Customizing polyelectrolyte complex shapes through photolithographic directed assembly.

Polyelectrolyte complexes (PECs) form through the association of oppositely charged polymers and, due to their attractive properties, such as their mild/simple preparation and stimulus-sensitivity, attract widespread interest. The diverse applications of these materials often require control over PEC shapes. As a versatile approach to achieving such control, we report a new photolithographic directed assembly method for tailoring their structure. This method uses aqueous solutions of a polyelectrolyte, an oppositely charged monomer and a photoinitiator. Irradiation of these mixtures leads to site-specific polymerization of the ionic monomer into a polymer and, through this localized polyanion/polycation mixture formation, results in the assembly of PECs with 2-D and 3-D shapes that reflect the photoirradiation pattern. In addition to generating macroscopic PECs using photomasks, this photodirected PEC assembly method can be combined with multiphoton lithography, which enables the preparation of custom-shaped PECs with microscopic dimensions. Like other PECs, the custom-shaped structures formed through this photodirected assembly approach are stimulus-responsive, and can be made to switch shape or dissolve in response to changes in their external environments. This control over PEC shape and stimulus-sensitivity suggests the photopolymerization-based directed PEC assembly method as a potentially attractive route to stimulus-responsive soft device fabrication (e.g., preparation of intricately shaped, function-specific PECs through photolithographic 3-D printing).

Photolithographically assembled polyelectrolyte complexes as shape-directing templates for thermoreversible gels.

Preparation of soft materials with diverse, customized shapes has been a topic of intense research interest. To this end, we have recently demonstrated photolithographic directed assembly as a strategy for customizing polyelectrolyte complex (PEC) shape. This process uses in situ photopolymerization of an anionic monomer in the presence of a cationic polymer, which drives localized PEC formation at the irradiation sites. Here, we show how such photolithographically assembled PECs can serve as structure-directing templates for tailoring the shapes of other soft materials; namely, thermoreversible gels. These templated hydrogels are prepared by adding a thermogelling polymer (agarose) to the anionic monomer/cationic polymer/photoinitiator precursor solutions so that, upon irradiation, custom-shaped PECs form within agarose gel matrices. Once these PECs are formed, the surrounding agarose gels are melted (through heating) and washed away which, upon returning the samples to room temperature, produces interpenetrating PEC/agarose gel networks with photopatterned shapes and dimensions. Dissolution of these sacrificial PEC templates in concentrated NaCl solutions then generates photolithographically templated agarose gels, whose shapes and dimensions match those of their PEC templates. Besides tuning their shapes and sizes, the mechanical properties of these gels can be easily tailored by varying the initial agarose concentrations used. Moreover, this PEC-templated gel synthesis appears to not adversely affect hydrogel cytocompatibility, suggesting its potential suitability for biological and biomedical applications. Though the present study uses only agarose as the model gel system, this PEC-based strategy for customizing gel shape can likely also be applied to other thermoreversible gel networks (e.g., those based on methylcellulose, poloxamers or thermoresponsive chitosan derivatives) and could have many attractive applications, ranging from drug delivery and tissue engineering, to sensing and soft robotics.