MFT/EOM/M-2.12!
JM Miller
Extraocular Muscle Force Transducer
Fabrication, Testing & Implantation
Type MFT-2.12 (MFT/EOM/M-2.12)" 2014.08.12" Joel M Miller, PhD"
[email protected]"
! “Left” and “Right” MFT Versions. Same frame is used for both (dimensions are accurate, but surface mountings and wiring of current 2.12 design are no longer as shown in this 2.10 drawing; see photos below). Not shown in the figure is that “ears” would be folded upwards before components are mounted.
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MFT/EOM/M-2.12!
JM Miller
Contents" MFT Fabrication, Testing & Implantation –
Summary Description"..................................................................................................................3" Frame"..................................................................................................................................................................3" Gauging & Wiring"................................................................................................................................................3" Encapsulation"......................................................................................................................................................4" Testing & Calibration"...........................................................................................................................................4" Implantation".........................................................................................................................................................5"
MFT Fabrication & Testing –
Detailed Instructions"....................................................................................................................7" Materials"..............................................................................................................................................................7" Specialized Tools "................................................................................................................................................8" Routines"..............................................................................................................................................................8" Things to Worry About".........................................................................................................................................9" 1. Prepare Frames".............................................................................................................................................9" 2. Gauging & Wiring".........................................................................................................................................11" 3. Encapsulate in Epoxy & Serialize"................................................................................................................15" 4. Parylene C Moisture Barrier"........................................................................................................................16" 5. Remove Masking Tubes"..............................................................................................................................16" 6. A Case of Poor Parylene Adhesion".............................................................................................................17" 7. On-Board Electrical Test"..............................................................................................................................18" 8. Leak Testing & Localization".........................................................................................................................18" 9. Preparing Attachment Pins"..........................................................................................................................19" 10. Final Testing & Calibration"...........................................................................................................................20" 11. Connector Can"............................................................................................................................................20" 12. Other Types of MFTs"...................................................................................................................................22"
Appendices"................................................................................................................................24
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MFT/EOM/M-2.12!
JM Miller
MFT Fabrication, Testing & Implantation –
Summary Description" The MFT/M-2.12 Muscle Force Transducer for NHP extraocular muscle is described below in sufficient detail to understand how it is made and used. Detailed fabrication instructions appear in a subsequent section."
Frame" Our Muscle Force Transducer for NHP extraocular muscle, MFT/M-2.12, is fabricated from 10-mil half-hard stainless steel sheet. Blank frames are cut by photochemical etching (Elcon Co, San Jose, CA), electro-polished to remove the “knife edges” that etching forms in the middle of the sheet’s thickness, and manually finished to soften other edges that might damage muscle or tendon. Half-depth engravings on the top of the frame define lines along which the four “ears” are bent upward at right angles to form bearings for an Attachment Pin that directs the muscle through the frame. The side of the frame without bearings or wiring contains tiedown holes used to maintain MFT orientation during healing, and is bent down ~20° so the device better conforms to the globe, preventing the muscle from losing any of its 3 lines of contact with the device when wrapped around the globe. Bearing holes are sized to fit the 26 ga Attachment Pin after Parylene coating. To improve SG bonding and coating adhesion, frames are sandblasted."
Gauging & Wiring"
! Finished frame, held in custom forceps, also used for implantation. Wiring passes from bottom to top of frame through oblong holes at bottom. Attachment Pin (not shown) passes through upward-bent ears and under muscle at middle. Tie-down holes at top secure device to muscle margins to stabilize orientation during healing.
Matched, transducer-grade, U-shaped, semiconductor SGs (Entran ESU-025-500-4 www.meas-spec.com) were chosen for their small size and high gauge factor (sensitivity to strain). One gauge is bonded to the top surface of the frame and another to the opposite beam on the bottom. The two function differentially, with muscle tension compressing the top and elongating the bottom gauge. Wired as arms of a Wheatstone Bridge, this arrangement achieves high output, temperature compensation, and sums forces across the width of the muscle. Forty-gauge copper “magnet” wire joins one side of each gauge. This common wire, and the two free gauge wires are connected, via solderable terminals, to PTFE insulated stainless-steel multi-stranded lead wires (AS-631, Cooner Wire, www.coonerwire.com), such that, to lengthen paths of possible leakage, the points of attachment are as far as possible from where the lead wires depart the encapsulated frame."
A thin insulating “base coat” of M-Bond 610 epoxy adhesive (Vishay Micro-Measurements, (www.vishaypg.com/ micro-measurements/installation-accessories) is applied by dipping, and is then baked. Solder connection terminals are shaped from MST-035 and MST-094 bondable terminals (Micron Instruments, www.microninstruments.com) to be as small as possible to minimize surfaces over which electrical leaks might develop, tinned with 430 °F solder, and grooved to position the fragile gold SG lead wires. An SG and a set of prepared terminals is cleaned in acetone to remove any contaminants that might impair bonding, a small amount of AE-15 epoxy (Vishay M-M) is brushed on the bonding surfaces of the frame, and SGs are positioned, forming a small fillet of epoxy around them. These silicon gauges will shatter if clamped, so we tie the SG down to the frame with a thin strip of PTFE tape, and bake for 2 hr at 65 °C. We then lay the SG’s gold lead wires into pewformed terminal grooves, and cut them to size. We repeat the process on the other side of the frame." We apply acid flux with a pin tool, and solder the wires to their terminals using a miniature screwdriver-shaped tip (Weller MT221) on a temperature-controlled 410 °F iron carrying a small amount of 316 °F solder, clean the
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MFT/EOM/M-2.12!
JM Miller
joint of flux, and neutralize residual acid. We then join the near sides of the top and bottom SGs via their terminals with magnet wire passing through one of the frame’s oblong holes." We condition the proximal ends of the FEP-insulated lead wires with Tetra-Etch adhesion promoter (Gore, Newark, DE), wash in warm water followed by acetone, strip, tin with 430 °F solder, clean, and neutralize. We position wires for soldering with a manipulator incorporating an E-Z-Micro-Hook X2015, attached to a photographer’s “Israeli Arm”, mounted on a weighted base. Lead wires are soldered to their connectors with the 410 °F iron and 316 °F solder. Two lead wires pass up through the oblong hole along with the copper interconnect wire, join the lead wire soldered to the connector on the free end of the top SG, and the 3 wires are braided and tied to the frame with 6-0 Prolene suture. "
Encapsulation" We clean the devices in alcohol and apply ResinLab EP-1121 Clear Epoxy Encapsulant (Ellsworth Adhesives, www.ellsworth.com) to a few mm of braided lead wire distal to the Prolene tie-down. EP-1121 cures to a rubbery solid, and is used to prevent the much harder AE-15, used to mechanically protect the SGs and their gold lead wires and to bury sharp protrusions, from insinuating and stiffening the leads. The distal side of the frame is then serialized with colored enamel paint dots, baked, coated with AE-15, and baked again." MFTs are then encapsulated with Parylene-C, up to but not including the FEP-insulated lead wires, to provide a biocompatible moisture barrier for SGs, on-board wiring, and the frame itself. Lead wires are masked with silicone rubber sleeves, open at their ends to allow air exchange, but long enough to prevent Parylene vapor intrusion. Devices are cleaned, and etched with cold plasma to improve adhesion. Parylene-C is then vapordeposited to a thickness of 20 microns (Plasma Ruggedized Solutions, San Jose CA)."
Testing & Calibration"
Finished MFT. A stainless steel frame carries two semiconductor SGs (one visible at the lower left, the other on the underside of the opposite beam, at the lower right), wired as a half Wheatstone Bridge, and positioned to average forces across the tendon and compensate for variations in temperature. The device is encapsulated in epoxy resin and Parylene-C. It is affixed by pushing a pin (not shown) through holes in the 4 upward-bent tabs, and under the muscle. Devices are fabricated in 2 orientations: “left” (shown), for implantation on left LR and right MR, and “right”, for right LR and left MR, the intent being that posteriorlyoriented lead wires course superiorly in the orbit as they depart the device.
We measure and record resistance of each SG at several points during fabrication. To test the integrity of encapsulation, we suspend each MFT in warm, stirred saline for several ! weeks, periodically testing for leaks between grouped lead wires and bath with a Fluke 189 Multimeter, which can detect leakage resistances greater than 500 M#. If a leak is found, we localize it by immersing the MFT in saline, connecting it to a cathode, the bath to an anode, and applying up to several hundred volts. At the cathode, H2 is liberated from H2O, and Na from NaCl. The liberated sodium pulls OH– ions from H2O, liberating more H2. H2 bubbles thus appear where the bath contacts electrically exposed components. Repair is sometimes possible, though this test has been more useful in refining design and fabrication." Each MFT is calibrated by attaching it to an SG amplifier with 2 V excitation, inserting an Attachment Pin through its bearings, threading a length of 1/8” wide Mylar tape through it as the muscle would go, and hanging weights 5–80g from the end of the tape in ascending and descending series. All functioning devices have similar sensitivities, and are almost perfectly linear."
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MFT/EOM/M-2.12!
JM Miller
Implantation" MFTs have been chronically implanted in NHPs on lateral and medial recti. Under general anesthesia, with body supine, we spread the lids with a speculum or traction sutures, and disinsert the conjunctiva at the limbus in quadrants containing muscles to be implanted, placing traction sutures in cut edges of the conjunctiva to aid in visualizing the muscle. We clear the muscle of attachments by blunt dissection to about 10mm posterior to the limbus, slide a standard ophthalmic muscle hook under it, and then transfer the muscle to an MFT Tendon Lifter (Fig 4)."
Typical MFT Calibration.
MFT Installation Tools. MFT Tendon Lifters (left) are essentially small muscle hooks, whose handles and hooks are sized to pass through an MFT’s aperture. MFT Forceps (right) are custom-made locking forceps that hold an MFT firmly, without damaging its coatings.
Holding an MFT in its Forceps, we pass the Tendon Lifter and muscle up through the MFT’s aperture. Pulling the muscle gently up, while holding the MFT down, we push an Attachment Pin through the proximal bearings, under the muscle, and through the distal bearings. With the MFT positioned a few mm behind the insertion, and oriented squarely on the muscle, we pass a 6-0 Prolene suture with a round, non-cutting needle through each tiedown hole and tie it to the underlying muscle margin. It is essential to stabilize the MFT until it becomes enveloped in connective tissue. We then position the lead wire in the orbit, allowing some slack, and route it to the scalp, following the method used to implant eye coil wires (Judge et al., 1980)." MFT Connector Can is fabricated by stereolithography from “MicroFine Green” resin, an ABS-like plastic, at Fineline Prototyping (www.finelineprototyping.com) and fitted with gold-plated IC pins with standard 0.1 in spacing. Leads from 2 MFTs can be accommodated. The leads enter the hole near the bottom, which is then sealed with RTV silicone, wrap around the internal pedestal, and are soldered to connector pins near their bottoms. Standard connectors plug into the free ends of the pins. A screw cap protects the pins when not in use.
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MFT/EOM/M-2.12!
JM Miller
Because SG signals are, essentially, small resistance changes, electrical connections must be scrupulously clean and protected from body fluids. We pass the braided MFT lead wires, uninterrupted, into a special plastic Connector Can, where they are soldered to easily cleaned pins, onto which SG amplifier connections are made. A successfully implanted device is stable for several months or longer."
! MFT (left-hand configuration) installed in lateral rectus muscle.
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MFT Fabrication & Testing –
Detailed Instructions" Stepwise instructions for fabricating, testing and implanting our Muscle Force Transducer for NHP extraocular muscle, MFT/M-2.12, follow."
Materials" Parts" Entran ESU-025-500-4 matched semiconductor strain gauges (www.meas-spec.com)." Micron Instruments MST-035 & MST-094 bondable solder terminals - from which are made the terminals for onboard wiring." Hypodermic tubing 26 ga (OD = 0.0183 in) and 27 ga (OD = 0.0165) thick-walled – for Attachment Pins." Cooner Bioflex AS-631, 40 gauge, 10 strand, 316-stainless, FEP-insulated wire – for lead wires." Heavy-duty (2 layer) build AWG 40 copper magnet wire (oemwire.com) – for onboard wiring." Silicone rubber tubing (0.025in OD x 0.012 in ID) – for lead wire braid terminator & lead wire mask." Silicone rubber tubing (0.037in OD x 0.020 in ID, 0.065in OD x 0.030in ID & 0.077in OD x 0.058in ID) – for lead wire mask." 6-0 Prolene suture – to tie down lead assembly." PTFE Tape (plumbers tape or Glide dental floss) – to tie-down SG for bonding."
Cleaners (use under fume hood)" Chemtronics Flux-Off CZ rosin-flux remover spray– to remove rosin soldering flux." Vishay CSM-2 degreaser (n-Propyl Bromide) – for degreasing." Isopropanol 91% - general cleaning & “lubrication” for sliding silicone rubber tubes onto braided leads." Acetone CP – for cleaning semiconductor SGs and prepared solder terminals prior to bonding." Vishay Micro-Measurements M-Prep Conditioner A – a mild phosphoric acid etchant used to prepare SS surface for bonding (www.vishaypg.com/micro-measurements/installation-accessories)." Vishay Micro-Measurements M-Prep Neutralizer 5A – an ammonia-based scrub for surface cleaning and to neutralize acid after using M-Prep Conditioner A." Techni-Tool Tech Duster non-residue cleaner – to remove solvents by air pressure, rather than evaporation."
Adhesives & Encapsulants" Vishay Micro-Measurements M-Bond 610 epoxy – for coating SS frame prior to SG and solder terminal bonding. M-Bond 610 was previously used for SG bonding as well, but is not so reliable without clamping, which is not possible here. M-Bond 610 cures in 2 hours at 165 °C (330 °F), which may burn other materials, including epoxies, which should therefore be applied afterwards." Vishay Micro-Measurements AE-15 epoxy adhesive – for bonding SGs and solder terminals, and covering the body of the MFT, including SGs and wiring. Has excellent water resistance, but cures very hard, which is desirable for SG bonding, but less so for encapsulation. Cures in 2 hours at 65 °C (150 °F)." Ellsworth Adhesives (www.ellsworth.com) ResinLab EP-1121 Clear Epoxy Encapsulant, 50mL 1:1 cartridge, prepared with an EA6.3-21SA mixing nozzle and a STATOMIX® MA Static Mixer 6.3 X 21 – EP-1121 has low mixed viscosity and cures to a flexible (40 Shore) solid. It cures in 24 hrs at room temperature or in 1 hr at 65
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°C. To ensure a good mix, discard the first 3 inches of dispensed material. Poor water resistance. Used to protect braided lead wires from insinuation with AE-15." Gore Tetra-Etch™ fluorocarbon etchant (obtainable from Vishay Micro-Measurements in a 2oz, 60mL can as TEC-1) – to make gauge-ends of lead wired bondable using epoxy adhesive (but not cyanoacrylate). Must be stored cold, blanketed with nitrogen or argon to prolong its useful life. Active etchant is dark green or black. Spent etchant is brown, tan, yellow, or white."
Soldering Materials (use fume collector when soldering)" Temperature-controlled soldering station: Weller MT1500, with MT221 0.010in miniature screwdriver tip." Wire solder: Vishay Micro-Measurements 430-20S-25 – to tin SS wire & solder pads." Wire solder: Vishay Micro-Measurements 361A-20R-25 – to attach all tinned wires to solder pads." Vishay Micro-Measurements M-Flux SS – to tin SS wire, and solder all wires to terminals."
Specialized Tools " Custom MFT Forceps." Pin Tool: a fine entomology pin in a pin vise – for manipulating gold wires and other small features." A lead wire manipulator is custom made from an E-Z-Micro-Hook X2015, attached to a photographer’s “Israeli Arm” and mounted on a weighted base. It is useful for holding wires for tinning, and essential for positioning lead wires for soldering to terminals." Scalpel blades #11 & #15." Drill #76 (0.020in) – for sizing holes in frame." Drill #77 (0.018in) – for clearing epoxy from holes in frame." Round toothpicks." Fluke 189 Multimeter or similar device able to measure at least 500 MOhm."
Routines" General Clean & Dry Routine" Remove soldering flux with Flux-Off. Clean MFT using short spurts of CFM-2 organic solvent. Remove solvent by aerosolizing with Tech Duster. Complete drying with heat gun, holding hand behind MFT to be sure it is not overheated (observe cleaning fluid evaporating from under PTFE insulation, if lead wires are in place). It is important for good epoxy adhesion that the MFT be completely clean & dry. Clean again with isopropanol applied with pipette, and dry as above."
AE-15 Epoxy Mix & Bake Routine" Weigh 2.5g epoxy resin, add 8 drops AE-15 hardener, and stir very thoroughly. Epoxy should have a water-like consistency, which helps it wick into small places for complete coverage. Apply with toothpick, be careful not to apply an excessive amount, if this does occur, draw away excess with clean toothpick. Avoid creating bubbles in the epoxy. Mixed AE-15 must be used within 90 min (at 75°F). Place in and remove from cold oven. For cure, set oven for 2.5hrs at 65°C. For post-cure stress-reduction, set oven for 1.5hr at 95°C."
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Things to Worry About" Bending Frame" When bending the ears of the frame in the initial steps of fabrication it is crucial to minimize repeated bending, which will cause stress fractures, weakening the."
Coating Adhesion" Parylene coating is designed to protect on-board strain gauges (SGs) and wiring from body fluids for several months, while under almost continually varying stresses associated with eye movement." No substance (egs: wax, silicone) that might impair adhesion of this coating can be allowed to contaminate surfaces!" Primers or plasma treatment must be used to achieve the best possible adhesion of Parylene C to the frame."
PTFE Lead Insulation" MFT lead wires are also continually flexed, while being bathed in body fluids. The FEP insulation of the AS-631 lead wire is easily cut or nicked, rendering a device unusable. PTFE “creeps” under pressure, so it is possible to squeeze a lead wire too tightly (eg, with the Prolene suture that cinches the outer edge of the strain-relief sleeve, or by pinching with one’s fingernails, etc), causing the SS wire itself to press through the PTFE from the inside." The exit of the lead wires from the silicone rubber sleeve must be carefully sealed against body fluids. Pressure of the sleeve against the FEP lead wire insulation alone is insufficient because of PTFE creep. We make the FEP surface bondable with Tetra-Etch, and bond it to the surrounding sleeve with silicone adhesive."
1. Prepare Frames" Recycle used frames, if possible, by removing as much of surface mountings and epoxy as possible with a #11 scalpel, soaking in hot water, burning with a propane torch (being careful not to overheat the frame) and sandblasting." The procedure for fabricating new frames follows."
1.1. Photochemical Etching & Electropolishing" The primate (MFT/M-2) and human frame designs (MFT/H-3) are fabricated from 10-mil half-hard stainless steel sheet by Elcon Co (San Jose, CA • cost ~ $600/lot), using our dimensioned drawings, converted to CAD files. Photochemical etching is followed by electro-polishing to remove sharp edges, although manual finishing is still required."
Portion of a sheet of Stainless Steel MFT frames (top view). Small engraved rectangles, defining where ears will be bent upwards, are barely visible.
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To remove frames from the stainless steel sheet use a toothpick to apply slight pressure to the “ears” of the frame, using a gentle rocking motion to break the frame free of the sheet. Smooth broken sprue with a broken-off piece of fine abrasive wheel."
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Bend frame to an approximately 20° angle, away from the half-depth engravings side, the side we’ll call “up”. Frame bend is made on the side of the “ears” with the suture tiedown holes."
Frame bent 20° to better conform to shape of eye. Ears and sides of frame that will carry strain gauges, solder pads, and wiring are held flat in pliers, while side with tie-down holes is pressed down with fingers. Half depth engravings, not seen here, face upward. (Side view)
1.2. Fold Frame to Shape"
Frame shown bent down, away from side with half-depth engravings. (Top view)
There are two steps to forming the MFT “ears”: first bend, then square. To bend, use specially made smooth jawed, angle-tipped pliers to cover the ear to the half-depth engraving that defined the bending edge. Gently bend ear toward the engraving, that is, toward the top of the frame. Bending adjustments (bending & unbending) must be held to an absolute minimum to avoid work-hardening and weakening the ears."
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! Bending inner ear using custom angle-tipped pliers (Side view)
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Bend outer ear in same manner, again keeping bending adjustments to a minimum." One inner ear bent
Square the ears, that is, bring them to 90° angles with the (Top view) frame, using special 0.9 mm wide, square-tipped pliers to pinch the frame between the ears (do not flatten the 20° frame bend), and a second pair of pliers (the Custom pliers positioned for angle-tipped pliers work well) to squeeze the ears squaring MFT “ear”’. Square firmly against the square tip pliers, always keeping tip pliers are positioned between bending adjustments to a minimum." the ears. Angle tipped pliers are
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used to bend the ears against the square tip pliers (Top view)
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1.3. Polish Frame Edges" Use a #11 scalpel blade & a miniature 3-edged scraper to remove all sharp edges from the oblong holes; these will otherwise cut through the Thermaleze insulation of the SG interconnect wire, and damage the PTFE insulation of the lead wires as they are pulled through. Cut 600 grit abrasive paper into thin strips of approximately 0.8 -1.0 mm width and 3.5 -4.0 mm width. Use abrasive paper to polish the inner edges of the frame (it is essential to remove all inside sharp edges, which could otherwise cut the muscle!)."
Bent ears (left) & squared ears (right)
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Squared ears
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Drill through bearing holes in all 4 ear tabs with #76 (20 mil) drill held in a pin vise. This will allow a 26 ga (18 mil OD) hypodermic Attachment Pin to be inserted without excessive force that might damage the Parylene coating (assuming 20 micron, 2 x .0008 in, Parylene coating)." Size “tie-down” holes with a #76 drill."
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1.4. Sandblast & Clean" Use 25µ Al2O3 at 40 psi. Wash off sand particles with running water, and clean with CMS degreaser."
Unpolished inner edge !
2. Gauging & Wiring"
Polished inner edge !
2.1. Apply Epoxy Base coat to Frame" Clean sandblasted frames with acetone, M-Prep Conditioner A, and M-Prep Neutralizer 5A. M-Bond 610 is solvent-thinned, and can provide a thin (~0.5mil) very hard coating. We grasp the frame by in the center of its uninstrumented side with narrow-tipped locking forceps, dip it into the bottle of M-Bond 610, withdraw it, and quickly shake the excess off sharply. We then examine it to verify that all surfaces (except those grasped in the forceps) are coated, including the edges of holes, that no holes are filled, and that no foreign materials have been incorporated. If OK, bake to cure. If not remove the uncured epoxy with solvent and repeat." It is essential that SGs, solder terminals & wires be electrically insulated from frame, or an encapsulation leak anywhere on the frame, which might otherwise be innocuous, will short the circuitry to surrounding tissue (see below)."
2.2. Fabricate Solder Terminals" Solder terminals must be large enough to accommodate wires, but no larger. A large terminal provides a large surface over which electrical leaks might develop."
Polished inner edge on left, and unpolished inner edge on right !
Shaped frame before sandblasting
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Frame after sandblasting
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Outer solder terminals (top & bottom) are modified MST-035 Outer solder terminals. A strip of terminals taped to a piece of cutting surface terminals to make it easier to hold and manipulate. It is then cleaned with acetone & prepared with acid (SS) flux. A 510 °F iron, carrying a ! small amount of 430 °F solid core solder is swept across the strip, leaving a blob of solder on each terminal. Groves are then cut with a #15 blade as shown for the gold SG wires, and the corners on one side chamfered. Depth of the solder blob and grove must be sufficient to secure the gold wire for soldering. “Right” & “Left” versions are the same." Bottom Inner solder terminals are cut using #11 and #15 scalpel blades from Micron Instruments MST-094 Bondable Terminals, as shown. Terminals are cleaned with acetone & prepared with acid (SS) flux. A 510 °F iron, carrying a small amount of 430 °F solid core solder is swept across the strip, leaving a shallow blob of solder on each terminal. Neutralize and clean. Groves are then cut through the solder blob with a #11 blade for the gold SG wires and Thermaleze interconnect wires, as shown. The fiberglass terminal base that extends over the oval hole is left in place, because all wires are routed to the top of the frame by way of the other oval hole. “Right” version is shown; “Left” version is a mirror image."
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Top Inner solder terminals are similar to the bottom inner terminals, except the lead wire region needs only to accommodate a single Thermaleze interconnecting wire, and so is smaller. The excess fiberboard above the neck of the cut terminal will cover the oval hole in the frame through which wires must pass, but is left in place until after bonding to the frame because removal now would leave the terminal too weak to hold its shape. The fiberglass terminal base that extends over the oval hole is removed later. “Right” version is shown; “Left” version is a mirror image."
Bottom Inner solder terminal. Damage to the terminal is minimized by making cuts in the order shown. “Right” version is shown; “Left” version is mirror image.
2.3. Bond & Connect SGs & Terminals" Complete one side at a time, except for attaching interconnect wire & leads. Use a matched pair of SGs for each MFT. Open a box of SGs. Gauges are very brittle and their 50 ga gold lead wires very soft & fragile. SGs must be handled by the ends of the gold wire leads only." Gently bend a gauge towards its leads so that the center of gravity of the cut leads will fall in the center of the gauge during epoxy curing. Cut the leads with a scalpel, lift the gauge with forceps, and place it in acetone to remove any contaminants that might impair bonding. Place previously prepared terminals in acetone." Mix a batch of AE-15 epoxy. Use a pin tool to distribute a small amount over the areas to which SG and solder terminals will be bonded. Use enough so that SG is completely bonded, and just enough to tack down the terminals, but not so much that epoxy flows into the solder terminal groves and impairs soldering." Cut a strip of PTFE tape about 1mm wide (Glide dental floss?) and loop it around the frame. It will be used to tie down the SG, pressing it into the epoxy to create a thin, complete glue line."
SG tied down to epoxy with PTFE tape
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Top Inner solder terminal. “Right” version is shown; “Left” version is mirror image.
Place an SG on the frame, making certain that a fillet of epoxy forms at the edges of the gage. Do not allow SG leads to fall into epoxy, which will make them impossible to solder."
SG Interconnecting wire: Heavy duty build AWG 40 copper magnet wire (oemwire.com) 4.5 mm long, 1/2 mm tinned at each end.
Tie down the SG as shown." Position the terminals on wet epoxy. Position the frame so that the gauged surface is horizontal and bake."
SG Interconnecting wire on Bottom: With iron at 410 °F and 360 °F solder, solder interconnect wire into groove as shown.
Gently lay the SG gold wires in their terminal grooves." Apply a small amount of SS flux with pin tool, and solder using a miniature screwdriver-shaped tip (eg, Weller MT211, 0.45in x 1in) on a 410 °F iron carrying a small amount of 316 °F solder (wipe rosin from the iron). Clean the joint of flux and neutralize residual acid."
SG Interconnect wire on Top: Similarly, solder other end to top pad as shown.
Repeat on other side." A bundle of lead wires, prepared for etching.
2.4. Solder Interconnect Wires to Terminals"
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Inspect terminals and add AE-15 epoxy, if necessary, but only enough to achieve firm attachment." Trim excess fiberglass from top solder pad only. Reapply AE-15 around oblong hole and bake" Next, prepare the Thermaleze insulated copper SG interconnect wire. Using a 750 °F soldering iron, heatstrip and tin 1/2 mm on each end of a 4.5 mm length of 40 ga magnet wire. Tinning must be done quickly (< 1 sec) to avoid heating the body of the wire and damaging its insulation." Solder the interconnect wire to the terminals, as shown. Thermaleze insulation may be unreliable, so position lead wires where they will not press the interconnect wire against the edge of the oblong hole. Route the interconnect wire so it does not touch the frame where it passes through the hole or elsewhere. To secure spacing, apply AE-15 and bake."
Custom MFT forceps holder and wire manipulator are used to position lead wire for attachment. Joint is prepared with a drop of SS flux, and the soldering iron is fitted with a micro-screwdriver tip.
2.5. Prepare Lead Wires" Lead wire sealing begins by making the initial segment of the PTFE insulation bondable, using Tetra Etch. To prevent the etchant from wicking into the multi-strand SS wire, we cut lengths of AS-631 wire, twice as long as desired for the final lead (~ 2 x 12 in), strip & tin free ends (~ 1 cm), bend the wires into a “hairpin” and dip the bend into etchant. FEP etching should take about 30 sec with fresh etchant, longer otherwise, and should leave the surface light brown and wettable by water. Excess etchant is then removed with MEK before cutting and 13 of 29
stripping the leads for soldering. It should be sufficient to etch about 2 cm of each lead wire. Etched PTFE is stored in a desiccator under vacuum, protected from UV light until bonding." Strip etched end of lead wires, tin with 430 °F solder & trim to 0.75 mm length. Clean lead ends of acid flux and neutralize with M-Prep Neutralizer 5A."
2.6. Attach Lead Wires to Terminals" Working first on the bottom and then on top hold the lead wires in place with the custom wire manipulator, and using an iron at 410 °F, solder them to the terminals with 361 °F solder. On the bottom, first solder the terminal shared with the SG interconnect wire, then the small corner terminal." Projecting wire tips and solder “peaks” tend to project through the encapsulant and must be kept from forming or removed." Clean device thoroughly with M-Prep Neutralizer to remove flux, which tends to wick into wire strands, then soak in alcohol and blow dry."
Bottom lead wires: Lead wires are prepared by tinning and treating with Tetra-Etch. With iron at 410 °F and 361 °F solder, solder lead wires as shown.
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Top lead wires: Attach upper lead wire. Then bring bottom leads up through oblong hole. Braid ~ 1/4 inch.
Etched silicone rubber tubing collar tied with a 6-0 Prolene suture. This will secure collar to tie-down and prevent encapsulant applied to frame spreading to braided leads.
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A second tie holds the collar to the frame.
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To prevent untying, suture
ends are made to ball-up by Within the oblong hole, position the SG approximating a 510 °F clean interconnect wire so it is not pressed against the soldering iron. edges of the hole. Then thread the 2 bottom wires ! through the oblong hole. Braid ~1cm of the lead wires. Cut a 5 in length of 6-0 Prolene suture and secure the lead wires to the frame at the more distal notch in the frame (see picture). Cut free ends, fuse to “mushrooms”, and secure knot with cyanoacrylate. This suture serves two functions:"
• Anchors the lead bundle to the frame, and" • Separates the AE-15 epoxy that protects the frame and its mountings from moisture from the EP-1121 epoxy that provides strain relief at the lead exit." The Tetra-Etched lead wires must be bonded within a few days unless protected from UV light and moisture."
2.7. Route Leads" PTFE-insulated lead wires can be effectively sealed to epoxy encapsulant by preparing them with Tetra-Etch (see table). We use two types of epoxy, both of which have low uncured viscosity and cure at 65°C in 2hrs:" • Vishay AE-15 – cures hard and seems highly resistant to moisture (tested by soaking in warm saline)." • ResinLabs EP-1121 – cures rubbery and has very poor moisture resistance; it seems to both transmit and absorb water."
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Material
Adhesive
Primer
Bond Peel Strength
Silicone
Cyanoacrylate
Tetra-Etch or 770
Exceeds strength of material
Silicone
Cyanoacrylate
Good
PTFE
Epoxy
Tetra-Etch
Good
PTFE
Cyanoacrylate
770
Fair
PTFE
Cyanoacrylate
Tetra-Etch
Poor
Silicone
Epoxy
Tetra-Etch or 770
Poor
PTFE
Epoxy
770
None
We use EP-1121 to provide strain-relief where the braided leads depart from the frame, and to prevent the harder AE-15 from wicking into and stiffening this region." First, we build a boundary at the Prolene suture with a small amounts of EP-1121 on the lead wire side and AE-15 on the inboard side. EP-1121 should lightly coat a few mm of the, while the tougher AE-15 should encapsulate the Prolene suture."
2.8. Finish Braiding Leads" Finish braiding lead wires, and slip on a silicone sleeve (25mil OD, 12mil ID, 3mm long) near the end to keep the braid from loosening. It will later be slid up to the MFT frame."
3. Encapsulate in Epoxy & Serialize" Parylene C will provide the primary moisture barrier for SGs, on-board wiring, and the frame itself, but the strain-relief assembly and surface wiring must first be fixed in place, and the gold SG wires protected, prior to Parylene coating. Protruding features must also be buried to prevent Parylene covering these spots from being selectively abraded." Clean frames by agitating in alcohol bath, blow-drying with compressed air & then warm air. Inspect for adherent fibers and other foreign material." Cover the entire frame with AE-15. Apply the initial coat slowly, and avoid trapping air bubbles, particularly in crevasses. Take particular care to completely coat terminals, SGs & wiring. Apply sparingly, in multiple coats (~ 5) to avoid pooling." Excessive thickness over the SGs may reduce their sensitivity. The aims are to [1] bond firmly to the Tetra-Etched PTFE insulation, producing a moisture barrier, [2] reduce the overall surface area needing to sealed with Parylene, and [3] provide a secondary moisture barrier in case of damage to the Parylene coating" Hold the MFT in a 1 ml syringe with a slot cut in the tip (see figure). Use a pin tool to apply enamel paint (Testors, Rockford, IL) code dots according to the following scheme:"
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Color code: 0-255.
Bake to dry. Protect the serialization dots from the pressure of MFT forceps jaws with a coating of AE-15." Measure resistances of SGs and record."
4. Parylene C Moisture Barrier" Check sizing of bearing holes in ears and tie-down holes with a #76 (20 mil) drill held in a pin vise. Verify that all holes are completely de-burred to ease insertion of Attachment Pins and tie-down suture needles." Construct a lead wire mask from 4 nested silicone rubber tubes, each slipped up from the end of the braid: [1] a 3.0mm length of 25 mil OD x 12 mil ID tube, [2] a 4mm length of 37 mil OD, 20 mil ID tube, [3] a 5mm length of 65 mil OD x 30 mil ID tube, and [4] a 2ft length of 77mil OD x 58mil ID tube. Use alcohol to clean frame & wires and to lubricate wires." The masking tubes must be left open at the end because Parylene is applied in a vacuum. Since the masking tubing has an inner diameter of 0.058 in, and Parylene-C has a creep ratio of 200:1, make the tubing about 12-14 in longer than the leads to avoid coating the leads from the open end." The masking tubes must not touch the frame (including the frame’s epoxy coating) because it would interfere with a complete Parylene coating. " Thoroughly clean the frame and masking tubes before putting them in clean plastic Petri dishes to be shipped for Parylene coating. " Devices are cleaned and “etched” with cold plasma to improve adhesion. Parylene-C is vapor-deposited to a thickness of 20 microns. Annealing, which would otherwise be useful to toughen the coating cannot be used because the high temperatures required (180 - 200 °C) would melt the solder joints (316 °F = 158 °C) and damage the epoxy."
5. Remove Masking Tubes" The Parylene coating does not adhere firmly to silicone, but care is required to remove the masking tubes from the MFT. Use a scalpel to cut the Parylene coating of the masking tubes at the point the largest tube joins the next largest tube. Slide the big tube off. Then use a small spring scissors to cut the smaller tubes open, and to cut the Parylene at the top of the smallest tube."
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! ! Nested silicone masking tubes abutted to strain relief sleeve to protect lead wires from Parylene.
!
Tubing-protected leads are coiled for shipping & coating. Drafting tape holds the coiled leads to the plastic dish to prevent damage.
!
After Parylene C coating surfaces appear dulled.
!
Removing Mask Tubes
Next use forceps to peel away the outer masking tube. Lift to move forceps around the junction of the nested masking tubes. Add alcohol; use gentle twisting motion to remove outer masking tube. Clean any excess Parylene from leads." Seal the torn parylene edge with EP-1121 and bake."
6. A Case of Poor Parylene Adhesion" Inadequate cleaning or failure to prepare the substrate with effective adhesion promoters will result in a coating that may look good, but will fail in use."
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Finished MFTs. From left to right: “Left” version viewed from top, “Right” version from bottom, “Left” version from top, all with Attachment Pins in place.
!
! !
Poor Parylene adhesion. Although a coating may look good, and even pass leak tests (left), improper surface preparation will result in poor adhesion and later failure. In this case, despite clear, written directions to use effective adhesion promoters, our coating contractor failed to do so, and the coating was easily separated from the substrate with forceps (center), and completely removed (right).
7. On-Board Electrical Test" Measure and record resistance of each strain gauge. Determine top from bottom strain gauge by applying gentle pressure to frame edge."
8. Leak Testing & Localization" Suspend MFTs in a warm, stirred water bath for several days or weeks, periodically testing for leaks between grouped lead wires and bath with Fluke 189 Multimeter."
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MFT Leak Detector. Up to 8 MFTs are suspended in a 40 °C, continuously stirred, tap water bath, such that the resistance between the grouped lead wires and the bath can be measured. Each set of lead wires is held in a clip, and the clips can be joined to measure multiple
Leaks can be localized by immersing the MFT in saline, connecting it to the negative terminal, the bath to the positive terminal of a suitable voltage source. At the cathode, H2 is liberated from H2O, and Na from NaCl. The liberated sodium tends to pull OH– ions from H2O, liberating more H2. Thus, H2 bubbles appear where the bath contacts electrically live regions.
An MFT with a 20 MOhm leak to the warm water bath is seen to have a leak local to the Prolene suture. MFT Leak Localizer, producing 0-1000 V. MFT in saline bath is at lower right.
An MFT with a 10 MOhm leak to the warm water bath is seen to have widespread leaks due to poor insulation of electrical components from frame.
9. Preparing Attachment Pins" Use thick-walled 26 ga & 27 ga hypodermic tubing (one or the other size will slide in easily enough to avoid damaging Parylene coating, but fit tightly enough to stay in place). Cut hypodermic tube to 6 mm, a length equal to the width of the frame. Hold hypodermic tubing in pin vise, cut to size with Dremel, smooth rough edges with abrasive paper, polish edges on an Arkansas stone."
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10. Final Testing & Calibration" Measure and record resistances. Test sensitivity (and, possibly, hysteresis and creep). We calibrate each MFT by threading a length of 1/8” wide cassette tape through it, as the muscle tendon would go, and hanging weights 5-80g, in ascending and descending series." Power-on the Strain Gage Conditioner and Amplifier System unit. Use small screwdriver to set excitation of leftmost channel to 2 Volts, zero the amplifier so that both balance lamps are off. Next turn excitation on and adjust the Balance control so that both lights are off. Record this value. Add 80g load, adjust gain so that output value reads approximately 1000mV, record value."
11. Connector Can" Device failure can be caused by moisture intrusion at the solder joint between the MFT lead wires and connector pins on the head. We now protect all connections with a Connector Can, into which the braided MFT leads run uninterrupted through a small hole sealed from body moisture with silicone. All joints and exposed metal are elevated inside the Can for protection and to enable inspection (see Figure)." 0.025" square gold-plated beryllium copper PCB Pins 1/2" long are press-fit as follows. Optionally, holes can first be sized with a #71 (0.026”) drill. Pins are cut to size and ends are chamfered. Press-fit a block of 6 pins, and remove the plastic matrix after, using a small saw or hot soldering iron to cut the block into sections (be careful not to heat the pins, which will loosen them from the connector can base), and a sharp knife to remove remaining plastic. "
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Bench Calibration, showing details of MFT positioning, weight application, and compensation for imperfect SG matching (better SG amps do not require external balancing resistors).
MFT Connector Can is fabricated by stereolithography from “MicroFine Green” resin, an ABS-like plastic, at Fineline Prototyping (www.finelineprototyping.com) and fitted with gold-plated IC pins with standard 0.1 in spacing. Leads from 2 MFTs can be accommodated. The leads enter the hole near the bottom, which is then sealed with RTV silicone, wrap around the internal pedestal, and are soldered to connector pins near their bottoms. Standard connectors plug into the free ends of the pins. A screw cap protects the pins when not in use.
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12. Other Types of MFTs" 12.1. MFT for Acute Procedures"
MFT to be used intraoperatively with snap-in Attachment Pin. As shown here, the space between the ears has been filled with epoxy, and a #76 drill is used to cut a clearance hole for the pin. (Top view)
MFT with completed cross bar preparation. A slight curve in ears must be maintained for the Attachment Pin to snap and lock into position. Notice the limited amount of epoxy outside the ears, leaving the strain gauge unobstructed. (Side view)
In addition to the areas of usual epoxy coating, build up the epoxy between the ears of the MFT so that there is a small hill of epoxy between them. Use #76 drill and low speed dremel tool to cut through epoxy between the ears so that a 27ga tube can easily slide through. File to flatten top of epoxy coating and ears with a fine triangle shaped file. Prevent file from scratching the epoxy by shielding strain gauges and wires with fingers. File one pair of ears at a time, check and recheck work, be sure the height of the ears is maintained by inserting a 27ga tube. Next, remove 27ga tube and use the major edge of a fine diamond shaped file to file through ears and epoxy, opening the gap to lightly fit a 27ga tube (see above photo on the right). Polish rough edges with a modified razor blade: one surface covered with 400 grit abrasive paper. A 27ga tube should lightly snap into position between finished ears without force, finished ears will look similar to photos. "
! !
MFT/H-3 frame with cross bar preparation complete: epoxy drilled, filed and polished to fit the Attachment Pin.
MFT/M-2.
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12.2. MFT for Intraoperative Use in Humans"
! Plan of human MFT (MFT/H-3).
"
! !
Device similar to MFT-2.10, but designed for human intraoperative use. Human device has a complete Wheatstone bridge on board, 2 strain gauges on top surface (one to measure strain and one for temperature compensation on the top surface which is exposed), and 2 strain gauges on bottom surface (one to measure strain and one for temperature compensation on bottom surface which is at body temperature). This requires four rather than three lead wires. (Top view left, bottom view right)
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12.3. MFT for Human Digit Flexor Tendon"
" Appendices" • Vishay M-M M-Bond 43B, 600 & 610 Adhesive Use" • Vishay M-M M-Flux SS Use Instructions
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