1. Introduction
Attributed to the three-dimensional polymeric networks and remarkable capacity to absorb water, hydrogels have attracted significant attention and have been applied in medical science, technology, and soft material fields [
1,
2,
3,
4]. One of the most widely used hydrogels, thermoresponsive hydrogel fabricated from
N-isopropylacrylamide (NIPAAm) and nanosized synthetic hectorite clay, are treated as material candidates for a soft robot, soft actuator, and 4D printing [
5,
6,
7,
8,
9]. Based on a temperature-responsive hydrogel, the photoresponsive hydrogel is fabricated by the addition of gold nanorods, carbon nanotubes, graphene oxide [
10,
11,
12], and so on to convert light into heat, which resulted in the size and shape change of hydrogel. Compared with temperature-responsive hydrogel, the photoresponsive one exhibits the characteristic of non-contact control, avoiding the restriction of environmental temperature. The photo-driving action can be actualized via the way of energy transformation based on light-absorbing nanoparticles embedded in a thermosensitive hydrogel. In virtue of practicality, convenience, and better-suited to biological applications, [
13] the near-infrared (NIR) light is treated as the manipulator of photoresponsive hydrogel. Recently, due to the high photothermal energy transformation and absorbance of NIR energy, [
14,
15] graphene oxide (GO) has been wildly utilized in photoresponse hydrogels.
Mechanical strength is a basic characteristic for the application of hydrogels. In order to improve the mechanical properties, as a kind of filler, nanofibrillated cellulose (NFC) has been added into the hydrogel matrix to provide a kind of hydrogel material with high strength for biomimetic 4D printing [
9]. As a kind of method for increasing mechanical strength, nanofiber reinforced composite material provided another way to increase the mechanical strength of hydrogels. With the development of a nanofiber fabrication method, electrospinning is regarded as a simple and versatile method for generating nanofibers with a unique advantage of high surface area, which has been treated as a best candidate in many different practical applications [
16,
17,
18]. As a result of a relatively high mechanical strength and anti-impact properties, polyvinyl butyral is treated as a kind of electrospinning material [
19]. Therefore, in the view of material composition and material structure, we try to add the nanofibrillated cellulose and the nanofibers obtained from electrospinning into the hydrogel with temperature and near infrared responses to improve the corresponding mechanical strength.
Structure modality is the key point for a hydrogel material to achieve bending/unbending ability. Wang and co-workers [
20] synthesized near-infrared light-driven hydrogels actuators by interfacing genetically engineered elastin-like polypeptides with reduced-graphene oxide sheets, which exhibited rapid and tunable motions controlled by light position, intensity, and path, including finger-like flexing and crawling. The common ground of the responsive bending of hydrogels are the anisotropic responses of layered structure. The model of two layers with different temperature or near infrared absorbing capacities is the simplest layered structure. In order to achieve the smart bending properties, some kind of bilayer structure consisting of a temperature and photoresponse hydrogel layer and a nonresponsive layer are fabricated via in-situ, free radical polymerization. Jiang and co-workers [
21] designed and fabricated a photoresponsive biomimetic microfish, which can move forward, backward, and turn around in water under near infrared irradiation. Yao and co-workers [
5] fabricated a double-layered temperature-controlled hydrogel with relative high responsive bending and elastic properties via a two-step photopolymerization. This kind of hydrogel can be treated as a temperature-controlled manipulator for various applications including encapsulation, capture, and transportation of targeted objects. Even though the temperature and photoresponse behaviors of hydrogels with layered structures were investigated [
5,
6], how to combine electrospinning and realizing the simple fabrication method of one-step, in-situ, free radical polymerization has not been investigated.
In this paper, by regulating the nanofibrillated cellulose content, we fabricated a series of polyvinyl butyral-containing bilayer hydrogel system with temperature and near infrared responses via a one-step, in-situ, free radical polymerization and electrospinning. Variation of nanofibrillated cellulose content affected cross-link density of hydrogels, which built the fabrication base for the one-step, in-situ, free radical polymerization. The incorporation of nanofibrillated cellulose and polyvinyl butyral nanofibers enhanced mechanical strength and maintained the bending/unbending properties under the stimulation of temperature and a near infrared laser. The bilayer hydrogel with different strength and bending degree can be treated as a kind of smart material for soft actuators and robots driven by temperature and near infrared laser.
2. Experimental Section
2.1. Material
Monomer N-isopropylacrylamide (NIPAAm, C6H11NO, Aladdin, Shanghai, China, 2% stabilizer) was recrystallized from toluene/n-hexane mixture and dried in vacuum at room temperature for 48 h. Nanosized synthetic hectorite clay, Laponite XLG, Mg5.34Li0.66Si8O20(OH)4 was purchased from Rockwood, Ltd., (Moosburg, Germany) and used after drying at 125 °C for 2 h. Initiator potassium peroxydisulfate (KPS, K2S2O8, Shanghai Aibi Chemical Reagent Co., Ltd., Shanghai, China, Analytical reagent AR), catalyst N,N,N′,N′-tetramethylethylenediamine (TEMED, Tianjin Weiyi Chemical Technology Co., Ltd., Tianjin, China, 98%), graphene oxide (GO, Suzhou Hengqiu Graphene Technology Co., Ltd., Suzhou, China, 95%), Nanofibrillated cellulose (NFC, Guilin Qihong Technology Co., Ltd., Guilin, China, 1342 nm) and methyl blue (Analytical reagent AR, Shanghai Aibi Chemical Reagent Co., Ltd., Shanghai, China) were used as received. The polyvinyl butyral (PVB, aircraft-grade) and ethyl alcohol (Analytical reagent AR) used for electrospinning were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pure water was obtained by deionization and filtration with a Millipore purification apparatus (resistivity ≥ 18.2 MΩ·cm).
2.2. Synthesis of PNIPAm–NFC Hydrogel
Before the synthesis of hydrogels, PVB nanofibers were fabricated via electrospinning (Model SS-2535H, Beijing Uclalery Technology Development Co., Ltd., Beijing, China). The homogeneous and stable PVB solution with concentration of 7% and weight of 20 g was prepared at room temperature. In order to ensure the oriented characteristics of PVB nanofibers, model of conjugate spinning was employed. The positive high voltage of 10.8 kV and negative high voltage of −10.8 kV were applied to the right and left syringe needles of the electrospinning machine, respectively. The distance between the tip of the needle and the grounded collector was fixed at 10 cm.
Figure 1 shows a typical scanning electron microscopy (SEM) image of electrospun PVB nanofibers. The nanofibers exhibited the oriented characteristics on the substrate, forming an interwoven network structure.
The PNIPAm–NFC hydrogel was synthesized via in-situ, free radical polymerization of NIPAm in the nanosized clay suspension with GO and NFC. Before the fabrication of PNIPAm–NFC hydrogel, the pure water was degassed in the continuous nitrogen-saturated atmosphere for 2 h. Then 0.5 mL of methyl blue solution with a concentration of 40 mg/mL was stirred for 0.5 h in 19 mL pure water for the fabrication of the hydrogel without GO and NFC. The graphene oxide was first dispersed in 19.5 mL of pure water by ultrasonic radiation for 30 min and stirred for 1 h via a magnetic stirrer (Model DF-101S, Changchun Jiyu Technology Equipment Co., Ltd., Changchun, China). The XLG clay was added into GO suspension, which was stirred for 1 h and ultrasonically radiated for 30 min. Then NFC, with a corresponding weight, was added and stirred for 1 h in an ice-water bath. The monomer NIPAm was added into miscible liquids of GO, XLG, and NFC under a nitrogen-saturated atmosphere in an ice-water bath for another 2 h. Finally, 0.5 mL of KPS solution with a concentration of 40 mg/mL and 27 μL of the catalyst TEMED was added under stirring. The solution was rapidly injected into a laboratory-made rubber mold of 70 mm × 20 mm × 2 mm (Length × Width × Thickness). The polymerization was conducted at 25 °C for 24 h to produce the NC hydrogel. The mole ratio of NIPAm monomer, initiator, and catalyst in all suspensions was kept at 100:0.370:0.638. Under the photothermal energy transformation property of GO, in order to investigate the effect of NFC content on the microstructure, mechanical properties and temperature, and near infrared response of the hydrogel, 0, 1, 2 and 3 mg/mL NFC was added in the hydrogel, respectively. In this paper, the PNIPAm–NFC hydrogels were defined as NFC0, NFC1–PVB, NFC2–PVB, and NFC3–PVB, where 0, 1, 2, and 3 represented the concentration of NFC. The compositions of the hydrogels are listed in
Table 1. In order to investigate the temperature and near infrared responses of the bilayer hydrogel system, the corresponding layered structure was fabricated, as shown in
Figure 2. In the bilayer structure, due to the existence of GO, the layer with NFC consisted of two NFC hydrogel layers and a PVB nanofiber layer, which presented black. Attributed to the existence of methyl blue, the layer without NFC consisted of two NFC0 hydrogel layers and a PVB nanofiber layer, which exhibited blue. The NFC contents were 1, 2 and 3 mg/mL, respectively. Due to the variation of NFC, the two layers of the bilayer hydrogel system exhibited different densities, which provided a method for fabrication of the bilayer hydrogel system efficiency via density difference in a one-step, in-situ, free radical polymerization. The layer with NFC was injected first in the laboratory-made rubber mold. The PVB nanofiber layer was paved in the middle of the NFC hydrogel layer. After the superimposition of NFC0 hydrogel layers and another PVB nanofiber layer, the laboratory-made mold was sealed tightly. The polymerization of the bilayer hydrogel system was also carried out at 25 °C for 24 h.
2.3. Characteristics
2.3.1. Microstructure
In order to observe the internal microstructure via a scanning electron microscope (SEM) (Model Evo18 Carl Zeiss, Oberkochen, Germany) and an environmental scanning electron microscope (ESEM-FEG) (Model XL-30, FEI Company, Hillsboro, OR, USA), the corresponding samples with dimensions of 5 mm × 5 mm × 2 mm (Length × Width × Thickness) were placed in a freeze drying oven (LGJ-10C, Beijing Four Ring Scientific Instrument Factory Co., Ltd., Beijing, China) to remove water thoroughly. After gold sputtering, the cross section of samples was observed with magnifications of 200×, 5000×, and 20,000×, respectively.
2.3.2. Infrared Spectrum and Differential Scanning Calorimetry (DSC) Analysis
Fourier transform infrared (FT-IR) spectra of hydrogels were analyzed via IRAffinity-1 FT-IR spectrometer (Shimadzu Corporation, Kyoto, Japan). The range of wavenumber was 500–4000 cm−1. Before measurement, the samples were ground into powder and pressed with KBr into a disc.
The volume phase transition temperature (VPTT) of the hydrogel sample was measured via differential scanning calorimetry (DSC) (Model Q2000, TA Company, Boston, PA, USA). The samples were heated from 20 to 50 °C, and then cooled from 50 to 20 °C at the rate of 10 °C/min under the nitrogen atmosphere. The VPTT of the hydrogel was determined at the onset of the endotherm peak during second heating.
2.3.3. Tensile Stress-Strain Analysis
To obtain the tensile stress-strain characteristic of hydrogels, a universal testing machine (Model C43, MTS Criterion, Eden Prairie, MN, USA) with the constant loading rate of 100 mm/min was employed to test the tensile properties. The sample size was 60 mm × 6 mm × 2 mm (Length × Width × Thickness). Average values of stress and strain were calculated from three individual measurements.
2.3.4. Temperature and Near Infrared Responses
In order to disclose the effect of NFC content and sample size on dynamic thermoresponsive bending behaviors, the completely swollen bilayer hydrogels with different NFC contents were cut into strips of 40 mm × 8 mm × 4 mm and 70 mm × 5 mm × 4 mm (Length × Width × Thickness), respectively. The bilayer hydrogels were placed in water of poikilothermy temperature ranging from 30 to 50 °C. The whole dynamic thermoresponsive bending process was recorded by a digital camera. The near infrared response of the hydrogel system was conducted via a near infrared illuminant with a wavelength of 808 nm and power of 3 W (Model FC-W-808-30W, Changchun New Industries Optoelectronics Technology Co., Ltd., Changchun, China). The distance between the light source and sample was 50 cm. The energy density delivered to the sample was 3.82 W/cm2. The deformation and recovery process of the hydrogel system were conducted in pure water at 25 °C. A digital camera was used to record the whole dynamic infrared driving bending process.
4. Conclusions
By regulating the NFC content, a novel kind of PVB nanofiber-containing bilayer hydrogel system with excellent temperature and near infrared responses was successively fabricated via the combination of a one-step, in-situ, free radical polymerization and electrospinning. The PVB nanofibers showed high combination with the hydrogel base with a typical honeycomb-like structure. The size of the microstructure became smaller with the increase of the NFC contents, indicating the increase in cross-linking density. The addition and variation of the NFC and PVB nanofibers presented inessential influence on the change of the chemical bond and volume phase transition temperature. The combination between the NFC and PVB nanofibers enhanced the mechanical strength and decreased the strain value, which built the base for high bonding strength between the two layers and an efficient thermoresponsive and near infrared responsive bending/unbending. With the increase of the NFC content, the bending degree became smaller. Sample dimensions affected the deformation degree. The longer the dimension in length and smaller the dimension in width, the higher bending degree the bilayer hydrogel exhibited. Different NFC-containing bilayer hydrogels own different deformation abilities, which can be designed as different parts of soft actuators and provide superior performance to satisfy various practical application demands.