**3. Electrodes**

Electrodes play a crucial role in bioimpedance measurements. The sensitivity to tiny impedance changes, as well as stability and repeatability of measurements depend on the quality of the electrodes. Bioimpedance variability during cardiac cycle is usually measured with two pairs of electrodes: two current-injecting electrodes and two voltage-sensing electrodes. This configuration cancels out electrode polarisation impedances and reduces dramatically the influence of skin-electrode contact resistance. However, quite frequently we cannot see this advantage, and the electrode-skin contact impedance remains prominent exceeding the actual bioimpedance of interest, which greatly affects the end signal quality [25]. This is especially important when measuring heartbeat-associated impedance variations from the wrist area, where they are minuscule (order of mΩ). Choosing appropriate electrodes increases the correct result probability, but the top skin layer (*stratum corneum*) against the electrode is very dry and badly conductive making electrode design extremely complicated. The main type to consider for bioimpedance measurements is disposable non-polarizable and pre-gelled silver/silver chloride (Ag/ AgCl) electrodes. Pre-gelled electrodes have usually the lowest skin-electrode impedance, low motion artefacts and low noise level [26]. Unfortunately, as they are suitable for single use only, we do not consider them for wearable devices. Dry electrodes are a more prospective choice, but due to lack of gel between the skin and the electrode, there exists a significant capacitive layer, which increases the total impedance and the probability of motion artefacts [27].

#### **3.1 Electrode placements and materials**

The total impedance Z of the wrist consists of the invariable basal impedance Z0 and a variable part ΔZ(t) that is caused by the pulse wave. As a result, the impedance expresses as

$$\mathbf{Z}(\mathbf{t}) = \mathbf{Z}\_0 + \Delta \mathbf{Z}(\mathbf{t}) \tag{1}$$

**19**

**Figure 9.**

*of the electrodes.*

**Figure 8.**

*the radial artery (reprinted from [27]).*

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform*

*Dimensions (a), design (b), and placement of a custom-made flexible four-electrode system in the case of a distal (c) and circular (d) locations on the wrist, where the thick red line denotes the approximate location of* 

*Frequency response of measured (a) ΔZ(t) and (b) Z of the wrist in the cases of distal and circular placements* 

*DOI: http://dx.doi.org/10.5772/intechopen.86065*

In order to detect the cardiac activity, the interesting variable is the ΔZ(t), assumedly reflecting the volume change of pulsating blood in arteries. A custommade flexible electrode (**Figure 8a** and **b**) was used, positioned distally (**Figure 8c**) and circularly (**Figure 8d**) on top of the location of the radial artery.

Suitable materials for electrodes must be found and thoroughly tested for truly unobtrusive and reliable pervasive monitoring. Easy applicability is paramount. They should not irritate the skin; their parameters should stay reasonably unchanged during the acquisition cycle and should be insensitive to motion-induced stress.

In order to evaluate the effect of distal and circular placement of electrodes on the radial artery to the measured values of Z and ΔZ(t), the experiments were carried out having the excitation signal with the amplitude of 500 mV in the frequency range of 10–5000 kHz. The results are visible on **Figure 9**. The ΔZ(t) is few times higher in the case of longitudinal placement of electrodes (**Figure 9a**, red line) than in the case of transverse placement (**Figure 9a**, blue line). The total impedance Z is on average about 2.7 Ω higher in the case of transverse placement than in the case of longitudinal placement. When Z is decreasing with frequency, the ΔZ(t) is maintaining its relative *Noninvasive Acquisition of the Aortic Blood Pressure Waveform DOI: http://dx.doi.org/10.5772/intechopen.86065*

#### **Figure 8.**

*Wearable Devices - The Big Wave of Innovation*

**3. Electrodes**

disturbance when making provisional experiments with simple stainless steel electrodes [24], and the role of movement artefacts was highly troubling. This implies that

Electrodes play a crucial role in bioimpedance measurements. The sensitivity to tiny impedance changes, as well as stability and repeatability of measurements depend on the quality of the electrodes. Bioimpedance variability during cardiac cycle is usually measured with two pairs of electrodes: two current-injecting electrodes and two voltage-sensing electrodes. This configuration cancels out electrode polarisation impedances and reduces dramatically the influence of skin-electrode contact resistance. However, quite frequently we cannot see this advantage, and the electrode-skin contact impedance remains prominent exceeding the actual bioimpedance of interest, which greatly affects the end signal quality [25]. This is especially important when measuring heartbeat-associated impedance variations from the wrist area, where they are minuscule (order of mΩ). Choosing appropriate electrodes increases the correct result probability, but the top skin layer (*stratum corneum*) against the electrode is very dry and badly conductive making electrode design extremely complicated. The main type to consider for bioimpedance measurements is disposable non-polarizable and pre-gelled silver/silver chloride (Ag/ AgCl) electrodes. Pre-gelled electrodes have usually the lowest skin-electrode impedance, low motion artefacts and low noise level [26]. Unfortunately, as they are suitable for single use only, we do not consider them for wearable devices. Dry electrodes are a more prospective choice, but due to lack of gel between the skin and the electrode, there exists a significant capacitive layer, which increases the total

The total impedance Z of the wrist consists of the invariable basal impedance Z0 and a variable part ΔZ(t) that is caused by the pulse wave. As a result, the imped-

Z(t) = Z0 + ∆Z(t) (1)

In order to detect the cardiac activity, the interesting variable is the ΔZ(t), assumedly reflecting the volume change of pulsating blood in arteries. A custommade flexible electrode (**Figure 8a** and **b**) was used, positioned distally (**Figure 8c**)

Suitable materials for electrodes must be found and thoroughly tested for truly unobtrusive and reliable pervasive monitoring. Easy applicability is paramount. They should not irritate the skin; their parameters should stay reasonably unchanged dur-

In order to evaluate the effect of distal and circular placement of electrodes on the radial artery to the measured values of Z and ΔZ(t), the experiments were carried out having the excitation signal with the amplitude of 500 mV in the frequency range of 10–5000 kHz. The results are visible on **Figure 9**. The ΔZ(t) is few times higher in the case of longitudinal placement of electrodes (**Figure 9a**, red line) than in the case of transverse placement (**Figure 9a**, blue line). The total impedance Z is on average about 2.7 Ω higher in the case of transverse placement than in the case of longitudinal placement. When Z is decreasing with frequency, the ΔZ(t) is maintaining its relative

and circularly (**Figure 8d**) on top of the location of the radial artery.

ing the acquisition cycle and should be insensitive to motion-induced stress.

the electrode design must be considered more seriously in further research.

impedance and the probability of motion artefacts [27].

**3.1 Electrode placements and materials**

ance expresses as

**18**

*Dimensions (a), design (b), and placement of a custom-made flexible four-electrode system in the case of a distal (c) and circular (d) locations on the wrist, where the thick red line denotes the approximate location of the radial artery (reprinted from [27]).*

#### **Figure 9.**

*Frequency response of measured (a) ΔZ(t) and (b) Z of the wrist in the cases of distal and circular placements of the electrodes.*

#### **Figure 10.**

*Modified dimensions of the standard ECG electrodes in utilised four-electrode system (a) and the placement on the wrist in distal (b) and circular configuration (c and d), where the thick red line denotes the approximate location of the radial artery (reprinted from [27]).*

value regardless of the excitation frequency. We can say that the longitudinal placement of electrodes possesses better results concerning the monitoring of cardiac activity in the wrist by using the prepared flexible electrode [28].

In order to verify the results of the custom-made electrode system, a similar research was performed by using the standard Ag/AgCl electrodes with foam tape (Type 2228 of 3M Health Care). Electrode dimensions were reduced physically (**Figure 10a**) and placed on the wrist distally (**Figure 10b**) and circularly (**Figure 10c** and **d**). The results in the case of distal placement of Ag/AgCl gel electrodes confirm the outcome of the results obtained with the custom-made electrodes.

Another custom-made electrode material was tested to try to improve the signal acquisition. Highly conductive carbon-based fillers added to the soft and flexible polydimethylsiloxane (PDMS) or silicone rubber matrix make a prospective dry electrode material. These fillers can be carbon nanotubes (CNTs), carbon nanofibres (CNFs), carbon fibres (CFs) and carbon black (CB). Previous researches have shown that these composites are biocompatible, and the existence of sweat and long-term wearing has little influence on the performance [27, 29]. We have developed a CNF/CF-PDMS material that could be used as electrodes for our wearable bioimpedance device due to its softness and stretchability [30]. Stratum corneum has very high impedance due to a large number of dead skin cells. Our hypothesis is that the developed electrode material can overcome this problem because the long fibres of carbon inside the silicone are sticking out and pressing a little bit into the skin layer (**Figure 11**).

We compared three different sets of electrodes: (a) Ag/AgCl gel electrodes, (b) carbon nanofibre electrodes (CNF-PDMS) and (c) carbon nanofibre together with carbon fibre electrodes (CNF/CF-PDMS). We abraded the skin slightly with a rough cloth for a better contact and placed the material on the wrist to register impedance variability with MFLI Lock-In amplifier (Zürich Instruments) on the frequency of 1o kHz. The results are shown on **Figure 12**. Pre-gelled commercially available electrodes showed good clean signal with impedance change of 0.1%. As skin-electrode

**21**

**4. Simulations**

**Figure 12.**

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform*

contact is worse, the CNF-PDMS and CNF/CF-PDMS soft electrodes gave slightly

*Three different impedance signals from the wrist with Ag/AgCl, CNF-PDMS and CNF/CF-PDMS electrodes.*

The development of mathematical and physical models of a haemodynamics is of great importance for the cardiovascular research [32]. The model is a simplified approximation of the real system, which incorporates most of the features. By using simulations, it is possible to predict the performance of the instrumentation,

During these preliminary experiments, we could conclude that the CNF/ CF-PDMS electrode material gave more stable results than CNF-PDMS over longer period of time. Further work needs to be done to establish whether silicone polymer together with carbon fibre and carbon nanofibre has a prospect to be used as electrodes for bioimpedance wearable devices. Also the question of the source of the signal arise—in what amount the blood itself contributes to the measured ΔZ(t) and in what amount it is caused by the rhythmical compression of tissues nearby [31].

*Carbon fibre strands sticking out of the base material (CNF/CF-PDMS polymer) (reprinted from [30].*

noisier signal, but the impedance change is clearly visible.

optimise and minimise the design and cost.

*DOI: http://dx.doi.org/10.5772/intechopen.86065*

**Figure 11.**

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform DOI: http://dx.doi.org/10.5772/intechopen.86065*

#### **Figure 11.**

*Wearable Devices - The Big Wave of Innovation*

*location of the radial artery (reprinted from [27]).*

*Modified dimensions of the standard ECG electrodes in utilised four-electrode system (a) and the placement on the wrist in distal (b) and circular configuration (c and d), where the thick red line denotes the approximate* 

value regardless of the excitation frequency. We can say that the longitudinal placement of electrodes possesses better results concerning the monitoring of cardiac

In order to verify the results of the custom-made electrode system, a similar research was performed by using the standard Ag/AgCl electrodes with foam tape (Type 2228 of 3M Health Care). Electrode dimensions were reduced physically (**Figure 10a**) and placed on the wrist distally (**Figure 10b**) and circularly (**Figure 10c** and **d**). The results in the case of distal placement of Ag/AgCl gel electrodes confirm

Another custom-made electrode material was tested to try to improve the signal acquisition. Highly conductive carbon-based fillers added to the soft and flexible polydimethylsiloxane (PDMS) or silicone rubber matrix make a prospective dry electrode material. These fillers can be carbon nanotubes (CNTs), carbon nanofibres (CNFs), carbon fibres (CFs) and carbon black (CB). Previous researches have shown that these composites are biocompatible, and the existence of sweat and long-term wearing has little influence on the performance [27, 29]. We have developed a CNF/CF-PDMS material that could be used as electrodes for our wearable bioimpedance device due to its softness and stretchability [30]. Stratum corneum has very high impedance due to a large number of dead skin cells. Our hypothesis is that the developed electrode material can overcome this problem because the long fibres of carbon inside the silicone are sticking out and pressing a little bit into the

We compared three different sets of electrodes: (a) Ag/AgCl gel electrodes, (b) carbon nanofibre electrodes (CNF-PDMS) and (c) carbon nanofibre together with carbon fibre electrodes (CNF/CF-PDMS). We abraded the skin slightly with a rough cloth for a better contact and placed the material on the wrist to register impedance variability with MFLI Lock-In amplifier (Zürich Instruments) on the frequency of 1o kHz. The results are shown on **Figure 12**. Pre-gelled commercially available electrodes showed good clean signal with impedance change of 0.1%. As skin-electrode

activity in the wrist by using the prepared flexible electrode [28].

the outcome of the results obtained with the custom-made electrodes.

**20**

skin layer (**Figure 11**).

**Figure 10.**

*Carbon fibre strands sticking out of the base material (CNF/CF-PDMS polymer) (reprinted from [30].*

**Figure 12.** *Three different impedance signals from the wrist with Ag/AgCl, CNF-PDMS and CNF/CF-PDMS electrodes.*

contact is worse, the CNF-PDMS and CNF/CF-PDMS soft electrodes gave slightly noisier signal, but the impedance change is clearly visible.

During these preliminary experiments, we could conclude that the CNF/ CF-PDMS electrode material gave more stable results than CNF-PDMS over longer period of time. Further work needs to be done to establish whether silicone polymer together with carbon fibre and carbon nanofibre has a prospect to be used as electrodes for bioimpedance wearable devices. Also the question of the source of the signal arise—in what amount the blood itself contributes to the measured ΔZ(t) and in what amount it is caused by the rhythmical compression of tissues nearby [31].
