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TETS for Powering Implantable
Biomedical Devices
T. Dissanayake, D. Budgett, A.P. Hu, S.
Malpas and L. Bennet
Problems
• Coupling between internal and external
components may vary according to
orientation and posture
• Insufficient power -- implanted device won’t
operate
– Charge an implanted battery
• Excess power – dissipated heat can cause
tissue damage
Power Regulation
• Internal
• Heat dissipation
issue
• Increased size
• Increased weight
• External
– Preferred
– Used in this design
Methods of Control
• Magnitude Control
– Input voltage is varied to
vary power delivered to the
load
• Most common
• Problem: mismatch of
resonant freq. of the sec.
resonant tank and
operating freq. of the
external power converter
• Result: miss-match in freq.
reduces power transferred
– Increases Vin required
– Decreases system efficiency
• Frequency Control
• Operating freq. is varied
to vary power delivered
• Tune/detune the secondary
pick-up
• Effective power
delivered is regulated
• RF link used to provide
wireless feedback from
implanted circuit to
external freq. controller
TETS System Architecture
nRF24E1 Nordic Transceivers
• Detect DC output voltage
and transmit 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
• Response time: 360 ms
TETS Design
• Internal coil and the resonant capacitor were
Parylene coated and encapsulated with
medical grade silicon
• Implanted total weight: < 100 grams
• Secondary coil held on sheep using three
loosely tied strings
• Displacement
– Axial: up to 10 mm
– Horizontal: 10 – 20 mm
– Power delivered: 5 – 25W (experiment: 10 W)
In Vivo Sheep
Testing:
ΔT = 3.8°C
Tmax = 38.4°C
Test Time:
24 hours
Thermistors
measure ΔT
Experimental Results
P = 10W; V = 23.5V;
I = 0.425A
Freq. = 163 – 173 KHz
ASAIO Journal (1994, vol. 40)
Adaptation of Tissue to a Chronic Heat Load
• Implanted constant heat flux devices into calves
next to lung and muscle tissue
– 0.04 W/cm2; 0.06 W/cm2; 0.08 W/cm2
• Initial:
– ΔT = 6.4±0.6°C; 4.5±0.2°C; 1.8±0.2°C
• After 7 weeks:
– ΔT = 3.7±1.2°C; 2.8±0.1°C; 0.8±0.1°C
• Adaptive response of the tissue to increase heat
dissipation through angiogenesis (development
of new blood vessels)
nRF24E1-REEL
Cost: $5
nRF24E1-EVKIT
Cost: $419
Transceiver EVKIT Features
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Single Chip RF transceiver + MCU +
ADC
nRF2401 2.4GHz RF transceiver
8051 compatible microcontroller
9 input 12 bit ADC 100Kspls/s
Internal voltage regulators
Ultra low current drain standby and
operation
Internal VDD monitoring
Supplied in 36 pin QFN (6x6mm)
package
Mask programmable version available
Very few external components
Ease of design
Data rate 0 to1Mbps
Multi Channel operation 125 channels
Support frequency hopping
Channel switching time <200us.
Power supply range: 1.9 to 3.6 V
Address and CRC computation
ShockBurst™ mode for ultra-low
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power operation
0 dBm output power
100% RF tested
Complements the nRF2401 transceiver
Cuts board space by 80% compared to
existing solutions
Eases design
High reliability
No need for external voltage regulator
No need for external SAW filter
Short time to market
Made for volume production - surface
mount and RF tested
Layout information available for free
Package Contains:
2 x nRF24E1
Boards- 2 x USB ConfigBoards
Software
2 Antennas
There are mainly two types of TET transformers:
1. Iron –core
Upside: Little flux leakage
No radial or axial misalignment
No magnetic flux cross-coupling by the nearby conductor
Downside: Difficulty in implantation
2. Air – core
Upside: Easy to implantation
Downside: Some flux leakage
Radial or axial misalignment leads to decreased efficiency even no power
output
Magnetic flux cross-coupling by the nearby conductor
Proposed: Air core transformer to easy implantation,
Suggestion: can we put button magnets inside wound coils circle to realize self-alignment
of primary and secondary coils both radially or axially?
The coils center is filled with polyurethane which also wraps the button magnets inside
skin
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skin
Charging
Circuit
Battery
DC/AC
Converter
Primary Coils
Secondary
Coils
Rectifier
Battery
AMB Amplifier
Motor Controller
HESA
Heart
Pump
Outside Body
Inside Body
Further
• Current power of the pump needed: 20 W?
• What kind of coil to use? How do we choose
one? Material?
• What electronics needed to make the
transceiver work? Difference b/w REEL and
KIT.
• Which components can be bought, which
have to be designed?