Patches are no longer just for nicotine. Health-tracking sensors that stick to the skin are catching on and can measure more variables than wrist-worn health gadgets like Fitbits. These smart patches continuously scan the body and send data to a smartphone app that alerts people when there's a warning sign. As more consumers collect data on their own health, these patches might also help transform health care, say some experts, by helping prevent chronic diseases, like diabetes or heart disease. "This is an evolutionary technology," said Kate McCarthy, a senior analyst at Forrester Research. She said the potential patch market is huge. Since patches are worn all the time, unlike wrist gadgets, patches spew data continuously. Patches are also relatively inexpensive — costing a few hundred dollars. "There doesn't need to be a big commitment of time and money," she said.

Justin Solomon | CNBC

The global wearable patch market is expected to grow quickly through 2020, driven mainly by the U.S. market, according to Technavio. Smartpatch, start-ups like Kenzen and Lief Therapeutics, are among the companies that hope to drive sales among insurers, employers and consumers. They are betting that the patches reinforce an existing economic logic in health care: that prevention can be a lot cheaper than treating long-term illnesses.

Kenzen's ECHO smartpatch analyzes sweat to predict dehydration or cramping. The data is sent to a smartphone for analysis, and it alerts a user when they are in a critical zone. Kenzen has raised more than $3 million in grants and funding from investors. More from Modern Medicine:

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Several sports teams, like the San Francisco 49ers and the FC Dallas soccer team, have signed up to test the patch. The patch, which costs $300, will be launched in sports stores and online at the end of 2017. "You might want to monitor vital signs and biomarkers that you don't want to advertise," said Dr. Sonia Sousa, co-founder and CEO of San Francisco-based Kenzen. "By putting on a patch, people can get real-time data on dehydration level, heart rate, respiration and core body temperature to make decisions about their health."

We have much more information about the health of our car than about our bodies. When data is available to more people, we're leading more informed lives. Dr. Aenor Sawyer associate director at the UCSF Center for Digital Health Innovation

Studying sweat can be used to predict health, Sousa said. The effects of losing fluids and electrolytes through sweat have been studied for nearly 50 years. Gatorade, for example, began as a study related to sweat after heat-related illnesses sidelined football players at the University of Florida. In the past, individuals had to go to performance centers to get sweat tests. "But the measurements are only for a specific point in time," said Jake Ireland, managing partner, Hickory VC, whose firm invested $500,000 in Kenzen. Patches overcome two of the biggest drawbacks of that legacy approach: They monitor body signals 24 hours a day and can be applied by the individual. Other experts hope that patches will also overcome some of the criticism of wearables like Fitbit devices. "We need more stickiness in wearables," said Dr. Aenor Sawyer, associate director of strategic relations for the UCSF Center for Digital Health Innovation. "And sustaining adoption has been a dilemma, since wrist wearables can be taken on and off or may be forgotten."

Weakling wearables

Americans are still buying fitness trackers, but they no longer wear them as much. Fitbit, which recently reported disappointing earnings and layoffs, is down more than 63 percent in the past year and more than 80 percent all-time since its first day of trading. Shares of watchmaker Fossil dropped by as much as 20 percent on Wednesday morning after a big earnings miss, despite a much-hyped move into wearable technology. Recent growth for the niche has been lackluster. The wearables industry grew only 3.1 percent year-over-year in the third quarter of 2016, according to IDC. While Warren Buffett's conglomerate Berkshire Hathaway recently invested in wearables, a move that caught attention, the famed investor explained it as a bet on the future of the centuries-old jewelry business (of which Buffett is already a big investor) and a bet on what will be stocked on jewelry shelves in the future, not a health-tech play.