Global sales of smartwatches and bracelets are expected to reach 100 million units in 2016, led by the release of a second-generation Apple Watch in the second half of the year, according to an estimate by industry sources.

Although the first-generation Apple Watch has made Apple the top vendor of smartwatches globally, follow-up sales have not been able to pick up significantly, possibly affected by its small display size and the requirement to pair with other mobile devices to utilize some functions, said the sources.

While Android Wear has received support from a number of vendors, restrictions to Google apps on Android Wear, and difficulties to differentiate products under the platform have combined to slow down the popularity of Android Wear-based devices, commented the sources.

As some handset vendors have chosen to use either in-house developed or other closed operating systems for their wearable devices, the market share of Android Wear-based products declined to 2.7% in 2015 from 11.9% a year earlier, according to an estimate by Taipei-based Market Intelligence & Consulting Institute (MIC).

However, Android Wear is expected to start supporting Chinese and to add voice connection and other functions in 2016, which may attract more vendors to join the club, said the sources, adding vendors including Motorola (Lenovo), LG Electronics, Asustek Computer and Huawei are expected to perform better in the segment in 2016.

For the smart bracelet market, Xiaomi has launched its updated tracker with a built-in optical heart rate sensor, but has continued to keep its price low. Other vendors have to differentiate their devices from Xiaomi's to avoid direct competition with the China-based smartphone vendor.

For example, China-based BBK, the vendor of Oppo brand smartphones, has rolled out a smartwatch for kids. The vendor sold 700,000 units of its XTC Y01 smartwatches during the third quarter of 2015, making it the fifth biggest wearable device vendor globally in the quarter, according to IDC.

Other China-based vendors, including Tencent and Qihoo 360, are expected to roll out similar products to compete with BBK, the sources indicated.