A taxonomy of shape-changing behavior for 4D printed parts using shape-memory polymers

Additive manufacturing (AM) is a process of building components in layers directly from 3D CAD data, without the need for complex and costly tools and with minimal material. It is a process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies [19]. Additive manufacturing (AM) uses methods such as material extrusion, material jetting, binder jetting, and sheet lamination to build solid three-dimensional structures through the controlled deposition of materials one layer after another. Most AM processes today enable the production of mechanically stable parts that are rigid and static to achieve its intended form. Consequentially, most of these parts are not designed to actuate or transform. Moving parts such as hinges or actuators involve assembling multiple parts together after being produced [14]. The post-processing of putting together separate parts requires specific tolerances and also takes time to assemble. One answer to this is to use materials that can morph in a controlled manner over time when it is exposed to external stimuli.

4D printing technology capitalizes on the rapid development of smart materials such as SMAs and SMPs with freeform digital manufacturing processes. An AM produced, self-changing structure that responses to an external trigger such as heat, light, electricity, or other stimuli is called a 4D printed part [39, 40]. 4D printing uses time as the fourth dimension, in the sense that the object can change over a duration, dependent on environmental stimuli [37]. The biggest advantage of utilizing 4DP is size reduction due to the computational folding that can be achieved. In AM parts, most machine build sizes have to accommodate the surface area and volume of the part. However, 4D printing does not have such constraints because large parts can be “folded” or “compressed” in sections and printed to fit into the constraints of the machine bed. Farhang et al. [10] highlighted that the fundamental elements of 4DP include the AM process, the stimuli, the stimuli-responsive material, interaction mechanism, and mathematical modeling. Figure 1 describes that the AM process is necessary for the production of freeform parts; and the stimuli, in turn, trigger the stimuli-responsive material into its new form. The interaction mechanism is the programming process in which the material is deformed into a temporary shape. Finally, the mathematical modeling process defines the material distribution and its structure needed to achieve the desired change in its shape [10].

The mathematical modeling process is intrinsically linked to the overall design of the part which is in turn linked to the computer-aided-design model. For example, the shape-shifting behavior can be enabled by incorporating targeted smart hinges at precise locations within the structure [10, 14, 36, 50]. The hinges are made from smart materials and all the other parts constitute non-reactive materials (Fig. 2).

Instead of stacking uniform layers to exhibit different dimensional changes that respond to the stimuli, the shape-memory material could be arranged by controlling the amount, location, and pattern, so that different effects such as swelling ratios could be generated upon activation when exposed to stimuli. Figure 3 shows the results of the material when different swelling ratios are achieved [23, 49, 55].

4D printing is still in its early stages of development. Below are the current examples of emerging 4D printing applications that demonstrate the potential to reshape new product development in several ways. Figure 4 is a 4D printed smart valve. The team at the ARC Centre of Excellence for electro-materials’ science described that the use of shape-memory materials can be transformed from one form to another and could be applied for use in medical soft robotics. This combination of technology and materials was used to create a valve that works in response to the temperature of the water surrounding the valve.

Figure 5 illustrates that MIT’s Tangible Media Group has developed a novel “pasta” that can respond in a variety of ways that fold when in contact with heat and water. The printed Pasta is flat, and when boiled, it can achieve different shape deformations such as bending, folding, and rolling. To achieve this effect, Wang et al. [53] 3D printed a piece of edible cellulose over the top layer of the gelatin.

According to a market-research report by Frost and Sullivan [12], 4D technology and equipment are expected to grow rapidly in the medical, aerospace, defense, and automotive industries. 4D printing can produce a variety of products to support components for use in automotive parts to human organs. Benefits of 4D printing include improved functionality of mechanical products, new uses of adaptive materials, additional manufacturing efficiency, and reduced manufacturing costs and carbon emissions. In addition, according to the Market research future [34], 4D printing technology is expected to be widely used in aerospace and defense, healthcare, automotive, construction, clothing, utilities, and others.

Smart materials have a dual-shape capability that can transform from one defined shape to another when stimulated. These stimuli-responsive materials can be classified into several sub-categories, as shown in Fig. 6. However, shape-memory alloys (SMAs) and shape-memory polymers (SMPs) are the most commonly used materials for 4D printed parts [22]. The idea of using stimuli-responsive materials was initially based on thermally actuated shapes and today’s applications range from heat shrinkable tubes to smart medical devices. The application of shape-memory materials requires a combination of suitable materials and accurate programming. A temporary shape is achieved through a deformation process. In 4D printing, shape-memory polymers (SMPs) are commonly used as compared to SMAs. SMPs have a wider range of glass transition temperatures from − 70 to 100 °C. SMPs also have the potential to achieve configuration recovery properties of up to 400% of the plastic strain, while the recovery properties of SMAs are approximately 7–8%. SMAs are considered to be disadvantageous due to complex production, high costs, toxicity, and limited recovery (Table 1). Therefore, SMPs and hydrogels are preferred over the use of shape-memory alloys (SMAs) [9].

Although SMPs were developed over 30 years ago, the past few years have seen SMP research progressing significantly in which material scientists have developed from single-shape to multi-shape properties, and to design completely reversible transformative capabilities during this short span of time. SMPs possess at least two phases—a stable phase which stabilizes the polymer and this is used to recover the original shape; as well as a second (or temporary) phase. The deformation of the first phase is the force required to recover the original shape, and this is achieved through the interpenetrating networks, chemical crosslinks, or crystalline phases of the material [35]. The second-phase momentarily fixes the temporary shape by the glass transition (T_g) or crystallization state between the different liquid crystalline phases, through covalent or non-covalent bonds such as Diels–Alder reactions, which is a thermo-reversible reaction. Other stimuli-responsive methods such as switching by redox reactions can also be applied. The mechanism of SMPs is commonly achieved through thermal transitions, thermo-responsiveness, and chemo-responsiveness. The thermal transition of SMPs is due to molecular switches/net points that are physical and chemical crosslinks. Phase-segregated morphology and formation are the foundational mechanism behind the state of material change [25]. In thermosets, the network chains between net points consist of switch segments of chemical crosslinks. The shape-memory switch is achieved through a thermal transition of the polymer segments. Thermosets display less creep and, therefore, show a less irreversible change during transformation when compared to thermoplastics. They also display better shape, mechanical, and thermal memory than thermoplastics [26]. For thermo-responsive SMPs, a dual-component system is used in polymers that are excited by heat. The matrix remains elastic throughout and the fibers reversibly change in material stiffness [32, 41]. Thermo-responsive SMPs use the Glass Transition or the melting point as the threshold temperature. The SMP has two stages, in which the first is the deformation to a temporary shape (or the programming stage); and the second stage is the recovery phase [18]. Finally, for chemo-responsive SMPs, immersion in a chemical stimulates the plasticizing effect of the polymer [43]. This effect often reduces the glass transition temperature (T_g) of the material and there is a need to heat the material over the T_g. There are alternatives for shape recovery due to this chemical responsiveness. It can be triggered by the ionic strength, the pH value, or the concentration of the agent [29]. Currently, thermo-responsive SMPs are most applicable for 4DP parts, followed by chemo-responsive SMPs particularly using hydration. The methods that can be used to print thermo-responsive SMPs include material extrusion, material jetting, and stereolithography. As for chemo-responsive SMPs, usually hydrogels are used and bio-extrusion is the most common method of fabrication [25].

Materials that can respond and change its configuration according to the environmental conditions are often selected for use in 4DP. Global deformation of the material can be achieved when exposed to external changes such as water and temperature that are commonly used as the activation stimuli. When combined with the other less-exploited triggers such as magnetism, light, and pH, they open up new avenues to design smarter devices [2]. Thermal-responsive SMPs, through heating from a source such as heated gas or water, can also trigger the SME. Other methods for SMP actuation that have shown potential for application include light and electricity sources. SMPs are characterized by their ability to recover their original shape from a temporary form that they take on. They remain in the temporary shape until an external stimulant is used and then it reverts back to its original shape. This process is known as the shape-memory effect (SME). The geometrical design and responsiveness of the material influences the transition speed of the material from one form to another. The amount of shapes that can be retained in the memory of the material depends on its network elasticity. The strain recovery rate (R_r) and the strain fixity rate (R_f) determine the “intensity” of the SME. The strain recovery rate (R_r) is the ability of the material to memorize its permanent shape; and the strain fixity rate (R_f) refers to the ability of the switching segments within the mechanical deformation. Materials can be either reversible or irreversible. Reversible materials usually have two-way memories, since they have two permanent stages, although most existing SMPs are almost limited to one-way memory, needing reprogramming after each recovery. Two-way SMPs have an SME that can change back and forth from a temporary shape back to a permanent shape. Smart or stimulus-responsive material: stimulus-responsive material is one of the most critical components of 4D printing. Stimulus-responsive materials can be classified into several sub-categories, as shown in Fig. 6 [47].

As an emerging, future-ready technology, 4D printing has added an extra dimension to the progress of additive manufacturing where structures can morph after being exposed to triggers such as light and heat. The manufactured constituent is capable of small or radical changes that can result in totally new structures. We classify shape-change behaviors into three categories—basic shape change, complex shape change, and a combination of shape change. Basic shape changing involves only a process of single deformation. The main characteristic is that it is a single-step process that produces a wholly uniform effect. Basic shape-changing behaviors include bending, rolling, twisting, helixing, buckling, curving, topographical change, expansion, and contraction. Basic shapes can be programmed to undergo a specified sequence of deformations over time to subsequently achieve complex shapes, sometimes also known as “sequential” shape-shifting [31]. Complex shape-changing behaviors consist of multiple deformations which can be derived from extending an earlier form of a basic shape change or as a completely multi-faceted form. For example, multiple folding, multiple bending, multiple rolling, multiple twisting, multiple helixing, multiple buckling, multiple topographical changes, and curvature changes are extended behaviors of the basic shape change. Other complicated shape-change behaviors include waving and curling. Finally, a combination type of shape changing is an amalgamation of different types of shape-changing behaviors in which two or more constituent behaviors can be programmed to occur in the component either simultaneously or in a carefully timed sequence. In terms of mathematical analysis, all shape deformation is a resultant of different stress along the plane. It is influenced by different strains, gradients, and shapes. Taking a step further, Table 2 summarizes the differences of shape-changing behaviors through a mechanical analysis lens.

Basic shape-changing behavior results in a single deformation and the transformation occurs as a one-step process. This includes folding, bending, rolling, twisting, helixing, buckling, curving, topographical change, expansion, and contraction.

Complex shape-changing behaviors consist of multiple deformations that can be derived from extending an earlier form of a basic shape or as a completely multi-faceted form. These characteristics of multiple deformation shape-changing have one or two more steps’ deformation. For example, multiple folding, multiple bending, multiple rolling, multiple twisting, multiple helixing, multiple buckling, multiple topographical changes, and curvature changes are extensions of the basic form. Other complicated structures include waving and curling.

Combination shape-changing behaviors consists of merging different types of shape-changing behaviors that result in an amalgamation of different deformation results to the component.

As each author provides each different shape deformation with separate explanations and descriptions, it has become difficult to gain an overview of the types of shape-change behaviors that can be realized using 4D printing processes. Furthermore, the current knowledge in the literature is not only sparse, but also fragmented. The purpose of the taxonomy is an attempt to classify the different shape-change behaviors and to distinguish their corresponding characteristics. Understanding the types of shape-change behavior is essential for designers and engineers to implement suitable shape deformations within the CAD and programming process. To identify the types of shape-change behavior for 4D printed parts, we conducted a thorough review of the literature, and analyzed the types of shape deformations before summarizing this into a tabulated form. To develop the taxonomy, we first grouped all the types of shape deformation that were similar, paying a special attention to those types that were identical, but were named differently. The second step was to assign names to the taxons. In the process, we realized that all of shape-change behaviors are related in number of “sequential” shape-shifting process. Therefore, these shape-change behaviors were named “basic shape-change behaviors” and “complex shape-change behaviors.” It can be mutually related and we assigned a new group known as a “Combination Shape-Change Behavior”. We continued to proceed in this way to determine all the groups and categories.

For the first category, we classify basic shape-changing as a single deformation that occurs from a single step or process. This includes folding, bending, rolling, twisting, helixing, buckling, curving, topographical change expansion, and contraction. The second category involves complex shape-changing behaviors in which the deformation can take place in either a single or multiple steps. Those that involve two more steps of deformation are also known as a “sequential” shape-shifting process whereby certain changes occurred at specific points of time. Complex shape-changing behaviors include multiple folding, multiple bending, multiple rolling, multiple twisting, multiple helixing multiple buckling, and multiple topographical and curvature changes are extensions of the basic form. Other even more complicated structures include waving and curling. The third category of shape-changing behaviors takes place in a combined form, whereby two or more constituent behaviors can be programmed to occur within the component. The classification table is illustrated in Fig. 26.

Taking a step further, we observe that basic shape-changing behaviors are often single-deformations, where the changes occur simultaneously. For more complex shape-changing behaviors, multiple deformations or “sequential” shape-shifting process can be programmed to occur. We illustrate this phenomenon in Fig. 27, in which basic shape changes are limited to a single step of deformation and are located at the bottom corner of the matrix. Consequentially, complex shape changes that have the capability to have multiple deformation stages integrated into the design are located on the top right corner of the matrix.