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In recent years, there has been rising interest in additively manufactured shape memory polymers (SMPs) and their multi-functional composites. Four-dimensional (4D) printing is an intriguing additive manufacturing field, which uses time-responsive programmable materials. Stimuli-responsive polymers present an ability to retain their original shapes from a programmed temporary shape upon exposure to different external stimuli including heat, light, electricity, humidity, or magnetism. In recent years, rapid technological developments have been made in the field of smart and multi-functional materials by integrating 4D printing (4DP) and shape memory polymer composites (SMPCs). 4DP of SMPCs helps in exploring the myriad engineering applications including automotive, soft robotics, food, smart textiles, biomedical devices, mechatronics, and wearable electronics. Herein, this review article presents important 4DP technologies in conjunction with the underlying functionalities of stimuli-responsive polymer composites. This review also elucidates the future opportunities of 4D-printed SMPCs in terms of preprogramming knowledge, multi-way SMPCs, multi-material printing, sustainability, and potential applications. Finally, based on the characteristics of the SMPCs employed for each technology illustrative examples of the principal applications are provided. It is anticipated that the insightful exploration of this novel field could support future advancements as well as spur innovations in the 4DP field.

In this paper, a thermo-mechanical analysis of shape memory polyurethane foams (SMPUFs) with aiding of a finite element model (FEM) for treating cerebral aneurysms (CAs) is introduced. Since the deformation of foam cells is extremely difficult to observe experimentally due to their small size, a structural cell-assembly model is established in this work via finite element modeling to examine all-level deformation details. Representative volume elements of random equilateral Kelvin open-cell microstructures are adopted for the cell foam. Also, a user-defined material subroutine (UMAT) is developed based on a thermo-visco-elastic constitutive model for SMPUFs, and implemented in the ABAQUS software package. The model is able to capture thermo-mechanical responses of SMPUFs for a full shape memory thermodynamic cycle. One of the latest treatments of CAs is filling the inside of aneurysms with SMPUFs. The developed FEM is conducted on patient-specific basilar aneurysms treated by SMPUFs. Three sizes of foams are selected for the filling inside of the aneurysm and then governing boundary conditions and loadings are applied to the foams. The results of the distribution of stress and displacement in the absence and presence of the foam are compared. Due to the absence of similar results in the specialized literature, this paper is likely to fill a gap in the state of the art of this problem and provide pertinent results that are instrumental in the design of SMPUFs for treating CAs.

This research demonstrates how a combination of two-way shape memory alloy (SMA), low-temperature liquid epoxy cure composites, and fibre reinforced plastic (FRP) may be utilised to create a novel reversible actuator with built driven functionality. The novelty of this work is that the actuator can reverse its original shape and be mounted on different customized structures. The strategy is based on a knowledge of SMA wires and the manufacturing principle underlying composite structure, as well as experiments to see how soft SMA-FRP can be programmed to bend. The folding mechanism is studied in terms of fabrication factors such as SMA training and strong interfacial bonding between SMA and epoxy resin, which influence the programming process and shape change. The two-way SMA wires are trained using the pre-straining method to programme the SMAs. The technique has been used to assemble the SMA wires with bond reliability to enhance the actuator interface's thermal behaviour. The SMA elements are directly inserted into FRP strips and epoxy resin is used as an adhesive, resulting in dynamic hybrid composites. The module is actuated using an electrical board with a current value between 3 and 6 A. The robustness, controllability, mechanical properties, and 500 life cycles of the actuator are tested. Results indicate a bending angle of 58° with 30 mm of deflection in 7 s after actuating the module. Also, 3D printing is used to print a gripper inspired by human fingers and a structure to lift various weights. The actuator’s performance as a soft gripper is reliable in terms of grasping objects of different shapes.

Drug delivery systems are advanced methods that aim to deliver a targeted drug to a specific location or release it at a controlled rate. Many methods have been proposed for drug delivery systems, among which microfluidic systems present unique advantages. In contrast to bulk methods, in this work, by considering the unique capacity of microfluidic-based drug delivery systems, including controllability of fabricated chip geometry and flow rate of multiphase fluid, highly stable particles with higher encapsulation efficiency can be generated. Employing additive manufacturing in biomedical applications has enabled researchers to propose novel and accurate microfluidic systems. In this paper, by employing stereolithography (SLA) and fused deposition modeling (FDM) 3D printing, a microfluidic-based drug delivery system for generating polycaprolactone (PCL) droplets loaded with dexamethasone drug is fabricated. Scanning electron microscope (SEM) images and microscopic images show the effectiveness of this method in generating such droplets.

Soft magnetic composites are exceptional because they can be controlled remotely, move quickly, conform to hard things, and interact with people safely. However, even with all these features, magnetic elastomers suffer a lack of stability due to the high softness of the elastomer. This issue affects their controllability and repeatability. This article introduces a novel conceptual design of magneto-responsive shape memory polyurethane (SMP) foam composites with high stability and reversibility. The fabrication technique is based on the silicone resins filled with strontium ferrite magnetic particles and a thin SMP foam placed onto one side. Material properties, room-temperature shape recovery features, and magnetization conditions necessary for the process are determined by experimental studies of composite actuators. As a result, a workable, light, stable, soft composite gripper with programmable magnetic patterns is created, which can carry out activities like grabbing, holding, and moving objects in horizontal and vertical directions when a low magnetic field is applied. The SMP foam increases the contact surface and decreases the weight by up to three times providing better stability compared to the magnetic elastomer without SMP foam. The shape-recoverable gripper with a small contract area can lift objects eight times heavier than its weight.

This article introduces novel 3D zero Poisson's ratio (ZPR) metamaterials for reversible energy absorption applications fabricated by 4D printing technology. The designs are introduced based on piecemeal energy absorption (PEA) and conventional energy absorption (CEA) approaches. Topologically, the design of the 3D metamaterials is founded on star-shaped unit cells herein. To achieve the PEA behavior, horizontal bars are merged into the parent star-shaped unit cell. This leads to introducing multistiffness unit cells (controllable unit-cell densifications) to provide stability and different peak force levels during compression. For further evaluation, finite element analysis (FEA) is employed. To illustrate the design functions during physical operation and validate the FEA, lattice-based metamaterials are fabricated from resin with a shape recovery property by an SLA 3D printer and tested mechanically. Close coincidence is observed between the FEA and the experiments, showing the accuracy of the modeling. A thermal test, via a heating–cooling process, is also carried out to display the shape recovery capability of metamaterials where plastic deformations are fully released, and samples get back to their original shapes. Finally, the newly proposed ZPRs are compared with conventional 3D reentrant metamaterials in terms of energy absorption capacity, demonstrating their considerable mechanical performances.

Tissue engineering, whose aim is to repair or replace damaged tissues by combining the principle of biomaterials and cell transplantation, is one of the most important and interdisciplinary fields of regenerative medicine. Despite remarkable progress, there are still some limitations in the tissue engineering field, among which designing and manufacturing suitable scaffolds. With the advent of additive manufacturing (AM), a breakthrough happened in the production of complex geometries. In this vein, AM has enhanced the field of bioprinting in generating biomimicking organs or artificial tissues possessing the required porous graded structure. In this study, triply periodic minimal surface structures, suitable to manufacture scaffolds mimicking bone’s heterogeneous nature, have been studied experimentally and numerically; the influence of the printing direction and printing material has been investigated. Various multi-morphology scaffolds, including gyroid, diamond, and I-graph and wrapped package graph (I-WP), with different transitional zone, have been three-dimensional (3D) printed and tested under compression. Further, a micro-computed tomography (μCT) analysis has been employed to obtain the real geometry of printed scaffolds. Finite element analyses have been also performed and compared with experimental results. Finally, the scaffolds’ behavior under complex loading has been investigated based on the combination of μCT and finite element modeling.

The biomechanical and hemodynamic effects of atherosclerosis on the initiation of intracranial aneurysms (IA) are not yet clearly discovered. Also, studies for the observation of hemodynamic variation due to atherosclerotic stenosis and its impact on arterial remodeling and aneurysm genesis remain a controversial field of vascular engineering. The majority of studies performed are relevant to computational fluid dynamic (CFD) simulations. CFD studies are limited in consideration of blood and arterial tissue interactions. In this work, the interaction of the blood and vessel tissue because of atherosclerotic occlusions is studied by developing a fluid and structure interaction (FSI) analysis for the first time. The FSI presents a semi-realistic simulation environment to observe how the blood and vessels' structural interactions can increase the accuracy of the biomechanical study results. In the first step, many different intracranial vessels are modeled for an investigation of the biomechanical and hemodynamic effects of atherosclerosis in arterial tissue remodeling. Three physiological conditions of an intact artery, the artery with intracranial atherosclerosis (ICAS), and an atherosclerotic aneurysm (ACA) are employed in the models with required assumptions. Finally, the obtained outputs are studied with comparative and statistical analyses according to the intact model in a normal physiological condition. The results show that existing occlusions in the cross-sectional area of the arteries play a determinative role in changing the hemodynamic behavior of the arterial segments. The undesirable variations in blood velocity and pressure throughout the vessels increase the risk of arterial tissue remodeling and aneurysm formation.