Spacer fabric-based exuding wound dressing – Part II: Comparison with commercial wound dressings

Fabrication of electrospun nanofibrous membranes

Two kinds of spacer fabrics, C4-1 and T4-1, were selected in Part I of this study. These fabrics were produced by using a single polyester/spandex (100D/40D) yarn to knit the two outer layers, and a 32S/2 bleached cotton or Tencel yarn to knit the spacer layer with a connecting distance of 4 needles, respectively. After knitting, the spacer fabrics were steamed and then conditioned at 20℃ and 65% relative humidity (RH) for a week. As mentioned above, PU and PS were selected for covering nanofibrous membranes onto their outer layer surfaces, respectively. While the wt 20% PS electrospinning solution was prepared by dissolving 20 g PS (average Mw ∼ 350,000, average Mn ∼ 170,000, purchased from Sigma-Aldrich Co., LLC) in 80 g DMF (GR grade, purchased from Duksan Pure Chemicals Co., Ltd); the wt 11% PU electrospinning solution was prepared by dissolving 10 g PU (average Mw ∼ 600,000, purchased from Hong Kong Hi-Tech Enterprises Ltd) in 90 g DMF. The fabrication process of the nanofibrous membrane on a spacer fabric is schematically shown in Figure 1. It was performed on an electrospinning apparatus purchased from Micro & Technologies Expert (TL-Pro), Shenzhen, China. As shown in Figure 1, the prepared solution was added to a 20 ml syringe, and the flow rate of 1.1 ml/h was controlled through a syringe pump. A high positive voltage of 16 kV was applied to the needle while the solution was spraying out of the needle to form nanofibers. The PS was sprayed for 30 min, and the PU was sprayed for 1 h. The PS or PU nanofibers were connected on the spacer fabric that was adhesive to a drum rotating at a rotating speed of 500 r/min. The electrospinning process was conducted under an environment of 25℃ and 65% RH. After completing the electrospinning process, the nanofibrous membrane covered on the spacer fabric was air-dried for about 24 h. Combining two types of materials, PU and PS, and two spacer fabrics C4-1 and T4-1, four different spacer fabric-based dressings were obtained. They were named Dressing A (C4-1 covered with PS membrane), Dressing B (C4-1 covered with PU membrane), Dressing C (T4-1 covered with PS membrane) and Dressing D (T4-1 covered with PU membrane). Because the thickness and weight of the electrospun nanofibrous membrane could be negligible, the thickness and areal mass of Dressings A and B were the same as those of spacer fabric C4-1, which were 3.253 mm and 540.4 g/m² respectively, and the thickness and areal mass of Dressings C and D were the same as those of spacer fabric T4-1, which were 3.343 mm and 616.4 g/m², respectively. OPEN IN VIEWER

Characterization of electrospun nanofiber membranes

The chemical structures and function groups of electrospun nanofiber membranes were characterized via a Perkin-Elmer Spectrum 100 on a Fourier transform infrared (FT-IR) Spectrometer made in the USA in the range of 650–4000 cm⁻¹ at room temperature.

The morphologies of electrospun nanofibrous membranes were observed by field emission scanning electron microscopy (FE-SEM) on a Jeol 6490 microscope. Prior to observation, samples were fixed on the sample stage using an electroconductive double-tape and then a layer of gold was sputtered on the surface of samples. These samples were observed in vacuum condition by FE-SEM and their images were recorded.

Comparison with commercial wound dressings

In order to confirm the performance of the spacer fabric-based dressings developed, a comparison was conducted with three types of commercial dressings for heavily exudate wounds, Foam A, Foam B and Alginate. The details and photos of these commercial dressings are shown in Table 1 and Figure 2, respectively. Foam A was a soft and conformable PU foam dressing that could effectively absorb and retain wound exudate. It ensured an optimal moisture balance for healing of exuding wounds. Foam B was a type of hydrocellular foam dressing. It could absorb and retain enough fluid to maintain an ideal moist wound environment and could transpire enough fluid to keep the dressing comfortable and conformable. Alginate dressing contained salts of alginic acids that could interact with physiological fluids to form a gel. All the dressings were non-adhesive, that is, there was no adhesive film on their surface to make the comparison under the same condition. As can be seen from Table 1, the foam dressings were thicker than all the spacer fabric-based dressings, but the alginate dressing was thinner and lighter than all the spacer fabric-based dressings. Thick dressings offer good absorption and good protection for wounds, but dressings that are too thick may make patients uncomfortable. OPEN IN VIEWER

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Table 1.

Details of the commercial wound dressings

Type	Base material	Product name & brand	Size	Areal mass (g/m2)	Thickness (mm)	Surfaces and cross-sections
Foam A	Hydrophilic PU foam	Biatain non-adhesive foam dressing from Coloplast	10 cm × 10 cm	754 (±23.1)	4.65 (±0.06)	Outer layer    Wound contact layer      Cross-section
Foam B	Hydrocellular PU foam	Allevyn non-adhesive foam dressing from Smith & Nephew	10 cm × 10 cm	680 (±14.2)	5.70 (±0.05)	Outer layer     Wound contact layer     Cross-section
Alginate	Calcium alginate	Algisite from Smith & Nephew	10 cm × 10 cm	136 (±6.5)	2.28 (±0.02)	Surface (two surfaces   are the same)     Cross-section

Note: Standard deviations are given in parentheses.

PU: polyurethane.

The following performance indicators were used for the comparisons between the spacer fabric-based dressings and commercial products. They are the water contact angle, wettability, absorbency, air permeability and water vapor transmission rate (WVTR). When applying to wounds, dressings should be water resistant in their outer layer, and should absorb liquid quickly from the wound contact layer. While the water contact angle of the outer layer of a dressing defines its ability to preserve the wound from contaminated fluids, the wettability indicates the time needed for fluid to go through the wound inner contact layer. Absorbency shows the capacity of dressings for liquid retention, which presents the exudate absorptive capacity. Moreover, water vapor transmission defines the capacity of moist management. Since a moist environment is beneficial to wound healing, the WVTR is a crucial index for the moisture maintaining property. Air permeability shows the breathability of dressings. Except the water contact angle, the testing conditions and methods for wettability and air permeability were maintained the same as used in Part I. Conditions were modified for absorbency and WVTR tests considering real applications of dressings. All the samples were conditioned for 24 h and all the tests except the absorbency tests were carried out at 20℃ and 65% RH.

Absorbency test

For the comparison of dressings, liquid containing Na⁺ and Ca²⁺ ions was used to replace the distilled water in the absorbency test according to BS EN 13726-1 – Test method for primary wound dressings. The test solution, which was prepared by dissolving 8.298 g/L sodium chloride and 0.368 g/L calcium chloride (AR grade, purchased from Oriental Chemicals & Lab. Supplies Ltd) in distilled water, had an ionic composition comparable to human serum or wound exudate. A 5 cm × 5 cm (25 cm²) sample was first put in a warmed solution (37℃) having 40 times the mass of the dressing, and then transferred to an oven to stand for 30 min at 37℃ and finally drained for 30 s. Five specimens were tested for each type of fabrics and dressings. The absorbency was calculated according to equation (1)

Absorbency ( g / 100 cm 2 ) = ( W 1 - W 0 ) / 25 × 100

(1)

where W₀ is the mass of the dry sample (g) and W₁ is the mass of the sample after absorbing (g).

Structures and morphologies of PS and PU nanofibrous membranes

PS and PU solutions were treated onto the surface of spacer fabrics by the electrospinning method to form the designed dressings. As shown in Figure 2, a continuous membrane was formed and covered on the outer layer surface of the spacer fabric. FT-IR spectra of PS and PU nanofibrous membranes are shown in Figure 3. In the FT-IR spectra of PS, the absorption bands at 2922 and 2852 cm⁻¹ were the asymmetric and symmetric stretching vibrations of –CH₂, respectively. The peaks at 1492 and 1452 cm⁻¹ may result from the vibration of C–C of the benzene ring. The absorption bands in the 690–790 cm⁻¹ region are caused by C–H of the benzene ring. In the FT-IR spectra of PU, a strong absorbing peak appears at 1729.26 cm⁻¹ owing to the stretching vibration of C=O bond in carbamate (R–NH–C(=O)–O–R). There was a peak at 1703.49 cm⁻¹ attributed to C=O stretch of the amido ester. The wide 1105–1066 cm⁻¹ region was caused by the C–O–C asymmetrical flexing vibration. OPEN IN VIEWER

SEM images of the PS and PU nanofibrous membranes are shown in Figure 4. It can be seen that nanofibers were interlaced together. The diameters of PS nanofibers were larger than 2 µm. They were large but were distributed evenly. PU nanofibers were fine with diameters less than 1 µm. The PU nanofibers were not uniform in their diameter, because the viscosity of PU solution and the electrospun voltage (15 kV) were relatively high. However, no obvious beads and beaded nanofibers were found in either type of membrane. The denser structure of the PU membrane made its air permeability lower than that of the PS membrane. OPEN IN VIEWER

Water contact angle

The water contact angles of the dressings are shown in Figure 5. The images of water drops contacting with dressings after 0, 10 and 30 s are presented in Figure 6. The water contact angle is an index that shows the waterproof ability of materials. A high water contact angle indicates that the material has a good waterproof property. At the same time, a slow decreasing of water contact angle within the same period shows good water resistance of the material. As shown in Figure 5, all the spacer fabric-based dressings had much larger water contact angles than the commercial dressings. While the contact angles of Foam A and Foam B were less than 100°, those of all spacer fabric-based dressings were high than 120°. Alginate dressing is an absorbent without a hydrophobic surface, and therefore the drop of water spread out immediately. As shown in Figure 6, the water contact angles of the spacer fabric-based dressings decreased more slowly than those of the foam dressings. In particular, the contact angles of Dressings A and C (covered with PS nanofibrous membrane) changed little after 30 s, which indicated that the PS nanofibrous membrane had very good water resistance. Dressings B and D, covered with PU nanofibrous membrane, had an inferior waterproof property compared with Dressings A and C, but they still had much better water resistance than foam dressings and Alginate dressing. It is also found that Dressings A and B (made of cotton spacer yarns) had slightly higher water contact angles than Dressings C and D (made of Tencel spacer yarns). OPEN IN VIEWER

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Figure 6.

Images of water drops contacting with dressings after 0, 10 and 30 s: (a) Dressing A; (b) Dressing B; (c) Dressing C; (d) Dressing D; (e) Foam A; (f) Foam B; (g) Alginate.

When water was dropped on the surface of spacer fabric-based dressing, the water firstly contacted with the electrospun nanofibrous membrane. As this membrane was continuously covered on the surface of the spacer fabric (as shown in Figure 2), the water contact angle was mainly determined by the membrane. Surface energy and surface roughness are the dominant factors for water resistance. ³⁰ Since both the PS and PU membranes were obtained by electrospinning, their morphology was nanofibers, which means that their surface roughness was similar. Therefore, the reason the PS membrane had better water resistance than the PU membrane is that the surface energy of the PS membrane is lower than that of the PU membrane, which is determined by the chemical groups. According to the chemical formula shown in Figure 3, PS has a benzene ring in its structure, whereas PU has no benzene ring but carbamate instead. These different chemical groups lead to the difference of water contact angles between PS and PU electrospun nanofibrous membranes. The spacer yarns slightly affected the water resistance, as the tuck loops appearing on the spacer fabric surface could influence the uniformity of membranes. The different distribution of nanofibers on cotton spacer-yarn fabrics and Tencel spacer-yarn fabrics led to a slight difference of the water resistance. As the spacer fabric-based dressings had better water resistance than the commercial dressings, they could protect wounds better from contaminated fluids or even infection.

Wettability

The wetting times of the spacer fabric-based dressings and commercial dressings are shown in Figure 7. Wettability is the ability of a material to absorb liquid into its body. A shorter wetting time indicates that fluid can be absorbed into dressing faster, and therefore the material has better wettability. It can be seen that Foam B required the longest time, more than 60 s to be wetted, which means Foam B performed the worst in wettability. Although Foam B is made from hydrocellular PU foam, there is a non-adherent film on its wound contact layer (Table 1). The non-adherent film lowers the adherence and reduces the pain while removing the dressing, but it also blocks liquid from entering into the dressing directly. Foam A is also made of PU foam, but its wetting time was much shorter than that of Foam B, because no film covers the wound contact layer of Foam A. The wetting time of Alginate was the shortest, because its main ingredient, calcium alginate, is a type of salt. So, water could spread on the surface quickly and lose its reflectivity directly while contacted. OPEN IN VIEWER

The wetting time of the spacer fabric-based dressing mainly depends on the type of spacer yarn. Because Tencel spacer yarn absorbed water very quickly, the wetting times of Dressings C and D were quite short (around 1 s). As the wettability was tested on the wound contact layer of the spacer fabric-based dressings, the effect of the electrospun fibrous membrane was very low. The wettability of Dressings A and B was slower than that of Foam A, but much faster than that of Foam B. A better wettability is important for wound care while exudate comes out, and the dressing should absorb it quickly to avoid fluid collecting around the wound and adjacent intact skin. The results show that the wettability of spacer fabric-based Dressings C and D was much better than that of foam dressings.

Absorbency of liquid containing Na⁺ and Ca²⁺ ions

Figure 8 shows the absorbency of liquid containing Na⁺ and Ca²⁺ ions of all the dressings. It can be seen that Alginate had the lowest absorbency, which was 19.66 g liquid absorbed by 100 cm² of material. The main reason for this is that the thickness of the Alginate wound dressing was only 2.28 mm, much thinner than the others (the thickness of foams was higher than 4.65 mm and that of spacer fabric-based dressings was higher than 3.3 mm). It is therefore found when calculating the areal absorbency that the value of Alginate dressing was very low. However, Alginate dressing could react with liquid by exchanging sodium ions with calcium ions to create a fibrous gel, which is helpful for absorbing ionic liquid. OPEN IN VIEWER

Foam A absorbed 74.8 g liquid per 100 cm², while Foam B absorbed just over 30 g. Both Foam A and Foam B were made of hydrophilic PU foams. Hydrophilic PU foam was produced by grafting hydrophilic compounds on the PU surface. These compounds are macromolecules with unsaturated double bonds, such as acrylic acid, acrylamide and so on. The hydrophilic groups on the polymer chains would form hydrogen bonds with water molecules. Foams swell when absorbing water and store water in the material. In the absorbency test, Foam A obviously swelled more than Foam B, so that the PU matrix of Foam A could contain more liquid inside its body. In addition, the absorbency of Foam B was reduced by the non-adherent film on its wound contact layer.

As for the spacer fabric-based dressings, their absorbency was better than that of Foam B and Alginate. It is noted that although Dressings C and D absorbed liquid faster than Dressings A and B, Dressings A and B could lock more ionic solution within their structure than Dressings C and D. This result was consistent with the water absorbency of spacer fabrics. Although Tencel spacer yarn absorbs liquid faster than cotton spacer yarn, the cotton yarn could retain more liquid inside the spacer fabric-based dressing. The large amount of liquid absorption extends the dressing period and reduces the frequency of dressing change. Furthermore, all extrudes emitted from the wound should be entirely absorbed by the dressing and liquid retention sustains a moist environment for wound healing. Consequently, the spacer fabric-based dressings were suitable to be used as absorbent dressings for exuding wounds.

Air permeability

The testing results of air permeability of all the dressings are presented in Table 2. It can be seen that the air permeability values of Foam A and Foam B were smaller than 0.020 ml/s/cm² at 100 Pa, exceeding the minimum value that the SDL M021S tester could measure. This implies that the air permeability of Foam A and Foam B was very poor. As shown in Figure 9(a), the foam dressings are quite thick. They were made from dense PU foams with a few small irregular pores inside. It is difficult for air to pass through the PU matrix. In addition, the non-adherent film on the surface of Foam B further reduces the air permeability. As Alginate dressing had the lowest thickness and a loose non-woven structure, its air permeability was very high, exceeding the maximum test value of the tester, which is 78.740 ml/s/cm² at 100 Pa. However, too high air permeability of Alginate makes it a non-occlusive dressing. In this case, a secondary dressing is required, and the most commonly used secondary dressing is gauze. OPEN IN VIEWER

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Table 2.

Air permeability of wound dressings

Sample code	Air permeability (ml/s/cm2 at 100 Pa)
Dressing A	21.260 (±0.574)
Dressing B	20.118 (±0.906)
Dressing C	31.890 (±1.710)
Dressing D	29.370 (±2.181)
Foam A	<0.020
Foam B	<0.020
Alginate	>78.740

Note: Standard deviations are given in parentheses.

The values of air permeability of the spacer fabric-based dressings could be measured by the tester. They were higher than those of Foam A and Foam B, but lower than that of Alginate. The air permeability of the spacer fabric-based dressings was much better than that of the foam dressings due to the lower thickness and density of the fabric structure. As shown in Figure 9, the spacer fabric-based dressings consisted of repeating units, and these regular units generated regular channels (Figure 9(b)) rather than the irregular structure of the PU matrix (Figure 9(a)). The openings regularly arranged in a spacer fabric structure can reduce air resistance and facilitate air circulation. On the contrary, the small pores irregularly arranged in a foam structure cannot form smooth passages to let air flow freely.

The spacer fabric-based dressings were thicker and denser than Alginate, so that it was more difficult for air to pass through the spacer fabric-based dressings than in Alginate. In addition, the electrospun nanofibrous membrane could reduce their air permeability. As shown in Table 2, Dressings A and B (both fabricated with spacer fabric C4-1) had lower air permeability than Dressings C and D (both fabricated with spacer fabric T 4-1). This is caused by the air resistance property of cotton and Tencel spacer yarns. As explained in Part I, the fine hairs on the cotton fiber made it difficult for air to pass through, while the surface of Tencel fiber was smooth, which facilitated air passing through. As for the dressings made of the same base spacer fabric, the air permeability of Dressing A (C4-1 covered with PS) was higher than that of Dressing B (C4-1 covered with PU), and the air permeability of Dressing C (T 4-1 covered with PS) was higher than that of Dressing D (T 4-1 covered with PU). This indicates that the air permeability of PS the electrospun nanofibrous membrane was better than that of the PU membrane, because the structure of the PU membrane was denser than that of the PS membrane (Figure 4).

Anaerobic bacteria are one of the main causes of wound malodor when severe colonization or infection happens. Air permeable wound dressing allows oxygen in and to contact with the wound. As a result, the amount of anaerobic bacteria can be reduced and the wound malodor can be eased. However, it is difficult to make a material have good air permeability and good water resistance at the same time. Most wound dressing materials have good air permeability but poor water resistance, such as Alginate; or have good waterproof properties but poor air permeability, such as foam dressings. The combination of spacer fabrics and the electrospun nanofibrous membrane has provided a good solution to this difficult problem. The spacer fabric-based dressings had quite good air permeability and very good waterproof properties. Therefore, they are suitable for applying as wound dressings.

Water vapor permeability

The results of the WVTR of the dry dressings are shown in Figure 10. The same as the air permeability, foam dressings had the lowest water vapor permeability. This is also due to their high density and the irregular pores of the PU matrix. As quoted in Part I, low WVTR may cause skin maceration and even infection, so that the WVTR of wound dressings is suggested to be higher than that of normal skin, which is 200–500 g/(24 h*m²). From Figure 10, the WVTR of Foam B was lower than 500 g/(24 h*m²), while the WVTR of Foam A was around 550 g/(24 h*m²). There is a greater chance for patients who are using foam dressings to get skin maceration or infection. OPEN IN VIEWER

Alginate dressing is highly permeable for water vapor with a WVTR of 1267 g/(24 h*m²). It was made from calcium alginate fibers by non-woven technology. As the non-woven structure was very loose and the thickness of Alginate dressing was low, the Alginate dressing had a very high permeability. However, high permeability may cause too much evaporation and make the wound dressing barely maintain the required moisture environment for wound healing. For Alginate, a gauze secondary dressing is required in most situations.

The WVTR of all the spacer fabric-based dressings was around 900 g/(24 h*m²) between the foam dressings and Alginate dressing. On one hand, the WVTR of the spacer fabric-based dressings was higher than that of foam dressings, because of similar reasons as the air permeability. The lower density and regular channels in spacer fabric-based dressings were beneficial to the transmission of water vapor. On the other hand, the WVTR of spacer fabric-based dressings was much lower than that of Alginate. This is due to their higher thickness, and also the covering of the electrospun nanofibrous membrane on their surfaces. The membrane was not only waterproof, but also permeable for air and water vapor.

A material with a high rate of water vapor transmission can transport extrude or sweat from the surrounding skin in order to maintain ventilation. It helps to minimize the risk of maceration and infection to the wound and tissue. At the same time, keeping a moist environment for the wound to avoid escharosis and to accelerate wound healing needs a relatively low WVTR. In other words, it should not be a non-occlusive dressing such as Alginate if no secondary dressing will be used. In this regard, the combination of absorbent spacer fabric and a waterproof and permeable membrane brought many benefits to wound healing.

The testing results of the WVTR of the wet wound dressings are presented in Figure 11. Contrary to the dry condition, the foam dressings had the highest WVTR, and the WVTR of spacer fabric-based dressings was lower. There are two reasons for this. The first is that Foam A could absorb more water (Figure 8) than spacer fabric-based dressings in the same period of time, which means that more water was exposed in the air for evaporation. However, Foam B absorbed a similar amount of water (Figure 8) but transmitted more water vapor than spacer fabric-based dressings under the wet condition. This may be explained by the second reason: the specific area of PU foam was much larger than that of cotton or Tencel fibers. Materials with a larger specific area cause evaporation more easily after absorbing. OPEN IN VIEWER

When being applied on an exuding wound, a dressing should absorb exudates and blood quickly. The dressing would become wet in a short period. So the low WVTR in the wet condition is necessary and important because fluid inside a dressing has great effects on water vapor permeability and a moist healing environment. Accordingly, the spacer fabric-based dressings had an advantage in keeping a moist environment for wounds, which is the most important thing for an absorbent dressing.