ABSTRACT

Introduction: A significant need exists for miniaturized systems capable of facilitating wireless electrical stimulation and electrophysiological recording in ambulatory settings. Implantable stimulation / data logging systems may be particularly suited for modulating and monitoring peripheral nerve activity following injury. Chronic, serial monitoring of peripheral nerve function provides an accurate assessment of functional recovery as well as the efficacy of various surgical interventions. Utilization of wireless nerve activation and monitoring further enables a less invasive means of assessing functional recovery and lower the risk of infection associated with trancutaneous approaches. Inductive coupling was utilized to design a thin film wireless device capable of simultaneously delivering electrical stimuli to proximal tissue and wirelessly transmit recorded electrophsyiologic signals. Implantable wirelesss systems were examined in ambulatory Lewis rats in the setting of sciatic nerve repair.

Materials and Methods: Wireless power transmission was achieved using thin-film

implantable wireless receivers produced by Red Rock Laboratories, LLC (St. Louis, MO). Receivers were tuned to designed carrier frequencies generated by a modified class E oscillator. The transmitter coil and carrier frequency was established using a custom-design high-frequency timer circuit. Receiver coils were integrated into custom PCB fitted with XBee wireless chips enabling wireless data transfer. XBee receivers were attached to an Ardunio microcontroller to facilitate data recording. Low pass filter and voltage regulators were integrated into onboard circuitry. Wireless implants were incased in silicone and implanted in male Lewis rats within and without induced peripheral nerve injury (crush, transaction repair). Platinum-iridium lead wires were attached to the rats’ sciatic nerves to facilitate electrical stimulation. Sciatic nerve was stimulated at various frequencies (0- 80 Hz) and evoked EMG and muscle force measurement was recorded. Wireless measurement of various integrated and electrophysiological signal was performed in awake, ambulatory animals.

Results and Discussion: Implantable devices wirelessly delivered up to 27 V in vivo. Wireless data logging utilizing the XBee chip enabled chronic monitoring of power output and electrophysiological data. Electrical activation of rat sciatic nerve resulted in variable functional responses recorded and transmitted via XBee chip to external receivers. Wireless stimulation of healthy, injured sciatic nerve demonstrated graded functional outcomes post-operatively. Wireless stimulation / data logging systems were successfully utilized in ambulatory animals demonstrating the longevity and fidelity of implantable device.

Conclusions: Given the simplicity and efficacy, the present implantable wireless system demonstrates significant potential in a wide range of applications relating to wireless power and data transfer in implantable medical devices.

MATERIALS / METHODS Fabrication of Thin-Film Wireless Receivers:

• Flexible thin-film receivers were fabricated by Red Rock Laboratories (St. Louis, MO) • Receivers were fabricated from polyimide, copper, and gold using a modfiied process of

sacraficial photolithography enabling the production of 5-layer, dual sided devices • Receivers were fabricated in a number of sizes ranging from 10mm – 16 mm in diameter

Functional Nerve Recovery Modeling using Thin-Film Wireless Receivers for Electrical Stimulation

David Rubenstein1, Matthew R. MacEwan1,2, Daniel Moran2 1School of Medicine, Washington University, St. Louis, MO, USA, 63110, 2Department of Biomedical Engineering, Washington University, St. Louis, MO, USA, 63130

RESULTS: Nerve Stimulation

Thin-Film Receivers Facilitate Functional Stimulation of Rat Sciatic Nerve:

MOTIVATION

Development of Advanced Neuroprotheses Capable of Restoring Motor Function:

CONCLUSIONS 1.  Thin-film wireless receivers offer a flexible, low-profile, light weight,

implantable power supply for on-board bioelectric devices

2.  Implanted thin-film receivers enable chronic wireless electrical stimulation of various tissues in ambulatory animal models

3.  XBee system offers a way to transmit data wirelessly from implantable medical device

4.  A model for functional nerve recovery from cut or crush injuries can be made with a minimal number of surgeries

1.) Does flexible substrate enable suitable mechanical properties for in vivo use?

2.) Do flexible receivers offer adequate tuning and ample power transmission? 3.) Are flexible wireless receivers suitable for chronic in vivo applications?

Fabricated Thin-Film Wireless Receivers

OBJECTIVE: Design and characterize a flexible, implantable, thin-film wireless receiver suitable for chronic use in vivo

ACKNOWLEDGMENTS

• Presented studies were generously supported through startup funds provided by Washington University in St. Louis

• Thanks to Paul Gamble, and Kai Triplett at Washington University for their collaborative work on the backpack device.

Implantable Microelectrodes Support Selective Stimulation of Motor Axons:

RESULTS: Backpack Model

Thin-Film Receivers’ Voltage and Stimulation Potential

GOAL: Utilize understanding of wireless power and data communication to facilitate the design of more advanced, wireless systems capable of restoring motor function through applied functional electrical stimulation of peripheral nerve tissue

Specifications of Thin-Film Receivers:

• Diameter: 10 – 16 mm • Weight: 0.02 g •  Insulation: Epoxy, Silicone Elastomer • Design: Modular • Leads: Variable (eg. Pt/Ir) • Output: Bipolar

• Modular design facilitates use of wireless receivers in multiple animal models and experimental settings

• Modular design supports interconnection of multiple receivers and production of higher output voltages

INTRODUCTION

Existing Systems are Limited by Bulky Power Supplies:

• Implantable battery-operated power supplies have limited functionality, limited life-span, and bulky design ill suited for in vivo use

• Wireless receiver fabricated from silicon / copper previously demonstrated to facilitate transcutaneous power / data communication

• Rigid design sub-optimal for in vivo use

• Power output measured in ambulatory animal via wireless data communication from implantable device. • Obtained by rat walking under transmitter coil. • Model demonstrates capability for constant stimulation if transmitter coil is near

Receiver Coil

Output Lead Terminals

Receiver Layout Tuning

Components

• Numerous devices have been designed to facilitate electrical stimulation of peripheral nervous tissue

• Regenerative macro-sieve electrodes offer a unique approach to interfacing peripheral nerve tissue

• Macro-sieve electrodes support highly selective activation of motor axons and distal musculature

Macro-Sieve Electrodes Selective Muscle Activation

Battery-powered Constant Current Stimulator

Implantable Power Supply

Silicon-based Wireless Receiver

• 16mm flexible wireless receiver implanted subcutaneously in rat flank with dual Pt/Ir leads integrated into peripheral nerve cuff implanted around sciatic nerve

•  Implanted wireless receiver facilitated full-scale recruitment of sciatic nerve and distal musculature at amplitudes <1V

•  Implanted wireless receiver facilitated functional recruitment of peripheral nerve tissue and distal musculature

for >3 mo post-operatively

• Low profile of implanted device resulting in little irritation of distrurbance of ambulatory animal

Nerve Cuff Surrounding Sciatic Nerve

Evoked Tetanic Response in Rat Gastrocnemius

HP Function Generator

Tectronix Oscilloscope

Wireless Transmitter

Wireless Receiver

Coil Diameter (d)

Distance (h)

Number of Turns (n)

R = 5 kΩ Receiver Diameter (d)

Carrier Freq. (f)

Output (V) Tuning Capacitance (C)

MATERIALS / METHODS (continued)

System for Activation of Thin-Film Wireless Receivers:

• A 555-timer circuit was also used instead of a function generator for better portability. The custom design allowed for the manipulation of frequency and duty cycle within the desired limits. • The design allowed for about 1-100 Hz so that tetanus could be stimulated in receivers attached to rats’ sciatic nerves.

Wireless Data Communication:

• XBee wireless chips were used for wireless data transfer of voltage information of rat backpack device. A receiver XBee was connected to an arduino microcontroller.

• Schematic of backpack device included 3.6V rechargeable battery, voltage cap, and low pass filter.

• 5kΩ potentiometer • LED

• 20kΩ potentiometer • 470µF capacitor