The selective laser melting (SLM) of Ti–Ni mixed powder with atomic ratio of 1:1 was performed in the present work in order to elaborate shape memory alloy (SMA). The martensite phase of Ti–Ni alloy can be found by X-ray diffraction (XRD) analysis under temperature field, moreover, the Ti2Ni phase at a high scanning velocity. The crystalline phase images also show that the synthesized Ti–Ni alloy possessed a refined martensite microstructure. In order to evaluate the mechanical properties, the microhardness and porosity were measured. The microhardness is relatively high about 400HV0.2 with champ temperature. Besides, the porosity is quite low due to the excellent laser energy absorptivity and meltability of Ni element. The differential scanning calorimetry (DSC) analysis shows that the transformation temperature from austenite phase to martensite phase is relatively high and stable.

Microstructure and Transformation Behavior of in-situ Shape Memory Alloys by Selective Laser Melting Ti--Ni Mixed Powder

4D printing of shape memory polymer composites: A review on fabrication techniques, applications, and future perspectives

4D bioprinting is promising to build cell‐laden constructs (bioconstructs) with complex geometries and functions for tissue/organ regeneration applications. The development of hydrogel‐based 4D bioinks, especially those allowing living cell printing, with easy preparation, defined composition, and controlled physical properties is critically important for 4D bioprinting. Here, a single‐component jammed micro‐flake hydrogel (MFH) system with heterogeneous size distribution, which differs from the conventional granular microgel, has been developed as a new cell‐laden bioink for 4D bioprinting. This jammed cytocompatible MFH features scalable production and straightforward composition with shear‐thinning, shear‐yielding, and rapid self‐healing properties. As such, it can be smoothly printed into stable 3D bioconstructs, which can be further cross‐linked to form a gradient in cross‐linking density when a photoinitiator and a UV absorber are incorporated. After being subject to shape morphing, a variety of complex bioconstructs with well‐defined configurations and high cell viability are obtained. Based on this system, 4D cartilage‐like tissue formation is demonstrated as a proof‐of‐concept. The establishment of this versatile new 4D bioink system may open up a number of applications in tissue engineering.

Jammed Micro‐Flake Hydrogel for Four‐Dimensional Living Cell Bioprinting

Nature's material systems during evolution have developed the ability to respond and adapt to environmental stimuli through the generation of complex structures capable of varying their functions across direction, distances and time. 3D printing technologies can recapitulate structural motifs present in natural materials, and efforts are currently being made on the technological side to improve printing resolution, shape fidelity, and printing speed. However, an intrinsic limitation of this technology is that printed objects are static and thus inadequate to dynamically reshape when subjected to external stimuli. In recent years, this issue has been addressed with the design and precise deployment of smart materials that can undergo a programmed morphing in response to a stimulus. The term 4D printing was coined to indicate the combined use of additive manufacturing, smart materials, and careful design of appropriate geometries. In this review, we report the recent progress in the design and development of smart materials that are actuated by different stimuli and their exploitation within additive manufacturing to produce biomimetic structures with important repercussions in different but interrelated biomedical areas.

4D printing in biomedical applications: emerging trends and technologies

A review on four-dimensional bioprinting in pursuit of advanced tissue engineering applications

https://iopscience.iop.org/article/10.1088/1361-665X/abcc0a/pdf

4D printing and collaborative design of highly flexible shape memory alloy structures: a case study for a metallic robot prototype

This article presents a soft crawling robot prototype with a simple architecture inspired by inchworms. The robot functionally integrates the torso (body) and feet in a monolithic curved structure ...

A shape memory alloy–actuated soft crawling robot based on adaptive differential friction and enhanced antagonistic configuration:

Two-dimensional shape memory alloy (SMA) structures have great potential in intelligent and highly flexible robots. However, they have challenges in direct electric heating. This paper proposes a solution to heat SMA sheets using carbon fibres as surface heat sources and indirectly transmits Joule heat generated by the carbon fibres to the SMA sheets. The preparation process of the carbon fibre-based thermoelectric composite film is illustrated in this study. The effect of heat source (carbon fibre) distribution on heat transfer characteristics and the temperature field of the SMA surface is simulated. Then, a functional prototype integrated thermoelectric film and flexible SMA is fabricated, and its effectiveness is verified by local and global activation, respectively. The results show that the composite structure can recover deformation rapidly in 2 s with only 0.7 A running current (applied to each carbon fibre). In addition, the local temperature control method can also realize distributed (spatial dimension) and sequential (temporal dimension) control of the structure. The proposed solution is expected to expand further the design and control of complex, intelligent structures with the development of 4D printing technology.

A flexible carbon fibre-based electrothermal film for fast actuation of shape memory alloy sheets

ABSTRACT Leveraging advances in shape memory alloys (SMAs) and flexible thermoelectric devices (f-TEDs), this paper presents a structural and functional integrity composite sheet to address the inefficient and bulky activation of 2-D SMAs. A series of experimental tests were performed to reveal the generation, change, and transfer mechanisms of different types of electrically-induced heat in the f-TED, as well as the temperature-induced SMA phase transformation behaviors. The results show that the composite sheet exhibited good bidirectional thermal management capacity. The austenite deformation can be completed within 10s at an operating current of 2 A. The cooling recovery, in particular, performs much better than in conventional modes (the temperature declines exponentially with time). Finally, through two functional prototypes, a light switch and a flexible gripper, the application potential of the proposed composite was further experimentally demonstrated. Graphical Abstract

https://www.tandfonline.com/doi/pdf/10.1080/19475411.2022.2076754?needAccess=true

Tao Yao

Papers

4D printing and collaborative design of highly flexible shape memory alloy structures: a case study for a metallic robot prototype

A shape memory alloy–actuated soft crawling robot based on adaptive differential friction and enhanced antagonistic configuration:

A flexible carbon fibre-based electrothermal film for fast actuation of shape memory alloy sheets

A flexible and smart shape memory alloy composite sheet based on efficient and bidirectional thermal management