about people's daily behavior and physiological states. "And you can't manage what you can't measure," says Astro Teller, CEO of BodyMedia, the Pittsburgh, PA company that makes the Armband.

The Armband differs from single-sensor medical monitoring devices in terms of both how much data it collects and what it does with that data. As for the "how much," the device sports six different sensors. Two of them take temperature readings, one measures heat flux, a two-axis accelerometer on the unit's main board senses motion, and galvanic skin response (GSR) sensors measure current differentials across the skin. The unit also acts as a receiver for standard heart rate monitors and can communicate wirelessly with scales, blood pressure cuffs, and other medical systems. Teller contrasts this richness of data with the "one-dimensional" view provided by conventional monitors. "We see the Armband as a hub for gathering all kinds of body information rather than as a way to measure one thing at a time," he says.

The Armband also handles data differently than most medical monitors. Even though it captures its share of the raw physiological data, the device represents more than the sum of its sensors. A 16-bit Motorola processor and a set of proprietary algorithms enable the device to transform the raw data from multiple sensors into snapshots of the user's lifestyle. "Sometimes it takes information from five or six sensors to make a judgment," Teller explains. The monitor infers sleep, from temperature and motion readings. Composite data from multiple sensors likewise indicate exercising. Teller envisions capturing TV watching, eating, and other behaviors, as the company's algorithms develop further. For now, though, the unit also includes a manual time-stamp button that allows the user to mark events that the monitor can't yet distinguish automatically.

Unlike those medical monitors that work only in a lab environment, the Armband has been designed from the ground up to gather all its data while users go about their daily activities. Enclosed in a shock and splash proof thermoplastic housing, the monitor straps to the user's right upper arm and can remain there for up to three days of continuous monitoring before its 3.7V battery needs recharging. And at 0.8 inches tall by 3.4 inches long and 2.1 inches wide, the housing squeezes under all but the tightest shirt sleeves with barely a bulge. "Because it's wearable and unobtrusive, the Armband 'sees' people in the context of their natural daily activities rather than from the constrained viewpoint of the lab," Teller says.

For users who would otherwise be tethered to a lab machine or have to wear multiple monitors, the ability to wear an Armband promises to make life a bit easier. Yet as a design concept, wearability had just the opposite effect on the engineers responsible for the device. "A wearable product changes all the rules," says Chris Kasabach, BodyMedia's vice president of industrial and mechanical design. "We first had to ask what makes people comfortable and then design all the electronics, sensors, and packaging around those human needs." Wearability always triggers design limitations related to ergonomics and durability, but in this case, it also exerted a more subtle influence over the sensor and electronics design.

Two difficult sensors. Two of BodyMedia's proprietary sensors, the GSR sensor and a heat-flux sensor, illustrate the contentious relationship between a wearable mechanical design and sensor performance. According to Kasabach, these two sensors drove a surprising number of mechanical design decisions. "The GSR and heat flux both have a lot of work in them," he says.

The GSR sensor, whose measurements of current across the skin provide a glimpse of chemical changes within the body, consists of two electrodes that have to contact the skin to work. "In a lab, these contact sensors are normally attached to the body with an adhesive," he notes. "Our challenge was to maintain contact using pressure alone." This requirement only called for adjustable straps but influenced much of the Armband's housing design. The device's underside, for example, features a compound curve that allows it to maintain skin contact despite differences in human arm sizes and muscular consistencies. Kasabach says the curved surface, whose precise shape resulted from a series of physical prototypes, covers arms sizes from 5.5 to 24.5 inches around and consistencies from jiggly to rock hard. "The same housing fits a child or a weightlifter," Kasabach says. A dimpled surface also helps the monitor, and its bottom-mounted sensors, stay in contact with the skin as the user moves, he adds.

Because the GSR has to work in wet conditions-whether a rain shower or a sweaty jog through the park-its electrodes required some environmental sealing. BodyMedia and Nypro Corp., the molder, addressed this problem by insert molding the GSR electrodes into the monitor's ABS bottom housing. "There's more metal there than meets the eye. Some of it is encapsulated in the plastic," Kasabach says. This encapsulation step eliminated the need to assemble an expensive gasketed or potted assembly, but it did prove tricky from a manufacturing standpoint. Noting that that the stainless steel electrode stampings measure just 0.012 inch thick, Nypro manager Keith Poole says these thin metal parts have to hold up to molding pressures without sustaining any damage or even moving. "It could have been ugly," he adds. "Except for the fact that everyone involved in the product and tooling design was at the table long before we started molding anything." For the tooling design, BodyMedia worked with K Development, a design engineering firm in Erie, PA. "The main tooling issue was capturing the edge of the stamping, which has to act a shut-off in order to avoid flashing," recalls Jason Williams, one of K Development's principles.

The heat flux sensor, meanwhile, illustrates some of the thermal issues faced by BodyMedia's design team. This custom sensor takes rapid (32 Hz) measurements of heat flux across the skin at a resolution of 44 mW/m² . "Thermal management in the device was critical given the sensitivity of this measurement," says Scott Boehmke, senior engineer.

At first glance, putting this sensor in the confines of a wearable housing creates a design paradox: To measure convection, the sensor would ideally press on the skin on one side and be exposed to the air on the other. "But that arrangement would not have been possible with the sensor positioned on the bottom of the monitor," Kasabach says. So BodyMedia created a way to move the thermal energy through the monitor housing. Kasabach won't say too much about this proprietary design, other than to describe the sensor as thermocouples separated by a thermally conductive plastic and pads. "We monitor the heat flow through the thermally conductive elements," he says. BodyMedia engineers also located the heat flux sensor on the lower edge of the bottom housing. "It's less susceptible to heat build-up there while still being close to the skin," he notes.

Where to wear it. Wearable sensors don't mean much if people won't wear them. So one of the fundamental issues faced by BodyMedia early in its design process was where on the body to put its sensors. Part of the decision was a nod to the fashion-forward user. "We put a tremendous effort into making the device completely unobtrusive under clothing," says Kasabach. "And users had to put it on or take it off without removing their clothing." Comfort also played a role in the decision. Much of the design, however, again came down to sensor function. "It goes way beyond comfort," says Teller. "All places on the body are not created equal for the purposes of sensing." Motion and heat flux sensors, for instance, don't ideally share the same location on the body.

To identify the best compromise between sensor locations and user comfort, the design team went through an unusual prototyping step. They "mapped" the workable sensor locations on the body by building and wearing sensor prototypes. That process revealed that the upper arm provided "the most bang for the buck," says Kasabach.