Influence of Tensile Stress on Woven Compression Bandage Structure and Porosity

1.1 Modeling of fabric porosity

A lot of models for description of porosity in woven fabrics were presented, some of them described the porosity between yarns (the inter-yarn porosity) and the others described the porosity between fibers inside the yarn (the intra-yarn porosity). According to the theory of a two-dimensional (2D) model, the porosity (S) is defined as a complement to the woven fabric cover factor (CF; Figure 1).

Figure 1

Structure of interlacing cell in woven fabric crossing point of plain weave, warp and weft diameter (d₁, d₂), warp and weft distance (B, A).

Woven fabric CF is given on the basis of illustrated structure of woven fabric in Figure 1 by equation (1):

Based on the known parameters of warp and weft densities (D₁ and D₂), we can write equation (2):

An area of pores is calculated as the perpendicular projection of woven fabric (horizontal porosity) [2]. Real values can be measured as illustrated in equation (3):

where d_o and d_u are the diameters of warp yarn and weft yarn, respectively, and D_o and D_u are the sets of warp yarn and weft yarn, respectively.

While dealing with 2D fabrics, porosity is defined as the ratio of the projected geometrical area of the opening across the material to the total area of the material [10, 11]. A classical 2D model of porosity seems insufficient for a tightly woven fabric. Neighboring yarns are very close, and the projected area of inter-yarn pores approaches to zero. As air flows through the woven fabric, it flows around the yarns, and it does not flow only in the perpendicular direction [12, 13]. Gee introduced the well-known “ends plus intersection theory,” which he modified, and called the “curvature theory” [14]. Until then, a “maximum theory” had been the subject of several researches. Some researchers [15, 16, 17, 18, 19] used a more theoretical approach, whereas others [20] used more experimental means. M. Kienbaum successfully joined theoretical and experimental investigations and presented his own theory which can be applied to all weaves and different yarn structures [2].

1.2 Effect of mechanical tensile stress on woven CBs

The venous as well as lymph system disorders or lower and upper extremities are often treated by utilizing compression garments which can provide compression therapy. By compression therapy, we limit the flow of diseased surface veins and increase the flow through deeper veins and reduce swelling. Patients who are compliant with compression therapy have a significantly improved ulcer healing rate and a decreased rate of recurrence. Even if the ulcers do recur, the interval to recurrence is more prolonged. Compression is thought to either correct or improve venous hypertension, mainly owing to an improvement of the venous pump and lymphatic drainage. Compression also improves blood flow velocity through deep and superficial veins [21]. For the creation of optimal compression on the lower and upper limbs by CB, it is necessary to observe the rules for application. The condition and shape of the bandage during application to any body part are in a straightened position similar to the uniaxial stress (Figure 2). Applied force on the bandage is given by final compression effect on the body. Deformation of structure and increasing of woven fabric porosity are given by these forces. This study aims to analyze the effect of applied load as a function of bandage extension and weave angle on woven CB structure and porosity for both two- and three-layer systems at ankle and mid-calf positions, as illustrated in Figures 2–6. CB applied with spiral 50% overlap technique will overlay the leg with two layers of bandage, whereas CB applied with 66% overlap will result in three layers of bandage [22].

Figure 2

Application of CB on mannequin and real leg. CB, compression bandage

Figure 3

Measurement of weave angle (θ)

Figure 4

Relationship between extension, weave angle, and porosity for cotton bandage

Figure 5

Relationship between extension, weave angle, and porosity for cotton/polyamide/polyurethane bandage

Figure 6

Relationship between extension, weave angle, and porosity for viscose/polyamide bandage

2.1 Materials

Three types of woven CBs were used for the analysis of bandage behavior during mechanical stress and compression pressure measuring. These bandages were produced according to the specifications given in Figure 7.

Figure 7

Characteristics of woven compression bandage; OE, open end

2.2 Testing procedure

Uniaxial stress and tensile force are a priority for the application of CB and the creation of optimal compression effect. CB loading–unloading is evaluated according to the standard test method ISO 13934-1:1999(E) [23]. Testometric M350-5CT was used to measure the tension developed in the bandage during extension to its end of elasticity using constant traverse speed of 100 mm/min. The device gage length was set to 200 mm, and the load cell of 100 N was used. High-speed camera was used during mechanical stress for the analysis of structure deformation and changing of woven bandage porosity during different extensions, as illustrated in Figure 8. Based on this method, it is possible to predict the corresponding bandage porosity given by tension along CB. The influence of CB structure was tested for three basic types of woven CBs (Figures 4–6).

Figure 8

Setup of Testometric M350-5CT instrument and high-speed camera

The same three types of bandages were worn on both mannequin model and real leg to test and analyze the effect of woven CB extension and porosity on bandage pressure at ankle and mid-calf position in both static and walking conditions. Recalculations of tensile force based on a specified extension range during bandage application are illustrated. Practical bandage pressure is measured using PicoPress tester, which gives both digital readings and graphical charts. The ankle and calf positions were adjusted to leg circumference of 21.4 and 32.4 cm, respectively, for mannequin model, and 25.6 and 38.9 cm, respectively, for real leg as shown in Figure 2.

It is possible to predict the final compression effect as a function of bandage pressure. For the validation of pressure given by woven CBs, the measured results using PicoPress were compared with theoretical compression forces calculated by Laplace's law equations (4, 5) as follows [24, 25, 26]:

The level of pressure exerted on a medical device matches with the Laplace's equation stating that the pressure (P is expressed in Pascal) of a compression applied to the skin surface is directly proportional to the tension (T in Newton) of the compression material and number of layers and inversely proportional to the radius of curvature (R in meter) of limb surface to which it is applied and the bandage width (W in meter) [27].

Prediction of fabric properties is based on a combination of mathematical modeling and experimental research, including development and application of nonstandard methods for the definition and measuring of the fabric structure [28]. One of the main aims of this study is to analyze the relationship between applied tensions and weave angle, given by the changing of bandage porosity. During mechanical stress test, we can illustrate the fabric porosity as a function of weave angle (equation 6 and Figure 3).

where S is the fabric porosity and θ is the weave angle.

3.1 Loading–unloading uniaxial test for woven CBs

When the three bandage types were subjected to uniaxial stress at full extension (i.e., closer to end point of elasticity), CO-PA-PU bandage achieved approximately 300 N at 200% extension, whereas VI-PA and cotton bandages had 280 and 750 N at 150% extension, respectively. These results confirm the highest elasticity and comfort properties of the CO-PA-PU bandage compared with 100% cotton bandage. Figure 9 illustrates that all bandage samples recovered its original length after relaxation. This elastic recovery is due to the optimum extensibility of bandages in elasticity zone; moreover, there is no dwell time while stretching the CB samples. But when these bandages are worn or wrapped on human body, there will be a bit residual deformation due to higher applied bandage tension and longer treatment time.

Figure 9

Loading–unloading curves for cotton, cotton/polyamide/polyurethane, and viscose/polyamide bandages

3.2 Optimum fabric tension for woven CBs

As the optimum required bandage tension is approximately 10 N, that value is achieving the required bandage pressure (4,000 Pa or 30 mmHg) according to Laplace's equation (4) for two-layer bandaging at radius 5 cm and bandage width 10 cm. Figure 10 confirms that CO-PA-PU and VI-PA bandages require 110% and 92% extension, respectively, while cotton bandage requires only 60% extension to achieve the required bandage tension 10 N (Table 1). The cotton bandage extension depends on the highly twisted plied yarns (1,200 turns/m) that enables to achieve the required bandage stretch, but these bandages have lower extension compared with CO-PA-PU that contains a 6% of elastomeric filament (polyurethane) which gives higher extensibility. Although the VI-PA bandage consists of two types of yarns having different thermal and melting points, in which case the stretch is given by steaming then heat setting at the required percent of shrinkage.

Figure 10

Optimum fabric tension for cotton, cotton/polyamide/polyurethane, and viscose/polyamide bandages

3.3 Effect of bandage structure and weave angle on bandage porosity

The factors affecting bandage porosity such as warp and weft yarn count, density, twist, CF, and fabric structure are changing during bandage extension. The main variable during bandage application is the applied tension to achieve the required compression. Figures 4–6 confirm that bandage porosity is significantly increased by increasing weave angle and bandage extension. Statistical analysis of the obtained results using linear regression (ANOVA) concluded that the bandage extension and type have significant effect on weave angle (sig.P = 0 < 0.05; Table 2). Moreover, the combined effects of bandage type and extension are significantly influencing the bandage porosity (P = 0 < 0.05) as listed in Table 3.

3.4 Comparison between calculated pressure and measured compression using PicoPress

Hundred percent cotton, VI-PA, and CO-PA-PU bandages were worn one by one on real leg to test the real compression pressure at ankle and mid-calf position in both static and walking conditions at different extension levels. Ankle and calf positions were adjusted at leg circumference of 25.6 and 38.9 cm, respectively. Deviation percent was calculated as the difference between measured compression using PicoPress and calculated pressure by Laplace's equation, as listed in Tables 4 and 5.

The obtained results in Table 4 confirm that there is a significant deviation when applying Laplace's equation for two- and three-layer bandaging ranging from ±0.18% to ±20.82%. Although Jawad Al Khaburi developed equation (7) to include the increase in limb circumference due to multilayer bandaging, the equation has improved the deviation range from ±0.96% to ±17.91% as follows [28, 29]:

where

Results of Table 6 conclude that the deviation when applying Laplace's equation for mid-calf position is ranging from ±0.74% to ±11.87%, while the deviation range of Al Khaburi's equation is ranging from ±0.84% to ±10.10%.

3.5 Effect of bandage extension on its porosity

Figure 11 ensures the direct relationship between bandage extension and its porosity. Hundred percent cotton bandage achieved the porosity of 0.35, 0.41, and 0.47 at the extension of 50%, 100%, and 150%, respectively, whereas CO-PAPU reached 0.29, 0.41, and 0.5; moreover, VI-PA bandage achieved 0.45, 0.58, and 0.67, respectively (Table 8).

Figure 11

Effect of bandage extension on its overall porosity

3.6 Effect of bandage porosity on corresponding applied tension

Figure 12 illustrates the relationship between binary area fraction – which represents the bandage porosity – and the applied tension. Hundred percent cotton bandage only needs 0.38 porosity (at 60% extension) to achieve the required bandage tension of 10 N, whereas CO-PA-PU and VI-PA bandages require porosity of 0.43 (at 110% extension) and 0.57 (at 92% extension) to reach that tension which is required for optimum compression. Statistical analysis confirmed that there is a significant relationship between bandage porosity and applied tension, as concluded in Table 9.

Figure 12

Effect of overall porosity on bandage tension