  Wireless power transfer using inductive coupling is commonly used for medical
implantable devices. The design of the secondary coil on the implantable device
is important as it will affect the power transfer efficiency, the size of the
implant, and also the data transmission between the implant and the in-vitro
controller. In this paper, we present a design of the secondary coil on a
polyimide-based flexible substrate to achieve high power transfer efficiency.
Load shift keying modulation is used for the data communication between the
primary and secondary coils. A thorough analysis is done for the ideal and
practical scenario and it shows that a mismatched secondary LC tank will affect
the communication range and communication correctness. A solution to achieve
robust data transmission is proposed and then verified by SPICE simulations.
