Flexible boron-doped diamond spiral electrode for application in brain–computer interface based on steady-state visual evoked potential

•The flexible BDD spiral electrode and its 19-lead EEG cap are reported for the first time.•Strategy of polycrystalline BDD film equivalent to diamond crystal microelectrode array.•Flexible BDD spiral electrode has low skin-contact resistance and high SNR of EEG.•Flexible BDD spiral electrode-based brain-computer interface control robot arm drawing diamond.