Enhancing the Usability of Brain-Computer Interface Systems

Table 1

Summary of representative dry EEG electrodes.

Type	Fabrication	Flexibility	Ref.	BCI application	Strengths	Drawbacks
Electrode finger	Copper pin	Stiff	[29]	SSVEP	(i) Simple manufacturing procedure for mass production	Subjects felt pain or discomfort from the pressure
					(ii) Small size (d = 15 mm) for good spatial resolution
	Spring loaded pin coated with gold	Soft	[36]	N/A	(i) Small size (d = 15 mm) for the good spatial resolution	Equipment required for electrode fixation
					(ii) High level of geometric conformity between the sensor and the scalp surface
	Flexible polymer pin	Soft	[37]	N/A	Subjects reported that they were more comfortable than the conventional EEG system	Slight erythema was found after 10–35 h, but it faded rapidly after the electrode was removed
Toothbrush	Silver-coated bristles	Soft	[30]	(i) Motor imagery	Better comfort than wet or pin-based electrodes	(i) Requires multistep preparations
				(ii) P300		(ii) Some subjects felt prickling sensation
				(iii) N100
Reverse-curve-arch-shaped	Sterling silver using 3D printer	Stiff	[31]	N/A	Maximized contact area and dispersed pressure	(i) Obtrusive wiring interfaces.
						(ii) Equipment required for electrode fixation
Epidermal (tattoo)	Microfabrication with polyimide	Soft	[33]	(i) SSVEP	(i) Soft, stretchable, and lightweight	(i) Cannot be applied to hairy scalp
				(ii) P300	(ii) Conformal contact, adequate adhesion	(ii) Sensor design problem for people who do not want stick-on, temporary tattoos
Capacitive	CNT/aPDMS	Stiff	[32]	(i) SSVEP	(i) Electrode could be autonomously attached to the scalp without the need for additional equipment.	Require multistep preparations, obtrusive wiring interfaces
				(ii) N100	(ii) Small size (d = 6 mm) for good spatial resolution
	Custom-integrated AFE	Stiff	[40, 41]	SSVEP	No need for external G- or T-ohm for biasing network	(i) Weak for motion artifacts
						(ii) Poor coupling interface through dry hair
	Polymer foam surfaced	Soft	[34, 35]	(i) SSVEP	(i) EEG measurement through hair	(i) Weak for motion artifacts
				(ii) ASSR	(ii) Comfortable for users	(ii) Low spatial resolution due to relatively large size (r = 36 mm)
					(iii) Higher signal quality than with rigid capacitive electrodes