Table 2.

Performance comparison of different pain sensors and devices

Sensing type	Signal	Sensor	Mainstream material	Advantage	Limitation
Electrical	ECG, EMG, EEG, EDA	Electrodes	Ag/AgCl [164]	High performance and stability	Low biocompatibility
			Polypyrrole (PPy) [165]	High performance	Low stability
			Carbon nanotubes (CNTs) [166]	High performance and suitable for Flexibility	Low biocompatibility
	Respiration, PPG	Piezoelectric sensors	Polyvinylidene fluoride (PVDF) [167]	High performance and suitable for Flexibility	Low piezoelectric efficiency
			Piezoelectric nanomaterials [168]	High performance	High cost
Optical	PPG	Photodiode [169]		High sensitivity and low noise	No built-in gain
		Phototransistor [170]		High sensitivity	High noise and low responding speed
Thermal	Temperature	Thermal resistance	Platinum thermal resistor [171]	High accuracy but slow	Slow responding time
		Infrared temperature sensor		Low accuracy	Quick responding time

ECG, electrocardiogram; EMG, electromyography; EEG, electroencephalogram; EDA, electrodermal activity; PPG, photoplethysmographic