4D Printing of Hydrogels with Hiearchical Surface Wrinkling Patterns

Additive manufacturing of stimuli responsive materials leads to 3D printed constructs that can adapt their properties in response to an external stimuli. This process is referred as 4D printing. 4D printing is commonly used to fabricate shape-shifting constructs, which change their asprinted shape into a preprogrammed shape post-printing with response to an external stimuli. Although 4D printing of shape-shifting objects have significantly expand the applications of 3D printing, novel ink formulations are needed for 4D printing to create 2D and 3D structures that allow dynamic control of properties beyond shape change. In this study, we present smart inks for material extrusion-based direct ink writing (DIW) printing that display preprogrammed surface wrinkling patterns on 2D and 3D surfaces post-printing, which appear only when exposed to a solvent. Hydrogels displaying surface wrinkling patterns with user-defined order and shape are useful for a range of applications, including microfluidics, flexible electronics, smart adhesives, optical sensors, and cell culture platforms. However, conventional techniques to fabricate wrinkling patterns involve multiple steps and are significantly limited to create hierarchical patterns, wrinkles on 3D and non-planar structure. It is also very difficult to scale-up the manufacturing process and to integrate surfae patterning into a continuous manufacturing process. 4D printing is used in this work to overcome these issues. The use of photocurable ink formulations allow us to tailor flow properties and printability by controlling photopolymerization time and partial crosslinking time. The crosslinker concentration is varied to develop distinct ink formulations with preprogrammed wrinkling morphology, including lamellar, peanut and hexagonal patterns. We demonstrate 4D printing of optical devices, solvent sensors, anti-counterfeit devices and high-throughput cell culture platforms.