A Fluid Actuator for Thin-Film Electrodes

[+] Author and Article Information
Benjamin Arcand

 Boston Scientific, 1 Scimed Place, Maple Grove, MN 55331 and Mechanical Engineering—Engineering Mechanics Department, Michigan Center for Wireless Integrated Microsystems,  Michigan Technological University, Houghton, MI 49931

Sudeep Shyamsunder

Mechanical Engineering—Engineering Mechanics Department, Michigan Center for Wireless Integrated Microsystems,  Michigan Technological University, Houghton, MI 49931

Craig Friedrich

Mechanical Engineering—Engineering Mechanics Department, Michigan Center for Wireless Integrated Microsystems,  Michigan Technological University, Houghton, MI 49931craig@mtu.edu

J. Med. Devices 1(1), 70-78 (Aug 28, 2006) (9 pages) doi:10.1115/1.2358357 History: Received December 06, 2005; Revised August 28, 2006

Background. Cochlear implants have become an accepted and successful treatment for profound sensorineural deafness in both children and adults. Essential to the effective and efficient localized stimulation of the auditory nerves, is the position of the implant electrode array stimulating sites within the scala tympani (ST). However, the small size, delicate internal structures and helical shape of the cochlear chambers complicate the matter of precise positioning of the implant electrode array. The design, fabrication process, and in vitro testing of a fluid actuator to steer a thin-film electrode array is presented. The application chosen to show feasibility is for a cochlear implant, however, the actuator might be used for cortical electrode arrays, for example. Method of Approach. The actuator functions by a change of internal fluid pressure within one or more flattened and curled polymer microtubes, expanding the highly elliptical cross section of each tube thereby stiffening it and causing a change in its curling radius. The straightening from an initially helical shape allows insertion of an attached electrode array into the basal end of the in vitro cochlea and as the insertion proceeds the pressure is decreased allowing the straightened electrode array to controllably return to its initial helical shape. The allowable envelope of the scala tympani of a guinea pig was determined from published data and the actuator was designed and fabricated accordingly. Results. Multiple actuators were fabricated and tested in vitro. The insertion tests helped to confirm the viability of using this type of actuation to facilitate the insertion of an electrode array into the ST. These tests were performed in a minimal amount of time and often on the first attempt. The actuator reached the furthest extent of the in vitro cavity and achieved a position adjacent to the modiolus. Conclusions. In vitro insertion tests show that the actuator can deliver a thin-film electrode array to a depth of more than one turn into the in vitro scala tympani within a few minutes under open-loop, manual control.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Schematic cross section of the guinea pig cochlea and scala tympani chamber. The basal diameter of the ST centerline is approximately 2.7mm and the height is approximately 3.2mm.

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Figure 2

Design concept of multitube fluid actuator assembled with thin-film electrode array in the final modiolus-hugging configuration. Internal pressure causes the tube cross section to expand resulting in straightening of the actuator.

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Figure 3

Typical guinea pig unprocessed cross section data from Voie (see Ref. 16). Similar cross-sectional data in the other two orthogonal planes was used to determine equations for the ST geometry.

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Figure 4

Surgical picture of the guinea pig round window ledge and cochleostomy (normal implantation site). This is the basal end of the ST. Major tick marks are spaced 1mm apart.

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Figure 5

(Left) Definition of the curling radius (R), z-height, length (L), major section radius, minor section radius, and tilt (ϕ) of the ST envelope. (Right) Solid model of ST from data (see Ref. 16) used to fabricate 2D and 3D in vitro cavities.

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Figure 6

Summary of fabrication and assembly of three-tube actuator. (Top view) tube is flattened, sealed, and drawn if necessary, (side view) tubes of different lengths are added one by one to the electrode array, (right) assembly is wrapped around mandrel and heated to set the helical shape (see Ref. 22).

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Figure 7

A completed three-tube actuator without an electrode array attached. The performance of the actuator alone was determined from prototypes such as this (coin diameter is approximately 18mm).

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Figure 8

Three-tube actuator and attached electrode array with increasing internal pressure. The left picture is with no applied pressure, center picture shows pressure applied to the full-length tube (276kPa) resulting in the largest change of curvature toward the apical end. Nearer the basal end, the full-length tube is partially constrained by the nonpressurized shorter tubes. The right picture shows the addition of pressure (207kPa) to the two shorter tubes resulting in straightening of the assembly.

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Figure 9

Single tube actuator with attached silicon electrode array. The diameter is larger than that required for modiolus hugging and is due to a reverse curvature of the electrode array caused by residual stress from its fabrication.

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Figure 10

Single tube actuator and attached silicon electrode array with modiolus hugging diameter. A change in the electrode array fabrication process resulted in near zero residual stress and no initial curvature prior to attachment to the actuator.

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Figure 11

Modiolus hugging in vitro insertion into machined acrylic 2D guinea pig ST cavity. Insertion required 4min to complete. The actuator has been highlighted to increase visibility. The electrode array had zero initial curvature prior to attachment to actuator.

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Figure 12

In contrast to Fig. 1, in vitro insertion into machined acrylic 2D guinea pig ST cavity. The electrode array had reverse curvature prior to attachment to the actuator.




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