Photoreversible Covalent Hydrogels for Soft-Matter Additive Manufacturing

Abstract

Reversible covalent chemistry provides access to robust materials with the ability to be degraded and reformed upon exposure to an appropriate stimulus. Photoresponsive units are attractive for this purpose, as the spatial and temporal application of light is easily controlled. Coumarin derivatives undergo a [2 + 2] cycloaddition upon exposure to long-wave UV irradiation (365 nm), and this process can be reversed using short-wave UV light (254 nm). Therefore, polymers cross-linked by coumarin groups are excellent candidates as reversible covalent gels. In this work, copolymerization of coumarin-containing monomers with the hydrophilic comonomer N, N-dimethylacrylamide yielded water-soluble, linear polymers that could be cured with long-wave UV light into free-standing hydrogels, even in the absence of a photoinitiator. Importantly, the gels were reverted back to soluble copolymers upon short-wave UV irradiation. This process could be cycled, allowing for recycling and remolding of the hydrogel into additional shapes. Further, this hydrogel can be imprinted with patterns through a mask-based, post-gelation photoetching method. Traditional limitations of this technique, such as the requirement for uniform etching in one direction, have been overcome by combining these materials with a soft-matter additive manufacturing methodology. In a representative application of this approach, we printed solid structures in which the interior coumarin-cross-linked gel is surrounded by a nondegradable gel. Upon exposure to short-wave UV irradiation, the coumarin-cross-linked gel was reverted to soluble prepolymers that were washed away to yield hollow hydrogel objects.

The Role of Photochemical Reactions in the Development of Advanced Soft Materials for Biomedical Applications

“…This endeavor presents further challenges but also opportunities since it open the possibility of reversibly modifying hydrogels, following a multistep procedure or real‐time alteration to adapt it to changes required. In order to meet these demands and tackling the incorporation of coupling and cleavage in a single design, [2 + 2] reversible cycloaddition of coumarin derivatives have been at the center of such efforts . The reaction occurs via a dimerization process upon 365 nm irradiation that is reversed by irradiation with 254 nm light, cleaving the coumarin dimer to regenerate the coumarin monomer ( Figure a).…”

“…(Photographs in panels (e) and (f) show the last step.) Adapted with permission . Copyright 2018, American Chemical Society.…”

Supramolecular Competitive Host–Guest Interaction Induced Reversible Macromolecular Metamorphosis

“…The reversibility of the change in macromolecular architecture can also be important for adaptation in response to environmental changes, and draws the attention of researchers around the world. Different conditions like UV irradiation and heat have been used to attain structural reversibility . However, achieving architectural reversibility in the context of macromolecular metamorphosis still remains challenging.…”

“…Different conditions like UV irradiation and heat have been used to attain structural reversibility. [9] However, achieving architectural reversibility in the context of macromolecular metamorphosis still remains challenging. In this scenario, the reversibility of supramolecular chemistry (noncovalent interaction) paves the way for reversibly changing the polymer architecture.…”

Nanohydrogels Based on Self-Assembly of Cationic Pullulan and Anionic Dextran Derivatives for Efficient Delivery of Piroxicam

“…The obtained system was efficiently taken up by cells and was shown to be biocompatible.Pharmaceutics 2019, 11, 622 2 of 16 bioactive compound (e.g., low molecular weight drugs, proteins, DNA or RNA) and release it in a controlled way [5][6][7][8]. In order to adjust the release profile of a bioactive substance to the needs of the therapy the ability of nanogels to respond to the microenvironmental stimuli such as light, magnetic field, changes in ionic strength, pH, temperature, or redox potential, are often exploited [9][10][11][12][13]. For example, sodium alginate was combined with N-isopropylacrylamide to obtain a dual-responsive system for oxytetracycline delivery, where the drug release could be effectively controlled by both pH and temperature [14].…”