Why Electrodes Matter: Skin-Electrolyte Interface
The largest source of artefact in the ECG signal is without a doubt the skin. Due to its sizeable impedance and unstable potential it can create many problems when recording ECGs. The electrode–electrolyte interface impedance is minimal in comparison with the large impedance of the skin.
With the use of conductive gels, skin preparation techniques, and carefully selected environmental factors, we can reduce the impedance found in the skin considerably. However, the techniques you choose to put in place are dependent on the desired application of the electrode; long-term, short-term, or stress tests. These will be discussed below but first we must address the structure of the skin and why we see this huge impedance.
Skin Structure
The skin is a multi-layered organ that covers and protects the body. It is made up of three principal layers; the epidermis, dermis and subcutaneous layer.
Cells within the inner most layers are constantly multiplying, pushing the older skin cells up towards the skins surface. As these are pushed up, they undergo changes. As the cells are pushed outwards, the cell layers become flattened, compacted and dehydrated. These cells form the outermost layer, the stratum corneum. This layer is relatively non-conductive compared to the inner layers of the skin.
The stratum corneum is very thin (typically between 12-30 layers thick), and exhibits insulating properties which enable it to act as a dielectric between the highly conductive electrolyte and inner layers of the skin. Current between the two highly conductive areas therefore couples capacitively making the level of impedance subjected to the current frequency dependent. As the frequency increases, the reactance within the capacitor decreases exponentially (known as the capacitors complex impedance).
Due to the high reactance at low frequencies, the current tends to pass resistively through the skin-electrolyte interface at these low frequencies. This is possible due to the appendages present within this layer; the sweat glands, sebaceous glands and hair follicles extend deep into the skin providing a channel for the current to cross between the two highly conductive areas. However, this current does not pass easily and incurs a great deal of impedance. Skin preparation can be used to reduce this.
The flow of the current can be represented by the equivalent circuit model as you see above. The large resistance present in the skin can be represented by the parallel resistance’s, Rc, or Rp. The two of these combined would equal the overall resistance of the skin site. Ideally, a low resistance and high capacitance are strived for.
To achieve this, we can reduce the thickness of the non-conductive outer layer (the dielectric). As with capacitors, the capacitance increases with the decreasing in thickness of the dielectric. Not only does this increase the capacitance, it also reduces the resistivity we see at lower frequencies There are various different types of skin preparation which aim to remove some of the layers within the stratum corneum, allowing us to decrease the width of this dielectric area. These are described below.
Skin Preparation
The different types of skin preparation techniques commonly used are:
Abrasion pads
Puncture
Pre-rub Gel
Alcohol wipes
Each of these affect the skin in different ways and some are more suited to certain applications than others.
Abrasion and puncture work much the same, by removing some of the non-conductive top layers of skin. By reducing the width of the dielectric(Stratum corneum) we increase the capacitance while also revealing a less resistive and more conductive top layer. Using these techniques, we see a very quick drop in impedance. However, as the skin starts to grow back underneath the electrode, we see the impedance increasing again. The gradual increase in impedance and the aggressive nature of these techniques leaves them only suited to short-medium term monitoring applications. This is especially important if used in conjunction with a conductive gel as this will cause further irritation to the skin.
Pre rub gel is an electrolyte gel with abrasive sediments within. They usually have a very high concentration of electrolyte, when rubbed into the skin this forces the electrolyte ions absorb into the skin, reducing the resistivity of the skin. The abrasive properties work in the same way as the abrasion pads in removing the outermost skin cells. This helps the gel reach more conductive layers. Due to the aggressive properties of the pre-rub gel, it should also only be used for short-term and not to be used in conjunction with a pre-gelled electrode.
Alcohol wipes are also used to remove the loose outermost cells of the skin, and the non-conductive lipids present on the skin as well. The alcohol content however does cause the skin to dry out once used, creating an initial increase of impedance. As the skin rehydrates itself the impedance then reduces below its original value. This is suited especially for wet electrodes due to their hydrating properties, enabling a faster rehydration process. Hydrogel electrodes do not hydrate the skin so do not have the same affect. Due to their hydrophilic properties they can actually be seen to further dehydrate the skin. Alcohol wipes are best suited to medium-long term monitoring, as their initial increase in impedance can disrupt a short-term recording.
Skin Site
It is not only important to take the correct measures for skin preparation, but also to choose the correct site on the body to attach the electrodes. Throughout the body, there is huge variation in the skins potential and impedance levels. To reduce impedance as much as possible at the skin site, the electrodes should be placed strategically.
As mentioned above, the stratum corneum can range from 12-30 layers throughout the body. For example, the stratum corneum can be as thick as 400–600mm in the palm and plantar areas and as little as 10–20mm on the back, legs, and abdomen. The thicker this layer, the more impedance we see within the ECG. When placing electrodes, it is therefore preferable to place them on areas with a thinner stratum corneum (areas with a lower impedance and higher capacitance). It is also important to place all electrodes within the same area to ensure there isn’t a large impedance mismatch between the different electrodes, as the mismatch will be amplified by our circuit and cause massive distortions and power line interference in our ECG.
Gelled Electrodes
Two types of gelled electrodes exist, wet gels and hydrogels. The gelled electrolyte provides a conductive medium between the skin and electrode allowing the current to pass from the skin to electrode easier. As the major portion of electrolytes present in tissue fluids and sweat are sodium, potassium, and chloride, the electrolytes most commonly used in electrode gels are sodium chloride and potassium chloride. These not only ensure good electrical conductivity of the gel but also increase the conductivity of the skin as the electrolytes diffuse into it due to the existing concentration gradient.
Wet gel electrodes usually consist of a gel impregnated sponge, with adhesive around the outside to ensure hold. Wet gels have a very high water concentration, actively hydrating the skin and reducing the resistivity in the outer layers. They also typically have a higher concentration of electrolytes than hydrogels, which produces a much lower impedance. The disadvantage of this is the skin irritation caused. This is due to the high concentrations of both electrolyte and water within the wet gel. These high concentrations create a large concentration gradient between the gel and skin, causing the electrolyte to diffuse into the skin at a very fast rate. Even when both gel types have a similar concentration of electrolytes, the wet gels are still a lot harsher on the skin due to the higher water content within them that make them react faster and more aggressively on the skin. The main drawbacks of the wet gels are obviously the skin irritation, but another would be their inability to be repositioned; the wet consistency of the gel means we leave a deposit of gel on the skin when moved. Wet gel electrodes are ideal for short-term monitoring and stress tests uses due to there very low impedances, but shouldn’t be used long term due to the irritation they can cause. Due to their aggressive nature, it is also not advised that these be used in a paediatric setting.
Hydrogel electrodes serve principally to ensure a good electrical contact between the skin and the electrode without significantly affecting the outer layer of skin, as the wet gel does. Hydrogels provide the added benefit of being repositionable, and also less irritating to the skin. This is a result of the lower electrolyte concentration(usually) and the lower water concentration in comparison to wet gels. Although less irritating, the disadvantage to this is their inability to hydrate the skin. This causes hydrogels to produce a much higher resistivity; they typically have an impedance of 800-8ohmcm2, whereas wet gels have an impedance of 5-500ohmcm2. To combat this, we can use a larger electrode surface area to reduce the impedance. The overall size of the electrodes however, are still similar to that of the wet electrodes as the hydrogel acts as an adhesive and does not require a foam backing as the wet gels do.
The impedance with hydrogels behaves differently to both wet and dry electrodes when in contact with sweat. Wet and dry both see a decrease in impedance that does not rise again. However with hydrogel electrodes the impedance fluctuates with sweat gland activity. This is due to its hydrophilic properties; as the skin produces sweat and the impedance reduces, the hydrogel then absorbs the water molecules causing the skin to dehydrate and the impedance to increase again. Due to this, hydrogels are not ideal for stress-test, but due to their less irritating properties are recommended for long-term use.
Dry Electrodes
Dry electrodes do not possess the conductive electrolyte which we see in the pre-gelled electrodes. These electrodes can be particularly useful in home environments where the patient may not require an ECG with as high of a frequency range as those in clinical settings. One of the main benefits of these is that they can be reused again and again.
When using dry electrodes, it is particularly important to take into consideration the unevenness of the skin. As dry electrodes lack an electrolyte, when initially applied the dry flat electrodes will only come in contact with the peaks of the skin. This creates a very small surface contact area for the current to pass through. As the sweat builds up between the skin and electrode, it fills up the troughs within the skin increasing the contact between the two. The increase in contact area also increases the capacitance at this interface. The impedance seen within dry electrodes therefore decreases with time as more sweat accumulates between the electrode and skin.
Not only does the capacitance increase with time, the resistance decreases as well. Over time the sweat rehydrates the skin increasing the conductivity in the stratum corneum. It has these properties due to the small amount of sodium chloride found in sweat, acting as a weak electrolyte. To increase the amount of sweat, we can place the electrodes on sites with an abundance of sweat glands, or place the actual patients themselves in situations which cause them to produce more sweat (stress-tests).
