The biggest issue there is battery life, due to the difficulty of fitting a large battery into a small enclosure.
Making the battery flexible, so it can conform to the shape of the device, such as a wrist band, is one way to increase the size, and Nokia has patented a new, novel method of achieving that.
As electronic devices continue to become more sophisticated, these devices provide an increasing amount of functionality and features, and additionally these devices continue to have different forms, such as dynamically flexible devices or mechanically conformable devices, for example. As consumers demand increased functionality from electronic devices, there is a need to provide improved devices having increased capabilities while maintaining robust and reliable product configurations
The battery works by interleaving the cathode and anode in a special way as seen in the figure above.
The apparatus includes a single battery ribbon and vacuum packaging. The single battery ribbon comprising a first portion, a second portion, and an interconnecting portion between the first portion and the second portion. The first portion comprises a first block. The second portion comprises a second block. The first portion, the second portion, and the interconnecting portion form a continuous single layer comprising an anode and a cathode. The vacuum packaging surrounds the single battery ribbon. The vacuum packaging comprises a middle connecting portion configured to contact a first side of the interconnecting portion and a second opposite side of the interconnecting portion
If this design can be made thin enough we can easily see it fitting in the band of a smart watch, easily accommodating the regular opening and closing of the band and significantly extending the amount of battery capacity of such a device.
Maybe it will even look something like this Nokia design on the right?
See the patent here.