MATERIALS AND METHODS

2.3.1. Test objects and bathing media

Non-biological test objects: Polyacrylamide gel was made with ammonium persulphate O.OSg

sodium chloride 0.2g, 10 - 40 ml strength polyacrylamide diluted with distilled water for making 100 ml gel (Holder and Khan, 1994). A variety of gels were then made either with different

concentrations of polyacrylamide of 10, 20, 30,and 40% alone, or with non-biological materials into the 10% polyacrylamide gel. The non-biological materials included (1) Polystyrene

(microspheres, 7 - 40 pM); (2). Polymethylmethacrylate (microspheres, 100 - 500 nm); (3) Silica

fumed (particle size 14 nm, 99.8%, Sigma Chemical Co., UK); (4) Silica powder (particle size 0.5

- 10 pM, 99.9% silicon. Sigma Chemical Co., UK); (5) Barium Titanate. (Particle sizes 0.5 - 10

pM, Aldrich-Chemie Co. Germany); (6) Ballotini Beads (microspheres, 100 - 500 nm); (7) PMBA

and PMMA, suspensions of Polymethylmethacrylate (microspheres, 100 - 500 nm), and (8)

Polyurethane sponges with 8 densities (2.0 - 2.3, 2.4 - 2.7a, 2.4 - 2.7b, 2.8 - 3.2, 3.3 - 3.7, 3.8 - 4.2, 4.8 - 5.2 and 5.8 - 6.2% (w/v), were shaped into columns of 9 mm In diameter and 8 mm length, were soaked in the packed red cells. Before testing, those polyurethane sponges were

washed for 10 minutes in running water to remove fire retardant chemicals. Air bubbles were excluded by repeated manual compression under the bathing liquid.

Biological test materials: (1) Fresh bananas without skin, shaped as a column, 25 mm in diameter and 28 mm length. (2) Fresh cucumbers without skin shaped as a cylinder 10 mm in diameter and 5, 10, 20, 30, 40 or 50 mm in length. Samples of banana and cucumber were

covered with cling film in order to keep them moist between measurements. (3) Packed red blood cells (time expired and so unfit for human use, obtained from the hospital haematology

department). Each 500 ml packed red cells had a 60 - 70% haematocrit, 63 ml solution of sodium citrate 2.63g, anhydrous glucose 2.9g, citric acid monohydrate 327 mg, sodium

dihydrogen phosphate 251 mg, adenine 27.5 mg per 100 ml, added as preservative and anticoagulant for medical reasons.

Bathing solutions were (1) Potassium chloride solutions (99.5% KCI, SLR, Fisons Scientific Apparatus, UK) of 0.02, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3 and 0.5%. (2) Sodium chloride solutions (99.9% NaCI, BDH Laboratory Supplies, UK) of 0.2% (similar to the impedance value of the cerebral cortex) and 0.9% (physiological saline). Other liquids tested were pasteurised fresh milk (Tesco Stores Ltd, UK), Jif cleaning liquid (Lever Brothers Limited, UK), washing up liquid (Fairy Limited), shampoo (Silvikrin, Hampshire, UK) and Fairy liquid (Procter and Gamble Limited, UK).

2.3.2. Electrodes and measurement systems

Two sets of electrodes were used in this study. (1) For liquid samples, silver-silver chloride plate electrodes (termed “Type 1” below), 9 mm in diameter, were tightly sealed at the ends of polystyrene tubes of inner diameter 10.5 mm and length of 30, 50, 100, 200 mm. (2) Solid

samples were placed between two silver plate electrodes of 50 x 30 x 2 mm (termed “Type 2”

below). A silver foil of 50 x 30 x 10 mm with 25g in weight was glued to a non-measurement

surface of the upper electrode (Figure 2-1). Care was taken to ensure tight contact between measured objects and electrodes. Impedance of test objects was measured with a Hewlett Packed (HP) 4284A system (see section 1.4.4.) over a frequency range of 20 Hz -1 MHz.

HP system

Electrodes typel

Electrodes type2

Figure 2-1. Electrical impedance of liquid or solid test objects was directly measured using the HP system via a pair of silver electrodes of either type 1 or type 2.

2.3.3. Direct impedance measurements in vitro

Liquid test objects were separately inserted into a polystyrene testing tube. Air bubbles were eliminated, and the tubes were tightly sealed with an electrode (type 1 ) at each end. Solid test objects were placed between two plate electrodes (type 2) immediately after removal from a bathing solution and any excess solution was blotted off with filter paper. Good contacts were

ensured by the weight of the upper electrode. Resistance, reactance, impedance and phase

angle of test objects were then measured at frequencies from 1 kHz to 1 MHz with a current of 1 mA using the HP system. The ratios of reactance/resistance were calculated in some cases. All measurements were carried out at either room temperature of 25 ± 1°C or at 25®C controlled

by a water bath.

2.3.4. Evaluation of electrode impedance

Impedance of electrodes was evaluated by making measurements of differing lengths of a given test object. The series electrode impedance was taken to the offset value at the zero point of the regression curve of resistivity against length. Regression curves of electrodes type 1 and 2 were obtained with the polystyrene tubes of 30, 50,100 and 200 mm in length filled with NaCI solution or red blood cells and with a solid object of cucumber of 5, 10, 20, 30, 40 and 50 mm in length. This was subtracted from all values of impedance presented below.

2.3.5. Imaging test objects using the Sheffield Mark 1 system

EIT images were produced using the Sheffield Mark 1 system (IBEES, Sheffield, 1986) with an

applied current of 5 mA at 51 kHz. 16 equally spaced stainless steel electrodes were placed around a cylindrical Perspex tank of height 80 mm and inner diameter 88 mm (Figure 2-2).

Samples of either fresh cucumber in 0.09, 0.125 and 0.14% KCI solutions or polyurethane sponge of three different densities of 2.0 - 2.3, 3.8 - 4.2 and 5.8 - 6.2% (w/v) immersed in red

the bathing solution, a meniscus was created and gave erroneous values. Samples were placed in three different positions in the tank, 45 mm (centre), 27 mm (intermediate) and 10 mm (edge) from the edge of the tank. A common mode feedback electrode was placed near the centre of the tank. Screened leads of 4 mm in outer diameter and 300 mm length were connected between the Sheffield Mark 1 system and electrodes. EIT images of the test objects were referred to an image taken on the bathing liquid alone. Each image was acquired at 10 data sets

per second, averaged over 1 minute and reconstructed using the Sheffield Mark 1 algorithm (Barber and Seagar, 1987).

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