The last time your doctor asked how much you exercise, did you tell the truth? Do you even really know the truth—not just how many visits to the gym you’ve made this month, but how many hours you sit or how many calories you burn in a day?

What if your doctor had already received the information from a tiny device built into your cell phone, wallet, or undershirt? Sonny Vu believes a device like this could fundamentally change health care. “You can’t just lie to your doctor—it’s all there, recorded,” he says. “You cut right to the chase rather than having to tease out all that information.”

Vu is an entrepreneur who thinks a lot about how a well-designed mobile device can affect health. As a cofounder of the medical-device company AgaMatrix, he created the first FDA-approved glucose sensor that plugs into an iPhone; it hit Apple stores this month under the brand name iBGStar.

Now Vu is taking his ideas a step further, betting that the next phase for mobile computing is on our bodies. He’s heading a new company called Misfit Wearables, which is developing health monitoring devices that he says will fit unobtrusively into the clothing and objects we use every day.

Mobile health devices and software could change medicine profoundly, allowing people to continuously monitor vital signs and better track and modify behavior. That’s important because chronic diseases like obesity and diabetes are on the rise. “We’re seeing an infusion of mobile technologies into people’s lives,” says Susannah Fox, who studies technology and health care for the Pew Internet & American Life Project. “And we’re seeing a very rainy forecast in terms of people’s health.”

In health care, however, good ideas often succumb to the realities of human nature. “Health isn’t really top of mind for most of us,” says Fox. Yet many health-related apps and devices essentially ask people to make health a priority. Pew’s research has found that interest in health apps hasn’t been increasing among users.

Vu’s idea is to remove from the equation what he calls “intentionality”—the deliberate daily choice to use a health technology. Donning a pedometer or entering information into a calorie counter every day is asking too much of most people. “The best products are the ones that you really rely on but you don’t have to remember to use,” he says.

Vu says that realization came to him after many years of trying to understand why people with diabetes might forget to use their glucose meter, even though their health depends on doing so. (The meters use a drop of blood from a pinprick to measure blood sugar.) “If you have diabetes, what’s your main problem? It’s that you don’t want to have diabetes anymore,” says Vu. Carrying around a bulkier glucose meter is annoying and a constant reminder that one is ill. By creating meters that were closely integrated with a phone, something many people never leave the house without, “we enabled people to be closer to where they wanted to be, which was a little less diabetic,” he says.

Vu, 39, splits his time between several locations, including Cambridge and his native Vietnam, where he’s cultivated a software development team. He calls himself a “product person” who is happiest designing products and obsessing over their details. He founded Misfit Wearables last fall with John Sculley, former CEO of Apple, naming the company after Apple’s iconic “Think different” ad (“Here’s to the crazy ones. The misfits. The rebels …”).

The company raised $7.6 million this year from prominent investors, including Peter Thiel’s Founders Fund and Vinod Khosla, following seed investment from the Cambridge incubator IncTank Ventures. Vu says technology investors are seeking to understand when, and how, computers will become wearable.

As a developer of medical devices, Vu is accustomed to proving his products’ worth to the FDA. Now he’s bringing that experience into the much less regulated world of consumer health and gadgets. Devices that monitor weight, activity level, heart rate, or other vital signs could, in principle, lower health-care costs by aiding efforts to prevent chronic illnesses like diabetes and heart disease. They could make it possible to provide medical services such as remote monitoring of patients or automatic detection of falls. “Wearable sensor data is going to be the most complete you can get,” Vu says. It could make a yearly blood pressure measurement at the doctor’s office seem archaic.

But developing these devices is challenging. First there’s what Vu calls the “skin and silicon” problem. It’s technically difficult to create an interface that accurately collects physiological signals and transmits them to a small mobile device. It’s equally difficult to figure out what to do with the data. The people who obsessively analyze their own heart rate are a tiny minority, and even doctors don’t have time to wade through raw data about their patients. The key, he says, is to provide software that can hunt for patterns and provide usable insights—that your heart rate veers dangerously high at work, or that your activity level drops on certain days of the week. But even the best device can’t make someone follow its advice.

Vu is keeping quiet about the details of the product Misfit is planning to launch, which is still in development. It will function like current fitness monitors—he mentions the Fitbit pedometer and BodyMedia’s activity-tracking armband—but will add a novel measurement that no other wearable device supplies. Vu is aiming for a consumer product, but eventually he’d like to conduct a clinical study of its effectiveness and seek FDA approval for a medical application.

The primary goal, however, is invisibility. “You have wearable products right now—they’re just not that wearable,” he says. “And you have to remember to wear them.” He thinks a health monitoring device should would be unobtrusive enough to be incorporated into something you already wear or carry every day: socks, bra, undershirt, cell phone, wallet, keys.

That goal has brought Vu into the world of high-tech fashion. At a recent conference on smart fabrics, he mingled with designers and textile engineers making clothes that light up with fiber optics or heat and cool themselves. Vu believes the textile world could ultimately contribute more creative innovation to wearable computing than device companies do. “Those folks are thinking about clothing and about stuff you’re already wearing,” he says. “Not ‘How can we strap this thing to your body?’”