Piezoelectricity in native bones has been well recognized as the key factor in bone regeneration. Thus, bio-piezoelectric materials have gained substantial attention in repairing damaged bone by mimicking the tissue’s electrical microenvironment (EM). However, traditional manufacturing strategies still encounter limitations in creating personalized bio-piezoelectric scaffolds, hindering their clinical applications. Three-dimensional (3D)/four-dimensional (4D) printing technology based on the principle of layer-by-layer forming and stacking of discrete materials has demonstrated outstanding advantages in fabricating bio-piezoelectric scaffolds in a more complex-shaped structure. Notably, 4D printing functionality-shifting bio-piezoelectric scaffolds can provide a time-dependent programmable tissue EM in response to external stimuli for bone regeneration. In this review, we first summarize the physicochemical properties of commonly used bio-piezoelectric materials (including polymers, ceramics, and their composites) and representative biological findings for bone regeneration. Then, we discuss the latest research advances in the 3D printing of bio-piezoelectric scaffolds in terms of feedstock selection, printing process, induction strategies, and potential applications. Besides, some related challenges such as feedstock scalability, printing resolution, stress-to-polarization conversion efficiency, and non-invasive induction ability after implantation have been put forward. Finally, we highlight the potential of shape/property/functionality-shifting smart 4D bio-piezoelectric scaffolds in bone tissue engineering (BTE). Taken together, this review emphasizes the appealing utility of 3D/4D printed biological piezoelectric scaffolds as next-generation BTE implants.

The additive manufacturing (AM) of Ni-based superalloys has attracted extensive interest from both academia and industry due to its unique capabilities to fabricate complex and high-performance components for use in high-end industrial systems. However, the intense temperature gradient induced by the rapid heating and cooling processes of AM can generate high levels of residual stress and metastable chemical and structural states, inevitably leading to severe metallurgical defects in Ni-based superalloys. Cracks are the greatest threat to these materials' integrity as they can rapidly propagate and thereby cause sudden and non-predictable failure. Consequently, there is a need for a deeper understanding of residual stress and cracking mechanisms in additively manufactured Ni-based superalloys and ways to potentially prevent cracking, as this knowledge will enable the wider application of these unique materials. To this end, this paper comprehensively reviews the residual stress and the various mechanisms of crack formation in Ni-based superalloys during AM. In addition, several common methods for inhibiting crack formation are presented to assist the research community to develop methods for the fabrication of crack-free additively manufactured components.

The construction of electrode materials for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) has gradually been an appealing and attractive technology in energy storage research field. In the present work, a facile strategy of synthesizing ultrathin amorphous/nanocrystal dual-phase P-doped Bi₂MoO₆ (denoted as P-BiMO) nanosheets via a one-step wet-chemical synthesis approach is explored. Quite distinct from conventional two-dimensional (2D) nanosheets, our newly developed ultrathin P-BiMO nanosheets exhibit a unique tunable amorphous/nanocrystalline dual-phase structure with several compelling advantages including fast ion exchange ability and superb volume change buffer capability. The experimental results reveal that our prepared P-BiMO-6 electrode delivers an excellent reversible capacity of 509.6 mA·g⁻¹ after continuous 1,500 cycles at the current densities of 1,500 mA·g⁻¹ and improved rate performance for LIBs. In the meanwhile, the P-BiMO-6 electrode also shows a reversible capacity of 300.6 mA·g⁻¹ after 100 cycles at 50 mA·g⁻¹ when being used as the SIBs electrodes. This present work uncovers an effective dual-phase nanosheet structure to improve the performance of batteries, providing an attractive paradigm to develop superior electrode materials.

Mineral hydrogels have caught a lot of attention for their strong competency as artificial skin-like materials. Nonetheless, it remains a great difficulty in fulfilling in one hydrogel system a range of key functionalities that are needed for practical artificial skin applications, i.e., to be biocompatible, strain-sensitive, ion-conductive, elastic and robust, anti-swelling, and anti-freezing. Here we present a such type of versatile hydrogel that is not only capable to deliver all the above-mentioned key functionalities but also highly stable. This novel hydrogel is constructed by introducing a gelatinous and amorphous multi-ionic biomineral (denoted as Mg-ACCP, containing Mg²⁺, Ca²⁺, CO₃²⁻, and PO₄³⁻) into the network of biocompatible polyvinyl alcohol (PVA) and sodium alginate (SA). The presence of Mg²⁺ and PO₄³⁻ in this hydrogel helps prohibit the crystallization of the biominerals, leading to significantly improved stability. The hydrogel thus obtained delivers excellent mechanical performance due to the chelation between the mineral ions and the organic matrix, and high sensitivity even to subtle pressure and strain applied, such as slight finger bending and gentle tapping. Furthermore, the novel hydrogel features high ionic conductivity, high resistance to swelling, and extraordinary anti-freezing property, holding great promise for applications in different practical scenarios, particularly in aqueous or cold environments.

Fabrication of single-crystalline metal-organic framework (MOF) hollow nanostructures with two-dimensional (2D) morphologies is a challenging task. Herein, twin-like MOF nanobricks, a quasi-hollow 2D architecture, with multi-metal nodes and replaceable organic ligands, are uniformly and firmly grown on conductive Ni foam through a generic one-pot approach. The formation process of twin-like MOF nanobricks mainly includes selective epitaxial growth of Fe-rich MOF layer and simultaneously dissolution of the pre-formed Ni-rich metal-organic frameworks (MOFs), all of which can be ascribed to a special self-templated mechanism. The fantastic structural merits of twin-like MOF nanobrick arrays, featuring highly exposed active sites, remarkable electrical conductivity, and hierarchical porosities, enable this material for efficient electrocatalysis. Using bimetallic NiFe-MOFs grown on Ni foam as an example, the resultant twin-like nanobrick arrays can be directly utilized as three-dimensional (3D) integrated electrode for high-performance water oxidation in 1 M KOH with a low overpotential, fast reaction kinetics (28.5 mV·dec⁻¹), and superb stability. Interestingly, the unstable NiFe-MOFs were served as an oxygen evolution reaction (OER) pre-catalyst and the single-crystalline NiFe-MOF precursor can be in-situ topochemically regulated into porous and low-crystalline NiFeO_x nanosheets during the OER process. This work extends the hollowing strategy to fabricate hollow MOFs with 2D architectures and highlights their direct utilization for advanced electrocatalysis.

Most of the challenges experienced by many engineering materials originate from the surface which later leads to total failure, hence affecting the resultant mechanical properties and service life. However, these challenges have been addressed thanks to the invention of a novel surface mechanical attrition treatment (SMAT) method which protects the material surface by generating a gradient-structured layer with improved strength and hardness without jeopardizing the ductility. The present work provides a comprehensive literature review on the mechanical properties of materials after SMAT including the hardness, tensile strength and elongation, and residual stress. Firstly, a brief introduction on the different forms of surface nanocrystallization is given to get a better understanding of the SMAT process and its advantages over other forms of surface treatments, and then the grain refinement mechanisms of materials by SMAT from the matrix region (base material) to the nanocrystallized layer are explained. The effects of fatigue, fracture, and wear of materials by the enhanced mechanical properties after SMAT are also discussed in detail. In addition, the various applications of SMAT ranging from automotive, photoelectric conversion, biomedical, diffusion, and 3D-printing of materials are extensively discussed. The prospects and recent research trends in terms of mechanical properties of materials affected by SMAT are then summarized.

Metal foams have been intensively studied as three-dimensional (3-D) bulk mass-support for various applications because of their high conductivities and attractive mechanical properties. However, the relatively low surface area of conventional metal foams largely limits their performance in applications such as charge storage. Here, we present a convenient electrochemical method for addressing this problem using Cu foams as an example. High surface area Cu foams are fabricated in a one-pot one-step manner by repetitive electrodeposition and dealloying treatments. The obtained Cu foams exhibit greatly improved performance for different applications like surface enhanced Raman spectroscopy (SERS) substrates and 3-D bulk supercapacitor electrodes.