4D Printed Composite Actuators

Robin Delbart, Laurane Roumy, Marine Harcourt, Colin Robert, Fabienne
Touchard, Thuy Quynh Truong Hoang, Francisca Martinez-Hergueta

► To cite this version:

Robin Delbart, Laurane Roumy, Marine Harcourt, Colin Robert, Fabienne Touchard, et al. 4D Printed Composite Actuators. Fabienne Touchard; Fabrizio Sarasini. Additive Manufacturing of Polymer-based Composite Materials: Materials, Processes, and Properties, Elsevier S&T Book, 2024, 978-0-443-15917-6. 10.1016/B978-0-443-15917-6.00005-0. hal-04477283

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Submitted on 16 Oct 2024

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4D Printed Composite Actuators

R. Delbart<fnref n="1" />, L. Roumy<fnref n="2" />, M. Harcourt<fnref n="2" />, C. Robert<fnref n="3" />, F. Touchard<fnref n="2" />, T.Q. Truong-Hoang<fnref n="4" />,
F. Martínez-Hergueta<fnref n="1" />*

Abstract

This chapter presents a literature review on 4D Printed Composite Actuators for engineering applications. The recent integration of particle and fibre reinforcements into liquid crystal elastomers, hydrogels, and shape memory polymers enables the development of next-generation actuators with advanced functionalities and significantly enhanced load-carrying capacity. Proofs-of-concepts considering the nature of the stimuli-responsive material are categorised, and the advantages of the particle and fibre reinforced devices are presented. The approaches are illustrated with examples, exposing their capabilities and limitations. Advanced functionalities and modelling techniques are summarised. The main challenges associated with implementing 4D printing technologies in two key engineering applications: soft robotics and morphing structures are discussed.

Keywords: Additive Manufacturing, Actuators, Shape Memory Polymer, Modelling

1. Introduction

4D printed devices are Additive Manufactured components able to transform energy
into mechanical motion in response to specific external stimuli [1]. The fourth dimension
addresses the change in the shape of the 3D printed component as a function of time. The
baseline technology consists of a 3D printed stimulus-responsive smart material that can
shape-shift between two different stable stress states when the appropriate stimuli are applied

Preprint submitted to Additive Manufacturing of Polymer-based Composite Materials September 20, 2023

<footnote n="1">¹ *School of Engineering, Institute for Infrastructure and Environment, The University of Edinburgh,*
*William Rankine Building, EH9 3FG, Edinburgh, UK.*</footnote>

<footnote n="2">² Institut PPRIME, Dpt Physique et Mécanique des Matériaux, CNRS-ENSMA-Université de Poitiers,
France.</footnote>

<footnote n="3">³School of Engineering, Institute for Materials and Processes, University of Edinburgh, UK</footnote>

<footnote n="4">⁴ ESTACA, Pôle Mécanique des Structures Composites et Environnement, Ecole d'ingénieurs, France</footnote>

<footnote>*francisca.mhergueta@ed.ac.uk</footnote>
[2]. 4D printed actuators are lightweight, low-cost and relatively simple in comparison to conventional hydraulic, pneumatic, electro-mechanical or electro-magnetic actuators [3]. These characteristics allow them to perform a range of unique tasks, such as self-assembling, self-repairing, and bioprinting, not achievable with conventional actuation technologies. This enables the development of novel technologies for a large variety of applications, including autonomous sensing soft robots or morphing structures.

Although many review articles have been published in 4D printing to date, the rapidly changing nature of this research field requires the publication of new reviews formulating the current open research questions in the field. The addition of particle and fibre reinforcements in conventional liquid crystal elastomers, hydrogels and shape memory polymers enables the creation of multifunctional materials with photothermal, magneto-thermal and electro thermal properties, facilitating the triggering of the actuation. Furthermore, the addition of a stiffer constituent increases the load-carrying capacity of the actuators, enabling the creation of next-generation actuators with superior load-carrying capacity.

This chapter book presents a literature review of different 4D printed composite proof-of-concept actuators. The different approaches are categorised considering the nature of their stimuli-responsive materials and are illustrated with different examples in the literature. Novel original experimental and numerical demonstrators created by the authors have been included. The chapter exposes the disruptive capacity of each technology while addressing the current challenges and limitations for their widespread adoption in engineering applications. The potential of advanced functionalities enabled by the reinforcement is also summarised. Finally, the future trends in two key engineering applications: soft robotics and morphing structures are discussed.

## 2. 4D printing definition and approaches

Different methods are available to create 4D printed objects using a variety of 3D printing techniques [1] but the most common widespread technique combines different smart materials in the three dimensional space resulting in at least two metastable states in a 4D printed structure: the temporary shape, and the original shape that is recovered thanks to
<figure><img src="image_1.png" /><figcaption>Figure 1: Summary of the different manufacturing techniques, materials and stimuli used in 4D printing.</figcaption></figure>

an appropriate stimulus [2]. A large amount of 3D printing technologies are available in the market compatible with 4D printing, summarised in Fig. 1. This includes Inkjet printing, Fused Deposition Modeling (FDM), Stereolithography (SLA), Selective Laser Sintering (SLS), Direct Ink Writing (DIW) and Digital Light Processing (DLP) [4, 5, 6, 7]. They have been combined with different types of stimuli, such as water [8, 9, 10, 11], heat [12, 13, 14, 15] or light [16, 2] to trigger the transformation. They can present multiple functionalities such as self-sensing (the capacity to perceive information without relying on external sensors), responsiveness (the capacity to react in response to external stimuli), shape memory recovery (the capacity to recover its original shape), self-adaptability (the capacity to autonomously adapt its behaviour, configuration or performance in response to changing conditions) and self-repair (the capacity to detect, diagnose and repair failure autonomously) [17, 18, 19].

The most critical component of a 4D printed structure is the stimulus-responsive material used. This section summarises the main approaches available in the literature, classified considering the different combinations of smart materials in one device and their nature. It presents the advantages of particle and fibre-reinforced materials over their non-reinforced counterparts. Emphasis is given to thermal-responsive approaches [20].
## 2.1. Shape Memory Polymers

Shape Memory Polymers (SMPs) are smart materials with the ability to change their shape in response to an external stimulus. This property is known as the Shape Memory Effect. The Shape Memory Effect is programmed by inducing internal residual stresses that result in a metastable temporary shape. Afterwards, the SMP is subjected to an external stimulus that allows the device to release the residual stresses and recover its original shape [10].

A series of programming steps lock the temporary shape to constrain the devices and induce residual stresses. As an illustrative example, in the case of thermo-responsive SMPs, the temporary shape is acquired due to the differences in material behaviour at the molecular level when exposed to high temperatures. Semicrystalline and amorphous polymers have a characteristic glass transition temperature $T_g$, which divides the response of the materials, from the original stiff response at low temperature, into a rubbery state [21]. The programming procedure takes advantage of these fundamental physics to introduce internal stresses in four consecutive steps illustrated in Fig. 2: (1) heating the structure above the glass transition temperature ($T_g$), (2) deforming the object into a new shape (applying external load) while the temperature is kept above $T_g$, (3) cooling the structure below $T_g$ while holding the external load and finally (4) releasing the structure to acquire a "temporary" shape that can recover its initial shape when (5) the structure is heated above the $T_g$ again. In some cases, the programming steps are not mandatory and further details will be explained in the following sections.

A large range of stimuli is available in the literature to trigger the shape change in shape memory polymers:

* **Temperature:** Thermally responsive SMPs are the most conventional in literature. Their main physics have been explained in the previous paragraph, and will be further explored in this section.

* **Light:** UV radiation is generally employed to achieve shape fixation after deformation, photo-inducing the formation of new chemical bonds [22]. This mechanism can be
<figure><img src="image_2.png" /><figcaption>Figure 2: Programming steps and actuation of a conceptual bending actuator.</figcaption></figure>

eventually combined with temperature-responsive motion mechanisms to achieve a
reversible trigger [23].

* Magnetic Fields: Magnetic nanoparticles can induce the shape change of a soft polymer by particle realignment in the presence of a magnetic field [24]. This approach produces patterned magnetic polarity devices compatible with soft elastomers. This method does not incur in an increment of temperature and therefore is suitable for bioengineering applications.

* Solvents and pH: Some SMPs respond to changes in solvent composition. This includes water-soluble SMPs with a relatively low critical solution temperature which defines the point at which the polymer changes from hydrophilic to hydrophobic behaviour, inducing substantial changes in mechanical properties, transparency and shape [25, 26]. These are usually coupled to pH-sensitive shape change mechanisms, when subjected to different acidity or alkalinity conditions. They have potential applications in drug delivery systems, such as encapsulators of metallodrugs [27, 28].

For the particular case of temperature-responsive SMPs, two kinds of heating stimuli are
available; (i) the classical external sources of heat such as an oven, hot water or radiation [29, 30], and (ii) the indirect sources of heat such as the photothermal effect [31, 32, 33], the induction heating effect [34] and Joule's effect [35]. The following paragraphs describe the underlying physics of the different indirect heating triggering methods, illustrating them with different 4D printing examples and analysing the main limitations of each technology.

Photothermal responsive SMPs are based on the phenomenon of transforming the electromagnetic radiation of light into thermal energy. To provide photothermal properties to polymers, previous authors have employed SMPs reinforced with carbon nanotubes (CNTs), black ink, gold nanoparticles or nanorods [36]. These nanoparticles have a strong absorbance in the near-infrared region, high photothermal conversion efficiency and are low cost in the case of carbon nanoparticles (CNTs, graphene or carbon quantum dots). Photothermal responsive SMPs have been successfully manufactured by Ge et al. [7] that used Digital Light Processing and a methacrylate-based resin to create a shape-shifting object such as an Eiffel tower able to recover from a temporary bent shape, and different objects including a grip, see Fig. 3. One of the main limitations of this technology is the ability to impose high strains above the $T_g$ due to the brittle nature of these polymer blends. This is crucial to achieve large changes in shape and allow the implementation of technologies where large rotation angles or extensions are required. Such limitations have been found in the literature when using stereolithography or Digital Light Processing manufacturing methods since they are using photopolymers that are not very stretchable due to their high cross-linking density, which stands for the number of cross-link molecular bonds in the polymer.

Temperature-responsive SMPs can also be triggered by the induction heating effect, which appears when a polymer reinforced with ferromagnetic nanoparticles such as $Fe_3O_4$ is exposed to an alternating magnetic field. The rapidly alternated magnetic field goes through the object and generates an electrical current, the so-called eddy current, that flows through the material to heat it by the Joule effect, shifting to the second metastable shape. One remarkable example of this technology includes the concepts developed by Wei et al. [6] who produced a thermally and remotely actuated shape-changing SMP by direct ink writing, printing ultraviolet cross-linking Polylactic acid/$Fe_3O_4$ composite inks, see Fig. 4.
<figure><img src="image_3.png" /><figcaption>Figure 3: 4D printed photothermal responsive Eiffel tower that recovers from the temporary bent shape (left side) and the snapshots of a grabber that picks a screw (right side) [7]. Creative Commons CC BY license.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 4: (a) Demonstration of the remotely-actuated shape-changing performance of a 4D printed nanocomposite cylinder in a 30 kHz alternating magnetic field. Reproduced with permission from [6].</figcaption></figure>

This actuator has potential biomedical applications such as intravascular stents, as it is magnetically guided and remotely triggered. However, the stent requires substantial heat to be triggered and can cause unpleasant side effects in patients, which currently limits its implementation.

In 2018, Kim et al. [24] created fast-transforming 3D printed soft materials actuated by magnetic fields. The nozzle was modified to apply a magnetic field during the 3D printing process of an elastomer composite containing ferromagnetic microparticles allowing the reorientation of the particles along the applied field to create a patterned magnetic polarity device. This method does not require raising the temperature of the structure to trigger the actuation, but can be only applied to soft materials with limited load-carrying capacity.

Very recently, few publications have reported investigations on Joule's heating [37]. This approach relies on a multifunctional SMP able to change the temperature when subjected to
<figure><img src="image_5.png" /><figcaption>Figure 5: Electro-thermo-mechanical actuator. Conceptual design.</figcaption></figure>

a potential difference. The concept is illustrated in Fig. 5. When a large potential difference is induced, and the structure is self-heated over the $T_g$, the stiffness drastically drops such that it can be deformed into a temporary new shape that can be locked once the electrical generator is switched off and the structure cools down below the $T_g$.

The multifunctional SMPs are reinforced with conductive fillers such as metal nanoparticles, carbon black, graphene or carbon nanotubes [38, 39, 40, 41, 42] to form a segregated conductive network and acquire their electrical properties. The physical principle relies on the distribution of the conductive particles to drive electricity from one pole to another, this is called percolation [43]. Electrical conductivity develops through two mechanisms in conductive polymers [44]: (i) electron tunnelling, referring to the “jump” of electrons from one particle to the next one across distances through the polymer matrix and/or (ii) direct contact of conductive particles, referring to the electron transfer via conductive filler pathway. The Percolation theory establishes there is a minimum volume fraction of conductive filler required for the composite to transition from insulation to conductive nature, which is known as the percolation threshold. This threshold depends on many factors, including the chemical structure of the matrix, and the shape, size, distribution, and conductive properties of the filler [45, 46, 47, 48, 49, 50].

The electrical properties of a polymer composite can be defined by its resistivity, the fundamental physical property that measures the inherent resistance of the material to the
<figure><img src="image_6.png" /><figcaption>Figure 6: Snap shots of an electro-thermo-mechanical multifunctional 3D printed actuator with folding capabilities recovering its initial flat shape. Representative thermal camera temperature measurements during actuation.</figcaption></figure>

flow of an electric current. When the electric current flows through a conductor with resistance, the collisions between the moving charges and the atoms in the conductor cause the electric energy to be converted into heat. This phenomenon is known as Joule heating. These physical principles have been experimentally exploited recently to program polylactic acid (PLA) 3D printed devices reinforced with CNTs and carbon black particles, increasing the temperature above the $T_g$ indirectly with an electric generator [51, 37, 52]. The studies demonstrate the feasibility of the technology to unfold layers and recover their original flat shape. Fig. 6 shows an electro-thermo-mechanical 3D printed folding actuator manufactured in carbon black PLA able to recover its initial flat shape after being heated by an electric current. The main current limitation of this approach is the difficulty to control the temperature gradients and avoid irreversible damage and degradation of the electrical properties when overheated.

Further efforts in the systematic characterisation of thermo-electro-mechanical properties are crucial to ensure the successful integration of self-heated particle reinforced SMPs in engineering applications and control the motion of the devices during actuation with accurate spatial and time resolutions. As reported in the literature, the resistivity of a polymer increases with the temperature, showing drastic changes in trends above the $T_g$ [53, 54]. Furthermore, the response of conventional actuation technologies can vary with the number of applied cycles [55]. This progressive deviation hinders the development of a robust actuation technology. The performance of the actuator may degrade over time or with repeated
use, which can affect its functionality and long-term reliability [51]. Addressing these issues
and improving the durability of 4D printing is crucial for advancing the field and achieving
reliable actuation technologies.

## 2.2. Liquid Crystal Elastomers

Liquid Crystal Elastomers (LCEs) are soft materials that combine the elastic properties of rubbers with the anisotropic properties of liquid crystals [56]. The physical principle behind the reversible shape variation of LCEs relies on the differences in stiffness between molecular chains along different preferential directions. Side-chain or main-chain LCEs with liquid crystal mesogenic molecules (or mesogens) can undergo a large contraction along the direction of the mesogens under external stimulus [57, 58], resulting in a prescribed deformation and motion.

The most conventional stimuli employed to trigger the shape-shifting functionality of LCEs are heat, organic solvents or light [59]. The exposure to such stimuli induces phase transitions of the molecular chains, aligning the molecules to modify the stiffness of the material. The main drawback to implementing this technology in engineering applications relies on the impracticality of applying the external stimuli (e.g., heating the surrounding air, immersion in hot water or exposure to infrared light) to induce the actuation process.

To overcome such limitations, current research efforts in LCE 4D printing are focused on the development of technologies with integrated self-triggering systems. For instance, Urayama et al. [60] developed a novel magnetic device able to align the polymer chains when subjected to an electric field. This approach led to a variable stiffness as a function of the particle orientation, and different electrooptical and electromechanical effects as a function of the cross-linking density. However, this technology currently presents a very low energy transformation efficiency, way lower than the conventional 40% ratio, since the energy applied with the magnetic field is much larger than the mechanical energy produced during the deformation of the device.

Alternatively, temperature gradients can be controlled by electrical circuits that can provide local heating to the structure. Yuan et al. [61] demonstrated the potential of LCEs
<figure><img src="image_7.png" /><figcaption>Figure 7: Thermo responsive fibre reinforced LCE actuator [66]. (a) and (b) recovery of the original shape from different twisting angles. (c) Combination of the previous actuators in a proof-of-concept grip holding a foam. Reproduced with permission from Elsevier.</figcaption></figure>

in 4D printing to achieve reversible actuation with an integrated electric-control system. In their work, LCE films were pre-fabricated [62, 63] and manually installed on 3D printed objects [64, 65]. To achieve thermal actuation, conductive silver nanoparticle ink wires were also printed at the locations where the LCEs would be installed. After inducing an electrical current, the printed circuit produced Joule heating, increasing the temperature of the LCE strip and resulting in an overall bending deformation of the 3D printed structure. When the current was switched off, natural convection gradually cooled the actuator, leading to the recovery of the original flat shape.

Recently, Xia et al. [66] developed fibre-reinforced LCE composites to eliminate the need for secondary conductive wires, developing temperature-responsive grippers with modest load carrying capacity, see Fig. 7. This multifunctional composite actuator has a simpler design in comparison to previous approaches, showing the potential of composite LCEs over conventional LCEs to facilitate actuation. The reliability of actuation after cyclic electro-thermal loading is a current open research question.

All the previous LCE approaches demonstrated reversible actuation, being a promising solution to fully develop the field of soft robotics. However, the technology still presents
several limitations that hinder its implementation. First, the actuation speed which is very slow and limits its use for dynamic responsive applications. Second, the low TRL of the composite actuators, and the additional complexity of the self-triggered systems which rely on secondary elements such as metallic wires that can be damaged after multiple cycles. Finally, due to the rubbery nature of LCEs, their load-carrying capacity is limited, resulting in very compliant hinges not suitable for applications with large stress solicitations such as heavy-duty robots or morphing of primary structural components.

## 2.3. 4D printed shape-shifting multimaterials and hydrogels

The possibility to print objects with more than one material has allowed huge progress in 4D printing. This technology induces local strain gradients when combining materials with different mechanical properties, localising the change of shape on a specific area [67]. Nowadays a large number of commercial 3D printing systems enable multi-material printing including composite hydrogels (water-responsive [8, 68] or thermal-responsive [69, 70]), SMP composites (fibre-reinforced active composites [71, 72] or bilayer SMP composites [73, 74]), multi-material SMP [14, 75] and desolvation-induced multi-materials [76, 77].

Hydrogels are hydrophilic materials that can be used in combination with other polymers to create an actuator. The main physical principle relies on their ability to imbibe large amounts of water making them swell. They have been used by Tibbits and coworkers [8] to create multi-material 3D printed structures composed of rigid polymer beams and hydrophilic rubber hinges that swelled when exposed to water. The hydrogel expansion created when drawn into water triggered the movement of the hinge that caused the joints to fold.

Hydrogels have also been used by Mao et al. [68] to create a reversible shape-shifting structure, see figure 8. Their technology combines hydrogels and SMPs in a multilayered structure, composed of an SMP top layer, an elastomer layer at the bottom, and the hydrogel layer in the middle. This hydrogel layer is 3D printed with holes to let the water go through and trigger the shape transformation. When the structure is drawn into cold water, the stiffness of the SMP is high enough to constrain the deformation induced by the swelling of
<figure><img src="image_8.png" /><figcaption>Figure 8: The reversible actuation component where the hydrogel is confined by the SMP and the elastomer layers. Schematic plots for a typical two-way actuation: In S1, the sample is immersed in water at low temperature; in S2, it is brought to a high-temperature environment and bends; in S3, it is cooled down to a low temperature; in S4, it is allowed to dry; in S5, it is heated to recover the original printed shape [68].

Creative Commons Attribution 4.0 International License.</figcaption></figure>

the hydrogel, however, when drawn in hot water the SMP's stiffness drops drastically and releases the hydrogel stresses, inducing the bending of the structure. As soon as the structure is subsequently cooled and dried, it is locked in a temporary shape that is triggered with a second heating cycle that makes it recover its initial shape, resulting in two different shapes that can be inverted without the help of any mechanical forces. Similar methods have been used with thermal-responsive hydrogels to create 4D printed structures [69, 70, 78]. The main disadvantage of hydrogels consists of the reliance on water immersion to acquire the new shape, which drastically limits the application of the technology for real world devices.

Unlike hydrogels, multilateral SMP actuators need to be programmed [79, 71]. Ge et al. [12] printed a composite based on a rubbery polymer (TangoBlack, an acrylic-based material created by Stratasys) with embedded rigid polymer fibres (veroWhite, another acrylic-based resin created by Stratasys) using a polyjet 3D printer. With a glass transition temperature of around 60°C, the fibres were used as SMPs and programmed to a temporary shape. Thus, depending on the fibres distribution/fraction/orientation, complex movements were obtained
<figure><img src="image_9.png" /><figcaption>Figure 9: SMP composite actuator (a) CAD design. (b,c) The deformation behaviour of the active insect when put in hot water with temperatures of 30°C. (d) Rotated view of the structure in hot water. (e) Recovered shape in 60 °C water [15]. Creative Commons CC BY 4.0 License.</figcaption></figure>

such as bending and twisting, to acquire coiled and folded final shapes. This concept has been further investigated by Wu et al. [15] where different combinations of rigid and rubbery polymers were employed to change the glass transition temperature and stiffness, resulting in a laminated composite structure capable of achieving multiple shape changes at different temperatures. Figure 9 shows a structure able to mimic an insect thanks to the combination of SMPs with different glass transition temperatures. The main drawback of this technology is the reliance on an external heat source such as hot water, which can limit its applicability in various engineering applications.

## 3. Advanced functionalities

4D printing refers not only to shape-shifting but also to changes in material properties or functionality over time. For example, 4D printing has transformed electronic devices into shape-shifting functional devices [80, 77], produced devices with optical or electrical shape-dependent properties [81], and allowed the development of devices with time-dependent functional properties such as tissue maturation [82], biodegradability [83, 84], self-healing [85, 86, 87] or colour shifting [88]. This section summarises a variety of advanced functionalities.

### 3.1. *Programmable free, shape-shifting devices*

Despite all the advantages reported for SMPs, thermomechanical programming can be restrictive, so removing these programming steps by incorporating the internal stresses di-
<figure><img src="image_10.png" /><figcaption>Figure 10: (a) The direct 4D printing approach exploits the ability to print multimaterial composites with internal stresses [89]. Creative Commons BY-NC 4.0 License. (b) Temporary shape of the folded 2D lattice taken out from 70°C water, then recover the original shape of the 2D lattice when plunged into 70°C water again, exhibiting a shape-memory effect [90]. Creative Commons Attribution 4.0 International License.</figcaption></figure>

rectly during 3D printing presents a clear breakthrough in the development of autonomous 4D printing. Ding et al. [89] used this process to create a bilayer laminated strip combining a soft elastomer and a variable stiffness SMP. Internal stresses were controlled with the printing parameters and were released as soon as the structure was heated above $T_g$, changing its shape into a new permanent position, see figure 10(a). An et al. [74] printed a so-called thermorph, a bilayer actuator composed of a PLA SMP active layer and a thermoplastic polyurethane constraining layer. The composite was printed flat and was triggered to self-fold into complex 3D geometries (up to 70 faces) such as a chair.

Other techniques that do not require programming steps employ the heat-shrinkable properties of SMPs. Indeed, when heated above the glass transition temperature, internal stresses induced during the 3D printing process are released, triggering the change in shape [91]. Zhang et al. [90] used FDM to print PLA 2D lattice structures to fold/unfold and trigger the pattern transformation in a controllable way, see figure 10(b). The main disad-
<figure><img src="image_11.png" /><figcaption>Figure 11: Triple-SME in the printed SMP box. Snapshots during the shape memory cycle, where $K_1$ denotes the curvature of the two horizontal sides, and $K_2$ denotes the curvature of the two vertical sides [92]. Reproduced with permission from Elsevier.</figcaption></figure>

vantages of these techniques are the possibility to trigger the actuator only once. They are also very sensitive to manufacturing parameters, making them barely reproducible.

Multi-shape actuators usually rely on a variety of transition temperatures (e.g., glass, melting) or a broad transition range. Yu et al. [92] introduced a new method that did not require a programming protocol, resulting in automatic shape change to multiple metastable positions. They used an epoxy-based photocurable resin with a broad temperature range of glass transition events around its $T_g$ (approx 40 °C). The range of glass transition temperature was used to create two temporary shapes, hence creating a multi-SME according to the temperature [93, 94, 95, 96]. A triple shape memory square is shown in Figure 11. Upscaling this technique is a current open research question due to the technical difficulties to 3D print this polymer blend.
## 3.2. Reversible actuation

Most of the reversible actuation technologies available rely on Liquid Crystal Elastomers (LCEs, see section 2.2). They present a reliable solution for reversible actuation but with limited load-carrying capacity [59]. Very few examples of reversible SMP actuators have been reported in the literature. Wang and Li [97] combine the active SMP layer with a rubbery polymer to obtain a reversible actuation. They took advantage of the internal stresses induced by the 3D printing process when printing at high speeds (60 mm/s) to obtain a temporary bent SMP, avoiding the initial programming steps, able to shift to a flat shape at high temperatures when heated in an oven. The actuator included a passive confinement rubbery layer to induce internal stresses after cooling due to the different thermal expansion coefficients between the materials. As a result, the actuator recovered the post-printing bent shape, and the actuation could be triggered multiple times. The main disadvantage of this technology is the poor reproducibility of the residual stresses induced by the 3D printer, and the loss of functionality after several actuation cycles due to the progressive relaxation of the viscoplastic materials [98].

Reversible actuation can also be achieved by combining two SMPs with different glass transition temperatures and thermal expansion coefficients. Figure 12 (a) shows an example of a morphing structure 3D printed in conductive carbon black PLA and Acrylonitrile butadiene styrene (ABS). The structure was composed of three beams which can be programmed at different bending angles. Figures 12 (b), (c) and (d) show the performance of a beam initially programmed by Joule's heating at 80°C, above the $T_g$ of the PLA, but below the $T_g$ of the ABS polymer. Afterwards, it was subjected to three consecutive warming and cooling cycles at 100°C, the beginning of the ABS $T_g$. Figure 12(e) shows the evolution of the bending angle as a function of the time and the temperature. The first motion mechanism registered during actuation was the shape memory effect of the PLA, which straightened the beam from the temporary 130° bent angle, see Fig. 12(b) to 162°, see Fig. 12(c). Afterwards, during cooling, the difference in thermal expansion coefficient produced a second bending of the beam, generating a new temporary position, see Fig. 12(d). Consecutive cycles triggered reversible bending with changes of 8° in bending angle.
<figure><img src="image_12.png" /><figcaption>Figure 12: (a) Morphing structure prototype. (b), (c) and (d) response of the bipolymer beam at different time frames during the actuation stage. (e) Evolution of the bending angle as a function of temperature.</figcaption></figure>

### 3.3. 4D bioprinting

The term 4D bioprinting refers to the 3D printing of biocompatible materials or living cellular structures that evolve over time after being printed [99]. This includes both shape-shifting biomaterials and the maturation of engineered tissues [82]. Kuribayashi-Shigetomi et al. [100] developed a self-folding origami cell formed by 3D cell-laden microstructures thanks to the cell traction force. Using microplates without a flexible joint, hollow microstructures with relatively low bending stiffness can be self-folded into shapes such as a cube, see figure 13(a). This process requires advanced 3D printers capable of manufacturing microarchitectures with resolutions of 3 to 4 µm and must be totally bacterial free according to bioprinting standards.

3D printing of porous bioscaffolds shows tremendous potential for soft tissue reconstruction [101, 102, 103]. Kang et al. [104] reported an integrated tissue-organ printer that can fabricate stable, human-scale tissue constructs of any shape, such as in vivo bioprinted ear
<figure><img src="image_13.png" /><figcaption>Figure 13: (a) Regular tetragon, regular dodecahedron and cylindrical tube with self-folding capabilities [100]. Creative Commons CC BY 4.0 License.</figcaption></figure>
constructs. However, in addition to the previous limitations, this method has limitations in the size and stability of the printed organs, and it is currently unaffordable in comparison to conventional prosthetics.

### 3.4. 4D self-healing

In order to extend the lifetime of material systems, self-healing materials have been fabricated to enable structural restoration and function recovery [105, 106]. Healing occurs at molecular level due to dynamic bond exchange reactions. These are typically achieved by using different types of dynamic covalent bonds, such as the Diels-Alder (DA) reaction [107, 108], transesterification [109, 110], Schiff base chemistry [111, 112] and disulfide metathesis [113, 114].

Recently, Kuang et al. [87] developed UV-assisted direct ink writing (DIW) printing to fabricate a semi-interpenetrating elastomer network with a complex structure, high strain shape memory effect, and repeated self-healing capability. The diffusion and re-entanglement of linear polycaprolactone (PCL) chains in crack interfaces upon heating led to repeated crack healing, see figure 14(a). In addition, the effect of high strain shape memory could help heal large cracks in Vitrimers or covalent adaptive networks, changing their network topology through bond exchange reactions [115].

Yang et al. [117] developed a reversibly cross-linked thermoset based on furan-maleimide DA links for FDM printing. DA thermosets could be de-cross-linked and melt during printing at elevated temperatures (>90 °C) and re-cross-linked at lower temperatures due to their entropically favourable state [118, 107]. Appuhamillage et al [86] blended a DA polymer with commercially available PLA to achieve remarkable mechanical properties by using the reversible DA reaction to improve the interfacial bonding properties after printing.

More recently, Zhang et al. [116] used a two-step polymerization strategy to develop 3D printed reprocessable thermosets that impart reshapeability, repairability, and recyclability into 3D printed structures see figure 14 (b). These recyclable and healable 3D printing methods contribute to alleviating environmental challenges associated with the continuous increase in consumption of 3D printing material.
<figure><img src="image_14.png" /><figcaption>Figure 14: (a) Self-healing process for printed Archimedean spiral structure. Reprinted with permission from [87]. Copyright 2018 American Chemical Society. (b) Demonstration of the ability of the material to repair flawed printed structures: surgery on a damaged rabbit [116]. Creative Commons Attribution 4.0 International License.</figcaption></figure>

To date, the development of composite actuators with self-healing capabilities has been poorly reported in the literature. Delbart et al. [53] investigated the self-healing capacity of a multifunctional carbon black-reinforced PLA composite subjected to Joule heating, see Fig. 15. The evolution of void content was analysed by tomography, showing a progressive reduction in void content when increasing the temperature of the device above the $T_g$ due to the resin flow. Reductions of 63 % in void/material pixel ratios were registered after one Joule heating cycle, with the possibility to reduce up to 96 % of void content for annealing processes. Despite the potential, the use of the previous technology to remove and heal larger cracks is a current open research question, but it could be employed to repair these actuators, for instance, after low-velocity impacts.

## 4. Modelling

Considering the complexity of the couple electro/magnetic/photo/thermal-mechanical response of these novel actuators, modelling can play a crucial role to gain deeper insight into
<figure><img src="image_15.png" /><figcaption>Figure 15: Representative tomographies showing the progressive reduction of void content in a carbon black PLA 3D printed component subjected to successive Joule heating cycles.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Figure 16: Numerical simulation of the hydrogel actuator [119]. Creative Commons CC-BY-NC-ND license.</figcaption></figure>

the underlying physics involved in the mechanical response during the different programming
steps and actuation.

Sosson et al [119] developed a modelling framework for simulating smart materials considering a voxel basis. The modelled smart materials were limited to nonprogrammable shape-shifting SMs, including piezoelectric materials, electro-/magneto-/photostrictive materials, and hydrogels. They used the material properties characterised by Westbrook and Qi [120] to predict hydrogel shrinking and imposed bending. This motion was created by patterning the hydrogel (red) into another non responsive hydrogel (blue) as shown in Fig. 16. For instance, this framework has been employed to optimise the final shape of a plane wing [121], see figure 17.

Other approaches based on the Finite Element Method (FEM) focused on the development of actuators for soft robotics [122, 123] or morphing structures [124], providing an efficient method to optimise the topology and the motion of the devices. The FE models provide insights into the actuator behaviour under different inputs such as pressures [122] or temperature [125]. The bending angles and output forces of the actuator can be characterised experimentally and used to validate the models. In addition, FEM can predict the force distribution for different sets of design parameters, considering geometrical characteristics such as wall thicknesses and cross-sectional widths [122].

The level of complexity in the modelling approaches found in the literature is directly
linked to the complexity of the physical system being studied. Therefore, technologies that
<figure><img src="image_17.png" /><figcaption>Figure 17: Wing twist problem formulation [121]. Creative Commons CC-BY license.</figcaption></figure>

involve internal self-triggering mechanisms and couple the thermo-mechanical behaviour with electro/photo or magnetic properties pose greater technical challenges and demand additional efforts to achieve the necessary level of accuracy for engineering predictive tools.

For example, one action to improve the accuracy of the prediction of temperature- responsive SMPs is to populate the existing experimental database of material properties, with a focus on the plastic response of polymers across a large temperature range. This is challenging experimentally, in particular above the $T_g$ since the equipment for mechanical polymer testing does not conventionally adapt to the rubbery response on that range of temperatures. Most of the authors usually employ Dynamic Mechanical Analysis to characterize the storage modulus of the polymer, and afterwards obtain indirectly the equivalent Young's Modulus [97, 126], but that methodology does not provide information on the plastic response of the polymer, which is crucial in 4D printing applications undergoing large deformations. Conducting additional experiments and measurements to gather data points that accurately represent the material's behaviour under different temperature conditions is therefore necessary for the implementation of temperature-dependent constitutive models with improved predictive capabilities.
Another limitation to model 4D printing relies on the development of robust multiscale modelling approaches. The development of mesoscale models that discretise the individual filaments and their interaction with the adjacent ones is crucial to predict the internal stresses obtained during the 3D printing process which influences the behaviour of the sample during actuation [127, 128]. However, the computational cost of such models is currently prohibitive, thus homogenised macromechanical approaches are used instead, losing resolution of the physics occurring at microscale level [129, 130]. Furthermore, microscale models [131] that can account for the stochastic distribution of voids and defects in 3D printed samples are key to unravelling the role of each filament in the overall motion of the actuator, providing crucial information to optimise the internal architecture and design actuators with enhanced reliability.

## 5. Engineering applications and future trends

4D printed actuators offer several advantages over conventional hydraulic, pneumatic, electromechanical, or electro-magnetic actuators [3]. They are lightweight and compact, enabling the implementation of morphing technologies not compatible with complex mechanical systems. Compared to traditional actuators, 4D printed actuators are low-cost and require fewer parts, resulting in reduced material and energy costs. Their simplified design and construction also leads to shorter assembly times and lower maintenance requirements. Potential engineering applications may include the deployment of origami solar arrays [132, 133], soft robotics [134, 123], intravascular and trachea stents [135, 24, 136], reflector antennas [137, 138] or tissue engineering [139, 140].

Despite the previous advantages, it is important to note that the current capabilities of 4D printing do not match the performance level of conventional actuators [141]. While smart materials can adapt to the environment, their functionality is limited compared to the capabilities of mechanical actuators, which offer a reliable and adaptable motion for various applications. 4D printed actuators require careful design and customization for specific applications, and their versatility is limited compared to the wide range of applications that can be achieved by adjusting operational parameters of off-the-shelf mechanical actuators.
For example, mechanical actuators can vary rotational speed simply by adjusting intensity, and pneumatic actuators provide a holding force by adjusting the pressure. The following sections will outline the main challenges associated with implementing 4D printing in two key engineering applications: soft robotics and morphing structures, and how 4D printed composites can contribute to advance the field.

## 5.1. *Soft robotics*

The development of advanced autonomous soft robots requires the integration of a sensing control system that can dynamically adapt the response to different robotic operations such as gripping or human-machine interaction [142]. Conventional approaches in the literature incorporate electromechanical servos [143] or pneumatic actuators [144] into the 3D printed structure, however, the integration of these actuators results in complex components with relatively higher manufacturing, assembly, and maintenance costs than the counterpart emerging 4D printing technology.

The field of soft robotics can be potentially transformed by disruptive 4D printing technologies. Soft smart materials possess an adaptive shaping capacity, which is particularly advantageous for mimicking biological behaviours [145, 146]. To date, most of the concepts for soft robotics incorporate LCEs to create sensing robots that can shape-shift when subjected to external stimuli such as light [145] or heat [147, 61]. Despite the potential, LCEs on their own are not compatible with conventional control systems used in robotics. Considering the previous, composite LCEs with multifunctional characteristics can enable the implementation of closed-loop control technologies [66], without the need to rely on secondary heating circuits based on copper or silver to warm the structure and trigger the actuation [147, 61].

The development of alternative 4D printing technologies based on composite LCEs and SMPs integrated within an electrical circuit is crucial to exploit the future abilities of soft robots. 3D printing allows the implementation of electrical circuits [39, 148, 149, 150] and integrated sensors [151, 152, 153] that can be employed to control the actuators and provide autonomy to the devices through an integrated control system [134]. Current efforts to
<figure><img src="image_18.png" /><figcaption>Figure 18: Design of a 4D printed finger. (a) Example of phalanxes rotations. (b) 4D printed finger concept. (c) Numerical simulation of the mechanical response during a bending cycle, showing a good correlation with the physiological response.</figcaption></figure>

replace these conventional actuators are focused on the development of 4D printed joints using multifunctional SMPs [154]. Fig. 18 shows the conceptual design of a simplified 4D printed finger with two phalanxes composed of a rubbery matrix and SMP fibres. The virtual component is able to mimic the rotations of the phalanxes, showing a bending cycle during actuation. This technology can be potentially manufactured with multifunctional temperature-responsive composites to allow controlling the movement and rotations as a function of an applied voltage.

Despite the previous evidence of disruptiveness, further efforts are needed to ensure these technologies are (i) reliable when subjected to continuous cycles, (ii) fully reversible without the need for additional programming steps and (iii) responsive at given rates with improved actuation speed to adapt to dynamic robotic operations.

## 5.2. Morphing structures

Morphing structures refer to systems that can autonomously alter their shape, mimicking organisms in nature [155]. They can undergo controlled transformations, enabling them to adapt and optimise their performance to changing environmental conditions or user requirements. The change in shape can be triggered by external stimuli or internal control mechanisms that will induce either a tensile or flexural deformation of the material, or
<figure><img src="image_19.png" /><figcaption>Figure 19: Triangular deployable trusses. (a) Conceptual design. (b) Sketched of the hinge smart joints. (c) Load carrying capacity of the deployed structure. (d) Snap-shots of the deployment process [160]. Creative Commons Attribution-NonCommercial-ShareALike 4.0 International License.</figcaption></figure>

folding at creases [141].

The development of 4D printing plays a crucial role in providing the actuation capability necessary for hierarchical motion in a variety of applications such as variable aerodynamic drag [156, 157], variable stiffness [158] and deployable structures [159, 160], see Fig. 19. However, there are several challenges to address before 4D printing is implemented into those real-world applications. This section focuses on load-carrying capacity, recovery of folded creases and simultaneous actuation for large structures.

One of the main limitations of 4D printing is its overall poor load-carrying capacity. This limitation arises from the inherent trade-off between compliance and stiffness in traditional structural materials. Shape-changing objects require a certain level of compliance to undergo large rotations and deformations, which is often at odds with the requirements for structural integrity and load-bearing capabilities. The majority of morphing structures found in the literature employ either soft polymers [124], or Kirigami principles [161, 162]. Incorporating soft polymers or adding cuts and holes in stiff layers allow for increased compliance of
<figure><img src="image_20.png" /><figcaption>Figure 20: Example of mimetic morphing structures with energy absorption capacity based on dome-shaped creatures. (a) mushrooms and (b) sea urchins. (c) The recovery process of the deformed sample when the temperature is increased [165]. Reproduced with permission from Elsevier.</figcaption></figure>

hinges, enabling significant deformation and rotations in the morphing structure. However, these approaches may not meet the loading requirements of heavy-duty robots or structural applications.

To address this limitation, innovative approaches have been developed to combine the benefits of shape-changing materials with structural materials such as metals or composites. This includes the integration of hybrid materials, such as Shape Memory Alloys [163] or composites [164] that incorporate both compliant and stiff elements. Dezaki et al. [163] found a balance between load-carrying capacity and shape-changing capabilities to manufacture a gripper with improved load-carrying capacity able to hold a weight up to 300 gr. Biomimetic morphing structures have also been developed to improve the impact resistance [165]. Shape-morphing domes with variable mechanical properties have been produced and evaluated, finding a correlation between the number of circular units and the energy absorption capacity, see Fig. 20.

Aiming at the same objective but focusing on a different approach, certain authors are developing smart actuators that do not rely on the Shape Memory effect. Their rationale is based on the idea that by avoiding operational temperatures above the glass transition
<figure><img src="image_21.png" /><figcaption>Figure 21: Bending actuator presenting a bending cycle. Figures (a), (b) and (c) at temperatures below the $T_g$, exhibit a larger bending due to the differences in thermal expansion coefficients. Figures (d), (e), and (f) at temperatures above $T_g$, show the recovery of the original flat shape due to the shape memory effect.</figcaption></figure>

temperature ($T_g$), actuation can still be achieved while preserving the mechanical properties of the active polymer layer. Recent investigations focus on the difference in thermal expansion coefficient between adjacent materials that could produce a significant motion below the $T_g$. Compared with the Shape Memory Effect, this mechanism is reversible and quite repeatable, since it limits the degradation of the materials [37]. This principle has been employed by Zhang et al. [166] to create a self-folding lattice structure with variable global stiffness, and by Shao et al. [167] to manufacture grippers. The main disadvantage of this approach is the scalability of the bi-polymer layered structure since it usually relies on a passive rubbery layer that is difficult to produce. Fig. 21 shows a bilayer TPU/PLA bending actuator that presents both motion mechanisms. For temperatures below $T_g$, a larger bending is induced due to differences in the expansion coefficient. Above the $T_g$ the actuator recovers each original printed flat shape.

4D printing technologies also present limitations on folding and recovery capabilities, which is crucial for the manufacturing of autonomous self-deployable origami structures [168]. Origami allows folding truss or membrane structures in a relatively flat volume following a corrugation pattern, so they can be confined in small spaces. Some examples include car airbags [169], paper shopping bags or sophisticated solar panel arrays [170].
The term rigid origami applies to structures with non-negligible thickness and consists of rigid panels divided by creases that act as hinges [171]. The stress-state on a crease is not uniform and varies along its entire length. The magnitude of the strain gradient for a given material depends on the crease geometry in terms of angle and radius, which defines the sharpness of the fold. The main implications for 4D printing consist of the generation of large plastic irrecoverable deformations at the creases due to the large strain gradients when the structure is folded. This problem is worsened for thermo-responsive devices programmed at temperatures above the $T_g$ since the behaviour of the material is dominated by a viscoplastic response at that stage.

This plastic deformation avoids the full recovery of the original 4D printed shape. The recovery ratio is a trade-off between the compliance of the hinge and the curvature of the fold. For rigid SMP layers, full recovery for 4D printed layers is reported in the literature for very large curvature radii [172], and a progressive loss in the recovery function is reported as the curvature decreases [51]. This issue can be alleviated by lowering the programming temperatures, ensuring the plastic deformation imposed in the fold is minimised, but still allowing the development of the Shape Memory effect [37]. The optimisation of the programming temperatures to increase the sharpness of the fold and implement origami solutions with rigid 4D printed hinges is a current open research question.

Finally, the simultaneous triggering of multiple actuators in large self-deployed structures is also a field that requires further research efforts. Most of the proof-of-concepts in the literature focus on small devices that cannot be scaled to large structures such as solar origami arrays. For instance, it might be difficult to operate simultaneously thermal-responsive actuators distributed along large deployed structures due to the presence of substantial temperature gradients [8]. To conceive feasible 4D printing technologies for large morphing structures it is needed to further develop control systems that integrate actuators, sensors and control algorithms to actively drive and regulate the morphing process, ensuring autonomous, precise and reliable transformations.
## 6. Conclusions

The field of actuator technologies has seen significant advancements with the emergence of 4D printing and the utilisation of smart materials. 4D printed actuators offer lightweight, compact, and cost-effective solutions compared to conventional hydraulic, pneumatic, electro-mechanical, or electro-magnetic actuators. They provide the ability to achieve complex motions, shape changes, and adaptability in response to stimuli. However, the current capabilities of 4D printing and smart materials still fall short of the performance level offered by conventional actuators. In particular, they lack the versatility, accuracy, reliability, functionality and load-carrying capacity of conventional actuators.

Composite 4D printing has the capacity to alleviate some of the previous issues. Composite 4D printing refers to actuators reinforced with particles or fibres, or hybrid structures composed of more than one material. The incorporation of conductive and magnetic fillers into conventional soft materials, such as shape memory polymers, liquid crystal elastomers, or hydrogels, offers the potential to acquire multifunctional properties. These fillers enable self-triggering capabilities, eliminating the need for external stimuli or secondary triggering systems to activate the shape memory effect. In the case of thermal-responsive devices, this enables self-induced shape changes due to the photothermal, induction, or Joule effect. Electro-thermo-mechanical devices are of particular interest due to their possibility to be controlled by an electrical circuit compatible with the close-loop technologies developed for autonomous robots.

Hybridisation with a second constituent (responsive or not) with different physical properties has been employed to incorporate additional motion mechanisms. This includes differences in thermal or swelling expansion coefficients. Those motion mechanisms can be combined with the shape memory effect to create multi-shape actuators with advanced functionalities, such as programmable free, multi-shape and reversible actuators. Furthermore, soft materials have been hybridised with stiffer constituents such as carbon fibres to increase the load-carrying capacity, at the expense of limiting the motion of the devices.

Despite the previous significant developments, the current capabilities of 4D printing
still do not match the performance of conventional actuators, and further research efforts
are necessary to mature 4D printing technologies and implement them in fields such as soft
robotics and morphing structures. Several open research questions still exist in the field,
primarily focused on three key areas: the development of new materials, virtual optimisation
and design exploration and integration with sensing and control systems. Additional ther-
moplastic polymers, including unidirectional reinforced composites, should be considered in
future stages. Reinforcing with continuous fibres would increase the loading capacity, how-
ever, the continuous fibres will need to be strategically placed to maximise the motion of the
actuator. The reinforcement would also influence the electrical conductivity of the printed
actuator, therefore it needs to be carefully selected to ensure a low electrical resistance while
being compatible with the glass transition temperature of the polymer matrix. Numerical
approaches should focus on the development of high-fidelity element models. Multiscale sim-
ulation frameworks with temperature-dependent constitutive models need to be developed
to enhance the accuracy and predictive capabilities of actuator designs, and provide valu-
able insights into the behaviour and performance of these systems. Finally, multifunctional
4D printed composites could integrate 3D printed sensors to monitor the temperatures and
deformations during the programming and triggering stages. Future investigations can focus
on the development of a closed-loop control and autonomous operation, able to self-regulate
the applied voltages to achieve a desired thermo-response and motion. This technology
should enhance the actuator's responsiveness and precision.

7. Acknowledgements

The authors would like to acknowledge the economic support of the Defence Science
and Technology Laboratory (Dstl), UK (grant number DSTLX1000144021R), the Defence
Innovation Agency (AID) of the French Ministry of the Armed Forces (grant number AID-
2021 65 0045) and the UK Space Agency (grant number UKSAG23-0001M). The scientific
contributions of Marcelo Dias and Niamh O'Neill-Berest are kindly acknowledged.
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