Wireless Power Transfer System Design for

Implanted and Wom Devices Xiaoyu Liu, Fei Zhang, Steven A. Hackworth, Robert 1. Sclabassi and Mingui Sun

Laboratory for Computational Neuroscience, Departments of Neurosurgery, Electrical and Computer Engineering, and Bioengineering, University of Pittsburgh, Pittsburgh, USA

drsun@pitt.edu (Mingui Sun)

Abstract-Witricity, a highly efficient wireless power transfer method using mid-range resonant coupling, was reported in recent literature. Based on this method, we present a wireless power transfer scheme using thin film resonant cells for medical applications. The thin film cells, consisting of a tape coil in the exterior layer and conductive strips in the interior layer separated by an insulation layer, are made light and flexible to provide comfort for users during wear. The wireless power transfer scheme presented displays great potential for delivering energy to implantable and worn devices, with range much larger than the current technology for powering implantable devices.

Keywords: wireless power transfer, witricity, thin film cell

I. INTRODUCTION

Nowadays, electronic devices can be implanted inside or worn around the body to perform a variety of therapeutic, prosthetic, and diagnostic functions. However, efficiently powering these devices is still a problem undergoing study. Wireless power transfer technologies provide one solution to this problem. In the medical field, wireless power transfer technology can be used to transmit power to implantable devices from outside the body to eliminate battery replacement surgeries. Wireless power transfer technology can also be used to transmit power to body sensors for health monitoring applications as well as computer assisted rehabilitation.

In medical applications, currently, magnetic coupling is the primary choice of wireless power transfer due to its non­radiativity. However magnetic coupling suffers from drawbacks like short range, directionality and low efficiency; the source has to be placed close to the device to transmit sufficient power. An MIT study recently reported that magnetic resonant coupling using a non-radiative mid-range field can be used for wireless power transfer [1]. This scheme, also called "witricity", has been tested to be feasible and efficient in non-medical cases. In their experiment, they lit up a 60W bulb 2 meters from the power source with efficiency up to 40%. Although their experiment was successful in transmitting power in non-medical applications, their resonant

This work was supported in part by US Army Medical Research and Material Command contract No. W8IXWH-050C-0047 and National Institutes of Health grant No. UOI HL91736.

coil design is not feasible for transmitting power to medical devices due to its large size and inflexibility. Based on witricity, we develop our wireless power transfer system (WPT) and cells for recharging medical implants and powering worn devices. Our experiment has shown that the WPT system proposed is effective in transferring power efficiently to implanted and worn devices.

II. THIN FILM CELL AND WPT SYSTEM DESIGN

As proposed in [1], two helical resonant coils, serving as the transmitter and the receiver, are placed coaxially near the source and the load. The source and the load inductively couple with the transmitter and the receiver, respectively, each through a single loop. In our system, thin film cells are used as the transmitter and the receiver. The thin film cell consists of three layers (Fig. 1). The middle layer, made of polymer, is the insulator between the exterior and interior layers. The exterior layer consists of a helical conductor forming an inductor that captures and generates the magnetic field. To make the cell light weight and flexible, the conductor is made of copper tape. The interior layer consists of several conductive strips in parallel with the axis of the cell, forming capacitors with the overlapped parts in the exterior layer and dividing the inductor into segments. The cell thus forms a structure with multiple resonant frequencies. This enables the cell to be used for simultaneous power transmittion and data communication. Furthermore, this type of cell confmes most of the electric field inside the insulator, therefore reducing the possibility of harm to the human body.

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Figure I. Three panel views of the cell (left) and actual cell (right).

In our WPT system design, we use one transmitter and several receivers. The transmitter, inductively coupled with a power driving loop, is placed around the waist. The receivers have much smaller sizes, placed around the body near implanted or worn devices (Fig. 2). The driving circuit for the load, inductively coupled with a receiver, can be either a single loop or a coil with multiple turns. By adjusting the number of turns, one can control the voltage across the devices and hence the power transmitted.

The energy exchange between the two resonant cells can be expressed using the following equations [2]:

(1)

where ar (1) and a R (1) are the field amplitudes, whose squared absolute values equal the energy; rr, rR and rL are the decay rates of the transmitter, receiver and the equivalent decay rate of the load, which are inversely proportional to Q factors; K is the coupling coefficient and Fr (1) is the external

field. If one assumes ar (1) = Arej(l)rt kept by role of Fr (I) ,

a R (1) then has the following solution [3]:

Equation (2) indicates that maximum energy exchange occurs when resonance happens, i.e. (j)r = (j)R' Equation (2) also indicates that, to obtain high efficiency, K must be far greater than the decay rates, and their ratio determines the effective range of power transfer.

III. EXPERIMENT

The experimental platform includes one source, one trasmitter, one receiver and one load. The transmitter cell, consisting of 4 turns in the exterior layer and 8 strips in the interior layer, has a radius of 176 mm and a height of 29 mm. The receiver cell has 7 turns and 6 strips with 8.13 cm radius and 6.71 cm height. Both cells are made of the same copper tape that has a thickness of 40 J.lm. The width of the wires in the exterior layer and the width of the strips in the interior layer are 6.35 mm and 25.4 mm, respectively. The dielectric constant of the insulator is 3.74. Table 1 lists the measured resonant frequencies and Q factors of the cells.

TABLE I

RESONANT FREQUENCY AND Q FACTORS OF TRANSMITTER CELL AND RECEIVER CELL

Resonant Frequency Q factor

(MHz)

Transmitter 7.04 51.3

Receiver 7.02 62.0

The source, contammg a function generator, a power amplifier and a driving loop, outputs a 7 MHz sine wave. For convenience of measurement, we use a 1500-0hm resistor as the load and measure its voltage to calculate its power. To measure the power supplied, a 66.2-0hm test resistor is connected in series with the wire loop at its connection point to the source. We measure the voltages across the test resistor, across the wire loop and across both. From the voltages and the resistor, we obtain the total power supplied to the transmitter, the receiver and the load. The measured efficiency is shown in Fig. 3. The efficiency at 20 cm is 40.2% using witricity, while using megnetic coupling, the distance must be kept shorter than 4 cm to obtain the same efficiency. Note that the load resistor is not optimized for efficiency, we believe that the optimized efficiencies are greater than those shown in Fig. 3.

IV. CONCLUSION

We present a wireless power transfer scheme using thin film cells as transmitter and receiver. The system can be used to recharge implantable devices and power wearable devices around the body. Through an experiment, we verify that the proposed system is more effective in power transfer to medical implants than magnetic coupling.

REFERENCES

[I) A. Kurs, A. Karalis, R. Moffatt, 1. D. Joannopoulos, P. Fisher, and M. Soljacic, "Wireless power transfer via strongly coupled magnetic resonances," Science, Vol. 317, pp. 83-86, July 2007.

[2) H.A. Haus, Waves and fields in optoelectronics, Prentice hall: Englewood Cliffs, 1984, pp. 197-234.

[3) A. Karalis, 1.D. Joannopoulos, and M. Soljacic, "Efficient Wireless Non-radiative Mid-range Energy Transfer," Annals of Physics, Vol. 323, pp. 34-48, January 2008.

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Figure 2. WPT system setup.

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Figure 3. Power transfer efficiency vs. distance.