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Apple co-founder Steve "Woz" Wozniak has confidence the company can do with wearable devices what the iPhone did to smartphones back in 2007: make them compelling to a mass market.

But when it comes to wearables -- devices we strap to our wrists (and other body parts) that mimic many of the functions of current generation smartphones -- Woz says it may be harder to convince customers to buy these kinds of gadgets.

"Apple works very hard to produce exceptionally great products and doesn't quickly release things like a wearable. So if one is introduced I expect it to have a chance to set the direction and make the product category finally viable," Woz said Wednesday in an email exchange after a report that Apple may introduce a new wearable device in September along with new iPhones.

"I feel that wearables are a hard sell," Woz added. "They are go-betweens for your smartphone but are an extra piece and need special advantages that the smartphone doesn't have, in my opinion. If they are just a Bluetooth go-between then it could wind up in the category of Bluetooth headsets: Fun to wear and show off for a day."

Apple's entry into the wearables market, which has been rumored for years and is now expected for this fall, comes as its biggest competitors are also competing for mindshare and market share with consumers. Smartphone and tablet rival Samsung has already released -- and upgraded -- a few devices. Some of these gadgets now run a wearable-specific operating system designed by Google, called Android Wear, that aims to lure users away from Apple's iOS software.

For Apple to succeed, it will need to wow -- as it did with the iPhone's touchscreen and slim design -- in a still-unproven marketplace that has seen the biggest number of sales around sports and fitness bands.

What does Woz want? A device that provides a bigger screen than the current iPhone, which has a 4-inch display. (The yet-be-announced iPhone 6 is rumored to come in two, larger sizes. A 4.7-inch model is all but certain, while a phablet-style 5.5-inch model similar to Samsung's Galaxy Note line of smartphones may be in the works as well.)

Yet when it comes to current wearables like LG's G Watch and Samsung's Gear Live smartwatch -- which both sport displays slightly larger than 1.5-inches -- screen size is a tough nut to crack. Too large and the device is heavy and may be unattractive, but too small and sleek and its screen may be less visible and harder to interact with. There's also the debate over round versus square. LG and Samsung are both reportedly chasing Motorola's Moto 360 -- one of the only circular-faced smartwatch poised to hit the market -- with round-faced projects of their own.

"I personally want a larger screen that can do more of what my iPhone does," Woz said. "The small 1.5-inch by 1.5-inch screens don't hack it for me. If it serves as a speakerphone, the speaker had better be good. My Martian watch is usable for phone calls but not my Galaxy Gear, for this reason."

There's also the element of health and fitness. Apple's wearable is expected to come loaded with a bevy of sensors that work with the company's new HealthKit platform, which lets developers design apps that more easily talk to each other and work within Apple's iOS operating system. Woz finds that likely and, though not personally an appealing feature, sees it as a point of potential differentiation for Apple's wearable.

"I would not be surprised to find some personal health aids built in, Fitbit style," Woz said.

"I would be turned off slightly, but I'm sure millions would be astounded, if [Apple] become another to build in an EKG display," he added, pointing to the heart-reading technology now making its way to mainstream wearables.

For now though, Woz said, "I'm holding my breath," as he waits to see what Apple comes up with. He's not the only one.