Open Journal of Applied Sciences, 2012, 2, 35-46
doi:10.4236/ojapps.2012.21004 Published Online March 2012 (http://www.SciRP.org/journal/ojapps)
Mid-Range W ir eless Power Transfer and Its Application to
Body Sensor Networks
Fei Zhang1, Jianbo Liu2, Zhihong Mao1, Mingui Sun1
1Department of Neurosurgery, University of Pittsburgh, Pittsburgh, USA
2Department of Electrical and Computer Engineering, Michigan State University, Lansing, USA
Email: drsun@pitt.edu
Received January 10, 2012; revised February 9, 2012; accepted February 20, 2012
ABSTRACT
It has been reported that, through the evanescent near fields, the strongly coupled magnetic resonance is able to achieve
an efficient mid-range Wireless Power Transfer (WPT) beyond the characteristic size of the resonator. Recent studies
on of the relay effect of the WPT allow more distant and flexible energy transmission. These new developments hold a
promise to construct a fully wireless Body Sensor Network (wBSN) using the new mid-range WPT theory. In this paper,
a general optimization strategy for a WPT network is presented by analysis and simulation using the coupled mode theory.
Based on the results of theoretical and computational study, two types of thin-film resonators are designed and proto-
typed for the construction of wBSNs. These resonators and associated electronic components can be integrated into a
WPT platform to permit wireless power delivery to multiple wearable sensors and medical implants on the surface and
within the human body. Our experiments have demonstrated the feasibility of the WPT approach.
Keywords: Body Sensor Network; Strongly Coupled Magnetic Resonance; Wireless Power Transfer; Coupled Mode
Theory; Relay Effect
1. Introduction
Wireless Body Sensor Networks (wBSNs) have emerged
in recent years as a key enabling technology to address a
number of significant and persistent challenges in health-
care and medical research, including continuous, non-
invasive, and inexpensive monitoring of physiological
variables. Typically, a wBSN is composed of a number of
sensor nodes dedicated to different forms of measure-
ments, such as the electrocardiogram (ECG), electro-
myogram (EMG), body temperature, glucose, and blood
pressure. Each sensor node, which can be either inside (as
an implant) or outside (as a wearable device) the human
body, is usually composed of an analog readout front-end,
a microprocessor, a radio transmitter/receiver, and a
power supply. These sensor nodes, wire-connected to a
battery, transmit data continuously through a wireless
connection to a central node, typically a PDA or a smart
cellphone. The central node collects, visualizes and ana-
lyzes data and/or wirelessly relays the data or partially
processed results to a remote terminal for more advanced
off-line processing or evaluation by healthcare profess-
sionals. While longer battery capacity, lower power con-
sumption, smaller size of the battery and other circuit
components, and higher manufacturing volumes have
made wBSN data collection more continuous, non-inva-
sive, and inexpensive, the progress towards wireless
powering has remained to be a significant problem. A
great demand exists for developing efficient Wireless
Power Transfer (WPT) methods to reduce the dependency
on batteries and remove both the network of wire connec-
tions around the body and the percutaneous wires pene-
trating the human skin.
Since the wireless electricity was reported in Science
in 2007 [1], this technology has been widely dubbed as
“witricity” which joins the first and last a few letters of
“wireless electricity” and is the trademark of the Witri-
city, Inc. Wireless electricity is based on strongly cou-
pled magnetic resonance which has sparked widespread
interest in both industrial and academic communities
[2-14]. Recently, it has been reported by the same group
of the original paper that the approach of powering mul-
tiple devices simultaneously can result in a higher overall
WPT efficiency compared to the WPT efficiency to each
individual device [8]. We analyzed the relay effect [2]
which extends the original witricity from the source-
device(s) scenario to the source-relay-device scenario,
allowing more flexible and distant WPT. In addition to
the extended power transmission range, the relay WPT
reduces the transmission power at the power source due
to the increase in the overall efficiency of energy de-
livery. This provides additional human safety from that
provided by the wtricity which transmits energy using
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL.
36
the magnetic field; it is known that the biological tissue
responds more passively to the magnetic field than either
the electric or electromagnetic field [1].
In this work, we extend our previous experimental re-
sults in [9] to address the theoretical and system optimi-
zation issues of the witricity in a network fashion. In this
network, a group of resonators are involved working in
synchrony. We describe the resonator and prototype de-
signs specific for wBSN applications, and demonstrate
the performance of a proof-of-concept wireless power
platform for the construction of wBSNs.
The remainder of this paper is organized as follows:
Section 2 focuses on the theory of the witricity network.
In Section 3 we describe the design and fabrication of our
new witricity resonators. What follows demonstrates the
results of proposed platform for wBSN in Section 4. Sec-
tion 5 discusses and suggests the potential applications of
witricity. Finally, conclusions are drawn in Section 6.
2. Theoretical Background
A systematic overview about the operational mechanisms
of the witricity can be found in [1-3]. Briefly, as shown in
Figure 1(a), the basic relay witricity system consists of
three resonators: a source, a relay and a device. It also con-
sists of a driving loop and an output loop. The source re-
sonator is coupled inductively with the driving loop, while
the driving loop links to an oscillating power source. Simi-
larly, the device coil is coupled inductively with the output
loop to supply power to an external load. The resonant eva-
nescent strong coupling mechanism between source and
device can be mediated by the presence of the relay.
The most important property of the witricity is that it
operates on the strongly coupled magnetic resonance in
the mid-range, which enables the system to achieve a very
high frequency selectivity towards the energy receiver
[1-3,15]. This can be realized by “tunneling/concentrate-
ing” the energy of near-field magnetic fields in a resonant
energy avenue from the source resonator to the device
resonator via an invisible path over space, with or without
being guided by a relay, provided that all resonators have
the same frequency. This tunneled energy form will travel
between the resonators in an oscillatory fashion, where
their coupling time between resonators is required to be
much shorter than the loss time [1-3]. Since the resonant
wavelength (often in MHz) is usually much larger than
the volume of the resonators, the field can circumvent
extraneous non-resonant objects in the vicinity of the
transmission path, and thus this midrange energy transfer
scheme does not require an uninterruptible line-of-sight.
In this paper, we further extend the witricity scheme to
involving multiple resonators using the coupled mode
theory. The general form of the multi-resonator WPT
system can be modeled in the following coupled mode
matrix form: (see Equation (1))
where i and j is the decimal number ranging from 1 to n
(the number of resonators, ); denotes the
signal amplitude of the resonator; i
2n

i
at
2π
i
f
is the indi-
vidual angular frequencies; ij
k is the coupling coeffi-
cient between two resonators; and is the individual
intrinsic decay rate.
i
Equation (1) can be rewritten as

d
d
A
UAKA iWA
t
  (2)
where A is the amplitude matrix; K is the coupling ma-
trix; W is the frequency matrix; and Γ is the loss matrix.
Assuming that V is the modal matrix whose columns
are the eigenvectors of K, and that D is the canonical
form of K, i.e., D is a diagonal matrix with K’s eigen-
values on the main diagonal, we have
K
VVD, or
1
K
VDV
 . Using this canonical matrix representa-
tion, we can obtain
expBiKDiWt
  (3)
Thus, the analytic solution of (2) can be written as
10AVBVAt
 (4)
where
0At
is the initial amplitude matrix.
Above general solution can work for any number of
sources, relays and devices. In order to understand the
physical mechanism more easily, we will next focus on a
3 by 3 network as shown in Figure 1(b), where a maxi-
mum of 9 identical resonators are involved. For clarity,
we use red, yellow and green to represent “source”, “re-
lay” and “device” resonators, respectively, in all remain-
ing figures when these resonators are studied. We choose
resonant frequency fo = 1 MHz and intrinsic loss Γo =
1000 for all resonators, and k = 500,000 between reson-
ators 1 and 5. The fact that the coupling, in theory, is
inversely proportional to the cube of the separation dis-
tances to other elements in the coupling matrix. Substi-
tuting these parameters into Equation (4) will enable the
alculations of the following WPT scenarios. c









1 11213141
1 1
212 223242
2 2
31323 3343
3 3
1234
d
n
n
n
nn nnnn
n n
iikikikik
at at
ikiik ikik
at at
dikik iikik
at at
t
ikikik iki
at at
 
 

 


 

 



 

 

 

 



(1)
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL. 37
(a)
(b)
Figure 1. (a) Basic components of a relay WPT system; (b) 3 by 3 WPT network.
2.1. “Source-Relay(s)-Device” Scheme
2.1.1. Absence of Relay
In order to form a complete picture, the first case under
analysis is the original witricity scheme as shown in Fig-
ure 2(a). In this case, the coupling matrix is [0 1/8; 1/8 0]
k, and the initial amplitude matrix will be [1; 0] if we as-
sume that all the energy exists in the source side at the
starting time. As a key distance-dependent figure-of-merit,
the effective coupling parameter represents, intuitively,
the ratio of “how fast energy is transferred between the
source and device” to “how fast it is dissipated due to in-
trinsic losses in these resonators” [1-3]. Since the intrinsic
losses are fixed and assumed identical as aforementioned,
the speed of energy transfer becomes the evaluation met-
rics for the WPT network. In order to gauge the WPT per-
formance of the system, we provide a notion of energy
exchange time defined as the time required to complete one
energy exchange cycle between the source and the device,
(the period of the slowest energy exchange frequency), as
marked by the arrow in Figure 2, where the energy ex-
change time is measured to be 5.1E–5 second. In the gen-
eral case (Figures 3 - 14), the energy exchange time indi-
cates the period of the energy exchange envelope for the
device resonator. Using (1), it can be shown that the energy
exchange time is directly related to the coupling coefficient.
Apparently, the smaller it is, the higher the efficiency will
be. Next, how to minimize the energy exchange time by
engineering the coupling matrix is the primary interest to
boost the performance of the WPT network.
2.1.2. One Relay
The relay effect can greatly enhance the performance of
the WPT system with a high efficiency, longer distance
and flexible routing. We have analyzed the solutions for
one-relay witricity system in [2], where the weaker coup-
ling between source and device resonators was ignored to
observe the difference between original and relayed cases.
Here, we consider all coupling items with a coupling
matrix [0 1 1/8; 1 0 1; 1/8 1 0]k. After substituting it into
Equation (4), the energy exchanges among resonators are
plotted in the Figure 3, where the energy exchange time
is reduced to 8.8E–6 second. Compared to Figure 2(b),
we can see that the relay for a fixed separation between
the source and device resonators accelerates the energy
exchange and improves the efficiency, which conforms
to our reported experiments in [2] as well.
2.1.3. Two Relays
There are three two-relay cases under investigation,
shown in Figures 4-6, respectively. Their energy ex-
change time is 7.5E–6, 1.3E–5 and 3.2E–6 second, res-
pectively. The coupling matrix of Figure 4(a) is [0 1
1/2/ 2 1/8; 1 0 1 1; 1/2/2 1 0 1/2/2; 1/8 1 1/2/2
0] k. Due to the similarity, next coupling matrixes are
omitted. Comparing Figure 4 with Figure 3, we can see
additional relays for a certain separation will accelerate
the energy exchanges when the other relays are fixed.
While Figures 4-6 together indicate that different con-
figurations of the same number of relays will lead to dif-
ferent coupling matrix, and hence different energy trans-
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL.
38
fer performance. Apparently, the position strategy of
Figure 6(a) is the best among the three, which also
matches well with our experiments in [2]. The fact that
the energy exchange time of Figure 3(a) is smaller than
that of Figure 5(a), pointing out that more relays may
result in more loose couples if these relays are not well
positioned.
2.1.4. Three Relays
The three-relay case is investigated as shown in Figure
7(a), and its energy exchange time is 5.1E–6 second. The
comparisons between Figures 6 and 7 also de- monstrate
that less relay case may combat the more relay case if the
relay network is optimized to form a more strongly cou-
pling fashion.
0.0 2.0x10
-5
4. 0x10
-5
6. 0x10
-5
8. 0x10
-5
1. 0x10
-4
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0
1. 2
Device
Source
Normalized Energy
Time(Sec)
System
Energy
Exchange
Time
(a) (b)
Figure 2. (a) Resonator position for case 1; (b) Energy exchanges.
0.0 2.0x10
-5
4. 0x10
-5
6. 0x 10
-5
8. 0x10
-5
1. 0x10
-4
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0
1. 2
Device
Relay(5)
Sour ce
Normalized Energy
Time(Sec)
System
(a) (b)
Figure 3. (a) Resonator position for case 2; (b) Energy exchanges.
0.0 1. 0x 10
-5
2. 0x10
-5
3. 0x10
-5
4.0x10
-5
5.0x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Energy
Time(Sec)
Relay(6) De v ic e
Rela y ( 5 )
Source
System
(a) (b)
Figure 4. (a) Resonator position for case 3; (b) Energy exchanges.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL. 39
0.0 1. 0x 1 0
-5
2.0x1 0
-5
3.0x1 0
-5
4.0x10
-5
5.0x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Energy
Time(Sec)
Dev ic e
Rela y(4 ,6 )
Source
Sys t e m
(a) (b)
Figure 5. (a) Resonator position for case 4; (b) Energy exchanges.
0.0 1. 0x10
-5
2.0 x10
-5
3.0x 10
-5
4.0x 10
-5
5.0x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Energy
Time (Sec)
Relay2 Device
Relay1
Sour c e
Sys te m
(a) (b)
Figure 6. (a) Resonator position for case 5; (b) Energy exchanges.
0.01.0x10
-5
2.0x10
-5
3.0x10
-5
4.0x10
-5
5.0x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
Normalized Energy
Time (Sec)
Relay(5) De vic e
Relay(4,6)
Sour c e
Sys tem
(a) (b)
Figure 7. (a) Resonator position for case 6; (b) Energy exchanges.
2.1.5. Seven Relay s
The seven-relay case in Figure 8(a) is also investigated,
and its energy exchange time is 3.1E–6 second as shown
in Figure 8(b).
2.1.6. C omparison s
Figure 9 lists all the energy exchange time for cases 1 - 7
for easy comparisons and understanding. It can be ob-
served that the resonator network optimization is of great
importance for improving WPT system performance. The
general solution in Equation (4) can well serve this opti
mization purpose.
2.2. “Source-Device(s)” Scheme
2.2.1. On e Device
The typical “Source-device” witricity scheme as shown in
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL.
40
0.0 1.0x10
-5
2.0x10
-5
3.0x10
-5
4.0x10
-5
5.0x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Energy
Time(Sec)
Relay (7, 8)
Relay (4,6)
Relay (5)
Device
Relay (2,3)
Sourc e
Sys te m
(a) (b)
Figure 8. (a) Resonator position for case 7; (b) Energy exchanges.
1234567
0.0
1.0x10-5
2.0x10-5
3.0x10-5
4.0x10-5
5.0x10-5
Time (Sec)
Case Number
Figure 9. Comparisons of energy exchange time for cases
1-7.
Figure 10(a) is included here for easy understanding.
The energy exchange time is 6.3E–6 second in this case.
Compared to Figure 2, the exchange time is eight times
less, which agrees well with the fact that the coupling is
inversely proportional to the separation distance.
2.2.2. Two Devices
The first multiple-device scheme is shown in Figure
11(a), where two device resonators are symmetrically
distributed around the source resonator. The energy ex-
change time is around 4.2E–6 second. Compared with
6.3E–6 second from Figure 10(b), this means that the
configuration in Figure 11(a) boosts the WPT perfor-
mance and improves the system efficiency, which mat-
ches with the experimental results in [8] that powering
multiple devices simultaneously can result in a higher
overall efficiency. Our comparison clearly illustrates the
physical mechanism behind this efficiency improvement.
2.2.3. Four Devic es
Figures 12 and 13 give the results of four device resona-
tors with the energy exchange times of 3.06E-6 and
1.09E-6 seconds, respectively. These two similar cases
with different energy exchange speeds repeat the early
conclusion that the optimization of power network sig-
nificantly affects the performance of the whole energy
ransfer with the same number of resonators. t
0.05.0x10
-6
1.0x10
-5
1.5x10
-5
2.0x10
-5
2.5x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Device
Sour ce
Normalized Energy
Time(Sec)
Syst e m
(a) (b)
Figure 10. (a) Resonator position for case 8; (b) Energy exchanges.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL. 41
0.0 5.0x10
-6
1.0x10
-5
1.5x10
-5
2.0x10
-5
2.5x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2 Device (1,9)
Source
Normalized Energy
Time(Sec)
Syste m
(a) (b)
Figure 11. (a) Resonator position for case 9; (b) Energy exchanges.
0.0 5.0x10
-6
1. 0x10
-5
1.5x10
-5
2.0x10
-5
2.5x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
Normalized Energy
Time (Sec)
Device
So urce
Sys te m
(a) (b)
Figure 12. (a) Resonator position for case 10; (b) Energy exchanges.
0.0 5.0x10
-6
1. 0x10
-5
1.5x10
-5
2. 0x10
-5
2. 5x10
-5
0. 0
0. 2
0. 4
0. 6
0. 8
1. 0
Normalized Energy
Time (Sec)
Device
So urc e
Sys te m
(a) (b)
Figure 13. (a) Resonator position for case 11; (b) Energy exchanges.
2.2.4. Eight Devices
The WPT case with eight device resonators is shown in
Figure 14(a), where the energy exchange time is 7.20E–7
for devices 2, 3, 7, and 8, and 9.22E–7 second for devices 1,
4, 6, and 9, respectively. The series of Figures 10-14 rein-
forces the two arguments: 1) the more devices, the higher
total efficiency; and 2) the better network optimization, the
stronger system coupling, which necessitate the theoretical
optimization strategy discussed in this section before wire-
less power network is put into real construction.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL.
42
0.0 5.0x10
-6
1.0x10
-5
1.5x10
-5
2.0x10
-5
2.5x10
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Normalized Energy
Time (Sec)
So urc e
Device(2,3,7,8)
Device(1,4,6,9)
Sys te m
(a) (b)
Figure 14. (a) Resonator position for case 12; (b) Energy exchanges.
3. Resonator Design and Prototype
Implementation
For the wBSN application, a thin-film resonator is highly
desirable. This type of resonator can be imprinted or em-
bedded to the exterior or interior cover of the parent de-
vice to which the electric power will be transferred with-
out taking its interior space. This thin-film design not only
utilizes the maximum dimensions of the parent device to
capture magnetic flux, but also provides the maximum
space for the parent device itself. In addition, it facilitates
heat dispersion.
3.1. Cylindrical Resonator
3.1.1. C ylindrical Design
Our cylindrical resonator design consists of three layers of
films as shown in Figure 15(a): two layers of copper (red)
and one layer of insulator (blue). The top and middle
panels in Figure 15(a) show, respectively, the top and
side views of the resonator. The horizontal narrow copper
strips (red) in the middle panel represent a helical induc-
tor. The yellow lines represent the spaces between the
copper strips. The bottom panel shows the side view from
the interior. Several vertical copper strips (red) are affixed
to the insulator film (blue). These vertical strips form
physical capacitors with the coil conductors in the exterior
layer. Clearly, this thin-film design represents a compact
LC tank circuit. In our design, energy transfer is primarily
provided by the magnetic field, while the electric field is
mostly confined within the physical capacitors. This fea-
ture effectively prevents the leakage of the electrical field
and helps reduce health concerns since the biological tis-
sue (and other electrically conductive objects) interacts
much more strongly with electric fields than with mag-
netic fields. Another motivation of our design is to obtain
a compact size. By increasing the capacitance using the
strips while keeping the same inductance in the LC tank
resonator, the operating frequency of the witricity system
can be reduced, which is desirable in many practical ap-
plications where the size of the parent device is small.
In practical applications, the size and shape of the
source resonator have fewer restrictions and can be larger
than that of the device resonator. A larger source reson-
ator can produce stronger magnetic fields for a longer
transmission range. Conversely, the size of the device
resonator, though preferred to be as large as possible, is
usually limited by the size of the parent device.
3.1.2. Implementation of Cylindrical Design
As shown in Figure 15(b), in our experiments, we made a
larger source resonator in the form of Figure 15(a). It was
a large flexible ring with diameter 350 mm, thickness
0.35 mm, and width 29 mm. The insulator had a dielectric
constant of 3.74. The exterior copper tape (width 0.635
cm) formed a 4-turn coil. Eight copper strips with 2.54-cm
widths were affixed to the insulator film on the internal
side. The resonator was incorporated into the waist belt,
and the one-turn driving coil (a loop) linked to the RF
power source can was attached on the belt to make the
system wearable. Using vector network analyzer (8753ES),
we set its resonant frequency to be 7 MHz and Q value
was measured to be 51.62.
Likewise, based on the design in Figure 15(a), a smaller
receiver resonator was made as shown in Fi gure 15(c). The
exterior copper strip with 0.635-cm width formed a 6-turn
coil, and six copper strips with 2.54-cm widths were af-
fixed to the insulator film on the internal side. The radius
and height of the cell were about 8.1 cm and 5 cm, respec-
tively. Similarly, we also designed much smaller reson-
ators for other body parts such as arms. A seven-turn coil
was used as the output coil to connect and power the load.
We also set its resonant frequency to be 7 MHz and mea-
sured its Q value as 56.38. For the purpose of observation,
an LED was first used as the load, simulating the load ef-
fect of electronics devices and allowing visual examina-
tion. This LED was then replaced by a resister from which
quantitative measurements was performed.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL. 43
(a) (b) (c)
Figure 15. Cylindrical thin-film resonator: (a) Structure; (b) Source resonator; (c) Device resonator.
3.1.3. P er formance o f Cy lindric a l Design
Figure 16(a) shows an experimental 7 MHz witricity
system on a workbench. Wireless energy fully illuminated
the low-power LED. When we misaligned the axes of
source and device resonators, the power was still be trans-
mitted efficiently as shown in Figure 16(b). This pheno-
menon differs from that of the conventional magnetic
induction methods. In order to evaluate system perfor-
mance quantitatively, we replaced the LED with a re-
sistor whose resistance approximates the impedance of
the output terminals at resonance. Utilizing a diode-based
RF detector, we measured the RF energy at the input ter-
minals of the driving loop and the energy at the load ter-
minals of the output loop. Then the power transfer effi-
ciencies with and without misalignment are measured and
plotted in Figure 17. It can be seen that, the efficiency
without misalignment can reach approximately 80% at a
15-cm separation between the transmitter and receiver. It
can also be seen that certain misalignments (e.g. 5 cm)
between transmitter and receiver cause only a slight drop
in efficiency. This tolerance in misalignment is highly
desirable in practical applications since, for example, it
allows energy to be transmitted to a moving target or
multiple locations as in the case of wBSN.
3.2. Planar Resonator
3.2.1. Pl a nar Design
In order to satisfy the requirement of commonly used
electronics devices, we believe that the planar structure
will be desirable since it will be easily implemented by
printing this resonator on the internal or external cover, or
embedded in the clothes. As shown in Figures 18(a) and
(b), the planar thin-film receiver resonator also includes
three layers: the top metal layer (black) formed a planar
spiral rectangular coil by attaching the metal strip on an
insulated thin film (middle layer, transparent), underneath
which there are several separated vertical metal strips
(a)
(b)
Figure 16. Witricity system working at 7 MHz: (a) Align-
ment setup; (b) Misalignment setup.
1520 25 30 35 40 45
0.0
0.2
0.4
0.6
0.8
1.0
Power Transfer Efficiency
Distance (cm)
Center Alignment
5 cm shift
10 cm shift
Figure 17. Measured transmission efficiency vs. distances of
separation for cylindrical de sign.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL.
44
(red). The top and bottom metal strips form the capacitor
via the insulated middle layer. Clearly, this thinfilm de-
sign represents a compact LC tank circuit. This design is
very similar to the source resonator design, and the dif-
ference lies in changing the belt shape into a planar struc-
ture.
3.2.2. Implementation of Planar Design
The constructed device resonator is shown in Figures
18(c) and (d) with length 20.5 cm, width 20 cm and
thickness 0.35 mm. The coil inductor consists of 6 turns
of copper strips (0.635 cm width) and the bottom layer is
formed with 8 vertical copper strips (2.54 cm width). We
have measured its Q value as 40 using a vector network
analyzer (8753ES). Since the Q value is the important
factor in evaluating the energy loss, we may further
change the spiral inductor into symmetric [16], step
width [17] or patterned floating structures [18,19] in the
future as the alternative designs to improve the Q value
of the resonator.
3.2.3. Perform ance o f Pla na r Design
Using the same measurement method with the cylindrical
design, the power transfer efficiency of our planar design
is plotted in Figure 19. It can be seen that the efficiency
reaches above 80 % at a 20 cm separation of transmitter
and receiver.
4. Platform for wBSN Applications
In our wBSN design, the large cylindrical resonator can
be integrated with a waist belt. A separate device con-
taining a battery and electronic circuits is attached to the
belt to generate the required RF signal and host electronic
circuits. Several smaller cylindrical and planar receiver
resonators could be embedded in the clothes or implanted
within the body, such as the head, chest, abdomen, arms
and/or legs, after being integrated with the implantable
devices as described previously.
In a separate experiment, we utilized a source resonator
and several device resonators to simulate the body sensor
network. In order to eliminate the health concerns, we use
a maximum 500 mW power as the source. Each of these
device resonators is magnetically coupled with an energy
pick-up coil which is connected to a low-power LED as a
load. Figure 20 shows that the LEDs, located on the hand,
the head (Figure 20(a)), the limbs (Figure 20(b)) and the
belly (Figure 20(c)), are lit by the energy transmitted
from the waist belt transmitter. If each LED is replaced by
a rectification and regulation circuit, the power produced
can be used to operate a microsensor which is either out-
side of the body or implanted inside so that we could by-
pass the batteries needed by these devices to operate.
Similarly, if the resonant signal is modulated appropri-
ately, the wireless system can also perform communica-
tion tasks. Clearly, this qualitative experiment shows that
it is feasible and convenient to use the witricity as a new
tool to construct wBSN systems to perform a wide variety
of diagnostic or monitoring functions.
5. Discussions
Although, currently, long life lithium ion batteries and
ethanol fuel cells have been persuaded as ways to make m
(a) (b) (c) (d)
Figure 18. Schematic and real picture of planar thin-film device resonator: (a) & (c) Top view; (b) & (d) Bottom view.
20 30 40 50 60
0.0
0.2
0.4
0.6
0.8
1.0
Efficiency
Separation distance (cm)
(a) (b)
Figure 19. (a) The measured setup for planar design; (b) Transmission efficiencies vs. distances of separation.
Copyright © 2012 SciRes. OJAppS
F. ZHANG ET AL. 45
(a) (b) (c)
Figure 20. Wireless power transfer platforms with three schemes for wBSN.
the electrical components more mobile, consumers’ ex-
pectations are still far to meet due to the added weight and
expensive replacement price of batteries. The discovery of
witricity, as a new option, is revolutionizing the wireless
industry, and holds great promise to leave batteries as a
thing of the past.
With a lock-and-key mechanism, the witricity enables
energy to be transferred only to the intended target, in
which the source and device are both tuned to the same
resonant frequency [20]. The fact that strongly coupled
regime through interaction functions when the two objects
is separated by a large distance and even blocked by an
additional object brings people considerable confidence
for the exploration of practical applications. Its recent
success of preliminary applications to consumer electron-
ics such as TV, laptops, electrical cars and cellphones
[12,14] and biomedical sensors [9,13] have received con-
siderable attention in wireless powering and recharging.
In addition, nonradiative fields ensure that little energy is
lost or would adversely affect the environment. The relay
[2] and resonance enhancement [8] effects of witricity,
with the network optimization concept presented in this
paper, will further enhance the robustness of building a
successful WPT network.
6. Conclusion
In the past decade, the veritable feast of electrical devices
in modern electric era has been enabled by the techno-
logical advances. Numbers of unwanted power cords
stimulate people to develop the concomitant powering
solution from wired to wireless ways. As a new method of
wireless energy transfer, witricity has a high potential in
medical applications. In this paper, we investigate the fea-
sibility of witricity in providing wireless power to wBSN
systems. Theoretical analysis has been presented to fully
understand the oscillatory behaviors during in the WPT
system operation when multiple resonators are involved.
Two types of thin film resonators have been specially de-
signed, fabricated and quantitatively measured for the con-
struction of wBSN systems. A prototype WPT platform
has been built and qualitatively evaluated for the wBSN.
Our experimental results have indicated that the witricity
technology will provide a powerful solution to power mul-
tiple sensors for fully wireless BSN applications.
7. Aknowledgements
This work was supported in part by US Army contract No.
W81XWH-050C-0047 and National Institutes of Health
grant No. U01 HL91736.
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