ADVANCED MATERIALS & PROCESSES | APRIL 2024 54 FEATURE twisting, and buckling in a precise manner—enabling their collective choreography to achieve emergent properties. Material architecture, which can be positioned between the material microstructure at the nano and micro scale and structural components at the meter scale, arises when morphology/dimensional control aligns with the control of architectural features across length scales (Fig. 2). Advances in manufacturing, especially additive processes such as two-photon techniques, blur the line between microstructure and architecture, pushing boundaries into the nanoscale. As such, architected materials open avenues for diverse material functionalities. Some are useful for structural functions, boasting strength, toughness, impact resistance, and adaptive flexibility. Others are opening new fields for multifunctional architected materials, with capabilities like unit cell actuation and optical modulation, as well as electronic, thermal, magnetic, and acoustic properties. Responsive architected materials display adaptability, shape-changing, reconfigurability, and actuation—exhibiting features such as sensing, self-healing, thermo-responsiveness, photo-responsiveness, electrical tunability, and acoustic actuation. NATURE-INSPIRED DESIGN With the ability to control material placement and geometry, the natural next step is to explore how to design these materials effectively. Several design methods pave the way forward. Mathematical and computational modeling tools, including topology optimization and machine learning, offer avenues for exploration. Leveraging known architectures such as octets, honeycombs, and crystal structures, and experimenting with variations and combinations alongside new materials, presents exciting possibilities. Bioinspiration or biomimicry provides insight, allowing observation of how nature ingeniously crafts multifunctional materials from diverse and abundant resources. The exploratory and discovery phase involves delving into unit cells exhibiting peculiar behaviors and unconventional properties along with exploring different combinations of ideas. The landscape of materials design is vast, offering numerous avenues for discovery and innovation (Fig. 3). Nature is a testament to ingenious materials design, with remarkable examples that surpass expectations based solely on constituent elements. What sets nature apart is its mastery in utilizing local materials found in the environment, ingeniously combining them to create novel tradeoffs, and defying traditional paradigms in materials science. Across the natural world, there are myriad instances of materials possessing not only strength and toughness but also durability, accompanied by optical and/or thermal properties. Nature is skilled at manipulating basic elements at different length scales resulting in the emergence of properties that challenge conventional materials science, offering a rich source of inspiration for innovative design approaches. Moreover, nature has evolved materials with inner channels and connected pores for fluid transport (e.g., nutrients and minerals), which may be perceived as defects and stress concentrators. Yet these materials exhibit a remarkable combination of strength, toughness, rigidity, and lightweight properties, distinct from conventional brittle materials, by incorporating soft biopolymers alongside stiff minerals. Biomineralized composites, in contrast to monolithic brittle materials, employ strategies that enhance toughness while preserving stiffness and strength. Such strategies encompass geometric arrangements, weak interfacial Fig. 1 — Components of the materials science tetrahedron. Fig. 2 — Graphic depicting how material architecture arises when morphology control aligns with the control of architectural features along various length scales. 8
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