In the last months of Steve Jobs's life, the Apple co-founder fought cancer while managing diabetes.
Because he hated pricking his finger to draw blood, Mr. Jobs authorized an Apple research team to develop a noninvasive glucose reader with technology that could potentially be incorporated into a wristwatch, according to people familiar with the events, who asked not to be identified because they were not authorized to speak on behalf of the company.
It was one of many medical applications that Apple considered for the Apple Watch, which debuted in 2015. Yet because many of the health features proved unreliable or required too many compromises in the watch's size or battery life, Apple ended up positioning the device for activity tracking and notifications instead.
In September, Apple announced that the Apple Watch would no longer need to be tethered to a smartphone and would become more of a stand-alone device. Since then, a wave of device manufacturers have tapped into the watch's new features like cellular connectivity to develop medical accessories -- such as an electrocardiogram for monitoring heart activity -- so people can manage chronic conditions straight from their wrist.
What's happening with the Apple Watch is one of the first signs of a leap forward in the utility of wearable devices. Many people had wondered why they would need a smart watch at all when most already carried more powerful smartphones with them. But as the Apple Watch becomes capable of handling more medical tasks on its own, they may now have an answer.
''This is an important step in the evolution of wearables,'' said Tim Bajarin, president of Creative Strategies, an advisory and research firm. ''The Apple Watch can now be on you all the time doing this type of medical monitoring.''
The Apple Watch has dominated competitors in the nascent smart watch market, but it has not taken a place alongside the iPod, iPhone and iPad as the next breakthrough Apple product. Apple does not disclose specific sales numbers for the watch, but the company has said its sales have risen 50 percent compared with a year earlier for three straight quarters. An Apple spokeswoman declined to comment further.
A digital health revolution has been predicted for years, of course, and so far has been more hype than progress. But the hope is that artificial intelligence systems will sift through the vast amounts of data that medical accessories will collect from the Apple Watch and find patterns that can lead to changes in treatment and detection, enabling people to take more control of how they manage their conditions instead of relying solely on doctors.
Vic Gundotra, chief executive of AliveCor, a start-up that makes portable electrocardiograms, said this would put patients on a more equal footing with doctors because they would have more information on their own conditions.
''It's changing the nature of the relationship between patient and doctor,'' he said, adding that doctors will no longer be ''high priests.''
Last month, AliveCor introduced a band for the Apple Watch with a built-in electrocardiogram, or EKG, to detect irregular heart activity such as atrial fibrillation, a form of arrhythmia and a potential cause of a stroke. AliveCor uses the watch's heart rate monitor to alert patients to take an EKG when their pulse quickens or slows unexpectedly.
The band, known as the KardiaBand, was the first Apple Watch accessory approved by the Food and Drug Administration. When a thumb is placed on the band's sensor, EKG readings are taken in 30 seconds and sent wirelessly to the patient's cardiologist. That helps resolve one of the biggest headaches in detecting atrial fibrillation -- catching it during an episode so a cardiologist can properly assess it.
Elena Remus, 36, a digital marketing consultant from Alameda, Calif., has experienced what the KardiaBand can do. She had suffered heart palpitations on and off for the last five years, but there was little her doctors could do because they were never able to catch the incidents in real time.
Shortly after buying a KardiaBand for her Apple Watch a few weeks ago, Ms. Remus felt the onset of palpitations as she was making morning coffee. She looked down at her watch and saw that her normal heart rate of around 75 beats a minute had surged to 205 beats a minute. She started taking an EKG reading on her wrist and was finally able to record the activity, allowing her doctor to diagnose her condition as atrioventricular nodal re-entrant tachycardia -- or in nonmedical speak, an abnormally fast heart rhythm -- and plan a course of treatment.
''Now, I don't feel like I'm going to pass out at home alone,'' Ms. Remus said.
Apple is also looking for its own medical breakthroughs. Last month, the company announced a joint research study with the Stanford University School of Medicine to see whether the Apple Watch's heart rate sensors could detect irregular heart activity without an electrocardiogram to notify people who might be experiencing atrial fibrillation.
Apple is also looking at potentially building an electrocardiogram into future models of the Apple Watch, according to a person familiar with the project, who spoke on the condition of anonymity because the details were confidential. It is unclear whether the EKG development, earlier reported by Bloomberg, would be introduced; such a product would most likely require F.D.A. clearance.
Separately, Apple is continuing research on a noninvasive continuous glucose reader, according to two people with knowledge of the project. The technology is still considered to be years away, industry experts said.
The current solution used by many diabetics is also coming to the Apple Watch. Dexcom, a maker of devices measuring blood sugar levels for diabetics, said it was awaiting F.D.A. approval for a continuous glucose monitor to work directly with the Apple Watch. Continuous glucose monitors use small sensors to pierce the skin to track blood sugar levels and relay those readings through a wireless transmitter.
Kevin Sayer, Dexcom's chief executive, said that patients could opt for a monitor to communicate directly with the watch for convenience, but that the big payoff could come with combining sleep or activity data from the watch with glucose readings from its device to find correlations.
''We're just beginning to unlock the potential of that data,'' Mr. Sayer said.
Health care professionals said there was a risk of too much information. Dr. Khaldoun Tarakji, a cardiac electrophysiologist at Cleveland Clinic, said doctors needed to guide patients about who would benefit from more information and how much data was the right amount.
''We need to think about when too much information becomes just noise,'' said Dr. Tarakji, who has worked with AliveCor to test KardiaMobile, a portable electrocardiogram that works with iPhones.
A flood of information, albeit potentially useful, may overwhelm doctors instead of assisting them. Initially, KardiaMobile patients sent their doctors an email each time they took an EKG reading. The flood of emails became confusing and raised questions about where the doctor should store that information. In response, AliveCor introduced a software platform for doctors that could populate the readings from patients and help detect early signs of atrial fibrillation.
''A wearable device does not equal a wearable physician,'' Dr. Tarakji said. ''As a medical community, we have to be intelligent about how to use these new technologies.''