Discussions

Problem Statement

The request for a design of a rodent cochlear electrode array fabrication system was submitted as a senior design project by Dr. Clement.  Currently, the process of creating a rodent cochlear electrode is fabricated completely by hand.  Due to inconsistency and human error, each electrode is inherently different.  A more consistent and refined method would optimize each testing array, and therefore cochlear array research. 

Dr. Ryan S. Clement is an assistant professor of bioengineering at The Pennsylvania State University.  His research covers neural engineering and neuroprostheses.  One such neuroprosthetic included in his research is the cochlear implant.  While cochlear implants have been in development for quite some time, improvements to this particular medical device are constantly in progress.  Generally, rodents have been used as test subjects for such improvements due to their well understood auditory system.  The cochlear arrays currently used in Dr. Clements lab are painstakingly created through a long and arduous fabrication process. 

Scope of Sponsor Objectives

             The primary objective of this project is to provide an efficient and less time consuming fabrication system for rat cochlear electrode arrays.  The current method of production requires that the array to be fully fabricated by hand.  This leads to potential inaccuracies in data collecting due to the lack of reproducibility of hand-fabricated cochlear arrays.  The design of a new fabrication system will significantly improve the quality and quantity of future rat cochlear arrays and decrease the variability between arrays.

The new rat cochlear electrode fabrication system must accomplish the following objectives:

      Design Specifications, Constraints, and Limitations

To achieve a more reliable, functional and time efficient cochlear electrode array the following design specifications have to be considered.

Design

Ease of manufacturing and ease of use are two of the biggest concerns for the team when considering design approaches.  One design that simplifies both the manufacturing procedure and user friendly characteristics of the mold is to create a one- sided mold.  This mold design will still employ the use of two parts, but in this design, the top surface will only be used to compress silicone into the electrode array mold as it cures. 

Some basic dimensional changes must be made from the two sided mold design to the one sided design.  The main design change is that the main channel for the electrode array is 0.3 x 0.3 mm, while five individual 0.2 mm diameter divots are drilled into this channel 0.4 mm apart.  This increased depth will account for the fact that there is no complimentary channel on the top surface.  In this design, the channel will be cut using a dicing saw and the divots will be drilled using a 0.2 mm drill bit in a drill press.

Dime Mold With Engraved Channels -Third Design Approach.

Instead of using 6061-T6 Aluminum as in the first two design approaches, this method employs the use of Teflon for the mold.  The mold design is cut into a circular piece of Teflon that is 17 mm in diameter and 1.5 mm thick.  Using Teflon as the mold material allows for easier release of silicone from the mold.  This makes it easier to clean the mold after use.  Using Teflon also eases the machining process, because Teflon is softer than Aluminum.

Ideally this design will not have to employ the use of a negative pressure pump to distribute silicone to the electrode array.  Instead, the user will able to apply silicone directly on the top of the channel and use double sided tape as a top surface. Sliding motion of the tape relative to the mold surface will add enough pressure to distribute silicone evenly inside the channel. This tape is adhered as it is slid across the surface, where it will then stay in place until the silicone is cured. To account for excess silicone that may be squeezed between the mold and the tape without a negative pressure pump, five side channels are cut perpendicular to the main channel with the dicing saw.  These side channels acts as passages for excess silicone.

Technical Specifications.

The dimensions of the round Teflon dime mold should be as follows:

    Mold Diameter: 17 mm

Mold Height: 1.6 mm

The major channels where the electrodes are placed should have dimensions below:

  Electrode Channel Width: 0.3 mm  

 Electrode Channel Depth: 0.3 mm

The picture below shows the isometric view of the final mold prototype. This figure also displays the above dimensions at their respective positions.

Isometric View of Final Mold Prototype

The divots for the electrode ball tip need to have the specified dimensions given below to make the ball tip fit snugly into respective divots to prevent silicone to seep below the electrode ball tip.

Drilled Electrode Hole Diameter: 0.2 mm

Drilled Electrode Hole Depth (in relation to top of mold): 0.5 mm

The side channels, which are machined perpendicular to the main channels, need to have specified dimensions so they can hold the dummy wires that anchor the electrode ball tips at the right position.

          Side Channel Width: 0.15 mm

        Side Channel Depth: 0.3 mm

Top View of Final Mold Prototype

Theoretical Analysis

            An electrode mold was fabricated under specific geometric constraints.  Teflon was the primary material used for the mold as it prevents the silicone from adhering to the surface during the curing process.  Initially, a 0.3 x 0.3 mm main channel was cut through the center of the mold to fit five electrode wires and to accommodate enough space for silicone coverage.  Five cavities of 0.2 mm in diameter were drilled through the main channel to place the electrodes in.  This dimension was used in order to snuggly fit all the electrodes into the cavities and to prevent half of the electrode surface from being covered in silicone.  In addition, five 0.15 x 0.3 mm side channels were cut across the main channel and the cavities.  These side channels were cut to secure each electrode with dummy wires of diameter of less than 0.15 mm.  This prevents electrodes from displacing from the cavities during the injection of silicone.  Once the electrodes are placed in the mold and silicone is injected, clear double sided tape is placed on top of the mold to easily detach the silicone from the mold.  After the fabricated electrode array is detached, excess silicone is cut off for the final perfect 0.3 x 0.3 mm electrode with five half sphere electrodes exposed.

Experimental Analysis Plan

There were two types of impedance tests performed for the experiment.  The initial testing was performed in order to determine the diameter of the electrode ball that would meet the design specifications for the impedance range of 20 kΩ ± 5 kΩ.  Using a 0.38 mm diameter 90% Platinum and 10% Iridium wire, several spherical electrodes were fabricated with varying diameters.  These electrodes were fabricated by melting a specified length of 90% Platinum and 10% Iridium wire with a micro flame butane torch.  In order to obtain a specific diameter of the sphere, this equation was used calculate the amount of the wire that needed to be melted:

                                               (1)

Once the electrode was made, silicone was applied to cover half of the sphere under a microscope.  A heat gun was used to reduce the curing time of the silicone.  Impedance for each electrode was tested using an Impedance Meter.  In order to test the impedance of the electrode ball, a complete circuit was made by grounding one lead to a paper clip placed in saline solution. The other lead was connected to the end of the electrode trailing wire (the insulation was burned off to make a site for contact) and the electrode ball was inserted into the saline solution. This completed the circuit and the impedance of the electrode was obtained using the Impedance meter.

Manufacturing Process Plan

            Steps for fabricating the mold, docking platform, and array can be found here

Quantitative Evaluation of Design Performance

            The five-channel electrode array fabricated by the mold was subjected to both in vitro and in vivo testing.  The performance of the design is evaluated by the functionality of the electrode array.  The array was soldered into place on a pin board before it was subjected to impedance and stimulus testing.  Although the electrode array fabricated by the mold did not meet all the original design specification stated, the results did show that the mold design has the potential of being a full success.

            After the electrode array was soldered into the pin board, the impedance of each individual electrode was tested.  The table below shows the impedances of each electrode in the array.  The results showed that two of the five electrodes had impedance values close to design specifications.  Electrode 3 had no impedance value because the wire connected to the electrode broke off when it was being removed from the silicone mold.  Electrodes 1 and 2 had extremely high impedance values (in the MΩ).  This could be attributed to the fact that silicon may have covered most or all of the electrode sphere.  Despite these issues, the sponsor was impressed enough with the device to allow it to be implanted into the cochlea of a rat.

            In vivo testing was conducted in the cochlea of a rat by Dr. Clement.  The electrode array was implanted into the cochlea and stimulus current was sent through each electrode to test its functionality.  During the implantation procedure, however, Dr. Clement noticed that some of the silicone had fallen off of the first two electrodes.  This could have been responsible for decreasing the impedance values of these electrodes, allowing them to record good readings.  Also, after the array had been implanted, it seemed that the last two electrodes, #4 and #5, did not seem to be fully implanted in the cochlea. Despite these difficulties, the first two electrode channels were successfully tested by sending a stimulus signal through the electrode. Both channels were able to elicit a good response from the rat cochlea and showed that the electrodes were fully functional.

 Testing Results