Browsing by Subject "Thiol-ene"

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    Facile preparation of photodegradable hydrogels by photopolymerization
    (Elsevier, 2013) Ki, Chang Seok; Shih, Han; Lin, Chien-Chi; Biomedical Engineering, Purdue School of Engineering and Technology
    Photodegradable hydrogels have emerged as a powerful material platform for studying and directing cell behaviors, as well as for delivering drugs. The premise of this technique is to use a cytocompatible light source to cleave linkers within a hydrogel, thus causing reduction of matrix stiffness or liberation of matrix-tethered biomolecules in a spatial-temporally controlled manner. The most commonly used photodegradable units are molecules containing nitrobenzyl moieties that absorb light in the ultraviolet (UV) to lower visible wavelengths (~280 to 450 nm). Because photodegradable linkers and hydrogels reported in the literature thus far are all sensitive to UV light, highly efficient UV-mediated photopolymerizations are less likely to be used as the method to prepare these hydrogels. As a result, currently available photodegradable hydrogels are formed by redox-mediated radical polymerizations, emulsion polymerizations, Michael-type addition reactions, or orthogonal click chemistries. Here, we report the first photodegradable poly(ethylene glycol)-based hydrogel system prepared by step-growth photopolymerization. The model photolabile peptide cross-linkers, synthesized by conventional solid phase peptide synthesis, contained terminal cysteines for step-growth thiol-ene photo-click reactions and a UV-sensitive 2-nitrophenylalanine residue in the peptide backbone for photo-cleavage. Photolysis of this peptide was achieved through adjusting UV light exposure time and intensity. Photopolymerization of photodegradable hydrogels containing photolabile peptide cross-linkers was made possible via a highly efficient visible light-mediated thiol-ene photo-click reaction using a non-cleavage type photoinitiator eosin-Y. Rapid gelation was confirmed by in situ photo-rheometry. Flood UV irradiation at controlled wavelength and intensity was used to demonstrate the photodegradability of these photopolymerized hydrogels.
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    Interfacial thiol-ene photo-click reactions for forming multilayer hydrogels
    (ACS, 2013) Shih, Han; Fraser, Andrew K.; Lin, Chien-Chi; Biomedical Engineering, Purdue School of Engineering and Technology
    Interfacial visible light-mediated thiol-ene photoclick reactions were developed for preparing step-growth hydrogels with multilayer structures. The effect of a noncleavage type photoinitiator eosin-Y on visible-light-mediated thiol-ene photopolymerization was first characterized using in situ photorheometry, gel fraction, and equilibrium swelling ratio. Next, spectrophotometric properties of eosin-Y in the presence of various relevant macromer species were evaluated using ultraviolet-visible light (UV-vis) spectrometry. It was determined that eosin-Y was able to reinitiate the thiol-ene photoclick reaction, even after light exposure. Because of its small molecular weight, most eosin-Y molecules readily leached out from the hydrogels. The diffusion of residual eosin-Y from preformed hydrogels was exploited for fabricating multilayer step-growth hydrogels. Interfacial hydrogel coating was formed via the same visible-light-mediated gelation mechanism without adding fresh initiator. The thickness of the thiol-ene gel coating could be easily controlled by adjusting visible light exposure time, eosin-Y concentration initially loaded in the core gel, or macromer concentration in the coating solution. The major benefits of this interfacial thiol-ene coating system include its simplicity and cytocompatibility. The formation of thiol-ene hydrogels and coatings neither requires nor generates any cytotoxic components. This new gelation chemistry may have great utilities in controlled release of multiple sensitive growth factors and encapsulation of multiple cell types for tissue regeneration.