Lorna Zadravec Zaletel, Matjaz Popit and Marjan Zaletel
measured from the modification of the arterial diameter between the systolic and diastolic phases on CCA segments. Carotid diameter waveforms were assessed by means of an ultrasound and converted to carotid pressure waveforms using an empirically derived exponential relationship between pressure and arterial cross section. Blood pressure measurements were obtained simultaneously with ultrasound measurements. The derived carotid pressure waveform was calibrated from brachial end-diastolic and mean arterial pressures by iteratively changing the wall rigidity coefficient
Sepideh M. Moqadam, Parvind Grewal, Majid Shokoufi and Farid Golnaraghi
and modulus of elasticity of in-vivo and in-vitro tissue were measured during electrical and mechanical experimental procedures by two different setups, which will be explained in the following sections.
Electrical impedance setup and procedure . Bioimpedance measurements of various types of tissue were conducted while incremental pressure levels were applied to the in-vivo and in-vitro tissue. For this experiment an impedance spectroscope HF2IS along with a transimpedance amplifier HF2TA (TA) by Zurich Instrument (Zurich Instrument Inc., Switzerland) were
long it takes for the system to settle).
More advanced models of viscoelasticity exist. Their relevance is discussed in the Conclusions section of this work. For the purposes of this investigation, however, the model will suffice and the parameters (the elastic modulus and the characteristic relaxation time constant) are satisfactory to describe any trends that arise.
It is our contention that there will be a corresponding admittance relaxation curve to the stress relaxation equation presented above ( Eq. (8)) . This will serve to
]. However, a more common method, the patch-clamp technique [ 13 , 16 ], uses a pipette or microfabricated aperture into which a small section of the cell membrane (the ‘patch’) is drawn by way of an applied suction. The pipette is filled with an ionic solution and contains a single electrode to record currents through either individual ion channels or over the entire cell membrane. The whole-cell patch-clamp technique [ 16 ] requires a larger suction to be applied so that the membrane is ruptured. This ensures better access to the electrical properties inside the cell
which is an invasive method and cannot readily measure beat-to-beat changes. Electrical impedance measurements at the forearm provide a possible way to characterize hemodynamics, and in particular changes in the amount of blood in the arm as a result of vasodilatation and/or the cardiac cycle.
Although the simulation perspective to bioimpedance plethysmography is a rarity there have been several investigations pertaining to the impedance response at forearm section. Some works [ 13 , 14 , 15 , 16 ] related to multi-frequency electrical bioimpedance (MF-EBI) for
Tushar Kanti Bera, Nagaraju Jampana and Gilles Lubineau
circuit model of a single cell [ 6 , 13 , 64 , 65 , 66 , 67 , 68 , 69 , 70 ,], as described in the following sections.
Animal cells can be modeled with an equivalent circuit concept as shown in Figure 4 . The resistive path created by the ICF in biological cells to an electric signal ( Figure 4a) is represented as a resistive element called ICF resistance (R ICF ). Alternatively, the capacitance [ 74 ] offered by the protein-lipid-protein structure [ 62 , 63 ] of the cell membrane is modeled as cell membrane capacitance (C CM ) [ 5 , 6 ] ( Figure 4a
s1 , Z s2 , Z s3 e Z s4 which are the contact impedances between the electrodes and the inputs of the measuring system (see the previous section). Z0 1 is the output impedance of the voltage to current converter, Z0 2 is the output impedance of the transimpedance amplifier HF2TA, Zd 2 and Zd 3 are the input differential impedance of the amplifier, Zin 1 and Zin 2 are the input impedance of the amplifier.
Proposed equivalent model
The model shown in figure 1(c) is equivalent to the one of figure 1(d) . Preliminary tests have shown similar results
voltage (V) and current (I), applies to alternating current (AC). Resistive (R), capacitive (C) and inductive (L) components of the tissue, all contribute to the measured impedance;
Z = V/I
As Z is a complex function, it can be expressed by the modulus |Z| and the phase shift Φ or by the real part R which represents resistance, and the imaginary part X representing capacitance. Important to note that the imaginary part would be zero when direct current is applied.
Admittance (Y) is the inverse of impedance which means