I have been investigating different tDCS devices, thinking a lot about what makes a device or kit good for tDCS use, and I came up with some points for people to consider when they are deciding to build or purchase a device or kit. Any comments or additions would be welcome.
1. Current regulation
The idea is to maintain the level of current that you want. As the battery drains and battery voltage drops, a circuit with current regulation will make sure the output current does not change. If the electrode resistance is not optimal (maybe not enough water or saline, or head band not tight enough), then the current regulator will make sure the output current does not change (as long as the resistance isn’t too high)
There are DIY devices and designs with resistance only designs (and no current regulators) with an external potentiometer to vary the resistance to get the right current. These will work, but any fast changes of battery or electrode resistance will not have an automatic response by the circuit as happens with current regulation.
2. Multiple, selectable, output current levels approximately in the range 1.0mA to 2.0mA
All the research shows the benefits of tDCS in the 1.0mA to 2.0mA range, with a slight warning that side effects and undesirable symptoms may start to happen greater than 2mA.
Microcontroller based systems will have control over smaller iterations of current (commonly 0.1mA) or the current levels may be switched in.
3. Quality of electrodes
The rubber electrodes that are commonly used in TENs are one type that appears to be working and sponge electrodes with a steel plate behind and rubber encasing. I much prefer the sponge electrodes, there is less change of hot spots (as long as you keep them wet). Note that the electrodes may be at least $10 each, so if a device does not include electrodes, know that you will be adding at least $20 to the overall prices.
4. Maximum output voltage on the electrode.
This is a safety concern that comes straight from Ohms law. If the maximum voltage at the electrode is 18V (2 x 9V battery) and if there is a fault in the current regulation/board/resistor chain, and saying there is a good connection of the electrodes on the head of R = 5kW, then even in fault condition, the maximum current would be 3.6mA (not pleasant sounding to me but not immediately dangerous). If the device is designed for other uses, it can provide higher voltages at the electrodes to reach the higher currents. Hence if there is 50V at the electrode, with our same R = 5kW, then the maximum current would be 10mA, a more dangerous level for the brain.
5. Current limiting and fuse protection
There are two scenarios that need to be considered. Firstly, if there is a fault in the regulation/board/resistor chain you can have the maximum voltage of the unit at the electrodes (this is when you want a battery maximum of 18V at the electrodes rather than a switched upregulation). Secondly, if the user makes a very large mistake and puts the electrodes over an open wound or a way for the resistance to be much lower, and if the current regulation is working, the level should still be limited to current regulation. But if both scenarios happen at once, then the current can be dangerously high, and hence special microcontroller current limiting, or hardware current limiting through a resistor is recommended. Also a fuse is often added for the slow response.
6. Turns off when resistance too high (or only turns on when resistance is within a certain range) or when resistance is too low
Higher end devices will have the capability to automatically turn off the output current when the electrode resistance is too high (for example if the electrodes slide off the headband). They will also turn off when the resistance is too low, if the electrodes are touching on a table or a person mistakenly places on an open wound, the resistance can be too low and the device will protect both itself and the user and turn off the output current.
