Direct-Write Assembly of 3D Hydrogel Scaffolds for Guided Cell Growth

The ability to pattern soft materials at the microscale is critical for several emerging technologies, including tissue-engineering scaffolds, photonic crystals, sensors, and self-healing materials. Hydrogels are an important class of soft materials that can be fabricated in the form of 3D microperiodic structures by colloidal templating or interference lithography. However, neither approach allows one to omnidirectionally vary the spacing between patterned features over length scales ranging from sub-micrometer to tens of micrometers. By contrast, direct-write assembly enables a wide array of materials to be patterned in nearly arbitrary shapes and dimensions. Here, we report the fabrication of 1D and 3D microperiodic hydrogel scaffolds by direct-write assembly of an acrylamide-based ink. For the first time, we combine direct ink writing with in situ photopolymerization to obtain hydrogel scaffolds with micrometer-sized features (see Fig. 1). By plating 3T3 murine fibroblasts onto one-, two-, and four-layer hydrogel scaffolds, we demonstrate their cytocompatibility and, hence, potential suitability for tissue-engineering applications. Direct ink writing (DIW) is a layer-by-layer assembly technique in which materials are patterned in both planar and 3D forms with lateral feature sizes that are at least an order of magnitude smaller than those achieved by ink-jet printing and other rapid prototyping approaches, and nearly comparable in size to those produced by two-photon polymerization and interference holography. Central to our approach is the creation of concentrated inks that can be extruded through fine deposition nozzles in filamentary form, and then undergo rapid solidification to maintain their shape even as they span gaps in the underlying layer(s). Unlike prior efforts on polyelectrolyte inks that required reservoir-induced coagulation to enable 3D printing, we report the creation of hydrogel inks that can be printed directly in air, where they undergo solidification via photopolymerization (see Fig. 1a and b). The ink is created by first mixing monomeric acrylamide, glycerol, and water. Upon ageing for several hours under ambient conditions, the monomeric species polymerizes to yield a gel composed of 30w/o polyacrylamide chains. H NMR reveals that peaks associated with acrylamide, which are initially present, disappear after polymerization, followed by the emergence of two new peaks that correspond to alkyl chains (data not shown). Concomitantly, as the solution ages, sharp rises in both the shear elastic, G0, and loss, G00, moduli are observed, suggesting that the resulting gel is composed of physically entangled polyacrylamide chains (see Fig. 2a). To determine their degree of polymerization, N, the intrinsic viscosity, [h]0, of diluted polymer solutions is measured by capillary viscometry, and found to be [h]0 270mL g 1 (see Fig. 2b). Using the Mark– Houwink relation, h 1⁄2 01⁄4 KM, their molecular weight is determined to be 8.9 10 gmol , where K is 9.3 10 3 and a is taken to be 0.75 for polyacrylamide dissolved in an aqueous solution (0.5M NaCl). Hence, this initial polymerization process yields polyacrylamide chains with an average degree of polymerization N1⁄4 1.3 10 that is well above the entanglement value of Ne1⁄4 128. To further optimize the ink for direct-write assembly, this polymerizedmixture is diluted by addingmonomeric acrylamide, a crosslinking agent, N, N methylene bisacrylamide, a photoinitiator, diethoxyacetophenone, and deionized water at weight ratios (w/w) of 0.480, 0.036, 0.004, and 0.480, respectively. Notably, the initial polymerization step could be eliminated simply by adding high-molecular-weight polyacrylamide chains