Smart Polymer Materials for 4D Printing

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This entry is adapted from the peer-reviewed paper 10.3390/molecules29020319

Among the innovative materials gaining attention are smart materials, characterized by their ability to undergo changes in properties, such as shape, color, or size, in response to external stimuli like light, heat, humidity, or electric and magnetic fields. This class of programmable materials introduces a unique dimension to 3D printing, referred to as 4D printing. In 4D printing, the same process as 3D printing is used, but the printed objects possess the remarkable capability to dynamically transform their shape or properties over time in response to external stimuli. Some smart polymers exhibit minimal responses over extended periods or possess limited reversibility in their transformations. Despite the need for further advancement in achieving swift and precise transformations in 4D-printed objects, the realm of 4D printing presents novel opportunities across diverse applications such as textiles, aerospace, medical industries, electronics, and robotics.

additive manufacturing 4D printing polymers shape memory

1. Shape Memory Polymers

Shape memory polymers (SMPs) have emerged as remarkable candidates for 4D printing applications due to their versatile processing capabilities, aligning perfectly with the requirements of various additive manufacturing (AM) technologies. These polymers exhibit elastic behaviors, making them well-suited for the intricate geometric transformations involved in 4D printing ^[1]. What sets SMPs apart is their ability to endure substantial deformation strain across a range of temperatures, allowing for dramatic and impressive shapeshifting. It is no wonder that polymeric materials have found their way into the realm of AM due to their exceptional processing attributes, making the transition to 4D printing a seamless one.

An illustrative example is a shape-changing structure under a magnetic field, depicted in Figure 1a. This structure, composed of poly(lactic acid) and magnetic iron oxide (Fe₃O₄) nanoparticles, undergoes remote heating under magnetic fields through hysteresis ^[2]. Utilizing direct ink writing (DIW) printing with UV curing facilitates the production of these shape-changing structures. While the transformation mechanisms align with other examples using thermal response, this structure exhibits fast, remotely actuated behaviors and magnetically guidable properties.

Figure 1. (a) A collection of polymer composites designed for 4D printing, featuring various innovative applications. Schematic and demonstration of a restrictive shape recovery process triggered by an alternating magnetic field ^[2]. (b) A flower-like 4D structure transforming from a flat sheet to a final flower structure ^[3]. (c) A time-lapse illustration showcasing the folding sequence of a tulip ^[4]. (d) Examples of 4D-printed flower morphologies with different bilayer directions (90°/0° and 45°/45°) ^[5]. (e) CAD models of multi-material sea stars (left), and demonstration of swelling over time in water (right) ^[6]. (f) An illustration of an initial joint and folding of bars with spring–mass systems ^[7]. (g) Illustration of the Liquid Crystal Elastomer (LCE) ink states during hot-DIW: disordered LCE ink within the barrel (i), aligned LCE ink as it moves through the nozzle (ii), and the resulting crosslinked LCE filaments post-printing (iii). Additionally, a visual representation of the printed LCE actuator with a meander-line print path showcases distinct elongation states corresponding to different temperatures ^[8]. (h) A schematic illustrating the printing of a core–shell microstructure with a highly aligned shell using fused filament fabrication (FFF) ^[9]. Figures reproduced with permission from corresponding references.

Another approach to activate SMPs involves leveraging a heat-shrinkable property, obviating the need for a shape-programming step. Figure 1b illustrates a 3D configuration transforming from a planar sheet to a final flower structure in response to temperature changes ^[3]. The release of internal strain in the polymer, generated during FFF, maintains the printed composite in a flat state when heated. Upon cooling to room temperature, the structure transforms into a flower configuration due to the mismatch in the coefficient of thermal expansion (CTE) between different materials. Additionally, Figure 1c presents another example using time-lapse to showcase the evolution of shape with temperature change ^[4]. In this instance, both geometry and printing patterns, including dimensions, the number of grooves, and active elements, were controlled during printing to induce the transformation of floral leaves into different shapes at specific times.

SMPs can be readily tailored to specific environments and applications by manipulating the crystalline content of the polymer. This fine-tuning enables the programming of critical transition temperatures, such as the glass transition temperature (T_g) and the melting temperature (T_m) ^[10]^[11]. A notable example of utilizing SMPs for 4D printing is the work of Ge et al., who pioneered a multi-material system for printing shape memory polymers using micro-stereolithography. Their ingenious ink formulation incorporated methacrylate-based monomers, along with essential components like the photoinitiator Phenylbis(2,4,6-trimethyl benzoyl)phosphine oxide (BAPO), Sudan I, and Rhodamine B. By adjusting the proportions of PEGDMA, BPA, DEGMA, and BMA, they could precisely tailor the T_g to achieve fixity control at distinct temperatures, such as 43 °C and 56 °C (Figure 1d). This precision allowed the bloom motion of their printed flower to unfold in two distinct phases ^[5].

Smart hydrogel composites, with their ability to swell upon water immersion while maintaining structural integrity, hold significant promise as smart materials. Their applications span diverse fields, from biomedicine including drug delivery devices ^[12], artificial organs, and tissue engineering ^[13]^[14], to agriculture ^[15]^[16]. In 4D printing, these hydrogel composites present an innovative method for various applications such as custom-designed sensors and robotics ^[13]^[17]^[18]. For example, Figure 1d illustrates biomimetic hydrogel composites in the form of functional folding flowers capable of being folded and twisted ^[5]. The shape transformation is governed by the anisotropic swelling behavior of the hydrogel composite, which is controlled by the alignment of cellulose fibrils along the printing directions. Another approach involves incorporating different components with distinct swelling properties within a single hydrogel structure. Figure 1e showcases sea stars that actuate via spatially controlled swelling produced with Vat Polymerization (VP) ^[6]. The sea stars exhibit different swelling behaviors in their center region (strain-limiting area in purple) and arms (more swellable materials in white), causing gradual curling toward the center regions over time in water. Lastly, Figure 1f depicts evolving bars composed of rigid materials (white) for bars, disks, and bottom parts of links, with the top of the links made of hydrogel (red), enabling the folding of the structure ^[7].

Recent advancements in the realm of 4D printing have introduced the fascinating world of cross-linked liquid crystalline polymers (LCPs) and liquid crystal elastomers (LCEs). These materials have sparked innovation in the creation of actuators and soft robotics using the process of direct ink writing (DIW) ^[19]^[20]. LCEs, in particular, are a class of materials characterized by their anisotropic nature, where their properties are finely controlled by the orientation of their molecular structure, a property that can be tailored by temperature variations ^[21]. Their remarkable capacity for shape transformation allows them to craft intricate structures like cones, paraboloids, and even origami-like folds ^[22]. Moreover, external stimuli, including heat, light, and electrical fields, rendering them dynamic and versatile, trigger the mechanical responses of LCEs.

The fascinating behavior of LCEs hinges on the arrangement of rigid molecules known as mesogens, and this behavior can be fine-tuned by altering the temperature. LCEs shift from an ordered, anisotropic state to a disordered, isotropic state at a specific transition temperature, often referred to as the isotropic transition temperature (T_i or T_m) ^[23]. Lopez-Valdeolivas and colleagues embarked on an endeavor to create thermo-actuators using LCEs with an extrusion-based printer. In contrast to traditional methods that rely on heat-induced mesogen orientation, their innovative approach involved the extrusion process, which aligned polymer chains, effectively establishing orientational order within the mesogens. The result was an impressive 50% contraction of the printed parts when heated to 90 °C, followed by a rapid recovery to their original length upon cooling ^[24]. LCEs’ distinct advantage lies in their rapid recovery rates, a characteristic not commonly observed in the swelling behavior of hydrogels.

Notably, LCEs offer distinctive attributes in the world of 4D printing. For instance, Yang et al. harnessed the power of infrared (IR) light (808 nm) to fabricate, repair, and assemble carbon nanotubes (CNTs) and xLCEs for soft robotic actuators designed for low-temperature environments ^[25]. xLCEs boast the remarkable ability to self-heal micro-cracks through photo-healing, ensuring a sustainable and efficient material life cycle. Furthermore, CNT-xLCEs are manufactured with transesterification processes carried out at temperatures exceeding 180 °C. While many of the mentioned shape memory polymers (SMPs) thus far have been thermoplastic in nature, designed to regain their shape through temperature-induced transitions involving the glass transition temperature (T_g) and melting temperature (T_m), there exists a realm where robustness is paramount. In scenarios where extreme and harsh environments demand chemically and thermally stable materials, thermoset plastics have often been the choice due to their strong chemical covalent cross-links, ensuring excellent chemical and thermal stability.

Figure 1g depicts variations in the morphologies of photopolymerizable liquid crystal elastomer (LCE) ink during direct ink writing (DIW) and the diverse elongation of the printed LCE based on the aligned printing path, depending on the temperature relative to the nematic–isotropic transition. In another extrusion printing process using solid filaments, liquid crystal polymers (LCPs, Vectra A950) ^[9], as shown in Figure 1h, exhibit a notable alignment of nematic domains through the nozzle due to shear forces during extrusion. Despite the initial alignment, the printed filament tends to lose its orientation and solidifies from the surface, resulting in a core–shell microstructure with a highly aligned shell. The mechanical properties of this printed hierarchical structure can be further strengthened with thermal annealing, facilitating chemical crosslinking of chain ends between filaments.

A study conducted by Zhang et al. demonstrated a fascinating correlation between color transmittance and the nanostructure of a 4D-printed shape memory polymer (SMP) ^[26]. Using microstereolithography (MPL), the group 4D printed submicron-scale grids using VeroClearTM. When these grids were illuminated with white light, they selectively transmitted a limited range of wavelengths due to the differential scattering of the spectrum by the grid (see Figure 2). The dimensions of the grid determined which wavelengths could pass through. Upon programming the structure, i.e., heating it above the glass transition temperature (T_g) to 80 °C, distorting it, and cooling it, the structure became transparent to all wavelengths, achieving an “invisible” state. Reheating above T_g restored the nanostructure to its as-printed state, allowing only a specific range of wavelengths to pass through once again.

Figure 2. Illustration depicting the color and shape transformation of a constituent nanostructured element in the “invisible ink” 3D printed with a shape memory polymer. The as-printed structures feature upright grids on the left, acting as a structural color filter that selectively transmits a limited wavelength range of visible light. Deformation of the structures at elevated temperatures flattens the nanostructures on the right, resulting in a colorless state, which persists as an invisible state after cooling to room temperature. Heating restores both the original geometry and the color of the nanostructures, showcasing a submicron demonstration of 4D printing. Figure reproduced with permission from ^[26].

LCEs have also been successfully 3D printed in their isotropic state, providing the user with the flexibility to manually determine the nematic director of the mesogens post-printing. The resulting 4D-printed smart LCE object demonstrates the capability to switch between the high-order post-printing programmed shape and the low-order shape determined by the printing process. In the study by Barnes et al., a specially formulated LCE ink was used, comprising RM257 mesogens and chain extenders 2,2′-(ethylenedioxy)diethanethio (EDDET) and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP) (Figure 3a) ^[27].

Figure 3. Schematic illustration of the reactive 4D printing process and shape programming of liquid crystal elastomers (LCEs). (a) Depiction of the LCE synthetic scheme, highlighting the network-forming components and three distinct reactive steps during the fabrication process. (b) Schematic representation of the 4D printing of LCEs, involving printing in a catalyst bath, followed by deformation and UV curing for shape programming. The resulting LCE demonstrates reversible shape changes between the printed and programmed structures when heated and cooled, respectively. Figure reproduced with permission from ^[27].

The ink was partially crosslinked and DIW-printed into a catalyst bath, where further crosslinking occurred, resulting in a 3D LCE structure without a defined director (Figure 3b). The printed object contained residual unreacted acrylate functional groups. By stretching the object in a specific direction, a nematic director was established parallel to that direction, and UV light was applied to crosslink the residual acrylate groups, setting the programmed shape. Upon heat treatment to the transition to nematic isotropy (TNI) at 75 °C, the mesogens relaxed, and the programmed structure reverted to the original printed structure. This shape evolution could be reversed by cooling the object.

Azobenzene moieties undergo isomerization from the trans to cis state upon UV irradiation, causing an expansion of the polymer matrix. This phenomenon has been utilized in 4D printing to create light-responsive actuating silicone bilayers ^[28]. In the case of 4D printable LCEs, azobenzene moieties contribute to light responsiveness at ambient temperature, inducing stress and mesogen misalignment. Ceamanos et al. demonstrated the 3D printing of an azobenzene-containing LCE strip that contracted under UV light, lifting a weight, and recovered its initial length under blue light ^[29]. However, spontaneous relaxation of azobenzene moieties over time posed stability challenges. Lu et al. addressed this issue by incorporating 2-ureido-4[1H]-pyrimidinone (UPy) physical crosslinkers into the LCE ink, allowing the structure to change shape via photoisomerization of azobenzene moieties under UV light. The shape was then stabilized by the reformation of UPy crosslinks when the UV light was removed ^[30], as shown in Figure 4.

Figure 4. (a) Illustration and (b) photographs depicting the photo-switchable deformations of a flower-like actuator 4D printed with light-sensitive LCE. Under 365 nm UV light, the flower curled, and the process was reversed with 450 nm blue light. After UV irradiation, the curled shape was stabilized at room temperature via the formation of UPy crosslinks. The original flat shape was recovered by heating to 65 °C, breaking the crosslinks, and allowing the LCE to entropically switch back to the isotropic state. Figure reproduced with permission from ^[30].

2. Self-Healing Polymer Materials

In the context of 4D printing, self-healing materials present a remarkable capability. These materials have the unique ability to adapt and evolve over time, offering increased longevity and reliability to printed components in a variety of applications, including electronics and healthcare monitoring. Self-healing materials are a valuable addition to the toolkit of 4D printing, offering the potential for more resilient and long-lasting printed structures. By integrating self-healing properties into 4D printing materials, engineers and designers can create structures that can endure harsh environments, maintain their functionality over extended periods, and reduce the environmental impact associated with the disposal of damaged items. This remarkable feature of self-healing materials represents a significant advancement in 4D printing technology, with widespread potential across multiple industries and applications. Self-healing materials operate by autonomously initiating the repair process when defects like cracks or scratches occur. This spontaneous healing occurs as the material fills the void created by the defect with fresh material. The ultimate goal of an ideal self-healing material is to fully restore the original mechanical and chemical properties of the pristine material ^[31]. These self-healing materials can be broadly categorized into two main types based on their healing mechanisms. Autonomous materials are capable of immediately commencing the healing process upon sustaining damage. This spontaneous repair is facilitated through mechanisms like reversible hydrogen bonds, non-covalent bonds, or the release of self-healing substances that are embedded within the host material and triggered by damage ^[32]. Ton-autonomous materials, in contrast, require external stimuli to initiate the healing process. These stimuli can include factors such as exposure to UV light, application of heat, or mechanical activation. These external influences are necessary to trigger repair ^[32].

2.1. Microvascular Self-Healing Mechanisms

Self-healing mechanisms using microvascular systems incorporate a network of channels within a material, filled with a healing agent, traversing the material’s matrix. Upon the occurrence of a crack or damage, capillary action initiates the release of the healing agent into the damaged area, where it solidifies to seal the crack. This concept draws inspiration from the natural arterial system ^[33]^[34]. Li et al. ^[35] introduced an innovative biomimetic 3D vascular design inspired by nature for self-healing cementitious systems (Figure 5a–c). The 3D printed design featured a polylactic acid (PLA) vascular network produced on an Ultimaker^® 3D printer (FFF). This network incorporated the healing agent sodium silicate. In instances where a crack formed in the cement matrix, the vascular design adhered to Murray’s law for circulatory blood volume transfer, thereby facilitating the healing process. Another approach was taken by Wu et al., who developed a microvascular network using a fugitive ink and hydrogel reservoir to create a self-healing printed material tailored for applications in healthcare microvascular devices ^[36].

Figure 5. (a) Representation of a vascular beam crack pattern and microscopic imagery documenting the healing process following a 4-point bend ^[35]. (b) A CT grey level image unveiling the concrete matrix, cracks, PLA vascular tubes, and sodium silicate self-healing agent. The 3D system was reconstructed to illustrate the vascular mechanisms, with the yellow area symbolizing the cement, violet sections representing the PLA vascular structure, blue section representing sodium silicate gel, and the green area depicting the gel filling in cracks ^[35]. (c) Depictions of colored self-healing polyurethane samples that were cut, connected, and healed at 80 °C for 12 h. The healed sample is showcased while being stretched and supporting a 5 kg weight ^[37]. Figures reproduced with permissions.

Balancing the characteristics of printable inks with multiple functionalities is a delicate task, as it involves finding the right balance between printability properties and functional properties. In this instance, thiol groups play a role in the photocuring process, while disulfide groups contribute to self-healing properties. Li et al. also documented the printing of a photocurable elastomeric material using DLP (Figure 5c) ^[37].

2.2. Encapsulation Self-Healing Mechanisms

Encapsulation-based self-repair mechanisms utilize micro- or nano-sized capsules filled with a healing agent, such as a polymer or catalyst. Each capsule is shielded by a case or coating made from an inert material to prevent interference with the bulk material. This ensures that the healing process only commences when damage occurs, not prematurely. The capsule’s shell is designed to be fragile, enabling easy rupture upon damage. These capsules are distributed throughout the material’s matrix to ensure a rapid release of the healing agent upon rupture. The healing agent inside the capsules should have low viscosity to facilitate capillary action into the damaged area. For efficient healing, the healing agent should polymerize quickly at room temperature without shrinkage ^[38]. Davami et al. used SLA 3D printing to create structures that entrap photocurable resin within unit cells, acting as a self-healing agent. When damage occurs, the self-healing agent flows out of the cell through capillary action to the damaged site, where it is cured using UV light. Davami et al. reported a healing efficiency of 52% based on fracture toughness ^[39].

2.3. Autonomous Self-Healing Polymers: Supramolecular Polymers

Most polymers typically require the presence of a host network containing a self-healing agent, such as capsules or vascular systems, while supramolecular polymers constitute a distinct class of materials. Characterized by reversible non-covalent bonds, these polymers can reform and repair after being cleaved ^[40]. The inherent self-healing properties of supramolecular polymers distinguish them, allowing for repeated healing without depleting a host healing agent. Often described as “solid-liquids,” these polymers consist of associated groups typically covalently bonded to chain ends or side chains of a polymer. These groups bring together liquid-like polymers into a network of non-covalently cross-linked polymers, exhibiting plastic behaviors. The reversible reforming process of supramolecular polymers involves non-covalent crosslinks like π-π stacking, hydrogen bonds, host–guest interactions, ionic interactions, and metal coordination. Evaluating the effectiveness of restoring a self-healing material involves examining various properties, such as mechanical, electrical, and thermal properties, before and after healing. This assessment, known as healing efficiency, is calculated using the ratio of these properties before and after healing ^[40].

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