The global wearable device market is going to double over the next four years. According to research firm IDC, the market will see its sales double by 2021. Those sales will be helped by a rise in clothing and "hearable" products, although smartwatches will remain on top.

Smartwatches on their own are expected to grow from 71.4 million sales to 161 million in 2021. Lower-tech wristbands from companies like Fitbit and Jawbone will see a smaller rise from 47.6 million to 52.2 million units.

Smart clothing won't be a major contributor, but that sector will grow significantly, rising from 3.3 million sales to 21.6 million.