*Implantable Wireless Systems: A Review of Potentials and Challenges DOI: http://dx.doi.org/10.5772/intechopen.99064*

identification. The most popular way of transmitting power and data to passive implants [13] is the inductive power transfer between coupling coils, one on the implant and one outside the body on a reading device. As illustrated in **Figure 3**, the coil of the transmitter (TX) is placed adjacent to the skin and is a time variable magnetic field produced by a power source. This magnetic field induces an electromotive strength (EMF) inside the receiver (Rx) body that is processed using an RX system-based silicone rectifier [13]. In order to increase the PTE [14], the Rx coil should be tuned to the same working frequency as the Tx coil.

In passive systems, the connections have four categories for resonance: the SET (serial- to parallel) topology (SP), the serial-to-serial (SS) topology and the parallelto-parallel (PP) topology as shown on **Figure 4**. In order to guarantee better efficiency in the transmission of power of the inductive connective transmission, both sides are tuned with the same resonant frequency *f0*. In most cases, the principal circuit (reader) is tuned to series resonance, which gives the transmitter coil an impedance load which is almost always parallel to the secondary circuit and uses the LC circuit for driving a load of a not-linear corrective device [15].

The number of loops can be changed in practice based on wiring characteristics and coil form. A more practical approach consists of the measurement of inductance at construction and strange turns to achieve the specified inductance. However, a highly specialized and expensive inductance meter requires accurate measurement of inductance [15]. In practice, Equation (4) [16] can be used to calculate the resonance frequency f0. Many formulas can be used to estimate the number of turns necessary to achieve a specific inductance L. For example, in **Table 2**, the (N)

**Figure 4.** *Inductive coupling with four possible resonance circuits [15].*


**Table 2.**

*Formulas approximate the number of turns needed to achieve a given induction.*

turnings on the radius of the loop (a), on height of the loop (h), on width of the loops (b), on width (d), on the loop radius (r) and on magnetic inductivity (L). But only approximations to ideal conditions [20] could be made in such equations.

$$K = \frac{M}{\sqrt{L\_T L\_R}}\tag{4}$$

The mutual inductivity (M) and coupling coefficient with LT and LR, as proposed by [21] are other parameters to be examined during inductive coupling design.

$$f\_0 = \frac{1}{2\pi\sqrt{LC}}\tag{5}$$

The resistor R1 is the effective resistance series LT with the SP topology given in **Figure 4** which shows the transferred spindle losses and the power amplifier's output resistance, whereas R2 is the effective LR series resistance given in [21] and in [22]. CT and CR capacitors are used on both sides of the link to create resonance.

The frequency of resonance (Wo) of an LC tank may be calculated for both sides, as shown (6).

$$W\_o = \frac{1}{\sqrt{L\_T C\_T}} = \frac{1}{\sqrt{L\_R C\_R}}\tag{6}$$

The quality factor (Q) in (7) is presented for the primary and secondary coils.

$$Q\_1 = \frac{wL\_T}{R\_1} \text{ and } Q\_2 = \frac{wL\_R}{R\_2} \tag{7}$$

The performance on both sides of the connection should be maximized for high efficiencies and this can occur when.

$$\mathbf{1} \ll \mathbf{K}^2 \mathbf{Q}\_1 \mathbf{Q}\_2 = \frac{\mathbf{K}^2 \mathbf{L}\_\Gamma}{\mathbf{L}\_\mathbb{R}} \ast \frac{\mathbf{1}}{\mathbf{R}\_2 \mathbf{C}\_\Gamma} \tag{8}$$

**Figure 5** shows total efficiency (K2Q1Q2) as a function of increasing efficiency with the increasing coupling and quality factor (9) [23].

$$\eta\_{\text{max}} = \frac{\mathbf{K}^2 \mathbf{Q}\_1 \mathbf{Q}\_2}{\left(1 + \sqrt{\mathbf{K}^2 \mathbf{Q}\_1 \mathbf{Q}\_2}\right)^2} \tag{9}$$

The resistance of implanted devices is another factor that directly affects overall efficiency (loaded case). The total efficiency is also raised proportionately with the load increases, depending on the implanted resistance proposed, according to (10) [24].

*Implantable Wireless Systems: A Review of Potentials and Challenges DOI: http://dx.doi.org/10.5772/intechopen.99064*

**Figure 5.**

*Maximum achievable link efficiency as a function of (K2Q1Q2) [23].*

**Figure 6.**

*Variants for the coil design: squared printed spiral coil (left), circular printed spiral coil (middle), and solenoid wire wound coil (right) [8].*

$$\eta\_{Total} = \eta\_T \eta\_R$$

$$\eta = \frac{K^2 Q\_1 Q\_2^3 R\_{LR} R\_{Load}}{\left(K^2 Q\_1 Q\_2^3 R\_{LR} R\_{Load} + K^2 Q\_1 Q\_2 R\_{Load}^2 + Q\_2^4 Q\_{LR}^2 + 2 Q\_2^2 R\_{LR} R\_{Load} + R\_{Load}^2\right)} \tag{10}$$

The design of a coil with several possibilities, as outlined in **Figure 6**, is another relevant parameter. A first grade between printed spiral coils (PSC) and wounded coils (WCs) [25] is established. A first classification is given. PSCs are characterized by high reliability and production ease, especially with micro and nano-production processes. However, PSCs have a lower quality factor than WWCs [20]. There are also different key parameters for the two geometries. For a PSC, *d*<sup>0</sup> and *di* are the external and internal diameters of the spiral respectively, n is the number of turns where w and s are both the distance and the distance between them. For solenoid WWC, else, *d* is the diameter of the solenoid, constants during *n* rotations, *l* is the length of the driver, *d*<sup>0</sup> is the diameter of the wiring and *p* is the twisting pitch [8].

#### **4.3 Magnetic resonance coupling**

As illustrated in **Figure 7**, magnet resonance coupling is based on evanescent wave-coupling which generates and transfers electric energy through various or varying magnetic fields between the two resonant spins. As two resonant coils are strongly coupled with the same resonant frequency, high efficacy can be achieved. The advantage of the magnetic resonance connection are also immunity to the

**Figure 7.** *Magnetic resonance coupling [26].*

surrounding environment and the need for a free space transfer [26]. The quality factors are normally high, because magnetic resonance coupling usually works within the megahertz range. The high quality factor helps to mitigate a sharp reduction in connection effectiveness and thus loading efficiency, by increasing the loading distance. As a result, it is possible to extend the effective transmission power distance to meters [27].

With the declaration of the Wireless Power Consortium (WPC) on extending the transfer distance from 5 mm to 40 mm in 2012, new research works are expected to focus on new magnetic winding schemes and configurations [28]. Based on the new developments in improving the transfer distance, a new planar design would be able to charge the devices on desks and tables. To address the poor transfer efficiency defects of a two-coil energy transfer mechanism, as considered in [29], midrange WPT techniques, such as relay resonators (**Figure 8a**), four coils (**Figure 8b**), U coils (**Figure 8c**), domino coils (**Figure 8d**), array coils (**Figure 8e**), and dipole coils (**Figure 8f**) are proposed in previous studies and fused into future planar WPT chargers with increased distances or air gaps. Based on these studies, the transfer distances were 20, 60, 100, 180 (for seven resonator coils), 20, and 500 cm respectively) [30]. Each configurations have superior performance

#### **Figure 8.**

*Different WPT mechanisms: (a) relay coil; (b) four coils; (c) U-coil; (d) domino coils; (e) dipole coils; and (f) array coils [30].*

characteristics than the NRIC in several aspects: (a) better impedance matching capability to optimize the system power transfer, (b) higher Q-factor enabled by the primary and secondary coils, which can compensate for the sharp decline of PTE caused by the reduced coupling coefficient due to the increasing separation distance and (c) higher bandwidth of operation [31].
