Table 4.

Details of relevant studies on dry electrodes and evaluation.

	Mechanical	Electrical	Evaluation
Griss 2002	Fundament: array of microneedles. Material: silver or silver-/silver chloride-coated microneedles glued onto a circular double-sided printed circuit board. Dimensions: micrometers. Comfort: Comfortable. Skin showed some redness.	Active electrode: No. Impedance: minimum 0.65–16 k at 1 kHz and 0.6 Hz. Unstable at frequencies below 0.8 Hz. Noise:	Application: anesthesia monitoring systems. Biosignals: EEG. Measure: in-vivo. Procedure: Comparison with Zipprep electrodes. Features: Spontaneous EEG. Location: forehead. Evaluation: visual comparison dry Zipprep electrode.
Harland 2002	Fundament: Non-contact sensor. Material: metallic. Dimensions: disk of diameter 1 to 2 cm. Comfort: remote registering at 3 mm from the scalp, no physical contact.	Active electrode: Yes. Impedance:1015 Ω and. Noise: 70 nV/Hz−1/2 at 1 Hz	Application: multichannel EEG monitoring and real-time imaging of the brain. Biosignals: EEG, ECG. Measure: in vivo. Procedure: Closed and open eyes. Features: Alpha and Beta. Location: P3 and O1: Evaluation: Visual plots of Alpha and Beta modulation. No dry-wet contrast.
Sullivan 2007	Fundament: Non-contact. Material: Metallic plate on the bottom of a PCB. Dimensions: A quarter dollar. Comfort: No physical contact, 3 mm gap. ECG records through clothing.	Active electrode: Yes. Electrode impedance: Not reported. Noise: 2 μVrms at 0.2 mm sensor distance, and 17 μVrms at 3.2 mm distance over 1–100 Hz frequency range	Application: Biopotential recording systems and human machine interfaces. Biosignals: EEG, ECG. Measure: in vivo. Procedure: Open-closed eyes paradigm. Features: Alpha. Location: back of the head, reference behind the ear. Evaluation: visual plots. No dry-wet contrast.
Oehler 2008	Fundament: Capacitive electrodes. Material: metallic electrode plate. Dimensions: Diameter of the electrode plate is 26 mm, sensor height is 15 mm. Comfort: Electrodes fixed by means of a helmet.	Active electrode: Yes. Impedance: 106 GΩ. Noise: 2 μV/Hz−1/2 at 10 Hz and 70 nV/Hz−1/2 at 1 kHz.	Application: BCIs. Biosignals: EEG. Measure: in vivo. Procedure: Gazing at visual structured stimuli (checkerboards). Features: SSVEP. Location:O1, O2 and Oz, reference at FCz. Evaluation: Accuracy of SSVEP detection, ITR (bpm). No dry-wet contrast.
Matthews 2008	Fundament: A bioelectrode composed of 32 “fingers” capable of measurements through hair. Material: Metallic contact. Dimensions: US 5 cents approximately. Comfort: Use of harness. It permits subject motion.	Active electrode: Yes. Impedance: 10 MΩ per “finger”. Noise: 400 nV/Hz−1/2 (DAQ).	Application: Real time classification of workload during motion, Military applications. Biosignals: EEG, ECG, EMG, EOG. Measure: in vivo. Procedure: closing eyes. Classification of workload and engagement. Features: Alpha. Location: Cz, C3, C4, Fz, F3, F4, and Pz, reference at P4. Evaluation: Correlation with wet electrodes. Classification accuracy of cognitive workload.
Ruffini 2008	Fundament: Multiwalled carbon nanotube arrays. Material: Carbon without coating. Dimensions: Diameter of ca. 50 nm, length of 10–15 μm. Comfort: No side effects or pain even after 6 months.	Active electrode: Yes. Impedance: Not reported. Noise: It is low and rather similar to that of the commercial electrodes.	Application: General EEG recordings. Biosignals: EEG, ECG, EOG. Measure: In vivo. Procedure: Open-closed eyes paradigm. Auditory Evoked responses. Features: Alpha, N1 ERP. Location: Fp2, reference at nose. Evaluation: visual comparison dry-wet. Contrast hypothesis.
Dias 2010	Fundament: Array of 4 × 4 microtips. Material: silicon substrate coated with IrO. Dimensions: Each tip has a width in the range 150–200 μm, height of 100–200 μm, inter-microtip spacing of 2 mm. Comfort: Not reported	Active electrode: No. Conductivity: Similar to wet electrodes above 3 Hz. Noise:	Application: Human–machine interfaces. Suitable for electro-tactile stimulation. Biosignals: EEG, ECG, EOG, EMG. Measure: In vivo. Procedure: Eye movements, EOG measure. Features: EOG amplitude. Location: At the canthi of the eyes. Evaluation: Visual comparison dry-wet
Grozea 2011	Fundament: Flexible conductive bristles. Material: flexible metal-coated polymer bristles. Dimensions: 12 × 12 mm, and 10 mm long. Comfort: Pressure of the electrodes and the mounting frame found annoying. prickling sensation	Active electrode: No. Impedance: 80 KΩ (150–200 KΩ after 10 months of use). Noise:	Application: BCI. Biosignals: EEG. Measure: In vivo. Procedure: Eyes open/eyes closed conditions. Auditory evoked potentials. Odd ball paradigm and motor imagery. Features: Alpha rhythm. N100 AEP. P300, μ-rhythm (8–14 Hz). Location: P3, {Oz, Alpha}, {Fz, N100}, {Fz, Cz, P1, P300}, {Fz, C4, FC2, CP2, P3, μ-rhythm}. Evaluation: Spectral coherence (7–44 Hz) between dry-wet. T-test for detection of potentials.
Liao 2011	Fundament: Electrode with 17 spring contact probes. Material: BeCu. The probe head was coated with gold. Dimensions: 13 mm diameter, 20 mm height. Comfort: Not reported.	Active electrode: No. Impedance: Similar and better to wet electrodes on forehead and hairy site respectively. After 1 h, dry electrode impedance was better. Noise:	Application: BCIs, monitoring of human EEG states. Biosignals: EEG, EOG. Measure: Pre-recorded EEG data and in vivo. Procedure: Eye blinking. Features: Amplitude of EOG. Location: On the forehead (F10) and a hairy site (POz). Evaluation: Correlation with wet recordings.
Chin-Teng 2011	Fundament: Foam-based electrode. Material: foam covered by a conductive fabric and coated with Ni/Cu. Dimensions: 14 × 8 × 8 mm. Comfort: The foam electrode is soft enough to contact the skin properly.	Active electrode: No. Impedance: Similar and better to wet electrodes in forehead and hairy site respectively. After 1h dry electrodes impedance was better. Noise:	Application: monitoring of human EEG states, BCIs applications, clinical and research applications. Biosignals: EEG, EOG. Measure: Pre-recorded EEG data and in vivo. Procedure: EEG records and eye movements. Features: EEG and EOG amplitudes. Location: On the forehead (F10) and a hairy site (POz). Evaluation: Correlation with wet recordings.
Forvi 2012	Foundation: 8 × 8 pyramidal microneedles. Material: silicon. Dimensions: Contact surface 1 cm2.Micro scale. Comfort: Minimal invasiveness, easy and fast to use.	Active electrode: No. Impedance: 13 KΩ when SC is penetrated. Noise:	Application: monitoring of ECG signal in dynamic conditions. Biosignals: EEG, ECG, EMG. Measure: In vivo. Procedure: blinking eyes and in teeth grinding. Features: EEG, EMG, ECG. Location: EEG Fp1, Fp2, T3 and T4, ground at Fpz, reference at right mastoid. Evaluation: Visual comparison wet-dry.
Salvo 2012	Fundament: 3D printed dry electrode. Material: insulating acrylic based photopolymer. Coated with titanium and gold. Dimensions: millimeters. Comfort: Better than wet electrodes. It does not penetrate the SC.	Active electrode: No. Impedance: 1–3 KΩ and 2–5 KΩ at Fp1 and Fp2, respectively, whereas 4–7 KΩ and 1–3 KΩ are found at O1 and O2. Noise:	Application: Low cost biosignal applications. Biosignals: EEG, ECG. Measure: In vivo. Procedure: closed eyes. Features: Alpha. Location: O2, O1, Fp2, Fp1, reference at T6. Evaluation: visual comparison of dry vs. bridge electrodes.
Mota 2013	Fundament: Electrode with a gel reservoir inside. Material: Polymer coated with AgCl. Dimensions: 8 mm diameter, 11 mm height. Comfort: Not reported.	Active electrode: No. Impedance: 39 Ωcm2 at 10 Hz. 45 Ωcm2 at 1 Hz. Noise:	Application: Biopotential monitoring. Biosignals: EEG, EMG, ECG). Measure: \. Procedure and Features: Resting state EEG, Alpha activity EEG, Eye open/close test, Eye blink test. Location: Fp1, Fp2, O1, O2, ground at Fpz. Evaluation: RMSD, correlation, visual.