PEG-based cross-linked films with aligned channels: combining cryogenic processing and photopolymerization for the design of micro-patterned oriented platforms

Abstract

Poly(ethylene glycol)-based cross-linked films with aligned micrometric channels were obtained by applying ice-templating processing and cryo-photopolymerization to aqueous solutions containing a methacrylate monomer and a visible light photo-initiator system. Aqueous solutions containing poly(ethylene glycol) dimethacrylate (PEG-dma), camphorquinone (CQ) and ethyl-4-dimethyl aminobenzoate (EDMAB) were cast between glass slides and unidirectionally frozen (horizontally) by imposing a temperature gradient along the ends of the sample holder, keeping one of the sample ends at sub-zero temperature and the other one at room temperature. Immediately after freezing, samples were cryo-photopolymerized and air-dried for obtaining patterned films with micrometric channels aligned in the freezing direction. Crosslinking enabled producing polymer films with high mechanical and chemical stability that did not dissolve or collapse by contact with solvents, allowing efficient flow of solutions along their oriented micro-structure. Due to the high anisotropy of the topography, flow was clearly unidirectional, as determined from microscopic observation of the liquid front movement after drop seeding, an effect absent in non-patterned films prepared from the same precursors but under isotropic freezing conditions. Aqueous solutions perfused the films forming a unidirectional front that advanced very fast along the freezing direction whereas hydrophobic solutions limited their flow to well-defined channels. Addition of nanostructures to the initial aqueous formulations allowed easy transferring of photothermal response to the aligned porous platforms. Through this strategy, remote localized heating of the films was attained by using a laser beam, which could be used to enhance the potentiality of these materials as chemical micro-reactors, responsive scaffolds and/or advanced microfluidic platforms.