Assembly of Materials Building Blocks into Integrated Complex Functional Systems (2020)

It is hard to overstate the importance of developing novel functional materials and surfaces, which are often essential for meeting challenges in diagnostics, biotechnology, tissue engineering, optics, microfluidics, and many other fields. A central question is how to generate novel complex functionality or properties that would go beyond those of existing materials and surfaces. This question is being approached from numerous angles, among them inspiration from biological systems (biomimetics), combinatorial synthesis and high-throughput screening of novel materials, synergistic combination of different components or processes, as well as the assembly of microscopic building blocks into macroscopic materials. In this Special Issue of Advanced Functional Materials, “Assembly of Materials Building Blocks into Integrated Complex Functional Systems” we received 22 excellent papers presenting original research as well as reviews on this topic. These articles demonstrate the growing diversity of strategies for fabricating novel classes of materials in which complex functionalities emerge from the assembly of materials building blocks or the synergistic combination of components and processes. This editorial highlights a subset of representative examples to give a flavor of published research articles and the future potential of this scientific field. One example where large complex 3D functional materials are created through the assembly of small materials building blocks (voxels) is discussed in an article by Wegener et al. (article number 1907795), in which the authors review 3D additive manufacturing approaches in terms of maximum voxel printing rate and minimum voxel size. The authors also present a new multi-focus two-photon 3D printing technology that approaches total printing speeds of ten million voxels per second at sub-µm voxel sizes, which significantly surpasses previous top printing speeds. Fabrication of functional responsive 3D materials is reviewed by Blasco et al. (article number 1907615). The article discusses 3D printing of adaptive and dynamic structures (4D printing), where the additional dimension refers to the ability of the structures to change their shape in response to a stimulus. One example of such dynamic properties is based on the formation of hetero-microstructures combining materials with different swellability or different response characteristics. Assembling such microscopic “hetero” building blocks into a macroscopic structure can lead to interesting dynamic macroscopic properties. In nature, tissues are built through the hierarchical organization of microscopic “building blocks” into progressively larger and more complex structures. Bioinspired tissue engineering often relies on the assembly of microscopic artificial soft building blocks into macroscopic functional tissue-like structures. Stevens et al. (article number 1909009) give an overview of the recent progress and trends in the fabrication and assembly of living building blocks, with a key highlight on emerging bioprinting technologies that can be used for modular assembly and complexity in tissue engineering. Such living building blocks include single cells, cell fibers, cell sheets, cell spheroids and cell organoids, which can be assembled into various complex functional living structures (tissue engineering). Development of methods to create functional soft materials is essential due to the complexity of living tissues and organs that have to be mimicked to meet need in areas such as regenerative medicine and novel functional implants. Thus, cell-instructive multiphasic gel-in-gel materials based on combination of different types of hydrogel building blocks are reviewed by Werner et al. (article number 1908857). The final functionality and properties of such materials depend, to a great extent, on the architecture of such heterogeneous and multiphasic materials, for example, layered, embedded or bundled organization. Design of adhesives for affixing soft hydrogel building blocks is challenging and crucial for creating complex hydrogel structures. Lee et al. (article number 1908497) demonstrate how alginate-boronic acid-based glue has been used to attach diverse hydrogel building blocks to create complex soft macroscopic hydrogel structures. In another related progress report, Khademhosseini et al. (article number 1909882) review components of synthetic biology that can serve as building blocks to engineer cells in tissues with higher degrees of cellular complexity and function. Bottom-up self-assembly is another approach to assembling complex functional structures from small building blocks. Thus, Noorduin et al. (article number 1908218) demonstrate the bottom-up self-assembly of simple nanoscopic mineral (barium carbonate/silica) building blocks into complex architectures (double helices) with specialized and finely controlled optical properties. Due to such double helical organization, these self-assembled architectures emit highly directional light along their long axes, while affecting a differential refraction of left and right circularly polarized light. Li et al. investigate the assembly of nano- or microscopic objects into more complex packed structures. The presented approach harnesses the crack formation as a patterning tool to fabricate microscopic photonic structures with controlled sizes and geometries (article number 1908242). Steady-state, light-adaptive reconfiguration of mechanical patterns under dissipative out-of-equilibrium conditions through the combination of heterogeneous activation of a photo-thermal effect is reported by Walther et al. (article number 1905309). Assembly of molecular building blocks into integrated MOF/COF-based functional systems is reviewed by Bräse et al. (article number 1907625). Vogel et al. (article number 1907730) report an interesting study on controlled colloidal assembly inside of droplets, resulting in structural color. In addition, the rotational motion, dynamics and crystallization of such micron-scale clusters suspended in a liquid could be followed in real time via their anisotropic coloration, demonstrating that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters. Ionov et al. (article number 1908028) discuss the principles of the formation of complex functional devices based on shape memory materials. Combination of shape memory effects with different stimuli and different shapes and architectures leads to applications in the field of soft-robotics, microfluidics, sensors, smart textiles, medicine or drug delivery. In addition to the combination of building blocks, novel complex functionality can be generated through the synergistic combination of opposing processes (rather than properties). Scheiger et al. (article number 1909800) demonstrate hydrogels with pre-programmable lifetime, which was achieved through the combination of UV-induced polymerization with UV-induced photodegradation in the same material. Incorporation of reactive dynamic bonds into a coating can lead to novel macroscopic surface properties, such as the possibility to repair the functionality, change its properties such as hydrophobicity or hydrophilicity, or create patterns of properties. In this realm, Butt et al. (article number 1907605) review reconfigurable surfaces based on photo-controlled dynamic bonds including thiol-quinone methide, disulfide exchange, thiol-disulfide interconversion, diselenide exchange, and photosubstitution of Ru complexes. These are just a sampling of the many creative research directions discussed in this Special Issue. Also included are excellent research articles and reviews discussing topics such as microsphere design via visible-light cross-linking of functional prepolymers (article number 1905399), multi-functional (super)wetting surfaces (article number 1907772), challenges of self-healing in anti-fouling materials (article number 1908098), cell encapsulation systems for modular tissue regeneration (article number 1908061), or self-assembly in hopper-shaped crystals (article number 1908108). Finally, Lahann et al. (article number 1907865) investigated the magnetically directed spatiotemporal self-assembly and switching of magnetic janus particles. Progress in the development of nanogeneratores is discussed by Wang et al. (article number 1908252), while Greiner et al. (article number 1907555) reported fabrication of flexible low-resistance membranes with the reversible change in resistance, potentially useful for smart wearables. Together, this collection of papers provides both an inspiring display of the many complex functions—and beauty—that can arise from assembling and combining even well-known building blocks, and a wealth of insight into the growing variety of routes to accessing and discovering such novel and critical materials.