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?