Introduction

Recently, research of renewable resources has been listed as one of the 24 international frontiers due to global energy and sustainable development issues, and the utilization of natural polymers related materials has attracted much attention (Murugan and Ramakrishna, 2004; Saito et al. 2014; Anitha et al. 2014). Cellulose is the most abundant biopolymer in nature and can be dissolved and converted into regenerated functional materials with various shapes such as films, hydrogels, microspheres and fibers etc. (Cai et al. 2007; He et al. 2014a; Duan et al. 2014; Zhao et al. 2016; Wendler et al. 2009; Jedvert and Heinze 2017). Due to the intransigent nature of cellulose, materials made from it usually exhibit good mechanical properties, supporting their applications in various fields such as packaging and clothing manufacture (Mokhothu and John 2015). However, cellulose has poor bioactivity and no antimicrobial activity, which restricts its applications as biomaterials and antibacterial packaging. To solve this problem and make full use of cellulose, cellulose has usually been blended or modified with other polymers such as chitosan (Cao et al. 2016), collagen hydrolysate (Pei et al. 2013) and soy protein isolate (SPI) (Li et al. 2016a). Among these biopolymers, chitosan is the deacetylation product of chitin, the second most abundant biopolymer widely available from seafood processing waste (Chen et al. 2017; Rejinold et al. 2011). Chitosan exhibits good biocompatibility, hemostasis, antimicrobial activity and adsorptive property, which has been widely used in the biomedical, cosmetic and water treatment fields (Yang et al. 2016; He et al. 2017b; Yan et al. 2017). The poor mechanical property of chitosan greatly restricts its application, which is urgently needed to be improved to broaden the range of application. Thus, a worthwhile endeavor would be to blend chitosan with cellulose to realize the complement of each other’s advantages. The reported common solvents for cellulose and chitosan were N-methylmorpholine-N-oxide (NMMO), ionic liquids and ethylene diamine/potassium thiocyanate (Isogai and Atalla 1992; Hasegawa et al. 1994; Fink et al. 2001; Chaoming et al. 2009; Zhou et al. 2015), obviously, these solvents are usually expensive and hard to remove after the composite materials fabrication. Although cellulose and chitosan could be dissolved in NaOH/thiourea after chain depolymerization (Morgado et al. 2011), the exploiting of novel green solvents for directly dissolving cellulose and chitosan is still a challenge. Luckily, novel aqueous solvents such as LiOH/urea and KOH/LiOH/urea solutions were developed for cellulose and chitosan dissolution without depolymerization, respectively (Duan et al. 2015; Cai and Zhang 2005), and the regenerated cellulose and chitosan based materials were fabricated accordingly, which exhibited fascinating properties such as high strength, good biocompatibility and stimuli responsive (He et al. 2014b; Duan et al. 2016). However, as far as we know, bulk homogeneous cellulose/chitosan composite materials such as hydrogels or films from these novel alkali/urea aqueous solutions have never been reported.

In the present work, both chitosan and cellulose were dissolved in alkali/urea aqueous solutions, and then blended together in ice bath with different ratios to prepare homogeneous mixed solutions, which were coagulated in an ethyl acetate gaseous phase to prepare chitosan/cellulose composite hydrogels (CCG). The composite hydrogels were air-dried to obtain the corresponding films (CCF). The structure and properties of CCG and CCF were characterized, the results indicated that there is strong hydrogen bonding interaction between cellulose and chitosan. CCG exhibited homogeneous structure, indicating good miscibility. The mechanical strength of CCF increased with the cellulose content, while the equilibrium swelling ratio of CCG and antibacterial activity of CCF increased with the chitosan content. Therefore, CCG and CCF showed potential applications in the facial mask, antimicrobial packaging and water treatment fields.

Experimental section

Materials

Chitosan (viscosity, 100–200 mPa s) with deacetylation degree of above 95% was purchased from Aladdin Reagent Company. The cellulose sample (cotton linter pulp) was supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China). Its weight-average molecular weight (Mw) was determined by static laser light scattering (DAWN DSP, Wyatt Technology Co., US) to be 10.9 × 104 (Cai et al. 2006). The NaOH, KOH, LiOH·H2O, ethyl acetate, ethanol and urea (Shanghai Chemical Reagent Co. Ltd., China) were used as received. All the chemical reagents were of analytical grade and used without further purification.

Preparation process of CCG and CCF

As we reported in our previous work (He et al. 2014a), cotton linter pulp was completely dissolved in the pre-cooled 8 wt% LiOH·H2O/15 wt% urea aqueous solutions at − 12.8 °C to obtain 4 wt% cellulose solutions. Chitosan powder was dispersed into 7 wt% KOH/7 wt% LiOH·H2O/8 wt% urea aqueous solutions with stirring for 15 min and treated for 5 min through ultrasonic treatment (ultrasonic instrument KQ5200E). The resultant suspensions were completely frozen in a refrigerator at the temperature below − 30 °C, and then thawed at room temperature to get 4 wt% chitosan solutions. The cellulose and chitosan solutions were blended together in an ice bath under mechanical stirring with different weight ratios of 9:1, 7:3, 5:5, 3:7 and 1:9. The blended cellulose/chitosan solutions were centrifuged at 7000 rpm for 10 min at 0 °C to remove the bubbles. The resultant cellulose/chitosan solutions were cast on a glass plate with the solution layer thickness of about 0.2 mm, which was controlled by glass tubes with two loops wrapped around. They were immediately coagulated for 0.5 h in a closed container filled with an ethyl acetate gaseous phase. The blended solutions could be transformed into hydrogel sheets, which were rinsed thoroughly with running water, and then washed with deionized water to remove the excess residuals. The resultant hydrogel sheets were then fixed on a polymethyl methacrylate (PMMA) plate and dried in air at room temperature to obtain the composite films. The composite hydrogel sheets were coded as CCG-10, CCG-30, CCG-50, CCG-70 and CCG-90, according to the weight percentage of chitosan. The resultant composite films were coded as CCF-10, CCF-30, CCF-50, CCF-70 and CCF-90 accordingly.

Characterization

The CCG hydrogels were frozen in liquid nitrogen, immediately snapped and then freeze-dried for SEM observation (FESEM, Zeiss, SIGMA). The acceleration voltage for the FESEM observation was 5 kV. The cross-section of the freeze-dried hydrogels was sputtered with gold, and then observed. FT-IR spectra were carried out with a FT-IR spectrometer (1600, Perkin–Elmer Co., MA) in the wavelength range from 4000 to 400 cm−1. The powdered and vacuum-dried CCF was obtained, and the test specimens were prepared by using a KBr disk method. Wide angle X-ray diffraction measurements (WAXD) were carried out with a WAXD diffractometer (D8-Advance, Bruker, USA). The patterns with Cu Kα radiation (λ = 0.15405 nm) at 40 kV and 30 mA were recorded in the region of 2θ from 5 to 40°, scanning rate was 2°/min. The samples were cut into powder and dried in a vacuum oven for 48 h before testing.

The tensile strength (σb) and elongation at break (εb) of the CCF films in the dry state were measured on a universal tensile tester (CMT 6503, Shenzhen SANS Testing machine Co. Ltd., Shenzhen, China) according to ISO527-3-1995 (E) at a speed of 2 mm/min. The light transmittance of the CCF films was analyzed with a UV–Vis spectrophotometer over a wavelength range of 300–900 nm. The transmittance spectra were acquired using air as the background.

The heavy metal ion adsorbability of CCG-70 was also preliminarily studied, 3 cm × 6 cm CCG-70 hydrogel strip was immersed into a 50 mL 0.3 mol/L CuCl2 aqueous solution, and then soaked for 0.5 h, the adsorption process was photographed. The adsorption performance of CCG-70 was calculated by a gravimetric method accordingly.

Antibacterial test and L929 cell viability assay

The antibacterial test was performed similar to our previous work (Li et al. 2015). Briefly, culture medium, Petri dishes and others were sterilized in high-pressure steam for 20 min. Specimen, spirit lamp and test tube rack were sterilized for 30 min by UV in a clean bench. The culture medium was dumped into a Petri dish and UV sterilized for 30 min during solidification. Staphylococcus aureus or Echerichia coli colonies were scraped with inoculating loop, and then transferred to a centrifuge tube with 3 mL normal saline solution, which was vortexed to disperse bacteria evenly. 60 μL broth was added to the medium and then scratched. To test the anti-bacterial activities of the CCF films, implanted S. aureus and E. coli strains were placed in a Petri dish, thin disks of the CCF-10 and CCF-90 films with the diameter of 1 cm were stuck on the Petri dish to culture for 48 h at 60 °C in an incubator. The inhibition zones were then photographed.

To conduct cell viability assay, CCF films were cut into powder, sterilized by autoclaving, and then used to prepare the extract. According to ISO 10993-5, a cell line of L929 was re-suspended in the culture medium and plated (200 μL/well) into 96-well micrometer plates at 1 × 104 cells/well, which were incubated at 37 °C in a 5% CO2 atmosphere for 24 h. The medium was then replaced by 50 μL/well sterilized extract of CCF films, using the culture medium itself as a control. The cells were treated with 5 mg/mL MTT in PBS (20 μL/well) after incubating for 24, 48 and 72 h. A final concentration of 0.5 mg/mL MTT was obtained, and incubated for a further 4 h. At this stage, MTT was removed out and 150 μL dimethyl sulfoxide (DMSO) was added each well to dissolve the formazan crystals. The plates were placed in an incubator at 37 °C and shaken for 15 min. The absorbance values were measured in triplicate against a reagent blank at a test wavelength of 570 nm (Tecan GENios, Tecan Austria GmbH, Salzburg, Austria). Cell viability was calculated using Eq. 1:

$${\text{Cell}}\;{\text{viability}}\;\left( \% \right) = \left( {{\text{A}}_{\text{test}} \div {\text{ A}}_{\text{control}} } \right) \times 100\%$$
(1)

where Atest and Acontrol corresponded to the absorbance values of the test and control groups, respectively.

Results and discussion

Appearance and structure of CCG and CCF

All the CCG-n hydrogels could be fabricated facilely, no obvious phase separation occurred during their preparation process. In our findings, an ethyl acetate gaseous phase could coagulate the cellulose/chitosan blend solutions homogeneously and slowly by continually providing acetic acid and ethanol as direct coagulants through hydrolysis in the blend solutions, which was much milder for the macromolecular aggregation than common aqueous coagulants such as an acetic acid or H2SO4 solution. Figure 1 shows the photographs of the CCG-10, CCG-30, CCG-50, CCG-70 and CCG-90 hydrogels. Obviously, all the CCG hydrogels were homogeneous, transparent and flexible, indicating good miscibility between chitosan and cellulose due to their structure similarity and the mild coagulation process. Moreover, the CCG hydrogels exhibited good skin affinity due to high hydrophilicity of cellulose and chitosan.

Fig. 1
figure 1

Photographs of the CCG-n hydrogels (n = 10, 30, 50, 70 and 90) wrapped on one finger

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The micro structure of the composites could reflect the miscibility between the components (Li et al. 2016b), which could be characterized by SEM. Figure 2 shows the SEM images of the freeze-dried CCG-n hydrogels (n = 10, 30, 50, 70 and 90). It was noted that all the CCG hydrogels exhibited homogeneous pores, further indicating the good miscibility between chitosan and cellulose. The pore size of CCG-10 was 0.44 μm, which increased with the chitosan content and reached 0.72 μm for CCG-90. The increase of the pore size for the CCG hydrogels with the increase of chitosan content was possibly due to the electrostatic repulsion of chitosan molecules, which was caused by a protonation of –NH2 groups on chitosan chains with acetic acid from ethyl acetate (Anitha et al. 2014; Duan et al. 2015). Figure 2f shows the swelling ratio of the CCG hydrogels. The swelling ratio of all the CCG hydrogels was above 12.5 g/g, indicating high hydrophilicity. Interestingly, the swelling ratio increased with the chitosan content, which was mainly caused by micro structure change through the electrostatic repulsion of chitosan molecules (Fig. 2a–e) (Qin et al. 2010; Duan et al. 2015).

Fig. 2
figure 2

SEM images of the cross section of freeze-dried CCG-n hydrogels (a–e correspond to CCG-10-90 hydrogels, respectively), and the swelling ratios of the CCG-n hydrogels (f, n = 10, 30, 50, 70 and 90)

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The CCG hydrogels could be transformed into films by changing aggregation states of cellulose and chitosan through the inter and intra macromolecular chain rearrangement during the drying process (Wang et al. 2013; He et al. 2017a). Figure 3 shows the photographs of CCF and the corresponding light transmittance. The light transmittance value for CCF-10 was about 40% at 800 nm, which was lowest in CCF possibly due to the largest optical scattering and reflecting resulted from its relative heterogeneous structure caused by relative strong micro-phase separation (Fang et al. 2017). Other CCF films exhibited higher light transmittance (above 58% at 800 nm), indicating the formation of more homogeneous structures from inter-macromolecular rearrangement. It was noted that the light transmittance value of CCF-70 could reach 75% at 800 nm due to the formation of relative homogeneous structure, showing high light transparency. Therefore, all the CCF films exhibited relative high light transmittance, indicating good miscibility on the whole.

Fig. 3
figure 3

Photographs (a) and light transmittance spectra (b) of the CCF-n films (n = 10, 30, 50, 70 and 90)

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Figure 4 shows the FTIR spectra of the CCF-n films (n = 10, 50 and 90). The band at 3439 cm−1 was assigned to –OH or –NH of cellulose and chitosan in CCF-10, which shifted to 3432 and 3426 cm−1 for CCF-50 and CCF-90 respectively, indicating the formation of new inter-molecular hydrogen bonds between cellulose and chitosan (Xiong et al. 2010). The characteristic bands of amide-I and amide-II for chitosan appeared at 1663 and 1594 cm−1 for CCF-50 and CCF-90 respectively, whose intensity increased with the chitosan content. Those two bands weren’t found in CCF-10 because its chitosan content was quite low. The band at 1418 cm−1 was attributed to the –NH deformation vibration of chitosan. The strong peak at around 1065 cm−1 was attributed to the C–O–C stretching of the pyranose ring (Lima et al. 2005).

Fig. 4
figure 4

The FTIR spectra of the CCF-n films (n = 10, 50 and 90)

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Figure 5 shows the WAXD patterns of the CCF-n films (n = 10, 50 and 90). There were three obvious crystal peaks at 2θ = 12.0°, 20.0° and 21.8° appeared in CCF-10, which were assigned to crystal planes (\(1\bar{1}0\)), (110) and (020) of cellulose II respectively (Isogai et al. 1989; French 2014). The new peak at 2θ of about 29.5° for CCF-50 and CCF-90 was attributed to the (130) crystal plane of chitosan, the peak intensity increased obviously with the chitosan content. Two peaks at 2θ = 20.0° and 21.8° for cellulose merged into one peak at 20.4° for CCF-50 and 20.2° for CCF-90 with the increase of chitosan content, and the peak at 12.0° for CCF-10 changed to 11.5° and 9.89° for CCF-50 and CCF-90 respectively, further indicating the formation of new inter-molecular hydrogen bonds between –OH and –NH2 groups.

Fig. 5
figure 5

The WAXD patterns of the CCF-n films (n = 10, 50 and 90)

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Properties and applications of CCG and CCF

Figure 6 shows the stress–strain (σ–ε) curves of the CCF-n films (n = 10, 30, 50, 70 and 90). The tensile strength values of all the CCF films were above 48 (MPa) and much higher than pure chitosan films reported in the previous work (Rubentheren et al. 2015; Xu et al. 2010; Tang et al. 2008), showing good mechanical strength due to strong hydrogen bonding interaction between cellulose and chitosan. The mechanical strength increased gradually with the cellulose content in the CCF films, indicating that cellulose could be used effectively to strengthen chitosan. The tensile strength value of CCF-10 could reach about 90.0 (MPa), which was even higher than those of pure cellulose films reported in other work (Qi et al. 2009; He et al. 2011, 2013). Interestingly, the elongation at break increased with the chitosan content (≤ 30%) firstly and decreased obviously with further increase of chitosan content (> 30%). The Young’s modulus values for CCF-90, CCF-70, CCF-50, CCF-30 and CCF-10 were 3518.7, 4015.0, 4577.2, 4725.0, and 5035.1 (MPa) respectively, which increased obviously with the cellulose content. Therefore, compared with CCF-90, the mechanical strength, Young’s modulus and elongations at break were improved significantly with the increase of cellulose content in CCF, so cellulose could be used to improve the mechanical property of chitosan.

Fig. 6
figure 6

Stress-Strain (σ–ε) curves of the CCF-n films (n = 10, 30, 50, 70 and 90)

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The antibacterial activity of CCF-10 and CCF-90 was evaluated using E. coli and Staphyoccocus aureus. Figure 7 shows the photographs of antibacterial test for CCF-10 and CCF-90. E. coli bacteria colonies appeared on both CCF-10 and CCF-90, the number of bacteria colonies was much smaller for CCF-90 than that of CCF-10 (Fig. 7a, c), indicating better antibacterial activity of CCF-90 against E. coli. There was one Staphyoccocus aureus bacteria colony appeared on CCF-10, nearly no bacteria colony appeared on CCF-90, indicating better antibacterial activity of CCF-10 and CCF-90 against Staphyoccocus aureus than E. coli (Fig. 7b, d). Thus, the antibacterial activity increased with the chitosan content, so chitosan could be used to improve the antibacterial activity of cellulose. Figure 7e shows the experimental results of the cytotoxicity of the CCF films by MTT assay. The cell viability value for the CCF-10 film could reach 93% at 24 h, which increased obviously with the incubation time and reached 109% at 72 h, indicating no cytotoxicity to the L929 cells. The cell viability value increased obviously with the chitosan content (Fig. 7e) because the presence of –NH2 groups on chitosan is beneficial for cell proliferation (Dash et al. 2011; He et al. 2017b), which reached above 125% for CCF-90, showing high biocompatibility. Thus, chitosan could be used to improve the antibacterial activity and biocompatibility of cellulose (Fig. 7).

Fig. 7
figure 7

Antibacterial test photographs of the CCF-10 (a, b) and CCF-90 (c, d) films, a, c correspond to Echerichia coli and b, d correspond to Staphyoccocus aureus respectively, and the results of the cytotoxicity tests of CCF films (e). Scale bar: 2.5 mm

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Fig. 8
figure 8

Photographs of a glass mold (a) and the corresponding CCG-70 facial mask

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As we all know, chitosan could be used as a facial mask because it exhibits good moisture absorbability and moisture retentivity, heavy metal ion and dye adsorbability, antibacterial activity and nontoxicity (Crini and Badot 2008; Dash et al. 2011). As we mentioned above, the CCG hydrogels exhibited good flexibility, processibility, light transmittance and skin affinity (Fig. 1), so we managed to prepare cellulose/chitosan facial masks. Figure 8 shows the photographs of a glass mold for the fabrication of facial masks and the CCG-70 facial mask. As we expected, the CCG-70 facial mask could be facilely fabricated according to the mold shapes. It was noted that the fabrication process for the CCG hydrogels was mainly physical, so the CCG hydrogels could retain the intrinsic properties of cellulose and chitosan. Moreover, the addition of a little edible vinegar could improve the antibacterial activity of CCG significantly by protonation of the –NH2 groups of chitosan (Anitha et al. 2014), showing wide potential as antibacterial facial masks.

Fig. 9
figure 9

Photographs of Cu2+ adsorption process by CCG-70: before adsorption (a), in adsorption (b) and after adsorption (c)

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As we all know, there are some heavy metal ions left in the cosmetic, which threatens human health. So the adsorbability of heavy metal ions is a critical factor to evaluate the function of facial masks. The preliminary heavy metal ion adsorbability of CCG was evaluated. CCG-70 was soaked into the CuCl2 solution to study the Cu2+ adsorbability. Figure 9 shows the photographs of Cu2+ adsorption process by CCG-70. Obviously, CCG-70 exhibited good Cu2+ adsorbability, and the Cu2+ adsorption amount reached 2.98 mmol/g after 0.5 h adsorption, so the CCG-70 hydrogels could be used as a facial mask to remove the residual heavy metal ions left on the face or as portable water treatment adsorbents.

Conclusions

Novel flexible cellulose and chitosan composite hydrogels and films were successfully constructed by using a facile blending and gaseous coagulation method. The composite hydrogels exhibited homogeneous porous structures and the resultant films exhibited relative high light transmittance (above 40% at 800 nm), indicating good miscibility between cellulose and chitosan due to their structure similarity. The mechanical strength and Young’s modulus for CCF increased obviously with the cellulose content, which could reach about 90.0 and 5035.1 MPa, respectively. The equilibrium swelling ratio, antibacterial activity and biocompatibility increased with the chitosan content. The Cu2+ adsorption amount for CCG-70 reached 2.98 mmol/g after 0.5 h adsorption, CCG-70 facial mask could be facilely fabricated by using glass molds. Thus, the cellulose/chitosan composite materials showed potential applications in the facial mask, antimicrobial packaging and water treatment fields.