1. 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 ofNeurosurgery, Electrical and Computer Engineering, and
Bioengineering, University ofPittsburgh, 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 non-
radiativity. 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 ofHealth 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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VI("" ofthc
M idJk P.and
Figure I. Three panel views ofthe cell (left) and actual cell (right).

2. 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 ofturns, 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 aR(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 ofFr (I) ,
aR (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 Qfactors ofthe cells.
TABLE I
RESONANT FREQUENCY AND QFACTORS 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.
0.8
0.7
0.6
>. 0.5
g
.~ 0.4
"w 0.3
0.2
0.1
~o 15
Implanted or worn
device
Wire loop
Figure 2. WPT system setup.
~ H m D W g ~
Distance (em)
Figure 3. Power transfer efficiency vs. distance.