1 Introduction

Within the dynamic realm of additive manufacturing, 4D printing emerges as a revolutionary progression [1, 2], augmenting the functionalities of conventional 3D printing (material extrusion [3], vat photopolymerization [4], powder bed fusion [5], etc.) through the incorporation of the time dimension [2]. The term “4D Printing” was first coined in 2013 [6]. This groundbreaking technological advancement enables the fabrication of intelligent materials and structures that can undergo predetermined changes in shape, property, or functionality [7] in response to environmental stimuli including temperature [8], humidity [9], light [10], magnetic fields [11], etc. By facilitating the development of objects capable of self-assembly [12], repair [13], or adapt [14] to external conditions, this technology has the potential to significantly transform numerous industries, including biomedical engineering [15] and aerospace [16] industries. The immense potential of 4D printing to facilitate innovative applications and sustainable solutions underscores its importance as a manufacturing technology of the next generation.

The growing number of 4D printing applications indicates the criticality of comprehending the fatigue characteristics of materials—specifically, their ability to endure prolonged periods of stress or strain. Fatigue strength is one of the highest priorities, particularly for 4D printed materials that are intended to operate in dynamic environments and endure programmed transformations [17]. In practical applications, the reliability, safety, and durability of materials are directly impacted by their capacity to maintain their adaptive properties amidst cyclic loading conditions [18]. When designing and selecting materials for 4D printing, fatigue behavior becomes a crucial factor to consider; the durability of the adaptive response over time is equally as important as the initially demonstrated characteristics.

Extensive research has been dedicated to reviewing the studies of the fatigue behavior of 3D printed materials in the last 5 years [18,19,20,21], which signifies an increasing concern for their ability to withstand cyclic loading conditions and maintain their long-term dependability. The fatigue properties of a wide range of 3D printed materials, such as metals [19], polymers [20], and polymeric composites [18], have been the subject of extensive research. This has unveiled how printing parameters, layer orientation, and post-processing procedures affect the fatigue life of these materials. These inquiries yield significant knowledge regarding the optimization of 3D printing procedures to achieve improved mechanical characteristics, specifically for implementations in the aerospace [22], automotive [23], and biomedical [24] applications. Nevertheless, although extensive research has been conducted on the fatigue behavior of 3D printed materials, the investigation of fatigue properties in 4D printed materials is still largely unexplored. This gap in research highlights a critical need for comprehensive studies on the fatigue behavior of 4D printed materials, considering the additional complexity introduced by their time-dependent responses. There are more than 50 review articles found in Google Scholar for the year 2023. However, there is no review article in the literature solely focused on the fatigue behavior of 4D printed materials. Some research have been found in the literature evaluating 4D printed materials for fatigue behaviors. Despite this progress, the area of research concerning the fatigue characteristics of 4D printed materials is still in its early stages, where substantial deficits in knowledge persist.

The goal of this review paper is to bring together what is known about fatigue behaviors of 4D printed materials and point out the key study results, problems, and areas that could be explored further in the future. This paper will undertake a critical analysis of prior research and identify knowledge gaps to make a positive contribution to the wider implementation and progress of 4D printing technology.

2 Fatigue behavior of 4D printed materials

Zhang et al. [25] studied 4D Printable shape-memory polymer (SMP) using the DLP (Digital Light Processing)-based 3D printing method. In this process, a digital screen projects a single image of each layer onto the build platform to cure photopolymer resin into three-dimensional object (Fig. 3a) [25]. The objective of this study is to improve the mechanical robustness, deformability, and fatigue resistance of a newly designed tert-butyl acrylate (tBA) aliphatic urethane diacrylate (AUD) SMP specifically designed for 4D printing purposes. Superior fatigue resistance is considered as an important factor for SMPs and, consequently, 4D printed materials, as it guarantees their functionality and long-term dependability, particularly in applications that require the material to endure multiple shape transformations throughout its lifetime. The research methodology comprises an extensive set of tests that have been specifically developed to assess the performance of the SMP across a range of conditions. Mechanical tensile tests are employed to evaluate the strength and elasticity of the material, while fatigue testing determines its shape-memory and endurance, respectively, over thousands of loading cycles. In contrast to previously documented UV-curable SMPs, the tBA–AUD SMP exhibits exceptional fatigue resistance, as evidenced by its ability to endure more than 10,000 loading cycles without damage (Fig. 1a). On the other hand, other compounds such as Vero, tBA–PEGDA (PEGDA: Poly (ethylene glycol) diacrylate) (Fig. 1b), and BMA–PEGDMA (PEGDMA: Poly(ethylene glycol) dimethacrylate) (Fig. 1c) exhibited a lifespan of under 100. This finding suggests that the tBA–AUD SMP system has made substantial strides in terms of durability and suitability for dynamic applications.

Fig. 1
figure 1

Comparative fatigue resistance of SMP samples. a The endurance of a tBA–AUD SMP sample with 10 wt% AUD, showcasing its ability to withstand over 10,000 loading cycles. b The fatigue behavior of tBA-PEGDA (9:1, w/w) at 74 °C, highlighting a strain range of 10–20%. c The fatigue performance of BMA-PEGDMA (9:1, w/w) at 87 °C, with a strain tolerance between 20 and 40%. Reproduced with permission from [25]

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In another work, J.N. Chapuis and K. Shea [26] describe the application of PLA (Polylactic Acid) in the direct 4D printing of bilayer actuators to fabricate polymer wave springs. By integrating fused filament fabrication (FFF) with direct 4D printing, PLA bilayer actuators can be produced. In FFF, the material is formed into a filament, and the filament is pushed through a nozzle, which is heated to its melting point and then deposited layer by layer to create a three-dimensional (3D) object (Fig. 3b) [27]. By effectively integrating a pre-strain imbalance between active and passive layers, this technique enables the induction of bending or twisting. The principal material utilized is PLA (Polylactic Acid), which was selected due to its desirable printability, thermoplastic shape-memory characteristics, and compatibility with cold and hot programming methods. Taking advantage of a single material, the actuators are constructed to prevent delamination and guarantee robust adhesion, thereby improving cyclic performance. Active and passive layers are incorporated into the design to regulate deformation via the pre-strain imbalance. The print direction and process parameters are considered to regulate the nature and extent of deformation. Passive layers remain stationary during activation, whereas active layers undergo controlled deformation by contracting. Printed in a planar configuration, the actuators are patterned into a wave ring; upon activation, it takes on the form of a wave. This exemplifies the capability of direct 4D printing to produce durable, and functional components that are complex. The results of the research concerning the fatigue characteristics of direct 4D printed PLA bilayer wave springs provide important information regarding the structural soundness of the springs when subjected to cyclic loading. The springs exhibited remarkable resistance to fatigue during rigorous testing, as evidenced by the minimal damage observed even after 10,000 cycles (Fig. 2b). On closer inspection, however, initial fracture nucleation was observed (Fig. 2e), which signifies the initiation of fatigue damage. However, cracks (Fig. 2c) and fatigue striations (Fig. 2f) have been observed after deployments. This observation highlights the significant influence that thermal stresses have on the durability of the material during the redeployment process. The failure mechanisms of the 4D printed springs were not associated with layering, which contrasts with the early fatigue failure observed in conventional FFF parts. This finding emphasizes the engineering benefit of employing this printing technique to prolong the lifespan of components subjected to cyclic stress. The significance of material behavior and failure mechanisms in the design and fabrication of 4D printed components for applications involving recurrent loading is highlighted by this analysis.

Fig. 2
figure 2

Fatigue and Redeployment Analysis of 4D Printed PLA Wave Springs. a The spring post-initial deployment, showing its original condition. b The wave ring's tension side after 104 cycles without redeployment, revealing no visible damage. c The tension side of a spring subjected to redeployment tests, where active PLA has experienced cracking and failure. d The tension side in a different section of the wave spring in its printed state. E The tension side of the same wave ring post-10^4 cycles, with a small crack nucleation visible upon detailed inspection. f Fatigue striations on the tension side of a spring after redeployment tests, indicating fatigue damage. Reproduced with permission from [26]

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In another work, Yousuf et al. [28] utilize a thermoplastic shape-memory polymer (SMP) to 3D print an array of samples, including dogbone samples for testing and multi-cell and simple honeycomb structures. This particular polymer, which is suggested for deposition at a temperature of 200 °C, enables the fabrication of structures whose mechanical properties can be adjusted. Precise layer adhesion and infill density are key considerations during the printing process to ensure that the material's functionality corresponds to the intended experimental objectives, especially when analyzing local strain in particular structural regions. As a result of the accumulation of residual strains, repeated programming and recuperation cycles have a substantial impact on the mechanical properties of 4D printed structures. The deterioration is subject to the influence of both cycle frequency and programming intensity, highlighting the crucial significance of functional fatigue concerning the durability and dependability of 4D printed materials.

One research [29] describes a 4D printing technique for composite materials that embeds long fibers in polymeric resins to produce composites that are rigid, fatigue-resistant, and robust. The procedure entails the automated application of thin layers featuring a range of fiber orientations. These layers undergo differential rates of contraction as a result of the distinct thermal contraction coefficients after curing and chilling. This process initially transforms planar layers into curved structures without requiring complex molds. In particular, it employs carbon/epoxy materials, automated fiber positioning, and a cure temperature of 177 °C to demonstrate how the anisotropic properties of layered materials can be utilized to efficiently fabricate complex composite structures via 4D printing. The article also analyses the fatigue characteristics of composite materials produced via 4D printing by analyzing a 24-inch-long specimen that underwent a fatigue test involving three-point deformation. The experiment encompassed 175,000 cycles and recorded maximum and minimum displacements of 2.4 mm and 0.24 mm, respectively. The results demonstrated that the spring constant of the laminate remained unchanged throughout the fatigue test, indicating that the material possesses exceptional fatigue resistance. The significance of this result lies in the fact that it illustrates the capacity of these 4D printed composites to retain their mechanical integrity and functionality despite extended cyclic loading. This property is critical for the implementation of the composites in sectors where materials endure repetitive strain, such as the automotive and prosthetic device industries.

Another research [30] investigates the evolution, mechanical characteristics, and degradation structure of polylactic acid (PLA) peripheral vascular stents produced via 4D printing. The study centers on the performance of the stents when subjected to fluid shear stress after deployment. It provides evidence that degradation is expedited more significantly by microstructural damage resulting from deployment compared to shear stress in isolation. The stents exhibit commendable mechanical strength and hemocompatibility; the degradation characteristics are notably impacted by dynamic environmental conditions and microstructural damage. This research offers significant contributions to the understanding and assessment of forthcoming biodegradable stents, emphasizing the criticality of incorporating degradation characteristics and mechanical attributes during their formulation. The fatigue resistance of the stents was assessed to replicate the vasodilation effects. The findings suggest that stents featuring wall thicknesses of 0.2 mm and 0.3 mm exhibit considerable resistance to fatigue. Oppositely, the fatigue resistance of the stent measuring 0.4 mm in thickness is significantly lower. The observed discrepancy can be ascribed to the wall thickness of the stents when subjected to identical deformation conditions. Additionally, plastic deformation took place in the 0.4 mm stent when exposed to 37 °C. Also, it has been found that the fatigue resistance test results reveal a smooth curve with the compression force and recovery ratio experiencing a decline of approximately 21% and 13%, respectively, after 100 cycles.

For the manufacturing of Magnetoactive Soft Material (MASM) objects, the article [31] describes an innovative 4D printing strategy that combines conventional direct-ink-writing (DIW) with an origami-based magnetization technique. Similar to FFF, in this process, material(ink) is extruded through a syringe under controlled flow rate and deposited over a build platform layer by layer (Fig. 3c) [27]. The structures can undergo programmable changes and locomotion subjected to magnetic fields. By avoiding issues such as particle agglomeration and streamlining the fabrication process, this approach facilitates the production of objects featuring intricate three-dimensional magnetization profiles. The authors also found that when subjected to cyclic endurance tests at 40% compression strain and 50% tensile strain, the 4D printed MASM demonstrated an exceptionally high fatigue resistance (see Table 1).

Fig. 3
figure 3

Printing technologies for 4D printing. a Digital light processing. Reproduced with permission from [27]. b Fused filament fabrication Reproduced with permission from [27] c Direct-ink-write. d Femtosecond laser direct writing. Reproduced with permission from [32]. e Liquid substrate electric field driven microscale 3D printing. Reproduced with permission from [34]

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Table 1 Summary of fatigue behavior of 4D printed materials
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Another paper [32] describes the advancement of femtosecond laser direct writing (Fig. 3d) for the microscale fabrication of hydrogel materials in botanical-inspired 4D printing. The process utilizes ultra-short laser pulses to fabricate complex micro structures [33]. This methodology permits the fabrication of hydrogel structures that are responsive to changes in pH. Such structures demonstrate complex and quick transformations in shape, such as contraction, expansion, and torsion, when subjected to varying pH levels. The materials exhibit considerable promise for implementations in drug delivery systems, micromanipulation, and single-cell analysis on account of their exceptional biocompatibility, modifiable characteristics, and rapid response times. Multiple expansion and contraction cycles are utilized to determine the expansion ratio of the cubic plate to assess the fatigue resistance of the printed hydrogel. This validates the hydrogel’s ability to repeat successfully.

Flexible transparent electrodes (FTEs) with embedded metal filaments were created using a novel, low-cost, and efficient liquid substrate electric-field-driven (LS-EFD) microscale 3D printing technique, as described in this article [34]. In this technique, a layer of liquid is spin-coated on a flexible surface, and the material is extruded through a nozzle under high voltage (Fig. 3e) [34]. By utilizing this methodology, liquid film substrates can be imprinted directly with ultrathin metal meshes, resulting in FTEs that exhibit exceptional optoelectronic characteristics such as minimal sheet resistance and substantial transmittance. Enhanced mechanical stability and environmental adaptability are results of the embedded metal structure, which qualifies the material for use in severe environments. The technology's versatility in flexible electronics and optoelectronic devices is exemplified through its use in resistive transparent strain sensors and thermally driven 4D printing structures, among other applications. The reliability and consistent functionality of the flexible strain sensor are validated through an extensive array of tests conducted in the study. The resistance change of the sensor was specifically assessed when subjected to a strain rate of 60% for 1000 cycles to evaluate fatigue resistance. The findings indicate that the strain sensor exhibited consistent performance and stability during the entire testing phase, suggesting its dependability and suitability for uses requiring prolonged operation and robustness. This demonstrates the robustness of the sensor, rendering it appropriate for a wide range of practical applications that involve anticipated repetitive stress and strain.

3 Research gaps in the fatigue study of the 4D printed materials

Insufficient exploration of fatigue testing The principal aims of fatigue testing comprise evaluations of the material testing, the structure testing, and the actual service testing [35]. The objective of material testing is to clarify the behavior of substances when subjected to cyclic stresses, taking into account a range of environmental conditions, surface treatments, and geometric configurations. In contrast, structural type testing is dedicated to the assessment of various materials or structural configurations to determine the effects of stress concentration, estimate fatigue life, and analyze the consequences of fabrication methodologies. Lastly, the primary goal of service testing is to ensure quality and dependability by simulating real-world applications [20]. Although the testing methodologies are illustrated clearly on the literature, a substantial knowledge gap has been detected regarding the fatigue testing of 4D printed materials. Comprehension of the performance and durability of 4D printed materials under cyclic loading conditions, which is crucial for their application in engineering and design fields, is limited by the lack of research.

Insufficient exploration of influence of printing parameters on fatigue behavior The advent of 3D Printing technology offers the distinctive advantage of precisely controlling numerous printing parameters. In the context of fused filament fabrication (FFF), parameters such as raster angle, layer thickness, build orientation, raster width, print speed, infill density, air gap, infill pattern, extrusion temperature, platform temperature, nozzle diameter, and filament diameter can be meticulously adjusted [36]. These parameters significantly influence the mechanical properties and fatigue behavior of the printed parts [37]. Similar considerations apply to other 3D printing methodologies, each characterized by a unique set of controllable factors affecting the part's performance under cyclic loading. The impact of these printing parameters on the fatigue behavior of 3D printed materials necessitates thorough investigation to optimize their applications in various engineering domains. This exploration is crucial for advancing the understanding and utilization of 3D printing technologies, particularly in designing components with enhanced durability and reliability under operational stresses.

Limited numerical analysis for 4D printed materials In light of the complex structure and financial implications of 4D printing, the application of numerical simulations may present an efficient and economical approach to comprehend and forecast the performance of such materials across diverse circumstances. The fatigue behavior of 3D printed materials has been significantly investigated through numerical investigations, which have yielded valuable insights into fatigue parameters and material performance [38, 39]. Likewise, by implementing these numerical methodologies into 4D printing, it would be possible to simulate fatigue behavior, providing engineers and researchers with a valuable instrument for enhancing material formulations and printing processes without necessitating extensive physical prototyping. By adopting this methodology, not only are resources conserved but also the development of more resilient and dependable 4D printed materials is expedited, thereby expanding the limits of what is conceivable with existing technology.

4 Conclusion

The purpose of this literature review was to provide a thorough synthesis of prior research concerning the fatigue characteristics of 4D printed materials, emphasizing significant findings. Additionally, the study aimed to identify areas where further research are needed in the field, thereby guiding future investigations to tackle these obstacles. Significant research gaps remain to be filled in the comprehension of fatigue behavior in 4D printed materials. Present constraints involve an absence of thorough fatigue testing methodologies, inadequate investigation into the impact of 3D printing parameters on fatigue behavior, and a significant void in numerical studies that seek to forecast the performance of these materials across diverse conditions. To effectively tackle these obstacles, a comprehensive strategy is necessary, which integrates progress in mechanical engineering, material science, and computational modeling. This integration enables the complete realization of the capabilities of 4D printed materials in applications that require exceptional durability and adaptability.