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ISSCC 2014 Short Course Transcription The Biomedical Electrode-Tissue Interface: A Simple explanation of a complex subject

Instructor: Eric McAdams

1. Introduction

So, my name is Eric McAdams. I live in France, but you’ve probably worked out from the accent that I’m not French: I come from Northern Ireland. And I’m going to give you an Irish engineer’s explanation of electrodes. What that means is: I’m going to actually make it really simple (because otherwise I wouldn’t be able to understand it), and I think it’s Einstein (I’m just name dropping), Einstein said make it simple, but no simpler. So, I am going to try to make it very simple: I’m going to present very simple models.

In my defense, I think those models will actually help you understand all of the key things that you need to know, and, when you want to learn more than that, then you can delve into it more deeply. So I’m going to give you a good working model for most of the things that you’re going to come across.

For those of you who are electrochemists, I would like to apologize at the very beginning: I’m going to totally massacre your subject area, so hopefully there’s not that many electrochemists present. So, I’m going to give you a very simple explanation of what is a very complex subject.

2. Sensors

If you look, you’ll see me many years ago (probably twenty or thirty years ago), with hair: I’m the good looking one on the right. I have been working in, or with, electrodes and electrical properties of tissues for thirty-five-whatever-it-is years. And, as you can see from some of the examples that I’ve put here, you can see that we have worked on patches, we’ve worked on smart clothing, we’ve worked on putting sensors into wallets, mobile phones, wristwatches, whatever. So we have worked a lot in this area, and we have patented a few of the designs as well.

The other thing is that I helped start a company called Intelesens in Northern Ireland, and you can check out some of their products on the internet if you so wish. So that’s my background.

3. Outline

This is an outline of my presentation. I’m going to talk to you about signal distortion first of all (because I want to put it all in context), and I want to show you some of the problems that can happen if you don’t get your sensors or your electrodes designed correctly. Then I’m going to move on to the electrical properties of the electrode-electrolyte interface, and then I want to move on from there to the electrical properties of the skin.

So I want to just reiterate that, make it very clear: I’m going to talk about the contact between the electronics and the patient, and I’m going to subdivide that into electrical properties of the electrode itself and the electrolyte – electro gel, for example – and then I want to talk separately about the electrical properties of the skin.

And what you’ll see, and what you’ll find out, is that the one that’s the most important is, actually, the electrical properties of the skin.

And, then, finally, I’m going to draw some conclusions.

4. Biosignal Distortion

So, let’s start with biosignal distortion.

5. Biosignal Distortion: Input Signal Characteristics

Here are a range of measureands: biosignals that we can measure from the body for various applications, various studies. What I want you to notice is that, especially the electrical signals, there are lots of them, and they all have very small amplitudes, and they tend to have fairly low frequency ranges.

So one of the big problems is that they’re very low amplitude: much smaller than what you’re used as electronic engineers – you’re used to seeing, and also they’re very low frequency, so 50Hz tends to be smack in the middle of the frequency range that you’re wanting to look at. As good electronic engineers you know it’s better to get a good signal at the very beginning, rather than trying to filter it afterwards. Because if you’re trying to filter out 50, sorry 60 (I’m in the U.S.), 60Hz noise, and the big problem is that you’re going to remove part of the biosignal that you’re trying to measure. Therefore, try and get good, noise-free signals in the first place, rather than trying to just remove the signal afterwards (from) the noise.

One of the other problems is that you have competing signals; so, when you try to measure the ECG for example (electro cardiogram, EKG in the U.S), you’re going to measure the EMG, you’re going to measure the EOG, the ERG – all those G’s – you’re going to be measuring all of them at the same time, and their frequency ranges overlap, so, you’re going to be measuring lots of things that you don’t actually want to measure.

What I’m going to show you later on is that there are significant problems with the measurement sites that you’re wanting to put your electrodes on, and they will cause you lots of problems not to be ignored; and also external interference: the 50, 60Hz, for example, that surrounds us at the moment.

6. Electrode-Skin Contact

So, I’m going to, for the moment, group the electrode gel, interface, and the skin impedance, and call it, for the moment, the contact impedance; I’ll then subdivide it later on. But, for the

moment, let’s just look at the problems that arise due to a wrong design in an electrode, and the resultant contact potential, and contact impedance.

So, you have a nice biosignal, nice EKG, coming out of the patient. It arrives here at the interface between the human body and the electronic circuit at this contact, and what tends to happen is that what comes out into your circuit is something that looks not all like a nice, clean EKG.

That’s the problem that you’re going to experience when you start to get into the area of wearable monitoring. It’s going to be a can of worms. And if you don’t think simple, and eradicate simple sources of problems, then you’re going to have great difficulty, no matter how wonderful your circuits are, your ideas are, no matter how cool your gadget, you’re going to have problems. The reason why we don’t see so many cool gadgets that are commercialized is simply because people run into the stormy weather of interface problems, and then they abandon their design and then they go on to something else, some other cool project. And that’s why we don’t have too many systems that are commercialized successfully.

7. Contact Potential

So let’s look at the contact potential, nice and simple. In theory, you have a contact potential – it’s a bit like a battery – at contact 1; you also have a battery at contact 2 (sorry to insult your intelligence), and if you have a nice differential amplifier, et violà, you don’t have a problem because the differential amplifier only amplifies, by definition, the difference, and you don’t have a problem: there is no problem with your electrodes, you’re not going to see any contribution from your electrodes in your biosignal. If only.

8. Baseline Wander

What happens in reality, if we look just at the electrodes; for example, if you take two standard ECG or EKG electrodes and you stick them gel to gel? In theory, they should have exactly the same potential, therefore, you should see nothing. If only. And what you’ll find is you already have slight inadvertent commas: differences between one electrode and the other. We’re not talking about electrodes even on the patient, we’re just talking about two electrodes stuck together, and, what’s even worse is that some of those potentials actually change with time.

If you have a constant potential, as you know, you can remove it very simply with your circuit. But if you have something like this, which is not only large – there it’s 60mV, remember the EKG is about 1mV, so you have something which is many tens times bigger – but not only that, it actually changes with time, so you’ve got a big problem; you can’t just simply remove it. Starting to see the problem.

One of the reasons why we use Ag-AgCl is because it tends to have a very stable potential. Very low, not zero, but if you put two Ag-AgCl electrodes generally, in the correct manner, you’ll actually get very low mismatch, and it’s stable, hence, most of the EKG electrodes that you will see around the world tend to be Ag-AgCl. We’ll go into that with more depth later.

9. Baseline Wander - Skin

Another source of contact potential and your biggest problem when you start to work on wearable is the problem of the skin potential. Generally it’s called “motion artefact” and it’s due to fluctuations in the skin potential due to stretching of the skin, deformation of the skin, pressing on the skin.

So if you put something on a patient, and they start to move around, their skin stretches and the skin behaves somewhat like a piezoelectric, and as you deform it or stretch it, then you’re going to have terrible problems. As you can see from the little diagram at the top there: you have an EKG which is about 1 mV; you have fluctuations in the skin potential of about 10 mV. Go try and interpret something from that EKG: it’s going to be very difficult. So that is a major problem for the technician if they’re wanting to do stress testing: you have a problem with your heart, they bring you into the hospital, they want to stress you so they get you to exercise. Big problem is that as you start to jog around, your skin is going to stretch; you’re going to have lots of artefact. You try and put something on a patient and they put their clothes over your system and they move around, you’re going to have lots of artefact. You try and put your electrode underneath your electronics, your electronics are going to move, and as your electronics start to move against the skin, you’re going to have lots of artefact. That’s probably going to be your biggest can of worms.

10. Contact Impedance

That was potential. Now let’s look at the contact impedance; and you’re going to have problems there: either due to the size of the impedance, or due to the mismatch of impedance.

The most trivial one, most obvious one, is obviously if you have intermittent contact: again, if someone is wearing something under their clothes and the electrodes start to pull off, you’re going to have a problem, that’s trivial. More significant is you’re going to have signal attenuation: your skin is dead (that’s why we have dandruff), and it’s dead skin, very, very, very high impedance – mega ohms, depending on the frequency range – and that, coupled with your amplifier, is going to cause signal attenuation. Not a big problem. Sure, if it’s all decreased by 10%, who cares? But what is even worse is that it is selectively attenuated.

Your contact impedance is an impedance: it has a capacitive component, and hence, it changes with frequency. So therefore certain frequencies in your signal will be attenuated and certain frequencies won’t, which means that your whole signal is skewed: it becomes deformed, distorted.

And, again, depending on the arrangement, if you have impedance mismatch, what’s going to happen is that you’re going to have interference: you want to have 50 or 60 Hz pickup. Major problem.

11. Signal Attenuation

So, I’m going to deal about this very simplistically; you’re going to get more of this in the next speakers’ talks. But, for example, if you have a signal coming from the heart which is 1 mV, let’s say that the skin contact impedance is 100 kΩ (it’s actually quite small, it could be far bigger),

and for sake of argument, just to keep the sums and calculations simple, you have an input impedance of your amplifier of 1 MΩ. Very difficult mathematics, fortunately I have it on this slide, but you’re going to have your signal attenuated to 0.9 (90 percent). As I say, that isn’t normally a big problem; it is a problem however, when certain frequencies in your signal are attenuated more than others.

12. Signal Distortion

So signal distortion arises because your contact impedance, as we shall show, is a resistance in parallel with a capacitance. Hence, when you look at the impedance of that circuit, and we will, you will find that it is frequency dependent, and the impedance is larger at low frequencies.

That means that you’re going to get more attenuation of low frequency components of your EKG (or whatever signal that it is that you’re interested in).

Here’s an EKG. The low frequency components are the P wave (that tends to vary quite slowly with time), T wave, but also this part here which is called the S-T segment which looks almost DC, hence, low frequency.

The big problem is that S-T segment is actually very important to your condition. If you have a heart attack – myocardial infarction – that S-T segment will actually change. But if you have bad electrodes or bad skin preparation your circuit isn’t too good; then what’s going to happen is you’re going to deform the signal and the technician could actually misinterpret the EKG, and think that the patient is having a heart attack. Therefore, it is very important that you get your electrodes and your circuit correct.

So what tends to happen is the signal is distorted (the low frequency components), and what you get is something that looks somewhat like that. And as I said, the S-T segment is what’s very important to technician and what you can see here is that it is totally distorted, and it could at least mask something which is important, or it could be interpreted as something which it isn’t.

13. 50/60 Hz Pickup – Impedance Mismatch

Lastly, from the biosignal distortion, we have 50 or 60 Hz pickup. Now there are lots of different combinations of this, and you can look at some of the books by Webster, for example, and some of his publications to find more detail.

But just to illustrate the point. Here we have – I’m standing beside power lines that are around me. I’m coupled capacitively to those power lines: you don’t see it, I don’t feel it, but there is a current flying through the air into me and down to ground. It’s a very large impedance, it’s a capacitive impedance, and the current flows down through the cables, through me, and to ground if I was connected to an EKG monitor. This is just one scenario.

What we assume is that these two impedances that couple me to the power lines are very, very large (which they are if you do the calculations), and we’re going to assume that they are both the same, so hence we can ignore them because they’re both in series with these two contact

impedances which is Z1 and Z2, that’s the electrode contact. And what we find is that because of the current flowing through Z1 and Z2, we get this ∆V: we get a voltage difference generated due to the 50 or 60 Hz noise at the inputs of our amplifier.

Both these currents send flow through the patient (or me), down to ground, and since they’re both common, we can ignore them. So the two sources of difference in impedance – or of potential – are Z1 and Z2, and if we do the calculations again, we want to keep it nice and simple, is that we get ∆V is equal to the difference in voltage dropped across each of the impedance contacts.

Just to make the calculation even more simple, we’re going to assume that both currents are the same – which is close to the truth because the currents are going to be determined by the very large series resistance coming through the air to my body – and what you get then is that your 50 Hz signal that’s picked up at your amplifier, is due to impedance mismatch (Z1 - Z2).

As I said, skin impedance can be MΩ. So if you have 1 MΩ here, a half a MΩ here – I will show that in a bit more detail later on – you have a half a MΩ, and even if the current is very, very, very small, multiplied by half a million, tends to make it sizeable. And, in fact, it can be bigger than the biosignal that you’re trying to monitor.

So impedance mismatch is one of the sources of 50 or 60 Hz pickup; hence, you have to be very careful with the impedance of your contact.

14. Biosignal Distortion

So I’ve dealt very briefly, simplistically, with biosignal distortion, just to show you that it is actually relevant. You’re going to suffer all of these problems when you try to monitor on patients when you have your new device and you’re wanting to monitor a patient.

Let’s now move to the electrical properties of the electrode–electrolyte interface.

15. Electrode Interface

As I mentioned before, it has two aspects: it has the electrode potential, but also the electrode impedance; and so I’m going to start with the electrode potential.

16. Electrode Interface: Potential

Normally, if I was in university with my students, what I would do is I would take a cup of tea (I’m Irish, so I drink lots of tea), so you’re just going to have to imagine that this is a cup of tea, and I have my spoon in my cup of tea (let’s assume it’s silver, I’m very rich), and I have silver ions in my cup of tea: it’s a rather toxic cup of tea, okay? So we’re just going to have to imagine because I don’t even have a pen. But there I have my metal inside of an electrolyte.

That was all just a bluff so that I could have a wee drink.

But what happens is that my silver spoon starts to lose ions and they drift into the electrolyte (my cup of tea), and, at the same time, you have silver ions in my cup of tea – very toxic tea – which come and deposit onto the silver electrode (in this case my silver spoon), and they deposit silver metals.

So you have two electrochemical reactions: I didn’t tell you that it would be nice and simple. One of them is oxidation, where you have the metal becomes a metal ion, and you have electrons liberated into the metal, like so. And you have reduction, where you have the ion comes, sucks out from the metals some electrons, and it then forms a metal atom on the electrode.

So, in my cup of tea, there is no net flow of current. The current that goes in one direction due to oxidation is equal and opposite – I hope you can see – to the current flowing in the opposite direction. So there is no net current, however, there is a current going in one direction and a current going in the opposite direction, and that current, at equilibrium, is called the exchange current density. That’s actually going to be very important when we start to talk about electrodes. So it’s a key one to understand.


So, what we said is we have oxidation. With oxidation you have electrons which are liberated into the metal. So, not surprisingly, the electrode becomes negative. But, you have another reaction called reduction, and in it you have electrons being sucked out of the metal, and hence, the metal becomes positive.

And what you end up with is a reversible potential, it’s called, in electrochemistry (because they like big words): you have an equilibrium potential, due to the balance between these two reactions. One of them makes the electrodes negative compared to the electrolyte, the other one makes it positive. And depending on the equilibrium between the two, you get a net voltage difference.

Generally, in most cases, as you will find, metals are negative. And, the term for this potential is, the intuitively obvious one is equilibrium, because it’s my cup of tea with one electrode in it; nothing is going anywhere, so it’s under equilibrium. Electrochemists like to say it’s reversible potential, because what goes in one direction is reversed in the opposite, so they talk about reversible.

And if you’re a biomedical engineer – I don’t know if there any present – they talk about half-cell potential; sounds grand. Half-cell is very simple; it means that if you wanted to have a chemical cell – electrochemical cell – you would have normally two electrodes and a battery: two different metals that have two different potentials and that’s what makes you your battery. Because in my case I only have one electrode in my cup of tea or in my electrolyte, then biomedical engineers call it a half-cell. Go figure.

So, biomedical engineers talk about half-cells, electrochemists – they talk about reversible potential, so we’re learning some of the jargon.


I would normally, at this stage, ask a trick question, and I would say, okay, here’s my cup of toxic tea with an electrode in it, how do we actually measure this reversible potential? And the trick is that the students normally say: well, you get a multi-meter and you connect to this electrode and then you get the other connector from the multi-meter and you put it also into the electrolyte.

What they don’t realize, generally, at this stage is that by putting another metal into the electrolyte, you have another interface, you have another potential, so what you now have is a full-cell and you’re measuring the difference between the potential of the electrode you want to measure and the new potential due to the interface of the measurement that you’re using. So it’s one of those Murphy Laws of life. You can’t actually measure a single half-cell potential by itself. The moment you use another piece of metal, you’ve got a full-cell.

So what happened in the past is that they decided to make one electrode to be the standard, and we will measure every other electrode relative to that standard.

This is hundreds of years ago when universities used to have glass blowers. I’m old enough to remember universities that had glass blowers: I don’t know if anyone knows, I’m very old. Anyway, so, everywhere in the world they could have glass blowers. So what they did was they said, right, we’re going to make an electrode which is the gold standard (play of words) and what they decided was that they would make electrodes where you had glass tubes, and you bubbled hydrogen on top of a platinum electrode. That’s electrochemistry, don’t go there.

But it meant that you had a potential that was always reproducible whether you were in Australia, or you’re in the U.S., or you’re back in Ireland. So that was their gold standard, in fact, that was their platinum standard. And it meant that anybody could compare anything anywhere in the world. So it became the gold standard, and even though you’re measuring the potential of both the electrodes, because one is taking a standard, everything is measured relative to the standard hydrogen electrode as it’s called. So it’s called SHE, the standard hydrogen electrode.

Nowadays, people tend not bother with standard hydrogen electrode, they use Ag-AgCl, but because a hundred-odd years ago, that was what was used, then all of the electrode potentials that we’re going to talk about, are normalized to the standard hydrogen electrode. It’s taken as zero.


I did say that I wasn’t going to deal too much with electrochemistry (as an electrical engineer, it makes me come out in spots), but that’s the Nernst equation and all I want you to do is sit back, relax, and see two things. It’s made of two components. So forget about the Greek symbols, just concentrate on two things.

One is that it has a constant component, E0, so for any electrode, if it’s, for example, Ag-AgCl, it’s measured relative to platinum electrode, you’ll always have this constant.

But, there’s another component which varies, and it varies with the concentration of your electrolyte. So, for example, if you have a Ag-AgCl electrode and a nice EKG gel, you have another Ag-AgCl electrode but its gel is slightly different, then the potential will be different, and if you use those two electrodes on a patient, you’re going to actually have a big potential mismatch. So it’s not just the metal that counts, it’s the electrolyte concentration. So, because of that, we do get these slight differences, even though we tend to be using Ag-AgCl.

20. Values

So, these are typical values from some of the old books. Hydrogen reaction is taken as 0 by definition, and, for example, if you wanted to make a battery – this is an Irish explanation of a battery – what you’d do is you would take one electrode which has a very positive value relative to your electrolyte, one which is very negative, and you would put both of them in, and therefore you would get a very big potential difference, which is what they do. It’s a bit more complicated than that, but it’s pretty close.

So you would find, for example, lots of electrodes are based on zinc, for example. If you could get one that was -1 here, and one that was +1 there, you would then have a cell which has a voltage of 2 volts.

If you notice, most batteries are based on multiples of 1.5 because some of the early batteries that they made – two different electrodes: one positive, one negative, they got a difference of 1.5; so most of your batteries tend to be multiples of 1.5: 3 volts, 6 volts, 9 volts, 12 volts.

Just interesting aside: in the U.K., they tended to put the cells and accumulate them sideways, so they call them a battery of cells. So, for example, we talk battery of drums, so, battery of cannons, so the British talk about batteries of cells. The French, vive la différence, they like to be different, so they used to put theirs up the way, so it’s a pile of cells. So the French have the pile, we have the battery, we’ve all both forgotten about the word cell, which is actually what it’s all about, but the French call them des piles and in the U.K. we talk about batteries, but it’s batteries of cells, piles de cellules, okay? Just a little bit of French there, to keep you going.

So, for example, silver has a voltage of .8 volts, .8 volts! That’s big. We’re talking about EKGs of 1mV, and we’re going to use silver electrodes, .8 volts, hey, that’s big. But, if you look, Ag-AgCl – we’ll deal with this in a little more depth later on – it has a potential which is actually very small, .2 volts. Still big, but it’s actually very constant – relatively constant – and relatively constant with time.

So, again, if you want to make a battery, you would take two materials that are very distant on this table, however we don’t want batteries: we want to have 0 potential if that was possible. So what we’re going to go for is something like Ag-AgCl, which is relatively very low, but also, is very stable.

21. Electrodes

So back to that slide: if you stick two electrodes together, face to face, what you get, unfortunately, is a mismatch, because you not only have Ag-AgCl, you also have gel which may vary from one electrode to the other – there might be imperfections, etc. So, instead of having which is 0, which it should be, you can actually get sizable offset potentials, mismatch potentials, of 40, 50, 60, mV.

22. Electrode-Electrolyte Impedance

So, I have talked to you about electrode-electrolyte potential. Now I’m going to move on to talk to you about electrode-electrolyte impedance.

23. Reminder

Just to remind you, the reason why we’re doing this is because, if we have large, or mismatched contact impedances, we have signal attenuation, signal distortion, and interference.

24. Impedance

So here’s a rather colorful slide. I said that metals tended to be negative. So we have a negative layer of charge on our metal (could be an EKG electrode, it could be your electrode put inside someone’s head), some electrolyte, and you have a layer of negative charge.

What happens is that positive charge goes: “well, that’s very nice, I’m attracted,” and it will diffuse and come and stick to the surface of the negative charged electrode. Electrochemists, they use big words, they don’t stick, they use the word adsorb. The difference between adsorb and absorb is that if you get the minced pie on your face, that is adsorb, if you eat the minced pie, it’s absorb: just a way of remembering it.

So what happens is that all these positive charge come along and stick to the interface. And, as good engineers we know that if you get a layer negative charge, a layer of positive charge with a gap in between, we have a capacitor.

Okay. And the electrochemists, although they like to use big words, they like to use pretty simple ones in this case, and they call that the double layer capacitance, (pretty obvious). So you have a layer of charge, a layer of charge, with a separation, double layer capacitance.

What then happens is that we have the ions close to the surface; we’re now going to get those electrochemical reactions that we talked about, oxidation and reduction. So what happens is that you start to have charge being transferred. If you remember in the electrolyte, in the patient’s brain, in the gel, current is carried by ions, whereas in your circuits, in the metal part of the electrodes, is carried by electrons. So what you have now is a transfer of charge from one to the other.

We have this charge transfer, and it proceeds but with difficulty, unfortunately, and hence we call that the charge transfer resistance, and that is in parallel with the double layer capacitance. So we now have a very simple circuit, pretty good circuit, where we have the reversible

potential, we have the double layer capacitance, we have the charge transfer resistance, and we have the electrolyte resistance which is in series.

I have massacred electrochemistry. You can say things which are far more complicated than that, but for what you need, that’s actually pretty close to what you need, in my opinion. You can understand most of what we’re going to present.

25. Impedance, Rct

So the charge transfer resistance is actually given by a Butler-Volmer equation which involves things that look a bit like exponentials. You’ll notice the mysterious exchange current density – i0 again – and you will see that at the reversible potential – equilibrium potential – you actually have 0 current. You have current going in one direction i0, you have current going in the opposite direction -i0, but added both together, they’re 0. As you go away from the reversible potential, you then start to get very large currents on either side. We’ll look at that.

So the charge transfer resistance is taken as the slope of that for relatively small signals – you’re not going to be applying, hopefully, not going to be applying large voltages to your patients, I hope not – so because you’re measuring EKGs, you’re operating in a small signal size, so therefore we can approximate the complicated equation by this linear portion.

26. Impedance, Rct Equation

That’s the equation, lots of Greek symbols, again, I told you it was electrochemistry. And if you replace that Greek symbol, and think of it just as voltage, you’re pretty close.

So what I want you to notice is that the current is proportional to the exponential of the voltage. In fact, this symbol is over potential and it means simply the voltage drift away from the reversible potential. It’s called the over and above potential, so it’s over, or extra from the reversible potential.

So, what you can see is you have two exponentials: one is for the anodic reaction, and one for the cathodic reaction, and you have base. So the key thing is, that’s from an engineering point of view, you have a current which is proportional to the exponential of the voltage. So your resistance is going to be nonlinear, but you can simplify it for small signals, and you get a very simple explanation of the resistance.

27. Impedance, Rct Terms

So that’s some of the terms. The key thing that I want you to see is you have the exchange current density. We have overpotential, which is E (the potential that you’ve applied) minus the reversible potential. So it’s just a difference: the distance that you’re away from equilibrium, the overpotential, and that’s all you really need to know from that slide.

28. Impedance, Rct Dependences

So if you do the calculations and simplify an exponential, you end up with a charge transfer resistance which is proportional to 1/(exchange current density). That’s very important. If you understand that, you understand everything.

If you remember, exchange current density was what happens under equilibrium conditions. Let me give you a very Mickey Mouse – is what we say in Ireland, no disrespect to Disney – but let me give you a Mickey Mouse explanation of what goes at the electrode interface.

As I said, the ions diffuse, they come and they stick to the electrode surface. It’s a bit like boats, ions, boats in the water, they come, and they come up to the harbor, and they anchor or whatever the term is, onto the harbor. At the harbor, you have all these lorries – I don’t know if in America you call them lorries, vans, I don’t know. England and America are two countries separated by the same language. So, yes, whatever you call them, okay? So what we have done, we have the boats come with all the cargo, as you can see in all your wharfs, and then we’re going to charge and discharge the lorries.

So, we’re not actually going to transfer anything out of San Francisco port; what we want to do is just test to see how good the guys are at charging and discharging the boats. So what we do is we charge the boat, and we time, and then we discharge the boat.

The point is that’s under equilibrium conditions: we’re not actually going anywhere, we’re just seeing what our capacity is. And then we go gee, yeah, we can load a boat in half an hour, great.

That means that when we actually have to do any real work, we can assess whether we’re going to be able to do it with comfort, or it’s going to be very difficult, or impossible. So, the exchange current density is a measure of the capacity of the electrode with current. If it, normally, even when it’s at rest, it can send a current of i0 that way, an i0 that way, and it can keep you at 0 very easily, then when you apply a current which is around i0, it’s going to cope very easily. So it’s a measure of how well the electrode can cope with passing current through.

If you have an electrode which has an enormous capacity to pass current, an enormous i0, well then, no problem. If, on the other hand, you have an electrode that can, you know, sort of move one box per R, then you’re not going to be able to load your vans or lorries, and unload the boats very quickly.

That’s important to see. Exchange current density is therefore related to the resistance, the impedance you have to the flow of current. A very good electrode will have a very large charge transfer resistance. A very bad electrode will have a very large charge transfer resistance and a very low exchange current density.

29. Total Impedance

So, back to the model. Just to remind you again: we have a charge transfer resistance which is a measure of how good the electrode is often, the double layer capacitance, and your reversible potential with this series resistance due to underlying fluids, your tissues, your electro gel, whatever.

30. Impedance Plot

I’m going to presume to give you a lecture on electrode impedance. Again, I hope that I’m not insulting your intelligence. It’s just I’m going to show you the results and it’s important that you can see for yourself what they mean. So I’m going to show you complex impedance plots, I’m also going to show you Bode diagrams. And also for some of the lectures that are going to come afterwards, it’s important that you know what a Bode diagram is and what a complex impedance plot looks like. Unfortunately in many publications, that may not necessarily be the case.

So it’s the resistance – series resistance – plotted against a series reactance; negative, because most of the things that we’re going to look at: tissues, electrodes, skin, are capacitive, and so we have this plot. It’s called a complex impedance plot – some people call it Nyquist, all sorts of strange names – complex impedance plot says what it says.

So, for example, if I have a simple resistance, I can change the resistance – the applied frequency – forever. Nothing is going to change: I’m going to have pure resistance, and it’s going to be represented by a point on the resistive axis R. No matter what I do with a frequency, it’s still a resistance of 10Ω, it’s never going to change, there’s no capacitive component, nothing is going to happen. So a simple dot is for resistance.

If, on the other hand, you have a capacitor, the impedance of a capacitor is 1/jωC. So, I don’t have any resistance, I can change the frequency from today to kingdom come, it’s never going to change, so I have absolutely no resistive value. But, depending on the frequency, I’m going to go up and down on this line – vertical line – happy enough?

Moving ahead gracefully then. If I have a resistance and a capacitance in series, again, I can change the frequency all I want; the resistive value will always be on this part, so if it’s10Ω, it’s 10Ω. However, the capacitive component will change due to the fact that the frequency changes and the reactance depends on the value of the frequency ω.

Making it slightly one more step in advance: if we have a resistor and a capacitor in parallel, what happens is that if I apply current, dc current, dc can’t go through a capacitor, as I remember, so what happens is that all the current goes through that resistance, and the dc you have a pure resistive value equal to R. If you go to a very high frequency, the current goes right through the capacitor, 1/ωC, at very high frequencies, the impedance of a capacitor is zero, and you have a purely resistive value of zero.

But in between, for frequencies in between you have this capacitance, lets current through, and hence, you have two impedances in parallel. And as you know if you have two impedances in parallel, it gradually decreases. So what happens is that if you start off here at ω = 0, gradually you start to have the impedance, the magnitude of impedance, which is from here to here, is gradually decreasing. If we go to this dot, you can see the magnitude of impedance has decreased, until from here to here, but also you’ve got an imaginary term now because current is going through the capacitor. Say that again, or is that okay?

Purely dc, you have pure resistance. As you start to increase the frequency, more current starts to go through the capacitor, so you start to get a capacitive component, and because you actually have two impedances in parallel, your magnitude of your impedance decreases: here it was maximum, equal to R, and gradually starts to decrease until it goes right down to zero.

Now I can start to present to you some of the curves.

31. Impedance Plot-Curves

So the overall circuit is a resistor in series, and all that changes is the high frequency term, resistive term. The very high frequency, what happens is that you have the current goes through here, R1, it goes through the capacitor (which has an impedance of zero, and you therefore have only the R1 that is seen. At very low frequencies, you go through R1, you can’t go through the capacitor, so it goes through the resistor, and you have R1 + R2. Et voilà!

32. Bode Plot

Stretching you slightly, it is the morning and you’ve all had your coffee, so I’ll stretch slightly more and then I’ll stop, honest.

You then have to have the Bode diagram, you need to have both and you’re going to make big mistakes if you just use Bode diagrams. Let’s face it, come on, as an electrical engineer, if your results are awful, really bad, what do you do? You put them on a log-log plot, and they all look straight, they all look nice, and everybody’s happy. That’s the truth.

So be very careful that you don’t use l log-log plots by themselves. Recently we in France had a series of experiments on fibrosis in vivo, exciting stuff. The Bode diagrams looked wonderful until we actually looked at the complex impedance plot and we realized that the results were total rubbish because we had negative resistors, wow! But on the Bode diagram, you know, it all just came out as the amplitude, so it all looked good, and we kept on turning the handle and doing the experiments, only to find that it was total rubbish. So be very careful about doing Bode diagrams. It’s a very nice way of massaging your data to make it look as if it means something, be careful. Do both.

So if you look at a Bode diagram, this is a Bode diagram here, its log of its impedance, at DC you have again, current goes through the resistance R1 and the resistance R2, so up here you have R1 + R2. At very high frequencies, it goes through R1 and then through the capacitor which has zero impedance, so you have done here the limiting value of R1.

In between, you have this capacitive component coming through so the magnitude of the impedance gradually starts to decrease. As I said, you can see it from here: the magnitude at DC is from here to there, and as you start to increase the frequencies – let’s say to that point – your magnitude of your impedance is now from that point, to there. It’s actually a lot smaller, and as you continue to increase your frequency, the magnitude to any point then starts to get smaller and smaller until you eventually have reached this limiting value of R1.

I’m doing my best with my finger on the mouse, I hope that was clear. Normally I’d be dancing around with my arms, but anyway.

The same thing with the phase. Phase is important to look at because what happens at DC, you have pure resistance, therefore the phase is zero as shown. But what happens is the phase starts to increase, increase, increase, probably until about there, and then you have a phase angle of, what’s that look like, about 40? So it increases, increases, increases, until it’s about 40, and then as you continue down your phase angle actually gradually decreases until it gets to zero at very high frequency.

So, you now know, hopefully, a little bit about Bode diagrams, and Nyquist plots. In the reference section in the very end of my talk, I’ll give you books that you can look up to get more information.

It is important because a lot of people who use impedance, a lot of what they present are total artefact, and it’s embarrassing, so, please make sure that you do understand a Bode diagram and a complex impedance plot.

33. Surface Electrode

So, surface electrodes. We’re talking about impedance, and they asked me to talk about not only electrode impedance on the surface, but also implant electrodes; they’d like to make my job difficult.

So, what do you want for a surface electrode? What we would like is that the current, the biosignal that is coming out from my heart, flows directly out through my electrode, totally unimpeded. I don’t even want to see my electrode, the current just flows straight out and into your circuit. No impedance, no voltage drop, no signal attenuation, none of that business. That’s what we would like.

So we want a charge transfer resistance of 0. We want a harbor where it goes straight through: no impedance, no delay. That’s what we want.

34. Implant Electrode

What about an implant electrode? It’s a catch question. What do we want? Well, I want, if I have a pace maker electrode in my heart, I want the current to go straight through. So I want a charge transfer resistance of 0. What’s that mean? That means that there’s an electrochemical reaction, and the electrochemical reaction is so fast that the current flows through. But do you really want in your heart, an electrode chemical reaction where there are metal ions going into your heart? Do you want that in your brain? The answer is no, you don’t.

So you have a problem in that, from an engineering point of view, we want electrodes that have zero charge transfer resistance. Rather, you want something that is biocompatible? What’s that mean? It means you want a charge transfer resistance that’s really, really, really, really big, so

there isn’t any electrochemical reactions. You don’t want electrochemical reactions in your brain, trust me.

35. Implant Electrode

So you have a paradox. What happens is that people use noble materials where you don’t get any reaction. That’s not quite true: work with me, I did say it’s an Irish engineer’s explanation simplified. It’s nearly true. So it’s biocompatible, and it’s got a really, really, really, really, really, really, big charge transfer resistance.

So from an electrical point of view, you’ve got a big problem because as an electrode it’s rubbish. It’s nice and biocompatible, it’s nice, but it doesn’t work.

So, this is, again, the Irish engineer’s explanation of all implant electrodes, all implant electrodes have rough surfaces. There, I can go home now.

If you take away all the pseudoscience and complex terminology and things, boil it all down, it means they have rough surfaces – activated for trace carbon, pores, this, blah, blah, blah. It all sound good, it gives you mystique; but the bottom line is that it’s generally a material which is biocompatible and hence terrible from an electrical point of view, and you roughen the surface.

Irish engineer’s explanation. And it’s often referred to as “using a depolarising layer:. And, I want to stop for a wee while and amuse myself with the word depolarising. Depolarising, what the heck does polarising actually mean, really? Have you any idea?

Most biomedical engineers – I know you’re not biomedical engineers, you’re much, much more intelligent than us, biomedical engineers – biomedical engineers, we haven’t actually any idea what it means.

I’ve worked in hospitals, you talk to medics and they talk Greek and Latin, and we haven’t a clue, I mean, I don’t know what big words they use for their knee cap, they can’t call it a knee cap, they have to call it something fancy. So, we engineers, we have an inferiority complex. So we defend ourselves as best we can. We’re using a few, you know, it’s the multiplex, you know, we talk a few alliterations, so people don’t understand us, and it gives us an aura of intelligence.

Depolarising and polarising are perfect examples, trust me. I’m being humorous but try it, you’ll see it works.

36. Impedance, Polarisation

What is electrode “Polarisation”? It is truly the greatest mystery in the universe. And because we’re being filmed, I’m not going to show you examples to prove my case, you’re just going to have to – you know, do I look trustworthy? Do you believe me? And I don’t need the embarrassed textbooks and websites and famous companies, by showing you examples, okay? Please trust me.

Anything a biomedical engineer doesn’t understand about electrodes is polarisation. It’s a magic term. Go back to your lab and when you are talking about your electrode, there’s something that you haven’t a clue about, pause, look knowledgeable, and say ah, its polarisation. It will shut their mouths, no one will dare ask you a question, and it will give you an aura of absolute profound intelligence, and that means absolutely nothing. But it’s a very good term.

Often it’s used to describe the double layer capacitance, for historical reasons, which I’ll show you. Sometimes it’s used to describe the fact that the double layer capacitance that I’ve telling you about is actually not a capacitance. It is frequency dependent: instead of having a phase angle of 90° as capacitors should, it doesn’t. So, people talk about that as polarisation. You’ll find in the literature, the fact that the impedance is nonlinear, I told you the charge transfer resistance is due to the Butler-Volmer equation (so it’s an exponential), so if you apply larger signals, the charge transfer resistance decreases, and you get all sort of complicated things. That’s polarisation.

You have electrode noise, that’s polarisation. The list goes on. Anything you don’t understand or looks puzzling, it’s polarisation.

37. Impedance, Polarization 2

I think where it started was with Herman Schwan, who was the grandfather of electrodes, a very able physicist, and he tended to use the word polarisation in its classical sense, which means that you have, you know, if this an audience is polarised, it means that one lover over there, and one lover over there and they probably don’t get on too well. And that’s what it tends to mean in classical physics and chemistry is that you have a separation of charge, and hence, you can see why the double layer capacitance is called a polarisation element, because it’s got to do with charges that are separated.

And it became associated with this rather strange empirical observed impedance, where we saw that the double layer capacitance doesn’t behave like a capacitance, it behaves with something with a strange phase angle. So instead of it being 90°, if that was (jω)1 there, then you would have a standard capacitor.

But what you’ll find in life is that tissues, interfaces, batteries, anything you look at, behave in ways that they’re not supposed to, and they have strange capacitors. C’est la vie, it’s a big area of study.

So it could be that the pseudo capacitance, the strange capacitance, the double layer capacitance which is rather distorted, is the polarisation. So you’ll hear a lot of biomedical engineers talking, after Schwan, of the polarisation impedance being the double layer capacitance. That leads to extreme complications, confusion, because in electrochemistry it means the exact opposite.

So that’s a complex impedance plot. Instead of it being a nice semicircle, you’ll see that here, it joins and hits the real axis with an angle which is less than 90, and you’ll find that if you study impedance in just about anything, you’ll find this strange behavior. It’s very interesting; it’s my thesis and many years of research.

38. Polarization in Electrochemistry

However, in electrochemistry, and we are talking about electrochemistry, it means the change of the potential of an electrode from its equilibrium potential upon the application of faradaic, they like to use big words, faradaic, DC.

What does that mean to an average engineer? It’s something very simple. If you have a reversible potential here, or equilibrium, or if you’re a biomedical engineer, half-cell potential, and you pass a current through it, lo and behold, something very strange happens: a current goes through a resistance and you get a voltage drop. You get an extra voltage so that your potential is no longer at the reversible potential, it has actually an extra, or overpotential, which is what the electrochemists call it. So overpotential is where you get this extra voltage because you have a current flowing through your simple resistor, and that is called polarisation: it’s when the potential of the electrode deviates away from its equilibrium, or reversible potential. That’s it, very simple.

39. Impedance, Polarization Summary

But, just in case you didn’t grasp it because it was so difficult, V = IR, I’ll do it again. You have your reversible potential, DC current can’t go through the capacitor so we’ve taken off the capacitor, and if you pass the DC current, it must go through the charge transfer resistance, and hence you have a voltage dropped across that charge transfer resistance I x R, and that is called the over-potential, and it’s that change in potential which is polarisation. Et Voilà! Now you know.

40. Ideal Surface Electrode

So, if you wanted to have an ideal surface electrode, you would want to have an electrode that has zero charge transfer: charge just flies straight through as if you’re on the freeway, I think you say, motorway, straight through. So what would happen is you could pass any amount of current and nothing would ever change. The electrode would always stay at its reversible potential. That would be an ideal nonpolarisable electrode. I was going to say they don’t exist, in biomedical engineering, they don’t exist, so if you see an electrode advertised as nonpolarisable, take it with a pinch of salt. They don’t actually exist in our area.

What does exist is that you have an electrode which has a slope, and if you pass current through the electrode, then the voltage will to some extent or the other, change. And the slope of that gives you your value of charge transfer resistance, so a very good EKG electrode is one that has a very small charge transfer resistance; a very bad one is one that has a very large transfer resistance. You would have to be careful when you start to use electrodes and you’re doing it in the wearable area that you don’t pick bad electrodes. And Murphy’s Law is if you’re going to pick bad ones: you’re going to pick nice shiny electrodes because they look nice and they’re clean, so you’re going to pick stainless steel, for example. And you’re going to make the same mistakes that everyone else in the universe has made. You want to pick nice clean electrodes and they’re nice and conductive. So if they’re conductive, they must be good electrodes, and when you put them on a patient, they don’t work, but that, we’ll get on to that later on.

So be very careful, you’re probably going to pick very bad electrodes. And if we want an implant electrode, what we want, ideally, is an electrode where the charge transfer resistance is infinite. Which is not true either.

41. Impedance, Polarization - Cole and Curtis

So, because we’re being filmed, I haven’t shown you a rogues gallery of all the big companies and textbooks, they got it wrong, you’re just going to have to take my word for it, I have skipped to the conclusion which is one by Cole and Curtis in the thirties, which is brilliant:

“…the use of the term polarization for describing the unexplained effects occurring at the metal-electrolyte interface is only an admission of our ignorance.”

Mega, very good quotation.

42. Surface Electrode Metals

So, let me now move on to actual metals that people have used, and people will use.

43. Surface Electrodes

What we would like ideally, is to have zero electrode potential. We would like to have it nonpolarisable – current goes straight through – zero interface impedance (which means almost the same thing, because if you have zero charge transfer resistance, you have your double layer capacitance shorted out so, zero interface impedance), and you would like zero noise. That’s what you would like.

44. Early Electrodes

What happened in the past is that people used whatever was in the lab, and that’s happening again today, so things repeat every one hundred or so years. So, Eindhoven, for example, had a very bad, well it was very good at his time, but was a very bad amplifier. It had a very, very low input impedance. So, if he wanted to get good EKGs, he had to have extremely low contact impedances. So he used dirty great big buckets of salty water. And the only thing that you could put into dirty great big buckets of salty water are your arms, legs, and possibly your head, but patients tend not to appreciate that too much, so it’s arms and legs. That is why, still today, when you go for your gold standard EKG, where do they put the electrodes? On your arms and legs. Why? It’s because that’s the only limbs that Eindhoven could actually put in buckets of water. Often the truth is somewhat strange.

45. Stainless Steel, etc.

What happened though is that Eindhoven and other people started to improve on their design of amplifier, and that meant that you could start to use smaller area electrodes: you didn’t have to use great big buckets, which is fortunate because otherwise today, you wouldn’t be able to do much ambulatory monitoring unless you put skid boards underneath the buckets. So they started

to design smaller electrodes, again, strapped to the wrists; stainless steel because they’re nice and shiny, conductive. And, eventually, they started to develop even smaller area electrodes for sticking on the chest, called welsh cup. They weren’t invented by the Welsh, it was a man called Welsh, and it was a suction cup. You still see them in some hospitals today. Maybe not in the U.S., who knows?

46. Metals: Surface Electrodes

So early electrode materials were readily available: ease of manufacture, low cost, and generally very poor performance. Be very careful when you start to work in the wearable area, you don’t make the same mistakes.

Again, classic one, hopefully it’s now past – the fad has past – but everyone started to use stainless steel, because, if you put stainless electrodes into a t-shirt, then you can wash it. And after all, stainless steel is conductive, therefore, it must be a very good electrode, mustn't? The answer is no.

So more demanding applications require better performances, and that’s when silver/silver chloride started to come along.

47. Surface Electrodes - Ag-AgCl

The advantages of Ag-AgCl are that it has very low stable potential (it’s not zero), small interface impedance (not zero), it’s relatively non-polarisable (it is polarisable but it is less than most of the other rogues gallery), and it has very low noise. So it tends to be the gold standard of electrodes at the moment.

48. Electrochemistry

I’m not going to go in too much to the electrochemistry, just to give you an Irish explanation, is that if you have silver, for example, as your metal (it’s nice and conductive), and you have the patient (who is made of an electrolyte: sodium chloride), then you have chloride ions here, you have a metal, and you have this buffer bridge of silver chloride, which sort of has half/half, properties. I did say that I was going to massacre electrochemistry. But for length of time, to simplify it, you have this intermediate layer which facilitates the transfer of charge, and hence, the potential is very low, and the charge transfer resistance is very low as well.

I’ve recommended some books which go into it with a bit more detail than that.


So, if you use silver electrodes, gold, they tend to not react, so the potential tends to vary (it’s actually quite difficult to know what potential it’s going to have and it varies with time). If you use Ag-AgCl electrodes, it tends to have a very stable potential, especially once in the presence of an electrolyte that has chloride – that’s what it works on – and since the patient, or the gel, has lots of chloride, hopefully, then the potential tends to be rather stable, and very small.


Now this is why I showed you some of the diagrams about Bode diagrams. If you remember, we have a charge transfer resistance in parallel with a double layer capacitor, so, a Bode diagram at zero (or at very low frequency) you will get something. The magnitude of the impedance due to your series resistance, plus your parallel resistance (which in our case is the charge transfer resistance), and then you get this capacitive roll-off down to this limiting value of the series resistance.

And as you can see, all of these metals have this part here, and you can see that, with the eyes of faith, that eventually they will sort of limit the way up here, and hence they have very large charge transfer resistances. Way off the scale.

What’s shocking is if you can actually read that, it says that the impedance of Ag-AgCl electrodes is less than 10Ω for these frequencies. That means that the charge transfer resistance and the impedance of a silver/silver chloride electrode is off the scale, it’s so small.

So compared to gold, platinum, stainless steel, tin, all of these have massive charge transfer resistances, and silver chloride is so small, it doesn’t even fit onto a log-log plot. Charge transfer resistance of silver chloride is massively smaller than a lot of these other materials, which is why it’s used so widely in biosignal monitoring.

51. Implant Electrode Metals

Let me talk briefly about implant. Again, I was asked to cover that, just in case you’re looking at implant materials and implant electrodes.

52. Implant Electrodes

What we want are electrodes which are perfect, and hence, we want them to be biocompatible, we want them not to couple through the charge transfer resistance, but that’s bad, that’s electrochemistry and reactions and things, that’s not nice. So what we want to do is that they couple capacitively through the double layer capacitance, and not through the charge transfer resistance.

We also want our cake and eat it: so we want to have, not only does it couples capactively, but that the interface impedance is very small, because we don’t want to waste our energy for stimulating or we don’t want to distort this signal if we’re recording with large interface impedances.

53. Biocompatible

So here are a list of some of the materials that used to be used, or still are used, especially in pacemaking and things like that, where you have lots of noble materials tend to be used, alloys of noble materials, but also some other materials which tend to act like noble materials, either due to surface layers or whatever.

54. Couple Capacitively with Tissue

This is a transient response. If you apply a pulse of current to an electrode, you tend to get something that looks like this: your exponential response, looking at the voltage response.

What I was wanting to show is that this part is what interests us: we don’t want the current to limit, so if you apply a pulse to an electro inside someone’s brain and you see that it limits, then you know that the current, from an engineering point of view, is going through the resistor. If it starts to limit here and level off, then it’s purely resistive.

But you now know from an electrochemical point of view, albeit, Irish explanation, that if you’ve got a current going through the resistor, that means you have electrochemical reactions, and you will start to find things happening which are not nice.

So, lots of experiments. If you have the signal leveling, means it’s going through the resistance, means that you’ve got nasty things happening at the tissue interface.

So what you want to do is to couple capacitively, and you’ll always want your waveforms to look like that. So, there’s several ways of doing it. Either you always keep your pulse duration very short, so that you’re always there, that’s one solution.

Another solution is to use noble materials which have very large charge transfer resistances, and these are some very old classic diagrams, but you can see here, this is platinum, and platinum iridium, but you can see that they are coupling capacitively to tissue.

And what’s interesting is that the reaction at the anode and the reaction at the cathode are identical, meaning you’ve passed the current in one direction, you’re going through the capacitor. If you pass the current in the opposite direction, you’re going through the capacitor as well, you have something which is the same in both directions, and that’s a mark. The fact that it doesn’t level off, and the anode and cathode give you the same sort of traces, you’re in the safe area.

55. Small Interface Impedance

So what we said was that you want noble materials which have very large charge transfer resistances, however, that means they’re going to have very large impedance. So let me repeat that what you’re going to do, then, is to use very rough surface electrodes, and most electrodes that are implanted for one reason or another, are rough surfaced.

They made them rough surfaced in pacing in the very early days simply so fibrosis would grow into the pores, and that would hold the electrode in place, rather than to continue to move and cause lots of tissue trauma and excessive growths of fibrosis. But they then noticed by accident…whoo, the impedance is a lot smaller…gee, that’s cool, wonder why, and it’s got to do with the fact that your surface is very rough. More than that, the pores on the electrode behave like transmission lines and it means that you get a square root term so the impedance is drastically reduced.

So it increases the surface area, simplistically, increases the double layer capacitance, decreases the capacitive impedance, and increases your response time. So that, because it increases the response time constant, it actually flattens it out and makes it look as if you have an electrode which is non-polarisable, and yet it is polarisable.

So that is one of those optical illusions where, when you roughen the surface, you’ve actually changed the time constant and it actually looks as if you’ve hit a pure resistance when you haven’t.

56. Electrical Properties of Skin

So let me move on now to skin.

57. Skin Electrical Properties

And we did mention again that we have two aspects of the skin which are of interest to us: we have the contact potential and the contact impedance, as before with the electrodes.

So you have a nice signal which comes in, and because of the contact potential and the contact impedance, what comes out is something that you don’t actually want.

So let’s start with the skin potential.

58. Potential - Reminder

Reminder, why we’re interested in skin potential, is that you’re going to get this baseline wander, or motion artefact, if you have skin being deformed, stretched, in any way.

59. Potential - Human

The skin potential varies between about -10 and -60 [mV], and depends on a whole range of things, including skin site: I’ll show you some nice diagrams of that, some of the earlier work, whether or not you’re sweating, gel concentration, skin thickness, skin condition, and skin deformation.

So you need to be very careful and be aware that skin potential will vary from site to site; skin impedance will vary from site to site, and if I have time, I want to ram home that message, because if you test several electrodes, and, on different sites, you’re going to find that one’s better than the other and you’re going to go “Yeah! My electrode is better than all the rest,” and you’re probably only assessing the skin site, and not your electrode. That is a classic, classic, classic, classic mistake. The fact that I’m repeating that is that most papers that I review, make that mistake. Be very careful.

60. Potential - Skin Site

So here’s some nice old diagrams that are illustrated quite well. You tend to have, on sweaty areas, you tend to have very large potential – so here you have -50 – and on the feet as well, you have very large potentials.

So, again, depending on where you’re putting the electrode on the body, you’re going to have potential mismatch.

61. Potential - Skin Deformation

Another old slide/diagram is that here we have a potential of -30, well -25 [mV], and along here, it’s the number of sandpaper strokes with which you rub the patient’s skin, and the skin potential gradually decreases as you start to remove layers of the epidermis. What’s more interesting, certainly to the researcher, maybe not to the patient who is getting this skin rubbed, but anyway, is that the artefact, the fluctuation due to pressing on the electrode which is normally about 5mV, actually decreases. So if you rub the skin with glorified sandpaper what you’re doing is effectively rubbing away the source of your problem. And if you are (I assume you are), you’re all wanting to work in wearable and such things, you’re going to suffer a lot of motion artefact. So you can take a wire brush and you could rub down the patient, and remove their stratum corneum. They may not thank you for it, but, you’ll get very good results, you know.

62. Potential – Skin Prep

This was a system. It’s a glorified, in English we would say, Black & Decker, I don’t know, whatever, drill, and it has like a wire brush on the end, and I tried it on one of my Oriental students. And, the skin impedance of Irish – sweaty Irish men – and Orientals is very different, as you may or not may know. So this system has a nice light that comes on when you get to the right, the right, skin impedance. So I had a student called Chung, and I started to use this; the light didn’t come on, he was complaining – ‘ah come on Chung, are you a man or a mouse?’ – and I kept going. But I started to feel a bit embarrassed, it was getting for quite a long time, and when I took the system off him, and the electrode off him, it was a bit of a bloody mess literally, bloody mess. So he never did any more research with me, that guy. Hi Chung, sorry.

So, you can get systems like that, where you abrade the skin, and you will get very good traces. You can’t use those on a lot of different applications, and I’ll go into that in more detail.

63. Potential – Skin Preps - Comparison

So, there are a range of things that you can do: you can abrade the skin, you can rub it with alcohol, or you can puncture the skin, different ones. One of Murphy’s law of life is that a nurse in a hospital will take alcohol wipe and will rub the skin with alcohol. I was going to make a joke about, what a waste of alcohol, but that’s just an Irishman.

But, the point is, when you start to rub the skin with alcohol, you dry it out, you make it worse. Longer term it would be better because it’s nice and clean and the gel will penetrate, but initially, it’s worse. So, be very careful, again, these are just things that people do and they don’t think about it.

If you rub the skin with alcohol, rub it hard so that you remove some of the outer layers, then you’re going to have good results. But if you just rub the skin and dry it with alcohol, then initially you’re going to have very bad performances. It’s just, so that if you’re wanting to get that grant and you want to give a demonstration of your new system, or you want to get a company interested; I’m just giving you tips on how to make sure that your results are good, okay?

What also happens is, if you puncture the skin, you have the least artefact, but what tends to happen is that the skin regrows, and, it tends, therefore, not to be as good. So you need to weigh up and you need to think about your application and how long it’s going to be on the patient.

If it’s going to be on for a long time, there’s no point abrading the skin, because hopefully, hopefully, the skin will regrow – unless you’ve really destroyed the patient, then you’ve got a big problem. So you’re not going to abrade the skin for long term applications, it’s pointless. You will use skin abrasion for short term, and if I can say, tongue and cheek, you measure the patient’s EKG, you get them on a treadmill, and then you get them out of the hospital and away from you and that way if they have skin irritation, it’s not your problem. But if they’re going to be long term monitored in the hospital, you definitely don’t abrade the skin, because you’re going to cause trouble, for them and for you.

64. Potential – Solutions

The other thing is that you need to selectively place the electrodes, and if you look at the example of either stress test or some other Holter monitoring, they tend to put the electrodes away from wobbly bits, for two reasons: you take it away from muscles so that you don’t get EMG as the person exercises (every time I move); you don’t want EMG. But the other thing is that if you put it on part of the body that’s going to vibrate and move, than weight of the electrode and the cable, as you move, will stretch and deform the skin and cause lots of motion artefact.

So you need to think outside the box and start to put electrodes in areas of the body where there are no muscles and there’s no fat or flab that’s going to be moving.

If, for certain short term applications, you can use skin prep, again be very careful you don’t do that in young children/neonates below their skin isn’t totally formed; you’re going to cause terrible skin irritation problems.

You can use concentrated gel; the gel will penetrate into the skin and remove your problem, but that takes time, and, you wouldn’t use concentrated gel for long term applications, because long term, it’s going to cause skin irritation problems.

One of the best things to do is to delay before recording: so if you’re wanting to do that demonstration – I have been there and done all the t-shirts and all the rest of it – if you want to have really good results with your new gadget or gismo (dry electrode, whatever it is), put it on several hours before you do your demonstration. I know all the tricks. And then, by that stage, your skin has become well hydrated, even if the dry electrodes sweat has built up, and you’re

going to have beautiful traces, okay? What you don’t do, is you don’t put the electrodes on at the last minute.

So if you’re going to, for example, go to your hospital to do measurements, you will first of all put the electrodes on the patient, and then fiddle with your device, and get then to fill in forms, meanwhile, back at the ranch, the electrodes are actually improving in performance.

65. Potential – Solutions

So what you want to avoid, also, is direct pull on the skin; that’s going to cause skin deformation, stretching. So you will use stress loops, you will see that where they have the cable coming to the electrode is taped remotely from the electrode, that way if there is any pull due to the monitoring cable, it’s going to be here, and not directly on the electrode.

There are lots of different electrode designs, so you can get electrodes which have offset centers; so you connect here, you’re not connecting on a snap fastener directly on the electrode, and if there’s any pull, it’s going to be here and it will not pull directly on the sensor site. So there’s lots of different permutations.

With your new gismo monitoring device, whatever it is that you’re interested in, be very careful that you don’t put a weight on top of your sensors. So if you have your device, keep the electrodes at a distance away from your electrodes. If put your device directly on top of the electrode, as that moves, then you’re going to get lots of deformation of skin and you’re going to have lots of problems. Been there, got all the t-shirts.

66. Outline: Skin Impedance

So, let’s talk about skin impedance.

67. Skin:: Impedance

Just to remind you, the reason why we’re interested in skin impedance is because if you have a mismatch, you’re going to have signal attenuation, signal distortion, and interference.

Skin impedance is much, much, much, much, much, much, much bigger than your electrode impedance. So when you measure electrode skin impedance, you’re probably measuring skin impedance. And when you’ve come to the conclusion when your electrode is better than somebody else, your electrode metal is better than somebody else, you’re probably fooling yourself. Be very careful.

68. Skin: Impedance

So if you think of the skin – that’s the outer layer – as being dead; you have your nice, juicy electrode gel, you have your underlying juicy tissues, then you have two conductive layers sandwiching a rather dry dielectric there. You have, again, a capacitor, and we can represent that by a skin capacitance. You have lots of holes through your skin, appendages, hair follicles, sweat

glands, etc., and current can flow through that capacitor, so you represent that by a resistance in parallel. And then you have the underlying tissue resistance.

So you can see that I’m not very innovative: we have the same parallel circuit as we had for the electrode, we had the double layer capacitance charge transfer resistance, the resistance of the gel (which can be significant, bare that in mind when you’re doing your measurements, it can throw you, especially if you use hydrogel electrodes, you will get actually sizable, or can get sizable resistances). And this is the epidermal layer; it’s the double layer of the parallel capacitance and the parallel resistance of the skin.

Because skin impedance is much larger than electrode impedance, as I said, it tends to dominate many, many fold; so be careful in how you interpret your data.

69. Skin: Impedance

These are some results – complex impedance plots – of my skin, but again what you can see is that instead of it coming down here at 90°, it tends to come down at an angle of about 70 or 80°. That’s actually important information of you want to diagnose skin complaints, things like that. So that phase angle is relevant: don’t throw the information out if you’re wanting to characterize tissue, for example.

70. Skin: Impedance - Dry

Skin impedance, if you don’t have gel, if you just put a dry electrode on, then it’s going to depend on the patient: people with dark skin tend to have far higher impedance than people with light skin. I designed electrode systems for potty Irishmen like myself and they worked fantastically well, and we then had an Indian student, we tried it on him, and lo and behold, it didn’t work, and that was back to the drawing board. We were sending it out to the Middle East, and it was just as well, we tried it out on a patient with very dark skin. So be very careful.

Skin site, I’m going to show you that. Again, be very careful where you put electrodes; your skin site impedance will vary and you will then get large impedance mismatch.

Electrode area. Again, the larger the electrode, obviously, the better the impedance, but you can’t make very big electrodes, you don’t want to make very big electrodes.

Contact pressure. Be very careful when you’re testing a dry electrode against somebody else’s if you want to fudge the results, not that you’d ever want to. But if you want to fudge the results then you press very heavily on your dry electrode and you don’t press too heavily on the wet gel electrodes, and lo and behold, they come out almost the same, and everybody’s happy and you get the grant and you get your publication, but when it’s going to be put out in the real world, it doesn’t work. Contact and pressure is one of the areas where you can fudge your results.

Skin condition. Again, if the person has eczema or whatever, it can either mean that the person has dry skin or the fact that their skin has breaks in the outer layer, means that it would be more conductive, depending on which.

Time after application. Again, if you want to fudge your results, very good technique is that you take your electrodes and you put them on: now that I’ve just rolled up my shirts sleeve, my skin’s nice and warm, and I’m going to put it near here because it’s nice and sweaty, especially on me, and I’m going to measure mine there, and I’m going to wait a wee while until my arm is nice and cool, and then I’m going to put the oppositions electrode probably at the back of my arm because that’s high skin impedance, and lo and behold, my electrode is always better than theirs. Not that you would ever do that, but, people seem to do it accidently – well, they assume it’s accidently.

The protocol on how you assess your electrodes is most important. You can fudge your results and make them say anything you want. The problem is, they’re not going to work in real life. So it’s up to you.

Emotional state of the patient. You could actually fudge the results by stressing your patient, I’ll show you some results on that, but if you make them stressed, they sweat, and you get very good results. Give them a cup of tea, hot tea would probably make them sweat, or show them that machine that I had there, that Black & Decker drill, that would probably stress them.

Applied signal amplitude as well.

71. Skin: Impedance – Dry Electrodes

Here are some results. People have assessed different dry materials, stainless steel, aluminum and titanium, and they’ve come to the conclusion that some metals are better than others. That’s not true.

If I put stainless steel here, where it’s nice and sweaty on my arm, and I put aluminum here and titanium somewhere else, whichever one is closest to my fold in my arm will have the best results. It’s got nothing to do with the metal, it’s got to do with my skin site. And again, if I put this one on first, do all the measurements for half an hour, and then I do this one, then I’ve definitely got the best results that I wanted.

So be very careful. Dry electrode contact impedances vary: often, mostly, nearly all the time, it’s not due to the metal; it’s due the skin site. So be very careful on your experimental protocol, that’s what I’m wanting to get through; I’m being humorous by saying you can fudge your results. The point is I’m wanting you not to fudge your results and not waste your time by coming to wrong conclusions. It’s a very common mistake.

72. Skin: Impedance – Gelled

Gel composition. If you have a wet gel, obviously it depends on the composition, it depends on the concentration (the more you make it concentrated the more quickly it diffuses into the skin), and also skin preparation.

73. Impedence - Patient-to-Patient Variations

What I found in my results that were published a few years ago is that it’s generally the skin resistance that varies from one patient to another, from one site to another, and the skin capacitance tends not to vary from one site to another or even from one patient. Again, as I mentioned, people with dark skin tend to have much higher skin impedance.

74. Impedence - Site-to-Site Variations

This is, again, some of your earlier results that were published. And it was found bizarre that you have this, sort of, almost, part of an ark, and these are mono frequency, I think it was 10 Hz, impedances of different parts of the body. And if you actually plot them, you can actually form a semi-circular ark whose center is on the imaginary axis. If you do the calculations, it means that it’s the skin resistance that’s changing and the skin capacitance is constant.

For all of these sites, the skin capacitance is constant, and it’s only the skin resistance which is changing.

These two parts of the body, the palms of the hands and fingers, and also the soles of the feet, lie on a different semicircle because they have a different capacitance. You can guess why, because there, the skin is much thicker, and hence their skin capacitance differs. But most parts of the body, or for most patients, you’ll find that it’s the skin resistance which changes.

So, I’ve drawn here a semicircle. So, for example, the chest, this point of 10 Hz, is on the semicircle, but here, whereas on the back, you can see that we’re on a larger diameter semicircle. So the resistance of this semicircle is much bigger than the one with the chest. And that’s what you’ll find is that the inside of the body is more delicate and has lower impedance, the back tends to have a higher impedance, inside of your arm, lower impedance, outside of your arm, higher impedance.

So, again, if you want to fiddle your results, you can put electrodes on different sites. What you need to be careful of, however, is if you’re putting electrodes on different sites, you’re going to have some impedance mismatch; that’s the key point if you’re wanting to monitor. So outside of the body tends to have a higher impedance than the inside of the body. Hairy, sweaty areas tend to have a very low impedance, and your skin on the soles of the feet, and on your hands, tend to have much thicker skin, and hence their impedance is different.

75. Skin Impedance- Pressure

You need to be careful of pressure. Acupuncture points, for example, are detected by pressing and wiggling probe, and you will detect a low impedance spot, but the point is you’re actually making the acupuncture point where you’re trying to find it, unfortunately.

So be careful, pressure, depending on how you do your measurement, you actually can be distorting your results.

76. Skin: Impedance - Electrode Gels

Standard electrodes. You can find that you either have wet gels or solid hydro gels, and the general function of an electrode gel is to ensure good electrical contact; that sounds pretty obvious, but if you think of this skin having a very rough surface, and you have a flat metal, then you’re not going to make very good contact.

So the gel actually helps to make a good electrical contact with the gel’s conductive.

Decreases the skin impedance, and also decreases motion artefact: by wetting the skin, you remove your problem.

77. 'Wet' Gel Composition

Wet gels tend to be based on the thickener, electrolyte, and they also have lots of other things added on to wet the skin, or to, even sometimes, abrasives. So depending on your application you would use different gels: you would use a very mild gel on babies, for example, not an aggressive one.

78. 'Wet' Gel Concentration

Time constant, if you put an electrode on a patient, if it’s a fairly high concentration electrolyte, the impedance would decrease and it would take about 10 minutes time constant; so the longer you wait, the better your results.

79. Wet Gel Concentration - Results

Here’s some results. I’ve run out of time so I’m going to skip that, but it’s just to show you that normally, when you put an electrode on a patient, what happens is that it tends to be due to this change in resistance of the skin: the ark diameter gradually decreases pseudo exponentially. I’ll miss that out.

80. Wet Gel Design

Wet gel design tends to be – nowadays they’re mass produced, but they used to be, anyway, more complex in design – you have a gel impregnated sponge, retaining ring, and you have a snap fastener that you can see through there and at the back.

81. Wet Gel Advantages

The advantages of a wet gel is that it actually wets the skin, and, hence, you get higher performances.

82. Wet Gel Disadvantages

The big problem, however, with hydro gel, is that it tends to not hydrate the skin, so be very careful what gel you’re using.

Disadvantages of wet gels are that they tend to be a bit more complicated designs, so they can be a bit more expensive. They’re not repositionable because if you put it on the skin and then move it, the wet gel remains on the first site. And, because of the big adhesive foam, especially if you use concentrated gel, you then get lots of problems with skin irritation.

83. Hydrogel Composition

Hydrogels are hydrocolloid, so they look a bit like a jelly. They are reasonably solid and that makes them v

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