Nanostructured gradient gels with unique bending properties due to their deswelling characteristics are presented. These gradient gels are readily fabricated via electrophoresis followed by photo‐polymerization, and subsequent silica extraction. Differences in the physical properties between both sides of the gradient gels are the driving force behind the bending of the gels.
We propose a new approach to fabricate reversible self-bending actuators utilizing a photo-triggered pH jump reaction. A photo-initiated proton-releasing agent of o-nitrobenzaldehyde (NBA) was successfully integrated into bilayer hydrogels composed of a polyacid layer, poly(Nisopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) and a polybase layer, poly(N-isopropylacrylamide-co-N,N 0 -dimethylaminopropylacylamide) (P(NIPAAm-co-DMAPAAm)), where the adhesion of both layers was achieved via electrophoresis of semiinterpenetrating polyelectrolyte chains. The NBA-integrated bilayer gels demonstrated quick proton release upon UV irradiation, allowing the pH within the gel to decrease below the volume phase transition pH in 30 seconds. By controlling the NBA concentration and the gel thickness, the degrees and the kinetics of bending were easily controlled. Reversible bending was also studied with respect to the NBA concentration in response to 'on-off' UV irradiation. Additionally, self-bending of the non-UV irradiated region of the gel was also achieved because the generated protons gradually diffused toward the non-irradiated region. The proposed system can be potentially applied in the fields of mechanical actuators, controlled encapsulation and drug release, robotics and microfluidic technologies because control over autonomous motion by both physical and chemical signals is essential as a programmable system for real biomedical and nano-technological applications.
Surface-modified hydrogels with a polyion complex composed of poly(vinylamine) (PVAm) and poly(acrylic acid) (PAAc) were prepared in order to control the release of drug molecules without a volume change of the hydrogel. We first prepared a poly(N-vinylacetamide)-co-poly(N-vinylformamide) (poly(NVA-co-NVF)) hydrogel, and then used a hydrolysis reaction to produce a cationic PVAm layer on the surface of the hydrogel. The polymerization of AAc to the surface-cationized hydrogel resulted in a hydrogel that possesses a polyion complex (PIC) of PVAm and PAAc only on the surface. This surface-PIC hydrogel (sPIC gel) could suppress the release of a model drug (Fluorescein isothiocyanate labeled Dextran, M w = 9500) under neutral pH conditions because of the tight PIC surface layer, and repeatedly controlled the drug release against the pH conditions depending on the formation and dissociation of PIC. Controlled release was achieved without a large volume change, because the PIC layer was thin enough to maintain the original size of the hydrogel. Furthermore, the sPIC gel retained a larger amount of model drug as compared to the PIC gel, which possesses the polyion complex from the surface to the inside of the hydrogel. Consequently, the surface-modified hydrogel with PIC, that is sPIC gel, is useful for controlled drug delivery systems that require a constant volume and large drug loading.
The adhesion of stimuli-responsive hydrogels was achieved via electrophoresis. The adhered gels were quite stable in water during repetitive swelling and shrinking processes, respectively.
Rapid self-healable and biocompatible hydrogels were prepared using the selective formation of metal-ligand interactions between selected metal ions and phosphate end groups of poly(ethylene glycol) (PEG). The phosphate-terminated branch of PEG was synthesized via a substitution reaction of the hydroxyl end groups using phosphoryl chloride. The gelation and gel properties including rheological properties can be tuned by the careful selection of metal ions, branch numbers, and temperature. Especially, the gels rapidly formed by trivalent metal ions such as Fe(3+), V(3+), Al(3+), Ti(3+), and Ga(3+) have relatively small ionic radii. The ligand substitution rates also affected the repeatable autonomic healing ability. We have also demonstrated a gel-sol/sol-gel transition by switching the redox states of Fe(3+)/Fe(2+) ions. Learning from biological systems, the proposed phosphate-metal ion based self-healable hydrogels could become an attractive candidate for various biomedical and environmental applications.
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