**5. Wireless power transfer**

the block diagram of the sensor system. The coil antennas and the tuning capacitors form the resonant circuits. Relaxation oscillators are used as the frequency converters in the transpond‐ er. The system is designed to operate at 1.34 MHz since the recommended maximum permis‐ sible exposure of magnetic fields is the highest in the frequency range of 1.34 MHz to 30 MHz [20]. A coil antenna is made using a 34-AWG magnet wire wound around the printed circuit board. The energy harvesting circuit consists of a series of diodes and capacitors (100 pF) in a voltage multiplier circuitry that builds up the DC voltage from the received RF signals [21], [22], [23]. To maintain a constant DC level of 2.5 V for biasing the circuits, a voltage regulator

Data Transmission

**Figure 6.** Block diagram of the gastroesophageal reflux disease (GERD) monitoring system [19].

Reader Implant

Cheong *et al.* [24] presented an inductively powered implantable blood flow sensor microsys‐ tem with bidirectional telemetry. The microsystem is comprised of silicon nanowire (SiNW) sensors with tunable piezoresistivity, an ultra-low-power application-specific integrated circuit (ASIC), and two miniature coils that are coupled with a larger coil in an external monitoring unit to form a passive wireless link. The implantable microsystem operates at 13.56- MHz carrier frequency. It receives power and command from the external unit and backscat‐ ters digitized sensor readout through the coupling coils. Cheong *et al.* fabricated the ASIC in a standard 0.18-μm CMOS process and the chip occupied an active area of 1.5 × 1.78 mm2 while consuming only 21.6 μW of power. The overall system architecture consisting of an implant‐ able wireless sensor microsystem and an external hand-held device is shown in Figure 7. The ASIC consists of a sensor interface circuit, an analog-to-digital converter (ADC), a digital baseband (DBB), a low-dropout (LDO) regulator, and front-end circuits for wireless powering and bidirectional telemetry. The external monitoring unit needs to be placed in close proximity to the implant microsystem to initiate the passive sensing operation. The RF power is trans‐ mitted by the external unit through the carrier at 13.56 MHz. The parallel resonant LC tanks and the rectifiers convert the received RF signal to a DC signal, and the LDO regulator powers the ASIC with regulated DC supply. Following the demodulation of the incoming modulated carrier, it is de-spread by the DBB to configure the system parameters such as integration time, amplifier gain, selection between two sensors, resonance tuning, and modulation index. At the same time, the clock is extracted from the incoming carrier and is provided to the DBB. Once the system parameters are set according to the received commands, the sensing operation

RF Powering

Tissue

Impedance Sensor

> pH Sensor

Frequency Generator

Energy Harvesting

> Time/ MUX

is used.

160 Advances in Bioengineering

Power Amplifier

Frequency Generator

Memory Card/Wireless Module

Envelope Detector & Filter

> Previously, transcutaneous power cables were used in clinical implantable applications [25] at the expense of the introduction of a significant possibility for infection. Another alternative for power cables is an implanted battery. The use of batteries is usually intended to be avoided in implantable biomedical sensors as battery replacement is cumbersome and there is always a probability of leakage which can have serious health consequences. For this reason, various energy harvesting schemes and wireless powering techniques are employed in implantable sensors for battery-less operation. Energy harvested from body heat, breathing, arm motion, leg motion or from the motion of other body parts during walking or any other activity can be converted into a usable voltage to power up the sensor.

> There are several possible sources of energy for sensorsincluding kinetic and thermal energy harvesters such as piezoelectric and pyroelectric transducers, photovoltaic cells etc. A sum‐ mary is provided in the following table highlighting their sizes, produced energy or power and respective applications [26].


**Table 1.** Comparison of Different Sources of Energy

All the above methods presented in Table 1 have their benefits as well as disadvantages. The method which is free of most of these disadvantages is the wireless power transfer (WPT). WPT is clean, controllable, independent of patient's movement, always available and more efficient than all the sources of energy that have been mentioned in Table 1. Although WPT has lower efficiency compared to the battery, it does not have the risk factors that are associated with a battery. Especially for sensors that come in direct contact with blood, any leakage can cause chemical burning, poisoning etc. and may eventually lead to death. A battery usually lasts for 5 to 7 years and then surgical procedure is required for its removal and replacement. On the other hand, WPT usually lasts for 15 to 20 years and consequently it is much cheaper than the batteries.

WPT is a technique for supplying energy from the source to the destination without any interconnecting wires. Nicola Tesla first demonstrated WPT using his resonant transformer called 'Tesla coil'. In this design, resonant inductive coupling was used to excite the secondary side of a transformer. With the passage of time, many researchers came up with different applications for the use of WPT and now WPT is used when a wire interconnection is incon‐ venient, risky or impossible. WPT is now used in induction heating coils, wireless chargers for consumer electronics, biomedical implants, radio frequency identification (RFID), contact-less smart cards etc. Several types of wireless power transfer techniques have been briefly dis‐ cussed in the following section.

#### **5.1. Classifications of wireless power transfer**

There are two major methods for wireless power transfer – electromagnetic induction and electromagnetic radiation. Electromagnetic induction can be subdivided into three categories – electrodynamic, electrostatic and evanescent wave coupling. Electromagnetic radiation such as microwave power transfer and laser are also used for transferring power wirelessly. Figure 8 illustrates various types of wireless power transfer techniques.

**Figure 8.** Types of wireless power.

**Method Density Advantage Disadvantage Piezoelectric** 200 μW/cm3 No energy required from outside Dependent on movement **Thermoelectric** 60 μW/ cm3 No material to be replenished Low efficiency less than 5% **Kinetic** 4 μW/ cm3 No material to be replenished Dependent on movement

wave

**Visible light** \*100 mW/cm2 Free Not available at night and in

**Airflow** \*1 μW/cm2 No material to be replenished Implantation is difficult **Heel strike** \*7 W/cm2 Good source of energy Dependent on movement.

**Temperature variation** 10 μW/ cm3 No material to be replenished Low efficiency, Energy storage

All the above methods presented in Table 1 have their benefits as well as disadvantages. The method which is free of most of these disadvantages is the wireless power transfer (WPT). WPT is clean, controllable, independent of patient's movement, always available and more efficient than all the sources of energy that have been mentioned in Table 1. Although WPT has lower efficiency compared to the battery, it does not have the risk factors that are associated with a battery. Especially for sensors that come in direct contact with blood, any leakage can cause chemical burning, poisoning etc. and may eventually lead to death. A battery usually lasts for 5 to 7 years and then surgical procedure is required for its removal and replacement. On the other hand, WPT usually lasts for 15 to 20 years and consequently it is much cheaper

WPT is a technique for supplying energy from the source to the destination without any interconnecting wires. Nicola Tesla first demonstrated WPT using his resonant transformer called 'Tesla coil'. In this design, resonant inductive coupling was used to excite the secondary side of a transformer. With the passage of time, many researchers came up with different applications for the use of WPT and now WPT is used when a wire interconnection is incon‐ venient, risky or impossible. WPT is now used in induction heating coils, wireless chargers for consumer electronics, biomedical implants, radio frequency identification (RFID), contact-less smart cards etc. Several types of wireless power transfer techniques have been briefly dis‐

There are two major methods for wireless power transfer – electromagnetic induction and electromagnetic radiation. Electromagnetic induction can be subdivided into three categories

Depends on EM wave availability

cloudy days.

required

**harvesting** \*1 μW/cm2 Harvesting energy from ambient EM

**Ambient RF energy**

162 Advances in Bioengineering

\* Energy Density per Unit Area

than the batteries.

cussed in the following section.

**5.1. Classifications of wireless power transfer**

**Table 1.** Comparison of Different Sources of Energy

Electromagnetic Induction

By varying the magnetic field an electromotive force can be produced across a conductor. This is called electromagnetic induction. The three possible ways to achieve that are summarized below:


#### Electromagnetic Radiation

After an electromagnetic radiation is emitted, it can be absorbed by some charged particles. This type of radiation can propagate through vacuum at the speed of light. It has a time varying electric field component as well as a magnetic field component, which oscillates perpendicu‐ larly to each other and perpendicularly to the direction of energy and wave propagation. Two ways in which wireless power transfer using electromagnetic radiation can be accomplished are:

**a.** *Microwave power transmission:* It is the transmission of energy using electromagnetic waves with wavelengths ranging from 30 cm down to 1 cm or equivalently a frequency range of 1 GHz to 30 GHz. It is used for directional power transmission to a remote destination.

**b.** *Laser:* In this technique electricity is first converted into a laser beam which is then directed towards a photovoltaic cell. The receiver is an array of photovoltaic cells designed to convert the light back to a usable electrical energy. This method is also known as optical coupling.

Since inductive link is the most commonly used wireless power transferring technique for biomedical sensors, it is discussed in more detail in the following section.

**Figure 9.** Basic inductive link caused by alternating electromagnetic field.

#### **5.2. Inductive link**

An inductive link comprises of a loosely coupled transformer consisting of a pair of coils as shown in Figure 9. An alternating source (AC) drives the primary coil and generates the desired electromagnetic field. A portion of the generated magnetic flux links the secondary coil and according to Faraday's Law of electromagnetic induction, the temporal change of magnetic flux induces a voltage across the secondary coil. The voltage induced in the secondary coil is proportional to the rate of change of magnetic flux in the secondary coil and the number of turns in that coil.

**Figure 10.** Schematic of basic inductive link based on series parallel resonance.

Figure 10 illustrates a basic inductive link based on series-parallel resonance. The maximum value of the mutual inductance, *M* that can possibly be achieved between the two coils of inductance of *L*1 and *L*2 is *(L*1*L*2*)* 1/2 and this occurs when all the flux generated in the primary coil links with all the turns in the secondary coil. The ratio of mutual inductance to its maximum value is called the coupling coefficient *k*, which is a dimensionless quantity ranging from 0 to 1 and can be determined using the following equation:

$$k = \bigwedge\_{\{\Gamma\_{\alpha\_1}, \Gamma\_{\alpha\_2}\}} \bigwedge\_{\pi} \tag{2}$$

The performance of the inductive link is dependent on the link efficiency, which is defined as the ratio of the power delivered to the load to the power supplied to the primary coil. For a parallel resonant circuit, the link efficiency of the secondary side can be written as [46],

$$\eta = \frac{k \, ^2Q\_iQ\_2}{\left\{1 + \frac{Q\_2}{a} + k \, ^2Q\_iQ\_2\right\} \left(a + \frac{1}{Q\_2}\right)}\tag{3}$$

For a series resonant circuit, the link efficiency of the secondary side turns out to be,

**b.** *Laser:* In this technique electricity is first converted into a laser beam which is then directed towards a photovoltaic cell. The receiver is an array of photovoltaic cells designed to convert the light back to a usable electrical energy. This method is also known as optical

Since inductive link is the most commonly used wireless power transferring technique for

An inductive link comprises of a loosely coupled transformer consisting of a pair of coils as shown in Figure 9. An alternating source (AC) drives the primary coil and generates the desired electromagnetic field. A portion of the generated magnetic flux links the secondary coil and according to Faraday's Law of electromagnetic induction, the temporal change of magnetic flux induces a voltage across the secondary coil. The voltage induced in the secondary coil is proportional to the rate of change of magnetic flux in the secondary coil and the number of

M

Figure 10 illustrates a basic inductive link based on series-parallel resonance. The maximum value of the mutual inductance, *M* that can possibly be achieved between the two coils of

coil links with all the turns in the secondary coil. The ratio of mutual inductance to its maximum value is called the coupling coefficient *k*, which is a dimensionless quantity ranging from 0 to

> (*L* <sup>1</sup>*L* 2) 1

*k* = *<sup>M</sup>*

C1 L1 L2 C2 RL

1/2 and this occurs when all the flux generated in the primary

<sup>2</sup> (2)

Air Core

RLoad

biomedical sensors, it is discussed in more detail in the following section.

coupling.

164 Advances in Bioengineering

**5.2. Inductive link**

turns in that coil.

inductance of *L*1 and *L*2 is *(L*1*L*2*)*

AC

**Figure 9.** Basic inductive link caused by alternating electromagnetic field.

**Figure 10.** Schematic of basic inductive link based on series parallel resonance.

1 and can be determined using the following equation:

$$\eta \eta = \frac{k \, ^2Q\_1 \alpha \,}{\left(1 + \frac{1}{\sqrt{Q\_2}} + k \, ^2Q\_1\right) \left(\alpha + \frac{1}{\sqrt{Q\_2}}\right)} \tag{4}$$

In both Equation 3 and 4, *Q*<sup>1</sup> is the quality factor of the primary coil, *Q*<sup>2</sup> is the quality factor of the secondary coil, *k* is the coupling factor between the coils, *α* is a unit-less constant which is equal to *ωC*2*R*L, where ω is the angular frequency, *C*<sup>2</sup> is the capacitance of the secondary coil and *R*L is the load resistance.

Inductive link is a common method for wireless powering of implantable biomedical elec‐ tronics and data communication with the external world. WPT and data telemetry using inductive link have been demonstrated for various biomedical applications including visual prosthesis, cochlear implant, neuromuscular and nerve stimulator, cardiac pacemaker/ defibrillator, deep-brain stimulator, brain machine interface, gastrointestinal microsystem and capsule endoscopy[27-37]. A summary of various inductive link wireless power transfer applications and their respective carrier frequencies is presented in Table 2:


**Table 2.** Wireless Power Transfer for Different Biomedical Implants

The design of an inductive link is required to meet the power requirement of any of the above mentioned applications. There are several parameters which play key roles in determining the performance of an inductive link. A qualitative analysis of these key factors is presented in the following section.

## *5.2.1. Performance dependence on different factors*

The factors that affect the performance of an inductive link wireless power transfer are as follows:


**Figure 11.** Indcutive coupling bewtween two coils with lateral misalignment and angular misalignment.


There are some major challenges associated with the implementation of an efficient wireless power transfer system in implantable sensors. The next section summerizes the major limitations.

#### **5.3. Limitations of wireless energy transfer**

The design of an inductive link is required to meet the power requirement of any of the above mentioned applications. There are several parameters which play key roles in determining the performance of an inductive link. A qualitative analysis of these key factors is presented in the

The factors that affect the performance of an inductive link wireless power transfer are as

**a.** *Diameter of coils:* The diameters of the receiver and the transmitter coils are important parameters affecting the voltage gain of an inductive link [28]. Both the self inductance and the mutual inductance are proportional to the diameters of the coils. Therefore increasing the diameters boosts the link efficiency. In case of an implantable system, due to the constraint on the size of the implant there are more stringent limitations on the

**b.** *Number of turns:* The number of turns is another important factor since the mutual inductance is proportional to the product of the number of turns in the transmitter and the receiver coils. Therefore increasing the number of turns will improve the performance.

**c.** *Spacing and alignment:* Spacing between the primary and the secondary coils and their alignment also significantly affect the coupling between them. Therefore the position of the implantable sensor in terms of the external wireless power transferring module is an important factor that needs to be taken in to account. Compared to an exact coaxial alignment, similar or even better performance can be achieved if the receiver coil is present within the circumference of the transmitter coil [28]. In case of implantable sensors, any movement of the patient can cause misalignment between the transmitter and the receiver which in turn can alter the mutual inductance and the link gain. Two types of misalign‐ ment that affect the link efficiency are lateral misalignment and angular misalignment [25] as illustrated in Figure 11. After a certain lateral or angular misalignment, the performance degradation of the wireless inductive link becomes proportional to the magnitude of the

Coil Spacing, da

**Figure 11.** Indcutive coupling bewtween two coils with lateral misalignment and angular misalignment.

ϕ

Receiver Coil

Angular Misalignment, ϕ

Transmitter Coil

receiver coil size compared to those of the transmitter coil.

following section.

166 Advances in Bioengineering

misalignment.

Coil Spacing, da

Δ

Lateral Misalignment, Δ

Receiver Coil

Transmitter Coil

follows:

*5.2.1. Performance dependence on different factors*

There are several factors that impose serious limitations on the widespread use of wireless energy transfer for implantable sensors. First of all, it is not often possible to scale the trans‐ mitter or the receiver down to a small enough size to make it suitable for its implementation in a miniaturized system. Secondly, the range of energy transfer has not yet been demonstrated to exceed a few meters, which poses a major challenge for its practical implementation. Ongoing research is focused on finding more compact solution for wireless energy transfer covering a greater range. Another problem with wireless energy transmission is that its typical efficiency varies between 45% and 80% falling short of a conventional battery or wire based technology. Future innovations resulting in the reduction in size as well as increase in efficiency and operating range will undoubtedly make wireless energy transfer suitable for plenty of new potential sensor applications.
