Wireless Charging System for an Implanted Sensor

Increasing the demand for developing implantable devices and sensors promotes the concept of the wireless power transfer. The implanted glucose sensors, for example, shall be built small enough to allow implanting it inside the patients' body [1] to indicate the readings easily. It grants the patients the ability to read data simply through a receiver located out of the body. However, there is a problem in most of these sensors in providing them with the necessary power by using traditional chargers because any direct contact with these devices is impossible. Therefore, scientists and researchers investigate new solutions and methods to maintain transferring enough power to charge the battery of the sensor. Among all of these methods, the inductive coupling proves its ability in transmitting the power wirelessly to the application with high efficiency. This paper presents a wireless charging system to transfer the power from an external charger to an implanted device based on the inductive coupling. It discusses different considerations and possibilities in designing and implementing the proposed charger to provide enough power to the largest possible distance.


INTRODUCTION
Wireless charging technology refers to the ability of an energy source in transmitting electromagnetic energy to a load without any connection between the transmitter and receiver [2,3]. Although the technology of the wireless charging may sound futuristic, it is not a new concept. This notion started in the late of 1800's when Nicolas Telsa proposed theories about wireless energy transmission [4,5]. His work was impressive, but without particular methodologies for the wireless power transmission early of 1990s, Braun Company used inductive charging to develop Oral-B rechargeable toothbrushes [7]. Then, in 1964 Brown demonstrated "a microwave-powered model helicopter" that uses a microwave beam to transmit the necessary power for the flight These devices are only some inventions in a long history of development in the idea of the wireless charging during more than a century. O these years, many innovations in this field have been discovered and developed, and many researchers have interested in investigating this technology to introduce new methods and techniques. Technological advances over the last century and increasing the demand for reducing the size and the power consumption in electronic devices have promoted the concept of Energy harvesting [10] and its applications in the electronics fields. This development has affected the biomedical field, especially the implantable sensors.
In the field of biomedical instrumentation, the most significant advantage of adopting the wireless charging technology is removing all of the connections between the devices, in particular for the implanted devices. Providing enough energy to these devices is one of the challenges that are associated with making a successful practical application because using the traditional techniques (cables) leads to many problems and disadvantages. In this direction, the wireless power transfer has considered as a convenience and flexible solutions for this problem. The second advantage of the wireless charging is reducing the cost and the hazards of direct connections [11]. With this technology, the transmitter (that is placed outside) transfers the power to charge the battery of the implanted sensor and, can receive data from this implanted device at the same time. Uniting the two functions in one system reduces the cost through minimizing the required instruments. Thirdly, the wireless charger can be more reliable than the Wireless charging technology refers to the ability of an energy source in transmitting electromagnetic energy to a load without any connection between the transmitter and receiver Although the technology of the wireless charging may sound futuristic, it is not a new concept. This notion started in the late of 1800's theories about . His work was impressive, but without particular methodologies for the wireless power transmission [6]. In the early of 1990s, Braun Company used inductive B rechargeable Then, in 1964 Brown powered model er" that uses a microwave beam to transmit the necessary power for the flight [8,9]. These devices are only some inventions in a long history of development in the idea of the wireless uring more than a century. Over these years, many innovations in this field have been discovered and developed, and many researchers have interested in investigating this technology to introduce new methods and techniques. Technological advances over the last emand for reducing the size and the power consumption in electronic devices have promoted the concept of Energy and its applications in the lds. This development has affected the biomedical field, especially the implantable biomedical instrumentation, the most significant advantage of adopting the wireless charging technology is removing all of the devices, in particular for the implanted devices. Providing enough energy to these devices is one of the with making a successful practical application because using the traditional techniques (cables) leads to many lems and disadvantages. In this direction, has considered as a convenience and flexible solutions for this problem. The second advantage of the wireless charging is reducing the cost and the hazards of . With this technology, the outside) transfers the power to charge the battery of the implanted sensor and, can receive data from this implanted device at the same time. Uniting the two the cost through the required instruments. Thirdly, the be more reliable than the traditional wired charger because it can ensure transferring the necessary power efficiently in different environmental conditions [ In this trend, the inductive coupling has been classified as an optimal solution to transfer the power wirelessly in the biomedical technologies in a way that can satisfy these demands. This paper aims to design a system that can transfer enough power into the implanted sensor and charge its battery wirelessly by using the inductive coupling. Therefore, two devices will be developed and implemented in this paper. The first one is the transmitter that is placed body and the second is the receiver be implanted besid the sensor.

The Inductive Coupling
In electronics and electromagnetism, the inductance refers to the ability of an inductor in storing energy by using the magnetic field With this capability, inductors can generate a voltage in opposing to the changing rate of the current [14]. This is also called a self to discriminate it from the mutual inductance that can occur between two circuits equation (1) defines the quantitative definition of the self-inductance L in SI units [16 where 'V' denotes the voltage (in Volt), and 'I' refers to the current (in Amperes). According to (1), a current can change linearly with a constant voltage. Based on the Ampere's law, i can be caused in response to the magnetic field that is generated by electric currents

Fig. 1. A model of a typical magnetic coupling
Exchanging an energy between the wires and the magnetic fields is called inductive coupling. It works on the principle of electromagnetism.
; Article no. BJAST.33867 traditional wired charger because it can ensure transferring the necessary power efficiently in [12].
In this trend, the inductive coupling has been classified as an optimal solution to transfer the power wirelessly in the biomedical technologies in a way that can satisfy these demands. This paper aims to design a system that can transfer the implanted sensor and charge its battery wirelessly by using the inductive coupling. Therefore, two devices will be and implemented in this paper. The is placed out of the body and the second is the receiver which would In electronics and electromagnetism, the inductance refers to the ability of an inductor in storing energy by using the magnetic field [13].
nductors can generate a voltage in opposing to the changing rate of the self-inductance to discriminate it from the mutual inductance that can occur between two circuits [15]. The equation (1) defines the quantitative definition of 16], as follows: (1) 'V' denotes the voltage (in Volt), and 'I' refers to the current (in Amperes). According to (1), a current can change linearly with a constant the Ampere's law, induction in response to the magnetic field by electric currents [17].

magnetic coupling
between the wires and the magnetic fields is called inductive coupling. It works on the principle of electromagnetism. Therefore, if two circuits are coupled magnetically, a magnetic field can be generated by interlinking two circuits. Then, this will facilitate transferring the energy among these circuits through transmitting the magnetic field. The model of two coupled inductors and a resistance is shown in Fig. 1 and identified in (2) and (3) [17]: where v1, i1 and v2, i2 are the voltage and the current of the primary circuit and the secondary circuit respectively; L1 and L2 are the selfinductance and M refers to the mutual inductance and can be calculated according to (4) [18], where K is the coupling coefficient of the windings. This factor refers to the quality of the magnetic circuit. The power transfer in the wireless charger can work efficiently when the coupling coefficient (k) equal to one or very close to one only. However, in the practical application, this factor cannot reach this value, especially with the large distances between the coils, because of the resistance of inductors. Therefore, the quality of the power transmission can be determined based on the value of the "quality factor" that is identified in (5), where ω is the resonance frequency, and R is the inductor's series resistance. Fig. 2 shows a block diagram of the proposed system which includes two separate devices. The first device is the external device which is connected to the power source and located out of the body. The power then will be received by the second device that is called the receiver (the internal device), which will be implanted inside the body and beside the sensor. In order to transfer a power with high efficiency, inductors can be placed in a series resonant LC circuit. Fig. 3 demonstrates a typical AC model for the series circuit of the inductive power transfer.

The Devices Setup
Basically, the external device includes four parts: an oscillator, a power amplifier, a coil and an envelope detector. The oscillator generates the carrier frequency, and then the power amplifier will drive the coil. A circuit of an envelope detector and an amplifier chain has been used to facilitating the data transfer. The circuit detects any change in the voltage's amplitude over L1 that may be caused by the data transmission of the implantable device.
Secondly, the implanted device includes a resonator (to receive the power) in addition to a transistor (for load modulation) and a rectifier (to convert the AC voltage to DC voltage). Fig. 4 and

The Crystal Oscillator
The Crystal Oscillator is an electronic oscillator that can be designed as a thin slice of quartz sandwiched between two electrodes provide a series or a parallel resonance. The crystal oscillator has been used in the external device to oscillate the circuit with high frequencies (above 10 MHz). Fig. 6 illustrates the circuit that has been used to oscillate

The Class-D Amplifier
The Class-D Amplifier is an amplifier can generate square pulses (square wave). It consists of an inverter and two transistors as shown in Fig. 7. The inverter works as a switch to control the two transistors. The Class Amplifier dissipates very little power with the other types of the amplifier. lead to decrease the heat and increase the battery life [21]. Therefore, the high efficiency and the low cost are its main advantages. Two NC7WZ04 transistors have been used to build the amplifier of this system because this type of transistors is able to achieve high speed and high output drive with low power dissipation

The Rectifier
The rectifier is one of the main components of the receiver (the implanted device). It uses high

Receiver Coil
Oscillator

4
. A block diagram to show the components of the transmitter

A block diagram to show the components of the receiver
The Crystal Oscillator is an electronic oscillator as a thin slice of quartz sandwiched between two electrodes [19]. It can parallel resonance. The crystal oscillator has been used in the external device to oscillate the circuit with high 6 illustrates the oscillate this device.

The circuit of the crystal oscillator
D Amplifier is an amplifier can generate square pulses (square wave). It consists of an inverter and two transistors [20], as shown in Fig. 7. The inverter works as a switch to control the two transistors. The Class-D power comparing with the other types of the amplifier. This can lead to decrease the heat and increase the . Therefore, the high efficiency cost are its main advantages. Two NC7WZ04 transistors have been used to build the amplifier of this system because this type of achieve high speed and issipation [22].
The rectifier is one of the main components of . It uses high-frequency power to generate DC power. Two types of rectifiers have been considered and tested in this system. The first type wave rectifier which converts both polarities of the wave to a constant polarity. It can using four Schottky diodes or ultrafast diodes to eliminate the loss effect [23]. The second rectifier is the half wave rectifier which has to be designed by single Schottky diodes. After some investigations and tests, the half wave rectifier has been used instead of the full wave rectifier to reduce the area. The size of the receiver is one of the important issues that have addressed because it would be implanted the human body. Therefore, the half rectifier replaces the full wave rectifier, and this will be shown in the next section. Finally, Fig.  and Fig. 8.b illustrate general circuits for a and half wave rectifiers respectively.

The Voltage Regulator
The voltage regulator is an essential component to control and stabilize the output voltage of the rectifier in the receiver (the implanted device). In this system, a linear voltage regulator is required to reduce the variation in the output of the rectifi this purpose, the Low dropout (LDO) regulator used. Therefore, there are only small differences between the supplied voltage and the load voltage [24].

Rectifier Regulator Load
Power Amplifier frequency power to generate DC power. Two types of rectifiers have been considered and type is the full wave rectifier which converts both polarities of the wave to a constant polarity. It can be built by using four Schottky diodes or ultrafast diodes to . The second rectifier alf wave rectifier which has to be designed by single Schottky diodes. After some investigations and tests, the half wave rectifier has been used instead of the full wave rectifier to reduce the area. The size of the receiver is one that have been be implanted inside the human body. Therefore, the half-waverectifier replaces the full wave rectifier, and this in the next section. Finally, Fig. 8.a 8.b illustrate general circuits for a full and half wave rectifiers respectively.

D amplifier
The voltage regulator is an essential component to control and stabilize the output voltage of the rectifier in the receiver (the implanted device). In this system, a linear voltage regulator is required to reduce the variation in the output of the rectifier. For this purpose, the Low dropout (LDO) regulator is . Therefore, there are only small differences between the supplied voltage and the load voltage

SYSTEM DESIGN
The system has been proposed to transmit an enough power to charge an implantable glucose sensor without any direct connection between the charger and the implanted sensor. According to [25], the implantable glucose sensors are connected directly to a microcontroller to facilitate transferring the data from the sensor to the external reader. Therefore, in addition to the power of 18 uW that are required by the sensor, an extra power needs to be provided to feed the microcontroller. Thus, the system has been designed and tested to provide the implanted part with a total of 650 µWatt to maintain an enough power that will be consumed by both of the sensor and the microcontroller.
It can be concluded from the proceeding that the system includes two devices: A transmitter (external device) and an implantable device (an internal device), as shown in Fig. 2. The external device provides and sends the power while the implanted device receives that power. The power will be transferred through the skin from the out of the body to the implanted device by using magnetic induction. Therefore, the receiver has a direct connection with the implantable sensor to feed it with required power.

The Transmitter Device
As it explained in the previous sections, this device is responsible for sending the power to the implanted device by using the inductive coupling. In the transmitter device, an RC oscillator has been used to oscillate the circuit by using a network of resistors and capacitors. Because it generates low frequencies, a power amplifier is required to be the second component of the transmitter, as illustrated in Fig. 4. The power amplifier is built of two MOSFET transistors (P-MOSFET and N-MOSFET) to establish the class-D power amplifier that is discussed previously.
Two air core coils (L1 in the transmitter and L2 in the receiver) are required to form the inductive coupling that is employed to transfer the power. These coils have actual air inside the windings. The reason for using air core coils is that they have lower inductance than the ferromagnetic core coil. These coils are handmade of a single strand copper with about 2 cm diameters and five turns.

The Receiver Device
The inductive coupling transmits the power that is generated in the first device (the external device) to the implanted device (the receiver). The secondary coils (L2) of the implanted device receives the transmitted power and passes it to the rectifier. The function of the rectifier is converting the received voltage from AC to DC by using Schocky barrier diodes, as it explained in the previous section.
In order to keep the resulted DC voltage stable, C4 will take its responsibility in holding this voltage. With these parts, the output voltage becomes ready to be provided to the sensor. Fig. 10 demonstrates the circuit design of the receiver and its configuration. In this circuit, the resistor R1 represents the load to determine the power consumption of the implanted sensor. Finally, a protection circuit can be added to the circuit to protect it by using two LEDs diodes connected in a series to dissipate any extra or unrequired power once the voltage over the coil become high enough to make the diodes forward-biased. Therefore, it also uses as a voltage limiter.

Implantation
The structure of the proposed system and its configuration have been discussed and detailed in the previous sections. Fig. 11.a and Fig. 12.a illustrate a schematic layout of the transmitter device and the receiver device respectively. They have been schemed by using the software which is available online in [26]. In this section, the proposed devices (the transmitter and the receiver) will be implemented on a Vero-board. This version has been made big to some extent (about 12 *4 cm for the transmitter and 4*7 cm for the receiver) because it is the experimental prototype for this project, and making it big and clear can be easier in testing the system and checking the connections and the signals. Fig.  11.b and Fig. 12.b show a photograph of the Veroboards implementation for the devices. During the design phase, the system has been built to do two functions: send power and receive data. However, only the power transmitter has been implemented and tested because this is the aim of this paper and its main contribution.
As an initial implementation, a function generator has been used to oscillate the circuit (with a power amplifier). The function generator has been adjusted to generate a sinusoidal wave with 5 V amplitude to feed the amplifier. For generating DC voltage, a DC power supply is used. Because of the significance effect of the frequency on the output voltage of the rectifier and the transmission distance, it has been examined by changing the frequency of the function generator (from 1 MHz to 12 MHz) and recording the results. However, the maximum frequency of the function generator is 10 MHz. Thus, the crystal oscillator has been used to oscillate the circuit and test its performance with the high frequencies. The circuit of the crystal oscillator that is shown in Fig. 6 is used to generate the high frequencies.
Testing the systems with different sizes of coils indicates that increasing the physical area of the coil rises the output voltage. The two coils of this system are implemented as copper handmade square coils with 4*4 cm size and six turns. It has two identical rectangular layers connected in parallel to allow using more turns on each layer.
Additionally, the efficiency (ƞ) of the system can be determining by the following equation, where Pout, Vout, Iout are the output power, voltage and current respectively, and Pin, Vin, Iin are the supplied power, voltage and current respectively.

The Experimental Results
During the experiment, the voltage at different nodes is measured by Oscilloscope, and the current is measured by a Millimeter. By applying an input power with 5 Volts and 42 mA current, the transmitter device will be equipped with 210 mW. The output of the power amplifier is about 5.2 peak to peak voltage.
The received current and voltage vary depending on the frequency of the source of energy. Fig. 13 and Fig. 14 Fig. 14.
During the experiment, the system has been tested with various distances (between the sender and the receiver). Therefore, different readings have been recorded from the receiver with these distances. This allows testing the impact of the range on the operation. In order to adjust the matching network, the output voltage of the envelope detector has been examined by Oscilloscope. It is clear from Fig. 15 that the highest voltage values are recorded when the distance was below 1.5 cm at 3 volts. Until to 35 mm distance, the output voltage was in the acceptable range (over 1.8 V). However, over 36 mm distance, the voltage dropped below the required value, then it went to approximately zero at 70 mm. The received current will take an opposite direction as it is shown in Fig.  16.
a. Schematic view b. Photograph  The purpose of testing the effect of changing the distance is to determine the maximum range between the receiver and the transmitter that can give sufficient power. According to the requirements of the system, 1.8 V is enough to charge the battery of the system.
In summary, these measurements indicate that the system is able to transfer enough power to large distances between the transmitter and receiver (compared with required distances). The amount of the energy that is transferred was sufficient within the range of 35 mm. The voltage regulator dissipates any extra or undesired voltage.
Characteristics of the coils (the primary and the secondary coils) are one of the confusion issues because the mathematical calculations cannot give the final decision about the size and number of turns of these coils. Several coils have been considered and examined during the design phase. All of these coils are handmade spiral square coils. The oscilloscope was connected in parallel to the receiver coil to measure the maximum voltage. The power transmission distance is measured between the closest edges of the coils.

CONCLUSION
The wireless power transmission is an effective and promising technology to supply the electric power without any direct connection. Nowadays, technological advances in the field of the biomedical promote the idea of the wireless power transfer with the increase of using the implantable devices that require being powered wirelessly [27]. This paper introduces a system to transfer the power wirelessly by using the inductive coupling. The produced system has two devices: an external device (transmitter) and an implantable device (receiver). The receiver harvests the power to charge an implanted battery with a regulated voltage. This battery is included to supply the implanted sensor that measures the glucose level in the human body and a microcontroller that is added to control the sensor.
A resistor has been used to represent the desired load (the implanted sensor) to exam whether the system is able to provide them with enough power or not. The previous tests and measurements show that the power that can be harvested is 360 µA current and 1.8 V voltage.
The tests prove that the system can transmit the required power upto 35 mm even with placing an insulator between the sender and the receiver.
The system has been implemented on Veroboards. This implementation is used for experimental purposes only. It has been designed and implemented to test the core circuit in sending and receiving the power to get the final configuration. It offers high quality and stability, but it has a big size compared with the PCB or ASIC. Therefore, for higher efficiencies and ranges, the implanted device can be implemented by using the Application Specific Integrated Circuit (ASIC), and this is one of the future directions for this work to introduce a practical implementation for these devices.