electroplating device engineering...

Electroplating Device Reverse EngineeringReport

Work by:Paul Aaron Bohn, Production EngineerNathan Darling, Mechanical Engineer

Consulting:Heitor Mourato, Scientific Instruments Facility Director

Edited By:Sam Damask

Erin Huelskamp

Boston University Electronics Design Facility

590 Commonwealth Avenue, Physics Rm. 255Boston, Massachusetts 02215T 617-353-4117 F 617-353-3331http://edf.bu.edu

Electroplating Device Reverse Engineering ReportLast Updated 2010-11-16

1

Introduction

Abstract

This engineering report describes a custom electroplating device designed by the"Instrument Development Group" based out of Johns Hopkins University. The purpose ofthe device is to plate neural microelectrodes for recording the brain activity in animals.

Background

See Appendix D for background information on the electroplating recording wire process.




2

Table of Contents

1. Introduction ....................................................................................... 21. Abstract ............................................................................................ 22. Background ....................................................................................... 2

2. Description ......................................................................................... 51. Purpose ............................................................................................ 52. Operation .......................................................................................... 53. Characteristics and Specifications ......................................................... 6

1. Identification................................................................................... 62. Physical Characteristics .................................................................... 63. Electrical Characteristics ................................................................... 64. General Observations ....................................................................... 75. Device Photographs and Figures ........................................................ 8

3. Disassembly & Reverse Engineering ................................................. 311. Disassembly .....................................................................................312. Reverse Engineering ..........................................................................31

4. Measurements and Performance....................................................... 321. Measurements ..................................................................................322. Performance .....................................................................................34

5. Evaluation......................................................................................... 346. Conclusions & Recommendations...................................................... 36

1. Circuit Design Improvements ...........................................................362. Mechanical Design Improvements .....................................................363. Manufacturability Improvements.......................................................374. Summary ......................................................................................37

7. References........................................................................................ 388. Appendices ....................................................................................... 39

1. Appendix A - Schematic .....................................................................402. Appendix B - Drawings .......................................................................413. Appendix C - Bills of Materials .............................................................424. Appendix D - Background Information ..................................................43




3

List of Figures

2.00 EPplate Device by The Johns Hopkins University2.01 EPplate Top View2.02 EPplate Front View2.03 EPplate Bottom View2.04 EPplate Side View2.05 Empty Hammond Enclosure Outside View2.06 Empty Hammond Enclosure Inside View2.07 Opened Enclosure Perfboard Unmounted2.08 Perfboard Bottom View2.09 Perfboard Top View2.10 Installed Vessel Holder Outside View2.11 Installed Vessel Holder Inside View2.12 Vessel Holder Side View2.13 Vessel Holder Bottom View2.14 Vessel Holder Assembly2.15 Disassembled Vessel Holder Bottom View2.16 Vessel Holder Cover2.17 Vessel Side View2.18 Vessel Bottom View2.19 Disassembled Vessel2.20 Vessel Bottom View No Cover2.21 Vessel Holder and Vessel Assembly2.22 Vessel Holder and Vessel Assembly Exploded Drawing2.23 Vessel Holder and Vessel Assembly Drawing Side View4.00 Output Current Histogram

List of Tables

3.00 Part Identification for Figure 2.224.00 Output Current Measurements4.01 Output Current Measurements Location4.02 Output Current Measurements Dispersion4.03 Output Current Measurements Confidence Intervals for Mean




4

Description

Purpose

The device described in this report provides 5uA constant current to 10 channels for platingelectrodes. The amount of plating depends on plating time and strength of plating solution.

Operation

The device is activated by single momentary push button that controls the application ofcurrent to an electroplating solution (platinum chloride) contained in a Teflon(polytetrafluoroethylene (PTFE)) vessel. This cylindrical vessel is appears in the right handside of Figure 2.01 and is mounted on a socketed holder. In the center of the vessel thereturn electrode that is made out of graphite rod protrudes. The 15 pin D-Subminiature(dsub) connector distributes 10 channels of constant current source that is to be connectedto individual recording electrodes that are to be plated. A status indicator LED lights upwhen the momentary push button switch is depressed giving the user indication that currentwill flow if a complete circuit is made.

Figure 2.00 - EPplate Device by The Johns Hopkins University




5

Characteristics and Specifications

Identification

Device Name: EPplateManufacturer/Origin:

Instrument Development Group (idg)Department of Physics and AstronomyThe Johns Hopkins University3400 N. Charles StreetBaltimore, MD 21218

Telephone (410) 516-7097 or (410) 516-7383Fax (410) 516-6664

http://hopkinsidg.com/

Physical Characteristics

Dimensions: 119.5mm X 94mm X 56.5mm (just enclosure)Weight: 480 grams (with batteries)Color: WhiteSurface Coating: Spray PaintEnclosure Material: Diecast AluminumConnector: 15 pin Female dsub

Electrical Characteristics

Power Source: two nine-volt batteries (PP3)Nominal Supply Voltage: 18VDCNominal Electrode Output Current: 5ųAOutput Impedance:Current Source Compliance:




6

http://hopkinsidg.com/

General Observations

The device appears to have been produced in small quantities and was hand made. Uponarrival the device was fairly dirty and it was clear that it was used in a wet lab environment.The device was not water proof. Liquid could easily gain access to the interior through the

holes on the top of the enclosure. Paint is beginning to chip off of the diecast aluminumenclosure that makes up the housing. The simple interface introduces the potential forhuman error that could lead to inconsistent plating results for the untrained. Duringoperation there was no clear indication of whether or not proper current amount wasfollowing or if the batteries had sufficient capacity for the device to operate. It was difficultto insert and remove the 9V batteries because the holders were either warped, misaligned,or were not designed for small variations in battery dimensions. The mounting screwthreads in the Teflon vessel and vessel holder assembly were worn and beginning to strip.




7

Device Photographs and Figures

Figure 2.01 - EPplate Top View




8

Figure 2.02 - EPplate Front View




9

Figure 2.03 - EPplate Bottom View




10

Figure 2.04 - EPplate Side View




11

Figure 2.05 - Empty Hammond Enclosure Outside View




12

Figure 2.06 - Empty Hammond Enclosure Inside View




13

Figure 2.07 - Opened Enclosure Perfboard Unmounted




14

Figure 2.08 - Perfboard Bottom View




15

Figure 2.09 - Perfboard Top View




16

Figure 2.10 - Installed Vessel Holder Outside View




17

Figure 2.11 - Installed Vessel Holder Inside View




18

Figure 2.12 - Vessel Holder Side View




19

Figure 2.13 - Vessel Holder Bottom View




20

Figure 2.14 - Vessel Holder Assembly




21

Figure 2.15 - Disassembled Vessel Holder Bottom View




22

Figure 2.16 - Vessel Holder Cover




23

Figure 2.17 - Vessel Side View




24

Figure 2.18 - Vessel Bottom View




25

Figure 2.19 - Disassembled Vessel




26

Figure 2.20 - Vessel Bottom View No Cover




27

Figure 2.21 - Vessel Holder and Vessel Assembly




28

Figure 2.22 - Vessel Holder and Vessel Assembly Exploded Drawing




29

Figure 2.23 - Vessel Holder and Vessel Assembly Drawing Side View




30

Disassembly & Reverse Engineering

Disassembly

The perfboard, vessel holder, dsub connector, and battery holders were fixed to theenclosure with standard screws and/or standoffs. This made disassembly relatively straightforward, and only required standard hand tools to remove the fasteners.

Part Description

1 Vessel

2 Vessel Cover

3 Vessel Holder

4 Vessel Holder Cover

5 Graphite Electrode

Table 3.00 - Part Identification for Figure 2.22

During the disassembly of the vessel shown in Figure 2.17 and Figure 2.22, the embeddedgraphite return electrode (Part 5 - see table 3.00 and Figure 2.22) was found to be severed.Small pieces of graphite and another substance appeared in and around the electrode and

the electrical contact that appears in Figure 2.19. The additional substance appeared to beconductive epoxy that was used to bond the electrode into the male dsub electrical contact.

Reverse Engineering

The reverse engineering process included identifying all of the parts, creating drawings ofnon-standard parts, tracing out the circuit board to create a schematic, and doing someanalysis to figure out how the device works.

The parts that made up the device were identified and listed in three separate bills ofmaterials that can be found in Appendix C. A bill of materials was created for the vesselassembly, perfboard, and enclosure. Drawings of the vessel holder and vessel assemblywere created and can be found in Appendix B. Three-dimensional models of the vesselholder and vessel assembly were also created. Drawings of similar power dsub contactsfound in the device are listed in Appendix B as well. The male pin power contact wasmodified for use in the vessel. The solder cup was cut off, the retaining clip was removed,and the center was bored out such that it could accept the graphite return electrode.




31

The custom perfboard circuit was traced out visually and with the use of a multi-meter. Theschematic of the circuit can be found in Appendix A. Circuit analysis was performed toverify operation of the circuit. The design consists of multiple copies of a simple single BJTconstant current source.

Measurements and Performance

Measurements

Output current and voltage measurements were taken with Keithley Model 6512Programmable Electrometer. Prior to testing, new batteries were installed and their outputvoltage was measured to confirm that they were fresh. The measured voltage of the twobatteries in series with power off was 18.744V. Under no load conditions with the powerbutton depressed the combined battery output voltage sagged to 18.481V. Referencing theschematic located in Appendix A the voltage regulator U1 had an output voltage of 5.031Vwhen the device was powered on. One Keithley input cable terminal was attached to femalecontact that was embedded in the vessel holder. The other Keithley input cable wasattached to the proper dsub contact. A measurement was taken for one pin and then thenext until all ten measurements were completed. Table 4.00 shows the recorded values ofthe current source output measurements and Figure 4.00 is a histogram of the data. Tables4.01 through 4.03 display statistics on the data set. Later measurements were taken at adifferent room temperature approximately 5 to 8 degrees Celsius warmer. The mean outputcurrent value for the warmer temperature measurement was 4.563uA.

Pin Current (uA)

1 5.383

2 4.504

3 4.566

4 4.378

5 4.481

6 4.404

7 4.450

8 4.408

9 4.412

10 4.415

Table 4.00 - Output Current Measurements




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Figure 4.00 - Output Current Histogram

Table 4.01 - Output Current Measurements Location

Table 4.02 - Output Current Measurements Dispersion

Table 4.03 - Output Current Measurements Confidence Intervals for Mean




33

Performance

The device clearly states that its nominal supply current is 5uA which is 9.6% different fromthe mean current measured. The difference between the measurements of pin 1 and pin 9listed in Table 4.00 is 1.171uA. This is a 23.42 percent difference from the nominal current.The mean percent error for the different temperature measurements was 0.951%.

Evaluation

The device was evaluated base on characteristics related to its operation, manufacturability,testability, reliability, repairability, and cost of production.

The operation of the device is inherently prone to human error, because the plating time iscontrolled by how long the user depresses the on button. The amount of current applied toeach channel varies quite a bit which contributes to the electrodes plating at different rates.That is why the user should avoid mixing up the channels during the plating process to

control the repeatability. Orientation and pin alignment of any cable harnesses that pluginto the dsub connector does matter for the aforementioned reason. The current sourcedesign has a temperature sensitivity that originates from the temperature dependence ofthe transistor base emitter junction which changes by approximately 2.1mV per degreeCelsius. This translates to some temperature generated error. The errors introduced maynot be so bad that it prevents the device from performing an adequate job.

For the most part the device is designed such that it would be cost effective to manufactureit in small quantities. Modern technology offers less time consuming options that canreplace the hand assembled perfboard, and the wire wrapped dsub connector. The use of anon-standard-sized graphite rod for the return electrode was unnecessary and adds laborexpense. The worn threads in the vessel and vessel holder assembly indicates that theywere not strong enough to handle their job. Labor expense is the cost driving factor for thisdevice. Most of the parts are under the $10 level.

The device was easy to test and due to its simplicity it should be reliable and repairable.However, two items should be pointed out. One, the open holes on the top of the device

does open up the opportunity for damage. In a wet lab environment, liquid could easily bespilled directly onto the device and it could make its way onto the perfboard ultimatelycreating a short. Two, the most difficult item to repair is the vessel shown in Figure 2.17.If the graphite return electrode or vessel is damaged it will require rebuilding/remaking the

vessel assembly. Repairing the vessel assembly could require machining time for Teflonparts, the dsub male power electrical contact, and graphite rod return electrode. Inaddition, the dsub male power electrical contact needs to be bonded to the graphite rodwith conductive epoxy which costs on the order of $20, and takes 24 hours to cure. Thegraphite return electrode had to be replaced during the reverse engineering process,because it was found to be broken upon disassembly.




34




35

Conclusions & Recommendations

The general simplistic nature of the device was a sign of good engineering. The choice anduse of standardized parts such as the enclosure, hardware, lock washers, dsub connector,and push button switch were examples of sound decision making. The large clear label andsimple interface was a nice feature. The use of dsub power contacts and the vessel holderand vessel assembly design was clever.

There are opportunities to improve this device in several areas such as the circuit design,mechanical design, and its manufacturability.

Circuit Design Improvements

• precision and accuracy• reduce temperature sensitivity• adjustable current magnitude• current application timing control• electrode resistance measurement display• user resistance set automated plating

With little effort, the 5% 1Mohm resistors that were used in the existing current sourcecircuit could be replaced with higher tolerance resistors. This would increase the precisionof the output currents across the different channels. An opamp based current source wouldincrease the output currents precision and accuracy while reducing the temperaturesensitivity.

Mechanical Design Improvements

The Teflon vessel holder and vessel could be improved in the following different ways. Werecommend using larger diameter mounting screws and heli-coil thread inserts to preventthe threads from stripping. It is also recommended to use 316 stainless steel hardware toprevent corrosion. The design will need to be adjusted to snugly fit the dsub powercontacts that are listed in the bill of material in Appendix C, because the contacts areslightly shorter in overall length. The male dsub power pin's solder cup should be cut off,and the pin should be bored out on the solder cup side to receive a standard sized graphiterod. The vessel design should then be adjusted to accept the new graphite rod size andprovide a snug water tight fit. The screws used to hold the vessel together could betamper-resistant to discourage somebody from disassembling and possibly damaging thegraphite rod after it has been epoxied in place.

Due to the wet lab environment in which this device is likely to operate, we suggest that itshould be designed to be water resistant.




36

Manufacturability Improvements

To increase the manufacturability, we recommend the use of standardized parts such as apre-coated (anodized, powder coat, etc.) enclosure, a custom printed circuit board, and astandard sized graphite return electrode.

Summary

The reverse engineering process went well. All major components were identified andcataloged, a sufficient amount of engineering documents were drafted that can be usedreproduce the device, and analysis was performed to determine how the device worked. Itis unfortunate that the graphite return electrode was found to be damaged after it wasdismantled. The electrode had to be replaced and bonded to the male dsub power contactusing conductive epoxy. Additionally, dsub power contacts were purchased on short noticeto confirm that they were properly identified by the data sheet, and to serve as a back up incase the existing male dsub power pin was determined to be unusable. This allowed for thefast turn around time to deliver the device back to the client.




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References

http://www.schoenbaumlab.org/nicoweb/box5.html




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http://www.schoenbaumlab.org/nicoweb/box5.html

Appendices

Appendix A - SchematicAppendix B - DrawingsAppendix C - Bill of MaterialsAppendix D - Background Information




39

Appendix A - Schematic




40

E:\pbohn\public_html\Electroplating_Report\Design_Files\eplate.sch - Sheet1

Appendix B - Drawings




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Part 1

Part 2

Part 3

Part 4

Part 5

Material (Parts 1-4): Teflon.All tolerances .005 unless specified.

0.67

0

0.06

5

0.750

.500 Ball End-millDepth: .295

4-40 Threaded

0.310

0.500

0.120 0.000

+0.001

Part 1

0.190 0.000+0.001

0.240 0.000+0.001

0.750

0.25

50.

055

4-40 Clearance82 degree

Part 2

0.31

5

0.49

0

0.29

5

0.19

5

0.11

5

0.28

0

0.80

5

6-32 Threaded

0.215

0.000+0.0

01

0.1880.000+0.001

Bottom

0.760

0.650

Top

Part 3

0.20

5

0.750

6-32 Clearance

0.145 0.000+0.005

Part 4

0.119 -0.0010.000

0.750

Part 5

Material: Graphite

Appendix C - Bills of Materials




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Bill of MaterialsBoston University Electronics Design Facilityhttp://edf.bu.edu/Reverse Engineering ReportEPplate Enclosure BOM; 2010-11-02

Item Qty Description Purpose

1 1

Enclosure Diecast Aluminum 4.72X3.70X2.07 with Cover and 4pcs. 6-32 flathead phillips screws -Hammond Part Number 1590C

2 2

HOLDER BATT 9V SNAPIN ALUMSOLDR - Keystone Part Number1290

3 4BUMPON HEMISPHERE RubberFeet .75X.38 BLACK

4 1 Custom Perfboard Circuit Assembly5 1 Custom Decal Sticker Label

6 1418-8 SS Button Head Socket CapScrew 4-40 Thread, 3/16" Length

Battery HolderMounting, PCBMounting

7 4

18-8 Stainless Steel Split LockWasher No. 4 Screw Size, .21" OD,.02" min Thick

PerfboardMounting

8 4

Zinc-Pltd Brass Female ThreadedHex Standoff 1/4" Hex, 3/8" Length,4-40 Screw Size

PerfboardMounting

9 2

18-8 SS Male-Female ThreadedHex Standoff 3/16" Hex, 3/16" BodyLength 5/16" Male Thread Length,4-40 Screw Size Dsub Mounting

10 1 Vessel Holder and Vessel

Bill of MaterialsBoston University ElectronicsDesign Facilityhttp://edf.bu.edu/Reverse Engineering ReportEPplate Vessel Holder and VesselBOM; 2010-11-02

Item Qty Description Purpose

1 4Type 316 Stainless Stl Socket HeadCap Screw 6-32 Thread, 3/8" Length Vessel Holder

2 4

Type 316 Stainless Steel Split LockWasher No. 6 Screw Size, .25" OD, .03" min Thick Vessel Holder

3 2Type 316 SS Flat Head Socket CapScrew 4-40 Thread, 3/8" Length Vessel Cover

4 1 Vessel (Part 1)5 1 Vessel Cover (Part 2)6 1 Vessel Holder (Part 3)7 1 Vessel Holder Cover (Part 4)8 1 Graphite Electrode (Part 5)9 n/a Silver-Filled Conductive Epoxy

10 1

CONN D-SUB PIN 14AWG SOLDERGOLD Amphenol Part NumberL17DM537457

Vessel MalePin Contact

Bill of MaterialsBoston UniversityElectronics Design Facilityhttp://edf.bu.edu/Reverse EngineeringReportEPplate Perfboard BOM;2010-11-02

Item Qty Reference Value Description

1 1 C1 0.1Ceramic Through Hole CapacitorCK05BX104K

2 1 D1 T3 Green LED3 1 J1 Female Dsub 15pin4 10 Q1-Q10 PN2222A NPN BJT5 1 R1 910 Through Hole Resistor6 10 R2-R11 1M Through Hole Resistor7 1 SW1 C&K Momentary Pushbutton EP11

Appendix D - Background Information




43

Schoenbaum Lab

Members

Research

Publications

Pictures

Movies

NYASEbriefings

University of MarylandSchool of MedicineDepartment of Anatomy &Neurobiology20 Penn StHSF-2, Room S251Baltimore, MD 21201

Office: HSF-1, Room 280KPhone: 410-706-3814Cell: 443-722-6746Fax: 410-706-2512

Metal Microelectrodes for Recording in Behaving Animals

Electrode impedance is a primary factor in the performance of any electrophysiological recording system. Electrode impedance describes the electrical characteristics of the complex interface between the metalwire microelectrode and the extracellular recording medium.

Modeling Electrode Impedance

The equivalent electrical circuit of a metal microelectrode immersed in an electrolyte solution is shown inFigure 1. It consists of both resistive and capacitive elements. The resistive portion represents the mobilityof charge carriers on each side of the solution/metal interface plus the weak exchange of ions across theso-called "double-layer".

Figure 1

The double-layer is created by polarized water molecules at the metal/brain interface. These watermolecules form a thin dielectric (Figure 2). Thus, the double-layer is modelled by a capacitor. Theexchange of ions across the double layer is weak and the interface is considered polarizable. That is, ionsdo not physically pass into and out of the electrode. Instead, changes in ionic concentration in theextracellular space attract (or repel) electrons to (or from) the interface.

Figure 2

Both the capacitance and the resistance of this interface (Figure 2) are dependent on the size of thesurface area of the metal in contact with the solution. In general, more surface area results in lowercontact resistance and higher double-layer capacitance. Recall that higher capacitance values havesmaller capacitive reactance at any given frequency. Thus, the overall effect of increasing surface area isto reduce electrode impedance.

Electrode impedance is related to Johnson (or thermal) noise. Johnson (or thermal) noise is generated byrandom movement of electrons in all resistive impedance elements. In general, at any given temperature,Johnson noise is proportional to resistive impedance. If the resistive impedance of the electrode is toohigh, these random fluctuations will interfere with the electrophysiological recording. Because electrode

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schoenbaumlab.org/nicoweb/box5.html 1/4

impedance is largely determined by surface area of the electrode tip, increasing tip diameter is one way toreduce the Johnson noise inherent to the electrode. Unfortunately, many applications require small tipdiameters to obtain sufficient single-unit isolation. An alternative approach to reducing Johnson noise is toincrease the surface area at the electrode tip without increasing tip diameter. This is the strategyemployed by both "bubbling" and electroplating, two methods that reduce electrode impedance withoutsacrificing the selectivity of the microelectrode.

Measuring Electrode Impedance

The impedance of an electrode is measured by passing a small AC current through the electrode,measuring the voltage drop across a known resistance placed in series with the electrode and using thisinformation to calculate the impedance. In our lab we use a battery-powered sine-wave generator, anoscilloscope and a beaker of saline arranged as shown in Figure 3. The sine wave generator is set to 100mV and 100 Hz. The oscilloscope sweep rate and sensitivity is adjusted to display several cycles of thetest waveform. The reference electrode should have a large surface area.

Figure 3

Electrode impedance may be calculated directly from the oscilloscope reading using the equation in Figure4 below. In our lab, we use a table to make quick determinations of electrode impedance from measuredvoltages. In this analysis, Rm is the internal impedance of the oscilloscope (1 megohm in our example),V0 is the output of the sine-wave generator (100 millivolts in our example) and Vm is the value displayedon the oscilloscope. The electrode impedance measured will be dependent on the frequency of the signalgenerator. Electrode impedance will decrease as frequency is increased consistent with the model shownin Figure 1.

Figure 4

See Als Table for Computation of Impedance

Practical ConsiderationsImmerse the electrode to a consistent depth. When a metal microelectrode is immersed in a conductiveelectrolyte, the wire and the solution form the plates of a capacitor (shown as Ci, above). The insulationcoating the electrode wire is the dielectric and the capacitance depends on the depth of immersion. Thus,it is important to immerse the electrode to a consistent depth for each test so as to keep Ci invariant

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between tests. I typically immerse the tips to a depth of 2 mm.

Check the voltage output of the signal generator. Make sure the sine wave is devoid of DC offset (zeromean) by checking it with an oscilloscope or DC voltmeter.

Check the frequency of the sine wave. Impedance is dependent on the frequency of the signal used tomeasure it. For impedance measurements to be comparable and consistent, they must be made using thesame frequency signal each time.

Procedures to Reduce Electrode Impedance

A small diameter tip is necessary to detect the local ionic changes that are generated extracellularly by anaction potential. Wire diameter is chosen based on the cellular properties of the particular brain area beingrecorded. The surface area of the tip of the electrode is a strong determinant of the impedance of theelectrode. Both capacitive reactance and contact resistance are reduced by increasing the surface area ofthe exposed electrode tip. Thus, procedures that increase surface area will result in electrodes with lowerimpedance. The impedance reduction procedures described below change the profile of the exposed tipwithout changing the diameter of the electrode tip. Thus, reducing impedance without sacrificing unitselectivity.

Method 1: the "bubbling" technique. One simple and inelegant method that has worked for us empirically isthe byproduct of a procedure known as "bubbling" used to test the continuity of each wire. This procedureinvolves passing anodal current through each wire from a DC source, such as a 9 V or 12 V battery, andback through a saline solution. The current causes bubbles to form in the saline solution at the tip of thewire. This "bubbling" process confirms the integrity of the connections inside the electrode assembly andalso results in a lowering of the impedance measured at the electrode tip. Impedance is reduced becausethe strong current that produces the bubbles in solution also causes etching of the electrode tip. Thisetching process increases the exposed surface area of the tip without changing the diameter, whichdetermines the recording characteristics of the electrode. Damage to the insulation may also occurhowever, so the goal of this method would be to minimize insulation damage, which can be viewed undera stereomicroscope, while etching the tips sufficiently to achieve a useable impedance on each wire. Toomuch damage to the insulation will result in a non-selective electrode that is unable to isolate single cellsfrom their neighbors.

Method 2: plating. Another more conventional method of reducing electrode impedance is throughelectrolytic deposition of an inert metal onto the electrode tip (electroplating). Electroplating also reducesimpedance by increasing the effective surface area without increasing the tip diameter. Platinumelectroplating is accomplished by placing the electrode tips into a solution of platinum chloride andapplying a small current such that the platinum in solution is reduced, causing platinum deposition at thetip of the metal electrode. I plate our electrodes using a solution of hydrogen hexachloroplatinate (8%PtCl4 by weight; Sigma Chemicals) with a multi-channel, constant-current plating device available fromEclectic Engineering Studio.

A simple constant-current plating circuit is formed using a large resistance (R) and a DC voltage source(V) as in Figure 5, below. The resistor used in this circuit must be at least ten times larger than the largestanticipated DC resistance of the electrode. The plating current is computed using Ohm's law.

Figure 5

In constructing such a circuit, the current (I) should be kept relatively small (1 to 10 microamps), and thereturn electrode should be either graphite or platinum to prevent contamination of the platinum chloridesolution.

Immersion procedure for platinum electrodeposition. Both the plating current and the concentration of theplating solution will affect the immersion time required for sufficient plating. Typically we plate ourelectrodes at 5 microamps for 4-6 seconds in two separate immersions. Prior to each immersion, theelectrodes are dipped into a 90% EtOH solution to reduce small air bubbles and remove dirt particles thatcan have a negative impact on electrolytic deposition. Wetting the tips with EtOH and repeating theprocess several times serves to both reduce and distribute these factors and results in a more uniformimpedance across the different wires. It is important to visually examine the tips of the wires whendetermining the optimal parameters for time, current, and platinum concentration. Prepare the electrodesfor plating by cutting the tips using fine surgical scissors. Prior to plating, the exposed electrode tip shouldbe “shiny” and the insulation should be intact. After plating to the desired impedance, the tips will have a

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rough “matte” appearance and impedance is typically reduced by 50-75%. If the insulation appearsragged or degraded, then the wires have been damaged, indicating that the current, duration orconcentration of the plating solution should be reduced.

Variability of results. Finally it is worth noting that the exact parameters change slightly between days andon different electrodes, so we typically practice plating each electrode several times before making a finaldetermination of the correct values. This approach is possible because extra wire is fed through the guidecannula to be cut off prior to surgery, thus several plating attempts are made before the wires are cut totheir final length. Some variation in the final impedance between wires is normal and likely results fromlocal variables in the plating environment or the mating of each wire through the electrode assembly.

Web Content from: Schoenbaum, G. Olfactory Learning and the Neurophysiological Study of Rat Prefrontal Function. In: CRC Series: Methods and Frontiers in Neuroscience. Edited by S.A. Simon and M.A.L. Nicolelis, CRC Press, NY,2000.

This web page coauthored by Kevin B. Austin, Ph.D., Eclectic Engineering Studio, www.EclecticStudio.com

Please send comments and suggestions regarding this site to Dr. Geoffrey Schoenbaum, Director of the Lab.

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Schoenbaum Lab

Members

Research

Publications

Pictures

Movies

NYASEbriefings

University of MarylandSchool of MedicineDepartment of Anatomy &Neurobiology20 Penn StHSF-2, Room S251Baltimore, MD 21201

Office: HSF-1, Room 280KPhone: 410-706-3814Cell: 443-722-6746Fax: 410-706-2512

Calculating Electrode Impedance Using a Table

Vm Ze

(mV) (kW)

50 1000

52 923

54 852

56 786

58 724

60 667

62 613

64 563

66 515

68 471

70 429

72 389

74 351

76 316

78 282

80 250

82 220

84 190

86 163

88 136

90 111

92 87

94 64

96 42

In our lab, we use this table (left) to compute electrode impedance (Ze) from ameasured voltage (Vm). The table applies to measurements made using the circuit inMetal Microelectrodes for Recording in Behaving Animals and these parameters:

Source voltage: V0 = 100 mV pk-pk, sinusoid (100 Hz)

Oscilloscope internal impedance: Rm = 1 megohm

You may wish to adapt this technique for your particular situation. To do this, create aspreadsheet using an equation to compute impedance from your setup-specificparameters. One possible variation from our setup would use a digital voltmeter insteadof an oscilloscope. This substitution would most likely require you to change Rm from 1megohm to 10 megohms as most digital voltmeters have 10 megohms internalimpedance. If you decide to use a digital voltmeter, make sure you are measuring "ACvolts".

Web Content from: Schoenbaum, G. Olfactory Learning and the Neurophysiological Study of Rat Prefrontal Function. In: CRC Series: Methods and Frontiers in Neuroscience. Edited by S.A. Simon and M.A.L. Nicolelis, CRC Press, NY,2000.

This web page coauthored by Kevin B. Austin, PhD., Eclectic Engineering Studio, www.EclecticStudio.com

Please send comments and suggestions regarding this site to Dr. Geoffrey Schoenbaum, Director of the Lab.

11/2/2010 Z table

schoenbaumlab.org/nicoweb/ztable.html 1/1

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