The electrodes which have been used for biomonitoring and their researched functionality has come a long way since the 19th century. However, the uses and understanding of these devices continue to grow with time, highly innovative ideas and significant advancement within the medical field. Our article features a brief history as well as the future growth of ECG sensors.

History Of ECG Sensors

During 1838, the genius findings of professor Carlo Matteucci a physics professor demonstrated that each heartbeat occurred with an electric current. After his findings, this drastically changed the sensors which were used to monitor the electric currents found within the human body. He demonstrated this with what was known as the Rheoscopic Frog, simply put, the nerve in a frog’s leg was cut and it was used as an electric sensor, as the muscle twitches, this was used to visually illustrate signs of electrical activity.

The Rheoscopic Frog

Carlo Matteucci went on to conduct further experimentation for a study which revolved around the electric current in muscles, this was done with the use of an astatic galvanometer. During this time, this particular process was performed by inserting a galvanometer wire into an open section of the dissected muscle, and, the other wire was then placed on the muscles surface. Later on in 1887, the first electrocardiogram was published by that of Augustus D. Waller a British physiologist. However, he firstly experimented on his bulldog, he placed the paws of his dog in a glass container which was filled with a saline solution and he used a collar that had brass studs; the saline sensors used in this experiment were connected by conductive wires to galvanometers.

Wallers technology was later refined in 1901 by Willem Einthoven, he used a string made of quartz which was silver coated in the galvanometer. This device was quite big since it weighed more than 600 pounds and required five, yes, five technicians for a single use. At the time, the sensors used were mainly buckets that had wires running to the string galvanometer, however, the buckets were of course filled with a consistent saline solution.

The arrangement as well as number of buckets were always the same. It was mainly three buckets that were used in a manner which gave one for each leg and one for each hand, and they were then arranged in the form of an equilateral triangle. Einthoven then went on to present his method, the Einthoven Triangle to the Chelsea Clinical Society of London in 1912. And, he won the Nobel Prize for his cleaver invention of the electrocardiograph.

Modern Biomonitoring Electrodes

The electrodes which are used today for Tens, EEG, ECG and various other applications are mainly constructed from a plastic substrate which is then covered in either silver or a silver chloride ionic compound. It should be noted that silver chloride is used because of its soluble properties in water resulting in a stable compound. The electrode then goes on to be assembled with the anion, chlorine, simply because of its electrolytic properties. The chloride anion is utilized because of the fact that the skin on the human body has an excess of chloride ions which are found in sweat.

The silver which is used to cover the electrode surfaces are later oxidized in order to form silver ions for interface. The silver ions are then combined with the chlorine as together they form silver chloride, and ionic compound. However, the newly formed compound reacts only slightly with water so it remains soluble, this then forms a silver chloride deposit since as a result of the precipitate on the silver electrodes. In addition, the sensors are always converted from the form of metallic silver to that of silver chloride via either chemical conversion or an electrolytic process.

Electrodes which are made of silver or silver chloride are mainly used for most applications which require any type of bio electrode system. In the past, electrodes were made from different materials such as silver coated brass, nickel and even tin. However, these resulted in undesirable frequencies, as such, all modern electrodes are made from either silver or silver chloride.

How Is Heart Activity Measured?

Firstly, the activities of the heart can be measure in two ways. These being with the use of an Electrocardiography as well as Photo-Plethysmography. First, we will look at the ECG or EKG method.


An ECG tends to record all the electrical energy of the heart as the muscle experiences depolarization. This is done with the use of electrical waves which are administered on the skin. Even though the electric current used it relatively small, the electrodes which are attached to the skin is still able to read them in the units of microvolt. Electrocardiographs are set up with about four or more electrodes, these are positioned on either the chest or four extremities.

These extremities are mostly the right arm, the left arm, the right leg and the left leg alike. This is simply to increase the flexibility of the procedure as well as a means of way to acquire your readings in a less intrusive way. However, the electrodes are typically that of wet sensors and they also require that of a conductive gel, this is done in order to increase the conductivity chances between the electrode and the skin.


During the cycle known as the cardiac cycle, the pressure of blood increases and decreases accordingly. This is also experienced within the smaller vessels of the outer layers of skin. As a result, the peripheral flow of blood can also be measured with the use of optical sensors when they are attached to a person’s ear lobe, fingertip or even another capillary tissue. However, the accuracy of this method isn’t as good as that from an ECG mainly for the reason of lower conductance experienced on contact.

As we conclude our article, we have just looked at the in’s and outs of an ECG. We have discussed the history as well as the difference in the composition when compared to that of olden times. And we have also looked at how it works, in addition, ECG electrodes from ¬†are also paired with various sensors to further increase the accuracy of the results obtained.