Rapid Accessible Fabrication and Engineering of Bilayered Hydrogels: Revisiting the Cross-Linking Effect on Superabsorbent Poly(acrylic acid).

Kyoung Min Lee,Yuree Oh,Doyoung Jung,Hyemin Lee,Hyungwoo Kim,Hea Ji Kim,Ji Young Chang,Changsun Han

doi:10.1021/acsomega.8b00079

Kyoung Min Lee, Yuree Oh + Show 6 more

Open Access

https://doi.org/10.1021/acsomega.8b00079

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Abstract

Superabsorbent hydrogels are significant not only in materials science but also in industries and daily life, being used in diapers or soil conditioners as typical examples. The main feature of these materials is their capacity to hold considerable amount of water, which is strongly dependent on the cross-linking density. This study focuses on the preparation of hydrogels by reweighing the effect of cross-linking density on physical properties, which provides green fabrication of bilayered hydrogels that consist of homogeneous structural motifs but show programmed responses via sequential radical polymerization. In particular, when two hydrogel layers containing different cross-linking densities are joined together, an integrated linear bilayer shows heterogeneous deformation triggered by water. We monitor the linear hydrogel bilayer bending into a circle and engineer it by incorporating disperse dyes, changing colors as well as physical properties. In addition, we demonstrate an electric circuit switch using a patterned hydrogel. Anisotropic shape change of the polyelectrolyte switch closes an open circuit and lights a light-emitting diode in red. This proposed fabrication and engineering can be expanded to other superabsorbent systems and create smart responses in cross-linked systems for biomedical or environmental applications.

Highlights

Since poly(2-hydroxyethyl methacrylate) was reported to form a hydrogel in the presence of a cross-linker,[1] a myriad of hydrogels have been widely researched for biomedical and environmental purposes
Di(ethylene glycol) acrylate and potassium persulfate (KPS) were added to provide a ready-topolymerize solution, which was transferred by a syringe to a glass template having a rectangular space inside (11.5 cm × 15 cm × 0.1 cm) for polymerization
The template had a high ratio of surface area to volume and could facilitate efficient heat transfer, which is similar to controlling autoacceleration process in the sheet-type mold

Summary

Introduction

Since poly(2-hydroxyethyl methacrylate) was reported to form a hydrogel in the presence of a cross-linker,[1] a myriad of hydrogels have been widely researched for biomedical and environmental purposes. Superabsorbent hydrogels (SHs) confine considerable amount of water into a threedimensional, cross-linked network, which is typically greater than hundreds to thousands times their dry weight, and swell while maintaining the original shape.[2−4] Acrylic acid (or acrylate) and acrylamide are extensively used as monomers, along with other monomers such as N-isopropylacrylamide, N,N-diethylacrylamide, and more interestingly, bioextractable sodium 4-hydroxy-2-methylenebutanoate[5] or polymers including poly(vinyl alcohol), which are further cross-linked covalently or noncovalently in situ or ex situ.[2,6] When exposed to aqueous conditions, hydrophilic groups in the polymer network are primarily hydrated by water molecules, and the additional water can be absorbed via capillary force and osmotic pressure for filling the space inside the network Such wet materials essentially possess small yet functionally crucial cross-linking density, which leads to water absorbency and modulates the physical properties of the entire material on demand, for example, elasticity or swelling ratio. From the perspective of polymeric materials, swellable materials have been used as elastomers, watery matrices, containers, supports, and films, which can be further applied in many fields such as soil conditioning, tissue

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