3D printing of smart materials: A review on recent progresses in 4D printing

ABSTRACT

Additive manufacturing (AM), commonly known as three-dimensional (3D) printing or rapid prototyping, has been introduced since the late 1980s. Although a considerable amount of progress has been made in this field, there is still a lot of research work to be done in order to overcome the various challenges remained. Recently, one of the actively researched areas lies in the additive manufacturing of smart materials and structures. Smart materials are those materials that have the ability to change their shape or properties under the influence of external stimuli. With the introduction of smart materials, the AM-fabricated components are able to alter their shape or properties over time (the 4th dimension) as a response to the applied external stimuli. Hence, this gives rise to a new term called ‘4D printing’ to include the structural reconfiguration over time. In this paper, recent major progresses in 4D printing are reviewed, including 3D printing of enhanced smart nanocomposites, shape memory alloys, shape memory polymers, actuators for soft robotics, self-evolving structures, anti-counterfeiting system, active origami and controlled sequential folding, and some results from our ongoing research. In addition, some research activities on 4D bio-printing are included, followed by discussions on the challenges, applications, research directions and future trends of 4D printing.     

Interstitial elements created via metal 3D printing

3D printing of metals, such as laser powder bed fusion (LPBF) printing of stainless steels, often leads to elevated oxygen content in the alloy substrate relative to conventional processing routes. Here we show that the extremely rapid cooling rate (10⁶–10⁷ K/s) during LPBF processing of austenitic stainless steel can trap a considerable fraction of oxygen and other elements in the interstitial sites of the metal lattice, at concentrations far exceeding the room temperature solubilities. High resolution characterization and atomistic simulations with density functional theory reveal that oxygen and other elements exist in octahedral interstitial sites of the metal lattice and bond with their neighboring metal atoms. Our findings suggest that additive manufacturing can be a potential strategy of incorporating beneficial interstitial elements into a metal substrate. Given the well-known effect of interstitial elements in conventional alloys, significant improvement of the physicochemical properties of printed alloys is possible.     

3D printing technology and its impact on Chinese manufacturing

For both developed and developing countries, manufacturing plays a crucial role in international competition. There is a growing consensus that 3D printing (3DP) technologies will revolutionise the development of global manufacturing. Although considerable research has previously been conducted to define the technological and economic benefits of 3DP on global manufacturing, minimal research has linked 3DP with Chinese manufacturing (CM). Therefore, to address this research gap and to investigate 3DP’s potential impact on alleviating CM’s development issues, this paper explores the definition, characteristics and mainstream technologies of 3DP, presents the current situation and the main problems of CM, and analyses the potential impact of 3DP on the development of CM. Then, this study introduces the current 3DP promotion and industrialisation situation in China as well as the issues with promoting 3DP in CM.     

3D打印技术探究

3D打印技术是将三维数字模型分解成若干层平面切片,然后由3D打印机把粉末状、液状或丝状可粘合材料按切片图形逐层叠加,最终堆积成完整物体的技术。文章对3D打印的技术原理、步骤、常用材料、主要技术、发展及建议进行了深层次的探讨。与传统制造技术相比,3D打印技术有很多优势,目前已广泛应用于建筑、工业设计等领域。该技术将会带来全球制造业经济的重大变革。     

3D打印AlSi10Mg合金组织性能研究

3D打印技术(3D printing)是20世纪90年代出现的一种新型快速成型技术,为提升我国与该技术相关的扫描路径、扫描宽度和扫描深度等关键参数的控制水平,本文利用金相显微镜和扫描电镜分析了用进口EOSM280金属粉末激光烧结系统制备的AlSi10Mg合金制品的组织,并检测了制品的力学性能。结果表明:3D打印试样平行构建方向形成典型的鱼鳞状组织,这些鱼鳞状组织是激光扫描熔池凝固后形成的;组织分析结果表明,扫描形成熔池深度约为180μm,扫描间距约为210μm,层与层间的扫描方向互成120°;凝固熔池内形成了典型的柱状晶,其生长方向与传热方向平行;柱状晶内的共晶组织呈"管状",Si颗粒尺寸细小均匀;平行于构建方向样品的抗拉强度和延伸率分别为487.5 MPa和9.0%,垂直于构建方向样品的抗拉强度和延伸率分别为490.0 MPa和7.0%.     

3D printing with particles as feedstock materials

3D printing has been applied in numerous research fields ranging from biomedical, mechanical engineering and chemistry to material science. 3D printing applications have driven innovations in particle technology, especially through tackling particle-related issues arising from the development of particle-based printing feedstocks across such application areas. Therefore, in this review, established 3D printing processes are described to include their prototyping mechanisms, advantages and limitations. Various particulate systems, including dry and wet systems, as printing feedstock materials are introduced. The main motivation for this paper is to outline the current state of particulate feedstock systems and to attempt to outline future directions for enhancing these particle applications. This paper would be valuable for individuals, researchers and companies who need adequate and comparative information regarding the state of particle applications in the AM industry.     

磁控触变流体的3D打印工艺研究

针对现有磁控智能流体无法应用于3D打印的问题,本文制备了一种在磁场调控下能够实现溶胶-凝胶可逆转变的新型磁控触变流体,并对其打印工艺进行了研究。采用全系统损耗润滑油、有机膨润土和Fe₃O₄颗粒制备了不同配比的打印样品,搭建了直写式3D打印实验平台,研究其可打印性。流变学实验表明:磁控触变流体有机膨润土含量越高,触变性越强,剪切变稀越明显;磁场强度越强,屈服应力越高,储存模量越高。3种挤出方式下的打印实验结果表明,采用匀料恢复挤出方式打印的结构具备最高分辨率和最大高度。磁控触变流体在根据其流变特性设计的挤出装置下具有良好可打印性,为该材料应用于柔性传感、软体机器人个性化复杂结构设计与驱动、微流控检测等领域奠定了基础。     

一种3D打印立体光刻快速成型光敏树脂的制备及性能研究

报道了一种应用于3D打印立体光刻快速成型的3DPSL-1型光敏树脂制备方法,并对该光敏树脂性能进行了研究。研究结果表明,该光敏树脂透射深度(Dp)为0.14 mm,临界曝光量(EC)为14.1 m J/cm2,30℃时,黏度为359 m Pa·s。在25℃,3DPSL-1型光敏树脂密度为1.17 g/cm3,固化后的密度为1.21 g/cm3,固化体积收缩率为3.31%。该光敏树脂固化物样条拉伸强度为21.0 MPa,弹性模量为1 108.2 MPa,断裂伸长率为10.6%,该固化物的玻璃化温度为47℃。把该光敏树脂作为打印材料应用于3D打印立体光刻快速成型设备上制作了活动钳子这个零件,其制作效果较好。该光敏树脂的研制将对推动国内3D打印立体光刻快速成型技术的进一步发展起到较好的积极作用。     

3D printing of living structural biocomposites

Nature fabricates organic/inorganic composites under benign conditions, yet, in many cases, their mechanical properties exceed those of the individual building components it is made from. The secret behind the evolutionary pivot is the unique ability of nature to control structure and local composition of its materials. This tight control is often achieved through compartmentalization of the reagents that can be locally released. Inspired by nature, we introduce an energy-efficient process that takes advantage of the compartmentalization to fabricate porous CaCO₃-based composites exclusively comprised of nature-derived materials whose compressive strength is similar to that of trabecular bones. The unique combination of nature-derived materials, 3D printability, and good mechanical properties is achieved through the formulation of these materials: We combine microgel-based granular inks that inherently can be 3D printed with the innate potential of engineered living materials to fabricate bacteria-induced biomineral composites. The resulting biomineral composites possess a porous trabecular structure that comprises up to 93 wt% CaCO₃ and thereby can withstand pressures up to 3.5 MPa. We envisage this system to have the potential to be used in art restoration, serve as artificial corals to help the regeneration of marine reefs, and, with additional work, might even allow the reparation of broken or partially disintegrated natural mineral-based materials such as certain parts of bones.     

Direct-write 3D printing of NdFeB bonded magnets

We report a method to fabricate Nd–Fe–B (NdFeB) bonded magnets of complex shape via extrusion-based additive manufacturing (AM), also known as 3D-printing. We have successfully formulated a 3D-printable epoxy-based ink for direct-write AM with anisotropic MQA NdFeB magnet particles that can be deposited at room temperature. The new feedstocks contain up to 40 vol.% MQA anisotropic NdFeB magnet particles, and they are shown to remain uniformly dispersed in the thermoset matrix throughout the deposition process. Ring, bar, and horseshoe-type 3D magnet structures were printed and cured in air at 100°C without degrading the magnetic properties. This study provides a new pathway for fabricating NdFeB bonded magnets with complex geometry at low temperature, and presents new opportunities for fabricating multifunctional hybrid structures and devices.     

Additive manufacturing of concrete in construction: potentials and challenges of 3D concrete printing

Additive manufacturing is gaining ground in the construction industry. The potential to improve on current construction methods is significant. One of such methods being explored currently, both in academia and in construction practice, is the additive manufacturing of concrete (AMoC). Albeit a steadily growing number of researchers and private enterprises active in this field, AMoC is still in its infancy. Different variants in this family of manufacturing methods are being developed and improved continuously. Fundamental scientific understanding of the relations between design, material, process, and product is being explored. The collective body of work in that area is still very limited. After sketching the potential of AMoC for construction, this paper introduces the variants of AMoC under development around the globe and goes on to describe one of these in detail, the 3D Concrete Printing (3DCP) facility of the Eindhoven University of Technology. It is compared to other AMoC methods as well as to 3D printing in general. Subsequently, the paper will address the characteristics of 3DCP product geometry and structure, and discuss issues on parameter relations and experimental research. Finally, it will present the primary obstacles that stand between the potential of 3DCP and large-scale application in practice, and discuss the expected evolution of AMoC in general.     

3D printing of tuneable agglomerates: Strain distribution and effect of internal flaws

The current study presents a novel and reliable method for producing 3D printed agglomerates with different colour distributions and material properties with 2-fold aims: providing feasible and accurate control on compression of agglomerates under different compression angles, and better tracking of individual particle position after agglomerate breakage. Multi-coloured agglomerates in cubic tetrahedral and random sphere shapes were printed with both rigid and soft bonds. The printed agglomerates were analysed thoroughly of their surface and structural properties including surface roughness and printing accuracy. The agglomerate breakage behaviours under static compression were analysed as a function of bond strength, loading rate and loading directions, with strain distribution plotted over the random sphere agglomerate structure. In addition, agglomerate structures with designed internal macro-voids in different positions and sizes were also created for breakage study, in an effort to better understand parameters governing the mechanical properties of agglomerates with cavities and voids which is inevitable in particle industry but poorly understood at present.     

Synthesis and characterization of photopolymerizable triblocks for 3D printing tissue engineering scaffolds

While previous research on polycaprolactone (PCL) and polyethylene glycol (PEG) triblock copolymers has focused on their use as hydrogels or with conventional scaffold fabrication methods, this work concentrates on producing viable photocurable resins from synthesized triblocks for use in a layer-by-layer 3D printer. After successful synthesis of PCL-PEG-PCL and PCL-PEG-PCL-diacrylate triblocks, they were combined with (hydroxyethyl)methacrylated polyethylene glycol (PEG-HEMA) and used as biomaterials in a dynamic masking 3D printing system to fabricate porous scaffolds. Diacrylation of the polymer (PCL-PEG-PCL-DA) revealed a substantial increase in mechanical strength and resulting compound resolved the re-dissolving issue significantly during the 3D printing process. Degradation tests were carried out by incubation in phosphate-buffered saline, and both biomaterials demonstrated their degradation resistance with steady pH levels and mass loss plateauing at 20% over a sixty day timeframe. Preliminary MG63 cell culture tests on the cross-linked 3D porous structures showed no significant cytotoxicity and MTT assay data verified cell proliferation on the photocured samples after three days. As a result, end-capping PCL-PEG-PCL with acrylates demonstrated advantages over PCL-PEG-PCL while keeping similar performance in degradation and biocompatibility. Overall results from this work demonstrate the suitability of the novel triblocks for use as biomaterials in tissue engineering scaffolds.     

Role of porosity defects in metal 3D printing: Formation mechanisms,impacts on properties and mitigation strategies

Metal 3D printing (3DP), a state-of-the-art manufacturing technology that brings the potential to fabricate complex structures at low cost and reduced energy consumption, has been extensively adopted in various industries. However, the porosity defects inherited from the printing process can significantly impede the mechanical properties and weaken the performance of as-printed components, potentially challenging this approach's reliability and reproducibility. The advancement of detection techniques currently opens up a more intuitive and deeper study of porosity defects. Given that, this review systematically states the 'restriction role' of porosity defects in metal 3DP by generalizing the detailed information on porosity defects, including their characterizations, formation and migration mechanisms, and their impacts on the performance of printed parts. Furthermore, feasible porosity mitigation measures are discussed to inspire more advanced methodologies for the next generation of metal 3DP.     

Mechanical properties of 3D printed interpenetrating phase composites with novel architectured 3D solid-sheet reinforcements

Interpenetrating phase composites (IPCs) are novel types of multifunctional composite materials. This work focuses on investigating experimentally and computationally the mechanical behavior of novel types of three-dimensional (3D) architectured two-phase IPCs. The current IPCs are architectured using several morphologies of the fascinating and mathematically-known triply periodic minimal surfaces (TPMS) that promote several multifunctional attributes. Specifically, the second hard reinforcing phase takes the architecture of one of the 3D non-intersecting and continuous TPMS-based solid sheets. The mechanical response of the 3D printed polymer-based IPCs is measured under uniaxial compression where the effect of varying the second-phase architecture and volume fraction is explored. Anisotropy induced by the 3D printing is also investigated. 3D finite element analysis has been performed and validated for predicting elastic properties of the various types of TPMS-based IPCs. The most effective TPMS architecture in enhancing the mechanical properties and damage-tolerance has been identified.     

Hierarchically self-morphing structure through 4D printing

Hierarchical self-morphing refers to the concurrent global and local changes in shape or structure. Previous research works have demonstrated 3D printed self-morphing structures and the sequential folding/unfolding behaviours. However, the shape change events occurred mainly at the global level in a water environment either through absorbing moisture or through heating. Concurrent global and local shape changes in an ambient environment have not been reported. In this paper, we report a hierarchically blooming flower that blossoms in an ambient environment. Our design considers the strain limit through understanding the effect of thickness on the local strain to avoid fracture and the appropriate allocation of multiple materials to achieve predefined global and local shape changes. This design approach of hierarchical 4D printing may be useful for a variety of applications that involve controlled self-morphing structures with complex geometries.     

三维打印碳纤维增强聚合物复合材料热残余解析解

三维打印碳纤维增强聚合物（CFRP）复合材料因层间材料失配和打印过程中梯度降温而产生热残余现象,影响工件成形质量。本文取代简单的同步降温假设,提出了符合实际制备工艺的梯度降温概念,据此建立了三维打印正交铺层复合材料板和梁的热残余变形和应力的解析解。为反映三维打印过程中随时序动态变化的制备和降温过程,考虑每层制备轮次的降温梯度并进行热残余分析,最后合成得到热残余变形和应力。讨论了4种梯度降温模式,覆盖了所有可能的三维打印工艺。以CFRP复合材料三维打印为例,验证了本文解析解的精度和可靠性,显示了同步降温假设会产生显著的误差,表明热残余水平与降温梯度成正比,讨论了铺层方式对热残余的影响。为优化三维打印CFRP复合材料的结构设计和制备工艺、降低热残余水平提供了可靠的分析方法。      

Development of Bioimplants with 2D, 3D,and 4D Additive Manufacturing Materials

Over the past 30 years, additive manufacturing (AM) has developed rapidly and has demonstrated great potential in biomedical applications. AM is a materials-oriented manufacturing technology, since the solidification mechanism, architecture resolution, post-treatment process, and functional application are based on the materials to be printed. However, 3D printable materials are still quite limited for the fabrication of bioimplants. In this work, 2D/3D AM materials for bioimplants are reviewed. Furthermore, inspired by Tai Chi, a simple yet novel soft/rigid hybrid 4D AM concept is advanced to develop complex and dynamic biological structures in the human body based on 4D printing hybrid ceramic precursor/ceramic materials that were previously developed by our group. With the development of multi-material printing technology, the development of bioimplants and soft/rigid hybrid biological structures with 2D/3D/4D AM materials can be anticipated.     

3维打印用聚乳酸材料的改性研究进展

聚乳酸材料具有环保可生物降解性的优点,故其经常作为3维(3D)打印的原材料使用,然而其自身的脆性大、玻璃化温度低和热稳定性差等缺点,限制了该类材料的进一步应用和推广。所以对聚乳酸进行改性研究,改善它的力学性能或者耐热性能,从而扩大其在3D打印领域的应用具有很重要的研究意义。综述了聚乳酸材料的改性方法以及相关研究进展,主要从物理改性和化学改性等两类改性方法来分析聚乳酸改性的研究现状,总结分析了两类改性方法面临的问题并展望其前景,还对改性后的聚乳酸材料的应用进展进行总结与展望。     

Mechanical properties of titanium-based Ti–6Al–4V alloys manufactured by powder bed additive manufacture

Additive manufacturing is currently a topic of considerable interest at both academic and industrial levels. While a significant amount of data exists on the mechanical properties and structure–property relationships of traditional wrought alloys, less information is available on alloys manufactured by additive manufacture. This review examines current state-of-the-art manufacture of titanium-based Ti–6Al–4V alloys by powder bed additive manufacture. Published mechanical properties to date are collected which include tensile strength, yield strength, hardness, wear, fracture toughness and fatigue. Differences in microstructure and properties compared to conventional wrought alloys of the same composition are described.