tets for powering implantable biomedical devices t. dissanayake, d. budgett, a.p. hu, s. malpas and...
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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
• External– Preferred– Used in this design
• Internal• Heat dissipation
issue• Increased size • Increased weight
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°CTmax = 38.4°CTest Time:24 hours
Experimental ResultsP = 10W; V = 23.5V; I = 0.425A
Freq. = 163 – 173 KHz
Thermistorsmeasure ΔT
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-REELCost: $5nRF24E1-EVKITCost: $419
Transceiver EVKIT Features
There are mainly two types of TET transformers:1.Iron –core Upside: Little flux leakage
No radial or axial misalignmentNo magnetic flux cross-coupling by the nearby conductor
Downside: Difficulty in implantation
2.Air – coreUpside: Easy to implantationDownside: Some flux leakage
Radial or axial misalignment leads to decreased efficiency even no power output
Magnetic flux cross-coupling by the nearby conductor
N N
NN
S S
S S
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
skin
DC/AC Converter
Primary CoilsSecondary Coils
RectifierBattery
Charging Circuit
Battery
Outside Body Inside Body
AMB AmplifierMotor ControllerHESA
Heart Pump
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?
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