Review of Development and Characterisation of Shape Memory Polymer Composites Fabricated Using Additive Manufacturing Technology ^†

Abstract

Structures as well as components are generated by depositing filaments on one another via the technique of additive manufacturing. Among the various processes of printing, 4D printing combines the technology of 3D printing with the passage of time, resulting in additively generated parts that are responsive to stimuli from the outside via modifications of their form, volume, size, or mechanical qualities. Thus, the materials of shape memory are used in 4D printing and respond to environmental factors including temperature, pH, and humidity. Shape memory polymers (SMPs) are materials with a shape memory effect that are best suited for additive manufacturing. Contrarily, the method named fused filament fabrication (FFF) is employed most frequently among all additive manufacturing methods. In this regard, the objective of the present study is to evaluate all investigations that have been conducted on 4D-FFF materials’ mechanical properties. The study offers an unparalleled overview that highlights the possibilities of 4D FFF printing across multiple applications in engineering while keeping the end structure’s or component’s structural integrity in consideration.

1. Introduction

The technique of adding and integrating materials layer upon layer to create products is known as additive manufacturing (AM). Using this technical procedure, 3D items can be produced directly from CAD models. The biggest sectors where AM is employed are industrial machinery, the automotive sector, and consumer products [1]. Considering this, however, there have been certain industries—including aerospace, the energy sector, and the medical sector [2]—where AM methods are increasing in popularity. These industries include the printing of living tissue in the medical sector [3] and the field of aerospace. For some application purposes, printed structures might have to undergo modifications in behaviour or shape. Among the many positive aspects of 3D and 4D printing, the reductions in manufacturing time and the amount of manpower needed to manually assemble equipment or other things are commendable. Moreover, the process of 4D printing is far more efficient and able to reduce logistics costs, transportation costs, and the amount of material that needs to be transported due to a reduction in the volume of parts [4]. Objects that are fabricated using four-dimensional printing are defined as a function of time in addition to three-dimensional printing. This means that the properties or characteristics of the objects that are being fabricated can change over time, for example, shape, appearance, and functionality [5]. The printed structures’ multifunctionality, ability to repair themselves, and self-assembly are made possible by this time-dependent, predictable, and printer-independent technology. The combination of active material technologies and 3D printing is known as four-dimensional (4D) printing [6]. The development of polymer composites through 3D printing technology is generally conducted via the FFF process. FFF follows a process of developing the models via the layer-wise addition of material. FFF is one of the most widely used 3D manufacturing techniques used for the development of polymer parts. The 3D model is developed through a CAD file. Any complex part can be easily developed through this process.

Shape memory materials (SMM) as well as shape-changing materials are two subdivisions of materials that can change their shape, according to Zhou et al. [7]. When a stimulus is introduced into a material that leads to an instant change in shape, this is termed as a shape-changing material. Similarly, when the stimulus is removed, the material instantly returns to its original shape. Simple adjustments like shrinkage and linear volume expansion are the only kinds of transformation permitted in this field. The transformations are more complicated in the shape memory effect (SME). Two processes are involved in SMM modifications: the initial one, referred to as the programming step, wherein the structure is deformed and kept in a transient, metastable state; the second one, referred to as the recovery step, which allows the structure to revert to its initial profile in response to an adequate stimulus. Contrary to materials that can change their shape, shape memory materials can adopt a tentative shape until the proper stimulus is delivered. One-way shape memory materials are the first type of SMM in which the transformation is irreversible and a fresh programming process is required to regenerate the tentative shape once the initial shape is regained. However, the shape can be changed reversibly in the second type, which is a two-way shape memory material [8].

As smart materials’ shape memory polymers become deformed to a tentative shape, the materials have the ability to regain their strain factor up to one hundred percent. Magnetic fields, heat, electric forces, and UV rays are examples of suitable stimuli that can activate recovery [9,10]. The substance is known as a thermo-responsive shape memory polymer if heat is used as the applied stimulus [10]. Thermoplastic and thermoset materials can be used in SMPs [11]. A resin plus a hardening agent make up thermosetting polymers that, if combined, result in the chemical cross-linking and polymerisation of the polymer chains. A thermoset is unable to melt for recycling after it has been cured. No chemical cross-linking is exhibited by thermoplastic shape memory polymers, and they can be melted as well as reformed several times. In the course of the manufacturing process, a thermoplastic SMP that is thermo-responsive is given its perpetual shape. Injection moulding or extrusion are examples of traditional polymer processing methods that can be used in this procedure, as well as more unusual methods like melt electrospinning.

The temporary shape is assigned using a programming method. A physical constraint is then used to deform the SMP to the desired temporary shape above its glass transition temperature (Tg). The temporary shape is then fixed by cooling the material below the Tg while continuing to enforce the constraint. The temporary shape is accomplished once the part has been cooled beneath the temperature of a glass transition, which releases the physical limitation. The sample will regain its original shape when heated over the Tg. Until further programming is conducted, the permanent shape will be maintained by the sample after it is recovered. Some of the SMP applications include temperature sensors, actuators, medical devices that are implantable, and toys [12].

According to the sorts of stimuli used to elicit the SME, nearly every presently extant SMP can be classified into three distinct groups: thermo-responsive SMPs, photo-responsive SMPs, and chemo-responsive SMPs (thermo-responsive SMPs are often obtained through being heated, such as via light heating, inductive heating, mechanical heating, joule heating, etc.; photo-responsive SMPs are those that have light with many wavelengths but no associated heat.; chemo-responsive SMPs contain substances like ethanol and water [13,14]). Among the numerous applications of 4D printing technology, potential case studies were carried out that were successful in establishing tissue engineering, drug delivery, stents, soft robots, weapons and aircrafts, uniforms, and textiles.

In actuality, in a specific SMP, only certain distinct stimuli may cause shape recovery. For instance, stimuli for polyurethanes (PUs) and their composites might involve ethanol, water, and heat. Highly strain-recovering and easily adjustable characteristics are just a few of the added advantages that SMPs have over their metal counterparts [15,16]. The majority of polymers are capable of being thermally activated or chemically or thermally responsive, according to a new research report, which also shows that practically all polymers intrinsically exhibit SME features that are also chemo-responsive or activated thermally.

Three-dimensional printing or additive manufacturing technology helps in the fabrication of complex shape geometries with ease. The development of parts using this technology is limited to static responses alone. To evolve into dynamic responses of materials, 4D technology plays an important role in transforming geometry with the application of external stimuli. This technology helps in the self-assembly of certain processes in the absence of conventional driving equipment. This review article focuses on understanding the various material processes of 4D printing technology applied for numerous applications.

2. Shape Memory Relationships

Shape memory materials (SMMs), also known as innovative materials, are materials that change phase and can change their shape when an appropriate medium is applied to them, changing their shape from their base shape to a temporary one [17]. This term generally refers to four distinct kinds of materials, which include the SMA, that provide high levels of strength, creep resistance, and stiffness but have restrictions on the strength-to-weight ratio and strain point (approximately an elasticity of eight percent). Shape memory ceramics (SMCrs) are a type of secondary material that offer a better modulus and high degree of hardness, but have not yet been described in terms of structural aspects. SMPs actually fall into the third group and offer a high degree of elasticity (of approximately 400%) while being highly stimuli-responsive. Initiated SMCs are a final class of SMMs, though they do not meet the requirements for strength and thus are covered in a distinct area of this overview. The shape memory effect (SME) can have varied shapes depending on interactions in either one way or two ways. This is represented in Figure 1.

There are a few properties that quantify polymeric shape memory material programming as a function of length; these are discussed below. In thermomechanical cycles, the relationship between the strain and length of samples can be used to evaluate the material programming or shape memory realisation phases of polymeric shape memory materials. These are the two components that shape memory polymers need to have to produce the shape memory effect.

2.1. Shape Fixity Ratio

The capacity of the shape memory polymer material to transform fragments to accept their intended mechanical deformation during this process is known as the shape fixity ratio (R_f). Equations (1)–(3) show how it is represented.

R_f = the strain in the fixed impermanent shape/the strain afterward the broadening step (before its cooling)

$$R_f=\frac{Strain in fixed impermanent shape}{Strain after broadening step}$$

(1)

$${R_f=(\epsilon }_u\left(N\right)/{\epsilon }_m\left(N\right))\times 100\%$$

(2)

$${R_f=\left\{\right(L}_u-L_i)/(L_t-L_i\left)\right\}\times 100\%$$

(3)

where ε_u = strain in the stable provisional shape; ε_m = strain after the broadening step in a sample (before it is cooled); L_i = initial sample length; N = cycle numerals; L_u = unloaded sample length; L_f = final recuperated sample length; L_t = temporary sample length; Serial shape memory cycles are used to take into consideration R_r and R_f.

2.2. Shape Recovery Ratio

The ability of a shape memory polymer substance to resume taking on its original form again is known as shape recovery (R_r). Equations (4)–(6) provide a series of equations that serve as its representation.

$$R_r=\frac{Starin produced after strectching in sample-strain in sample after recovery}{strain in sample after broadening-strain in stable provision shape of sample}$$

(4)

$${R_r=[\epsilon }_u\left(N\right)-{\epsilon }_p\left(N\right)]/{[\epsilon }_m(N)-{\epsilon }_p(N-1\left)\right]\times 100\%$$

(5)

$${R_r=\left\{\right(L}_u-L_f)/(L_t-L_i\left)\right\}\times 100\%$$

(6)

where ε_p = the strain after recovery in the sample [18,19].

3. Shape Memory Polymer Composites (SMPCs)

The development of intelligent or smart SMPs has completely changed how materials are produced and constructed for a variety of purposes. This material class, which is a recoverable shape substance, also has other beneficial characteristics like high shape recovery, biodegradability, low density, high reliability, lightweightness, deformation ability, variable glass transition temperature, affordability, and good manufacturability [20,21]. The advantage of SMPCs over other kinds of shape memory materials, such as SMCs and SMAs, and their ability to function as well as SMAs and SMCs do, were identified when these qualities were compared. They cause a rise in shape memory polymer (SMP) research. Polymers such as these also exhibit disadvantages, which include lesser mechanical characteristics consisting of stiffness, deformation, low stress recovery, and strength. The list of disadvantages differs based on the application, and this involves the structure of aerospace and its poor-quality UV radiation resistance [22]. In response to all of these disadvantages, investigators discovered the inclusion of various kinds of filler materials, which are additional additives like curing agents and photo initiators, into shape memory materials. It may be feasible to improve shape memory polymer properties while rendering them sufficient enough to 4D print effectively while concentrating on the creation and design of SMPCs. Figure 2 shows the overall roadmap followed for the development of shape memory polymer composites.

The shape memory polymer composite comprises the system of reinforcement materials with the base as the primary matrix material (SMP), which establishes magnitude sections of the matrixes (composite), appears to have excellent electrical, thermal, and mechanical properties, and boosts the matrix’s compatibility. These materials include fillers of fibres, Kevlar, carbon black, short and continuous carbon fibre additives, and carbon nanotubes (CNT). Yanju’s team previously released an evaluation of a specific class of SMPC used in aerospace applications [23]. Four-dimensional printing is capable of being performed by these SMPCs.

Table 1. Materials used in 4D printing technology.

Materials	Applications	Advantages	Challenges
Metals and its composites	Automobile, aerospace, biomedical, defence	Multifunctional optimisation. Mass customisation Reduced material waste Fewer assembly components Possibility to repair damaged or worn metal parts	Limited selection of alloys Dimensional inaccuracy and poor surface finish Post-processing may be required (machining, heat treatment or chemical etching)
Polymers and its composites	Automobile, aerospace, biomedical, toys, military	Fast prototyping Cost-effective Complex structures Mass customisation	Weak mechanical properties. Limited selection of polymers and reinforcements Anisotropic mechanical properties (especially in fibre-reinforced composites)
Concrete	Construction	Controlling porosity of lattices Printing complex structures and scaffolds for human body organs Reduced fabrication time A better control on composition and microstructure	Layer-by-layer appearance Anisotropic mechanical properties Poor inter-layer adhesion Difficulties in upscaling to larger buildings Limited number of printing methods and tailored concrete mixture design
Ceramics	Biomedical and chemical industries	Mass-customisation No need for formwork Less labour required, which is especially useful in harsh environments and for space construction	Limited selection of 3D-printable ceramics Dimensional inaccuracy and poor surface finish Post-processing (e.g., sintering) may be required

shows the various forms of materials used in 4D printing technology.

Nadgorny [24] had discovered that by adding acrylonitrile–butadiene–styrene (ABS) at a weight of 12 percent to poly (2-vinylpyridine, or P2VP), the composites’ processing performance and mechanical performance were greatly enhanced. The pH swelling characteristics of the 3D-printed samples are reversible. For instance, the decline in the swelling amount resulted from the cross-linking amount, which was higher and rose with 1-bromoethane quaternisation, while ABS inclusion significantly increased the mechanical stability of P2VP, which had been susceptible to damage from the process of 3D printing.

Ly et al. [25] have confirmed printing parameters’ impact by conducting experiments through fused filament fabrication on a shape memory polyurethane-based polymer, and similarly, through the addition of carbon nanotubes to the same polymer, carbon nanotubes are added. The printed samples were then placed in a basin of water before being used in voltage tests. The SMP samples of polyurethane-based materials created via 3D printing “FFF” kept their thermal response properties; however, because the stimulation temperatures were greater than the Tg, the samples’ recovery durations were noticeably shorter. Nevertheless, for a successful recovery time, the temperature of activation should not be more than 10 °C below the Tg. Moreover, some of the factors, which included filling ratios, higher printing, the thickness of the layer, printing temperatures, or lesser feed rates, encouraged reduced electrical resistance and a quicker time for recovery.

Zhao et al. [26] showed that 3D-printed materials’ shape memory behaviour may be obtained using FFF and an easily accessible olefin ionomer. Additionally, investigators made a comparison of the samples that were 3D-printed to those that were produced via compression moulding, observing that the initial rate of recovery was lower in the first case (R = 58%) compared to that of the compression-moulded samples (R = 83%). During the first cycle, poor network recovery was caused by the formation of polyethylene crystals that were resistant to permanent network recovery. At a fixed stress value, the impact proved to be better in the 3D-printed samples, but this could have been due to the larger strain. Compared to samples that were 3D-printed, samples acquired through compression had worse recovery in the following shape memory cycles. They came to the conclusion that using the FFF technology for 3D printing makes it feasible to generate intricate structures out of polymers that are thermoplastic and have a shape memory of three dimensions when the right trigger is used, like heat.

Kang et al. [27] combined shape memory polymers and shape memory alloys to produce a composite and performed an analysis of the developed composite. The phase transition that occurs between austenite and martensite phases resulting from a change in temperature is what produces the shape memory effect (SMA). However, SMPs exhibit a memory effect that is aided by variations in the ratios of soft and hard segments at temperatures near the temperature of glass transition. A Nylon 12 (PA 12) filament was employed in this instance as a material for 3D printing. The SMP formed the sample’s base, into which two lines of nitinol wire were placed to imitate an actuator. The SMA was then fastened in the lines and inset into the part of the SMP to create the SMC that had been subsequently wrapped in the PLA to further increase its flexibility. Based on the different ratios of SMA to SMP, it was found that the best ratio (SMA:SMP of 1:5) caused the greatest change in length (eight millimetres) in the overall dimensions, as well as the shortest response time (4 s). The Joule heating effect caused the SME to create a reversal force of bending whenever current passed via the SMA wire. For instance, a 1.5 A current flowing for a small period of 2 s caused a 7 mm movement with a 10 °C temperature differential. These authors showed that the shape memory effect was detectable in the 45–60 °C temperature range using DSC and DMA analysis. Tensile tests were employed to examine the reversible action processes, and it was found that SMP and SMA existed together in the form of fibre, while at higher temperatures, the shape memory polymer changed to a form of the matrix. Low tensile strength is possessed by each of the single materials; however, the SMA displayed the lowest strain for temperatures of 20 and 90 °C. At 20 °C, the SMP only showed minor strain; at 90 °C, it showed large strain. For the SMP and SMA, the bending moduli were determined to be 1180 MPa and 600 MPa, respectively.

Table 2. Polymer composites with their applications and glass transition temperature.

Materials	Tg (°C)	Parts and Applications
Polylactic acid (PLA)/ paper composite	65	Obtain complex 3D structure, reversible self-folding flower
Polyurethane-based SMP filament	60	Self-conforming subtracts, self-tightening surgical sutures
UV cross-linkable linear PLA/PCL copolyesters	60	Personalised and adoptable elbow protector, smart Kungfu panda and Terracotta warrior
ABS for ball and socket SMP for the middle active part	45	Variable stiffness hyper-redundant robotic arm
Polyurethane-based SMP filament	60	Dimensional increase in structure via self-bending feature

shows numerous polymer composites with their applications and glass transition temperature.

A wide range of innovative and distinctive applications are made possible via 4D printing by including temporal variations in part specification and design. Despite its novelty, 4D printing uses a fabrication process that is nearly identical to that of 3D printing. Temporal variations are made possible via the thoughtful design of a 3D-printed item and the selection of an appropriate printing material. However, one of the main obstacles to the ongoing development of 4D printing is the growth in material alternatives. It is not enough to find a material that reacts well to an external stimulus; some applications also call for materials that react well to more specific stimuli. The future of 4D printing will largely revolve around expanding the selection of materials that may be used for the process, both in terms of their mechanical qualities and accompanying stimuli. For the ability to precisely trigger and regulate form changes from a distance, the additional customisation of change-inducing stimuli must be accomplished. The difficulty of constructing a structure that will permit the intended transformation into a specific shape is a major source of user-level issues in 4D printing. Building auxiliary components to support material programming, printing with multiple materials, and putting new printing methods and interfaces in place are all necessary in practise to achieve complex transformations like folding, curling, twisting, linear expansion, and shrinkage. The use of multiple SMPs can offer a great opportunity for designing more complex 3D motion, such as that in mechanical meta-structures, and control the temporal development of the 3D structure in accordance with the dimensional demands of the created framework, such as 3D-created constructions with varying shape recovery behaviour. Since its recent invention, 4D printing has inspired a lot of anticipation and curiosity.

Researchers in the realm of science are deeply intrigued by 4D printing for its capacity to enable objects to alter their shapes over time. This metamorphosis is orchestrated from a predetermined form, triggered by external factors like pressure, temperature, wind, water, and light. The realm of intelligent materials further fuels fascination due to their inherent attributes. The convergence of these two aspects gives rise to self-transforming structures, captivating diverse industries including medicine, defence, aerospace, and more. Moreover, the realm of 4D printing can be precisely tailored to individual consumer demands, catering to unique designs and consumer products.

This analysis of FFF 4D-printed shape memory polymers leads to the conclusion that most investigations concentrate on conceptual designs for these materials in response to various stimuli. Regrettably, there is a scarcity of studies that approach the issue from a structural perspective. In fact, the existing studies found in the technical literature examine the influence of process parameters on mechanical traits or shape memory. However, these studies are still in their nascent stages, lacking a comprehensive grasp of the subject and potential structural applications.

4. Conclusions

Four-dimensional printing has received greater focus after the technology’s launch in all disciplines, particularly in the industries of aerospace and medicine, owing to its ease of integration with all sorts of actions required by these industries as well as the possibility of different options replacing current issues including processes, machining types, and materials. For use in the aerospace industry, shape memory polymers have been found to be more effective than shape memory alloys. The aerospace industry has benefited from the use of 4D printing methods for additive manufacturing owing to the support of SMPs and SMPCs, with the most important requirements being the development of these kinds of applications in the real world, which include aeromechanical properties, curtailed density, outstanding shape recovery, and high strength. This study, however, is not restricted to SMPCs utilised in the aerospace industries, space technology, automation, and soft robotics via stimuli-responsive materials; it focuses on 3D printing, giving rise to the 4D printing of these substances to obtain necessary properties along with the assistance of an externally generated stimulus for operations. There is now a phase in the process in which these techniques will be industrialised and used for production; however, they are still restricted to development as well as laboratory research only. Furthermore, they are expected to become more common in the real world as hurdles like life cycles, multiple stimuli, performance characteristics, two-way actuation, and properties are overcome by researchers with adaptable research methods. The paper also offers a thorough overview of shape memory polymers, filler interactions, shape memory composite modelling, manufacturing methods, and specific applications. The current challenges and innovative study directions in the 3D/4D printing of engineered structures are highlighted in the final section.