Nickel-titanium (NiTi) is an attractive alloy due to its unique functional properties (i.e., shape memory effect and superelasticity behaviors), low stiffness, biocompatibility, damping characteristics, and corrosion behavior. It is however a hard task to fabricate NiTi parts because of the high reactivity and high ductility of the alloy which results in difficulties in the processing and machining. These challenges altogether have limited the starting form of NiTi devices to simple geometries including rod, wire, bar, tube, sheet, and strip. In recent years, additive manufacturing (AM) techniques have been implemented for the direct production of complex NiTi such as latticebased and hollow structures with the potential use in aerospace and medical applications. It worth noting that due to the relatively higher cost, AM is considered a supplement technique for the existing. This paper provides a comprehensive review of the publications related to the AM techniques of NiTi while highlighting current challenges and methods of solving them. To this end, the properties of conventionally fabricated NiTi are compared with those of AM fabricated alloys. The critical steps toward a successful manufacturing such as powder preparation, optimum laser parameters, and fabrication chamber conditions are explained. The microstructural characteristics and structural defects, the influencing factors on the transformation temperatures, and functional properties of NiTi are highlighted to provide and overview of the influencing factors and possible controlling methods. The mechanical properties such as hardness and wear resistance, compressive behaviors, fatigue characteristics, damping and shock absorption properties are also reported. A case study in the form of using AM as a promising technique to fabricate engineered porous NiTi for the purpose of creating a building block for medical applications is introduced. The paper concludes with a section that summarizes the main findings from the literature and outlines the trend for future research in the AM processing of NiTi.

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Many globular and structural proteins have repetitions in their sequences or structures. However, a clear relationship between these repeats and their contribution to the mechanical properties remains elusive. We propose a new approach for the design and production of synthetic polypeptides that comprise one or more tandem copies of a single unit with distinct amorphous and ordered regions. Our designed sequences are based on a structural protein produced in squid suction cups that has a segmented copolymer structure with amorphous and crystalline domains. We produced segmented polypeptides with varying repeat number, while keeping the lengths and compositions of the amorphous and crystalline regions fixed. We showed that mechanical properties of these synthetic proteins could be tuned by modulating their molecular weights. Specifically, the toughness and extensibility of synthetic polypeptides increase as a function of the number of tandem repeats. This result suggests that the repetitions in native squid proteins could have a genetic advantage for increased toughness and flexibility.tandem repeat | high strength | protein | thermoplastic | squid ring teeth P roteins are heteropolymers that provide a variety of building blocks for designing biological materials (1). Proteins have several advantages as natural materials: (i) their chain length, sequence, and stereochemistry can be easily controlled, (ii) the molecular structure of proteins is well-defined (e.g., secondary, tertiary, and quaternary structures), (iii) they provide a variety of functional chemistries for conjugation to other biomolecules or polymers, and (iv) they can be designed to exhibit a variety of physical properties (2). Proteins are diverse but often display substantial similarity in sequence and 3D structure. Duplication of structural units is a natural evolutionary strategy for increasing the complexity of both globular and fibrous/ structural proteins (3). For example, collagen has polyproline-and glycine-rich helices, whereas silk and elastin have β-spiral [GPGXX], linker [GP(S,Y,G)], and 3 10 -helix [GGX] repeats. These repetitions are advantageous because of the intrinsic promotion of stability through the periodic recurrence of favorable interactions (4-7).A new family of repetitive structural proteins was recently identified in the tentacles of several squid species (8, 9). Squid have teeth-like structures inside their suckers that allow the animals to grip tightly on a diverse array of objects (10). Using the tools of molecular biology and proteomics, it has been shown that these squid ring teeth (SRT) proteins have segmented semicrystalline morphology with repetitive amorphous and crystalline domains. SRT-based materials were shown to have high elastic modulus: 4-8 GPa in air and 2-4 GPa underwater below the glass transition temperature (11). However, a clear relationship between the molecular structure and the mechanical properties of this material remains elusive. This problem is complex, because SRT proteins are polydispersed i...

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