Master's thesis presentation at Université Paris-Sud XI. Author: Quang-Trung Luu Advisors: Antoine Diet, Yann Le Bihan (Université Paris-Sud, France), and Stavros Koulouridis (University of Patras, Greece) Research carried out at Laboratoire de Génie Electrique Génie et Electronique de Paris (GeePs) - UMR 8507 CNRS, CentraleSupélec, Université Paris-Sud (Paris XI), Université Pierre et Marie Curie (Paris VI).

1. Wireless Power Transmission
for Implantable Medical Devices
Bures-sur-Yvette, September 19, 2016
LUU Quang-Trung | Master 2 CAT
Advisors: Stavros Koulouridis, Antoine Diet, and Yann Le Bihan
Laboratoire Génie électrique et électronique de Paris
(GeePs, UMR 8507 CNRS, UPMC, Univ. Paris-Sud)
Soutenance de Master 2

2. Contents| 1. Introduction
2. Theoretical basis
3. Simulation
4. Measurement
5. Conclusion

3. Contents| 1. Introduction
2. Theoretical basis
3. Simulation
4. Measurement
5. Conclusion

4. Context and goal of the study
4 [1] S. Koulouridis et al., “Investigation of Efficient Wireless Charging for Deep Implanted Medical Devices,” APS’16, pp. 1045–1046,2016.
General view on the system of IMDs and
external devices [1].
• Design of inductive coils which can establish
wireless power transfer link with external devices.
• Alternative approach:
o Using antennas to transmit signal (higher
distance)
o Using inductive coils to transfer energy
(close proximity)
 Comparative study of WPT system when
using only two coils and when combining coil
with antenna
• Evaluation
o Safety consideration
o Power budget
Implantable medical device
(IMD)
Monitoring
system
Data transfer
Power supply
Power transfer
Introduction

5. Context of the study
Transmitter: Dipole antenna;
Receiver: Planar-inverted F-antenna (PIFA).
Original work: Koulouridis et al. [1]
• An implanted PIFA antenna that exhibits double resonance at 402 MHz
(for wireless data telemetry) and 915 MHz (for wireless power transfer)
• Implanted depth = 10 mm
Skin
Muscle
Bone
Implanted
antenna
100 mm
300 mm
Implanted depth
= 10 mm
Implanted PIFA antenna
Introduction
5
Gain = -35.6 dB @ 402 MHz
BW = 33 MHz
Gain = -23.4 dB @ 915 MHz
BW = 50 MHz
Wireless Data Telemetry
Radiating Wireless Power Transfer
[1] S. Koulouridis et al., “Design of a Novel Miniature Implantable Rectenna for In-Body Medical Devices Power Support,” EuCAP’16, Davos, 10-15 Apr. 2016.
Reflection coefficient and 3D far-field gain radiation
pattern of the PIFA antenna
Size: 13.8 mm x 15.8 mm
Total occupied volume ~ 280 mm3

6. Context of the studyIntroduction
6
Coil with
matching
circuit
Rx coil
substrate
Roger 3210
εr=10.2 superstrate
feed
shorting pin
Investigation of inductive charging using two coils
• Coil is inserted into antenna
• Antenna is retuned (very easily) in order to implement the coil without being affected in its operation
S. Koulouridis et al., “Investigation of Efficient Wireless Charging for Deep Implanted Medical Devices,” APS’16, pp. 1045–1046, 2016.
Insert coil into antenna,
between the substrate
and superstrate
D
Rx coil + Antenna
Tx coil

7. Contents| 1. Introduction
2. Theoretical basis
3. Simulation
4. Measurement
5. Conclusion

8. Magnetic coupling of coils
Principle:
• Magnetic coupling between two coils, based on the electromagnetic induction phenomenon
8
Theoreticalbasic
henrys (H)
Nd
L
dI

Self-inductance of a N-turn coil: dϕ, change of magnetic flux [Webers]
dI: change of current [A]
Magnetic field lines through two coils in close proximity
Mutual inductance
Whenever a current flows through a wire loop, the loop will generate a magnetic field called magnetic flux.
• For two coils with N1 and N2 turns:
1 12 2 21
12 21
2 1
N N
M M M
I I
 
   
1 2M L L• Upper limit of M: (1)
Coupling coefficient
• In order to define the degree to which the mutual inductance
reaches maximum, we refer to the term coupling coefficient
1 2
M
k
L L
 From (1)  0 1k 
Rx coil
Tx coil

9. Magnetic coupling of coils
Mutual inductance
Approximated formulas [1]
𝑀 =
4
3
𝜇0 𝑅1 𝑅2
Τ3 2
𝑁1
ℎ1
𝑁2
ℎ2
ሻ𝑋(𝑘11ሻ − 𝑋(𝑘22ሻ − 𝑋(𝑘33ሻ + 𝑋(𝑘44
𝑋(𝑘ሻ =
1
𝑘
1 − 𝑘2
𝑘2
ሻ𝐾(𝑘ሻ − 𝐸(𝑘 +
3𝜌 − 4
2
𝐸(𝑘ሻ −
3
2
𝜌(1 − 𝑘2
ሻ𝛱
𝜌𝑘2
− 2
𝜌 − 2
, 𝑘
𝐿 =
8
3
𝜇0 𝑅3
𝑁2
ℎ2
1
𝑘
1 − 𝑘2
𝑘2
ሻ𝐾(𝑘ሻ − 𝐸(𝑘ሻ + 𝐸(𝑘 − 1
[1] M. Piri, V. Jaros, and M. Frivaldsky, “Verification of a mutual inductance calculation between two helical coils,” EPE 2015, 2015, pp. 0–5.
9
Theoreticalbasic
Self-inductance of coil:
Tx coil
Rx coil
K(κ), E(κ), Π(ρ, k): the complete elliptic integrals of the first and second kind, respectively.
Details can be found in [1]
where

10. Calculation program
[1] M. Piri et al., “Verification of a mutual inductance calculation between two helical coils,”
EPE 2015, 2015, pp. 0–5.
[2] D. Ahn et al., “Optimal Design of Wireless Power Transmission Links for Millimeter-Sized
Biomedical Implants,” IEEE Trans. Biomed. Circuits Syst., pp. 1–13, 2014.
Coupling coefficient vs. distance
Self-inductance of coils
Ahn et al. (Simulation) 58.8 nH 36.1 nH
Ahn et al. (Measurement) 60.9 nH 36.8 nH
Simulink 60.08 nH 33.29 nH
Results of coupling coefficient
Ahn et al. (Measured in air) 0.00216
Simulink 0.002255
Input variables Calculate inductance
Calculate mutual inductance &
coupling coefficient
10
Theoreticalbasic

11. Two-port network calculation
Power Transfer Efficiency (PTE) [1]
Coupling coefficient Q-factor of Tx coil Loaded Q-factor of Rx coil Rx internal efficiency
Equivalent circuit using Z-matrix as the wireless
link between Tx and Rx coil [Ahn et al., 2015]
Rx PartTx PartCoupled link
Receiver power reception susceptibility
How strongly the implant can receive power
under a given magnetic filed exposure
Transmitter figure-of-merit
How strongly the transmitter
coupled with the receiver11
Theoreticalbasic

12. Specific Absorption Rate (SAR)Theoreticalbasic
12
• Evaluate how strongly the power per unit mass is absorbed by the biological tissue mass when
exposed to the electromagnetic field [1]
[1] T. Wittig, “SAR Overview,” CST User Gr. Meet., pp. 1–25, 2007.
E: electric field density [V/m]
σ: electrical conductivity [Siemens/m]
ρ: mass density of tissue [kg/m3]
• Safety limits for maximum SAR: [1]
• In United States: 1.6 W/kg averaged over 1 g of tissue (regulated by U.S. Federal
Communications Commission)
• In Europe: 2 W/kg averaged over 10 grams of tissue (regulated by the European Committee for
Electrotechnical Standardization)
𝑆𝐴𝑅 =
𝜎|𝛦|2
𝜌
[𝑊/𝑘𝑔]

13. Contents| 1. Introduction
2. Theoretical basis
3. Simulation
4. Measurement
5. Conclusion

14. Simulation Setup | WPT using two coils
Configuration of the Tx and Rx coil
Parameter Tx coil Rx coil
Radius 12 mm 0.5 mm
Height 1 mm 1 mm
Number of turns 1 7
Distance between two coils 12 mm
Configuration of the tissue model
Parameter Skin Muscle Bone
Thickness (mm) 2.5 25 22.5
Mue 1 1 1
Rho (kg/m3) 1100 1041 1850
Therm. Cond. (W/K/m) 0.293 0.53 0.41
Blood flow (W/K/m3) 9100 2700 3400
Simulation
14
Configuration 1: Using only two coils
• Implanted depth: 10 mm beneath the skin-air interface

15. Simulation Setup | With antenna implementationSimulation
15
Dielectric housing
PIFA antenna
Rx coil
Tx coil
Rx coil
Tx coil
Tissue
PIFA antenna
feedingPIFA antenna
Tx coil
12 mm
Human arm
Rx coil
Configuration 2: Integrating the Rx coil into the PIFA antenna
• Rx coil was inserted into the antenna, inside the L-shape (between substrate
and superstrate)
• Implanted depth: 10 mm beneath the skin-air interface

16. Results | Coupling coefficient & self-inductance
Calculation vs. Simulation results (in air)
Simulation
16
1. Coupling coefficient
• Deviations stable at range of 13-14 %
 The Simulink model can be used to roughly
estimate k and can be seen as a validation of the
use of the CST simulation model
2. Self-inductances:
Coupling coefficient vs. distance
CST Air CST Tissue MATLAB Air
L1 50.945 nH 50.026 nH 60.08 nH
L2 32.686 nH 32.618 nH 33.29 nH
k
Tx coil Rx coil
Nb. of turns Radius Height
Tx coil 7 12 mm 1 mm
Rx coil 1 0.5 mm 1 mm

17. Results | Coil’s parametersSimulation
17
Q-factor
Rx internal
efficiency
Rx-PRS
κ = 0.0026
Tx-FoM
Rx coil
Tx coil
Q-factor
Increase of losses in tissue
with frequency
 Divergence between air
and tissue simulation

18. Results | Power Transfer Efficiency (PTE)Simulation
18
• Optimal frequency in tissue simulation is lower than in air simulation
 Probably due to the absorption of tissue when increasing frequency
Tx coil
Rx coil
Tx coil
Rx coil
& antenna

19. Results | PTE – Displacement of two coils
Fix Rx and move Tx
Fix Tx and move Rx
Rx coil
Tx coil
d
Tissue
move Tx
Rx coil
Tx coil
d
Tissue
move Rx
Simulation
19
Two cases: Moving Tx and moving Rx, when fixing the remaining coil.
 For examined values of distance (12-28 mm), the implanted depth of Rx coil
does not affect much the efficiency
Tx coil
Rx coil
Tx coil
Rx coil
& antenna

20. 1. Lateral misalignment
Results | Misalignment analysis (1/2)
z
y
x
d
Rx coil
Tx coil
z
y
x
Δ
Simulation
20
• Two cases: Moving Tx along and perpendicularly to the arm
• Lateral misalignment Δ Є [0 mm, 20 mm]
• Affect PTE much: drops ~ 70 % with 10 mm of misalignment
• Divergence of A and B due to the geometrical shape of the L-
shape where the Rx coil occupies
Tx coil
Rx coil
Tx coil
Rx coil
& antenna

21. 2. Angular misalignment
Results | Misalignment analysis (2/2)
z
y
x
d
Rx coil
Tx coil
z
y
x
θ
Simulation
21
Optimal rotating angle: 20o - 50o
• Two cases: Rotating Tx along and perpendicularly to the arm
• Angular tilt θ Є [0o, 90o]
• PTE is stable at range 0o - 50o
• Divergence of A and B due to the geometrical shape of the L-shape
where the Rx coil occupies
Tx coil
Rx coil
Tx coil
Rx coil
& antenna

22. Effect of Tx coil’s radius on PTE
• Evaluate PTE with change of Tx’s radius from 6-14 mm
Results| Transmitter optimizationSimulation
22
Tx coil
Rx coil
Tx coil
Rx coil
& antenna
• Optimal Tx’s radius: 7-10 mm (Tissue); 10-12 mm (Air)

23. PTE results of two different radius of the Rx coil
(0.5 mm and 1 mm).
Results| Receiver optimization
Effect of Rx coil’s radius on PTE
• Rx radius is doubled
• The slot of PIFA is altered in order to contain the coil
(1.3 to 2.3 mm)
Simulation
23
• Increase Rx’s radius  PTE increases, resonant
frequency increases
 Design consideration

24. Peak 1g-SAR (Pin = 1W or 30dBm)
• Implanted system @ 10mm
Results| Patient safety consideration
Inductive Power Transmission (coils)
@ 105MHz
𝑆𝐴𝑅 =
𝜎|𝛦|2
𝜌
[𝑊/𝑘𝑔]
Simulation
24
D
Rx
Tx
SAR exceeds the FCC restriction !

25. Contents| 1. Introduction
2. Theoretical basic
3. Simulation
4. Measurement
5. Conclusion

26. Fabrication and measurement of coil and phantomMeasurement
26
5-turn coil
coaxial cable with SMA
connector
Coil
with
protection
Phantom’s
properties
Note: 3 measures (numbered 1 to 3) were done in an
experimental glass recipient and the last one
(numbered 4) in a plastic bottle
0.00
50.00
100.00
150.00
0.01
0.06
0.11
0.16
0.21
0.26
0.31
0.36
0.41
0.46
0.51
0.56
0.61
0.66
0.71
0.76
0.81
0.86
0.91
0.96
ε'
Frequency (GHz)
permittivity vs freq
Eps_1 Eps_2 Eps_3 Eps_4
0.00
0.20
0.40
0.60
0.80
0.01
0.06
0.11
0.16
0.21
0.26
0.31
0.36
0.41
0.46
0.51
0.56
0.61
0.66
0.71
0.76
0.81
0.86
0.91
0.96
conductivity
Frequency (GHz)
conductivity vs freq
Cond_1 Cond_2 Cond_3 Cond_4
Initial step: Check the validity of simulation by larger coil: Tx and
Rx with the same geometry (1 mm radius, 5 turns, 1 mm height)
• Phantom: mix of 28.5 % triton-X, sodium chloride (3.5 g/l),
water, and representing the human muscle.
• Measure S11 and Z11 by VNA
Phantom made by 28 % triton X – water - saltVNA 4 ports
Rx coil Tx coil
5 cm

27. Results of S11 and Z11Measurement
27
• Some agreements:
o The match of resonant frequency point
between the real and imaginary part of
Z11
o The resonance inside the tissue is below
the resonance in air environment, for both
measurement and simulations.
• Results from simulation do not match the
ones from measurement, due to:
o Magnetic disturbance from the SMA,
coaxial cable, welds, glue and protection,
etc.
o Unexpected parasitic elements
S11 (air) S11 (tissue)
Re[Z11] Im[Z11]

28. Contents| 1. Introduction
2. Theoretical basic
3. Simulation
4. Measurement
5. Conclusion

29. Conclusion & Future workConclusion
29
Conclusion
• Design of miniature coils for WPT system for implantable medical devices
• Compare to WPT using antennas:
o Coil efficiency is strongly affected by the implementation of the antenna  It will also affect the ability to
communicate to external devices
o Coils are sensitive (as expected) by the misalignment (lateral, angular), and displacement
o Transmitting coil generates higher SAR
Future work:
o Improve measurement results:  Solution: using “De-embedding” technique: remove the effect of
unwanted portions of the structure that are embedded in the measured data by subtracting their
contribution [1]
o Various coil sizes will be fabricated and tested with respect to the value of coupling coefficient obtained
from theoretical calculation
o Measure with two coils (S21, Z21) in order to determine PTE
[1] Agilent Technologies, De-embedding Techniques in Advanced Design System