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Tough robust dual responsive nanocomposite hydrogel as controlled drug delivery carrier of asprin
Journal of the Mechanical Behavior of Biomedical Materials 92 (2019) 179–187
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Tough robust dual responsive nanocomposite hydrogel as controlled drug delivery carrier of asprin
T
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Yang Chen, Shuai Kang, Junrong Yu , Yan Wang, Jing Zhu, Zuming Hu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Robust Biocompatible Hydrogel Carboxymethyl chitosan Drug delivery carriers
Smart mechanical strong hydrogels have gained increasing attention in the last decade. A novel tough robust biocompatible and dual pH- and temperature- responsive poly (N-isopropylacrylamide)/clay (Laponite XLS)/ gold nanoparticles (Au-S-S NPs)/caboxymethyl chitosan (CMCTs) nanocomposite hydrogel was synthesized by a facile one-pot in situ free radical polymerization, using clay and Au-S-S NPs as the cross-linkers instead of toxic organic molecules. By tuning the crucial factors, concentration of Au-S-S NPs, CMCTs and clay, the obtained hydrogels exhibited the highest tensile stress of 535.5 kPa at the breaking deformation of 1579.5%. Furthermore, these synthesized hydrogels were tough enough and simultaneously owned a fast recoverability after unloaded in 15 min at room temperature. Moreover, effects of the above factors on swelling and swelling-shrinking behaviors of the prepared hydrogels were investigated in detail. In addition, these designed hydrogels also possessed a controlled drug release property of asprin by adjusting their inner crosslink density. Owing to this property, they could be used as the potential drug delivery carriers in future.
1. Introduction Polymer hydrogels are cross-linked 3D hydrophilic networks comprising a great deal of water (Ye et al., 2016), which have drawn considerable attention in recent years in various fields involving food production, tissue engineering, medicine, soft devices, and so on. Smart hydrogels can evidently alter their volume or other properties in response to a variety of external stimuli, such as light (Li et al., 2017), temperature (Chen et al., 2015a), pH (Gao et al., 2015a), and magnetic/ electric fields (Yang et al., 2015; Dai and Nelson, 2010). Thanks to the remarkable stimuli-responsive properties, smart hydrogels have been widely used in numerous applications such as intelligent sensors (Shi et al., 2015), tissue engineering scaffolds (Fan et al., 2016; Zhang et al., 2016), artificial cartilage (Hu et al., 2016), chemical valves actuators (Beebe et al., 2000), and drug delivery carriers (Chen et al., 2018), for most of which strong mechanical properties are needed. However, most of the conventional smart hydrogels generally have two critical defects: mechanical weak and brittle, and response to single stimuli, which seriously limits their high-end applications. Therefore, how to synthesize the dual/multi-stimuli hydrogels, with tough mechanical strong property, becomes one of the research hot-points in the last decade. To address the first defect, plenty of strategies are proposed for preparing tough mechanical strong hydrogels, such as hydrophobically ⁎
associated hydrogels (Insu et al., 2016), double network (DN) hydrogels (Chen et al., 2015), dual-crosslinked (DC) hydrogels (Cui et al., 2015), and nanocomposite (NC) hydrogels (Su and Chen, 2018). It is well known that the preparation of NC hydrogels is a simple efficient way to enhance the mechanical strength of the hydrogels, with the addition of reinforced nanofillers, such as carbon nanotubes, nanofibers, graphene oxide (GO) and clay nanosheets. Wang et al. (2012) have synthesized the poly(N-isopropylacrylamide) (PNIPAm)/Laponite XLS hydrogels, which have the highest breaking tensile strength about 380 kPa at corresponding failure strain of 960%. Hu et al. (2016) have fabricated the robust DC poly(acrylamide-co-acrylic acid) (PAm-co-Ac) NC hydrogels, using clay nanosheets and Fe3+ ions as the cross-linkers, which possess the highest tensile strength of 3.5 MPa at the breaking elongation of 2100%. Cui et al. (2015) have prepared the poly(acrylamide-costearyl methylacrylate) (PAm-co-SMA) DC NC hydrogels, using Laponite XLG and the hydrophobically associated interaction as the crosslinkers, which have a good mechanical property and a large deformation. All the NC hydrogels mentioned above have both good mechanical property and large deformation. For the other defect, manufacturing the semi-interpenetration (semi-IPN) or interpenetration (IPN) networks is a direct method to synthesize the dual/multi-stimuli-responsive hydrogels. Chen et al. (2015b) have prepared the pH- and temperature- responsive semi-IPN
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.jmbbm.2019.01.017 Received 28 August 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 30 January 2019 1751-6161/ © 2019 Published by Elsevier Ltd.
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2.2. Preparation of Au-S-S NPs
poly (dimethylaminoethyl methacrylate) (PDMAEMA)/Laponite RD/ carboxymethyl chitosan (CMCTs) NC hydrogel, which can be used as the drug delivery carriers. Shutong et al. (2015) have designed the pH/ temperature responsive NC PNIPAm/chitosan/GO IPN hydrogels with the enhanced mechanical performance. Chen et al. (2018) have synthesized the mechanical strong dual responsive PNIPAm/Laponite XLG/ CMCTs/genipin(GP) NC IPN hydrogels, which have the highest breaking tensile strength of 137.9 kPa at the failure strain of 446.1% and can be used as the drug release carriers of asprin. Ma et al. (2016) have fabricated novel multi-responsive anisotropic semi-IPN GO/ PNIPAm/poly (methylacrylic acid) (PMMA) hydrogels, which own the light-, thermo-, pH- and ionic strength- sensitive properties. Among all the smart hydrogels, the pH- and temperature- responsive hydrogels are the widespread investigated hydrogels in the last decade. Moreover, Nisopropylacrylamide is the common raw material to synthesize the temperature-responsive hydrogels, and the polyelectrolyte materials (such as polyacrylate, chitosan, carboxymethyl chitosan and poly (ethylene imine)) are usually used to prepare pH- responsive hydrogels. For the most physical cross-linked thermo-responsive NC PNIPAm/ clay hydrogels, although they have high mechanical strength, simultaneously they also are unstable and have a bad recoverability. To overcome these defects, introducing the chemical cross-linked points into the hydrogels is an effective approach (Shi et al., 2015). Besides, incorporation of more coordination interaction groups (such as -COO-/ Fe3+, H2PO4-/Fe3+, -SH/Au, etc.) into the networks, with the bond energy between non-covalent and covalent interaction, has become an effective method to further improve the toughness of the mechanical strong NC hydrogels (Peng et al., 2015; Meng et al., 2014; Qin et al., 2017). Based on the above description, we attempt to design a novel biocompatible much more tough mechanical strong pH- and temperature- responsive PNIPAm/CMCTs NC hydrogel with good fast recoverability through a simple one pot in situ free radical polymerization using clay (Laponite XLS) nanosheets and Au-S-S nanoparticles (NPs) as the cross-linkers instead of the common used toxic organic molecules. The optimum conditions for fabricating tough robust PNIPAm/clay/Au-S-S/ CMCTs NC hydrogels was achieved by adjusting the crucial factors: the concentration of Au-S-S NPs, CMCTs and clay. What's more, the synthesized hydrogels also have a good biocompatibility (all the raw materials used possess good biocompatibility, which has been proved before (Qin et al., 2017; Agarwal et al., 2016; Zhang et al., 2013). In addition, for exploring the potential utilization as the drug delivery carriers, the drug absorbing and releasing tests of the synthesized hydrogels were conducted using acetylsalicylic acid (commonly known as Asprin) as the objective drug.
Monodispersed Au NPs were prepared according to the previous research (with average diameter about 16 nm, as shown in Fig. S1) (Ji et al., 2007). Briefly, HAuCl4 solution (solution 1, 0.6 mmol/L) and Na3-citrate solution (solution 2, 210 mmol/L) were fabricated firstly. Then, under the stirred condition, 50 mL solution 1 was heated until boiling in a three necks flask and 1 mL solution 2 was gradually added in. As the reaction goes on, until a brilliant wine red color appear, the mono-dispersed Au NPs were obtained. At room temperature, 5 mg BACA were added into the above Au NPs solution (10 mL), and subsequently the mixture was ultrasonicated for 15 min to make sure that the BACA molecules have a fine coating on the surface of Au NPs. After the BACA molecules coated on the surface of Au NPs, color of the solution turns from red into blue and simultaneously the Au-S-S NPs were obtained. 2.3. Synthesis of the nanocomposite hydrogels 2.3.1. Synthesis of the PNIPAm/clay/Au-S-S/CMCTs NC hydrogels The synthesis process of PNIPAm/clay/Au-S-S/CMCTs NC hydrogels was schematically shown in Fig. 1, and the obtained hydrogel sample was shown in Fig. S2a. Firstly, under the stirred condition, a predetermined amount of dried Laponite XLS was gradually added into a breaker with a given amount of deionized water. After the clay nanosheets were already dissolved into water, 0.452 g NIPAm and a calculated amount of Au-S-S NPs solution were added in, respectively and then the mixture solution was stirred for another 3 h. Afterwards, 4.7 mg kPS, a design amount of CMCTs powder and 9 μL TMEDA were added in, and subsequently the mixture solution was stirred in icewater bath for the last 1 h to get a homogenous solution. Secondly, the above solution was transferred into cylindrical tube and was pumped in the ice-water bath with a vacuum pump, and then refilled with nitrogen, repeated this step in 3 times. At last, the tube was put into 20 ℃ water bath after sealed with a rubber plug, and the robust tough PNIPAm/clay/Au-S-S/CMCTs NC hydrgel was obtained after reacted for 20 h. The manufactured NC hydrogels are denoted as PcmNxCnH in this paper, where P, c, N, C and H represent for PNIPAm, clay, Au-S-S NPs, CMCTs and hydrogels respectively, and m, x and n stand for clay concentration, Au-S-S NPs concentration and CMCTs content respectively. For instance, Pc0.076N0.784C10H means that in these synthesized hydrogels, the concentration of clay is 0.076 g/mL, the concentration of Au-S-S NPs is 0.784 mg/mL, and the content of CMCTs is 10 mg. Among all the prepared hydrogels, the total water content is fixed at 4 mL.
2. Materials and methods 2.3.2. Synthesis of the PNIPAm/clay/CMCTs hydrogels The preparation process of PNIPAm/clay/CMCTs hydrogels was the same to the PNIPAm/clay/Au-S-S/CMCTs NC hydrogels besides no AuS-S NPs was added in.
2.1. Materials N-isopropylacrylamide (NIPAm, J&K Scientific Co., Ltd., China) was recrystallized from a toluene/n-hexane mixture and dried in vacuum at 40 ℃ before used. Synthetic hectorite clay of sol-forming grade Laponite XLS (Mg5.34Li0.66Si8O20(OH)4 Na0.66) was purchased from Rockwood Ltd., Germany, and used after dried in stove at 125 ℃ for 3 h. Carboxymethyl chitosan (CMCTs) (with the viscosity-average molecular weight of 361 kDa and substitution degree of 0.92) was prepared as mentioned in our previous work (Chen et al., 2017). Chemicals below were all analytical grade and used as received. Gold (III) chloride hydrate was supplied by Sigma-Aldrich Co., Ltd., N,N′-bis(acryloyl)cystamine (ABAC) was purchased from Alfa Aesar Co., Ltd., and acetylsalicylic acid (ASA) was bought from the Tokyo Chemical Industry Co., Ltd., Japan. N,N,N′,N′-tetramethyl-ethylenediamine (TMEDA) was supplied by J&k Scientific Co., Ltd., China. Potassium persulfate (KPS), sodium hydroxide (NaOH), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4) and phosphoric acid were all provided by Sinopharm Chemical Reagent Co., Ltd., China.
2.3.3. Synthesis of the PNIPAm/clay hydrogels The synthesis process of PNIPAm/clay hydrogels was same to the PNIPAm/clay/CMCTs NC hydrogels with no CMCTs was incorporated in. 2.4. Characterizations Images of the synthesized Au NPs were obtained by JEM-2100F transmission electron microscopy (TEM) (USA). Fourier transform infrared (FTIR) spectra of the dried hydrogel samples were conducted on a Nicolet 8700 spectrometer (USA) with an attenuated total reflectance accessory ranging from 4000 cm−1 to 600 cm−1. X-ray diffraction (XRD) measurements were performed on a SAXS instrument (Japan) using Cu Kα radiation (λ = 0.154 nm) in a step of 0.02° s−1 range from 5° to 60°. Morphology of the hydrogel cross-sections was carried out by 180
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Fig. 1. The synthesis process of PNIPAm/clay/Au-S-S/CMCTs semi-interpenetrated NC hydrogel.
following formula:
a Quanta 250 (FEI Company, U.S.A) environmental scanning electron microscope (ESEM) at the voltage of 10 kV.
ESR = (We − Wd)/Wd
(1)
where, We and Wd stands for the weight of the equilibrium and dried hydrogel samples, respectively. Each hydrogel sample was reproduced at least three times.
2.5. Tensile test of hydrogels For the tensile mechanical test, all the NC hydrogel samples were manufactured in glass tubes with length of 70 mm and diameter of 4.61 mm. After been put out from the tubes, the hydrogel samples were tested by INSTRON 5969 instrument (USA) immediately (images of the tested hydrogel sample was shown in Fig. S2b). Tensile test conditions are listed below: crosshead speed of 100 mm min−1, initial gauge length of 15 mm and test temperature of 20 ℃. In order to avoid the hydrgel sample sliding when tested, a layer of soft rough paper was pasted on the clamps of the instrument before tested. For the recycle tensile tests, the test conditions are below: the stretching speed is 100 mm min−1 while the unload speed is 150 mm min−1, the initial gauge length is also 15 mm and the test temperature is 20 °C. For these tensile tests, each hydrogel samples were repeated at least three times.
2.7. Swelling-shrinking behavior test At first, the dried hydrogels samples were immersed into large amounts of water at 25 ℃ for 48 h. Then, the swollen hydrogels were transferred from 25 ℃ to 50 ℃ water bath. These steps were repeated in three times. At the scheduled time interval, the swollen hydrogel samples were taken out from the water bath and weighted after the surface water was removed. To guarantee the reproducibility, at least three duplicate samples were used to do the swelling-shrinking behavior test. 2.8. Drug absorption and release tests In the drug absorption and release test, acetylsalicylic acid (ASA) was used as the model drug. The operation process of this test was similar to our previous work (Chen et al., 2018). Firstly, the dried hydrogels samples were immersed in 0.2 mg/mL ASA aqueous solution and the absorption test was carried out in a water bath reciprocal shaker (120 rpm) at room temperature for 15 h (contact time) until the absorption content not increase any more. At the predetermined time intervals, by detecting the absorbance of drug solution at wavelength of 275 nm with the UV–vis spectrophotometer (Thermo SCIENTIFIC, Biomate 3S, USA) instrument and using the calibration curve, the absorbed concentration of ASA was obtained. Then, the ASA-absorbed hydrogel samples were quickly transferred to a lyophilizer and lyophilized at −50 ℃, 15 Pa for 8 h until a constant weight achieved. The
2.6. Swelling behavior of hydrogels After immersed into DI water for a week, changing water three times a day, the synthetic hydrogel samples were used to conduct the equilibrium swelling ratios (ESR) test. First of all, the hydrogel samples were dried in a fume hood for 48 h until their weight didn’t change anymore. Secondly, a predetermined amount of dried hydrogel samples were put into different pH value phosphate buffers solutions (PBS) at the room temperature and different temperature DI water for 48 h until their weight not change anymore. Subsequently, the swollen hydrogels were taken out and surface water of hydrogel samples were wiped off with filter paper before weighted. ESR values were carried out from the 181
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vibration of -COO- group, respectively (Mohamed and Sabaa, 2014). The FTIR spectrum of PcH exhibits pronounced peaks at 3294, 2971, 1641, 1543, 1458 and 1387 cm−1, which represent O-H stretching vibration, C-H and N-H stretching vibration, C=O stretching of amide group, N-H deformation vibration, -CH3 and –CH(CH3)2, respectively (Guo and Gao, 2007; Verma et al., 2016). By comparing the FTIR spectra of PcH, PcCH and PcNCH, the typical peak of C=O stretching at 1641 cm−1 in PcH shifts to 1635 cm−1 in PcCH and to 1637 cm−1 in PcNCH, and meanwhile, the peak of N-H deformation vibration at 1543 cm−1 in PcH moves to 1540 cm−1 in PcCH and to 1539 cm−1 in PcNCH, implying that there existed strong hydrogen-bonding interaction between CMCTs, PNIPAm and Au-S-S NPs in the hydorgels. In addition, with the addition of more CMCTs and/or Au-S-S NPs，the corresponding peak near 3294 cm−1 becomes more intense and sharper, which possibly attributes to that there are more oxygen-containing groups added into the hydorgels.
absorption content of ASA in the hydrogel samples could be calculated from the following equation:
qt = V(C0 − Ct )/W
(2)
−1
where, qt (μmol g ) represents ASA absorption content at time t, V refers to the volume of the drug solution (L), C0 is the initial concentration of ASA solution (mmol L−1), Ct is the dynamic concentration at different time, and W is the weight of the dried hydrogel samples. Before the test, the calibration curve was achieved by the absorbance of a group of ASA standard solutions at wavelength of 275 nm, which was estimated below:
y = 1.03752χ − 0.071 (γ = 0.99952)
(3)
−1
where, χ (mmol L ) means the ASA concentration in aqueous solution, and y refers to the absorbance at 275 nm. The release test was executed in a water bath reciprocal shaker at the same peed in PBS (pH = 6.8) solution at 37 ℃. After the ASAloaded gels were immersed into PBS solution, three milliliters of each solution was collected at the same time intervals for detecting the drug concentration with the UV–vis instrument at the wavelength of 296 nm, and simultaneously an equal volume of the same PBS solution was add back to keep a constant volume. The standard curve of the drug in PBS solution was calculated by:
y = 0.82504χ − 0.00313 (γ = 0.9998)
3.2. XRD spectrum Fig. 2b shows the XRD spectra of clay, CMCTs, PcH, PcCH and PcNCH. The characteristic peaks of clay appear at 2θ = 20°and 35°, and the typical peak of CMCTs is located at 2θ = 20.5°. In the XRD curves of PcH and PcCH, the typical peaks of clay disappear, which reveals that the clay nanosheets have a good dispersability in the fabricated hydrogels. By contrasting the curves of PcNCH with different of Au-S-S NPs, CMCTs and clay, it is clearly to see that the peak (located at 2θ = 20.5°) becomes stronger and sharper with the addition of more Au-S-S NPs and CMCTs, however, weaker as the content of clay increases; it mainly owing to that the interaction between the CMCTs polymer chains increases with more oxygen-containing groups added in (Au-S-S NPs and CMCTs have plenty of oxygen-containing groups) and decreases with more clay nanosheets inserted in. Comparing the curves of PcCH and PcNCH, it is obvious to see that there is a very small new weak peak appear at 2θ = 35.4° in the PcNCH and it becomes a little stronger with more Au-S-S NPs, CMCTs, and clay added in. We infer that the new weak peak is probably related to the addition of Au-S-S NPs, resulting in higher crosslink density, and as more Au-S-S, CMCTs or clay added in, the crosslink density also increases to some extent.
(4)
−1
where, χ (mmol L ) is the ASA concentration in PBS, and y represents the absorbance of the drug at 296 nm. The accumulative ASA release amounted was estimated by the following equation:
Cumulative release(%) = (Mt /M0) × 100%
(5)
where, Mt and M0 means the content of ASA at time t in PBS solution and the loaded ASA content in the lyophilized hydrogel samples, respectively. 3. Results and discussion 3.1. FTIR analysis
3.3. Mechanical property tests
The FTIR spectra of clay, CMCTs, PcH, PcCH and PcNCH are shown in Fig. 2. As illustrated in Fig. 2a, there are two characteristic peaks appeared at 1635 and 1000 cm−1 in the FTIR curve of clay, corresponding to O-H deformation vibration of the absorbed water and Si-O stretching vibration, respectively (Chen et al., 2015b). In the spectrum of CMCTs, the peaks at 3450–3200 cm−1 are associated with both the O-H and N-H stretching vibrations, and the peak at 2900 cm−1 refers to the C-H stretching vibrations. Besides, the peaks appear at 1415 and 1598 cm−1 are assigned to the symmetric and asymmetric stretching
3.3.1. Tensile test of hydrogels In this research, we have comprehensively investigated the crucial factors on tensile property of manufactured hydrogels, such as the content of Au-S-S NPs, CMCTs and clay. As shown in Fig. 3a–c, it is clearly to see that with the introduction of more of Au-S-S NPs, CMCTs and clay, failure stain of the obtained hydrogels decreases, but the elastic modulus (εt = 10–25%) increases. Besides, as more the above
Fig. 2. (a) The FTIR spectra of clay, CMCTs, PcH, PcCH and PcNCH, (b) The XRD spectra of clay, CMCTs, PcH, PcCH and PcNCH. 182
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Fig. 3. Tensile stress-strain curves of the synthesized hydrogels with different content of (a) Au-S-S NPs, (b) CMCTs and (c) clay.
proceeding stretch (Chen et al., 2014). Besides, the additional stretch segments of the loading curves (with the end strain of 800%, 1200%) have a good overlap with the loading curve of an original hydrogel specimen. For better understanding the recoverability of synthesized hydrogels, the loading-unloading recycle tensile test was performed. As shown in Fig. 4c, the successive loading-unloading recycle tensile tests of Pc0.095N1.372C12.5H were conducted four times. Comparing with the 1st recycle curve, the energy dissipation area of the other three curves decreases greatly. Furthermore, as the recycle times increases, tensile strength at the strain of 800% and the initial elastic modulus decreases a little, and simultaneously the area of loading-unloading curves becomes smaller. These results reveal that polymer networks in the synthesized hydrogel were destroyed extensively and couldn’t recover in a short time. As displayed in Fig. 4d–f, after the 1st recycle curve, one sample was reloaded immediately, and another was executed 15 min later, which was put into a polyethylene bag after unloaded at room temperature. The tested samples in Fig. 4d–f refer to Pc0.095H, Pc0.095C12.5H and Pc0.095N1.372C12.5H, respectively and the difference in Fig. 4f is that the other sample was also implemented 15 min later with NIR light (λ = 808 nm, 1 W/cm2) illuminated on at the last 3 min. It is clearly to see that the 2nd hysteresis loop conducted immediately after the 1st one becomes much smaller. After restored for 15 min at room temperature, area of hysteresis loop of the hydrogel samples in Fig. 4d–f increases from the proportion (to the 1st hysteresis loop) of 17.33%, 16.98%, 16.87–71.09%, 74.83%, and 87.61% (91.94%), respectively. The predominate reason for these phenomenon may that with the addition of CMCTs，the physical interaction between the polymer chain in the hydrogels increases, so the recoverable efficiency increases a little; however, with the introduction of Au-S-S NPs, both the physical interaction and the chemical crosslink density in the hydrogels increase, leading to a higher improvement of recoverability; when the unloaded Pc0.095N1.372C12.5H was also put under the NIR light, part of the destroyed dynamic bond between Au and disulfide could be healed,
additives added in, the strain hardening behavior appears at a lower strain value, and the breaking tensile strength increases at first and then decreases. This phenomenon perhaps could be elucidated by the following reasons: as more Au-S-S NPs, CMCTs and clay added in, there are more cross-linked points in the hydrogels, resulting in shorter intercrosslink chain length, higher elastic modulus, smaller failure strain and relative strong tensile property in general; however, when the crosslink density is too high, there are more irregular dispersed polymer chains in the synthesized hydrogels, leading to a lower failure tensile strength. From the above description, it is easily to conclude that the concentration of Au-S-S NPs, CMCTs and clay has a pronounced effect on improving the tensile property of designed NC hydrogels, and the optimized hydrogel (Pc0.095N1.372C12.5H) has the highest tensile stress of 535.5 kPa at the breaking deformation of 1579.5%, which is approximately 1.5 times high of the PNIPAm/Laponite XLS hydrogel reported before (Wang et al., 2012). 3.3.2. Energy dissipation mechanism and recoverable property In order to understand the energy dissipation mechanism of fabricated NC hydrogels more clearly, recycle tensile tests were conducted, and the effective energy dissipation of the NC hydrogels can be represented by the area between the loading and unloading curves. From Fig. 4a, it is obviously to see that the dissipation energy of PcCH increases only a little (13%) contrasting to PcH, but the PcNCH increase greatly (68%), which suggests that the introduction of Au-S-S NPs can effectively improve the toughness of PcH. In Fig. 4b, three continuous loading-unloading tensile tests were carried out on the optimal hydrogel with the end strain ranging from 400% to 1200%. As the end strain increases, the energy dissipation area of the synthetic hydrogel becomes larger, which reveals that polymer networks in the prepared hydrogels, especially the physical cross-linked networks, are destroyed seriously at a higher tensile strain, and the hydrogels designed in this research are tough enough. By comparing all the three curves, the difference between the adjacent curves increases dramatically as the end strain increases, which refers the trend of damage degree in each 183
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Fig. 4. Recycle tensile stress-strain curves of the synthetic hydrogels, (a) Pc0.095H, Pc0.095C12.5H and Pc0.095N1.372C12.5H, (b) Pc0.095N1.372C12.5H, (c) Pc0.095N1.372C12.5H, (d) Pc0.095H, (e) Pc0.095C12.5H and (f) Pc0.095N1.372C12.5H.
in the hydrogel samples (shown in Fig. 5a–d and Fig. 5f–h) become shorter and denser, and the pore size decreases from 10 µm to 3.5 µm, and 7 µm to 1.5 µm, respectively, which is mainly due to the shorter polymer chains and more cross-linked points in the fabricated hydrogels. With the introduction of more CMCTs, pore size of the hydrogel samples increases from 2.5 µm to 4.5 µm, but there are much more smaller pores in the big pores. Interpretation for this phenomenon may be that the distance between clay nanosheets and Au-S-S NPs increases with the incorporation of more CMCTs, leading to larger cross-linked polymer chains and the bigger pore size; at the same time, the physical interaction between polymer chains and the nano-additives (especially the Au-S-S NPs) also increases, resulting in much more smaller pores in the big pores. These results are consistent with that only at the suitable concentration of Au NPs, CMCTs or clay, could the regular and dense porous structure be beneficial to enhancing the tensile strength of the prepared hydrogels; however, when the content of Au NPs, CMCTs or clay is too high, the achieved much more regular and compact inner structure usually make the breaking tensile strength of the synthesized hydrogels decrease (Table 1).
resulting in much higher recoverability. Polymer chains in the hydrogel samples (nanocomposite hydrogel) may form different attachments on the clay surface, accompany with different adsorption strength to clay sheets and the chain length between two adjacent clay sheets (Okay and Oppermann, 2007). During the loading process, the lower adsorption strength and shorter polymer chains were firstly detached from the clay surface, producing the dissipation energy, then the higher and longer polymer chains were detached, responding to higher dissipation energy, and so on (Gao et al., 2015b). So that during the recycle stress-strain tensile test (Fig. 4b), as the tensile strain increases, the dissipation energy also increases, resulting in a bigger closed curves. After unloaded, the lower adsorption strength and shorter polymer chains could recover quickly at the room temperature, however, the higher adsorption strength and longer polymer chains recover may need more time. Because the clay concentration in our synthesized hydrogel samples is high and their cooperative alignment is along with the loading direction, so part of the orientation and strain were retained even after long relaxation time (Haraguchi and Li, 2006). Generally speaking, the recovery process is mainly associated with the reversible dissociation of the physical crosslinking between the clay sheets in the hydrogel samples (Cui et al., 2015). Besides, recoverable images of the above three kind hydrogels are shown in Fig. S3. Length of the above hydrogels after unloaded in 0 and 15 min are measured, and it recovers from 44.8%, 41.3%, and 31% to about 24%, 22.1% and 6.8%, respectively, which are in line with the recoverability of the above mentioned hysteresis loop areas.
3.5. Swelling behaviors The swelling behavior of PcNCH in different pH values of PBS solutions at 25 ℃were exhibited in Fig. 6a–c. Generally speaking, as the pH value increases, ESR of the synthesized hydrogels increase at first, then decreases and at last also increases a little. When pH = 1, there are plenty of hydrogen bonds existed in the hydrogels, leading to the small ESR values of all the hydrogel samples. When the pH value is near 3 (pH = 2–4), there are existed large amount of NH3+ in the hydrogels and simultaneously, part of hydrogen bonds begin disappear, so the repulsion force between NH3+ makes the ESR value increase. As the pH value (pH = 5–7) increases, more NH3+ ions are disappeared and the repulsion force vanished immediately, leading to smaller ESR value. Most of the -COOH groups turn into –COO- ions, when the pH value is higher than 8, resulting in stronger repulsion force between these ions and higher osmotic pressure, so that the ESR increase a little at last. With the addition of more Au-S-S NPs or clay nanosheets, the increased crosslink density and smaller pores making lower ESR in general.
3.4. SEM images Firstly, the synthesized hydrogel samples were immersed into DI water for two weeks (change the water three times a day), then they were freeze dried and their internal cross-section morphology was characterized by ESEM. As shown in Fig. 5a–h, it is clearly to see that pores in the hydrogels are uniformly dispersed and their diameter is generally at the micrometer level. Besides, in contrasting with PcH and PcCH, plenty of smaller pores appear in the big pores in PcNCH with the incorporation of Au-S-S NPs. As more Au-S-S NPs or clay added, pores 184
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Fig. 5. SEM images of the synthesized hydrogels. (a) Pc0.076H, (b) Pc0.076C10H, (c) Pc0.076N0.784C10H, (d) Pc0.076N1.372C10H, (e) Pc0.076N1.372C5H, (f) Pc0.076N1.372C12.5H, (g) Pc0.038N1.372C12.5H, (h) Pc0.095N1.372C12.5H.
3.6. Swelling-shrinking behaviors of hydrogels
Table 1 Mechanical property of the synthesized hydorgels. Hydrogel samples
Failure strength (kPa)
Failure strain (%)
Elastic modulus (kPa)
Pc0.076H Pc0.076C10H Pc0.076N0.392C10H Pc0.076N0.784C10H Pc0.076N0.98C10H Pc0.076N1.176C10H Pc0.076N1.372C10H Pc0.076N1.568C10H Pc0.076N1.372C5H Pc0.076N1.372C12.5H Pc0.076N1.372C15H Pc0.038N1.372C12.5H Pc0.095N1.372C12.5H Pc0.114N1.372C12.5H Pc0.152N1.372C12.5H
198.9 223.2 271.2 341.3 368.4 418.3 448.1 354.1 376.8 490.8 399.7 327.2 535.5 501.3 369.1
2632.7 2697.6 2524.3 2297.3 2189.7 2141.1 2108.9 1623.2 2142.6 2056.4 1786.3 2205.3 1579.5 1642.1 1131.1
1.9 2.3 2.4 2.4 2.5 2.8 2.9 3.5 2.6 3.1 3.2 2.4 6.0 9.1 11.2
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.5 3.7 4.1 3.8 4.9 4.7 5.1 4.6 4.8 5.2 5.5 3.8 5.6 5.0 3.9
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.5 5.3 4.8 5.6 4.3 3.9 4.4 3.9 4.6 4.8 3.9 3.7 3.5 3.2 3.6
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
As illustrated in Fig. 7, swelling-shrinking behaviors of PcH, PcCH and PcNCH were performed by altering the water bath temperature between 25 and 50 ℃. Firstly, the dried hydrogel samples were put into the 25 ℃ water bath for 48 h and then these swellen hydrogels were altered into 50 ℃ water bath for 24 h. Successively, this operation was repeated for another two times. It is obviously to see that after swelling in 25 ℃ water bath for 48 h (the 3rd), the PcNCH samples immersed in 50 ℃ water bath for 24 h (the 2nd) could recover nearly 100% to its last swelling ratio in 25 ℃ water bath (2nd), however, the PcH and PcCH couldn’t. The predominate reason for these results may be that the incorporation of Au-S-S NPs is beneficial for improving the stability of the hydrogels’ inner structure, which can also be clearly seen in the hydrogel pores in Fig. 5c.
0.1 0.3 0.2 0.2 0.3 0.2 0.3 0.4 0.3 0.2 0.3 0.3 0.4 0.3 0.5
3.7. Drug absorption and release tests Fig. 8a displays the absorption capacity of ASA in different synthesized hydrogel samples as a function of contact time. It is obviously to see that the absorbed rate increases dramatically at the beginning of the test, and then decreases gradually until a final plateau was achieved. Evidently, with the incorporation of more Au-S-S NPs, CMCTs or clay, the maximum absorption capacity (qm) of ASA in the samples increases. The mainly reason for this regular pattern may that although the crosslink density in the hydrogels increases with the addition of more the above additive, there are lots of oxygen-containing groups in these additive, which have strong affinity to the ASA molecules, leading to the higher qm values. The release test of ASA-loaded hydrogel samples was executed at 37 ℃ in PBS solution, detecting the released drug content as a function of time. As shown in Fig. 8b, it is easily to find out that the ASA-loaded hydrogel samples exhibited a fast release rate in the beginning 2 h. Subsequently, the hydrogel samples exhibited a relative slow and steady release of ASA into the medium. The initial abrupt release of ASA may attributed to the free ASA on the surface or in the hydrogels, and the followed slower release speed mainly related with the physical interaction between the polymer chains, clay and ASA. As displayed in Table 2, with the incorporation of more Au-S-S NPs or clay, the maximum cumulative release percentage decrease, which is predominately
However, as more CMCTs added in, ESR value decreases at first and then increases to some extent. The mainly reason for this phenomenon may that with the incorporation of more CMCTs, the physical interaction between the polymer chains increases, resulting in smaller ESR value; along with the CMCTs continue increases, large amounts of strong water-absorbable oxygen-containing groups are added in, leading to a higher ESR value. Fig. 6d–f displays the swelling behaviors of PcNCH samples at different temperatures in DI water. As the temperature increases, the hydrogels samples’ weight decreases, and particularly, the weight percent decreases seriously at the volume phase transition temperature (VPTT). It is important to point out that with the incorporation of more Au-S-S NPs or clay, weight percent of the hydrogel samples decrease slower (T = 31–34 ℃), leading to smaller slope values; however, with the addition of more CMCTs content, the slope values increase. The proper interpretation for the above phenomena may that as more Au-S-S NPs or clay added in, the increased crosslink density in the hydrogels restricts the movement of the polymer chains and results in a higher VPTT; with the introduction of more CMCTs, these hydrophilic polymer chains promote the inner water's movement, leading to a higher waterloss speed and a lower VPTT value (Luo et al., 2017) 185
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Fig. 6. The swelling behaviors of synthesized hydrogels with different content of (a) Au-S-S NPs, (b) CMCTs and (c) clay in different pH values, and with different content of (d) Au-S-S NPs, (e) CMCTs and (f) clay in different temperatures. Table 2 The maximum absorption capacity (qm) and cumulative release percentage of the hydrogel samples.
Fig. 7. Swelling and shrinking behavior of the prepared hydrogels samples with different content of Au-S-S NPs, CMCTs and clay by altering the temperature between 25 and 50 ℃.
Samples
qm/μmol g−1
Maximum cumulative release percentage/%
Maximum cumulative release/μmol g−1
Pc0.076H Pc0.076C10H Pc0.076N0.784C10H Pc0.076N1.372C10H Pc0.076N1.372C5H Pc0.076N1.372C12.5H Pc0.038N1.372C12.5H Pc0.095N1.372C12.5H
75.07 76.95 81.92 91.58 90.97 92.81 62.49 93.55
82.43 78.71 70.96 68.90 63.39 66.50 86.64 52.86
61.54–62.22 60.23–60.90 57.80–58.46 62.74–63.46 57.35–57.99 61.40–62.04 53.81–54.47 49.15–49.76
± ± ± ± ± ± ± ±
0.27 0.29 0.26 0.24 0.23 0.25 0.24 0.31
± ± ± ± ± ± ± ±
0.16 0.14 0.18 0.21 0.19 0.17 0.20 0.15
associated with the more crosslink density in the hydrogels; however, with the addition of more CMCTs, the percentage value increases at first and then decreases, the most probable reason is that the interaction between ASA and CMCTs increases as more CMCTs added in, when the interaction is too strong, resulting in a relative smaller percentage value. From the above description, we can concluded that the ASA
Fig. 8. The absorption capacity (a) and the cumulative release (b) of the prepared hydrogels samples with different concentration of Au-S-S NPs, CMCTs and clay. 186
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release speed and the maximum cumulative release can be controlled by adjusting the content of the raw materials, which indirectly reveals that these designed hydrogel could be used as the controlled drug delivery carries in the biomedical applications in future. Taken tighter, the synthesized tough robust hydorgels in this paper not only have the good recoverability and evidently dual pH- and temperature- sensitive property and, but also can be used as the drug delivery carriers. These advantages make them suitable for the artificial muscle, chemical values actuators, drug delivery systems and biological sensors in the future. Because the reaction temperature, reaction time and the mole ratio of BACA/Au NPs may play a key role in enhancing the tensile strength of the prepared hydrogels, it is intriguing to some more work by adjusting these influence factors to obtain much more strong hydrogels.
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Dynamic Au-thiolate interaction induced rapid self-healing nanocomposite hydrogels with remarkable mechanical behaviors. Chem 3, 691–705. Shi, K., Liu, Z., Wei, Y.-Y., et al., 2015. Near-infrared light-responsive poly(N-isopropylacrylamide)/graphene oxide nanocomposite hydrogels with ultrahigh tensibility. ACS Appl. Mater. Interfaces 7, 27289–27298. Shutong, H., Jianfeng, S., Na, L., et al., 2015. Dual pH- and temperature-responsive hydrogels with extraordinary swelling/deswelling behavior and enhanced mechanical performances. J. Appl. Polym. Sci. 132. Su, X., Chen, B., 2018. Tough, resilient and pH-sensitive interpenetrating polyacrylamide/alginate/montmorillonite nanocomposite hydrogels. Carbohydr. Polym. 197, 497–507. Verma, N.K., Purohit, M.P., Equbal, D., et al., 2016. Targeted smart pH and thermoresponsive N,O-carboxymethyl chitosan conjugated nanogels for enhanced therapeutic efficacy of doxorubicin in MCF-7 breast cancer cells. Bioconjugate Chem. 27, 2605–2619. 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4. Conclusion In summary, we synthesized a novel tough robust biocompatible and dual pH- and temperature- responsive PNIPAm/clay/Au-S-S/ CMCTs NC hydrogels in this paper through a one pot in situ free radical polymerization using Laponite XLS and Au-S-S NPs as the cross-linkers. By modulating the pivotal influence factors, the synthesized hydrogels displayed the highest tensile stress of 535.5 kPa at the breaking strain of 1579.5%. These hydrogels could dissipate a great deal of energy in the stretching process and recover 91.94% of the 1st loop energy in 15 min under the NIR light at room temperature after unloaded. Furthermore, the synthesized hydrogels have a remarkably dual pH- and temperature- responsive property, and a good swelling-shrinking property. Additionally, the designed hydrogels also have a good controlled drug release property of asprin in the vitro release test by tuning their inner crosslink density. This valuable property will enable these fabricated hydrogels as potential drug delivery carriers in future. Acknowledgement This work was financially supported by Shanghai International S&T Cooperation Fund (16160731302), National Natural Science Foundation of China (No. 51473031) and Natural Science Foundation of Shanghai (Grant no. 17ZR1401100). Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Author contributions Y.C. and S.K. contributed equally. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jmbbm.2019.01.017. References Agarwal, T., Narayan, R., Maji, S., et al., 2016. Gelatin/carboxymethyl chitosan based scaffolds for dermal tissue engineering applications. Int. J. Biol. Macromol. 93, 1499–1506. Beebe, D.J., Moore, J.S., Bauer, J.M., et al., 2000. Functional hydrogel structures for autonomous flow control inside microfluidic channels. Nature 404, 588. Chen, Q., Zhu, L., Huang, L., et al., 2014. Fracture of the physically cross-linked first network in hybrid double network hydrogels. Macromolecules 47, 2140–2148. Chen, Q., Wei, D., Chen, H., et al., 2015. Simultaneous enhancement of stiffness and toughness in hybrid double-network hydrogels via the first, physically linked network. Macromolecules 48, 8003–8010. Chen, Y., Zhuang, L., Chao, Y., et al., 2015a. Poly(N-isopropylacrylamide)-clay nanocomposite hydrogels with responsive bending property as temperature-controlled
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Tough robust dual responsive nanocomposite hydrogel as controlled drug delivery carrier of asprin
T
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Yang Chen, Shuai Kang, Junrong Yu , Yan Wang, Jing Zhu, Zuming Hu State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Robust Biocompatible Hydrogel Carboxymethyl chitosan Drug delivery carriers
Smart mechanical strong hydrogels have gained increasing attention in the last decade. A novel tough robust biocompatible and dual pH- and temperature- responsive poly (N-isopropylacrylamide)/clay (Laponite XLS)/ gold nanoparticles (Au-S-S NPs)/caboxymethyl chitosan (CMCTs) nanocomposite hydrogel was synthesized by a facile one-pot in situ free radical polymerization, using clay and Au-S-S NPs as the cross-linkers instead of toxic organic molecules. By tuning the crucial factors, concentration of Au-S-S NPs, CMCTs and clay, the obtained hydrogels exhibited the highest tensile stress of 535.5 kPa at the breaking deformation of 1579.5%. Furthermore, these synthesized hydrogels were tough enough and simultaneously owned a fast recoverability after unloaded in 15 min at room temperature. Moreover, effects of the above factors on swelling and swelling-shrinking behaviors of the prepared hydrogels were investigated in detail. In addition, these designed hydrogels also possessed a controlled drug release property of asprin by adjusting their inner crosslink density. Owing to this property, they could be used as the potential drug delivery carriers in future.
1. Introduction Polymer hydrogels are cross-linked 3D hydrophilic networks comprising a great deal of water (Ye et al., 2016), which have drawn considerable attention in recent years in various fields involving food production, tissue engineering, medicine, soft devices, and so on. Smart hydrogels can evidently alter their volume or other properties in response to a variety of external stimuli, such as light (Li et al., 2017), temperature (Chen et al., 2015a), pH (Gao et al., 2015a), and magnetic/ electric fields (Yang et al., 2015; Dai and Nelson, 2010). Thanks to the remarkable stimuli-responsive properties, smart hydrogels have been widely used in numerous applications such as intelligent sensors (Shi et al., 2015), tissue engineering scaffolds (Fan et al., 2016; Zhang et al., 2016), artificial cartilage (Hu et al., 2016), chemical valves actuators (Beebe et al., 2000), and drug delivery carriers (Chen et al., 2018), for most of which strong mechanical properties are needed. However, most of the conventional smart hydrogels generally have two critical defects: mechanical weak and brittle, and response to single stimuli, which seriously limits their high-end applications. Therefore, how to synthesize the dual/multi-stimuli hydrogels, with tough mechanical strong property, becomes one of the research hot-points in the last decade. To address the first defect, plenty of strategies are proposed for preparing tough mechanical strong hydrogels, such as hydrophobically ⁎
associated hydrogels (Insu et al., 2016), double network (DN) hydrogels (Chen et al., 2015), dual-crosslinked (DC) hydrogels (Cui et al., 2015), and nanocomposite (NC) hydrogels (Su and Chen, 2018). It is well known that the preparation of NC hydrogels is a simple efficient way to enhance the mechanical strength of the hydrogels, with the addition of reinforced nanofillers, such as carbon nanotubes, nanofibers, graphene oxide (GO) and clay nanosheets. Wang et al. (2012) have synthesized the poly(N-isopropylacrylamide) (PNIPAm)/Laponite XLS hydrogels, which have the highest breaking tensile strength about 380 kPa at corresponding failure strain of 960%. Hu et al. (2016) have fabricated the robust DC poly(acrylamide-co-acrylic acid) (PAm-co-Ac) NC hydrogels, using clay nanosheets and Fe3+ ions as the cross-linkers, which possess the highest tensile strength of 3.5 MPa at the breaking elongation of 2100%. Cui et al. (2015) have prepared the poly(acrylamide-costearyl methylacrylate) (PAm-co-SMA) DC NC hydrogels, using Laponite XLG and the hydrophobically associated interaction as the crosslinkers, which have a good mechanical property and a large deformation. All the NC hydrogels mentioned above have both good mechanical property and large deformation. For the other defect, manufacturing the semi-interpenetration (semi-IPN) or interpenetration (IPN) networks is a direct method to synthesize the dual/multi-stimuli-responsive hydrogels. Chen et al. (2015b) have prepared the pH- and temperature- responsive semi-IPN
Corresponding author. E-mail address: [email protected] (J. Yu).
https://doi.org/10.1016/j.jmbbm.2019.01.017 Received 28 August 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 30 January 2019 1751-6161/ © 2019 Published by Elsevier Ltd.
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2.2. Preparation of Au-S-S NPs
poly (dimethylaminoethyl methacrylate) (PDMAEMA)/Laponite RD/ carboxymethyl chitosan (CMCTs) NC hydrogel, which can be used as the drug delivery carriers. Shutong et al. (2015) have designed the pH/ temperature responsive NC PNIPAm/chitosan/GO IPN hydrogels with the enhanced mechanical performance. Chen et al. (2018) have synthesized the mechanical strong dual responsive PNIPAm/Laponite XLG/ CMCTs/genipin(GP) NC IPN hydrogels, which have the highest breaking tensile strength of 137.9 kPa at the failure strain of 446.1% and can be used as the drug release carriers of asprin. Ma et al. (2016) have fabricated novel multi-responsive anisotropic semi-IPN GO/ PNIPAm/poly (methylacrylic acid) (PMMA) hydrogels, which own the light-, thermo-, pH- and ionic strength- sensitive properties. Among all the smart hydrogels, the pH- and temperature- responsive hydrogels are the widespread investigated hydrogels in the last decade. Moreover, Nisopropylacrylamide is the common raw material to synthesize the temperature-responsive hydrogels, and the polyelectrolyte materials (such as polyacrylate, chitosan, carboxymethyl chitosan and poly (ethylene imine)) are usually used to prepare pH- responsive hydrogels. For the most physical cross-linked thermo-responsive NC PNIPAm/ clay hydrogels, although they have high mechanical strength, simultaneously they also are unstable and have a bad recoverability. To overcome these defects, introducing the chemical cross-linked points into the hydrogels is an effective approach (Shi et al., 2015). Besides, incorporation of more coordination interaction groups (such as -COO-/ Fe3+, H2PO4-/Fe3+, -SH/Au, etc.) into the networks, with the bond energy between non-covalent and covalent interaction, has become an effective method to further improve the toughness of the mechanical strong NC hydrogels (Peng et al., 2015; Meng et al., 2014; Qin et al., 2017). Based on the above description, we attempt to design a novel biocompatible much more tough mechanical strong pH- and temperature- responsive PNIPAm/CMCTs NC hydrogel with good fast recoverability through a simple one pot in situ free radical polymerization using clay (Laponite XLS) nanosheets and Au-S-S nanoparticles (NPs) as the cross-linkers instead of the common used toxic organic molecules. The optimum conditions for fabricating tough robust PNIPAm/clay/Au-S-S/ CMCTs NC hydrogels was achieved by adjusting the crucial factors: the concentration of Au-S-S NPs, CMCTs and clay. What's more, the synthesized hydrogels also have a good biocompatibility (all the raw materials used possess good biocompatibility, which has been proved before (Qin et al., 2017; Agarwal et al., 2016; Zhang et al., 2013). In addition, for exploring the potential utilization as the drug delivery carriers, the drug absorbing and releasing tests of the synthesized hydrogels were conducted using acetylsalicylic acid (commonly known as Asprin) as the objective drug.
Monodispersed Au NPs were prepared according to the previous research (with average diameter about 16 nm, as shown in Fig. S1) (Ji et al., 2007). Briefly, HAuCl4 solution (solution 1, 0.6 mmol/L) and Na3-citrate solution (solution 2, 210 mmol/L) were fabricated firstly. Then, under the stirred condition, 50 mL solution 1 was heated until boiling in a three necks flask and 1 mL solution 2 was gradually added in. As the reaction goes on, until a brilliant wine red color appear, the mono-dispersed Au NPs were obtained. At room temperature, 5 mg BACA were added into the above Au NPs solution (10 mL), and subsequently the mixture was ultrasonicated for 15 min to make sure that the BACA molecules have a fine coating on the surface of Au NPs. After the BACA molecules coated on the surface of Au NPs, color of the solution turns from red into blue and simultaneously the Au-S-S NPs were obtained. 2.3. Synthesis of the nanocomposite hydrogels 2.3.1. Synthesis of the PNIPAm/clay/Au-S-S/CMCTs NC hydrogels The synthesis process of PNIPAm/clay/Au-S-S/CMCTs NC hydrogels was schematically shown in Fig. 1, and the obtained hydrogel sample was shown in Fig. S2a. Firstly, under the stirred condition, a predetermined amount of dried Laponite XLS was gradually added into a breaker with a given amount of deionized water. After the clay nanosheets were already dissolved into water, 0.452 g NIPAm and a calculated amount of Au-S-S NPs solution were added in, respectively and then the mixture solution was stirred for another 3 h. Afterwards, 4.7 mg kPS, a design amount of CMCTs powder and 9 μL TMEDA were added in, and subsequently the mixture solution was stirred in icewater bath for the last 1 h to get a homogenous solution. Secondly, the above solution was transferred into cylindrical tube and was pumped in the ice-water bath with a vacuum pump, and then refilled with nitrogen, repeated this step in 3 times. At last, the tube was put into 20 ℃ water bath after sealed with a rubber plug, and the robust tough PNIPAm/clay/Au-S-S/CMCTs NC hydrgel was obtained after reacted for 20 h. The manufactured NC hydrogels are denoted as PcmNxCnH in this paper, where P, c, N, C and H represent for PNIPAm, clay, Au-S-S NPs, CMCTs and hydrogels respectively, and m, x and n stand for clay concentration, Au-S-S NPs concentration and CMCTs content respectively. For instance, Pc0.076N0.784C10H means that in these synthesized hydrogels, the concentration of clay is 0.076 g/mL, the concentration of Au-S-S NPs is 0.784 mg/mL, and the content of CMCTs is 10 mg. Among all the prepared hydrogels, the total water content is fixed at 4 mL.
2. Materials and methods 2.3.2. Synthesis of the PNIPAm/clay/CMCTs hydrogels The preparation process of PNIPAm/clay/CMCTs hydrogels was the same to the PNIPAm/clay/Au-S-S/CMCTs NC hydrogels besides no AuS-S NPs was added in.
2.1. Materials N-isopropylacrylamide (NIPAm, J&K Scientific Co., Ltd., China) was recrystallized from a toluene/n-hexane mixture and dried in vacuum at 40 ℃ before used. Synthetic hectorite clay of sol-forming grade Laponite XLS (Mg5.34Li0.66Si8O20(OH)4 Na0.66) was purchased from Rockwood Ltd., Germany, and used after dried in stove at 125 ℃ for 3 h. Carboxymethyl chitosan (CMCTs) (with the viscosity-average molecular weight of 361 kDa and substitution degree of 0.92) was prepared as mentioned in our previous work (Chen et al., 2017). Chemicals below were all analytical grade and used as received. Gold (III) chloride hydrate was supplied by Sigma-Aldrich Co., Ltd., N,N′-bis(acryloyl)cystamine (ABAC) was purchased from Alfa Aesar Co., Ltd., and acetylsalicylic acid (ASA) was bought from the Tokyo Chemical Industry Co., Ltd., Japan. N,N,N′,N′-tetramethyl-ethylenediamine (TMEDA) was supplied by J&k Scientific Co., Ltd., China. Potassium persulfate (KPS), sodium hydroxide (NaOH), sodium dihydrogen phosphate (NaH2PO4), disodium hydrogen phosphate (Na2HPO4) and phosphoric acid were all provided by Sinopharm Chemical Reagent Co., Ltd., China.
2.3.3. Synthesis of the PNIPAm/clay hydrogels The synthesis process of PNIPAm/clay hydrogels was same to the PNIPAm/clay/CMCTs NC hydrogels with no CMCTs was incorporated in. 2.4. Characterizations Images of the synthesized Au NPs were obtained by JEM-2100F transmission electron microscopy (TEM) (USA). Fourier transform infrared (FTIR) spectra of the dried hydrogel samples were conducted on a Nicolet 8700 spectrometer (USA) with an attenuated total reflectance accessory ranging from 4000 cm−1 to 600 cm−1. X-ray diffraction (XRD) measurements were performed on a SAXS instrument (Japan) using Cu Kα radiation (λ = 0.154 nm) in a step of 0.02° s−1 range from 5° to 60°. Morphology of the hydrogel cross-sections was carried out by 180
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Fig. 1. The synthesis process of PNIPAm/clay/Au-S-S/CMCTs semi-interpenetrated NC hydrogel.
following formula:
a Quanta 250 (FEI Company, U.S.A) environmental scanning electron microscope (ESEM) at the voltage of 10 kV.
ESR = (We − Wd)/Wd
(1)
where, We and Wd stands for the weight of the equilibrium and dried hydrogel samples, respectively. Each hydrogel sample was reproduced at least three times.
2.5. Tensile test of hydrogels For the tensile mechanical test, all the NC hydrogel samples were manufactured in glass tubes with length of 70 mm and diameter of 4.61 mm. After been put out from the tubes, the hydrogel samples were tested by INSTRON 5969 instrument (USA) immediately (images of the tested hydrogel sample was shown in Fig. S2b). Tensile test conditions are listed below: crosshead speed of 100 mm min−1, initial gauge length of 15 mm and test temperature of 20 ℃. In order to avoid the hydrgel sample sliding when tested, a layer of soft rough paper was pasted on the clamps of the instrument before tested. For the recycle tensile tests, the test conditions are below: the stretching speed is 100 mm min−1 while the unload speed is 150 mm min−1, the initial gauge length is also 15 mm and the test temperature is 20 °C. For these tensile tests, each hydrogel samples were repeated at least three times.
2.7. Swelling-shrinking behavior test At first, the dried hydrogels samples were immersed into large amounts of water at 25 ℃ for 48 h. Then, the swollen hydrogels were transferred from 25 ℃ to 50 ℃ water bath. These steps were repeated in three times. At the scheduled time interval, the swollen hydrogel samples were taken out from the water bath and weighted after the surface water was removed. To guarantee the reproducibility, at least three duplicate samples were used to do the swelling-shrinking behavior test. 2.8. Drug absorption and release tests In the drug absorption and release test, acetylsalicylic acid (ASA) was used as the model drug. The operation process of this test was similar to our previous work (Chen et al., 2018). Firstly, the dried hydrogels samples were immersed in 0.2 mg/mL ASA aqueous solution and the absorption test was carried out in a water bath reciprocal shaker (120 rpm) at room temperature for 15 h (contact time) until the absorption content not increase any more. At the predetermined time intervals, by detecting the absorbance of drug solution at wavelength of 275 nm with the UV–vis spectrophotometer (Thermo SCIENTIFIC, Biomate 3S, USA) instrument and using the calibration curve, the absorbed concentration of ASA was obtained. Then, the ASA-absorbed hydrogel samples were quickly transferred to a lyophilizer and lyophilized at −50 ℃, 15 Pa for 8 h until a constant weight achieved. The
2.6. Swelling behavior of hydrogels After immersed into DI water for a week, changing water three times a day, the synthetic hydrogel samples were used to conduct the equilibrium swelling ratios (ESR) test. First of all, the hydrogel samples were dried in a fume hood for 48 h until their weight didn’t change anymore. Secondly, a predetermined amount of dried hydrogel samples were put into different pH value phosphate buffers solutions (PBS) at the room temperature and different temperature DI water for 48 h until their weight not change anymore. Subsequently, the swollen hydrogels were taken out and surface water of hydrogel samples were wiped off with filter paper before weighted. ESR values were carried out from the 181
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vibration of -COO- group, respectively (Mohamed and Sabaa, 2014). The FTIR spectrum of PcH exhibits pronounced peaks at 3294, 2971, 1641, 1543, 1458 and 1387 cm−1, which represent O-H stretching vibration, C-H and N-H stretching vibration, C=O stretching of amide group, N-H deformation vibration, -CH3 and –CH(CH3)2, respectively (Guo and Gao, 2007; Verma et al., 2016). By comparing the FTIR spectra of PcH, PcCH and PcNCH, the typical peak of C=O stretching at 1641 cm−1 in PcH shifts to 1635 cm−1 in PcCH and to 1637 cm−1 in PcNCH, and meanwhile, the peak of N-H deformation vibration at 1543 cm−1 in PcH moves to 1540 cm−1 in PcCH and to 1539 cm−1 in PcNCH, implying that there existed strong hydrogen-bonding interaction between CMCTs, PNIPAm and Au-S-S NPs in the hydorgels. In addition, with the addition of more CMCTs and/or Au-S-S NPs，the corresponding peak near 3294 cm−1 becomes more intense and sharper, which possibly attributes to that there are more oxygen-containing groups added into the hydorgels.
absorption content of ASA in the hydrogel samples could be calculated from the following equation:
qt = V(C0 − Ct )/W
(2)
−1
where, qt (μmol g ) represents ASA absorption content at time t, V refers to the volume of the drug solution (L), C0 is the initial concentration of ASA solution (mmol L−1), Ct is the dynamic concentration at different time, and W is the weight of the dried hydrogel samples. Before the test, the calibration curve was achieved by the absorbance of a group of ASA standard solutions at wavelength of 275 nm, which was estimated below:
y = 1.03752χ − 0.071 (γ = 0.99952)
(3)
−1
where, χ (mmol L ) means the ASA concentration in aqueous solution, and y refers to the absorbance at 275 nm. The release test was executed in a water bath reciprocal shaker at the same peed in PBS (pH = 6.8) solution at 37 ℃. After the ASAloaded gels were immersed into PBS solution, three milliliters of each solution was collected at the same time intervals for detecting the drug concentration with the UV–vis instrument at the wavelength of 296 nm, and simultaneously an equal volume of the same PBS solution was add back to keep a constant volume. The standard curve of the drug in PBS solution was calculated by:
y = 0.82504χ − 0.00313 (γ = 0.9998)
3.2. XRD spectrum Fig. 2b shows the XRD spectra of clay, CMCTs, PcH, PcCH and PcNCH. The characteristic peaks of clay appear at 2θ = 20°and 35°, and the typical peak of CMCTs is located at 2θ = 20.5°. In the XRD curves of PcH and PcCH, the typical peaks of clay disappear, which reveals that the clay nanosheets have a good dispersability in the fabricated hydrogels. By contrasting the curves of PcNCH with different of Au-S-S NPs, CMCTs and clay, it is clearly to see that the peak (located at 2θ = 20.5°) becomes stronger and sharper with the addition of more Au-S-S NPs and CMCTs, however, weaker as the content of clay increases; it mainly owing to that the interaction between the CMCTs polymer chains increases with more oxygen-containing groups added in (Au-S-S NPs and CMCTs have plenty of oxygen-containing groups) and decreases with more clay nanosheets inserted in. Comparing the curves of PcCH and PcNCH, it is obvious to see that there is a very small new weak peak appear at 2θ = 35.4° in the PcNCH and it becomes a little stronger with more Au-S-S NPs, CMCTs, and clay added in. We infer that the new weak peak is probably related to the addition of Au-S-S NPs, resulting in higher crosslink density, and as more Au-S-S, CMCTs or clay added in, the crosslink density also increases to some extent.
(4)
−1
where, χ (mmol L ) is the ASA concentration in PBS, and y represents the absorbance of the drug at 296 nm. The accumulative ASA release amounted was estimated by the following equation:
Cumulative release(%) = (Mt /M0) × 100%
(5)
where, Mt and M0 means the content of ASA at time t in PBS solution and the loaded ASA content in the lyophilized hydrogel samples, respectively. 3. Results and discussion 3.1. FTIR analysis
3.3. Mechanical property tests
The FTIR spectra of clay, CMCTs, PcH, PcCH and PcNCH are shown in Fig. 2. As illustrated in Fig. 2a, there are two characteristic peaks appeared at 1635 and 1000 cm−1 in the FTIR curve of clay, corresponding to O-H deformation vibration of the absorbed water and Si-O stretching vibration, respectively (Chen et al., 2015b). In the spectrum of CMCTs, the peaks at 3450–3200 cm−1 are associated with both the O-H and N-H stretching vibrations, and the peak at 2900 cm−1 refers to the C-H stretching vibrations. Besides, the peaks appear at 1415 and 1598 cm−1 are assigned to the symmetric and asymmetric stretching
3.3.1. Tensile test of hydrogels In this research, we have comprehensively investigated the crucial factors on tensile property of manufactured hydrogels, such as the content of Au-S-S NPs, CMCTs and clay. As shown in Fig. 3a–c, it is clearly to see that with the introduction of more of Au-S-S NPs, CMCTs and clay, failure stain of the obtained hydrogels decreases, but the elastic modulus (εt = 10–25%) increases. Besides, as more the above
Fig. 2. (a) The FTIR spectra of clay, CMCTs, PcH, PcCH and PcNCH, (b) The XRD spectra of clay, CMCTs, PcH, PcCH and PcNCH. 182
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Fig. 3. Tensile stress-strain curves of the synthesized hydrogels with different content of (a) Au-S-S NPs, (b) CMCTs and (c) clay.
proceeding stretch (Chen et al., 2014). Besides, the additional stretch segments of the loading curves (with the end strain of 800%, 1200%) have a good overlap with the loading curve of an original hydrogel specimen. For better understanding the recoverability of synthesized hydrogels, the loading-unloading recycle tensile test was performed. As shown in Fig. 4c, the successive loading-unloading recycle tensile tests of Pc0.095N1.372C12.5H were conducted four times. Comparing with the 1st recycle curve, the energy dissipation area of the other three curves decreases greatly. Furthermore, as the recycle times increases, tensile strength at the strain of 800% and the initial elastic modulus decreases a little, and simultaneously the area of loading-unloading curves becomes smaller. These results reveal that polymer networks in the synthesized hydrogel were destroyed extensively and couldn’t recover in a short time. As displayed in Fig. 4d–f, after the 1st recycle curve, one sample was reloaded immediately, and another was executed 15 min later, which was put into a polyethylene bag after unloaded at room temperature. The tested samples in Fig. 4d–f refer to Pc0.095H, Pc0.095C12.5H and Pc0.095N1.372C12.5H, respectively and the difference in Fig. 4f is that the other sample was also implemented 15 min later with NIR light (λ = 808 nm, 1 W/cm2) illuminated on at the last 3 min. It is clearly to see that the 2nd hysteresis loop conducted immediately after the 1st one becomes much smaller. After restored for 15 min at room temperature, area of hysteresis loop of the hydrogel samples in Fig. 4d–f increases from the proportion (to the 1st hysteresis loop) of 17.33%, 16.98%, 16.87–71.09%, 74.83%, and 87.61% (91.94%), respectively. The predominate reason for these phenomenon may that with the addition of CMCTs，the physical interaction between the polymer chain in the hydrogels increases, so the recoverable efficiency increases a little; however, with the introduction of Au-S-S NPs, both the physical interaction and the chemical crosslink density in the hydrogels increase, leading to a higher improvement of recoverability; when the unloaded Pc0.095N1.372C12.5H was also put under the NIR light, part of the destroyed dynamic bond between Au and disulfide could be healed,
additives added in, the strain hardening behavior appears at a lower strain value, and the breaking tensile strength increases at first and then decreases. This phenomenon perhaps could be elucidated by the following reasons: as more Au-S-S NPs, CMCTs and clay added in, there are more cross-linked points in the hydrogels, resulting in shorter intercrosslink chain length, higher elastic modulus, smaller failure strain and relative strong tensile property in general; however, when the crosslink density is too high, there are more irregular dispersed polymer chains in the synthesized hydrogels, leading to a lower failure tensile strength. From the above description, it is easily to conclude that the concentration of Au-S-S NPs, CMCTs and clay has a pronounced effect on improving the tensile property of designed NC hydrogels, and the optimized hydrogel (Pc0.095N1.372C12.5H) has the highest tensile stress of 535.5 kPa at the breaking deformation of 1579.5%, which is approximately 1.5 times high of the PNIPAm/Laponite XLS hydrogel reported before (Wang et al., 2012). 3.3.2. Energy dissipation mechanism and recoverable property In order to understand the energy dissipation mechanism of fabricated NC hydrogels more clearly, recycle tensile tests were conducted, and the effective energy dissipation of the NC hydrogels can be represented by the area between the loading and unloading curves. From Fig. 4a, it is obviously to see that the dissipation energy of PcCH increases only a little (13%) contrasting to PcH, but the PcNCH increase greatly (68%), which suggests that the introduction of Au-S-S NPs can effectively improve the toughness of PcH. In Fig. 4b, three continuous loading-unloading tensile tests were carried out on the optimal hydrogel with the end strain ranging from 400% to 1200%. As the end strain increases, the energy dissipation area of the synthetic hydrogel becomes larger, which reveals that polymer networks in the prepared hydrogels, especially the physical cross-linked networks, are destroyed seriously at a higher tensile strain, and the hydrogels designed in this research are tough enough. By comparing all the three curves, the difference between the adjacent curves increases dramatically as the end strain increases, which refers the trend of damage degree in each 183
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Fig. 4. Recycle tensile stress-strain curves of the synthetic hydrogels, (a) Pc0.095H, Pc0.095C12.5H and Pc0.095N1.372C12.5H, (b) Pc0.095N1.372C12.5H, (c) Pc0.095N1.372C12.5H, (d) Pc0.095H, (e) Pc0.095C12.5H and (f) Pc0.095N1.372C12.5H.
in the hydrogel samples (shown in Fig. 5a–d and Fig. 5f–h) become shorter and denser, and the pore size decreases from 10 µm to 3.5 µm, and 7 µm to 1.5 µm, respectively, which is mainly due to the shorter polymer chains and more cross-linked points in the fabricated hydrogels. With the introduction of more CMCTs, pore size of the hydrogel samples increases from 2.5 µm to 4.5 µm, but there are much more smaller pores in the big pores. Interpretation for this phenomenon may be that the distance between clay nanosheets and Au-S-S NPs increases with the incorporation of more CMCTs, leading to larger cross-linked polymer chains and the bigger pore size; at the same time, the physical interaction between polymer chains and the nano-additives (especially the Au-S-S NPs) also increases, resulting in much more smaller pores in the big pores. These results are consistent with that only at the suitable concentration of Au NPs, CMCTs or clay, could the regular and dense porous structure be beneficial to enhancing the tensile strength of the prepared hydrogels; however, when the content of Au NPs, CMCTs or clay is too high, the achieved much more regular and compact inner structure usually make the breaking tensile strength of the synthesized hydrogels decrease (Table 1).
resulting in much higher recoverability. Polymer chains in the hydrogel samples (nanocomposite hydrogel) may form different attachments on the clay surface, accompany with different adsorption strength to clay sheets and the chain length between two adjacent clay sheets (Okay and Oppermann, 2007). During the loading process, the lower adsorption strength and shorter polymer chains were firstly detached from the clay surface, producing the dissipation energy, then the higher and longer polymer chains were detached, responding to higher dissipation energy, and so on (Gao et al., 2015b). So that during the recycle stress-strain tensile test (Fig. 4b), as the tensile strain increases, the dissipation energy also increases, resulting in a bigger closed curves. After unloaded, the lower adsorption strength and shorter polymer chains could recover quickly at the room temperature, however, the higher adsorption strength and longer polymer chains recover may need more time. Because the clay concentration in our synthesized hydrogel samples is high and their cooperative alignment is along with the loading direction, so part of the orientation and strain were retained even after long relaxation time (Haraguchi and Li, 2006). Generally speaking, the recovery process is mainly associated with the reversible dissociation of the physical crosslinking between the clay sheets in the hydrogel samples (Cui et al., 2015). Besides, recoverable images of the above three kind hydrogels are shown in Fig. S3. Length of the above hydrogels after unloaded in 0 and 15 min are measured, and it recovers from 44.8%, 41.3%, and 31% to about 24%, 22.1% and 6.8%, respectively, which are in line with the recoverability of the above mentioned hysteresis loop areas.
3.5. Swelling behaviors The swelling behavior of PcNCH in different pH values of PBS solutions at 25 ℃were exhibited in Fig. 6a–c. Generally speaking, as the pH value increases, ESR of the synthesized hydrogels increase at first, then decreases and at last also increases a little. When pH = 1, there are plenty of hydrogen bonds existed in the hydrogels, leading to the small ESR values of all the hydrogel samples. When the pH value is near 3 (pH = 2–4), there are existed large amount of NH3+ in the hydrogels and simultaneously, part of hydrogen bonds begin disappear, so the repulsion force between NH3+ makes the ESR value increase. As the pH value (pH = 5–7) increases, more NH3+ ions are disappeared and the repulsion force vanished immediately, leading to smaller ESR value. Most of the -COOH groups turn into –COO- ions, when the pH value is higher than 8, resulting in stronger repulsion force between these ions and higher osmotic pressure, so that the ESR increase a little at last. With the addition of more Au-S-S NPs or clay nanosheets, the increased crosslink density and smaller pores making lower ESR in general.
3.4. SEM images Firstly, the synthesized hydrogel samples were immersed into DI water for two weeks (change the water three times a day), then they were freeze dried and their internal cross-section morphology was characterized by ESEM. As shown in Fig. 5a–h, it is clearly to see that pores in the hydrogels are uniformly dispersed and their diameter is generally at the micrometer level. Besides, in contrasting with PcH and PcCH, plenty of smaller pores appear in the big pores in PcNCH with the incorporation of Au-S-S NPs. As more Au-S-S NPs or clay added, pores 184
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Fig. 5. SEM images of the synthesized hydrogels. (a) Pc0.076H, (b) Pc0.076C10H, (c) Pc0.076N0.784C10H, (d) Pc0.076N1.372C10H, (e) Pc0.076N1.372C5H, (f) Pc0.076N1.372C12.5H, (g) Pc0.038N1.372C12.5H, (h) Pc0.095N1.372C12.5H.
3.6. Swelling-shrinking behaviors of hydrogels
Table 1 Mechanical property of the synthesized hydorgels. Hydrogel samples
Failure strength (kPa)
Failure strain (%)
Elastic modulus (kPa)
Pc0.076H Pc0.076C10H Pc0.076N0.392C10H Pc0.076N0.784C10H Pc0.076N0.98C10H Pc0.076N1.176C10H Pc0.076N1.372C10H Pc0.076N1.568C10H Pc0.076N1.372C5H Pc0.076N1.372C12.5H Pc0.076N1.372C15H Pc0.038N1.372C12.5H Pc0.095N1.372C12.5H Pc0.114N1.372C12.5H Pc0.152N1.372C12.5H
198.9 223.2 271.2 341.3 368.4 418.3 448.1 354.1 376.8 490.8 399.7 327.2 535.5 501.3 369.1
2632.7 2697.6 2524.3 2297.3 2189.7 2141.1 2108.9 1623.2 2142.6 2056.4 1786.3 2205.3 1579.5 1642.1 1131.1
1.9 2.3 2.4 2.4 2.5 2.8 2.9 3.5 2.6 3.1 3.2 2.4 6.0 9.1 11.2
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.5 3.7 4.1 3.8 4.9 4.7 5.1 4.6 4.8 5.2 5.5 3.8 5.6 5.0 3.9
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.5 5.3 4.8 5.6 4.3 3.9 4.4 3.9 4.6 4.8 3.9 3.7 3.5 3.2 3.6
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
As illustrated in Fig. 7, swelling-shrinking behaviors of PcH, PcCH and PcNCH were performed by altering the water bath temperature between 25 and 50 ℃. Firstly, the dried hydrogel samples were put into the 25 ℃ water bath for 48 h and then these swellen hydrogels were altered into 50 ℃ water bath for 24 h. Successively, this operation was repeated for another two times. It is obviously to see that after swelling in 25 ℃ water bath for 48 h (the 3rd), the PcNCH samples immersed in 50 ℃ water bath for 24 h (the 2nd) could recover nearly 100% to its last swelling ratio in 25 ℃ water bath (2nd), however, the PcH and PcCH couldn’t. The predominate reason for these results may be that the incorporation of Au-S-S NPs is beneficial for improving the stability of the hydrogels’ inner structure, which can also be clearly seen in the hydrogel pores in Fig. 5c.
0.1 0.3 0.2 0.2 0.3 0.2 0.3 0.4 0.3 0.2 0.3 0.3 0.4 0.3 0.5
3.7. Drug absorption and release tests Fig. 8a displays the absorption capacity of ASA in different synthesized hydrogel samples as a function of contact time. It is obviously to see that the absorbed rate increases dramatically at the beginning of the test, and then decreases gradually until a final plateau was achieved. Evidently, with the incorporation of more Au-S-S NPs, CMCTs or clay, the maximum absorption capacity (qm) of ASA in the samples increases. The mainly reason for this regular pattern may that although the crosslink density in the hydrogels increases with the addition of more the above additive, there are lots of oxygen-containing groups in these additive, which have strong affinity to the ASA molecules, leading to the higher qm values. The release test of ASA-loaded hydrogel samples was executed at 37 ℃ in PBS solution, detecting the released drug content as a function of time. As shown in Fig. 8b, it is easily to find out that the ASA-loaded hydrogel samples exhibited a fast release rate in the beginning 2 h. Subsequently, the hydrogel samples exhibited a relative slow and steady release of ASA into the medium. The initial abrupt release of ASA may attributed to the free ASA on the surface or in the hydrogels, and the followed slower release speed mainly related with the physical interaction between the polymer chains, clay and ASA. As displayed in Table 2, with the incorporation of more Au-S-S NPs or clay, the maximum cumulative release percentage decrease, which is predominately
However, as more CMCTs added in, ESR value decreases at first and then increases to some extent. The mainly reason for this phenomenon may that with the incorporation of more CMCTs, the physical interaction between the polymer chains increases, resulting in smaller ESR value; along with the CMCTs continue increases, large amounts of strong water-absorbable oxygen-containing groups are added in, leading to a higher ESR value. Fig. 6d–f displays the swelling behaviors of PcNCH samples at different temperatures in DI water. As the temperature increases, the hydrogels samples’ weight decreases, and particularly, the weight percent decreases seriously at the volume phase transition temperature (VPTT). It is important to point out that with the incorporation of more Au-S-S NPs or clay, weight percent of the hydrogel samples decrease slower (T = 31–34 ℃), leading to smaller slope values; however, with the addition of more CMCTs content, the slope values increase. The proper interpretation for the above phenomena may that as more Au-S-S NPs or clay added in, the increased crosslink density in the hydrogels restricts the movement of the polymer chains and results in a higher VPTT; with the introduction of more CMCTs, these hydrophilic polymer chains promote the inner water's movement, leading to a higher waterloss speed and a lower VPTT value (Luo et al., 2017) 185
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Fig. 6. The swelling behaviors of synthesized hydrogels with different content of (a) Au-S-S NPs, (b) CMCTs and (c) clay in different pH values, and with different content of (d) Au-S-S NPs, (e) CMCTs and (f) clay in different temperatures. Table 2 The maximum absorption capacity (qm) and cumulative release percentage of the hydrogel samples.
Fig. 7. Swelling and shrinking behavior of the prepared hydrogels samples with different content of Au-S-S NPs, CMCTs and clay by altering the temperature between 25 and 50 ℃.
Samples
qm/μmol g−1
Maximum cumulative release percentage/%
Maximum cumulative release/μmol g−1
Pc0.076H Pc0.076C10H Pc0.076N0.784C10H Pc0.076N1.372C10H Pc0.076N1.372C5H Pc0.076N1.372C12.5H Pc0.038N1.372C12.5H Pc0.095N1.372C12.5H
75.07 76.95 81.92 91.58 90.97 92.81 62.49 93.55
82.43 78.71 70.96 68.90 63.39 66.50 86.64 52.86
61.54–62.22 60.23–60.90 57.80–58.46 62.74–63.46 57.35–57.99 61.40–62.04 53.81–54.47 49.15–49.76
± ± ± ± ± ± ± ±
0.27 0.29 0.26 0.24 0.23 0.25 0.24 0.31
± ± ± ± ± ± ± ±
0.16 0.14 0.18 0.21 0.19 0.17 0.20 0.15
associated with the more crosslink density in the hydrogels; however, with the addition of more CMCTs, the percentage value increases at first and then decreases, the most probable reason is that the interaction between ASA and CMCTs increases as more CMCTs added in, when the interaction is too strong, resulting in a relative smaller percentage value. From the above description, we can concluded that the ASA
Fig. 8. The absorption capacity (a) and the cumulative release (b) of the prepared hydrogels samples with different concentration of Au-S-S NPs, CMCTs and clay. 186
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release speed and the maximum cumulative release can be controlled by adjusting the content of the raw materials, which indirectly reveals that these designed hydrogel could be used as the controlled drug delivery carries in the biomedical applications in future. Taken tighter, the synthesized tough robust hydorgels in this paper not only have the good recoverability and evidently dual pH- and temperature- sensitive property and, but also can be used as the drug delivery carriers. These advantages make them suitable for the artificial muscle, chemical values actuators, drug delivery systems and biological sensors in the future. Because the reaction temperature, reaction time and the mole ratio of BACA/Au NPs may play a key role in enhancing the tensile strength of the prepared hydrogels, it is intriguing to some more work by adjusting these influence factors to obtain much more strong hydrogels.
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4. Conclusion In summary, we synthesized a novel tough robust biocompatible and dual pH- and temperature- responsive PNIPAm/clay/Au-S-S/ CMCTs NC hydrogels in this paper through a one pot in situ free radical polymerization using Laponite XLS and Au-S-S NPs as the cross-linkers. By modulating the pivotal influence factors, the synthesized hydrogels displayed the highest tensile stress of 535.5 kPa at the breaking strain of 1579.5%. These hydrogels could dissipate a great deal of energy in the stretching process and recover 91.94% of the 1st loop energy in 15 min under the NIR light at room temperature after unloaded. Furthermore, the synthesized hydrogels have a remarkably dual pH- and temperature- responsive property, and a good swelling-shrinking property. Additionally, the designed hydrogels also have a good controlled drug release property of asprin in the vitro release test by tuning their inner crosslink density. This valuable property will enable these fabricated hydrogels as potential drug delivery carriers in future. Acknowledgement This work was financially supported by Shanghai International S&T Cooperation Fund (16160731302), National Natural Science Foundation of China (No. 51473031) and Natural Science Foundation of Shanghai (Grant no. 17ZR1401100). Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Author contributions Y.C. and S.K. contributed equally. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jmbbm.2019.01.017. References Agarwal, T., Narayan, R., Maji, S., et al., 2016. Gelatin/carboxymethyl chitosan based scaffolds for dermal tissue engineering applications. Int. J. Biol. Macromol. 93, 1499–1506. Beebe, D.J., Moore, J.S., Bauer, J.M., et al., 2000. Functional hydrogel structures for autonomous flow control inside microfluidic channels. Nature 404, 588. Chen, Q., Zhu, L., Huang, L., et al., 2014. Fracture of the physically cross-linked first network in hybrid double network hydrogels. Macromolecules 47, 2140–2148. Chen, Q., Wei, D., Chen, H., et al., 2015. Simultaneous enhancement of stiffness and toughness in hybrid double-network hydrogels via the first, physically linked network. Macromolecules 48, 8003–8010. Chen, Y., Zhuang, L., Chao, Y., et al., 2015a. Poly(N-isopropylacrylamide)-clay nanocomposite hydrogels with responsive bending property as temperature-controlled
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