For neuroscience research, the delivery of light for optogenetic stimulation and recording of numerous brain regions is often achieved through invaluable tools known as fiber-optic patch cords. These flexible fibers (similar to the data transmission lines that brought you this information via the interwebs) help researchers tease apart the various elements of neural circuitry. They are powerful tools to ‘patch’ an optic source (lasers) to a tissue of interest (brain). However, due to current fabrication techniques and general understanding, most patch cords are limited to a single fiber-optics. The data/information/light transmission, or in short the primary toolset available to neuroscience, would be greatly expanded if the simple patch cord were to allow for either the splitting or combining of data/information/light.
Enter the split patch cord: a simple yet highly effective modification of the standard patch cord that opens the door to these areas of research. By simply shoving two separate optic cords through the same ferrule or optic-source connector of your choice (SMA or FC/PC), you can now:
- simultaneously stimulate multiple brain regions with the same optic source
- combine multiple optic sources for the stimulation of a single brain region (excitatory and inhibitory stimulation?! Oh, the POWER!)
- record from multiple brain regions
Below, we will give you a step-by-step guide on how to build your own split patch cord for very low cost. Once you have gotten comfortable with making one (it might take a few tries, as some steps can go wrong and force you to step back or even start over), you can even try adding your own modifications to the design (dare you to try three different wavelengths on one brain region?!).
Building Your Own Split Patch Cord
If this is your first time building a fiber-optic patch cord, we recommend that you check out the Fiber Optic Fabrication video in our external resources page or email us to discuss training for building standard patch cords. While this guide may provide insight on how to make a standard patch cord, some techniques will be much easier to learn when you only have one fiber-optic cord to deal with, and therefore save you headaches and frustration down the road. If you are already a patch cord pro, then let’s get to making our very own split patch cord!
This design is so simple, we call it elegant. Two fiber optic cables are fabricated with their usual connections. Then the fiber optic cords are aligned on the other ends. These ends are then simply shoved into a new connector, of diameter large enough to accept both cords, and then both cords are cut and polished. There is expected to be an efficiency loss and care should be taken for using the correct connector types see Extras section below.
NOTE: When designing your own custom optogenetic patch cord, there are several key parameters to consider: numerical aperture, modes (if you want to get really fancy), auto-fluorescence (if recording), and core diameter. The split patch cord shown here is fabricated from two 200μm core diameter fiber-optic cords. These are fitted individually into 230μm ferrules. The other ends are combined into a single 440μm ceramic ferrule. You should adjust the sizes of the cord/ferrules to suit your needs, just remember that at the “joined” end of your split patch cord, the inner diameter of your connector must be larger than 2x the diameter of your optic cord. Ideally your “joined” connector has some wiggle room when both optic cords are run through it (about 20-50um), as even with this space it will be a tight squeeze. If your connector is smaller than this requirement, then you are doomed from the start.
In order to make this split patch cord, we will need the following things:
- 200μm optic-fiber cord x2 (the length of these optic-fiber cords should be at least 8-10cm longer than your desired length, but we recommend an even greater buffer in case your first attempt(s) to join them goes wrong)
- 10.5mm length ceramic ferrule with 230μm bore x2
- 10.5mm length ceramic ferrule with 440μm bore
- 2x45mm piece of heat-shrink tubing
- Optic-fiber cladding stripper (if using a fiber-optic cord with different diameter, be sure your stripper matches its dimensions)
- Loctite Epoxy Quick-Set
- Toothpicks and weighboat for mixing Epoxy
- Straight-edge razor or similar cutting tool
- A hair dryer or heat gun (use a low heat setting!)
Now that we have the needed materials, let’s begin!
Step 1: Prepare the Heat-Shrink Tubing
Cut the tubing into three parts, with one part double the length of the others. Simply cut the original piece in half, then cut one of those halves in half again.
Step 2: Strip the Cladding from the Ends of the Fiber-Optic Cords
(NOTE: If you are using a smaller diameter fiber-optic cord, this is a good time to thread both cords through a suitable sleeve, leaving at least 3-4 cm of bare cord sticking out at both ends. It is also a good idea to coil the sleeved cords individually around a couple fingers and use tape to hold them in place so that stripping the cladding is much easier.)
Using your cladding stripper, carefully strip the cladding from both ends of the fiber-optic cords. Do not try to pull it all off in one go; instead, pull off small bits from the edges at a time (about 1 cm increments) until you reach the desired length of bare cord (at least 2-3 cm). When pulling the cladding off, grip the stripping tool firmly and pull away quickly to cleanly strip the cladding from the cord, or else the tool might slip and only pinch the cladding. If you are finding it difficult to pull the cladding off without having the cord slide between your fingers, try loosely coiling the cord around a few fingers and pinch the joined sections of cord to increase friction and hence your grip.
The cladding on this optic fiber is blue.
Step 3: Feed One of the Stripped Fibers into One of the 230μm Ferrules
Take one end of your stripped fiber optic and feed it into the bore hole of the ferrule. If you are having difficulty in seeing the bore hole, we recommend using a microscope or magnifying glass to make it easier to see. There should be enough stripped cord so that the ferrule can easily sit in the middle of the stripped section and leave a good amount of bare cord on each side. You will need this for the next part. Repeat this step for the other fiber.
Step 4: Epoxy the 230μm Ferrules on to the Stripped Cords
In your weighboat, squirt out a small amount of the 2-part epoxy and mix using a toothpick. Once the epoxy has been fully mixed, use your toothpick to apply the mixed epoxy to the far end of the bare cord with the ferrule on it (moving the ferrule back toward the cladded part of the cord). You only need a thin layer of epoxy to do the trick, but try to make sure the epoxy coats the bare cord on all sides. Once the epoxy has been applied, slide the ferrule away from the cladded cord into the coated section of cord (keeping it on the cord!), and then apply more epoxy in the same fashion to the part of the bare cord where the ferrule just was. Slide the ferrule back to its original location (where you just applied more epoxy), and then continue sliding it back and forth to evenly push the epoxy into the space between the bore hole and the bare cord. If you begin to feel resistance, it means the epoxy is drying, and you should push the ferrule back toward the cladded section of cord, ideally having the ferrule and cladded end touching. Finally, use a Kimwipe or your fingers to gently wipe off any beaded epoxy on the ferrule tip facing away from the cladded section of cord (this does not need to be perfect, so don’t go overboard and risk snapping the extra cord sticking out!). Repeat these instructions for the second cord and ferrule, and let both cords sit for 5 minutes to let the epoxy dry.
Step 5: Feed Both Cords into the 440μm Ferrule
This is where things will start to get tricky, so be patient (and maybe have a spare 440μm ferrule on hand should these next two steps go wrong). Take the non-ferrule end of both cords, and carefully feed the non-cladded ends into the 440μm ferrule. Usually this is easiest to accomplish by partially feeding one cord into the ferrule first (not sticking out the other end yet), and then feeding the second cord in next. When both cords are inside of the ferrule, there will be a lot of resistance since the cords are wedged in there next to each other, but it is important not to twist or bend the cords while feeding them through the ferrule, as they are likely to snap. Instead, try to hold both cords close together between your thumb and index finger, and use the other hand to slowly but firmly drive the ferrule directly down toward the cladded section of the cords. Once you see both ends of the cords sticking out of the other end of the ferrule, you will find it easier to push the ferrule down. However, DO NOT push the ferrule all the way down to the cladded section of the cords. This will likely put too much pressure on the junction between bare and cladded sections of both cords, and have a high risk of snapping one or both cords and ruining your work. Instead, try to leave between 0.5-1cm of bare cord between the ferrule and cladded part of the cords. Be gentle at this point; you are almost done, so don’t rush and break your work!
Step 6: Epoxy the 440μm Ferrule on to Both Cords
Re-make a fresh, small mix of epoxy using another toothpick (don’t try to use the previous mix), and apply a thin layer of mixed epoxy onto the bare cords past the ferrule tip. Carefully slide the ferrule up into the epoxy, and apply another thin layer of epoxy onto the rest of the bare cord (including the part where the ferrule will not be for a stronger patch cord). Like before, slide the ferrule back and forth a few times to push the epoxy into the bore hole, but be extra careful not to push too hard. Ideally, you should have the ferrule back to its original position before the epoxy begins to dry, to prevent any risk of having to use too much force and break the cord(s). Once the ferrule is back to its final position, gently wipe off any beaded epoxy from the ferrule tip, and wait 5+ minutes for it to dry.
(NOTE: If one or both cords break while applying the epoxy, start this part over with a new ferrule. If that happens, simply use a straight razor to cut the cords where the epoxy ends, even if it is cladded cord, re-strip the cords to expose more bare sections, and repeat steps 5 and 6. This is also why it is a good idea to have more cord than is needed if you are new to making these!)
Step 7: Shrink-wrap the Ferrules to the Cladded Cords
Now that the hard part is over, time to give our split patch cord some stability between the ferrules and cords. To do this, take a piece of heat-shrink tubing (use the smaller pieces for the 230μm ferrule ends, and the large piece for the “joined” 440μm ferrule end) and slide it onto the end of the patch cord such that it covers the junction between the ferrule base and the cladded section of the fibers(s). Once it is in place, use a hair dryer or heat gun on a low setting to heat up the tubing, turning and moving the patch cord in the heat so that the tubing is heated evenly. Once the tubing stops shrinking, remove it from the heat and repeat these steps for the other ends of the cord.
Step 8: Cut Off the Extra Fiber Ends from Each Ferrule Tip
Using a straight razor, carefully score the remaining bare fiber from each ferrule tip as close to the ferrule tip as possible. This is achieved by using one light, slow cut in the fiber such that it does not actually snap the cord from the pressure. After doing this, there should be an imperceptible groove in the cord, called a score, which should allow for a clean break. To get this clean break, simply flick the remaining bare fiber from the same side where you scored it using your fingers (like kicking a field goal in tabletop football!), and the remaining fiber should snap off cleanly. Do this for all three sides.
Step 9: Polish and Be Proud!
With our split patch cord roughly completed, all that is left is to polish your ferrule tips so the optic light can come through cleanly. You can also check at this time if your cord had any unnoticed breaks occur during fabrication by passing a light through one end of the cord and seeing if the light can be seen from the other end. If both cords allow light to pass to the joined end, then congratulations, you have successfully built your own split patch cord! Be sure to check your efficiency from both cords after polishing to ensure that they will both provide a similar optic power to your tissue of interest, and be proud that you made this thing yourself!
Incorporation into a Setup
We provide this example as a split cord having all three ends of the cord ending in a bare ceramic ferrule. Bare ferrules are typically of 1.5 mm outer diameter, with the inner diameter (bore) varying in diameter to match the diameter of the optic fiber (we used 230μm fiber). These ferrules can be connected to other ferrules of the same outer diameter using sleeves. This allows for the split patch cord to be incorporated into an existing setup.
Alternatively, one or more of the bare ferrules can be replaced with a different connector type. Lasers typically use “FC/PC” connectors and LED’s typically use “SMA” connectors. By replacing one or more bare ferrules with another connector type, one can directly plug the split patch cord directly into an optic source. But these connector types are metal and heavy, thus adding an additional challenge during fabrication and polishing!
It may be helpful to show the incorporation of a split patch cord into an existing research setup, as shown below. A light source (either an LED or laser) generates optic light and passes through a collimator to focus the light to a small spot. A fiber-optic cable accepts this light and is held in an SMA (LED) or FC/PC (laser) connector. A jacket protects the fiber-optics which ends in a bare ferrule. The connector and ferrule have an inner diameter of 440μm, and the fiber optic cord is 400μm. This constitutes a “patch cord”. The ferrule has an outer diameter of 1.5mm (all ferrules typically are). A sleeve connects the patch cord to the split patch cord via the 1.5mm outer diameter ferrules. The split patch cord starts with a 440μm inner diameter ferrule with two 200μm fiber optic cords inside of the ferrule. These cords split/separate into two ends. These ends are of 1.5mm outer diameter, 230μm inner diameter ferrules. A sleeve connects this ferrule to a mirror ferrule of the same 230μm inner diameter. This ferrule and fiber optic are implanted into the tissue and is called an “optic cannula”.
It is expected that the split patch cord will result in bad optic efficiency. See below for the reasoning.
Luckily optic sources are powerful enough to compensate; the main thing you want to focus on is whether the efficiencies of both cords are reasonably similar enough to meet your needs, especially if you are using the same optic source to stimulate two different brain regions simultaneously (otherwise one brain region is going to get little-to-no stimulation, or the other is going to get fried).
ONE Core acknowledgement
Please acknowledge the ONE Core facility in your publications. An appropriate wording would be:
“The Optogenetics and Neural Engineering (ONE) Core at the University of Colorado School of Medicine provided engineering support for this research. The ONE Core is part of the NeuroTechnology Center, funded in part by the School of Medicine and by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under award number P30NS048154.”