But despite initial appearances, both medical and consumer wearables share a few important goals.

Broadly speaking, both types of wearables aim to fill gaps in human capacity. As Sara Hendren aptly put it, "all technology is assistive technology." While medical devices fill gaps created by disability or illness, consumer wearables fill gaps created by being human. For example, evolution hasn’t given us brain wi-fi, yet.

Both kinds of wearables also need to justify being attached to our bodies. This seems pretty obvious for hearing aids, but it is just as true for consumer devices. A wearable that serves as only a slightly more convenient screen for your phone is hardly reason for the average person to spend hundreds of dollars. Instead, wearables need to offer a feature that works best when in close contact with your body, like measuring heart rate or offering haptic feedback.

Also, both types of wearables need to embed themselves seamlessly into our experiences. If a wearable obstructs your experience of the real world, or is a distraction, it’s likely to end up on a shelf instead of your wrist. That’s not to say that they don’t take getting used to—even after a lifetime of wearing hearing aids, it still takes me several weeks to adjust to a new pair. But after that period, a well-made wearable should seem like a seamless extension of our bodies.

In my current role at the Berkman Center for Internet & Society at Harvard University, I’m lucky to be able to study something I care deeply about: technology’s impact on our lives. I’m sure my interest partly arises from how I’ve depended on technology for as long as I can remember. I don’t know with certainty how consumer wearables will develop, but what I do know is how much hearing aids have changed over the last 30 years. And I have some insight into what sensory-enhancing wearables—like hearing aids, and unlike data-recording wearables like pedometers—could someday become. Over the next few years, I expect that we will see four trends, rich in both opportunity and peril, shape the evolution of these wearables from toys into tools.

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1. Wearables will create substitute realities.

In order to justify being part of our bodies, wearables need to offer something beyond an additional screen or input device. This means that sensory-enhancing wearables will need to mediate between reality and our experiences, altering our perception of the world around us.

For hearing aids, that role is enhancing sound, replacing the too-soft sounds of the real world with louder, more comprehensible ones. But modern hearing aids don’t simply make everything louder; instead, they provide a substitute soundscape tailored to my needs and my environment. When I go into a loud restaurant, the devices can identify the clatter of glasses and the din of conversation, and tune out those sounds, while tuning into the sound of a nearby voice. The result is an audio experience that is substantially different from the objective reality; the device replaces a reality that would be challenging with a substitute that is easier to understand and utilize.