In recent years, stimuli-responsive hydrogels have gained tremendous interest in designing 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. Here, we show 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 that may potentially enable the design and fabrication of multi-responsive polymer-based cellular materials that process complex operations.

To build up discontinuous soft matter based on hydrogel building blocks, the inter-connection stability of such building blocks is equally crucial as their property since it is directly associated with the transfer of information between the building blocks. Although multiple approaches of inter-connection and assembly have been described, they either miss out on the controlled stacking of assemblies into 3D objects or mostly rely on unsaturated moieties of the respective base material to inter-connect their hydrogel structures instead on a strong building block inter-connection.

We address these challenges using a separate crosslinking mechanism based on UV-induced 2,3-dimethylmaleimide (DMMI) dimerization to inter-connect hydrogel-based building blocks. To demonstrate its versatility, we inter-connect acrylamide-based and N-isopropylacrylamide-based millimeter-sized cubic building blocks, respectively, yielding dual-crosslinked freestanding assemblies. With this work, we want to pave the way towards cellular materials based on hydrogel building blocks with the capability of tuning material properties at the micrometer-scale, allowing topologically complex, multi-functional materials.