That will help people around here guide you towards a good way to prototype it.
Because there may be a way to prototype what you want without knowing much about hardware design.
You've already be directed to Adafruit which is a great resource, not just as a store and product developers in their own right but for all their learn materials and years worth of weekly videos. Here's a breakout board for charging their little LiPo batteries via MicroUSB.
As for the wristband, have a look at hexiwear, and the hexiwear battery pack(which may be wearable). it has BLE and it does support "Firmware Over the Air update", i think.
https://news.panasonic.com/global/press/data/2016/09/en16092...
Flexible batteries you can embad in a tissue wristband for example. But I don't know if it's possible to do so.
Thanks for the link I will check it out asap.
Most of these devices have on-demand displays to save power.
Miniturising it and making the casework function is the hard part. You might be able to build a "desktop" version of it fairly easily..