Download Transcutaneous Energy Transfer System for Powering Implantable

Transcutaneous Energy Transfer System for Powering Implantable
Biomedical Devices
T. Dissanayake1, D. Budgett1, 2, A.P. Hu3, S. Malpas2,4 and L. Bennet4
1
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
2
Telemetry Research, Auckland, New Zealand
3
Department of Electrical and Computer Engineering, University of Auckland, Auckland, New Zealand
4
Department of Physiology, University of Auckland, Auckland, New Zealand
Abstract — Time varying magnetic fields can be used to
transfer power across the skin to drive implantable biomedical
devices without the use of percutaneous wires. However the
coupling between the external and internal coils will vary
according to orientation and posture. Other potential sources
of power delivery variations arise from changes in circuit
parameters and loading conditions. To maintain correct device
function, the power delivered must be regulated to deal with
these variations. This paper presents a TET system with a
closed loop frequency based power regulation method to deliver the right amount of power to the load under variable
coupling conditions. The system is capable of regulating power
for axially aligned separations of up to 10mm and lateral displacements of up to 20mm when delivering 10W of power. The
TET system was implanted in a sheep and the temperature of
implanted components is less than 38.4 degrees over a 24 hour
period.
of variation in coupling is due to posture changes of the
patient causing variation in the alignment between the primary and the secondary coils. The typical separations between the internal and external coils are in the range of 1020mm. If insufficient power is delivered to the load then the
implanted device will not operate properly. If excessive
power is delivered, then it must be dissipated as heat with
the potential for causing tissue damage. Therefore it is important to deliver the right amount of power matching the
load demand.
Primary Coil
Skin
Magnetic Secondary Coil
coupling
DC
Supply
Power
Converter
Pickup
Keywords — Magnetic field, coupling, Transcutaneous Energy Transfer (TET)
Load
Power
feedback
Controller
I. INTRODUCTION
High power implantable biomedical devices such as cardiac assist devices and artificial heart pumps require electrical energy for operation. Presently this energy is provided
by percutaneous leads from the implant to an external power supply [1]. This method of power delivery has the potential risk of infection associated with wires piercing through
the skin. Transcutaneous Energy Transfer (TET) enables
power transfer across the skin without direct electrical connectivity. This is implemented through a transcutaneous
transformer where the primary and the secondary coils of
the transformer are separated by the patient’s skin providing
two electrically isolated systems. A TET system is illustrated in figure 1. The electromagnetic field produced by the
primary coil penetrates the skin and produces an induced
voltage in the secondary coil which is then rectified to
power the biomedical device.
Compared to percutaneous wires, TET systems become
more complex to operate under variable coupling conditions
as it result in a variation in power transfer [2]. One source
Fig. 1 Block diagram of a TET system
Power can be regulated either in the external or the implanted system. However, regulation in the implanted system results in dissipation of heat in the implanted circuitry
[3]. Furthermore, it also increases the size and weight of the
implanted circuitry therefore power regulation in the external system is preferred over the implanted system. There are
two main methods of regulating power in TET systems,
magnitude and frequency control methods. In the case of
magnitude control, input voltage to the primary power converter is varied in order to vary the power delivered to the
load. This method of control is very common in TET systems however it does not take into account the miss-match
of the resonant frequency of the secondary resonant tank
and the operating frequency of the external power converter.
This miss-match in frequency reduces the power transferred
to the load, consequently, a larger input voltage is required
which results in a reduction in the overall power efficiency
of the system. Frequency control involves varying the operating frequency of the primary power converter to vary the
Chwee Teck Lim, James C.H. Goh (Eds.): ICBME 2008, Proceedings 23, pp. 235–239, 2009
www.springerlink.com
236
T. Dissanayake, D. Budgett, A.P. Hu, S. Malpas and L. Bennet
power delivered to the load. Depending on the actual power
requirement of the pickup load, the operating frequency of
the primary power converter is varied so the secondary
prick-up is either tuned/detuned, thus the effective power
delivered to the implantable load is regulated [4]. The system discussed in this paper uses frequency control to control
power delivery to the load, and a Radio Frequency (RF) link
is used to provide wireless feedback from the implanted
circuit to the external frequency controller.
II. SYSTEM ARCITECHTURE
The TET system is designed to deliver power in the
range of 5W to 25W. Figure 2 illustrates the architecture of
the overall system. A DC voltage is supplied to the system
with an external battery pack. A current fed push pull resonant converter is used to generate a high frequency sinusoidal current across the primary coil. The magnetic coupling
between the primary and the secondary systems produces a
sinusoidal voltage in the secondary coil which is rectified
by the power conditioning circuit in the pickup to provide a
stable DC output to the implanted load. As shown in figure
2, a DC inductor is added to the secondary pick up following the rectifier bridge in order to maximize the power
transfer to the load. The DC inductor aids to sustain a continuous current flow in the pick up [5].
Primary
resonant tank
Push-pull
resonant
Frequency
controller
V ref
Cp Lp
Secondary
resonant tank
Ls Cs
Magnetic
coupling
Biomedical
load
V dc
Internal
transceiver
Skin
Digital analogue
converter
RF Communication
channel
External
transceiver
Fig. 2 System architecture
Two nRF24E1 Nordic transceivers are used for data
communication. The DC output voltage of the pickup is
detected and transmitted to the external transceiver. The
external transceiver processes the data and adjusts the duty
cycle of the output PWM signal in order to vary the reference voltage (Vref) of the frequency control circuitry. The
_________________________________________
PWM signal is passed through a Digital to Analogue Converter (DAC), in order to obtain a variable reference voltage. This variable reference voltage is then used to vary the
frequency of the primary resonant converter which in turn
varies the power delivered to the implantable system. The
response time of the system is approximately 360ms.
A. Frequency controller
The frequency controller employs a switched capacitor
control method described in [7]. The controller varies the
overall resonant frequency of the primary resonant tank in
order to tune/detune to the secondary resonant frequency.
The frequency of the primary circuit is adjusted by varying
the effective capacitance of the primary resonant tank. This
is illustrated in figure 3.
L1
L2 Primary resonant tank
Secondary resonant
tank
CP LP LS
VIN
CV1
SV1
CS
Load
CV2
S1
S2
SV2
Fig. 3 System based of primary frequency control [4]
Inductor LP, capacitor CP and switching capacitors CV1
and CV2 form the resonant tank. The main switches S1 and
S2 are switched on and off alternatively for half of each
resonant period and changing the duty cycle of the detuning
switch SV1 and SV2 varies the effective capacitances of CV1
and CV2 by changing the average charging or discharging
period. This in turn will vary the operating frequency of the
primary converter. Each CV1 and CV2 is involved in the
resonance for half of each resonant period. The variation in
reference voltage (Vref) obtained from the DAC is used to
vary the switching period of these capacitors. This method
of frequency control maintains the zero voltage switching
condition of the converter while managing the operating
frequency. This helps to minimize the high frequency harmonics and power losses in the system. As shown in figure
3 the pickup circuitry is tuned to a fixed frequency using the
constant parameters LS and CS. The operating frequency of
the overall system is dependent on the primary resonant
tank which can be varied by changing the equivalent resonant capacitance [6], therefore the tuning condition of the
power pickup can be controlled.
IFMBE Proceedings Vol. 23
___________________________________________
Transcutaneous Energy Transfer System for Powering Implantable Biomedical Devices
A prototype TET system was built and tested in a sheep.
The internal coil and the resonant capacitor were Parylene
coated and encapsulated with medical grade silicon to provide a biocompatible cover. The total weight of the implanted equipment was less than 100g. As illustrated by the
cross sectional view in figure 4, thermistors were attached
to the primary and the secondary coils to measure the temperature rise caused by the system in the surrounding tissue.
x
x
x
x
x
x
Thermistor 1: Placed on top of primary
Thermistor 2: Placed under the skin
Thermistor 3: Placed on the muscle side
Thermistor 4: 1cm from the secondary coil
Thermistor 5: 2cm from the secondary coil
Thermistor 6: Near the subcutaneous tissue near the
exit of the wound.
Prior to experimentation, the thermistors were calibrated
against a high precision FLUKE 16 Multimeter temperature
sensor and a precision infrared thermometer.
Primary coil
1
Subcutaneous tissue
6
2
4
5
1cm
3
Secondary coil
Healthy tissue
2cm
also put on the site of the wound to reduce infection. Following the surgery the sheep transferred to a crate where it
was kept over a three week period. The primary coil was
placed directly above the secondary coil and held on the
sheep using three loosely tied strings. A PowerLab ML820
data acquisition unit and Labchart software (ADInstruments, Sydney Australia) was used for continuous monitoring of the temperature, the output power to the load and the
variation in input current of the system during power regulation. The data acquisition was carried out at a frequency
of 10 samples per second.
IV. EXPERIMENTAL RESULTS
Experimental results were obtained for delivering 10W
of power to the load when the system was implanted in
sheep. Figure 5 illustrates the closed loop controlled power
delivered to the load over a period of 24 hours. The input
voltage to the system was 23.5V. The controller is able to
control the power to the load for axially aligned separations
and lateral displacements between 10mm to 20mm. Beyond
this range the coupling is too low for the controller to provide sufficient compensation, and delivered power will drop
below the 10W set point. Evidence of inadequate coupling
can be seen at intervals in Figure 5.Variation in input current reflects the controller working to compensate for
changes in coupling using frequency variation between 163
kHz (fully detuned) and 173 kHz (fully tuned). When the
coupling between the coils is good, the primary resonant
tank is fully detuned in order to reduce the power transferred to the secondary. When the coils are experiencing
poor coupling, the primary resonant tank is fully tuned to
increase the power transfer between the coils.
Fig. 4 The placement of the temperature sensors
Graph of closed loop control power at 10 W
12.0
_________________________________________
10.0
0.75
Power (W)
Prior to the surgery all implantable components were
sterilized using methanol gas. The sheep was put under
isoflurane anesthesia and the right dorsal chest of the sheep
was shaved. Iodine and disinfectant was applied over the
skin to sterilize the area of surgery. Using aseptic techniques a 5 cm incision was made through the skin on the
dorsal chest. A tunnel was created under the skin approximately 20 cm long and a terminal pocket created. The secondary coil and the thermistors were placed within this
pocket. The thickness of the skin at this site was approximately 10mm. The secondary coil was then sutured in place
and the power lead from the coil and leads of the thermistors were tunneled back to the incision site and exteriorised
through the wound. The wound was stitched and Marcain
was injected to the area of the wound. Iconic powder was
0.8
8.0
6.0
0.7
4.0
Input current (A)
III. EXPERIMENTAL METHOD
237
0.65
2.0
Outp ut Vo lt ag e
inp ut current
0.0
0.6
0
200
400
600
800
1000
1200
1400
Time (mins)
Fig. 5 Regulated power to the load and the input current to the system
Figure 6 shows the temperature recorded from the six
thermistors. It takes approximately 20 minutes for the tem-
IFMBE Proceedings Vol. 23
___________________________________________
238
T. Dissanayake, D. Budgett, A.P. Hu, S. Malpas and L. Bennet
perature to reach a steady state after turn-on. The maximum
temperature was observed in the thermistor placed under the
secondary coil on the muscle side. The maximum temperature observed in this thermistor over the 24 hour period was
38.10C. The maximum temperature rise observed was 3.80C
in the thermistor placed under the skin. The large variation
in the primary coil temperature is due to the changes in
current through the coil from the frequency control mechanism. When the system is in the fully tuned condition, the
current in the primary coil is at a maximum to compensate
for the poor coupling. The temperature rise in the thermistors 1cm and 2cm from the secondary coil is well below
20C.
Temperature against time when delivering 10W
40.0
Temperature (Celcius)
38.0
36.0
34.0
32.0
30.0
28.0
26.0
Q
R
ZL
(1)
Where R is the load resistance, (2\f) is the system angular operating frequency, and L is the secondary coil inductance. A larger Q will enable the system to be more
tolerant. This benefit is traded off against the need for a
more sensitive and faster feedback response from the control system.
VI. CONCLUSIONS
We have successfully implemented a system that is capable of continuously delivering power to a load in a sheep.
The results have been presented for delivering 10W of
power to the load with closed loop frequency control technique for a period of 24 hours. The external coil was loosely
secured to lie over the region of the internal coil and subjected to alignment variations from a non-compliant subject.
The maximum temperature observed in this system is
38.10C on the thermistor placed on the muscle side under
the primary coil. The maximum temperature rise was 3.80C
on the thermistor placed under the skin.
24.0
22.0
REFERENCES
20.0
0
200
400
600
800
1000
1200
1400
1600
1.
Time (mins)
unde r s kin
m us c le s ide
2 c m fro m s e c
ne a r wo und e xit
prim a ry s urfa c e
1 c m fro m s e c
Fig. 6 Temperature profile of the thermistors
2.
V. DISCUSSION
3.
Although the system performs well at delivering 10W
over a 24 hour period, there are short intervals when this
power level was not delivered. These intervals correspond
to times when the coupling is too low for the controller to
compensate. A variety of approaches can be taken to solve
this problem. The first is to tighten the coupling limitations
to prevent coupling deteriorating to beyond the equivalent
limit of 20mm of axial separation. The second approach is
to allow occasional power drops on the basis that an internal
battery could cover these intervals, (patient alarms would
activate if the problem persists). The third approach is to
increase the controller tolerance to low coupling. The ability
to tolerate misalignments of the frequency controlled system is mainly determined by the systems quality factor (Q
value), which is defined by:
_________________________________________
4.
5.
6.
Carmelo A. Milano, A.J.L., Laura J. Blue, Peter K. Smith, Adrian F.
Hernadez, Paul B. Rosenberg, and Joseph G. Rogers, Implantable Left
Ventricular Assist Devices: New Hope for Patients with End stage
Heart Faliure. North Carolina medical journal, 2006. 67(2): p. 110115.
C. C. Tsai, B.S.C.a.C.M.T. Design of Wireless Transcutaneous Energy Transmission System for Totally Artificial Hearts. in IEEE
APPCAS. 2000. Tianjin, China.
Guoxing Wang, W.L., Rizwan Bashirullah, Mohanasankar Sivaprakasam, Gurhan A. Kendir, Ying Ji, Mark S. Humayun and James
D.Weiland. A closed loop transcutaneous power transfer system for
implantable devices with enhanced stability. in IEEE circuits and systems. 2004.
Ping Si, P.A.H., J. W. Hsu, M. Chiang, Y. Wang, Simon Malpas,
David Budgett Wireless power supply for implantable biomedical device based on primary input voltage reglation. 2nd IEEE conference
on Industrial Electronics and Applications, 2007.
Ping Si, A.P.H., Designing the DC inductance for ICPT Power pickups. 2005.
Ping Si, A.P.H., Simon Malpas, David Budgett, A frequency control
method for regulating wireless power to implantable devices. IEEE
ICIEA conference, Harbin, China, 2007.
Author:
Institute:
Street:
City:
Country:
Email:
IFMBE Proceedings Vol. 23
Thushari Dissanayake
Auckland Bioengineering Institute
70, Symonds Street
Auckland
New Zealand
t.dissanayake@auckland.ac.nz
___________________________________________
Transcutaneous Energy Transfer System for Powering Implantable Biomedical Devices
Author:
Institute:
Street:
City:
Country:
Email:
David Bugett
Auckland Bioengineering Institute
70, Symonds Street
Auckland
New Zealand
d.budgett@auckland.ac.nz
_________________________________________
Author:
Institute:
Street:
City:
Country:
Email:
IFMBE Proceedings Vol. 23
239
Patrick Hu
University of Auckland
38, Princess Street
Auckland
New Zealand
a.hu@auckland.ac.nz
___________________________________________