  Wireless power transfer (WPT) technologies have been widely used in many
areas, e.g., the charging of electric toothbrush, mobile phones, and electric
vehicles. This paper introduces fundamental principles of three WPT
technologies, i.e., inductive coupling-based WPT, magnetic resonant
coupling-based WPT, and electromagnetic radiation-based WPT, together with
discussions of their strengths and weaknesses. Main research themes are then
presented, i.e., improving the transmission efficiency and distance, and
designing multiple transmitters/receivers. The state-of-the-art techniques are
reviewed and categorised. Several WPT applications are described. Open research
challenges are then presented with a brief discussion of potential roadmap.
