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2024 Vol. 6, No. 1

Two-photon polymerization-based 4D printing and its applications
Bingcong Jian, Honggeng Li, Xiangnan He, Rong Wang, Hui Ying Yang, Qi Ge
2024, 6(1): 012001. doi: 10.1088/2631-7990/acfc03

Two-photon polymerization (TPP) is a cutting-edge micro/nanoscale three-dimensional (3D) printing technology based on the principle of two-photon absorption. TPP surpasses the diffraction limit in achieving feature sizes and excels in fabricating intricate 3D micro/nanostructures with exceptional resolution. The concept of 4D entails the fabrication of structures utilizing smart materials capable of undergoing shape, property, or functional changes in response to external stimuli over time. The integration of TPP and 4D printing introduces the possibility of producing responsive structures with micro/nanoscale accuracy, thereby enhancing the capabilities and potential applications of both technologies. This paper comprehensively reviews TPP-based 4D printing technology and its diverse applications. First, the working principles of TPP and its recent advancements are introduced. Second, the optional 4D printing materials suitable for fabrication with TPP are discussed. Finally, this review paper highlights several noteworthy applications of TPP-based 4D printing, including domains such as biomedical microrobots, bioinspired microactuators, autonomous mobile microrobots, transformable devices and robots, as well as anti-counterfeiting microdevices. In conclusion, this paper provides valuable insights into the current status and future prospects of TPP-based 4D printing technology, thereby serving as a guide for researchers and practitioners.

Advances in memristor based artificial neuron fabrication-materials, models, and applications
Jingyao Bian, Zhiyong Liu, Ye Tao, Zhongqiang Wang, Xiaoning Zhao, Ya Lin, Haiyang Xu, Yichun Liu
2024, 6(1) doi: 10.1088/2631-7990/acfcf1

Spiking neural network (SNN), widely known as the third-generation neural network, has been frequently investigated due to its excellent spatiotemporal information processing capability, high biological plausibility, and low energy consumption characteristics. Analogous to the working mechanism of human brain, the SNN system transmits information through the spiking action of neurons. Therefore, artificial neurons are critical building blocks for constructing SNN in hardware. Memristors are drawing growing attention due to low consumption, high speed, and nonlinearity characteristics, which are recently introduced to mimic the functions of biological neurons. Researchers have proposed multifarious memristive materials including organic materials, inorganic materials, or even two-dimensional materials. Taking advantage of the unique electrical behavior of these materials, several neuron models are successfully implemented, such as Hodgkin-Huxley model, leaky integrate-and-fire model and integrate-and-fire model. In this review, the recent reports of artificial neurons based on memristive devices are discussed. In addition, we highlight the models and applications through combining artificial neuronal devices with sensors or other electronic devices. Finally, the future challenges and outlooks of memristor-based artificial neurons are discussed, and the development of hardware implementation of brain-like intelligence system based on SNN is also prospected.

Characterization, preparation, and reuse of metallic powders for laser powder bed fusion: a review
Xiaoyu Sun, Minan Chen, Tingting Liu, Kai Zhang, Huiliang Wei, Zhiguang Zhu, Wenhe Liao
2024, 6(1) doi: 10.1088/2631-7990/acfbc3

Laser powder bed fusion (L-PBF) has attracted significant attention in both the industry and academic fields since its inception, providing unprecedented advantages to fabricate complex-shaped metallic components. The printing quality and performance of L-PBF alloys are influenced by numerous variables consisting of feedstock powders, manufacturing process, and post-treatment. As the starting materials, metallic powders play a critical role in influencing the fabrication cost, printing consistency, and properties. Given their deterministic roles, the present review aims to retrospect the recent progress on metallic powders for L-PBF including characterization, preparation, and reuse. The powder characterization mainly serves for printing consistency while powder preparation and reuse are introduced to reduce the fabrication costs. Various powder characterization and preparation methods are presented in the beginning by analyzing the measurement principles, advantages, and limitations. Subsequently, the effect of powder reuse on the powder characteristics and mechanical performance of L-PBF parts is analyzed, focusing on steels, nickel-based superalloys, titanium and titanium alloys, and aluminum alloys. The evolution trends of powders and L-PBF parts vary depending on specific alloy systems, which makes the proposal of a unified reuse protocol infeasible. Finally, perspectives are presented to cater to the increased applications of L-PBF technologies for future investigations. The present state-of-the-art work can pave the way for the broad industrial applications of L-PBF by enhancing printing consistency and reducing the total costs from the perspective of powders.

Energy beam-based direct and assisted polishing techniques for diamond: A review
Zhuo Li, Feng Jiang, Zhengyi Jiang, Zige Tian, Tian Qiu, Tao Zhang, Qiuling Wen, Xizhao Lu, Jing Lu, Hui Huang
2024, 6(1) doi: 10.1088/2631-7990/acfd67

Diamond is a highly valuable material with diverse industrial applications, particularly in the fields of semiconductor, optics, and high-power electronics. However, its high hardness and chemical stability make it difficult to realize high-efficiency and ultra-low damage machining of diamond. To address these challenges, several polishing methods have been developed for both single crystal diamond (SCD) and polycrystalline diamond (PCD), including mechanical, chemical, laser, and ion beam processing methods. In this review, the characteristics and application scope of various polishing technologies for SCD and PCD are highlighted. Specifically, various energy beam-based direct and assisted polishing technologies, such as laser polishing, ion beam polishing, plasma-assisted polishing, and laser-assisted polishing, are summarized. The current research progress, material removal mechanism, and influencing factors of each polishing technology are analyzed. Although some of these methods can achieve high material removal rates or reduce surface roughness, no single method can meet all the requirements. Finally, the future development prospects and application directions of different polishing technologies are presented.

Device design principles and bioelectronic applications for flexible organic electrochemical transistors
Lin Gao, Mengge Wu, Xinge Yu, Junsheng Yu
2024, 6(1) doi: 10.1088/2631-7990/acfd69

Organic electrochemical transistors (OECTs) exhibit significant potential for applications in healthcare and human-machine interfaces, due to their tunable synthesis, facile deposition, and excellent biocompatibility. Expanding OECTs to the flexible devices will significantly facilitate stable contact with the skin and enable more possible bioelectronic applications. In this work, we summarize the device physics of flexible OECTs, aiming to offer a foundational understanding and guidelines for material selection and device architecture. Particular attention is paid to the advanced manufacturing approaches, including photolithography and printing techniques, which establish a robust foundation for the commercialization and large-scale fabrication. And abundantly demonstrated examples ranging from biosensors, artificial synapses/neurons, to bioinspired nervous systems are summarized to highlight the considerable prospects of smart healthcare. In the end, the challenges and opportunities are proposed for flexible OECTs. The purpose of this review is not only to elaborate on the basic design principles of flexible OECTs, but also to act as a roadmap for further exploration of wearable OECTs in advanced bio-applications.

Engineering vascularized organotypic tissues via module assembly
Zhenzhen Zhou, Changru Liu, Yuting Guo, Yuan Pang, Wei Sun
2024, 6(1) doi: 10.1088/2631-7990/acfcf2

Adequate vascularization is a critical determinant for the successful construction and clinical implementation of complex organotypic tissue models. Currently, low cell and vessel density and insufficient vascular maturation make vascularized organotypic tissue construction difficult, greatly limiting its use in tissue engineering and regenerative medicine. To address these limitations, recent studies have adopted pre-vascularized microtissue assembly for the rapid generation of functional tissue analogs with dense vascular networks and high cell density. In this article, we summarize the development of module assembly-based vascularized organotypic tissue construction and its application in tissue repair and regeneration, organ-scale tissue biomanufacturing, as well as advanced tissue modeling.

A brief review on the recent development of phonon engineering and manipulation at nanoscales
Siqi Xie, Hongxin Zhu, Xing Zhang, Haidong Wang
2024, 6(1) doi: 10.1088/2631-7990/acfd68

Phonons are the quantum mechanical descriptions of vibrational modes that manifest themselves in many physical properties of condensed matter systems. As the size of electronic devices continues to decrease below mean free paths of acoustic phonons, the engineering of phonon spectra at the nanoscale becomes an important topic. Phonon manipulation allows for active control and management of heat flow, enabling functions such as regulated heat transport. At the same time, phonon transmission, as a novel signal transmission method, holds great potential to revolutionize modern industry like microelectronics technology, and boasts wide-ranging applications. Unlike fermions such as electrons, polarity regulation is difficult to act on phonons as bosons, making the development of effective phonon modulation methods a daunting task. This work reviews the development of phonon engineering and strategies of phonon manipulation at different scales, reports the latest research progress of nanophononic devices such as thermal rectifiers, thermal transistors, thermal memories, and thermoelectric devices, and analyzes the phonon transport mechanisms involved. Lastly, we survey feasible perspectives and research directions of phonon engineering. Thermoelectric analogies, external field regulation, and acousto-optic co-optimization are expected to become future research hotspots.

Recent progress in bio-inspired macrostructure array materials with special wettability—from surface engineering to functional applications
Zhongxu Lian, Jianhui Zhou, Wanfei Ren, Faze Chen, Jinkai Xu, Yanling Tian, Huadong Yu
2024, 6(1) doi: 10.1088/2631-7990/ad0471

Bio-inspired macrostructure array (MAA, size: submillimeter to millimeter scale) materials with special wettability (MAAMs-SW) have attracted significant research attention due to their outstanding performance in many applications, including oil repellency, liquid/droplet manipulation, anti-icing, heat transfer, water collection, and oil–water separation. In this review, we focus on recent developments in the theory, design, fabrication, and application of bio-inspired MAAMs-SW. We first review the history of the basic theory of special wettability and discuss representative structures and corresponding functions of some biological surfaces, thus setting the stage for the design and fabrication of bio-inspired MAAMs-SW. We then summarize the fabrication methods of special wetting MAAs in terms of three categories: additive manufacturing, subtractive manufacturing, and formative manufacturing, as well as their diverse functional applications, providing insights into the development of these MAAMs-SW. Finally, the challenges and directions of future research on bio-inspired MAAMs-SW are briefly addressed. Worldwide efforts, progress, and breakthroughs from surface engineering to functional applications elaborated herein will promote the practical application of bio-inspired MAAMs-SW.

Influence of heat treatment on microstructure, mechanical and corrosion behavior of WE43 alloy fabricated by laser-beam powder bed fusion
Chenrong Ling, Qiang Li, Zhe Zhang, Youwen Yang, Wenhao Zhou, Wenlong Chen, Zhi Dong, Chunrong Pan, Cijun Shuai
2024, 6(1) doi: 10.1088/2631-7990/acfad5

Magnesium (Mg) alloys are considered to be a new generation of revolutionary medical metals. Laser-beam powder bed fusion (PBF-LB) is suitable for fabricating metal implants with personalized and complicated structures. However, the as-built part usually exhibits undesirable microstructure and unsatisfactory performance. In this work, WE43 parts were firstly fabricated by PBF-LB and then subjected to heat treatment. Although a high densification rate of 99.91% was achieved using suitable processes, the as-built parts exhibited anisotropic and layered microstructure with heterogeneously precipitated Nd-rich intermetallic. After heat treatment, fine and nano-scaled Mg24Y5 particles were precipitated. Meanwhile, the α-Mg grains underwent recrystallization and turned coarsened slightly, which effectively weakened the texture intensity and reduced the anisotropy. As a consequence, the yield strength and ultimate tensile strength were significantly improved to (250.2 ± 3.5) MPa and (312 ± 3.7) MPa, respectively, while the elongation was still maintained at a high level of 15.2%. Furthermore, the homogenized microstructure reduced the tendency of localized corrosion and favored the development of uniform passivation film. Thus, the degradation rate of WE43 parts was decreased by an order of magnitude. Besides, in-vitro cell experiments proved their favorable biocompatibility.

Toward understanding the microstructure characteristics, phase selection and magnetic properties of laser additive manufactured Nd-Fe-B permanent magnets
Bo Yao, Nan Kang, Xiangyu Li, Dou Li, Mohamed EL Mansori, Jing Chen, Haiou Yang, Hua Tan, Xin Lin
2024, 6(1) doi: 10.1088/2631-7990/ad0472

Nd-Fe-B permanent magnets play a crucial role in energy conversion and electronic devices. The essential magnetic properties of Nd-Fe-B magnets, particularly coercivity and remanent magnetization, are significantly influenced by the phase characteristics and microstructure. In this work, Nd-Fe-B magnets were manufactured using vacuum induction melting (VIM), laser directed energy deposition (LDED) and laser powder bed fusion (LPBF) technologies. The microstructure evolution and phase selection of Nd-Fe-B magnets were then clarified in detail. The results indicated that the solidification velocity (V) and cooling rate (R) are key factors in the phase selection. In terms of the VIM-casting Nd-Fe-B magnet, a large volume fraction of the α-Fe soft magnetic phase (39.7 vol.%) and Nd2Fe17Bx metastable phase (34.7 vol.%) are formed due to the low R (2.3 × 10-1 °C s-1), whereas only a minor fraction of the Nd2Fe14B hard magnetic phase (5.15 vol.%) is presented. For the LDED-processed Nd-Fe-B deposit, although the Nd2Fe14B hard magnetic phase also had a low value (3.4 vol.%) as the values of V (<10-2 m s-1) and R (5.06 × 103 °C s-1) increased, part of the α-Fe soft magnetic phase (31.7 vol.%) is suppressed, and a higher volume of Nd2Fe17Bx metastable phases (47.5 vol.%) are formed. As a result, both the VIM-casting and LDED-processed Nd-Fe-B deposits exhibited poor magnetic properties. In contrast, employing the high values of V (>10-2 m s-1) and R (1.45 × 106 °C s-1) in the LPBF process resulted in the substantial formation of the Nd2Fe14B hard magnetic phase (55.8 vol.%) directly from the liquid, while the α-Fe soft magnetic phase and Nd2Fe17Bx metastable phase precipitation are suppressed in the LPBF-processed Nd-Fe-B magnet. Additionally, crystallographic texture analysis reveals that the LPBF-processed Nd-Fe-B magnets exhibit isotropic magnetic characteristics. Consequently, the LPBF-processed Nd-Fe-B deposit, exhibiting a coercivity of 656 kA m-1, remanence of 0.79 T and maximum energy product of 71.5 kJ m-3, achieved an acceptable magnetic performance, comparable to other additive manufacturing processed Nd-Fe-B magnets from MQP (Nd-lean) Nd-Fe-B powder.

Oxygen vacancy boosting Fenton reaction in bone scaffold towards fighting bacterial infection
Cijun Shuai, Xiaoxin Shi, Feng Yang, Haifeng Tian, Pei Feng
2024, 6(1) doi: 10.1088/2631-7990/ad01fd

Bacterial infection is a major issue after artificial bone transplantation due to the absence of antibacterial function of bone scaffold, which seriously causes the transplant failure and even amputation in severe cases. In this study, oxygen vacancy (OV) defects Fe-doped TiO2 (OV-FeTiO2) nanoparticles were synthesized by nano TiO2 and Fe3O4 via high-energy ball milling, which was then incorporated into polycaprolactone/polyglycolic acid (PCLGA) biodegradable polymer matrix to construct composite bone scaffold with good antibacterial activities by selective laser sintering. The results indicated that OV defects were introduced into the core/shell-structured OV-FeTiO2 nanoparticles through multiple welding and breaking during the high-energy ball milling, which facilitated the adsorption of hydrogen peroxide (H2O2) in the bacterial infection microenvironment at the bone transplant site. The accumulated H2O2 could amplify the Fenton reaction efficiency to induce more hydroxyl radicals (·OH), thereby resulting in more bacterial deaths through ·OH-mediated oxidative damage. This antibacterial strategy had more effective broad-spectrum antibacterial properties against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus). In addition, the PCLGA/OV-FeTiO2 scaffold possessed mechanical properties that match those of human cancellous bone and good biocompatibility including cell attachment, proliferation and osteogenic differentiation.

Near-zero-adhesion-enabled intact wafer-scale resist-transfer printing for high-fidelity nanofabrication on arbitrary substrates
Zhiwen Shu, Bo Feng, Peng Liu, Lei Chen, Huikang Liang, Yiqin Chen, Jianwu Yu, Huigao Duan
2024, 6(1) doi: 10.1088/2631-7990/ad01fe

There is an urgent need for novel processes that can integrate different functional nanostructures onto specific substrates, so as to meet the fast-growing need for broad applications in nanoelectronics, nanophotonics, and flexible optoelectronics. Existing direct-lithography methods are difficult to use on flexible, nonplanar, and biocompatible surfaces. Therefore, this fabrication is usually accomplished by nanotransfer printing. However, large-scale integration of multiscale nanostructures with unconventional substrates remains challenging because fabrication yields and quality are often limited by the resolution, uniformity, adhesivity, and integrity of the nanostructures formed by direct transfer. Here, we proposed a resist-based transfer strategy enabled by near-zero adhesion, which was achieved by molecular modification to attain a critical surface energy interval. This approach enabled the intact transfer of wafer-scale, ultrathin-resist nanofilms onto arbitrary substrates with mitigated cracking and wrinkling, thereby facilitating the in situ fabrication of nanostructures for functional devices. Applying this approach, fabrication of three-dimensional-stacked multilayer structures with enhanced functionalities, nanoplasmonic structures with ~10 nm resolution, and MoS2-based devices with excellent performance was demonstrated on specific substrates. These results collectively demonstrated the high stability, reliability, and throughput of our strategy for optical and electronic device applications.

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures
Qiang Liu, Wei Xu, Xiaoxi Li, Tongyao Zhang, Chengbing Qin, Fang Luo, Zhihong Zhu, Shiqiao Qin, Mengjian Zhu, Kostya S Novoselov
2024, 6(1) doi: 10.1088/2631-7990/acfbc2

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ~440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

Towards a new avenue for rapid synthesis of electrocatalytic electrodes via laser-induced hydrothermal reaction for water splitting
Yang Sha, Menghui Zhu, Kun Huang, Yang Zhang, Francis Moissinac, Zhizhou Zhang, Dongxu Cheng, Paul Mativenga, Zhu Liu
2024, 6(1) doi: 10.1088/2631-7990/ad038f

Electrochemical production of hydrogen from water requires the development of electrocatalysts that are active, stable, and low-cost for water splitting. To address these challenges, researchers are increasingly exploring binder-free electrocatalytic integrated electrodes (IEs) as an alternative to conventional powder-based electrode preparation methods, for the former is highly desirable to improve the catalytic activity and long-term stability for large-scale applications of electrocatalysts. Herein, we demonstrate a laser-induced hydrothermal reaction (LIHR) technique to grow NiMoO4 nanosheets on nickel foam, which is then calcined under H2/Ar mixed gases to prepare the IE IE-NiMo-LR. This electrode exhibits superior hydrogen evolution reaction performance, requiring overpotentials of 59, 116 and 143 mV to achieve current densities of 100, 500 and 1000 mA·cm−2. During the 350 h chronopotentiometry test at current densities of 100 and 500 mA·cm−2, the overpotential remains essentially unchanged. In addition, NiFe-layered double hydroxide grown on Ni foam is also fabricated with the same LIHR method and coupled with IE-NiMo-IR to achieve water splitting. This combination exhibits excellent durability under industrial current density. The energy consumption and production efficiency of the LIHR method are systematically compared with the conventional hydrothermal method. The LIHR method significantly improves the production rate by over 19 times, while consuming only 27.78% of the total energy required by conventional hydrothermal methods to achieve the same production.

Additively manufactured Ti–Ta–Cu alloys for the next-generation load-bearing implants
Amit Bandyopadhyay, Indranath Mitra, Sushant Ciliveri, Jose D Avila, William Dernell, Stuart B Goodman, and Susmita Bose
2024, 6(1) doi: 10.1088/2631-7990/ad07e7

Bacterial colonization of orthopedic implants is one of the leading causes of failure and clinical complexities for load-bearing metallic implants. Topical or systemic administration of antibiotics may not offer the most efficient defense against colonization, especially in the case of secondary infection, leading to surgical removal of implants and in some cases even limbs. In this study, laser powder bed fusion was implemented to fabricate Ti3Al2V alloy by a 1:1 weight mixture of CpTi and Ti6Al4V powders. Ti-Tantalum (Ta)–Copper (Cu) alloys were further analyzed by the addition of Ta and Cu into the Ti3Al2V custom alloy. The biological, mechanical, and tribo-biocorrosion properties of Ti3Al2V alloy were evaluated. A 10 wt.% Ta (10Ta) and 3 wt.% Cu (3Cu) were added to the Ti3Al2V alloy to enhance biocompatibility and impart inherent bacterial resistance. Additively manufactured implants were investigated for resistance against Pseudomonas aeruginosa and Staphylococcus aureus strains of bacteria for up to 48 h. A 3 wt.% Cu addition to Ti3Al2V displayed improved antibacterial efficacy, i.e. 78%–86% with respect to CpTi. Mechanical properties for Ti3Al2V–10Ta–3Cu alloy were evaluated, demonstrating excellent fatigue resistance, exceptional shear strength, and improved tribological and tribo-biocorrosion characteristics when compared to Ti6Al4V. In vivo studies using a rat distal femur model revealed improved early-stage osseointegration for alloys with 10 wt.% Ta addition compared to CpTi and Ti6Al4V. The 3 wt.% Cu-added compositions displayed biocompatibility and no adverse inflammatory response in vivo. Our results establish the Ti3Al2V–10Ta–3Cu alloy’s synergistic effect on improving both in vivo biocompatibility and microbial resistance for the next generation of load-bearing metallic implants.