tDCS Electrode Guide
Caputron primer on electrodes for tDCS
Numerous types of electrodes exist in the market today that can be utilized for tDCS, CES, TENS, Iontophoresis, and other electrical stimulation techniques. To help you better understand these electrodes, the engineers at Caputron have outlined the different types of electrodes, as well as the advantages and disadvantages of each electrode type.
Parts of the electrode: The electrodes used for tDCS are, along with the device, the critical components for a reliable and quality stimulation. Understanding what makes a good electrode starts with understanding the components of an electrode. The term “electrode” itself can be a little confusing because technically it refers to the surface where the metal (or conductive rubber) touches the saline (or gel). But in common tDCS parlance the electrode actually refers to the entire assembly that includes 1) the electrode proper (conductive rubber, or metal mesh/sheet), 2) any sponge material used to hold saline (preferably 0.9%), 3) any support material such as non-conductive /silicone used to support the shape, help connect it to the strap, and any sticky material used to hold the electrode, 4) any conductive gel that comes on the electrode. When people add saline or conductive gel, they often refer to the now wet assembly as the electrode itself as well. To help simplify the electrode selection process, Caputron made the simple comparison table below, where we further explain each style of electrode you might see suggested for tDCS. We also list electrode parts. Without a doubt, sponge electrodes are the most common and reliable complete electrodes, and Caputron designed our model to provide a clinical/research grade and affordable option.
Don't touch the wire: Why not just use a dry electrode or put the metal of the electrode (or even the pin of the electrode cable) or the conductive rubber on the skin when you run tDCS? Because that is a guaranteed way to get a burn and something that is never done in tDCS human trials. The basic reason is this: inside your tDCS device and down the wires to the electrodes, current is carried by electrodes. In your body current is carried by ions. So at some point, the nature of the charge carrier needs to be converted from electrodes to ions. This happens at the electrode proper (see above what that is). It is right at the electrode that this conversion happens and as a result, you get a chemical reaction. Chemical reactions mean “bad things” can be created like pH changes that irritate the skin. So what the sponge, or gel do is they push back the electrode proper away from the skin. They keep the electrode proper (the material or conductive rubber) from touching the skin. The “bad things” are created inside the saline/sponge or gel and don't make it to the skin. Better tDCS electrodes might be electrodes that are better at keeping the “bad things” from reaching the skin, and this is why thick sponge-electrodes might work better than thin adhesive electrodes.
Don't forget the cable: One more thing. In addition to your device and electrodes, you have the cable (or two cables) that attaches them together. It's a mistake to think any “old” cable will do since a bad cable can completely compromise the performance of the electrode and also the device (see the Caputron device primer). For example, if you use an exposed alligator clip on the sponge there is no real control over the shape of things or anything stopping the clip from touching the skin. Caputron provides a range of medical-grade shielded and snug connectors matching devices to electrodes. And Caputron packages a range of kits to help you make sure you have everything you need to simply start right.
Sponge Electrodes
Sponge electrodes are the most common and prominent type of electrodes preferred by leading researchers and clinicians for applying tDCS.
Caputron tDCS electrodes are comprised of a conducitve carbon-rubber electrode with 4 mm banana connection, a sponge insert, and a non-conductive silicone housing. Caputron electrodes come in both a 2" x 2" and 3" x 3" size. Our tDCS electrodes are commonly used in clinics and leading universities worldwide. Requiring no maintenance, just replace the sponge inserts periodically to maintain an effective tDCS session. Our design uses a strong flexible material that can more easily contour to the shape of the head than the Amrex tdcs electrode, providing better contact quality between the electrode and the user. Combine these electrodes with the Caputron Universal Strap for an easy to set up and reproducible tDCS session.
Advantages:
- Low cost and reusable
- Allow the best connection quality, even with areas containing thick hair, providing uniform current delivery
- Sponge based electrodes are preffered in clinical trials
- Conductive carbon-rubber is preffered by researchers over metal mesh
- Metal mesh electrodes can potentially cut the electrode housing or your finger if not carefully removed
- Saline is an excellent conductor of current providing minimal skin irritation
Disadvantages:
- Drip if over saturated. An application of ~7 mL for the 2" x 2" and ~15 mL for the 3"x 3" is recommended.
Made directly by Caputron, we are able to offer these high quality tDCS sponge insert replacements at a much lower cost than Amrex. These sponges currently come in the conventional 2" x 2" and 3" x 3" sizes that fit their respective size tDCS electrodes.
Advantages:
- Low Cost
- Easy to replace
- Even current distribution
- Work well against areas with thick hair
- Are offered in an Extra Absorbent configuration, allowing for longer session run time without having to apply more saline
Disadvantages:
- Drip if over saturated. An application of ~7 mL for the 2" x 2" and ~15 mL for the 3"x 3" is recommended.
Amrex electrodes are comprised of a sponge insert, a metal mesh with 4mm banana connection and a rubber housing. Amrex electrodes come in a variety of sizes, with 2x2 and 3x3 being the most common, and have a higher up front cost but a cheaper cost to own over time as all they require are replacing the sponge inserts. Combine them with the Caputron Universal Strap for an easy to set up and reproducible tDCS session.
Advantages:
- Allow excellent connection quality, even with areas containing thick hair, providing uniform current delivery
- Sponge based electrodes are preffered in clinical trials
- Saline is an excellent conductor of current providing minimal skin irritation
Disadvantages:
- Drip if over saturated
- Rigid design makes them difficult to contour to the shape of the head
- Metal mesh electrodes can potentially cut the electrode housing or your finger if not carefully removed
Amrex sponge insert replacement come in a variety of sizes to fit into their respective rubber housing. Try the Caputron brand sponge insert replacement pads as a higher quality and economically friendly alternative.
Advantages:
- Easy to replace
- Relatively low cost
Disadvantages:
- Drip if over saturated
The Soterix Medical EasyPads provide for simple and consistent set-up. Made to be used either with the EASYstrap or Caputron Universal Strap, these sponge pockets use a rivet design to both hold the necessary conductive carbon rubber electrode in place as well as minimize current concentration at electrode edges that lead to current-density "hot-spots".
Advantages:
- Allow the best connection quality, even with areas containing thick hair, providing uniform current delivery
- Allow easy addition of more saline during a session due to the sponge being readily accessible
- Combining both saline and carbon rubber electrodes, EASYpads are the preferred sponge type of researchers and clinicians
- Easily reproducible results when combined with the EASYstrap or Caputron Universal Strap
- Low start-up cost
Disadvantages:
- Drip if over saturated
- High cost over time due to the recommended one time use of these sponge electrodes
Self-Adhesive Electrodes
Sponge electrodes are the most common and prominent type of electrodes preferred by leading researchers and clinicians for applying tDCS.
Self-adhering are flexible and comfortable, come in a variety of sizes and can be extremely cheap. They come pre-gelled so all you have to do is peel and stick. Most will have a 2mm female pin connection.
Advantages:
- Easy to use - simply peel and stick
- Excellent for CES and targeting the cranial nerves when placed on the temple
- Wide variety of shapes and sizes
- Low Cost
Disadvantages:
- Limited stimulation areas as they are unpractical for areas containing thick hair, such as on the head, and therefore not useful for tDCS
- Not recommended for DC stimulation
- Can fall off easily if overused or not secured with a strap, potentially leading to skin irritations and small burns
Self-adhering and reusable, hydroPods are the simplest of the electrodes to use. They come pre-gelled so all you have to do is peel and stick. hydroPods use a 2mm female pin connection, compatible with the mindGear stimulator. These electrodes are CE marked, FDA 510K cleared, and Health Canada approved.
Advantages:
- Easy to use - simply peel and stick
- Excellent for CES and targeting the cranial nerves when placed on the temple.
- Wide variety of shapes and sizes
- Low Cost
- CE Marked
- FDA 510K Cleared
- Health Canada Approved
Disadvantages:
- Limited stimulation areas as they are unpractical for areas containing thick hair, such as on the head, and therefore not useful for tDCS
- Not recommended for DC stimulation
- Can fall off easily if overused or not secured with a strap, potentially leading to skin irritations and small burns
Recommended for use with TENS and muscle stimulators
Advantages:
- Easy to use - simply peel and stick
- Low Cost
Disadvantages:
- Not recommended for DC stimulation
This electrode has an extra dense sticky gel that provides a very good connection with the skin.
Advantages:
- Easy to use - simply peel and stick
- More reliable than your standard sticky electrodes for areas containing hair
- Thick adhesive gel
- Can handle DC stimulation to some extent
Disadvantages:
- Higher than average cost for a sticky electrode
Carbon Rubber Electrodes
Excellent conductivity and a very good electrode if used correctly. These electrodes should NOT be placed directly on the skin!! They require the use of either a sponge pocket or a conductive gel. Depending on method of use they can optimized for either tDCS or TENS.
These carbon rubber electrodes utilize either a 4 mm bananan or 2 mm pin connection. To be optimized for tDCS, they are placed into a Caputron tDCS electrode or Soterix EASYpad. Do not place these electrodes directly on the skin.
Advantages:
- Excellent conductive material
- Very long lasting lifespan
- Come in a variety of shapes and sizes
- Can be easily made into custom sizes
Disadvantages:
- Require the use of a liberally applied conductive gel or sponge pocket
Flextrode Electrodes require the use of a conductive medium to be used. Do not place these directly on the skin. Combine these with disposable pad covers. and apply gel liberally.
Advantages:
- Excellent conductive material
- Very long lasting lifespan
- Come in a variety of shapes and sizes
- Has the option to use disposable pad covers for sterilization between patients
Disadvantages:
- Require the use of a liberally applied conductive gel
- Require a strap to keep them in place
- Some are hard to contour to the shape of the area being stimulated
High-Definition (HD) Electrodes
The electrode of choice for HD-tDCS stimulation. These electrodes are used in combination with an HD-Holder as well as a conductive gel.
Advantages:
- Small and accurate stimulation area
- HD-tDCS is naturally compatible with EEG
- Can be used for multiple sessions before they must be replaced
- Their small size allows the user to easily place many electrodes and direct current flow to optimize brain targeting
Disadvantages:
- HD-Electrodes are expensive
- Fragile and can easily be broken if not handled carefully
- Require the use of a conductive gel
Animal Electrodes
Comprised of various combinations of conductive metals. These electrodes are great for animal studies.
Advantages:
- Small and accurate stimulation area
- Its small size makes it excellent for animal studies, especially on mice
- Can be used for multiple sessions before they must be replaced
Disadvantages:
- Metal electrodes can be expensive
- Fragile and can easily be broken if not handled carefully
- Require the use of a conductive gel or saline
