The synthesis and simulations of solvent-responsive bilayer hydrogel
Graphical abstract
Introduction
Polymeric gels [1,2], which are three-dimensional polymeric network hydrogels, have received lots of attention because of their substantial volume change in response to the external stimuli such as solvent, temperature, electric field, or light [[3], [4], [5]]. Compared to polymer-based nanoparticles or microparticles and polymer films, hydrogels exhibit a simple preparation process, good biocompatibility, and excellent application characterizations (mechanical properties, structure stability, and swelling-deswelling properties) [[3], [4], [5], [6]].
The ability of hydrogels to serve as artificial muscles, drug-delivery devices, chemical valves, actuators, and magnetic resonance monitoring agents has been widely explored [[7], [8], [9]], especially for bilayer hydrogels consist one positive layer and one negative layer with different swelling ratios [[10], [11], [12]]. Compared to single layer hydrogel, bi-layer hydrogels exhibit higher similarity with nature creatures. Trough dispersing responsive properties into two layers rather than one layer makes bi-layer hydrogel exhibiting extended adaptability and applicability. Cheng et al. [13] designed the poly(N-isopropylacrylamide) and poly(2-(dimethylamino) ethyl methacrylate) based bilayer hydrogel actuator to be used as circuit switch. The bilayer actuators exhibited anisotropic swelling characteristics with repeatable and reversible shape memory ability under temperature and pH stimulus, which also showed potential applications in environmental and biomedical areas. Wei et al. [14] prepared bilayer hydrogel catalyst with two different temperature responses layers. The smart catalyst exhibited catalyzation reaction under temperature higher than 50 °C, which can be applied for controlling tandem reactions. Zhang et al. [15] formulated the poly (N-isopropylacrylamide) (PNIPAm) based bilayer hydrogel actuators to be used as sensors, wearable devices, medical devices, and bio-mimetic robotics. The bilayer actuators demonstrated reversible solvent and thermo-responsive performance, photo induced color changing behaviors, and fluorescent triggered shape deformation characteristic. There are also several excellent review papers summarized temperature responsive bilayer hydrogel [10], biomimetic bilayer gel [16], and double network hydrogels with controlled shape deformation [17].
The smart bi-layer hydrogels have been widely applied in different fields [[18], [19], [20], [21], [22]], because they are similar to natural analogs, such as tendrils, bracts, leaves, and flowers, which can respond to environmental stimuli [23,24]. Based on their swelling nature, the volume of hydrogels can be changed by three orders of magnitude in response to an external stimulus such as pH, irradiation, electrical potential, light, solvent type, or temperature [25,26]. The functional materials determining the stimuli-responsive performance of hydrogels are polymers and functional matters. The mechanism of solvent type stimuli-responsive hydrogels is the effect of swelling and water absorbency as well as diffusion and osmosis, which lead to volume variations [[27], [28], [29], [30]]. The mentioned mechanism of swelling and shrinking of hydrogels is attributed to the water and ion affinity functional groups, for example –COOH, –OH, –SO3H, CONH, and –CONH2 [27,31]. The commonly used functional polymers include poly(N-isopropyl acrylamide) (PNIPAm), poly(vinyl alcohol) (PVA), poly(hydroxyethyl methacrylate) (PHEMA), poly(methyl methacrylate) (PMMA), polyacrylamide (PAAm), poly(vinyl acetate) (PVAc), poly (ethylene glycol) dimethacrylate (PEGDMA), and poly(acrylic acid) (PAA) [[32], [33], [34], [35]]. Among these, PAAm is a lightweight and economical polymer with excellent light transmission, swelling ability, and high resistance to UV light and weathering [32,36]. Interestingly, PAAm also has poor resistance to solvent and temperature [37], which makes it an excellent material for solvent-responsive hydrogels.
In this paper, a bi-layer hydrogel was developed via casting method and photopolymerization. Based on the swelling difference of the two layers under external stimuli, the bi-layer hydrogel bent dramatically with solvent transformation due to internal stress. In addition, simulation of bi-layer hydrogel was conducted using ANSYS software to visualize the bending deformation of the bi-layer hydrogel. Importantly, the potential of bi-gel to be used as gel switchers and cross structured bi-gel was demonstrated. Theoretically and practically, the bi-gel designing method prepared in this manuscript exhibits high market potential.
Section snippets
Materials
The sodium alginate (Protanal® LF 120 LS, pH of 5.0–7.5 at 1% solution, synthetic) was kindly supplied by the FMC biopolymer (Drammen, Norway). Agar, acrylamide (for synthesis, a molecular weight of 71.08 g/mol), N, N′-Methylenebis acrylamide (MBAA, 99%, a molecular weight of 154.17 g/mol), 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (irgacure 2959, 98%), and calcium chloride (anhydrous, powder, ≥97%) were purchased from Sigma-Aldrich (Saint Louis, MO).
Fabrication of bilayer gels
As shown in Scheme 1, Sodium
Single layer and bilayer hydrogels
The bilayer gels will be fabricated by a simple casting method and photopolymerization, as shown in Scheme 3(a). Structurally, the achieved hydrogels had an internally heterogeneous structure consisting two different layers. The first layer is made of Ag/CA hydrogel, and second layer is made of Ag/CA/PAAm hydrogels. Due to swelling and deswelling ability difference between these two layers, a driving force is produced during swelling or deswelling process leading to bend of bi-layer hydrogel.
Conclusion
In summary, bilayer gel actuators were successfully achieved via a simple casting method, sensitive to solvent stimuli. The Ag/CA/PAAm gels dramatically de-swell in acetone/water mixture and swell in DI water, whereas the Ag/CA gels were relatively stable in both acetone/water mixture and DI water. The driving force for the de-swell of hydrogel in acetone/water mixture is due to the van der Waals interaction. Comparatively, the swell in DI water is because of the capability of PAAm to form
CRediT authorship contribution statement
Jilong Wang: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing - original draft. Xintian Zhang: Formal analysis, Data curation, Investigation, Methodology. Ao Wang: Formal analysis, Data curation, Investigation, Methodology. Xuefeng Hu: Formal analysis, Data curation, Investigation, Methodology. Linfeng Deng: Formal analysis, Data curation, Investigation, Methodology. Lihua Lou: Supervision,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the financial support from the Fundamental Research Funds for the Central Universities (No. 2232020G-01, 2232019D3-15, and 2232019D3-12) and the National Natural Science Foundation of China (No.51903034).
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