RF Communication for Implantable Devices

Perry Li St. Jude Medical IESD

Sylmar, CA

Overview

2

• Applications – CRM, Neuromodulation

• History of Implantable Devices and Telemetry

• Benefits and Uses of RF Telemetry

• Standards Requirements

• RF System Design for CRM Devices

• Challenges Unique To Implantable Devices

• Future Opportunities

Electrical Signals in the Heart

3

Cardiac Tissue:

- Automaticity

- Excitability

- Conductivity

Main Cardiac Conditions

4

• Bradycardia – slow heart rate – Pacemaker

• Tachycardia – ventricular fibrillation –

Implantable Cardioverter Defibrillator (ICD)

• Heart Failure – ventricular dissynchrony –

Cardiac Resynchronization Therapy (CRT)

Tachycardia: Reentry

5

Pacing Pulse and Defib Pulse

6

Time

Defibrillation Voltage

0

~850V

Vf_pos

Vi_neg

Vf_neg

tpos tneg

Pacing Voltage

Time

Vrtn

Vrtn 0

0.5V- 8V

Charge Balancing

Bi-Phasic Shock

Pacing Defibrillation

tpw

Device, Leads, Programmer, Patient

7

Neuromodulation Therapy

8

• Spinal Cord Stimulation (SCS) – Chronic pain

(eg: back pain)

• Deep Brain Stimulation (DBS) – Parkinson’s,

Tremors, OCD, possibly mood disorders

• Peripheral Nerve Stimulation

(PNS) – Chronic migraine

SCS Therapy

9

• Implant in lower back with leads

along epidural layer of spine

• Electrodes along leads send

electrical pulses to the spine

• These pulses interfere with nerve

impulses that cause pain

DBS Therapy

10

• Electrodes target specific areas

of the brain to stimulate

structures responsible for

chemical or motor deficiencies

• Directly modulates brain activity

in a controlled manner

• Unlike other surgical techniques,

the effects are reversible

PNS Therapy

11

• Used to treat chronic

migraine patients who have

failed to respond to

pharmaceutical treatments

• Mild electrical pulses to

stimulate specific nerves, eg:

the occipital nerves which

are located behind the head

just above the neck area

Device, Leads, Programmer, Patient

12

History of Pacers/ICD/IPG

13

Pacemakers:

First implant 1958

Transvenous & demand pacing 1960’s

Dual chamber 1970’s

ICD:

First implant 1980

FDA Approval 1985

Abdominal implant pectoral implants

IPG:

First implant 1982

Physical Design

14

Pacemaker ICD

Header

Titanium Case

Ports for leads

RF Antenna

Set Screws

Feedthru

Huge Dynamic Ranges

15

-Battery currents: ~A (background)

~Amps (charging)

-Voltages: < mV (ECG/sensing)

> 850V (shocking)

- Device currents: Many 10’s of Amps

(shock delivery)

Electrical Block Diagram

16

Sensing

HV

Protection

EMI

Filters

Sense

Multiplexing

Pace

Drivers

Pace

Multiplexing

ADC’s

MCU and

Programmable

Digital Functions

Clock

Generators

Memory

Telemetry

Rate Sensor

Patient Notifier

Battery-

and

Power-

Management

Battery

Header and

Case

Connections

Charge

Pumps

Magnet

Detect

Charger

Shock

Driver

Caps

Unique to ICD

Evolution of Telemetry

17

• Magnet / Trim-pots

– Device settings

– Battery voltage monitoring

• Inductive

– Short range magnetic field coupling

– 10’s of kbps

• RF

– Range of several meters

– 100’s of kbps

Benefits of RF Telemetry

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- At implant

- Remote monitoring

- At followups

- Therapy control

- Device control/configuration

- Real-time ECG streaming

- Firmware code downloads

Non-RF RF

Home Monitoring

19

- Bed-side Monitor

- Scheduled Device

Interrogations

- Patient Initiated

Device Interrogation

- Landline or Cell Connection

- Interface to Online Patient

Care Network

Non-RF RF

Administering Therapy

20

- Handheld programmer

- Turn on and off therapy

- Adjust level of therapy

- Wireless battery charging

MICS Standard

21

• Medical Implant Communication Service (MICS)

• FCC (1999) and ETSI (2004) – adopted world-wide

• Licensed by Rule – no individual application

• 402-405 MHz (shared with weather balloons)

• Tissue penetration with low power

• 300kHz channels, < 25uW EIRP

• Communication Initiated Only By Base Unit

– Listen Before Talk / Clear-channel Assessment (LBT/CCA)

– Emergency Device Transmission

• Standard does not require interoperability

• Modulation: FSK, 2FSK, 4FSK

ISM Band

22

• Industrial, Scientific, and Medical (ISM) radio bands

• 1985 – Unlicensed for commercial use

• 12 frequency bands from 6.7MHz to 246GHz

• Popular bands:

– 902-928 MHz

– 2.4-2.5GHz

• Interference from commercial

devices:

– 900MHz - GSM, Zigbee

– 2.4 - Bluetooth, Wifi, microwave

oven, etc

Clear Channel Assessment

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• Monitor Channel for > 10ms

• Power Below Determined Threshold

– Otherwise: Channel with lowest ambient power

– Second-quietest channel chosen as alternate

– Switch to pre-selected alternate in case of interference

– After 10ms monitoring of alternate channel

• Channel Can Remain In Use Unless Silent Period > 5sec

• Allowance For Switch To Pre-Selected Alternate Channel

• Emergency Transmission By Implant

FDA Classification

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• Class I – General Controls

– Not intended for supporting/sustaining life

– Bandages, gloves, hand-held instruments

• Class II – General/Special Controls

– Pre-market notification

– Powered wheelchairs, infusion pumps

• Class III – Pre-Market Approval

– Support or sustain life

– Pacemakers, ICD’s, replacement heart valves

Equipment Authorization

25

• FCC: – Testing – showing compliance

with FCC standards

– Radiation Exposure – SAR

– Device Labeling

• FDA:

– Pre-Market Approval (PMA)

– Investigational Device Exemption (IDE)

– EMC, Robustness, Coexistence, QOS, Data Integrity

and Latency, Security, Risk Management

Wakeup

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• Duty-cycle communications

• 2.45 GHz ISM band

– 100mW EIRP in US,

10mW in some countries

– OOK modulation

– Duty-cycled “sniffing”

• MICS band

– “sniffing” using RSSI

• Inductive Wand

RF IC

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• Microsemi ZL70102 chip – 400MHz Tx and Rx, 2.45GHz Rx only for wakeup

– Max Tx Power = -1dBm, Rx sensitivity = -85dBm

– Transmit Current = 5mA, Sleep current = 10nA

RF System Block Diagram

28

• Typical RF Block Diagram

– Match 1&2 on-chip capacitors for dynamic tuning

– SAW filter to allow only MICS band

– 2.4GHz notch filter to reject wakeup signal

Challenges Unique to Implantable

Medical Devices

29

• Design for patient safety

• FDA, TÜV, and other international medical regulatory bodies

• FCC, ETSI, and other international wireless communications

regulatory bodies

• Medical reimbursement

• Long design/approval cycles

• Relatively low volume

• Long-lived designs and implants

• Deployed base of RF infrastructure

• Backward compatibility

RF Technical Challenges

30

• Performance Requirements

• Size and Form-Factor

• Antenna Limitations

• Variable Implant Environment

• Power Limitations

• Interference

• Dual band functionality: 400MHz and 2.4GHz – Higher tissue losses at 2.4GHz

– Free space path loss 8.7dB higher at 2.4GHz than 400MHz

• Minimum distance requirement

• Antenna radiation pattern – Main lobes should be focused

through front and back of

patient

– Good flip performance

Performance Requirements

31

Size and Form-Factor

32

• Size

– Increasingly smaller header and can

size

• Pacemaker: 4 x 0.5 x 1.5 cm

• ICD: 3 x 1.3 x 1 cm

– Non-optimal header shape

– Seek high levels of integration

– More functions, channels, leads, etc.

– Hermetic feedthru connection

into can

• Hermetically Sealed Device

– Final calibrations without electrical

contacts

Antenna Limitations

33

• Antenna Type

• Antenna Location – Internal - Header

– External

• Metal in header – Leads

– Connectors

– Anchors

• At 400MHz, wavelength = 75cm in air, 9cm in body

• Entire header coated in dielectric material

• Bio-degradable material

• Manufacturability

Variable Implant Environment

34

• Implanted at variable

depths in human body

• Large antenna match and

body losses (40-45dB)

• Lead wrapped around

implanted device

• Impact of different tissue

layers. Dielectric

boundaries between layers.

Variable Implant Environment

35

• Patient physiology

– Body type

– Age

– Gender

• Implant location

Power Limitations

36

• Limited power level permitted outside the body in MICS

band

– 25uW or -16dBm per ITU-R SA.1346

– Downlink limitation

• FCC regulated SAR limit

– Partial body SAR Max 1g:

1.6 W/kg

– Full body SAR: 0.08 W/kg

– Uplink limitation

• Limited Tx output power requires better Rx sensitivity

System Power

37

• Battery power is finite. Tx/Rx typically on short

periods of time. Wakeup sniffing every few

seconds.

• Average power – battery longevity

• Typical lifetime of CRM device > 7 years

• Background/listen current few 100’s of nA

• Peak power – battery ESR and chemistry

• Particularly limited in pacemakers

• Peak current limited to several mA

Interference

38

• Interference from Implantable Device functions

– Pacing & Sensing

– Defibrillation shock

– Neuro stimulation

• Instruments in Operating

Room/Clinics

• Wireless Coexistence

Future Opportunities

39

• Improved antennas – Electrical vs. Magnetic antennas

• Alternate frequencies – S. O’Driscoll, “Operating Frequencies for Wireless

Power Transmission to Implantable Medical Devices,”

IEEE International Microwave Symposium, 2011.

• Wireless power transfer

• Body Area Networks (BAN)

• Remote processing – Telemetry power vs. processing power

and capability

– Latency

Thank you

40

Questions?