This chapter outlines the equipments and measurement protocols used for this study. Protocol 1 will be used in chapter 4 and 5 while protocol 2 will be used in chapter 6. This chapter also verified the impact of skin-electrode impedance and motion artefacts on the electrode which provided useful insight that assisted in the choice of the electrodes.
Figure 3.13: Galvanic-coupling circuit on the lower left arm, the four terminal silver-silver chloride Noraxon electrodes are attached to the body and connected to the VNA via a balun. The output signal shows on the laptop screen fitted
A New Circuit Model of Real
Time Human Body Hydration
Changes in human body hydration leading to excess fluid losses or overload affects the body fluid’s ability to provide the necessary sup- port for healthy living [32]. Conditions leading to excess fluid losses in the body usually result in problems such as dehydration, while fluid overload can lead to heart failure and death in some cases [9,97]. In chapter 2, hydration was referred to as the process of gaining tissue water and the rate of hydration as the amount of change in the level of tissue water with respect to time. It is also a symptom for diseases associated with excess fluid or low level fluid in a human body [98]. Two most common techniques for measuring body hydration are bio- electrical impedance analysis [99] and urine specific gravity [40, 100]. These techniques assume a constant hydration factor and are not easily applied on a specific area of the body, except in the case of segmented BIA which has been discussed in chapter 2. For instance,
the bioelectrical impedance analysis is based on a hypothetical rela- tionship between impedance and the electrical volume. It assumes that the entire human body is a cylindrical conductor and tissues are electrically isotropic with no reactive component. However, it has been shown that human tissue should be modelled as having both resistive and reactive components, since cell membrane capaci- tance contributes significantly to the effective impedance of electrical signals across tissues [33]. Schwan [56] further showed that biologi- cal tissues have frequency dependent electrical properties that could classify them into three frequency regions (α, β and γ). Current treatment of body fluid disorders such as lymphoedema are mostly monitored by changes in body weight, circumferential limb measure- ments, limb volume measurements, and water displacement methods which have issues with hygiene and problems with tracking sequen- tial changes in weight and limb circumference [101]. Therefore, a new method is required to measure body fluid levels effectively This thesis will adopt the approach of modelling the human body as a transmission channel, and propose a new time dependent compo- nent to model fluid changes. This will facilitate tracking of hydration in real time by predicting the attenuation of a propagating electri- cal signal. The resulting circuit model has the varying component that models body impedance changes due to fluid level changes. The validation of this model is through measurement of signal attenua- tion when a known signal is transmitted. The attenuation can be
measured making it possible to quantify hydration effects empiri- cally and hygienically while tracking changes in the fluid volume on a specific area of the body. The rest of this chapter is organised as follows: Section 4.1 is the variable tissue impedance on IBC signal propagation. Section 4.2 is the hydration model with subsections 4.2.1 highlighting discussions on previous circuit models followed by the proposed hydration model as a time-dependent circuit in section 4.2.2, estimation of the fluid component in the arm in section 4.2.3 and a first order model of the changes in human body impedance due to hydration in section 4.2.4. Section 4.3 will discuss the ef- fects of changing body fluid levels on intrabody signal propagation while section 4.3.1 is the simulation results and 4.3.2 is the empirical measurements. Finally, the chapter concludes with a discussion and summary in section 4.4
4.1
Variable Tissue Impedance on IBC Signal
Propagation
An electrical signal passing through the human body is strongly af- fected by the dynamic changes in the volume of tissue containing fluid and its dielectric properties. Tissues have high ability to store electrical energy in an electric field at low frequency. High frequen- cies are affected by human body antenna effects and possible radia- tion. Therefore this study shall concentrate more on frequency range
lying between 800 kHz to 1.2 MHz which lies within the β disper- sion region. Frequencies in the β dispersive region is also related to the cellular structure of biological materials [56, 77] (section 2.1.1). Fig.2.9 and Fig.2.10 depict the dielectric relationship with frequency of common human tissues and Fig.2.11 the penetration depth across frequency of the same tissues. Since relative permittivity of tissue is higher at low frequencies (Fig.2.10), this study shall observe signal attenuation on tissues at low frequencies (800 – 1.2 MHz) to mea- sure variabilities in human body hydration with respect to time. An electrical signal propagating through tissues changes in proportion to changing tissue impedance. Dehydration increases the impedance of tissues [35] and hydration, conversely, does the opposite. Conse- quently, this thesis will use the observed signal attenuation on tissues at low frequencies to measure variabilities in human body hydration with respect to time. In literature, previous models of signal propa- gation across human tissue, either by finite-element method [71, 72], finite difference time-domain methods [73], equivalent electric cir- cuit [69, 70] and quasi-static dielectric principles [74] are all based on the assumption of static tissue impedance. This assumption is contestable considering that the average quantity of water required by a normal person to replace lost fluid in a temperate environment is between 2600 - 2700 mL per day [29]. These dynamic changes in the body fluid level, which also changes the impedance of the body, and the high relative permittivity at low frequencies, is the motiva- tion for a real-time human body circuit model to describe human
body hydration.