Additive Soft Matter Design by UV-Induced Polymer Hydrogel Inter-Crosslinking

Abstract

In recent years, stimuli-responsive hydrogels have gained tremendous interest in designing complex smart 4D materials for applications ranging from biomedicine to soft electronics that can change their properties on demand over time. However, at present, a hydrogel’s response is often induced by merely a single stimulus, restricting its broader applicability. The controlled hierarchical assembly of various hydrogel building blocks, each with a tailored set of mechanical and physicochemical properties as well as programmed stimulus response, may potentially enable the design and fabrication of multi-responsive polymer parts that process complex operations, like signal routing dependent on different stimuli. Since inter-connection stability of such building blocks directly accompanies the transmission of information across building blocks and is as important as the building property itself to create complex 4D materials, we provide a study on the utility of an inter-crosslinking mechanism based on UV-induced 2,3-dimethylmaleimide (DMMI) dimerization to inter-connect acrylamide-based and N-isopropylacrylamide-based millimeter-sized cubic building blocks, respectively. The resulting dual-crosslinked assemblies are freestanding and stable against contraction–expansion cycles in solution. In addition, the approach is also applicable for connecting microfluidically fabricated, micrometer-sized hydrogel spheres, with the resulting assemblies being processable and mechanical stable, likewise resisting contraction–expansion in different solvents, for instance.

Combining parallelized emulsion formation and sequential droplet splitting for large‐scale polymer microgel production

“…All large‐scale approaches have a process inaccuracy beyond conventional microfluidics, allowing emulsion droplets to be produced with CV < 1% 3 . For fluorescent labeling, FITC dextran (M = 2000 kDa) is physically entrapped in the network during radical polymerization 14 . The fluorescence microscopy images indicate that the FITC dextran is stably entrapped in the microgel after purification (Figure 6A,B).…”

“…Our 3D‐printed high‐throughput flow cells address the continuously growing demand for functional, tailored, and uniform particulate systems in life and materials sciences, for example, as granular hydrogels in tissue engineering. These promising systems are characterized by microporosity (inter‐ and intramolecular voids), linkage of microgels with different functionality, 25 and can be used to prepare particle‐based inks for extrusion‐based 3D printing, 11 supragels 14 or multi‐material sheets 15…”

“…3 For fluorescent labeling, FITC dextran (M = 2000 kDa) is physically entrapped in the network during radical polymerization. 14 The fluorescence microscopy images indicate that the FITC dextran is stably entrapped in the microgel after purification (Figure 6A,B).…”

“…In addition, 3D‐printed microfluidic flow cells are easy to clean, show no surface fouling, and are well suited for reuse 1 . While additive manufacturing provides microfluidic devices in a simple, easy‐to‐apply fashion across life and materials sciences, the scale‐up of microfluidic production of emulsions and microparticles is increasingly important for applications such as jammed microgels in bioinks, 11,12 for advanced cell culturing platforms, 13 and functional particulate systems 14,15 . Various methods have been published on enhancing the production rates of microfluidic systems.…”

Fabrication of Thermoresponsive and Multimaterial Hydrogel Sheets by Spatially Controlled Aspiration and Interconnection of Microgel Building Blocks

“…[ 32 ] For fabricating micrometer‐to millimeter sized hydrogel assemblies based on this methodology, we first consider the material basis, microgel size as well as dimensions of the assembly device. Material‐wise, AAm is an easy‐to‐process base material to yield microgels in a fast and reliable fashion in droplet‐based microfluidics, e.g., via UV‐induced photo‐crosslinking based on free radical polymerization, [ 33 ] [ 34 ] with the microgels being ready to use after purification in isopropyl alcohol (IPA). For a proof of concept of the assembly process, we design a first generation of hydrogel arrays ranging from 400 × 400 µm to 1 mm × 1 mm that is sufficiently large to facilitate light and fluorescence microscopy characterization as well as for investigation of interconnection mechanisms and combinations of different materials, although theoretically the process could be upscaled toward larger structures in the millimeter range as well as adjusted to different microgel sizes, e.g., by lateral displacement of the assembly platform.…”