Basically, the biggest technical and design challenge was the battery life.
We wanted to make the lock in a way that the user does not have to charge the lock/battery nor has to worry about replacing the battery at least for a couple of years.
To accomplish that, we had to look across all of the components that consume power. On the radio side, bluetooth LE solved that problem for us. Since we are sending data in bursts and the amount of data transferred is so little, the radio power consumption is almost negligible. We also use an ultra low power micro-controller with intelligent power management to low power consumption when the lock is not in use. The biggest power sucker was the mechanical actuation system. So that became more of a mechanical design and engineering problem. Using a solenoid was our of equation. There are extremely inefficient. We design the lock so that the actuator movement is minimal and is driven by a very low power motor. However there was one last issue. Most of the off-the-shelf batteries leak energy and deplete even if you are not using them after a 1-2 years. We are now using an advance lithium battery technology that has a very high energy density as well as very low leakage rate (This technology is commonly used to power sensors in remote sensing and telemetry application)
Eventually when the battery level drops low after a few years, you can notification on your phone and you can then replace the battery.