Methods

7.2.1. Custom Testing Rig Development

An extrusion rig, custom designed to conduct these tests (Figure 7.1 and Figure 7.2), was attached to a Zwick Roell Z005 machine (Zwick/Roell GmbH Ulm, Germany) in which a displacement-controlled compression could be applied. The rig incorporated:

1. An aluminium base with legs and a 55mm diameter hole to allow for similar extrusion pressures as experienced during abdominal muscle activity, as per [41]. 2. A Perspex tube, inner diameter 55mm sitting flush on the aluminium base.

3. An aluminium plunger with a diameter of 54.5mm to allow for friction free movement.

4. A piece of 0.5mm thick silicone sheeting with a 13mm hole in the centre to simulate a 10mm trocar◊ _{hole with some enlargement due to operative manipulation.}

The silicone sheeting was placed between the aluminium base and the Perspex tube. The Perspex tube was then pressed hard against the aluminium base and secured with cables so that the silicone could not slide. The Perspex tube was filled with material (Section 7.2.2) and the system was attached to the Zwick machine. Displacement-controlled crosshead movement was used to apply a force to the content of the tube and cause it to extrude through the 13mm hole in the silicone sheeting. Silicone sheeting was selected as it deforms slightly, thus creating a conical shaped channel for the extruded material, somewhat replicating the slightly compliant nature of the tensed abdominal wall.

Chapter 7

Figure 7.1 – Line drawing of the extrusion rig. Figure 7.2 – Photograph of the extrusion rig.

7.2.2.Materials Tested

Tests were conducted on fresh non-cleaned small intestine, fresh cleaned small intestine, silicone gel, edible gelatine (jelly), dough (regular and oatmeal) and reconstituted powdered potatoes (RPP), see Table 7.1.

Material Number of Tests

1 Fresh non-cleaned small intestine. 3

2 Fresh cleaned small intestine 9

3 Silicone gel (EGel 3000, ACC Silicones, Bridgewater, England) 2

4 Edible gelatine (Chivers Brand, Premier Foods, Dublin) 2

5 Regular dough 9

6 Oatmeal dough 2

7 Reconstituted powdered potatoes (RPP) (Knorr Brand, Unilever, Dublin ) 9 Table 7.1 – List of materials tested.

Non-cleaned small intestines were harvested from three month old female pigs and cleaned small intestines were sourced from a local abattoir. The non-cleaned fresh small intestines

were harvested in-house8_{. There was no washing and the mesenteries}◊ _{were left intact. The}

cleaned small intestines were removed from slaughtered animals, washed inside and outside and the mesenteries were completely removed.

All other materials tested were composed of readily available food products. The regular dough mixture was composed of a combination of plain flour, tap water, table salt, sunflower oil and white wine vinegar (Table 7.2). The ingredients were mixed together in a saucepan on a low heat for 5-7 minutes until the dough was dry to touch. It was then kneaded for two minutes by hand.

Ingredient Quantity

Plain Flour 40g

Tap Water 400ml

Table Salt 495g

Sunflower Oil 20ml

White wine vinegar 6ml

Table 7.2 – Regular dough ingredients.

Oatmeal dough was made from a mixture of rolled oats, plain flour and tap water. The flour and oats were mixed together and water was slowly added to the mixture and stirred in. Table 7.3 shows the ingredients and their quantities.

Ingredient Quantity

Plain Flour 150g

Rolled Oats 110g

Tap Water 100ml

Table 7.3 – Oatmeal dough ingredients.

RPP was made using Knorr Powdered Mashed Potato (Unilever, Dublin) in a ratio of 5ml of tap water to 1g of powdered potato mixture.

8 _{Ethical approval for the use of porcine small intestine from pigs slaughtered in the bio-resources}

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Three samples of intestine, RPP and regular dough were tested at each extrusion rate (nine samples for each material in total). Only two samples of oatmeal dough, edible gelatine and silicone gel were tested, as the results were drastically different from small intestine. Three samples of non-cleaned small intestine were tested. These three samples were taken from a single three month old female pig. Samples were frozen until testing, defrosted at 4°C and tested at room temperature. The nine samples of cleaned small intestine were taken from one pig aged 26-28 weeks of unknown gender. Samples were frozen within four hours of harvesting, defrosted at 4°C prior to testing and tested at room temperature. Various regions of the small intestine were not distinguished for these tests.

7.2.3. Test Conditions

Crosshead movement rates of 7.5mm/min, 15mm/min and 30mm/min in the Zwick machine were used to investigate viscoelastic effects. These rates were chosen from a base rate of 7.5mm, which was considered significantly slower than any in vivo pressure generation, and was doubled for each test until the force required to extrude material exceeded the maximum force the machine could apply. Due to a strong odour, the fresh non-cleaned small intestine were only tested at 15mm/min for operator welfare reasons, which in itself is an argument for suitable surrogate intestine materials. Three tests were conducted on each of the other materials at the three rates listed above to test for repeatability. Room temperature during testing was constant at 20±1°C. Video recordings (Panasonic DMC-FT1, Panasonic, Osaka, Japan) were taken of each test to permit graphical analysis and force, crosshead displacement and time were recorded by the Zwick computer software.

7.2.4.Yield Strength Evaluation

In order to normalise the extrusion force and to account for the geometry of the system, the following equation was used to calculate the “yield strength” of the material. Although the equation assumes that the material is a continuous, incompressible solid, it does provide a measure of the pressure required to extrude the material regardless of the geometry of the system:

where 𝑌 is the yield strength of the material, 𝑃 is the pressure applied, and 𝑅 is the ratio of the initial diameter to the final diameter. This equation will be used to assess the yield strength of the materials tested.