tDCS is (as the name indicates) a constant current, whereas it looks like the temporal interference (TI) technique has two oscillating channels tuned for targeting deeper brain regions:
>TI stimulation is a novel brain stimulation strategy that employs two or more independent stimulation channels delivering HF currents (oscillating at f1 and f1 + Δf) within the kHz range, which are assumed to be inert in terms of inducing neuronal activity6,82. The two currents generate a modulated electric field, with the envelope oscillating at the low-frequency Δf (target frequency) where the currents join or cross. The peak of the envelope amplitude can be steered toward target areas located deeper in the brain by tuning the electrode position and current ratio across stimulation channels
The gear is pretty similar for tDCS, tACS (which uses alternating current at the "target" frequency"), and TI. Our data suggests that you can hit deep brain structures with all of them (https://www.pnas.org/doi/10.1073/pnas.1815958116) but TI has the advantage of focality: it avoids stimulating things between the scalp and the target.
The other major difference is how it affects the neurons. tDCS, tACS and even TMS create electric fields that electrically polarize the neurons. TI...doesn't. The stimulus itself changes too rapidly and only the envelope fluctuates at the "target frequency." However, that envelope isn't real: something has to extract/demodulate it from the carriers and it's not entirely clear how--or how well--that happens.
