Surface electrode measures the potential available from the surface of the skin. It senses the signal from heart, brain and nerves. Larger surface electrodes sense the ECG signals. Smaller surface electrodes sense the EMG and EEG signals. The types of surface electrodes are as follows.
Metal Plate Electrodes
ECG measurement technique uses either rectangular or circular shaped plate electrodes made of nickel, silver or German silver materials. It has a smaller contact area and do not seal completely on the patient. Electrodes are pasted on the skin using electrolyte paste. The electrode slippage and plate displacement are the two major disadvantages of this electrode type. They are very sensitive, leading to measurement errors.
Since it is suitable for application on four limbs of the body, they called limb electrodes. During surgical procedure since patient’s legs are immobile, limb electrodes are preferred. Chest electrodes interfere with the surgery, so not used for ECG measurement. At the same time for a long-term patient monitoring limb-electrodes are not used.
Suction Cup Electrodes or Welsh Cup Electrodes
To measure ECG from various positions on the chest, Suction cup electrodes are used. It suits well to attach electrodes on flat surface of the body and on soft tissue regions. They have a good contact surface. Physically they are large but the skin contacts only the electrode rim. It has high contact impedance. They have a plastic syringe barrel, suction tube and cables. Recently, due to infection and cleaning procedures, these electrodes are not used.
Adhesive Type Electrodes
In the surface electrode, the pressure of surface electrode against the skin squeezes out the electrode paste. To avoid this problem, adhesive electrodes are used. It has a lightweight metallic screen. They have a pad at behind for placing electrode paste. This adhesive backing hold the electrode on place and tight. It also helps to avoid evaporation of electrolyte present in the electrode paste.
Multipoint Type Electrodes
Multipoint electrodes are very practical electrode setup for ECG measurements. It has more than 1000 active contact points. This helps to establish low resistance contact with the human. Under any environmental condition, doctors can use multipoint electrode.
In metal plate or limb electrodes, the major disadvantage is the movement errors. Motion artefact occurs due to the motion at the interface between electrode and electrolyte. The interface gets stabilized usingFloating electrodes. The floating electrodes do not contact the human subject directly. They contact the subject via electrolytic paste or jelly. The advantage of this type is the mechanical stability.
Ear Clips and Scalp Electrodes
In the measurement of EEG, ear clip electrodes are used. Scalp electrodes provide EEG signals easily when placed on bare head. Generally,in 10 – 20 electrode system EEG measurement scalp electrodes are used. This type avoids measurement errors. During labor, fetal scalp electrode monitors baby’s heartbeat while still inside uterus.