  Citation: Neuendorf, T.A.; Weigel, N.; Vigogne, M.; Thiele, J. Additive Soft Matter Design by UV-Induced Polymer Hydrogel Inter-Crosslinking. Gels 2022, 8, 117. https://doi.org/ 10.3390/gels8020117 Academic Editor: Vijay Kumar Thakur Received: 25 January 2022 Accepted: 9 February 2022 Published: 14 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). gels Communication Additive Soft Matter Design by UV-Induced Polymer Hydrogel Inter-Crosslinking Talika A. Neuendorf † , Niclas Weigel † , Michelle Vigogne and Julian Thiele * Leibniz-Institut für Polymerforschung Dresden e. V., Hohe Straße 6, 01069 Dresden, Germany; neuendorf@ipfdd.de (T.A.N.); weigel@ipfdd.de (N.W.); vigogne@ipfdd.de (M.V.) * Correspondence: thiele@ipfdd.de; Tel.: +49-351-4658-492 † These authors contributed equally to this work. 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. Keywords: hydrogel; assembly; additive manufacturing; photopolymerization; crosslinking; building blocks; droplet microfluidics 1. Introduction The concept of controlled hierarchical assembly of building blocks is a well-known theme in our lives, e.g., when constructing stone walls, playing with interlocking plastic bricks, or looking at cellular organization in organisms. The idea of utilizing small building blocks with different colors or shapes to build up larger objects enables the constructor to tailor individually local and global properties, shape, size, and appearance of the final part. The transfer of this concept to the field of materials science and additive manufac- turing to create assemblies of building blocks exhibiting distinct properties and yielding complex multifunctional materials still remains a challenge. A key material basis for the design of these building blocks are hydrogels, which are widely used in rather different areas, like biomedicine [1] or sensor systems [2], where they exhibit a wide range of prop- erties being magnetic [3], electrically conductive [4], or thermo-responsive [5]. Several groups have already developed various assembly techniques and inter-connection con- cepts utilizing hydrogel-based building blocks towards multifunctional materials. One of the first works dates back to 2008, investigating the directed assembly of cell-laden microscopic hydrogels [6]. The authors fabricated polyethylene glycol methacrylate-based hydrogels by a photomask approach to yield cubic structures with approximate dimensions Gels 2022, 8, 117. https://doi.org/10.3390/gels8020117 https://www.mdpi.com/journal/gels