**5. WPT utilizing meta-surface structures**

Metasurface structures are also used to boost the PTE by confining the magnetic field in a narrow channel between transmitter and receiver by combing the evanescent waves from the Transmitter and redirect them into receiver direction due to the negative relative permeability characteristics of some kinds of the metamaterial surfaces. Metamaterials are artificial periodic structures that have negative reflective index characteristics. Metamaterials are classified into three types depending on the polarity of the relative permeability and relative permittivity of the structure: double negative (DNG), ε negative (ENG), and µnegative (MNG), as shown in **Figure 24**.

The inductive, resonance inductive, and strongly coupled magnetic resonance WPT systems rely on the magnetic field coupling between the transmitter and receiver. Thus, the MNG metamaterial category is used with WPT. When the magnetic field travels from the transmitter coil and incident on a metamaterial with MNG, the

**Figure 25.**

*(a) Metamaterial-based WPT system. (b) equivalent circuit model of applying metamaterial structures with WPT.*

outgoing magnetic fields are bent back toward the receiver coil, this increases the field strength between the two coils as revealed in **Figure 25**. Thus, the efficiency is enhanced, and the EMF leakage is reduced due to applying these metamaterial surfaces in the path between the transmitter and receiver coils. **Table 1** summarizes different metamaterial structures that are used in WPT systems [36–40].
