BARRIER EFFECT OF WOVEN FABRICS USED FOR SURGICAL GOWNS

1. Introduction

Surgical gowns must act as a barrier between the sources of infection (micro-organisms such as bacteria, and viruses of different size and geometry) and the user (i.e. a healthy person), and must also demonstrate good wearing comfort. The latter is important for the surgeon who often has to wear the surgical gown for several hours while doing hard work.

Hydrophobic wovens from polyester are used for shorter surgical operations with a small amount of liquid, and have so far been the only reusable surgical gowns which are currently able to fulfil both these contrary demands of barrier effect and wearing comfort.

There are many different fabric constructions on the market. Their barrier function against particle-loaded liquids has so far only indirectly been tested, e.g. with filtration tests. These tests have the disadvantage that it cannot be explained how the measured results come about.

The barrier function of such fabrics depends on the surface structure, and also on the number and size of the continuous pores running through the fabric – the pores which run both between the filaments in the filament yarn and also between the filament yarns. In this paper, these pore structures are characterised using optical images of the cross-sections of woven fabrics. The image analysis methods, linear analysis and QUANT of ImageC � by Aquinto show the pore size and distribution. They allow us to establish the reasons for the transfer of micro-organisms. It will thus be possible to

http://www.autexrj.org/No4-2003/0076.pdf

select parameters for filaments (fineness and cross-section), filament yarns (fineness) and fabrics (type of weave and fabric density) to have a better barrier effect. The results obtained in research work are reported in /1-6/.

2. Choice of fabrics

After having analysed 29 commercial polyester filament fabrics exclusively constructed of plain or twill weave, six typical fabrics have been characterised /5, 6/. In this paper we characterise two which contain only round filaments (table 1).

P 2 (Figure 1 left) is a plain woven fabric from the same warp and weft filament yarns, comprising of a blend of two filament diameters. P 6 is a twill-woven fabric with coarse round weft filaments and finer round warp filaments (Figure 1 right).

Table 1. Parameter of the selected woven fabrics

P 2 P 6

Figure 1. Light optical microscopy images of the wovens – above: warp section, below: weft section

3. Preparation of samples and light optical microscopy images of the wovens

It is necessary to prepare the chosen fabrics for the microscopic imaging of the cross-section of the textile fabric. For these purposes, the textile sample is vertically embedded in epoxy resin in a cylindrical sample support to cut either warp yarns (= warp section) or weft yarns (= weft section). After the sample has hardened, it is smoothened and polished. Subsequently, images of the cross-sections of the fabrics are obtained by using an optical microscope. Because of the resolution range, the microscope can only visualise filament distances up to 0.22 µm, which covers the range of all important bacteria (up to 0.5 µm), but not all viruses (0.01 to 0.3 µm). It is linked to a digital camera and a computer.

4. Determination of the pore structure of the woven fabrics

The images of the cross-sections of the fabrics show the micro-structure of the woven fabrics, i.e. the size and distribution of the pores in the filament yarn, and between the filament yarns with respect to the fabric weave and yarn density.

The microscopic images are binarised. The fabric is represented as a two-phase texture (filaments and pores). However, it is essential that the two components are clearly identified by contrasting them sufficiently. In order to determine the geometrical parameters of the pore structure (pore width and pore length, pore area and pore form factor), we use the linear analysis and the QUANT methods of ImageC ® by Aquinto. Measurements are made in rectangular measurement fields on the measuring line. Each sample is analysed in several measurement fields to obtain a pattern repeat by sampling at random. The measured data are accumulated accordingly.

Pore width and pore length are measured by using the linear analysis methods. The distances of cutting lines and the cutting direction (horizontal and vertical) are freely chosen. The distance between the individual filaments corresponds to the pore width (horizontal), and the pore length (vertical) is obtained automatically (Figure 2). A bar diagram gives the pore distribution.

horizontal vertical

Figure 2. Determination of filament distances (chosen distance of cutting lines: 2 µm)

QUANT is an extended object-related image analysis technique for particles. Its basis is a very efficient object search algorithm, the so-called contour tracing method. This method can be applied to determine the pore areas and form factors of closed pores (Figure 3) in accordance with the classification previously set.

Table 2. Pore area classification and pore areas for the P2 and P6 samples

form factor:

4 �p� F

f =

_U²

where:

F – area, µm²

U – perimeter, µm

form factor f=1 has a circle, f=0.87 – a square and f»0 – a long stretched pore.

Figure 3. Determination of pore areas and form factors for the P2 sample

The ideal pores offer the form factor 1, i.e. they are closed and can detain the micro-organism to passage through the woven fabric. The long stretch and ramified pores have a form factor value versus 0. Thus means that they are permeable for micro-organisms.

5. Results and discussion (examples)

5.1. Pores in the filament yarn

Figure 4 and Figure 5 demonstrate that the pore spaces are larger in the filament yarns of P 2.

This effect can be explained by the unfavourable filament arrangement because of the insufficient intermixture of both filaments’ finenesses.

P 2 P 6

Figure 4. Cross-section of filament yarns (above: warp yarn, below: weft yarn) and horizontal linear analysis

Figure 5. Relative cumulative frequencies of the pore classes P 2 and P6 (horizontal) – warp yarn (above) and weft yarn (below)

The mean pore diameters for P 2 are 4.85 µm in the warp section and 4.71 µm in the weft section. The mean pore widths for P 6 are 1.76 µm in the warp section and 1.96 µm in the weft section. The filaments in P 2 produce a pore share of 12.0% (warp yarn) and 12.4% (weft yarn) for pores larger than 10 µm, so the maximal pore widths are 73.56 µm (warp yarn) and 96.67 µm (weft yarn). For the warp yarn of P 6, it has been found that 93.5% (89.5%) of all pores in the warp yarn (weft yarn) are smaller than 4 µm. The maximum pore widths of P 6 are 8.45 µm in the warp yarn and 13.59 µm in the weft yarn. There are no pore channels; in other words, the filament yarn structure of P 6 prevents the transmission of bacteria.

P 2 does not have a barrier effect for bacteria. For this purpose, the transfer of micro-organisms is applied to particular image sectors. It is assumed that there are transfer paths for micro-organisms if continuous pore canals are to be found here. Ten sections of warp and weft yarns of P 2 were made, as illustrated in the example shown in Figure 6. All sections examined were continuous, that is, all of the pore surface could be marked with the chosen colour with the help of the program. The pore width was measured for the individual pore canals. These measurements show that the continuous pores present a considerable health hazard.

20 16 12 8 4 0

0.22-0.89

1.11-1.77

2.00-2.66

2.88-3.77

4.00-5.33

5.55-6.44

6.88-9.11

9.33-12.44

pore width (µm)

Figure 6. Selected pore channels in a warp yarn (P 2) The analysis of the pore areas (Figure 7, 8) confirms the results obtained by the linear analysis.

P 2 P 6

Figure 7. Cross-sections of filament yarns and determination of pore areas and form factors (above: warp yarn, below: weft yarn)

P 2 and P 6 differ in the pore areas. In P 2, 80.75% of all pores are larger than 40 µm². In P 6, this applies only to 1.9% of all pores. While the small pore areas of P 6 are not continuous, P 2 has very large continuous pore areas, for example the white pore area (Figure 7 top left) which includes exactly one pore greater than 500 µm. The relative frequency of this pore area is only 1.25%, but this pore area covers about one third of the warp yarn.

The larger the pore area, the less circular it is. The largest pore areas of P 2 have the lowest form factors, i.e., they are stretched long.

Figure 8. Relative cumulative frequencies of the pore areas and the associated form factors of the closed form areas (plotted against the bars) - P 2 and P 6 (warp section)

5.2. Pores resulting from the fabric structure

Plain weave (Figure 1): The weft yarns are fairly well stretched within the fabric, while the warp yarns have a high insertion. The warp yarns overlap each other, so that three filament yarns put on top of each other partially. This may improve the barrier effect.

Twill weave (Figure 9): On the left side, it is shown that in the twill weave (P 6) the pores between two weft yarns (12.6 tex) are large because the yarns are comparatively thick. Although thinner filament yarns are used as warp yarns, the spaces between the filament yarns are also wide (Figure 9 right). This applies to almost all pores between two warps (two wefts) and it is confirmed in Figure 10. The maximum filament yarn distance is 81.33 µm.

P6 - weft section P6 - warp section Figure 9. Pores between filament yarns in twill weave (P 6)

pore width in µm

Figure 10. Relative cumulative frequencies of the pore classes in the warp and weft section resulting from the fabric structure (P 6)

Additionally, another twill weave fabric K 2/2 (warp: 10 tex, 730 yarns/10 cm, weft: 20 tex, 590 yarns/10 cm) has been examined /5/. The low weft yarn density and the coarse weft yarns also

191

result in large pore spaces in the weft section. In the warp section, the two thinner warps (10 tex) are pressed tightly against each other (Figure 11).

weft section warp section

Figure 11. Pores resulting from the fabric structure - P 3 /5/

6. Penetration tests

The results of the structural barrier effect are compared with the results of penetration tests. Synthetic blood or particle-loaded liquids containing micro-spheres comparable with the sizes of bacteria are used. The time necessary to soak the fabric with liquids or the contamination on the back of the fabric is determined. Here are two examples:

1. micro spheres – Figure 12 (left) Fluorescent micro spheres with a mean diameter of 1 µm are used. They have a round shape, and are comparable to the size of staphylococcus aureus, a typical bacteria in the operating theatre. The contamination on the back of the fabric is affected by the reaction time, the type of solvent or DMSO (dimethyl sulphoxide) and the cylindrical roll-off movement on the surface of the sample.
2. latex micro spheres and micro polish suspensions – Figure 12 (centre and right). Latex micro spheres with a mean diameter of 5 µm or micro-polish particles with a mean particle size of 3 µm are used. Test suspensions were strongly diluted with distilled water. A micropipette was used to apply 20 µl of this suspension onto the right side of the woven fabric. Subsequently, the sample was subjected to a pressure of 0.33 N/cm² (= lightly leaning on the forearm) for 30 minutes on a Zwick 1445. The micro-spheres and micro-polish particles which appeared on the back of the fabric were made visible with the scanning electron microscope.

Conclusions

The barrier function of operating theatre textiles can be analysed using the present high-contrast section images of fabrics. It demonstrates a correlation between the micro-structure of the fabrics and the results of the penetration tests.

1. The image analysis methods, linear analysis and QUANT of ImageC� by Aquinto, are well suited to characterise the pore structure of woven fabrics. They allow good measurement of the
2. horizontal and vertical pore spaces, pore areas and form factors of closed pores with sufficient certainty.
1. It is necessary to optimise commercially available surgical woven fabrics which are made of PES filament yarns because they often do not fulfil the barrier function.
2. The results of the penetration tests with micro-spheres and the determination of the contamination on the back of the fabric confirm that most of the fabrics do not have a barrier effect.
3. Pores are found both in the filament yarn and in the woven fabric. Their size, geometry and number depend on the yarn and fabric parameters.
4. Round cross-sections of the filaments in the filament yarns are favourable for a maximum packing density. An insufficient intermixture of different filament finenesses causes an unfavourable filament arrangement.
5. The type of weave in connection with the yarn density is decisive for a better barrier effect. Twill weave is particularly critical, since it may have large pores between two weft yarns in the crossing points.
6. The research results show correlations between the micro-structure and the parameters of the woven fabrics such as filament cross-section, single filament fineness, yarn fineness, type of weave, warp and weft density.
7. It is our objective to model the theoretical density of the woven fabric. Assuming model fabrics that are woven from filament yarns with a calculated circular cross-section and with warp and weft yarn densities of 100% according to /6/, the 3D fabric structure is simulated. Subsequently, the yarn cross-sections and the yarn insertion are varied to improve fabric density.