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Design carboxymethyl cotton knitted fabrics for wound dressing applications: Solvent effects
Materials and Design 87 (2015) 238–244
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Design carboxymethyl cotton knitted fabrics for wound dressing applications: Solvent effects Jinchao Zhao a,b, Youhong Tang b, Yun Liu a, Li Cui a, Xianxin Xi a, Nannan Zhang a, Ping Zhu a,⁎ a b
School of Chemistry and Chemical Engineering, State Key Laboratory of New Textile Materials and Advanced Processing, Wuhan Textile University, Wuhan 430064, China Centre for NanoScale Science and Technology, and School of Computer Science, Engineering and Mathematics, Flinders University, South Australia 5042, Australia
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 18 July 2015 Accepted 21 July 2015 Available online 26 July 2015 Keywords: Carboxymethylation Solvent effects Cotton knitted fabric Substitution degree Water absorbency
a b s t r a c t Carboxymethyl cotton knitted fabrics (CM-CKFs) for wound dressings were fabricated with different solvents: water, ethanol–water, and isopropanol–water. The FTIR analysis showed that carboxymethylation reaction in the CM-CKFs occurred with the different solvents. With the water, ethanol–water and isopropanol–water order and the decrease of water ratio in the mixed solution, the degree of substitution (DS) increased, the crystallinity index of the CM-CKFs decreased, crystal structure of cellulose changed from type I to II, and appreciable increased in fiber diameter and gel formation. The water absorbency and water retention of the CM-CKFs increased with DS. The water absorbency of the CM-CKFs treated in water, ethanol–water (v/v = 3/1), and isopropanol–water (v/v = 3/1) was 1.86, 6.95, and 14.86 g/g, respectively. The breaking force of the CM-CKFs was lower than that of cotton knitted fabrics, but the breaking force increased with the DS, so did the breaking elongation. The water vapor permeability and water diffusibility decreased with the DS. The results demonstrated that the carboxymethylation of CM-CKFs occurring in isopropanol–water was most effective and uniform. The findings have theoretical and practical significance for the industrial uses of carboxymethyl cellulose dressing. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Wound management products are important protective barriers used to assist medical and pharmaceutical healing processes [1,2]. The wound therapeutic market is expected to grow by 5–6% per annum over the next five years, driven by an increase in the number of dressable wounds and a shift from traditional products to more advanced therapies [3–5]. Extensive studies have been carried out to develop functional dressings for different wound requirements [2]. According to moist wound healing theory, healing is accelerated when wounds are kept in a moist condition. Moist wound dressing can be removed easily without damaging the delicate tissues of newly healed wounds [6,7]. Moist wound dressings based on calcium alginates have been commercially available since 1981 [8], and varieties of moist wound dressing such as hydrocolloids, hydrogels, alginates, polyurethane or silicone foams, hydro-fiber, and collagens have been developed [9]. Although those moist wound dressings have obvious healing effects, they are more expensive per dressing than gauze dressings [10]. Carboxymethyl cellulose (CMC) synthesized by the alkali-catalyzed reaction of cellulose with chloroacetic acid is the most important cellulose derivative [11,12]. As a low price, readily available, and versatile reversible polymer system, CMC is widely used as a stabilizer and
⁎ Corresponding author. E-mail address: [email protected] (P. Zhu).
http://dx.doi.org/10.1016/j.matdes.2015.07.124 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
thickener in a wide range of applications in flocculation, drag reduction, detergents, textiles, paper, foods, drugs, and oil-well drilling operations [13]. The functional properties of CMC depend on the degree of substitution (DS) of the cellulose structure (i.e., how many hydroxyl groups have taken part in the substitution reaction) [3]. Modification of cellulose fibers and fabrics by a low degree of carboxymethylation can endow them with high water absorbency while still maintaining their fabric form when swollen in water, desirable properties for wound therapy [14–16]. Moist wound dressings based on CMC have shown effectiveness in pressure sores, leg ulcers, and surgical wounds, and produce warm, moist, local wound conditions for optimal wound healing [17]. In order to achieve best and most effective assists for different wound healing, many kinds of cotton fabrics were modified to make different types of carboxymethyl cotton fabric. In our previous research, carboxymethyl medical cotton gauze [18] and carboxymethyl viscose nonwoven [19] were successfully synthesized in water, demonstrating high water absorbency and hydrogel formation to provide moist conditions for wound therapy, and they could be used as highly absorptive materials for highly efficient, low cost medical dressings. Besides the low density cotton gauze and viscose nonwoven, the knitted fabrics are also good choice for high absorption property of moisture wound dressing. But the reaction efficiency could be low because the high density of knitted fabrics is not good for the reagents to penetrate. As for higher density fabrics, hand feeling of the modified fabrics depends on the uniform of carboxymethylation as well. The carboxymethylation of
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cellulose is a heterogeneous reaction by reacting cellulose with sodium monochloroacetate in the presence of NaOH. The reaction of high substituted degree CMC was widely investigated during the last decades [20–23]. It was proved that the presence of alcohol promotes an even distribution of sodium hydroxide and MCA during the reaction and by this favoring a more uniform etherification [24]. The US patent 09/720375 [25] disclosed that a carboxymethylated cellulosic article prepared in ethanol has degree of substitution in the range from 0.12 to 0.35 which were water-swellable in textile form other than water-soluable CMC in powder form. The patent also suggested that the presence of lower proportions of water than the specified minimum in water/ethanol mixture solution tended to result in uniform carboxymethylation. However, the effects of solvents on the preparation and properties of partly CMC fibers and fabrics as wound dressings have not yet been discussed in detail. In this work, three different kinds of reaction solvent, namely water, ethanol–water, and isopropanol– water, are proposed to synthesize carboxymethyl cotton knitted fabrics (CM-CKFs) for wound therapy applications. The preparation and properties of CM-CKFs were investigated, in order to improve the efficiency of carboxymethlation process, strengthen the properties and decrease the production cost of cotton fabric matrix wound dressing.
microscopy (SEM) was conducted to observe the textile surface on a JSM-6510LV scanning electron microscope instrument (Electronics Co., Ltd. Japan). The DS of all CM-CKFs samples was determined according to GB1904-2005. The characterization on the DS was carried out by ashing and titration. In detail, 1.5 g of CM-CKFs is weighed accurately (true amount m1 g), placed into a porcelain crucible, washed and filtered with 80% (v/v) ethanol/water solution which was preheated to 50–70 °C for 5 times and absolute ethanol at last time. The sample was dried for 2 h at 120 °C, cooled to room temperature in desiccators, weighted as m. Then the sample was placed into a porcelain crucible and ashed at 700 °C. The sodium oxide formed by the ashing is neutralized with 100 mL distilled water and 50 mL 0.1 mol/L H2SO4, and boiled for 10 min, then the excessive H2SO4 is titrated with 0.1 mol/L NaOH using methyl red as indicator. The end-point is marked by the red faded. The DS was calculated as follow.
2. Experiments
where:
2.1. Materials
cb
Cotton knitted fabrics (CKFs) of 190 g/m2 weight were obtained from Huilongxing knitting factory, China. Sodium hydroxide, ethanol, and isopropanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China and monochloroacetic acid was purchased from Tianjin Fuchen Chemical Reagent Factory, China. All reagents were used as purchased without further purification. 2.2. Preparation of carboxymethyl cotton knitted fabric Two reaction steps were used for the production of CM-CKFs, namely alkalization and etherification [26]. Experiments were carried out using seven different solvents: water, ethanol/water at 1/1, 2/1, and 3/1 volume ratios and isopropanol/water at 1/1, 2/1, and 3/1 volume ratios. Taking water as an example, 6 g CKFs was dipped into 150 g sodium hydroxide water solution (15 wt.%) at 30 °C for 15 min, and the fabric was then padded with 140 wt.% wet pick-up and batched up under tension at room temperature for 30 min. The obtained alkali cellulose fabric was padded twice, each time with 100 wt.% wet pick-up after being swollen in a mixed solution (9.6 g NaOH, 31.75 g MCA, 54.4 ml water). The textile was then baked at 70 °C for 3 h. After that, the textile was neutralized by acetic acid, washed three times in an ethanol/water (v/v = 4:1) solution using an ultrasonic washer for 30 min, and finally dried in vacuum oven. 2.3. Characterizations FTIR measurements were conducted on a Bruker TENSOR 27 spectrometer using the KBr pellet technique. XRD measurements were performed on an X'Pert PRO diffractometer system (PANalytical BV). Ni-filtered CuKα radiation (λ = 0.1541 nm) generated at a voltage of 40 kV and current of 50 mA was utilized. The scanning range was varied from 0° to 60° at a rate of 4°/min. The degree of crystallinity of the CKFs and CM-CKFs was calculated using Eq. (1) [27] as follows: CrI ¼
I002 −I am 100% I 002
ð1Þ
where I002 is the peak intensity of the (110) plane, Iam is the peak intensity attributed to the amorphous fraction at 2θ = 18°. Scanning electron
cb ¼
V 1 c1 −V 2 c2 m
ð2Þ
DS ¼
0:162cb 1−0:080cb
ð3Þ
V1 c1 V2 c2 m 0.162 0.080
millimoles of carboxymethyl group per 1 g CM-CKFs, 10−3 mol/g; the volume of 0.1 mol/L H2SO4, mL; the accurate concentration of H2SO4, mol/L; the volume of 0.1 mol/L NaOH required for titration, mL; the accurate concentration of NaOH, mol/L; the mess of CM-CKFs after purification and drying, g; the mess of 1.0 mmol of glucose unit in cellulose, g/10−3 mol; the mess of 1.0 mmol of sodium carboxymethyl, g/10−3 mol.
The reported results represent average values from two tests. Water absorbency (N1) and water retention (N2) [28] were tested by immersing weighed samples in an excess of distilled water (400 mL) at 25 °C for 1 h to reach swelling equilibrium. The swollen sample was then weighed as m1 until it was dripless. The sample was then centrifuged at 2500 rpm for 5 min and weighed as m2, followed by drying in an oven at 100 °C for 24 h and weighed again as m0. The N1 and N2 were calculated as follows. The reported results represent the average values from five samples. N1 ¼ ðm1 −m0 Þ=m0
ð4Þ
N2 ¼ ðm2 −m0 Þ=m0
ð5Þ
The water vapor permeability of the CKFs and CM-CKFs was determined using the “cup method” according to the gravimetric technique of ASTM E96-05. Water diffusibility was tested by adding 300 mL saline solution (0.9 wt.%) in a 500 mL flask. The sample was cut to a size of 10 × 100 cm2 by a blade and then placed vertically in the solution, ensuring that the 10 cm side of the sample just contacted the solution surface. After 5 min, the height that the water had reached was recorded as the water diffusibility. Mechanical properties of CKFs and CM-CKFs in the weft direction were tested with an YG062H electronic fabric strength meter (Laizhou electronic instrument Co., Ltd, China) according to ISO/DIS 3934.1-1994. Five specimens with dimensions of 250 mm (weft direction) × 50 mm (warp direction) have been tested. The distance between the two clamps before the test was kept to be 200 mm, and the testing speed was 20 mm/min. All tests were carried out under ambient atmosphere conditions. The reported results represent average values from five samples.
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Fig. 1. FTIR spectra of (a) CKFs and CM-CKFs treated in (b) water, (c) ethanol–water (v/v = 3/1), and (d) isopropanol–water (v/v = 3/1).
3. Results and discussion 3.1. FTIR FTIR spectra of CKFs (a) and CM-CKFs treated in water (b), ethanol– water (v/v = 3/1) (c), and isopropanol–water (v/v = 3/1) (d) are shown in Fig. 1. Compared with the IR spectra of CKFs (a), the absorption bands at 1602 cm−1 and 1425 cm−1 were the –COO− asymmetric and symmetric stretching vibrations, respectively. The absorption band at 1110 cm−1 was the asymmetric stretching vibration of aliphatic ether. The absorption band at 1070 cm−1 was assigned to acetal and – OH. The functional groups appearing in the FTIR spectra of the CM-CKFs treated in water (b), ethanol–water (v/v = 3/1) (c), and isopropanol–water (v/v = 3/1) (d), suggest the carboxymethylation reaction on the CKFs in presence of the three different reaction solvent types.
3.2. XRD XRD spectra of CKFs and CM-CKFs treated in seven different reaction solvent types: water, ethanol–water (1:1, 2:1, 3:1), and isopropanol– water (1:1, 2:1, 3:1), are shown in Fig. 2. It can be observed that the XRD spectra of the CKFs exhibit a sharp high peak at 2θ of 23.1° and two weaker diffraction peaks at 2θ of 15.1° and 16.7°, respectively,
which are assigned to cellulose I [29,30]. The XRD spectra of the CM-CKFs treated in water show similar diffraction peaks to those of the CKFs, which are again assigned to cellulose I. This phenomenon explains why the reaction of carboxymethylation in water occurred mostly in the amorphous regions of cellulose in the CKFs. In the CKFs, no effect of water without alcohol solvents on the conversion of the crystal form of cellulose was evident. The degree of reaction was low. However, a new peak appeared clearly at 2θ of 20.5° in the spectra of the CM-CKFs treated in different concentrations of ethanol–water and isopropanol–water solutions, indicating the structure of cellulose II, and the peaks belonging to cellulose I disappeared. This proved that type I cellulose in CKFs could be converted to type II by treatment in a mixed solution of ethanol–water or isopropanol–water. The crystallinity index of the samples is shown in Table 1. The crystallinity index of CKFs was 77.4%. This indicates that the crystallinity index was reduced with the water, ethanol–water and isopropanol– water order. Moreover, the crystallinity index of the CM-CKFs decreased with increases in the alcohol ratio and the DS. It is believed that the changes in the crystalline regions of cellulose from cellulose I to cellulose II were occurred in sodium hydroxide solution during the first step of alkalisation [31]. This result can be explained by the solvent effect that depends on the diluent polarity and ability to dissolve ions [24]. In the isopropanol–water case, where polarity is low, it seems that the Na+ and OH− ions do not go into solution, and the two phases existed in the alcohol/aqueous system which are the inner layer from the cellulose fiber consisting of sodium hydroxide, water and very small amounts of isopropanol and the outer layer consisting of isopropanol, water and very small amounts of sodium hydroxide. There is high concentration sodium hydroxide around the fiber in water-rich phase promoting swelling of fiber and decrystallizing cellulous. And there will be less side reactions to sodium glycolate since the low hydroxy group activity of the nonpolar solvent provides an indulgent environment for the monochloroacetic acid. So isopropanol is reported to be the most suitable organic diluent for carboxymethlation of cellulose with high efficiency and quality [24]. As aqueous content in isopropanol–water decreased, the crystallization degree decreased and swelling extent of fiber increased due to the increased NaOH concentration around the fiber. In the ethanol case, NaOH is more diluted and the NaOH concentration close to the fiber is lower than that in the isopropanol case. This is the reason for the lower reaction rate in ethanol compared that in isopropanol. However, ethanol is still commercially used due to the economical advantage in CMC production [24]. On the other hand, the curve (3)–(8) also shows a notable broadening of the peaks, indicating partial decrystallization of cellulose during the carboxymethylation in ethanol–water and isopropanol–water. The degree of substitution (DS) of CMC which is defined as a number of substituted sodium instead of –OH group on cellulose polymer has a maximum value of 3. The data in Table 1 demonstrate that the DS of the CM-CKFs increased with increases in alcohol ratio and the order of water, ethanol–water and isopropanol–water. The results also proved that adding alcohol could promote the reaction. The highest DS (0.65) of CM-CKFs was obtained by using isopropanol–water (v/v = 3/1) as the reaction solvent. Thus it was evident that the effectiveness of heterogeneous carboxymethylation of the cellulose depended on solvent effect of increased alkali concentration around the fiber.
Table 1 Crystallinity index of CKFs and CM-CKFs. Blank Water Ethanol–water [v/v] 1:1 Fig. 2. XRD spectra of CKFs and CM-CKFs treated in water, ethanol–water, and isopropanol/water.
DS 0 Crystallinity index /% 77.4
0.13 56.1
2:1
3:1
Isopropanol–water [v/v] 1:1
2:1
3:1
0.25 0.39 0.50 0.32 0.56 0.65 55.7 47.9 40.1 54.2 40.0 25.8
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Fig. 3. Images of (a) CKFs and CM-CKFs treated in (b) water, (c) ethanol–water (v/v = 3/1), and (d) isopropanol/water (v/v = 3/1) before (left) and after (right) absorbing water.
3.3. Morphology The macroscopic and microscopic morphologies of CKFs and CMCKFs treated in the three different reaction solvents were investigated,
and the images are shown in Figs. 3 and 4 respectively. From the macroscopic morphologies of CKFs (3a) and CM-CKFs treated in water (3b), ethanol–water (v/v = 3/1) (3c), and isopropanol–water (v/v = 3/1) (3d), it is evident that all the samples retained the knitted textile
Fig. 4. SEM images of surfaces of cellulose fibers in CKFs (a), with high magnification (b, c), and CM-CKFs treated in water (d), with high magnification (e, f), ethanol–water (v/v = 3/1) (g), with high magnification (h, i), and isoproponal/water (v/v = 3/1) (j), with high magnification (k, l).
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Table 2 Hygroscopicity and DS of CKFs and CM-CKFs. Blank
DS water absorbency/g · g−1 water retention /g · g−1
0 0.51 2.82
Water
0.13 1.86 ± 0.11 1.01 ± 0.10
Ethanol–water [v/v]
Isopropanol–water [v/v]
1:1
2:1
3:1
1:1
2:1
3:1
0.25 2.64 ± 0.14 1.52 ± 0.11
0.39 3.83 ± 0.16 2.75 ± 0.13
0.50 6.95 ± 0.31 4.07 ± 0.25
0.32 4.88 ± 0.26 3.23 ± 0.19
0.56 10.25 ± 0.69 7.12 ± 0.45
0.65 14.86 ± 0.87 10.07 ± 0.72
structure and morphology during the carboxymethylation. After absorption of water, there was a small amount of gel on the surface of the CM-CKFs treated in water. As with the wet CKFs, the structure of the CM-CKFs did not change obviously. The CM-CKFs treated in ethanol–water (v/v = 3/1) had higher DS than the CM-CKFs treated in water because of the gel coated on the surface. The fabric also retained its textile style. In the CM-CKFs treated in isopropanol–water (v/v = 3/1), the textile structure gradually disappeared during the absorption process, and a uniform gel formed after the absorption of water, contributing to visual inspection of wound healing. Thus, after the carboxymethylation process, the dry CM-CKFs retained their knitted fabric structure, but after the absorption of water, the wet CM-CKFs changed from an opaque textile to form a semitransparent gel, with increases in DS. SEM as shown in Fig. 4 was used to observe the morphology of the cellulose fibers in the CKFs (Fig. 4e) and the CM-CKFs treated in different types of reaction solvent. A woven structure typical of knitted fabrics can be seen in the CKFs image (Fig. 4a). The cotton fibers are neatly wrapped together (Fig. 4b). The cotton fiber in the CKFs is flat, with natural twists and grooves along its length (Fig. 4c). The diameter of the cotton fibers in the CKFs was 9 μm. With carboxymethylation, all fabrics retained their knitted structure, but the fibers of the CM-CKFs showed different degrees of swelling, with their diameter obviously increased. In Fig. 4d and e, which are the CM-CKFs treated in water, there is no difference from the CKFs treated in water. The fibers of the CM-CKFs treated in water also retained their natural twists and grooves (Fig. 4f) and were flat, but their diameter increased to 27 μm. The fibers of the CM-CKFs treated in a mixed solution became cylindrical. The diameter of fibers treated in isopropanol–water was 31 μm (Fig. 4l), which was greater than that of fibers treated in ethanol–water (23 μm) (Fig. 4i). The fibers in the CM-CKFs treated in mixed solution acquired gel form and some adhesion occurred (Fig. 4h and k), which could promote bonding between fibers and contribute to an increase in the mechanical properties of the CM-CKFs. However, the structure of the knitted fabric was retained (Fig. 4g and j). Thus the diameter of carboxymethyl fibers increased with the order of water, ethanol–water and isopropanol– water. The degree of carboxymethylation was the main reason for these dramatic morphological changes in carboxymethyl fibers, which were determined by the swelling and infiltration ability of the reagent to the reaction centers. When water was used as a solvent, the reagent reaction was mostly limited to amorphous regions. Under the same amount of reagent, however, the alcohol solvent has less polarity lead to high concentration of Na+ and OH− ions in aqueous which increase the penetration ability of reagent to the crystalline region. Thus the efficiency of carboxymethylation in cellulose fibers was enhanced. The swelling degree of fiber increased with increases in the order of water, ethanol–water and isopropanol–water and DS.
3.4. Hygroscopicity Hygroscopicity is the most important property of CMC fibers and fabrics used in the preparation of absorptive materials such as medical dressings. The hygroscopicity of the CM-CKFs was determined by DS. The hygroscopicity of the CKFs and CM-CKFs treated in the seven different types of reaction solvent is shown in Table 2. Table 2 shows that water absorbency and water retention of the CM-CKFs treated in ethanol–water and isopropanol–water were all higher than those of the CM-CKFs treated in water. Water absorbency and water retention of the CM-CKFs increased with the increase of DS. The water absorbency and water retention of the CM-CKFs treated in water were 1.86 g/g and 1.01 g/g, respectively. Isopropanol–water (v/v = 3/1) was the most efficient of the seven reaction solvents. The water absorbency and water retention of the CM-CKFs treated in isopropanol–water (v/v = 3/1) reached 14.86 g/g and 10.07 g/g, respectively. These results suggest that the mixture of water and isopropanol solution was an ideal solvent for the preparation of CM-CKFs compared with the other two types of reaction solvent. The hygroscopicity of CM-CKFs prepared in water–isopropanol (v/v = 3/1) was compared with previous works and the commercial product of CMC based wound dressing. As shown in Table 3, There are other three types of pure CMC wound dressings, including carboxymethyl viscose nonwoven (CM-VN) [19], CMC nonwovens [29] and AQUACEL® (ConvaTec Ltd) [32]. The CM-VN prepared in our lab [19] has water absorbency and water retention of 25.44 g · g−1 and 9.49 g · g−1, respectively. In Doh et al. research [29], the CMC nonwovens produced with the CMC fibers by a wet-laid nonwoven process has water absorbency of 8.3 g · g−1. The AQUACEL® (ConvaTec Ltd) [32,33] is the most popular CMC dressing used in clinical practice which has the water absorbency between 20 and 30 g · g−1. There are big differences between these four types of CMC wound dressing due to the properties of CMC wound dressing depend on not only the DS but also the source of raw material, preparation procedure, test method of hygroscopicity, and density and structure of the final CMC textile. Compare to knitted fabrics used in this work, the nonwoven possesses more fluffy structure and lower density. After absorbing water, the CMC fiber needs more space to swell and store water which can be provided by the nonwoven. And the nonwoven also has bigger space to store the water between fibers. So even the CK-CKFs have higher DS, its water absorbency is lower than that of the AQUACEL® and CM-VN. However, the water retention of CM-CKFs is higher than that of the CM-VN, because after centrifugation, the water stored between the fibers was detached, and the water between the fibers in CM-CKFs is less than that in the CM-VN. So the CM-CKFs could be used in the squeezed parts of body, like buttock, leg and foot, and it will have better effect than CM-VN for absorbing and retain more exudates. The differences between AQUACEL® and CM-VN are the source of raw material and preparation procedure. AQUACEL®
Table 3 Hygroscopicity of different 100% CMC wound dressing.
DS Water absorbency/g · g−1 Water retention /g · g−1 “–” the data not been tested or mentioned.
CM-CKFs
CF-CN [19]
CMC nonwovens [29]
Convatec Aquacel® [32,33]
0.65 14.86 ± 0.87 10.07 ± 0.72
0.35 25.44 ± 1.50 9.49 ± 1.12
0.88 8.3 –
b0.4 20–30 –
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Table 4 Mechanical properties of CKFs and CM-CKFs. Blank
DS Breaking force/N Breaking elongation/%
Water
0 362.7 ± 5.3 181.0 ± 3.2
Ethanol–water [v/v]
0.13 187.2 ± 3.7 343.8 ± 4.4
2:1
3:1
1:1
2:1
3:1
0.25 193.5 ± 5.9 149.7 ± 3.6
0.39 204.6 ± 4.1 168.2 ± 2.6
0.50 212.7 ± 1.8 183.4 ± 1.5
0.32 199.8 ± 3.0 162.2 ± 2.3
0.56 218.7 ± 3.2 190.9 ± 4.0
0.65 220.3 ± 2.9 195.0 ± 3.1
was prepared with the high cost regenerated cellulose fiber of Tencel and Lyocell by needled punched nonwoven method. But in our work [19], the viscose nonwoven which is cheap and easy to get was directly modified in aqueous medium through one bath process to produce the CM-VN. Meanwhile, the result of our CM-VN is almost the same as that in AQUACEL® with the same nonwoven density of 120 g/m2 [32,33]. For CMC nonwovens [29], the low water absorbency attributed to different test method which specimens were immersed in distilled water for 5 min during the water absorption test, but there are more than 1 h immersing time in other researches. They also use cheaper cotton fibers and traditional wet-laid nonwoven process which can decrease the cost of wound dressing as well. So researchers around the world made good efforts for the preparation of low-cost and high-performance medical dressings.
3.5. Mechanical properties Table 4 shows the mechanical properties of the CKFs and CM-CKFs. It confirms that the breaking force of the CM-CKFs clearly decreased after carboxymethylation, and that the CM-CKFs treated in water had the lowest breaking force and the highest breaking elongation among the samples. Because of the lower ion concentration in water and the heterogeneous reactions, the carboxymethylation occurred mostly in the amorphous regions of cellulose, causing a non-uniform degree of reaction within each of the knitted fabrics. The concentration stresses in the CM-CKFs caused by the non-uniform degree of reaction led to a decrease of breaking force. The significant increase of breaking elongation is thought to be the effect of intermolecular breakage of the hydrogen bond. However, the CM-CKFs treated in mixed solutions had higher breaking force and lower breaking elongation than the CM-CKFs treated in water. Both breaking force and breaking elongation increased with the increase of DS and the ratio of alcohol solution. It is known that carboxylation of cellulose fibers increases fiber–fiber bonding strength. The increase in fiber–fiber bonds seem to be a more likely cause for the increase in breaking force [34,35]. Another reason could be the mixture of alcohol and water, which was a more efficient reaction medium with higher ion concentration around the fiber than that in water, improved the uniformity of reaction in both crystalline and amorphous areas. Thus in the CM-CKFs treated with a mixed solution, there were few stress concentration, leading to a higher breaking force than that of CM-CKFs treated in water. With the increase of DS the fiber diameter increased, and length of the fiber elongation shrank, which strengthened the fibers. Moreover, the fibers in the gel form in mixed solutions adhered with each other, as shown in Fig. 4, which could also increase the mechanical properties of the fabrics, even the inter- and intramolecular hydrogen bonds in the fibers were broken.
Table 5 Permeability of CKFs and CM-CKFs.
3.6. Water vapor permeability It should be noted that the loss of water vapor from intact skin is 0.24–1.92 kg/(24 h · m2) and the water vapor loss from an open wound is about 4.8 kg/(24 h · m2) [36]. A wound dressing not only provides a moist environment but also transmits oxygen into and carbon dioxide and water vapor out of the dressing in case there is a risk of wound infection in the exudates. Therefore, the water vapor permeability of the CKFs and CM-CKFs was tested according to ASTM E96-05. The results in Table 5 show that the permeability of the CM-CKFs was lower than that of the CKFs. The permeability of the CM-CKFs decreased with the increase in DS. The spaces between fibers in the textile form the main channels for the evaporating water. The more and larger the spaces are between fibers, the better the permeability of the fabric has. During carboxymethylation, the fiber diameter in the fabrics increased in the alkaline condition, resulting in a decrease in the interfiber distance and the quantity of channels. The fiber diameter increased with an increase in alcohol ratio and the DS. Moreover, the fabrics became gel during the absorption process, preventing water from moving in and out of the dressing. Thus the permeability of the CM-CKFs decreased with increases in solvent alcohol ratio and DS. 3.7. Water diffusibility The gel-forming ability of CMC-based wound dressing is determined by the DS of carboxymethylation. Once formed, a carboxymethyl cotton gel consists of part solid and part solution, water, and other molecules such as exudate and bacteria are physically trapped within the CMC matrix by capillary forces. Therefore, low water diffusibility means good barrier properties. As an important factor in wound dressing, the water diffusibility of the CKFs and CM-CKFs was investigated and the results are shown in Table 6, demonstrating that the CKFs had higher water diffusibility than all the CM-CKFs. With increases in DS and alcohol ratio of the solvent, the water diffusibility of the CM-CKFs clearly decreases. The CMC hydrogel formed during the absorption process can effectively impede the diffusion of water. Then, during the healing process, bacteria could not migrate with exudate to infect normal tissues. The CM-CKFs treated in isopropanol–water solution had the best barrier property of the solvents tested. 4. Conclusions In this study, CM-CKFs were prepared with different solvents: water, ethanol–water and isopropanol–water. The results showed an obvious effect of the solvents on the carboxymethylation reaction. Isopropanol and ethanol promoted the destruction of cellulose I crystalline form and the formation of cellulose II structure, increasing the availability of
Table 6 Water diffusibility of CKFs and CM-CKFs.
Blank Water Ethanol–water [v/v] 1:1 DS 0 Permeability kg/(24 h · m2) 0.48
Isopropanol–water [v/v]
1:1
0.13 0.42
2:1
3:1
Isopropanol–water [v/v] 1:1
0.25 0.39 0.50 0.32 0.35 0.30 0.21 0.32
2:1
3:1
0.56 0.17
0.65 0.12
DS Diffusion distances/cm
Blank
Water
Ethanol–water [v/v]
Isopropanol–water [v/v]
1:1
2:1
3:1
1:1
2:1
3:1
0 5.93
0.13 5.23
0.25 4.47
0.39 3.14
0.50 2.08
0.32 4.06
0.56 1.69
0.65 1.05
244
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the reaction agent for carboxymethylation. The DS of the CM-CKFs increased with an increase of the alcohol ratio in mixed solvents and the order of water, ethanol–water and isopropanol–water. The CM-CKFs treated in isopropanol–water (v/v = 3:1) with a DS of 0.65 had the highest water absorbency and water retention of 14.86 g/g and 10.07 g/g, respectively, and it also had excellent mechanical properties, water vapor permeability, and barrier properties. Therefore, CM-CKFs treated in isopropanol–water solution is considered a good candidate for industrial production for wound therapy applications. Acknowledgment This work was supported by the Educational Committee Foundation of Hubei Province, China (Q20131606). References [1] M. Walker, J.A. Hobot, G.R. Newman, P.G. Bowler, Scanning electron microscopic examination of bacterial immobilisation in a carboxymethyl cellulose (AQUACEL®) and alginate dressings, Biomaterials 24 (2003) 883–890. [2] J.S. Boateng, K.H. Matthews, H.N.E. Stevens, G.M. Eccleston, Wound healing dressings and drug delivery systems: a review, J. Pharm. Sci. 97 (2008) 2892–2923. [3] D.V. Parikh, Carboxymethylation-cotton for wound care management, Bioact. Fibers Polym. ACS Symp. Ser. 792 (2001) 115–124. [4] B. Gupta, R. Agarwal, M.S. Alan, Textile-based smart wound dressings, Indian J. Fibre Text. Res. 35 (2010) 174–187. [5] M. Walker, D. Parsons, Hydrofiber® Technology: its role in exudate management, Wounds UK 6 (2010) 31–38. [6] D.V. Parikh, Antimicrobial silver/sodium carboxymethyl cotton dressings for burn wounds, Text. Res. J. 75 (2005) 134–138. [7] D.V. Parikh, T. Fink, A.J. DeLucca, A.D. Parikh, Absorption and swelling characteristics of silver (I) antimicrobial wound dressings, Text. Res. J. 81 (2010) 494–503. [8] Y. Qin, The characterization of alginate wound dressings with different fiber and textile structures, J. Appl. Polym. Sci. 100 (2006) 2516–2520. [9] L.I.F. Moura, A.M.A. Dias, E. Carvalho, H.C. de Sousa, Recent advances on the development of wound dressings for diabetic foot ulcer treatment—a review, Acta Biomater. 9 (2013) 7093–7114. [10] J.A. Fulton, K.N. Blasiole, T. Cottingham, M. Tornero, M. Graves, L.G. Smith, et al., Wound dressing absorption: a comparative study, Adv. Skin Wound Care 25 (2012) 315–320. [11] R. Moseley, M. Walker, R. Waddington, W. Chen, Comparison of the antioxidant properties of wound dressing materials—carboxymethylcellulose, hyaluronan benzyl ester and hyaluronan, towards polymorphonuclear leukocyte-derived reactive oxygen species, Biomaterials 24 (2003) 1549–1557. [12] H.D. Heydarzadeh, G.D. Najafpour, A.A. Nazari-Moghaddam, Catalyst-free conversion of alkali cellulose to fine carboxymethyl cellulose at mild conditions, World Appl. Sci. J. 6 (2009) 564–569. [13] D. Biswal, R. Singh, Characterisation of carboxymethyl cellulose and polyacrylamide graft copolymer, Carbohydr. Polym. 57 (2004) 379–387.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/jmad
Design carboxymethyl cotton knitted fabrics for wound dressing applications: Solvent effects Jinchao Zhao a,b, Youhong Tang b, Yun Liu a, Li Cui a, Xianxin Xi a, Nannan Zhang a, Ping Zhu a,⁎ a b
School of Chemistry and Chemical Engineering, State Key Laboratory of New Textile Materials and Advanced Processing, Wuhan Textile University, Wuhan 430064, China Centre for NanoScale Science and Technology, and School of Computer Science, Engineering and Mathematics, Flinders University, South Australia 5042, Australia
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 18 July 2015 Accepted 21 July 2015 Available online 26 July 2015 Keywords: Carboxymethylation Solvent effects Cotton knitted fabric Substitution degree Water absorbency
a b s t r a c t Carboxymethyl cotton knitted fabrics (CM-CKFs) for wound dressings were fabricated with different solvents: water, ethanol–water, and isopropanol–water. The FTIR analysis showed that carboxymethylation reaction in the CM-CKFs occurred with the different solvents. With the water, ethanol–water and isopropanol–water order and the decrease of water ratio in the mixed solution, the degree of substitution (DS) increased, the crystallinity index of the CM-CKFs decreased, crystal structure of cellulose changed from type I to II, and appreciable increased in fiber diameter and gel formation. The water absorbency and water retention of the CM-CKFs increased with DS. The water absorbency of the CM-CKFs treated in water, ethanol–water (v/v = 3/1), and isopropanol–water (v/v = 3/1) was 1.86, 6.95, and 14.86 g/g, respectively. The breaking force of the CM-CKFs was lower than that of cotton knitted fabrics, but the breaking force increased with the DS, so did the breaking elongation. The water vapor permeability and water diffusibility decreased with the DS. The results demonstrated that the carboxymethylation of CM-CKFs occurring in isopropanol–water was most effective and uniform. The findings have theoretical and practical significance for the industrial uses of carboxymethyl cellulose dressing. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Wound management products are important protective barriers used to assist medical and pharmaceutical healing processes [1,2]. The wound therapeutic market is expected to grow by 5–6% per annum over the next five years, driven by an increase in the number of dressable wounds and a shift from traditional products to more advanced therapies [3–5]. Extensive studies have been carried out to develop functional dressings for different wound requirements [2]. According to moist wound healing theory, healing is accelerated when wounds are kept in a moist condition. Moist wound dressing can be removed easily without damaging the delicate tissues of newly healed wounds [6,7]. Moist wound dressings based on calcium alginates have been commercially available since 1981 [8], and varieties of moist wound dressing such as hydrocolloids, hydrogels, alginates, polyurethane or silicone foams, hydro-fiber, and collagens have been developed [9]. Although those moist wound dressings have obvious healing effects, they are more expensive per dressing than gauze dressings [10]. Carboxymethyl cellulose (CMC) synthesized by the alkali-catalyzed reaction of cellulose with chloroacetic acid is the most important cellulose derivative [11,12]. As a low price, readily available, and versatile reversible polymer system, CMC is widely used as a stabilizer and
⁎ Corresponding author. E-mail address: [email protected] (P. Zhu).
http://dx.doi.org/10.1016/j.matdes.2015.07.124 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
thickener in a wide range of applications in flocculation, drag reduction, detergents, textiles, paper, foods, drugs, and oil-well drilling operations [13]. The functional properties of CMC depend on the degree of substitution (DS) of the cellulose structure (i.e., how many hydroxyl groups have taken part in the substitution reaction) [3]. Modification of cellulose fibers and fabrics by a low degree of carboxymethylation can endow them with high water absorbency while still maintaining their fabric form when swollen in water, desirable properties for wound therapy [14–16]. Moist wound dressings based on CMC have shown effectiveness in pressure sores, leg ulcers, and surgical wounds, and produce warm, moist, local wound conditions for optimal wound healing [17]. In order to achieve best and most effective assists for different wound healing, many kinds of cotton fabrics were modified to make different types of carboxymethyl cotton fabric. In our previous research, carboxymethyl medical cotton gauze [18] and carboxymethyl viscose nonwoven [19] were successfully synthesized in water, demonstrating high water absorbency and hydrogel formation to provide moist conditions for wound therapy, and they could be used as highly absorptive materials for highly efficient, low cost medical dressings. Besides the low density cotton gauze and viscose nonwoven, the knitted fabrics are also good choice for high absorption property of moisture wound dressing. But the reaction efficiency could be low because the high density of knitted fabrics is not good for the reagents to penetrate. As for higher density fabrics, hand feeling of the modified fabrics depends on the uniform of carboxymethylation as well. The carboxymethylation of
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cellulose is a heterogeneous reaction by reacting cellulose with sodium monochloroacetate in the presence of NaOH. The reaction of high substituted degree CMC was widely investigated during the last decades [20–23]. It was proved that the presence of alcohol promotes an even distribution of sodium hydroxide and MCA during the reaction and by this favoring a more uniform etherification [24]. The US patent 09/720375 [25] disclosed that a carboxymethylated cellulosic article prepared in ethanol has degree of substitution in the range from 0.12 to 0.35 which were water-swellable in textile form other than water-soluable CMC in powder form. The patent also suggested that the presence of lower proportions of water than the specified minimum in water/ethanol mixture solution tended to result in uniform carboxymethylation. However, the effects of solvents on the preparation and properties of partly CMC fibers and fabrics as wound dressings have not yet been discussed in detail. In this work, three different kinds of reaction solvent, namely water, ethanol–water, and isopropanol– water, are proposed to synthesize carboxymethyl cotton knitted fabrics (CM-CKFs) for wound therapy applications. The preparation and properties of CM-CKFs were investigated, in order to improve the efficiency of carboxymethlation process, strengthen the properties and decrease the production cost of cotton fabric matrix wound dressing.
microscopy (SEM) was conducted to observe the textile surface on a JSM-6510LV scanning electron microscope instrument (Electronics Co., Ltd. Japan). The DS of all CM-CKFs samples was determined according to GB1904-2005. The characterization on the DS was carried out by ashing and titration. In detail, 1.5 g of CM-CKFs is weighed accurately (true amount m1 g), placed into a porcelain crucible, washed and filtered with 80% (v/v) ethanol/water solution which was preheated to 50–70 °C for 5 times and absolute ethanol at last time. The sample was dried for 2 h at 120 °C, cooled to room temperature in desiccators, weighted as m. Then the sample was placed into a porcelain crucible and ashed at 700 °C. The sodium oxide formed by the ashing is neutralized with 100 mL distilled water and 50 mL 0.1 mol/L H2SO4, and boiled for 10 min, then the excessive H2SO4 is titrated with 0.1 mol/L NaOH using methyl red as indicator. The end-point is marked by the red faded. The DS was calculated as follow.
2. Experiments
where:
2.1. Materials
cb
Cotton knitted fabrics (CKFs) of 190 g/m2 weight were obtained from Huilongxing knitting factory, China. Sodium hydroxide, ethanol, and isopropanol were purchased from Sinopharm Chemical Reagent Co., Ltd., China and monochloroacetic acid was purchased from Tianjin Fuchen Chemical Reagent Factory, China. All reagents were used as purchased without further purification. 2.2. Preparation of carboxymethyl cotton knitted fabric Two reaction steps were used for the production of CM-CKFs, namely alkalization and etherification [26]. Experiments were carried out using seven different solvents: water, ethanol/water at 1/1, 2/1, and 3/1 volume ratios and isopropanol/water at 1/1, 2/1, and 3/1 volume ratios. Taking water as an example, 6 g CKFs was dipped into 150 g sodium hydroxide water solution (15 wt.%) at 30 °C for 15 min, and the fabric was then padded with 140 wt.% wet pick-up and batched up under tension at room temperature for 30 min. The obtained alkali cellulose fabric was padded twice, each time with 100 wt.% wet pick-up after being swollen in a mixed solution (9.6 g NaOH, 31.75 g MCA, 54.4 ml water). The textile was then baked at 70 °C for 3 h. After that, the textile was neutralized by acetic acid, washed three times in an ethanol/water (v/v = 4:1) solution using an ultrasonic washer for 30 min, and finally dried in vacuum oven. 2.3. Characterizations FTIR measurements were conducted on a Bruker TENSOR 27 spectrometer using the KBr pellet technique. XRD measurements were performed on an X'Pert PRO diffractometer system (PANalytical BV). Ni-filtered CuKα radiation (λ = 0.1541 nm) generated at a voltage of 40 kV and current of 50 mA was utilized. The scanning range was varied from 0° to 60° at a rate of 4°/min. The degree of crystallinity of the CKFs and CM-CKFs was calculated using Eq. (1) [27] as follows: CrI ¼
I002 −I am 100% I 002
ð1Þ
where I002 is the peak intensity of the (110) plane, Iam is the peak intensity attributed to the amorphous fraction at 2θ = 18°. Scanning electron
cb ¼
V 1 c1 −V 2 c2 m
ð2Þ
DS ¼
0:162cb 1−0:080cb
ð3Þ
V1 c1 V2 c2 m 0.162 0.080
millimoles of carboxymethyl group per 1 g CM-CKFs, 10−3 mol/g; the volume of 0.1 mol/L H2SO4, mL; the accurate concentration of H2SO4, mol/L; the volume of 0.1 mol/L NaOH required for titration, mL; the accurate concentration of NaOH, mol/L; the mess of CM-CKFs after purification and drying, g; the mess of 1.0 mmol of glucose unit in cellulose, g/10−3 mol; the mess of 1.0 mmol of sodium carboxymethyl, g/10−3 mol.
The reported results represent average values from two tests. Water absorbency (N1) and water retention (N2) [28] were tested by immersing weighed samples in an excess of distilled water (400 mL) at 25 °C for 1 h to reach swelling equilibrium. The swollen sample was then weighed as m1 until it was dripless. The sample was then centrifuged at 2500 rpm for 5 min and weighed as m2, followed by drying in an oven at 100 °C for 24 h and weighed again as m0. The N1 and N2 were calculated as follows. The reported results represent the average values from five samples. N1 ¼ ðm1 −m0 Þ=m0
ð4Þ
N2 ¼ ðm2 −m0 Þ=m0
ð5Þ
The water vapor permeability of the CKFs and CM-CKFs was determined using the “cup method” according to the gravimetric technique of ASTM E96-05. Water diffusibility was tested by adding 300 mL saline solution (0.9 wt.%) in a 500 mL flask. The sample was cut to a size of 10 × 100 cm2 by a blade and then placed vertically in the solution, ensuring that the 10 cm side of the sample just contacted the solution surface. After 5 min, the height that the water had reached was recorded as the water diffusibility. Mechanical properties of CKFs and CM-CKFs in the weft direction were tested with an YG062H electronic fabric strength meter (Laizhou electronic instrument Co., Ltd, China) according to ISO/DIS 3934.1-1994. Five specimens with dimensions of 250 mm (weft direction) × 50 mm (warp direction) have been tested. The distance between the two clamps before the test was kept to be 200 mm, and the testing speed was 20 mm/min. All tests were carried out under ambient atmosphere conditions. The reported results represent average values from five samples.
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Fig. 1. FTIR spectra of (a) CKFs and CM-CKFs treated in (b) water, (c) ethanol–water (v/v = 3/1), and (d) isopropanol–water (v/v = 3/1).
3. Results and discussion 3.1. FTIR FTIR spectra of CKFs (a) and CM-CKFs treated in water (b), ethanol– water (v/v = 3/1) (c), and isopropanol–water (v/v = 3/1) (d) are shown in Fig. 1. Compared with the IR spectra of CKFs (a), the absorption bands at 1602 cm−1 and 1425 cm−1 were the –COO− asymmetric and symmetric stretching vibrations, respectively. The absorption band at 1110 cm−1 was the asymmetric stretching vibration of aliphatic ether. The absorption band at 1070 cm−1 was assigned to acetal and – OH. The functional groups appearing in the FTIR spectra of the CM-CKFs treated in water (b), ethanol–water (v/v = 3/1) (c), and isopropanol–water (v/v = 3/1) (d), suggest the carboxymethylation reaction on the CKFs in presence of the three different reaction solvent types.
3.2. XRD XRD spectra of CKFs and CM-CKFs treated in seven different reaction solvent types: water, ethanol–water (1:1, 2:1, 3:1), and isopropanol– water (1:1, 2:1, 3:1), are shown in Fig. 2. It can be observed that the XRD spectra of the CKFs exhibit a sharp high peak at 2θ of 23.1° and two weaker diffraction peaks at 2θ of 15.1° and 16.7°, respectively,
which are assigned to cellulose I [29,30]. The XRD spectra of the CM-CKFs treated in water show similar diffraction peaks to those of the CKFs, which are again assigned to cellulose I. This phenomenon explains why the reaction of carboxymethylation in water occurred mostly in the amorphous regions of cellulose in the CKFs. In the CKFs, no effect of water without alcohol solvents on the conversion of the crystal form of cellulose was evident. The degree of reaction was low. However, a new peak appeared clearly at 2θ of 20.5° in the spectra of the CM-CKFs treated in different concentrations of ethanol–water and isopropanol–water solutions, indicating the structure of cellulose II, and the peaks belonging to cellulose I disappeared. This proved that type I cellulose in CKFs could be converted to type II by treatment in a mixed solution of ethanol–water or isopropanol–water. The crystallinity index of the samples is shown in Table 1. The crystallinity index of CKFs was 77.4%. This indicates that the crystallinity index was reduced with the water, ethanol–water and isopropanol– water order. Moreover, the crystallinity index of the CM-CKFs decreased with increases in the alcohol ratio and the DS. It is believed that the changes in the crystalline regions of cellulose from cellulose I to cellulose II were occurred in sodium hydroxide solution during the first step of alkalisation [31]. This result can be explained by the solvent effect that depends on the diluent polarity and ability to dissolve ions [24]. In the isopropanol–water case, where polarity is low, it seems that the Na+ and OH− ions do not go into solution, and the two phases existed in the alcohol/aqueous system which are the inner layer from the cellulose fiber consisting of sodium hydroxide, water and very small amounts of isopropanol and the outer layer consisting of isopropanol, water and very small amounts of sodium hydroxide. There is high concentration sodium hydroxide around the fiber in water-rich phase promoting swelling of fiber and decrystallizing cellulous. And there will be less side reactions to sodium glycolate since the low hydroxy group activity of the nonpolar solvent provides an indulgent environment for the monochloroacetic acid. So isopropanol is reported to be the most suitable organic diluent for carboxymethlation of cellulose with high efficiency and quality [24]. As aqueous content in isopropanol–water decreased, the crystallization degree decreased and swelling extent of fiber increased due to the increased NaOH concentration around the fiber. In the ethanol case, NaOH is more diluted and the NaOH concentration close to the fiber is lower than that in the isopropanol case. This is the reason for the lower reaction rate in ethanol compared that in isopropanol. However, ethanol is still commercially used due to the economical advantage in CMC production [24]. On the other hand, the curve (3)–(8) also shows a notable broadening of the peaks, indicating partial decrystallization of cellulose during the carboxymethylation in ethanol–water and isopropanol–water. The degree of substitution (DS) of CMC which is defined as a number of substituted sodium instead of –OH group on cellulose polymer has a maximum value of 3. The data in Table 1 demonstrate that the DS of the CM-CKFs increased with increases in alcohol ratio and the order of water, ethanol–water and isopropanol–water. The results also proved that adding alcohol could promote the reaction. The highest DS (0.65) of CM-CKFs was obtained by using isopropanol–water (v/v = 3/1) as the reaction solvent. Thus it was evident that the effectiveness of heterogeneous carboxymethylation of the cellulose depended on solvent effect of increased alkali concentration around the fiber.
Table 1 Crystallinity index of CKFs and CM-CKFs. Blank Water Ethanol–water [v/v] 1:1 Fig. 2. XRD spectra of CKFs and CM-CKFs treated in water, ethanol–water, and isopropanol/water.
DS 0 Crystallinity index /% 77.4
0.13 56.1
2:1
3:1
Isopropanol–water [v/v] 1:1
2:1
3:1
0.25 0.39 0.50 0.32 0.56 0.65 55.7 47.9 40.1 54.2 40.0 25.8
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Fig. 3. Images of (a) CKFs and CM-CKFs treated in (b) water, (c) ethanol–water (v/v = 3/1), and (d) isopropanol/water (v/v = 3/1) before (left) and after (right) absorbing water.
3.3. Morphology The macroscopic and microscopic morphologies of CKFs and CMCKFs treated in the three different reaction solvents were investigated,
and the images are shown in Figs. 3 and 4 respectively. From the macroscopic morphologies of CKFs (3a) and CM-CKFs treated in water (3b), ethanol–water (v/v = 3/1) (3c), and isopropanol–water (v/v = 3/1) (3d), it is evident that all the samples retained the knitted textile
Fig. 4. SEM images of surfaces of cellulose fibers in CKFs (a), with high magnification (b, c), and CM-CKFs treated in water (d), with high magnification (e, f), ethanol–water (v/v = 3/1) (g), with high magnification (h, i), and isoproponal/water (v/v = 3/1) (j), with high magnification (k, l).
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Table 2 Hygroscopicity and DS of CKFs and CM-CKFs. Blank
DS water absorbency/g · g−1 water retention /g · g−1
0 0.51 2.82
Water
0.13 1.86 ± 0.11 1.01 ± 0.10
Ethanol–water [v/v]
Isopropanol–water [v/v]
1:1
2:1
3:1
1:1
2:1
3:1
0.25 2.64 ± 0.14 1.52 ± 0.11
0.39 3.83 ± 0.16 2.75 ± 0.13
0.50 6.95 ± 0.31 4.07 ± 0.25
0.32 4.88 ± 0.26 3.23 ± 0.19
0.56 10.25 ± 0.69 7.12 ± 0.45
0.65 14.86 ± 0.87 10.07 ± 0.72
structure and morphology during the carboxymethylation. After absorption of water, there was a small amount of gel on the surface of the CM-CKFs treated in water. As with the wet CKFs, the structure of the CM-CKFs did not change obviously. The CM-CKFs treated in ethanol–water (v/v = 3/1) had higher DS than the CM-CKFs treated in water because of the gel coated on the surface. The fabric also retained its textile style. In the CM-CKFs treated in isopropanol–water (v/v = 3/1), the textile structure gradually disappeared during the absorption process, and a uniform gel formed after the absorption of water, contributing to visual inspection of wound healing. Thus, after the carboxymethylation process, the dry CM-CKFs retained their knitted fabric structure, but after the absorption of water, the wet CM-CKFs changed from an opaque textile to form a semitransparent gel, with increases in DS. SEM as shown in Fig. 4 was used to observe the morphology of the cellulose fibers in the CKFs (Fig. 4e) and the CM-CKFs treated in different types of reaction solvent. A woven structure typical of knitted fabrics can be seen in the CKFs image (Fig. 4a). The cotton fibers are neatly wrapped together (Fig. 4b). The cotton fiber in the CKFs is flat, with natural twists and grooves along its length (Fig. 4c). The diameter of the cotton fibers in the CKFs was 9 μm. With carboxymethylation, all fabrics retained their knitted structure, but the fibers of the CM-CKFs showed different degrees of swelling, with their diameter obviously increased. In Fig. 4d and e, which are the CM-CKFs treated in water, there is no difference from the CKFs treated in water. The fibers of the CM-CKFs treated in water also retained their natural twists and grooves (Fig. 4f) and were flat, but their diameter increased to 27 μm. The fibers of the CM-CKFs treated in a mixed solution became cylindrical. The diameter of fibers treated in isopropanol–water was 31 μm (Fig. 4l), which was greater than that of fibers treated in ethanol–water (23 μm) (Fig. 4i). The fibers in the CM-CKFs treated in mixed solution acquired gel form and some adhesion occurred (Fig. 4h and k), which could promote bonding between fibers and contribute to an increase in the mechanical properties of the CM-CKFs. However, the structure of the knitted fabric was retained (Fig. 4g and j). Thus the diameter of carboxymethyl fibers increased with the order of water, ethanol–water and isopropanol– water. The degree of carboxymethylation was the main reason for these dramatic morphological changes in carboxymethyl fibers, which were determined by the swelling and infiltration ability of the reagent to the reaction centers. When water was used as a solvent, the reagent reaction was mostly limited to amorphous regions. Under the same amount of reagent, however, the alcohol solvent has less polarity lead to high concentration of Na+ and OH− ions in aqueous which increase the penetration ability of reagent to the crystalline region. Thus the efficiency of carboxymethylation in cellulose fibers was enhanced. The swelling degree of fiber increased with increases in the order of water, ethanol–water and isopropanol–water and DS.
3.4. Hygroscopicity Hygroscopicity is the most important property of CMC fibers and fabrics used in the preparation of absorptive materials such as medical dressings. The hygroscopicity of the CM-CKFs was determined by DS. The hygroscopicity of the CKFs and CM-CKFs treated in the seven different types of reaction solvent is shown in Table 2. Table 2 shows that water absorbency and water retention of the CM-CKFs treated in ethanol–water and isopropanol–water were all higher than those of the CM-CKFs treated in water. Water absorbency and water retention of the CM-CKFs increased with the increase of DS. The water absorbency and water retention of the CM-CKFs treated in water were 1.86 g/g and 1.01 g/g, respectively. Isopropanol–water (v/v = 3/1) was the most efficient of the seven reaction solvents. The water absorbency and water retention of the CM-CKFs treated in isopropanol–water (v/v = 3/1) reached 14.86 g/g and 10.07 g/g, respectively. These results suggest that the mixture of water and isopropanol solution was an ideal solvent for the preparation of CM-CKFs compared with the other two types of reaction solvent. The hygroscopicity of CM-CKFs prepared in water–isopropanol (v/v = 3/1) was compared with previous works and the commercial product of CMC based wound dressing. As shown in Table 3, There are other three types of pure CMC wound dressings, including carboxymethyl viscose nonwoven (CM-VN) [19], CMC nonwovens [29] and AQUACEL® (ConvaTec Ltd) [32]. The CM-VN prepared in our lab [19] has water absorbency and water retention of 25.44 g · g−1 and 9.49 g · g−1, respectively. In Doh et al. research [29], the CMC nonwovens produced with the CMC fibers by a wet-laid nonwoven process has water absorbency of 8.3 g · g−1. The AQUACEL® (ConvaTec Ltd) [32,33] is the most popular CMC dressing used in clinical practice which has the water absorbency between 20 and 30 g · g−1. There are big differences between these four types of CMC wound dressing due to the properties of CMC wound dressing depend on not only the DS but also the source of raw material, preparation procedure, test method of hygroscopicity, and density and structure of the final CMC textile. Compare to knitted fabrics used in this work, the nonwoven possesses more fluffy structure and lower density. After absorbing water, the CMC fiber needs more space to swell and store water which can be provided by the nonwoven. And the nonwoven also has bigger space to store the water between fibers. So even the CK-CKFs have higher DS, its water absorbency is lower than that of the AQUACEL® and CM-VN. However, the water retention of CM-CKFs is higher than that of the CM-VN, because after centrifugation, the water stored between the fibers was detached, and the water between the fibers in CM-CKFs is less than that in the CM-VN. So the CM-CKFs could be used in the squeezed parts of body, like buttock, leg and foot, and it will have better effect than CM-VN for absorbing and retain more exudates. The differences between AQUACEL® and CM-VN are the source of raw material and preparation procedure. AQUACEL®
Table 3 Hygroscopicity of different 100% CMC wound dressing.
DS Water absorbency/g · g−1 Water retention /g · g−1 “–” the data not been tested or mentioned.
CM-CKFs
CF-CN [19]
CMC nonwovens [29]
Convatec Aquacel® [32,33]
0.65 14.86 ± 0.87 10.07 ± 0.72
0.35 25.44 ± 1.50 9.49 ± 1.12
0.88 8.3 –
b0.4 20–30 –
J. Zhao et al. / Materials and Design 87 (2015) 238–244
243
Table 4 Mechanical properties of CKFs and CM-CKFs. Blank
DS Breaking force/N Breaking elongation/%
Water
0 362.7 ± 5.3 181.0 ± 3.2
Ethanol–water [v/v]
0.13 187.2 ± 3.7 343.8 ± 4.4
2:1
3:1
1:1
2:1
3:1
0.25 193.5 ± 5.9 149.7 ± 3.6
0.39 204.6 ± 4.1 168.2 ± 2.6
0.50 212.7 ± 1.8 183.4 ± 1.5
0.32 199.8 ± 3.0 162.2 ± 2.3
0.56 218.7 ± 3.2 190.9 ± 4.0
0.65 220.3 ± 2.9 195.0 ± 3.1
was prepared with the high cost regenerated cellulose fiber of Tencel and Lyocell by needled punched nonwoven method. But in our work [19], the viscose nonwoven which is cheap and easy to get was directly modified in aqueous medium through one bath process to produce the CM-VN. Meanwhile, the result of our CM-VN is almost the same as that in AQUACEL® with the same nonwoven density of 120 g/m2 [32,33]. For CMC nonwovens [29], the low water absorbency attributed to different test method which specimens were immersed in distilled water for 5 min during the water absorption test, but there are more than 1 h immersing time in other researches. They also use cheaper cotton fibers and traditional wet-laid nonwoven process which can decrease the cost of wound dressing as well. So researchers around the world made good efforts for the preparation of low-cost and high-performance medical dressings.
3.5. Mechanical properties Table 4 shows the mechanical properties of the CKFs and CM-CKFs. It confirms that the breaking force of the CM-CKFs clearly decreased after carboxymethylation, and that the CM-CKFs treated in water had the lowest breaking force and the highest breaking elongation among the samples. Because of the lower ion concentration in water and the heterogeneous reactions, the carboxymethylation occurred mostly in the amorphous regions of cellulose, causing a non-uniform degree of reaction within each of the knitted fabrics. The concentration stresses in the CM-CKFs caused by the non-uniform degree of reaction led to a decrease of breaking force. The significant increase of breaking elongation is thought to be the effect of intermolecular breakage of the hydrogen bond. However, the CM-CKFs treated in mixed solutions had higher breaking force and lower breaking elongation than the CM-CKFs treated in water. Both breaking force and breaking elongation increased with the increase of DS and the ratio of alcohol solution. It is known that carboxylation of cellulose fibers increases fiber–fiber bonding strength. The increase in fiber–fiber bonds seem to be a more likely cause for the increase in breaking force [34,35]. Another reason could be the mixture of alcohol and water, which was a more efficient reaction medium with higher ion concentration around the fiber than that in water, improved the uniformity of reaction in both crystalline and amorphous areas. Thus in the CM-CKFs treated with a mixed solution, there were few stress concentration, leading to a higher breaking force than that of CM-CKFs treated in water. With the increase of DS the fiber diameter increased, and length of the fiber elongation shrank, which strengthened the fibers. Moreover, the fibers in the gel form in mixed solutions adhered with each other, as shown in Fig. 4, which could also increase the mechanical properties of the fabrics, even the inter- and intramolecular hydrogen bonds in the fibers were broken.
Table 5 Permeability of CKFs and CM-CKFs.
3.6. Water vapor permeability It should be noted that the loss of water vapor from intact skin is 0.24–1.92 kg/(24 h · m2) and the water vapor loss from an open wound is about 4.8 kg/(24 h · m2) [36]. A wound dressing not only provides a moist environment but also transmits oxygen into and carbon dioxide and water vapor out of the dressing in case there is a risk of wound infection in the exudates. Therefore, the water vapor permeability of the CKFs and CM-CKFs was tested according to ASTM E96-05. The results in Table 5 show that the permeability of the CM-CKFs was lower than that of the CKFs. The permeability of the CM-CKFs decreased with the increase in DS. The spaces between fibers in the textile form the main channels for the evaporating water. The more and larger the spaces are between fibers, the better the permeability of the fabric has. During carboxymethylation, the fiber diameter in the fabrics increased in the alkaline condition, resulting in a decrease in the interfiber distance and the quantity of channels. The fiber diameter increased with an increase in alcohol ratio and the DS. Moreover, the fabrics became gel during the absorption process, preventing water from moving in and out of the dressing. Thus the permeability of the CM-CKFs decreased with increases in solvent alcohol ratio and DS. 3.7. Water diffusibility The gel-forming ability of CMC-based wound dressing is determined by the DS of carboxymethylation. Once formed, a carboxymethyl cotton gel consists of part solid and part solution, water, and other molecules such as exudate and bacteria are physically trapped within the CMC matrix by capillary forces. Therefore, low water diffusibility means good barrier properties. As an important factor in wound dressing, the water diffusibility of the CKFs and CM-CKFs was investigated and the results are shown in Table 6, demonstrating that the CKFs had higher water diffusibility than all the CM-CKFs. With increases in DS and alcohol ratio of the solvent, the water diffusibility of the CM-CKFs clearly decreases. The CMC hydrogel formed during the absorption process can effectively impede the diffusion of water. Then, during the healing process, bacteria could not migrate with exudate to infect normal tissues. The CM-CKFs treated in isopropanol–water solution had the best barrier property of the solvents tested. 4. Conclusions In this study, CM-CKFs were prepared with different solvents: water, ethanol–water and isopropanol–water. The results showed an obvious effect of the solvents on the carboxymethylation reaction. Isopropanol and ethanol promoted the destruction of cellulose I crystalline form and the formation of cellulose II structure, increasing the availability of
Table 6 Water diffusibility of CKFs and CM-CKFs.
Blank Water Ethanol–water [v/v] 1:1 DS 0 Permeability kg/(24 h · m2) 0.48
Isopropanol–water [v/v]
1:1
0.13 0.42
2:1
3:1
Isopropanol–water [v/v] 1:1
0.25 0.39 0.50 0.32 0.35 0.30 0.21 0.32
2:1
3:1
0.56 0.17
0.65 0.12
DS Diffusion distances/cm
Blank
Water
Ethanol–water [v/v]
Isopropanol–water [v/v]
1:1
2:1
3:1
1:1
2:1
3:1
0 5.93
0.13 5.23
0.25 4.47
0.39 3.14
0.50 2.08
0.32 4.06
0.56 1.69
0.65 1.05
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