Recent Advances in the Field of Bioinspired Materials and Systems

Bioinspired and biomimetic materials have revolutionized many aspects in the engineering sciences, ranging from basic technologies to advanced applications such as the Gecko's nanostructure for sticky tapes or the sharkskin for swimming suits. Research in these subjects in Germany has been supported by several national priority research programs since 2009 (SPP 1420: Biomimetic Materials Research: Functionality by Hierarchical Structuring of Materials,[1] SPP 1569: Generation of Multifunctional Inorganic Materials by Molecular Bionics, 2012–2019[2]), which strongly accelerated developments and investigations in the field. Recently, also photonic applications of bioinspired strategies have come within reach, demonstrating the huge potential of the approach nature has chosen in many directions (SPP 1839: Tailored Disorder – A science- and engineering-based approach to materials design for advanced photonic applications, 2015–2024[3]). These developments have been accompanied by a biennial DGM Bioinspired Materials conference series, which was launched in 2012.[4] The present Advanced Functional Materials feature issue now proudly presents accompanying papers from the 6th DGM “Bioinspired Materials[5]” together with additional most recent and advanced research work pushing the limits of our exciting interdisciplinary research field. The collected perspectives, reviews and research articles represent the current state-of-the-art in the design of functional and structural materials and systems inspired by principles found in living nature. Main topics of this include the study and formation of hierarchical structures and the properties of complex-shaped biological materials. The transfer of biological principles into functional materials and systems made of organic and/or inorganic components represents a further main area. Beside synthesis and formation, the feature issue topics also include research on the functional and mechanical materials properties and the interaction of bioinspired materials with living entities. For example, van Nieuwenhoven et al.[6] give a perspective on how living nature manages a multi-purpose complex materials design on the way to high performance materials, which are also designed for an efficient and circular use of resources. Kennedy et al.[7] exemplify the importance of bioinspired fog (water) harvesting systems, which are related to the important Sustainable Development Goal SDG 6 “Clean Water and Sanitation”. Bioinspired materials also become more and more important in soft robotics as reported by Roh et al.[8] In a review on multifunctional materials inspired by sharkskin denticles, Ghimire et al.[9] report on sharkskin denticles with distinct implication for saving energy when applied to bioinspired engineering surfaces and that sharkskin denticles are inherently multifunctional comprising antifouling, protection, and bioluminescence characteristics. Biological photonics and their emulation into bioinspired approaches for advanced photonic application has become a vital field of research. This is especially true for biological systems deviating from a perfect order. It is shown in a research article by Bauernfeind et al.[10] how quasi-order is beneficial for producing stable colors with stunning spatial characteristics as observed in longhorn beetles (Sternotomis virescens). The follow-up perspective in the field of biological photonics by Vogler-Neuling et al.[11] highlights the interplay between nature's structural variety and the ultimately restricted artificial manufacturing possibilities. Since biological tissue is primarily built form biopolymers, the bioinspired adaptation to create advanced photonic materials from available biopolymers is of paramount importance for the development of environmentally benign and sustainable materials. In biopolymers, the polysaccharides cellulose and chitin take a very special role for various reasons. From a scientific point of view, their extensive capability of hierarchical structure formation is important for the design of bioinspired materials from these polysaccharides. The review on cellulose and chitin by da Rosa et al.[12] discusses nanoscale helicoidal architectures and the exploitation of chirality, which has often been ignored. The beneficial combination of two biopolymer classes – polysaccharides and proteins – in a functional device is shown by the fabrication of a low-energy light emitter by Hasler et al.[13] They exhibit superior properties compared to similar materials in the field and pave the road to fully biogenic functional materials for light applications. Another prospective conjunction of polysaccharides and proteins is the synthesis of covalently bound proteins within a polysaccharide based (agarose) hydrogel to form a stimuli-responsive hydrogel that can perform mathematical operations.[14] Apart from biohybrid materials, proteins alone can also exhibit highly interesting material properties, e.g., the protein resilin in insect flight systems, as shown in a review article by Appel et al.[15] Understanding resilin and its complex structures in insect wings are highly relevant for the development of resilient bioinspired materials. Also spider silk is a biological material with a highly complex fibrous protein architecture. Bioinspired artificial spiders silks outperforming engineering materials and methods for the fabrication are summarized by Schmuck et al.[16] In addition, research by Arndt et al.[17] on recombinant spider protein bioinspired hydrogels shows their beneficial application in 3D cell culture scaffolds, e.g., because bioactive molecules can readily diffuse through the spider silk hydrogel matrix. Many other protein assemblies also exhibit fascinating adhesive properties giving rise to the development of bioadhesives especially for biomedical applications.[18] Furthermore, underwater adhesive systems are highly relevant in soft robotics or biomedical applications. Eklund et al.[19] present a novel microchannel approach, demonstrating a high switching efficiency and the ability to carry light weights. The following topical block presents bioinspired materials from or with living cells. The first research article presents 3D matrices mimicking tumor-like properties in a microcapsule, thus inducing the phenomenon of cell polyploidy.[20] In addition, ciliates as unicellular organisms have very auspicious properties for the development of novel bioinspired functional materials and systems. These single-celled microorganisms as an inspirational source are reviewed with emphasis on complex mineralized structures, sensor materials as well as shape memory structures for bioinspired applications in Dörr et al.[21] Also immune cell signaling can be strongly controlled by mechanical forces and material stiffness, enabling the modulation of cell activation.[22] A bioinspired material approach exploiting adeno-associated viruses (AAVs) as a generic tool for therapeutic gene delivery is shown by Voigt et al.,[23] particularly how cellular mechanisms are used to deliver desired targeted cargo into cells. The influence of mechanical properties of the matrix on cell adhesion and mechanics is discussed. The last section of this feature issue covers the thrilling characteristics of biological hard tissue and bioinspired mineralization. A review by Rodriguez Palomo et al.[24] on the complex hierarchical structure of bone reveals its surprising complexity, which was unevolved so far. Recent results on the structure of the bone show that bone is more heterogeneous than usually described, especially when local variations in composition are thoroughly evaluated. The biominerals found in mollusk shells also show highly complex architectures,[25] demonstrating extensive twinning, which have important implications related to geological minerals such as Carrara marble. Hierarchical porosity is a highly interesting topic in many applications. Here, Yang et al.[26] report on a method that involves intrinsic foaming capabilities of water-glass using robocasting combined with a temperature treatment to create a micro- and macro-scale porosity, thus mimicking biomineral structures. We believe that this special issue vividly demonstrates the diversity of the rich and fascinating field of bioinspired materials. We want to thank our invited authors and all other contributors that have made this exciting issue possible with their magnificent perspectives, reviews and research papers. We are indebted to Dr. Jörn Ritterbusch and Dr. Mary De Vita for giving us the opportunity to be Guest Editors of this fantastic feature issue. We are exceptionally grateful for their enormous support. Financial support from the German Research Foundation in the frame of the Priority Program: DFG-SPP1839 “Tailored Disorder – A science- and engineering-based approach to materials design for advanced photonic applications” was gratefully acknowledged. The authors declare no conflict of interest. Cordt Zollfrank studied chemistry at the Technische Universität München (TUM, Germany) and earned his Ph.D. in forest sciences with a cellulose related topic. He was as a postdoctoral research fellow at the 2000 and 2002 University of Erlangen-Nuremberg (Germany) and established the Bioengineered Ceramics research group. In 2009 he finished his postdoctoral teaching qualification (habilitation) in material sciences. He became associate professor for Biogenic Polymers in 2011 at the TUM Campus Straubing for Biotechnology and Sustainability and was promoted to full professor in 2020. A key area of his work is the formation of bioinspired and biopolymeric structures and their conversion into composite materials for engineering and biomedical applications. C. Zollfrank has been awarded with a Reinhart-Koselleck-Project from DFG. He is a Fellow of the International Academy of Wood Science (IAWS). Christine Selhuber-Unkel is since 2020 a full professor for Molecular Systems Engineering at Heidelberg University (Germany). She studied physics at Heidelberg University (Germany) and Uppsala University (Sweden), and received her Ph.D. degree in physics at Heidelberg University (Germany) in 2006. She was a postdoctoral fellow at the Niels Bohr Institute (University of Copenhagen, Denmark), and an associate professor at the Institute for Materials Science at Kiel University 2011–2020 (Germany). Her main research interest is controlling the interaction of biological systems with materials, for example by structural and mechanical material properties. She also has a strong interest in the quantitative investigation of cell-material interactions. C. Selhuber-Unkel has been awarded with ERC Grants and is a member of the German National Academy of Science and Engineering (acatech)