Review
4D Printing of Multicomponent Shape-Memory Polymer
Formulations
Muhammad Yasar Razzaq *, Joamin Gonzalez-Gutierrez *, Gregory Mertz, David Ruch, Daniel F. Schmidt
and Stephan Westermann
Department Materials Research and Technology (MRT), Luxembourg Institute of Science and Technology
(LIST), ZAE Robert Steichen, 5 Rue Bommel, L-4940 Hautcharage, Luxembourg
* Correspondence: yasar.razzaq@list.lu (M.Y.R.); joamin.gonzalez-gutierrez@list.lu (J.G.-G.)
Citation: Razzaq, M.Y.; GonzalezGutierrez, J.; Mertz, G.; Ruch, D.;
Schmidt , D.F.; Westermann, S. 4D
Printing of Multicomponent
Shape-Memory Polymer
Abstract: Four-dimensional printing technology, as a next-generation additive manufacturing
method, enables printed objects to further change their shapes, functionalities, or properties upon
exposure to external stimuli. The 4D printing of programable and deformable materials such as
thermo-responsive shape-memory polymers (trSMPs), which possess the ability to change shape by
exposure to heat, has attracted particular interest in recent years. Three-dimensional objects based
on SMPs have been proposed for various potential applications in different fields, including soft
robotics, smart actuators, biomedical and electronics. To enable the manufacturing of complex multifunctional 3D objects, SMPs are often coupled with other functional polymers or fillers during or
before the 3D printing process. This review highlights the 4D printing of state-of-the-art multi-component SMP formulations. Commonly used 4D printing technologies such as material extrusion
techniques including fused filament fabrication (FFF) and direct ink writing (DIW), as well as vat
photopolymerization techniques such as stereolithography (SLA), digital light processing (DLP),
and multi-photon polymerization (MPP), are discussed. Different multicomponent SMP systems,
their actuation methods, and potential applications of the 3D printed objects are reviewed. Finally,
current challenges and prospects for 4D printing technology are summarized.
Formulations. Appl. Sci. 2022, 12,
7880.
https://doi.org/10.3390/app12157880
Keywords: 4D printing; additive manufacturing; layered structures; multi-material; polymer-based
composites; polymer blends; interpenetrated networks; shape memory polymers
Academic Editor: Domenico
Lombardo
Received: 6 July 2022
Accepted: 1 August 2022
Published: 5 August 2022
Publisher’s Note: MDPI stays neutral with regard to jurisdictional
claims in published maps and institutional affiliations.
Copyright: © 2022 by the author. Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
1. Introduction
Shape-memory polymers (SMPs), whose ability to recover from a temporary programmed shape to a permanent shape in response to an external stimulus, have drawn
increasing attention because of their scientific and technological significance [1-4]. Typical
stimuli to trigger the shape transition in SMPs include heat (i.e., thermo-responsive), light
(i.e., photo-responsive), chemicals (i.e., chemo-responsive, e.g., water/moisture, pH
change), or mechanical loading (i.e., mechano-responsive). Thermo-responsive SMPs
(trSMPs) have been most extensively studied because of their tailorable transition temperature (Tr) and the convenience with which their shape change may be triggered [5,6].
The activation of trSMPs can be carried out by either directly, by increasing the environmental temperature (Te), or indirectly, by Joule heating (i.e., passing electric current), inductive heating (i.e., exposure to an alternating magnetic field: AMF), or photo-thermal
effects (i.e., exposure to light) [6-8]. At the molecular level, trSMPs consists of net points
and a switching domain. The net points can either be covalent crosslinks (SMP networks)
or physical intermolecular interactions such as hydrogen bonds or crystallites (thermoplastic SMPs) and act as memory components of the network. On the other hand, the
switching domains are flexible polymer chains associated with a thermal transition (glass
transition temperature, Tg, or melting transition, Tm) and are responsible for the shape
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change process. To observe the shape-memory effect (SME), a thermomechanical programming procedure consisting of deformation to a temporary shape at elevated temperature (T > Tr) and subsequent cooling under constraint to a low temperature (T < Tr), is
required. The recovery to the original shape takes place by reheating the polymer to an
elevated temperature (T > Tr). The quantification of the SME is carried out by determining
the shape-recovery (Rr) and shape-fixity ratios (Rf), as commonly described in the literature [9-11].
A combination of SMPs and other functional materials is helpful to improve properties and increase relevance in technological applications. Such multicomponent SMPs provide high deformation capability, improved recovery force, faster shape recovery, and
better shape fixity/recovery ratios [1,12]. The excellent performance of multicomponent
SMPs has found potential utility in the aerospace, biomedical, electronic, and textile industries [13,14]. However, conventional processing techniques are limited in their ability
to produce complex structures with precise dimensions useful for the aforementioned
fields [15,16]. For this reason, additive manufacturing is being investigated.
Additive manufacturing, colloquially known as 3D printing (3DP), has attracted increasing interest of academic and industrial researchers and engineers due to its appealing
features, such as prototyping freedom and the ability to realize complex structures at high
resolution [17-19]. The 3DP of smart, deformable materials has resulted in the creation of
a new term, “4D printing (4DP),” which refers to the ability of materials to alter their form
after they are printed, with time as the fourth dimension that is controlled through the
design of dynamic structures [16,20]. Four-dimensional printing provides additional functional capabilities and performance-driven utility in a broad range of domains, such as
aerospace, biomimetic, biomedical, electronic, acoustic, textile, fashion, mechanical, defense, retail, and food industry applications [21]. The term “4D” in combination with
“printing” emerged for the first time in 2012 in a conference proceeding, where it referred
to a 4D barcode (color + 2D + time) [22]. However, the first journal publication related to
the 3DP of photo-polymeric inks with shape change capability appeared in 2013 [23].
Here, the initial configuration was created by 3DP, while the programmed action of the
polymer created a time-dependent configuration—the 4D aspect. Since then, hundreds of
publications related to 3DP with smart, programmable, and stimuli-responsive materials,
resulting in such 4D effects, have appeared in the literature, as shown in Figure 1. The
number of publications and patents based on “Web of Science”, “scopus.com”, and
“SciFinder” searches using the term “4D printing” and “4D printing of SMPs” have shown
an exponential increase since 2012.
SMPs are the most investigated 4D printed materials, and this can be attributed to
their tailorable elastic properties and immense potential for technological applications [2427]. The increasing number of publications related to the term “4D printing of SMPs” is
shown in Figure 1b. Various functions such as sensing, degradability, and self-healing
have been combined with shape-memory capabilities in 4D-printed objects using different
3DP techniques [24,28-31]. Furthermore, as SMPs can respond to various stimuli (such as
temperature, humidity, electric current, magnetic fields, and changes in pH), this opens
unprecedented opportunities for 4DP [27,32-34]. However, most current publications deal
with the 4D printing of trSMPs, with their shape-memory response activated by direct or
indirect heating methods [32,35]. With the growth in 3DP technology, it is expected that
SMP-based 4D objects responsive to other stimuli will also be developed, which will create
a myriad of scientific and technological possibilities [36].
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Figure 1. Publication statistics on the topic of (a) 4D printing and (b) 4D printing of SMPs (c). Data
were collected from Scopus (Elsevier, Amsterdam, The Netherlands), Web of Science (Clarivate Analytics, London, UK), and SciFinder (Chemical abstracts service CAS, Columbus, OH, USA). (c) Patent statistics on the topic of 4D printing and 4D printing of SMPs. Data were collected from
SciFinder. All searches were performed in June 2022.
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Recent advances in 3DP have focused on printing of multicomponent SMPs to produce complex 4D objects with enhanced properties along with the shape-memory effect
(SME) [16,28,37,38]. Generally, in a multicomponent system, polymers are combined with
metals or other inorganic materials to realize new functions or improve thermal or mechanical properties [1,34,39,40]. The fabrication of such heterogeneous systems presents
an exciting challenge for fundamental and applied research involving knowledge of physics, chemistry, mathematics, materials science, and process engineering [38,41-43]. Based
on growing interest in this area, we aim to review the recent advances in 4D printing of
multicomponent SMP formulations with more than one discernible phase (e.g., SMPbased composites and blends), as each of the phases brings new functionality to the
printed object.
The existing surveys and reviews covering the subject of 3D and 4DP of SMPs partially address the multicomponent aspect of SMPs [16,38,41,44,45]. Multicomponent SMPs
have unique functionalities but complex molecular architecture, including phase-segregated domains associated with different thermal transitions, as well as functional fillers
and/or additives [1,44]. Three-dimensionally printing these materials is challenging, and
it is crucial to show the limitations and possible improvements in the context of different
3DP techniques [46,47]. Therefore, the present review article highlights current literature
concerning the 4DP of multicomponent SMP formulations using different AM techniques.
The review starts with a general overview of multicomponent SMPs, fabrication strategies, limitations, and potential applications. The subsequent sections describe the different
3DP technologies that are particularly applicable for the printing of multicomponent SMP
formulations. We start with material extrusion (MEX) printing techniques such as fused
filament fabrication (FFF) (Figure 2a) and direct ink writing (DIW) (Figure 2b). Next, we
summarize the printing of multicomponent SMPs by different vat-photopolymerization
(VPP) techniques, including digital-light processing (DLP) (Figure 2c), stereolithography
(SLA), and multi-photon polymerization (MPP). Subsequently, a description of multi-material printing of SMPs by material jetting (MJT) (Figure 2d) is described. Finally, a conclusion with potential applications, limitations, and future direction in developing 3D/4D
printing of SMPs is provided. Printing methods such as powder bed fusion (PBF), binder
jetting (BJT), sheet lamination (SHL), and direct energy deposition (DED) are not discussed here as they are generally used with metallic shape-memory alloys. Furthermore,
to keep the focus on multicomponent SMP formulation, other aspects of 4DP such as one
component SMPs, self-folding/self-expanding systems, self-healing, and shape-changing
materials based on liquid crystalline domains have been excluded from this review.
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Figure 2. Schematic of commonly used 3DP techniques including their capacity to print different
multicomponent SMP architectures: (a) fused filament fabrication (FFF); (b) direct ink writing
(DIW); (c) digital light processing (DLP); (d) material jetting (MJT).
2. Multicomponent SMPs
SMPs are sometimes used in isolation as single materials; more often, they are combined with other functional materials. For example, polymers, metals, or other inorganic
materials, in the shape of particles, fibers, and laminates, are combined to create multicomponent systems. Since these multicomponent systems are built up from discrete
phases consisting of SMPs mixed with other polymers at the macroscopic or molecular
level or reinforced with other domains (e.g., inorganic particle), they are particularly
suited for the realization of multifunctionality [1,34,48-50]. Composites that consist of a
polymer matrix with solid filler or reinforcement are common multicomponent systems
to enable multiple functions in a single material [4,51]. Active fillers such as iron oxide
nanoparticles (IONP), carbon black (CB) and carbon nanotubes (CNT) are incorporated
into SMPs using conventional processing techniques [36,52,53]. These fillers introduce
new functions such as magneto-sensitivity, electric conductivity, or photosensitivity into
SMPs along with thermal and structural functionalities [50,51,54].
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Furthermore, when well-dispersed and compatible with the matrix, nanofillers provide much higher interfacial area per unit mass and much lower interparticle distances
vs. conventional fillers, resulting in fundamental changes in polymer microstructure and
behavior in the resultant nanocomposites. Additionally, high-aspect-ratio nanofillers are
less readily damaged than their larger equivalents, providing the potential for much lower
percolation thresholds in practice. However, being distinct phases, each constituent in
such a multicomponent system remains recognizable and retains its characteristics, and
well-defined adhesion levels between these phases are required for the successful realization of new functionalities [36,55]. Other possible arrangements to enable SMP-based multicomponent systems are blends, interpenetrating polymer networks (IPNs), and macroscopic layered architectures [56,57].
In the blending approach, instead of non-polymeric domains, SMPs are physically
mixed with other functional polymers with or without chemical bonding. The polymer
blend not only alters the thermal and mechanical properties of the base SMP but can also
offer new functionalities not found in the original polymers. Here, the compatibility of the
blend components is an important criterion in the development of high-performance multifunctional materials. However, most polymer blends are heterogenous systems, as
sometimes, the structural differences of the individual polymers are more important than
their composition. For instance, polyethylene and polypropylene technically have the
same composition, since they are both saturated hydrocarbons with the same C:H ratios,
but they do not mix [56,57]. Another approach with similar spatial organization is an IPN,
a thermodynamically stable arrangement of multicomponent crosslinked polymeric materials. An IPN is a combination of two or more polymer networks which are not covalently linked to each other and are obtained by two independent crosslinking reactions
occurring sequentially or simultaneously to form two different networks. Numerous IPNbased SMPs have been investigated in recent years, and some have exhibited improved
properties and new functionalities as well [57].
Among these different techniques, multilayer technology is considered one of the
most efficient and practical owing to its simplicity and versatility [58]. SMPs and other
functional polymers with desirable functionalities and architectures can be stacked together in a bi-/multilayer polymer system with a simple interface [59]. For example, combinations of SMPs with soft and rigid layers or layers with different coefficients of thermal
expansion have been produced to enable novel shape change behavior [60,61]. However,
successful integration of different functions depends on the individual layers, layer geometry and the character of the interface between layers [58].
Modern 3DP techniques provide a growing range of possibilities to print multicomponent systems (i.e., composites, blends, IPNs or layered structures) at different dimensional scales[21,30,46]. For example, SMP-based multicomponent systems have been used
for the 3DP of responsive mechanical devices, where part geometry has been scaled down
to micro- and nanometer levels to form metamaterials [62,63]. However, among the list of
widely adopted 3DP techniques, few have been used or customized to be suitable for the
3DP of multicomponent SMPs formulations. These mainly include MEX-based 3DP (e.g.,
FFF and DIW), VPP (e.g., DLP, SLA, and MPP) and MJT [38]. Here, the selection of a 3D
printer is related to its suitability to print either an ink, liquid resin, or filament. Furthermore, the printer should allow for the adjustment of key parameters affecting material
deposition and solidification, such as printing speeds, delivery volumes, material, and
chamber temperatures [37,41,42]. A schematic demonstration of some of most used 3DP
techniques along with their capacity to print different multicomponent SMP formulations
is shown in Figure 2.
MEX printers are commonly used to print SMP particulate composites or blends. Additionally, by using print heads with multiple nozzles, MEX systems can simultaneously
print several materials (i.e., layered structures), for example, multi-color objects or complex objects with dissolvable support structures [64,65]. Nevertheless, there are some lim-
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itations in MEX printing, as the printed parts have mechanical anisotropy in the z-direction (vertical direction) and produce parts with stepped structures at their surfaces [37,66].
Furthermore, a lack of strong inter-layer bonding can lead to mechanical failure of the
printed object, particularly after repeated shape changing cycles [16,67]. Nevertheless, the
printing parameters and the material characteristics have a significant effect on the fatigue
behavior of the printed objects [68].
VPP, on the other hand, is limited to the printing of low-viscosity (<10 Pa s) photopolymers that tend to have poor aging characteristics and limited thermal stability but can
provide objects with good resolution and surface finish [69]. The VPP of high-viscosity
resins is challenging, as the vertical detachment of the crosslinked layer from vat film can
occur. Nevertheless, there are some reports about the heat-assisted VPP of highly viscous
photopolymers, as heating can reduce the viscosity [70]. VPP techniques have also been
used to print multicomponent SMP resins with electric or magnetic functionalities [71,72].
Here, careful control over the ratio of the reinforcement and matrix is required as high
amount of reinforcement can absorb and scatter the light, resulting in partially crosslinked
systems with poor mechanical properties [73]. Nonetheless, by controlling the exposure
time or by using multiple vat systems with different photopolymers, multi-material printing can also be performed. However, changing materials during printing significantly
slows the printing process since a wiping/cleaning step is needed to avoid cross-contamination [74].
Compared to VPP, MJT printing has an intrinsic capability to print multiple materials
simultaneously, given the ease with which multiple ink reservoirs and multiple nozzles
may be implemented as in a conventional color ink jet printer [75,76]. The multi-material
printing of a variety of SMP resins via the MJT method is reported [31,77]. Such multimaterial systems provide novel shape memory functionalities along with tailorable thermal and mechanical properties of the printed objects.
Anisotropic polymer chain orientation, complex geometries that resemble natural
structures, and combinations of different materials in different layouts bring more flexibility to accomplish complex actuation not possible with other shaping methods such as
injection molding, extrusion, and thermoforming. One possible disadvantage of AM with
respect to other shaping technologies is that AM is prone to small defects that can change
the mechanical, optical, thermal, and electrical properties of the produced objects. However, since the scope of this review is on AM, other manufacturing techniques for SMPs
are not described. Therefore, the following sections described the different AM techniques
that can be used to shape SMPs with a tailored functionality due to the structures resulting
from 3D printing.
In sum, there have been significant efforts in the scientific community to fabricate
SMP based multicomponent systems with novel functions via a variety of approaches. In
the following sections, further details are given concerning the results obtained in each of
the 3DP categories selected for this review.
3. Extrusion Based 4D Printing
ISO/ASTM 52,900 defines MEX as an “additive manufacturing process in which material is selectively dispensed through a nozzle or orifice”. Based on this definition, fused
deposition modeling (FDM)/fused filament fabrication (FFF) and direct ink writing (DIW)
are MEX technologies. The following subsections of the review discuss these two extrusion technologies and their application to the 4D printing of multicomponent trSMP materials and macrostructures.
3.1. Fused Filament Fabrication (FFF)
MEX using filaments as feedstock materials was first patented in 1989 by Scott
Crump [78] and later commercialized as fused deposition modeling (FDM) by the company Stratasys, Inc. (Rehovot, Israel) As FDM is a registered trademark of Stratasys [79];
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therefore, the alternative name fused filament fabrication (FFF) was introduced by members of the RepRap project [80]. Some alternative names such as fused layer modeling
(FLM) [81] and layer plastic deposition (LPD) have also been proposed [82], but they are
not popular. In this review, FFF will be utilized to refer to the most widely used MEX
technique for processing thermoplastic-based materials. The versatility and low cost of
the equipment and materials, the range of available materials, and the simplicity of operation and troubleshooting are all factors that contribute to FFF’s popularity as a workshop
tool in industrial and hobby settings alike [83]. Therefore, it comes as no surprise that
there are numerous examples in the literature where FFF has been used for the 4D printing
of multi-material trSMP systems.
During the FFF process, the feedstock material is forced through a nozzle at temperatures at which it can flow. The generation of flow fields during both extrusion from the
nozzle and deposition from a moving head onto the build platform or on top of the previously deposited layers can cause orientation of any anisotropic species present, from
fillers down to individual macromolecules. When the material is rapidly (often directionally) cooled during the deposition process, residual texture and thermal stresses arising
from both the flow and the cooling processes can result [84]. Shape memory behavior has
been observed in thermoplastics available as filaments for FFF such as polylactic acid
(PLA) [25,83-85], polyurethanes (TPU) [26], acrylonitrile butadiene styrene (ABS) [30], and
poly(vinyl alcohol) (PVA) [86]. Heating these thermoplastics beyond their Tg accelerates
the stress relaxation process leading to self-deformation, which can be used to prepare
self-folding origami by programming the deformation direction based on the deposition
pattern of the extruded roads or strands. For example, some layers are printed axially,
others transversally and yet others in an isotropic manner to realize twists and bends in
selected sections of a printed part [30,83]. In this review, the emphasis is placed on the 4D
printing of trSMPs, which require thermo-mechanical programming to enable a shapememory effect (SME). Therefore, it is better to deposit layers in a way that leads to isotropic properties in the deposition plane (e.g., alternating layers with +45° and −45° raster
angles) at as high a density as possible to prevent delamination of the layers during the
forced deformation [87]. The shape memory response of thermoplastics processed by FFF
can also be changed by using different processing parameters, such as extrusion temperature and speed and layer height. It was observed that PLA processed at 210 °C and 40
mm/s and with a 0.3 mm layer height had a larger recovery ratio compared to when it was
processed at 180 °C and 80 mm/s with a 0.15 mm layer height [88]. Additionally, greater
shape recovery could be achieved when programming was performed at 85 °C compared
to 65 °C [15].
On certain occasions, neat thermoplastics need to be modified to increase their printability, in particular to prevent geometrical distortions during FFF [88], and provide additional functionalities, such as enhanced thermal [89] or electrical conductivity [90], magnetic properties [91], mechanical reinforcement [92], or drug release capabilities [93].
Therefore, thermoplastic blends and composites have been prepared to further enhance
the thermos-responsive shape memory capabilities of structures printed via FFF. On other
occasions, the deposition of two or more distinct materials in different sections of a 3D
object can help to realize the desired shape memory behavior. This is generally achieved
using FFF devices with multiple nozzles. Different multi-material SMPs and 3D objects
consisting of discernible phases, their properties and applications are described in the following sub-sections.
3.1.1. FFF of Polymer–Polymer Blends
As previously mentioned, PLA is a common thermoplastic material used in FFF that
shows a thermo-responsive SME. To modify the shape-memory properties, blending of
PLA with other thermoplastics is carried out. Jing et al. [94] prepared mixtures of PLA
and TPU at weight ratios of 90/10, 80/20, 70/30, and 60/40 using a twin-screw extruder.
The PLA/TPU mixtures were immiscible as observed by scanning electron microscopy
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images (SEM), with TPU droplets changing to fibrils as the content of TPU increased from
10 to 40 wt.% in the printed specimens thanks to shear elongation. All the samples were
able to be fixed into temporary shapes at room temperature without additional heating or
cooling cycles (Figure 3a). It was observed that the blend containing 80 wt.% PLA had the
highest 𝑅𝑅𝑓𝑓 (𝑅𝑅𝑓𝑓 = 82% in tension, 𝑅𝑅𝑓𝑓 = 91% in compression, and 𝑅𝑅𝑓𝑓 = 84% in bending)
at room temperature, but it also displayed brittle behavior and became more brittle as the
number of loading cycles reached three. Here, PLA crystalline domains acted as crosslink
points, retaining the original shape, while the TPU phase acted to accommodate strain,
preventing the PLA from breaking at high deformations. Once heated beyond their Tg (60
°C), the PLA molecules displayed increased mobility and released the stored stresses, enabling the specimens to return to their original shapes. In another study by Leng et al. [95],
80/20 TPU/PLA mixtures were compounded via twin-screw extrusion, converted to filaments, and processed via FFF. However, the high content of TPU made the obtained filaments very flexible and difficult to print, necessitating the use of a screw-based MEX system. Leng et al. also observed that PLA tended to fibrillate under certain extrusion conditions when the PLA/TPU viscosity ratio (𝜂𝜂𝑃𝑃𝑃𝑃𝑃𝑃 ⁄𝜂𝜂 𝑇𝑇𝑇𝑇𝑇𝑇 ) was close to 1, as observed at extrusion temperatures of lower than 190 °C in the 3D printer. The PLA fibrils were visualized
by dissolving the TPU in dimethylformamide (DMF) and taking scanning electron microscopy (SEM) images of the remaining material. It was observed that PLA fibrils with
large aspect ratios were formed in the nozzle of the printer and were oriented in the direction of deposition in the 3D printed specimens. The degree of crystallinity of the
printed specimens was much higher than for samples produced by compression molding
due to the different heating/cooling rates, and levels of polymer chain orientation. The
higher crystallinity resulted in a higher tensile strength and Young’s modulus for the
printed specimens. The shape memory capabilities of these TPU/PLA polymer–polymer
composites were later evaluated by Jiang et al. [96]. In this study, the content of PLA was
varied from 0 to 30 wt.%. As the content of PLA increased, 𝑅𝑅𝑓𝑓 improved, but the recovery
ratio (𝑅𝑅𝑟𝑟 ) decreased. The optimum overall shape memory behavior was achieved by adding 10 wt.% of PLA to TPU (𝑅𝑅𝑓𝑓 = 91%, 𝑅𝑅𝑟𝑟 = 71%). The orientation of the infill also played
a role in the shape memory ratios obtained. When the infill was parallel to the direction
of the applied force (i.e., 0°), the fixity ratio (𝑅𝑅𝑓𝑓 = 90%) was the highest, followed by when
the orientation was 45° from the loading direction (𝑅𝑅𝑓𝑓 = 72%). Using an infill pattern perpendicular to the applied force (i.e., 90°) yield the lowest 𝑅𝑅𝑓𝑓 (𝑅𝑅𝑓𝑓 = 65%). In contrast, 𝑅𝑅𝑟𝑟
was not significantly affected by the infill orientation, with all specimens having 𝑅𝑅𝑟𝑟 values between 71 to 75% [96] (Figure 3d).
Another blend of thermoplastic polymers that has been recently reported to display
shape memory capabilities is that of polybutylene succinate (PBS), with PLA at a mass
ratio of 9:1. This blend was prepared by melt compounding in a twin-screw extruder, with
the resultant filaments used directly for FFF. It was observed that the PBS/PLA mixture
had a recovery ratio of 92%, significantly lower than that of pure PLA (99%). It was hypothesized that PBS hindered the formation of PLA chain entanglements, thus diminishing the recovery ratio. A starfish object, an endoluminal stent, an embolization coil and a
porous scaffold were printed to demonstrate the actuation capability of the material and
its applicability in possible medical applications. The FFF printed objects were programmed in hot water at ~85 °C, after which their shape was fixed at room temperature.
As expected, the recovery time of the structures after heating was dependent on the shape
and volume of the objects (Figure 3e) [97].
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Figure 3. (a) Strips of PLA/TPU polymer–polymer composites, (b) room temperature shape programming of PLA/TPU composites, (c) shape recovery of PLA/TPU composites after heating to 70
°C. Reproduced with permission from reference [94] copyright 2014, John Wiley & Sons; (d) fixity
ratio (Rf) and recovery ratio (Rr) for TPU/PLA composites printed with different infill orientations
(0°, 90°, and 45°) Reproduced with permission from reference [96] copyright 2021, Elsevier; (e) FFF
produced PBS/PLA composite specimens after printing, programming in water at 85 °C, and shape
recovery in water at 85 °C (scale bar 10 mm). Reproduced with permission from reference [97] copyright 2021, Elsevier.
3.1.2. FFF of Polymer–Particulate Composites
Particulates can bring additional functionalities and can enhance the printability and
performance of SMPs; for example, they can be used to make electroactive [64,98-100],
photo-responsive [32,85], and inductive [101,102] materials for indirect heating of the
trSMP matrix. They can also be used to improve fixity and recovery ratios by creating
different morphologies in SMPs [103]. In this subsection, research work with particulate
filled thermoplastic trSMPs is summarized.
Using paper as a substrate, Wang et al. [98] used a PLA/graphene based filament to
print single layer structures that could be heated by passing an electric current through
them. The shapes were programmed at 70 °C, and as the electric current passed through
the composite and heated the structure, it recovered its original shape. Copper tape was
placed at the extremities of the printed structure to create electrodes, and wires were soldered to the copper tape to pass the electric current through the printed structure. It was
observed that the temperature of the composite and the angle of deformation of the
printed structure were proportional to the voltage applied. A voltage of 50 V gave rise to
a temperature of approximately 50 °C, and a voltage of 100 V increased the temperature
to around 110 °C. As the number of heating cycles increased, the angle of recovery decreased [98]. Another example was the use of PLA filaments filled with 53 wt.% carbon
black (CB), which were used to print structures with different infill orientations (0°, ±45°
and 90°); the effect of Joule heating was investigated by applying 30 V (DC) for 20 min. It
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was observed that specimens with infill orientations of 0° and ±45° could reach higher
temperatures of around 46 °C compared to specimens with an infill orientation of 90°,
which could only reach 40 °C. However, the temperature distribution along the specimen
was more homogeneous for the specimens with a 0° infill orientation [100] (Figure 4a).
Therefore, the overall part design and infill orientation must be considered when generating structures for 4D printing with electroactive materials, since these two parameters
can locally alter the response of such parts.
Figure 4. (a) Temperature distribution of FFF-produced electro-responsive specimens with different
infill orientations [100] (CC BY license); (b) temperature and shape of photo-responsive FFF-produced sunflower at different times after being exposed to 87 mW/cm2 of light. Reproduced with
permission from reference [32] copyright 2019, John Wiley & Sons; (c) shape recovery of a magnetoresponsive ASD occluder. Reproduced with permission from reference [101] copyright 2019, John
Wiley & Sons; (d) recovery of tracheal scaffold produced from a magneto-responsive PLA-Fe3O4
composite. Reproduced with permission from reference [102] copyright 2019, Elsevier.
Yang et al. [32] prepared TPU-CB composites that could be processed by a commercial FFF machine. CB has a 92% photothermal conversion efficiency; therefore, addition of
CB to a shape memory TPU can be used to fabricate photo-responsive shape memory materials. Exposure to sunlight with an intensity of 76 mW/cm2 for 160 s was enough to recover the shape of a hollow cube frame with a side length of 30 mm. CB content was varied
from 0.1 to 15 wt.%, and it was observed that the higher the content, the higher the temperature that was achieved. For example, when heating with a xenon light source with an
intensity of 60 mW/cm2 for 900 s, the unfilled TPU reached a maximum temperature of
29.3 °C, while TPU filled with 15 wt.% carbon black reached 40.5 °C. Thus, variations in
filler content can be used to tailor the response of FFF produced objects. A content of 10
wt.% was selected for 3DP because with this CB content, the filaments with desirable diameter could be extruded and was reliably printable by FFF. Using this material, a sunflower-like object was printed that opened when exposed to light (Figure 4b). Using solution casting, Hua et al. [85] prepared mixtures of PLA and multiwalled carbon nanotubes
(MWCNT) at concentrations from 0.1 to 0.6 wt.%; the material was then extruded into
filaments and used to 3D print objects by FFF. The light source used in this investigation
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was a 275 W near infrared lamp. A systematic investigation showed that exposing films
of the neat PLA to the infrared light for 15 s raised their temperature to approximately 90
°C, while the composites containing 0.6 wt.% MWCNT reached a temperature of approximately 170 °C; therefore, the photo-responsive nature of the composite was demonstrated. Rectangular actuators (60 mm × 6 mm) were fabricated by printing two layers of
the photo-responsive composite containing 0.5 wt.% MWCNT and a thickness of 200 µm
on top of a piece of paper with a thickness of 300 µm. The actuators were fabricated with
infill densities of 100, 66, 60, or 57%. It was observed that an infill density of 100% gave
the longest response and recovery times. Infill densities between 57 and 66% gave similar
response times, but an infill density of 66% gave the fastest recovery time. This highlighted
the effect of infill density on the performance of photo-responsive SMPs. A flower-like
structure was printed that opened and closed as the infrared light was turned on and off.
Lin et al. [101] added Fe3O4 particles to PLA by solution casting and prepared filaments that could be successfully processed using conventional FFF equipment. The addition of Fe3O4 particles made the resulting composite magneto-responsive, allowing for inductive heating by exposure to an alternating magnetic field (AMF). An AMF with a frequency of 27.5 kHz and field strength of 4 kA/m was used to investigate the magnetically
induced shape memory behavior of PLA composites with different iron oxide concentrations. The PLA composite with 10 wt.% of Fe3O4 experienced sufficient inductive heating
to initiate shape recovery. All composites had Rr values above 94% despite adding up to
20 wt.% of magnetic particles. Using a composite containing 10 wt.% iron oxide, it was
possible to print occluders to treat a congenital heart disease known as atrial septal defect
(ASD). It was observed that the printed occluders could recover their shape within 16 s in
the AMF described above (Figure 4c). In a similar study, Zhao et al. [32] prepared a PLA15 wt.% Fe3O4 composite filament via solution casting followed by twin-screw extrusion.
Using this composite filament, tracheal scaffolds were produced by FFF. These scaffolds
showed a recovery time of 35 s when exposed to a magnetic field strength of 4 kA/m and
frequency of 30 kHz (Figure 4d). The printed scaffolds were tested in vitro, and the results
showed these devices to be promising for artificial tracheal scaffold repair.
Hosseinnezhad et al. [103] added cellulose nanofibers (CNF) to polymer–polymer
composites of PLA and polybutylene adipate terephthalate (PBAT) to be used in FFF. The
introduction of CNF enhanced the conversion of PBAT droplets to PBAT fibrils during
the extrusion process; the thinner PBAT fibrils interacted more strongly with the different
phases present (i.e., PLA, CNF). The improved interfacial compatibility between the different components enhanced the strain recovery, strain fixity, and recovery rates. Furthermore, the addition of CNF increased the crystallization temperature of PBAT by 30 K,
which further separated the PBAT transition temperature from the Tg of PLA and enabled
a triple-shape effect that was otherwise impossible without the CNF.
3.1.3. FFF of Continuous Fiber-Reinforced Composites
Continuous fiber-reinforced composites can be used to provide lighter, stiffer parts
with greater energy absorption capabilities as compared to those provided by particulatefilled composites. FFF is a convenient way to prepare complex geometries with continuous fiber-reinforced thermoplastics using special FFF devices that allow for the co-extrusion and co-deposition of fibers and thermoplastics [104]. Besides mechanical reinforcement, electrically conductive continuous fibers can be used to heat trSMPs [33,104-107]. In
this section, examples of continuous fiber-reinforced thermoplastic structures that change
shape due to Joule heating are discussed.
Yang et al. [104] investigated the Joule heating of 3D printed structures consisting of
continuous carbon fibers (CF) and PLA or PEEK produced by a modified FFF machine.
Bar specimens were prepared with one layer of neat polymer and a second layer of polymer reinforced with the continuous CF. The FFF specimens showed a linear relationship
between electric power and temperature. The data showed that there were two distinct
slopes: one steeper from room temperature to Tg and another one less steep from Tg to Tm,
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indicating a diminution of the thermal conductivity of the respective polymer matrices.
Specimens with a PLA matrix needed 0.45 W of electric power to reach PLA’s Tg of 57 °C
and 2.4 W to reach the onset of melting (~155 °C), while specimens with a PEEK matrix
required 1.6 W to reach PEEK’s Tg of 143 °C and 5.1 W to reach the onset of melting (~282
°C) (Figure 5a). Bilayer specimens fixed as cantilever beams bent up as the temperature
increased and down as the temperature decreased due to the different stiffnesses and thermal properties of the layers. The bending degree was proportional to the power applied
and followed a similar trend to that of power vs. temperature. Nevertheless, not all specimens were able to fully recover from such deformations following heating and cooling.
The authors attributed this to the softening of the polymers and increased shear stress at
the fiber–matrix interface at temperatures higher than Tg. This study demonstrated that in
such printed bilayer structures, it is possible to control the deformation by controlling the
amount of applied power. Zeng et al. [33] reported the effect of printing parameters on
the mechanical properties of continuous CF reinforced PLA specimens produced by FFF.
Increasing the extrusion temperature promoted more complete densification, resulting in
higher bending performance and lower surface roughness of the printed components. In
contrast, higher printing speeds caused weak interfacial bonding, resulting in a poor mechanical response. By increasing the amount of deposited continuous fiber and using a
deposition angle parallel to the length of the bending specimen, the bending strength of
FFF specimens could be increased. The Joule heating and shape recovery of specimens
with the highest bending strengths was evaluated, and it was observed that applying 5 V
for 75 s increased the temperature to 75 °C and induced recovery of 95% of the original
shape. Here, fiber breakage and delamination were considered as major obstacles to 100%
shape recovery (Figure 5b). The infill geometry used in continuously reinforced structures
was also investigated in PLA-CF composites. Zeng et al. [106] prepared horseshoe lattice
structures with different sinusoidal wave amplitudes (Figure 5c). It was observed that all
metamaterial structures had the ability to fix their temporary shapes and freely recover
their original shapes. Of particular interest was the fact that the sinusoidal wave amplitude affected the shape recovery and shape fixity ratios of the prepared specimens. A
larger sinusoidal wave amplitude gave the highest shape recovery ratio (Rr) and the lowest shape fixity ratio (Rf).
Le Duigou et al. investigated a combination of Joule heating and hygroscopic actuation was investigated by adding continuous CF to polyamide [107]. Two types of polyamide (PA6 and PA6-I) were used as shape memory polymers. PA6-I-CF filaments were used
to prepare Joule heating elements in the printed structures. The 3D-printed specimens had
one layer of hygroscopically active PA6-based material and another layer of PA6-I-CF
based electrically active material in a U-shape configuration at the edge of the layer. Combining these two materials allowed the structure to be straight when fully saturated with
moisture and curled when in the dry state (Figure 5d). Joule heating was used to speed
up the drying process since the natural desorption mechanism of PA6 is very slow. The
thickness of the different layers was optimized using bimetallic strip theory calculations
in order to achieve maximal curvature actuation. The deposition pattern of the CF-reinforced composite was used to control the mechanical reinforcement (i.e., stiffness) and the
geometry of the electrically conductive path. The measured peak temperatures generated
by Joule heating did not exceed the Tg of PA6 or PA6-I; therefore, the thermal strains were
low, and the hygroscopic strains dominated the actuation in the printed structures. Nevertheless, the authors did not measure the drop in the Tg,s due to moisture uptake.
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Figure 5. (a) Temperature as function of applied electric power in PLA and PEEK reinforced with
continuous carbon fiber (CF). Reproduced with permission from reference [104] copyright 2017,
Taylor & Francis; (b) continuous CF-reinforced PLA specimen recovering initial shape after applying 5 V for 75 s, with post-recovery fiber breakage shown. Reproduced with permission from reference [33] copyright 2020, Elsevier; (c) horseshoe lattices with different sinusoidal wave amplitudes
printed with PLA and PLA reinforced with continuous CF. Reproduced with permission from reference [106] copyright 2022, Elsevier; (d) specimen printed with PA6-I and PA6-I reinforced with
continuous CF and its shape in atmospheres with two relative humidity (RH) values (9% and 98%).
Reproduced with permission from reference [107] copyright 2019, John Wiley & Sons.
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3.1.4. Three-Dimensional Objects Produced by Multiple Extrusion FFF
Multi-nozzle FFF printers can be used to print complex structures with variable stiffnesses and multiple functionalities by combining different SMPs and/or composites in one
3D object. Here, the printing parameters, the ratio between the different materials, and the
geometry of each layer can be used to control the shape-memory response of these multimaterial 3D objects. Some examples of multi-material printing of trSMPs are discussed in
this section.
Wang and Lin [108] prepared laminate structures with reversible shape-change properties by combining TPU and PLA using a dual-nozzle FFF machine. The printed laminates had layers of PLA that acted as an active material and layers of TPU that acted as a
passive material, providing the structure with a reversible shape memory capability without an external mechanical programming step. The TPU used had a glass transition below
0 °C but a Shore hardness of 90 A, ensuring that it could be printed as a filament. As
pointed out in previous investigations [83], the printing conditions used for depositing
PLA can be used to control the amount of thermal expansion and contraction, in order to
maximize bending deformation. By heating the bilayer system through the Tg = 63 °C of
PLA to a higher temperature Th = 90 °C and then cooling it to room temperature, a reversible bending behavior was observed. Furthermore, by changing the infill direction, PLA to
TPU thickness ratio and overall geometry of the printed specimen, it was possible to control the achievable bending deformation (Figure 6a).
Wang et al. [109] used a triple nozzle FFF machine to fabricate 3D objects with filaments consisting of PLA and thermochromic pigments. The 3D objects had shape changing and color changing capabilities. For example, a multi-material flower was produced
with orange and green petals that could open when heated to Tr = 80 °C, while at the same
time, all of the petals turned yellow. Other fabricated structures changed color to camouflage certain information in printed letters or QR codes. Chen et al. [64] combined an electroactive shape-memory composite with a color changing composite in a single 3D object.
The electroactive composite was CB-filled PLA, while the color-changing composite was
PLA containing a thermochromic pigment. A custom-built dual-nozzle FFF machine was
used to prepare a multi-material starfish object that could change color from purple to
pink by electric heating. The starfish consisted of three sections; the bottom and top sections were printed with the thermochromic composite, while the middle section was
printed with the electroactive composite. The middle layer had a different inner structure
as well, providing variations in electrical resistance and Joule heating capacity in different
parts of the printed starfish (Figure 6b,c). A similar strategy was used with thinner structures resembling the wings of a dragonfly that could change shape and color as an electric
current was applied. Using such an approach, control of both shape and visual appearance is possible, which could prove useful in combined sensing/actuating applications.
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Figure 6. (a) Printed structure with one layer of PLA and a second layer of TPU after heating the
flower opens. Reproduced with permission from reference [108] copyright 2021, Elsevier; (b) geometry used in different layers of a 3D printed star fish object. (c) printed object with a conductive
PLA composite and a color changing PLA composite as voltage was applied at different times. Reproduced with permission from reference [64] copyright 2021, Elsevier; (d) PLA gripper with silver
nanowires (Ag-NWS) in the hinges to locally heat PLA for actuating capabilities. Reproduced with
permission from reference [110] copyright 2020, Elsevier.
Another possibility is to combine FFF with other extrusion-based deposition techniques as reported by Shao et al. [110]. A gripper was printed with PLA and a suspension
of silver nanowires (Ag-NWS) was deposited via syringe at the hinges of the gripper. After the suspension dried, the layer of nanowires was covered by a conductive paste and
connected using electrodes and wires. Local Joule heating of the nanowires and supporting PLA film enabled indirect triggering of the gripper structure, as shown in Figure 6d.
3.2. Direct Ink Writing
Direct–ink–write (DIW) is another MEX technique used to print semi-liquid materials
with specific rheological characteristics; such concentrated suspensions are referred to as
inks [111,112]. These inks are deposited using dispensing robots and should have a suitable viscosity (suggested viscosity in a range between 0.1 to 103 Pa·s). Furthermore, inks
require an adequate reduction in viscosity under applied shear stress (i.e., shear thinning);
the extrusion of the inks from the printing head is typically followed by curing immediately following deposition [37,113]. Curing ensures that complex parts may be printed
without loss in dimensional stability. A variety of complex, high-resolution 4D printed
objects based on multicomponent SMP formulations are reported via this approach [29].
DIW of concentrated suspensions can be achieved by solvent casting or matrix crosslinking; therefore, both approaches for 4D printing of multicomponent SMPs are discussed
here.
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3.2.1. DIW of Thermoplastics in Solution
When DIW is used with thermoplastics in solution, rapid solvent evaporation occurs
during the printing process [114]. Bodkhe et al. reported solution-based DIW of a multifunctional particulate composites combining shape changing and sensing functionalities
in a single printed structure.[28] The printing ink consisted of a blend of PLA and polyesteramide (PEA) dissolved in dichloromethane (DCM), with piezoelectric barium titanate
nanoparticles (BTN: 0–30 wt.%) added as sensing elements. The composites were used to
print different shapes, and the shape-memory and sensing capabilities were investigated.
Shape memory investigations resulted in recovery ratios (Rr) of more than 95%. For all the
composite samples, the piezoelectric voltage output of the materials was studied by applying a compressive impact force in the range of 0.05–2 N at 1 Hz at room temperature.
The composite with 10 wt.% of BTN exhibited the highest piezoelectric output. The piezoelectric output voltage could be increased by poling the BNT in the printed specimen
(Figure 7a) and by increasing the dynamic compressive force (Figure 7b). The combined
effect of shape memory and piezoelectric sensing was also monitored in a rectangular
printed film which was clamped at one of its edges to act as a cantilever, with silver electrodes deposited on both surfaces at the same edge (Figure 7c). This beam was then placed
in an oven at 100 °C (higher than the Tg of the film). A 2 g calibration weight was placed
near its free edge. Output voltages were measured across the electrodes as the beam softened above its Tg (~26 °C) and the weight eventually fell. As seen from the insets in Figure
7d, the beam deformed with the rapid change in stiffness under the influence of increased
temperatures, with a proportional voltage output resulting.
Solution DIW printing of PLA–iron oxide-based magnetic inks enabled the generation of 4D shape-changing architectures in custom-defined geometries [115]. To improve
the shape-memory capabilities of PLA, a UV initiator was added to the PLA/iron oxide/dichloromethane (DCM) based ink. Exposure to UV irradiation and fast evaporation of
DCM facilitated the formation of a crosslinked shape changing 3D architecture (Figure
8a). These nanocomposites could be magnetically and remotely heated by exposure to
alternating magnetic fields (AMF). The printed film was folded into a temporary spiral
shape and placed in a tube, which restricted the full shape recovery upon heating. Exposing the tube to 30 kHz AMF, the spiral structure showed self-expanding behavior within
10 s as illustrated in Figure 8b. This restricted shape changing behavior could be useful in
biomedical applications such as expandable stents (Figure 8b).
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Figure 7. (a) Piezoelectric voltage output (root mean square, RMS) from poled and unpoled 10 wt.%
BTN/3.6 wt.% PEA/86.4 wt.% PLA film sensors upon dynamic excitation at 1 Hz under compressive
(impact) loads. Inset: (left) RMS voltage output from the sensors; (right) the test set up used. (b)
Piezoelectric response from poled 10 wt.% BTN/90 wt.% PLA subjected to dynamic compressive
loads at 1 Hz; inset: microscopic images of the 10 wt.% BTN/90 wt.% PLA films revealing porosity
and agglomeration at different magnifications. (c) Schematic of the test specimen and boundary
conditions used for the final multifunctional sensor demonstrator. (d) Piezoelectric voltage output
from the multifunctional actuator deforming under a 2 g weight at 100 °C (insets: deformation of
the material above its Tg). Reproduced with permission from reference [28] copyright 2020, Elsevier.
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Figure 8. (a) Schematic demonstration of 4D printing of a partially crosslinking PLA based SMPs via DIW
and some of the printed structures; (b) demonstration of the restricted shape recovery process triggered
by a 30 kHz alternating magnetic field and potential application of the 4D scaffold as an intravascular
stent. Reprinted with permission from reference [115]. Copyright 2016, American Chemical Society. (c)
Process of the preparation of solvent cast (SC) printable Ag@CNF/PLA based conductive polymer networks (CPNs) and a schematic demonstration of the SC printing process. (d) 3D printed smart gripper
structure based on PLA/Ag@CNFs CPNs. (e) Optical and SEM images of a freestanding 3D printed spiral
using PLA/Ag@CNFs ink. (f) High electrical conductivity of PLA/Ag@CNFs nanocomposites as the
printed components lighted up an LED light under a 2.5 V voltage. Reprinted with permission from reference [116]. Copyright 2019, American Chemical Society.
A similar strategy was used to fabricate lightweight, highly conductive nanocomposites by incorporating silver coated carbon nanofibers (Ag@CNF) as hybrid fillers into
PLA-DCM mixture (Figure 8c) [117]. One possible application for such composite materials could be smart grippers that open and close as electric current is applied (Figure 8d).
The achieved nanocomposites could be printed in open air under ambient conditions to
form freestanding coils (Figure 8e) and displayed a low percolation threshold (i.e., <6
vol%) and high electrical conductivity (i.e., >2.1 × 105 S/m) without any post-treatment.
Electric conductivity was confirmed qualitatively by incorporating an as-printed 3D cubic
scaffold and a freeform spiral into a circuit with a red light-emitting diode (LED). The LED
could be powered at a voltage of 2.5 V with a current of 100 mA, demonstrating utility
immediately after printing as shown in Figure 8f.
In another effort, the 3D and 4D printing of poly (D,L-lactide-co-trimethylene carbonate) (PLMC)/CNT nanocomposites with electro-responsive shape-changing capabilities was achieved by solution DIW [117]. To make PLA more useful for biomedical applications, the copolymerization of d,l-lactide and trimethylene carbonate was carried out,
and the Tg of the resulting PLMC copolymer was tuned to 49 °C. Four-dimensionally
printed structures displayed high electrical conductivity even when porous scaffolds were
printed (Figure 9a,b) thanks to appropriate levels of filler dispersion (Figure 9c). The scaffolds prepared with these composites displayed shape-changing behavior within 16 s following the application of 25 V, as illustrated in Figure 9d.
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Figure 9. (a) The usage of 3D-printed scaffolds as electrical components; (b) SEM image of the cut
electrical component; (c) SEM micrograph of a nanocomposite cross-section; (d) shape recovery of
the folded scaffold given an applied voltage of 25 V. Reproduced with permission from reference
[117], copyright 2019, Elsevier. (e) Shape-fixity and shape-recovery ratios of freeze dried (FD) and
3D printed (3DP) samples; (f) recovery time for each scaffold to reach the final recovery ratio; (g)
programming and recovery of a PU scaffold in water at 50 °C. Reprinted with permission from
reference [118], copyright 2018, American Chemical Society.
Addition of UV photoinitiators triggered the hydrogen abstraction from PLMC and
resulted a crosslinked system. UV crosslinking helped to significantly improve the mechanical strength and solvent resistance of c-PLMC/CNT, while the actuation temperature
and volume electrical conductivity remained almost the same.
The 3D printing of bone scaffolds based on waterborne shape-memory polyurethane
(PU) inks was reported by Wang et al. [118] The mixing of PU-based ink with polyethylene
oxide (PEO) or gelatin improved the biocompatibility of the scaffolds. However, using
gelatin reduced the recovery ratios and increased the recovery times (Figure 9e,f). The
addition of superparamagnetic iron oxide nanoparticles (SPIO NP) to the printed inks
promoted the osteogenic induction of the printed scaffolds. The printed scaffolds showed
better shape fixity and shape recovery in water than in air (Figure 9g).
3.2.2. DIW of Liquid Pre-Polymers
Kuang et al. reported the 3D printing of highly stretchable, shape-memory (SM) and
self-healing (SH) elastomers via UV-light-assisted DIW printing [47]. An ink containing
urethane diacrylate, linear semicrystalline PCL and fused silica nanoparticle (200–300 nm)
was utilized for UV-assisted DIW printing of semi-IPN elastomer composites. PCL crystalline domains with Tm = 55 °C acted as switching domains for shape-memory experiments and as healing agents for self-healing behavior. The 3D-printed structures could be
stretched up to 600 % and showed interesting functional properties, such as high strain
SM and SM-assisted SH capabilities (Figure 10a–c). Potential applications of the printed
structures for biomedical devices, soft robotics, and flexible electronics, were proposed.
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Figure 10. (a) Thermally induced shape change of a printed hollow vase based on a semi-IPN elastomer (scale bars: 6 mm). (b) Pictures showing a SM-assisted SH process for a printed Archimedean
spiral structure; insets are optical micrographs showing surface morphology with a scale bar of 1
mm. (c) Stress−strain curves for virgin, notched and healed semi-IPN elastomer samples; insets are
micrographs of deformation and crack growth of notched samples clamped in the tensile tester.
Reprinted with permission from reference [47], copyright 2018, American Chemical Society. (d) Resistive heating of a printed structure based on ESBO/BFDGE/CNF (5.6 vol% CNF). The application
of 20 V and 150 mA induces a change from the programmed shape to the printed shape after 180 s.
Thermal images were taken with an IR camera [119] (CC BY license).
Rodriguez et al. reported bio-based thermoset composites via 3D printing [119]. Renewable and eco-friendly epoxidized soybean oil (EBSO) was used as the matrix material.
To tailor the mechanical properties, different concentrations of bisphenol F diglycidyl
ether (BFDGE) were added into the matrix. Furthermore, carbon nanofibers (CNF) were
used as a filler to render the printed structures electroactive. By varying the ratio of the
individual components of the thermoset resin, mechanical, thermal (Tg) and transport
properties could be tuned. The 3D printing of the composite resin resulted in complex
origami-type multi-material architectures that exhibited programmable shape changes
triggered by either direct thermal heating or electric (Joule) heating. The programming
and shape recovery of a complex box-like printed structure is shown in Figure 10d.
The UV-assisted DIW approach also enabled the 4D printing of epoxy based SMP
IPN composites with high mechanical strength and thermal stability [120]. The base resin
consisted of commercial photocurable acrylates, thermally curable epoxy and silica nanoparticles in different weight ratios. The silica nanoparticles were added to modulate viscosity and to realize shear-thinning behavior. In the first curing step, an acrylate network
was formed during a UV-assisted printing process. The epoxy oligomer was thermally
cured in the second step to generate an IPN composite (Figure 11a). A variety of complicated architectures such as lattices, gear wheels, swirl bows and locked structures were
successfully printed. A lattice structure is shown in Figure 11b. These printed IPN composites exhibited clear shape memory effects, including complete shape change within 10
s at 104 °C (higher than Tg ≈ 76 °C) along with enhanced mechanical toughness.
The same group reported a multi-material 3D printing approach involving the use of
dynamic photomask-assisted DIW together with a two-stage curing hybrid ink [29]. In
particular, a nanocomposite ink containing a photocurable acrylated prepolymer, a thermally curable epoxy resin and fumed silica were utilized. Dynamic photomasks were generated by a nearby projector during the printing process to enable different degrees of
cure in different locations, which was further modulated by irradiation time. This enabled
the generation of local variations in the extent of cure in each layer being printed, resulting
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in a graded structure (Figure 11c(i)). The capability to print materials with dramatically
different properties was then utilized to give structures with controllable buckling or fracture behavior or sequential shape memory characteristics. The multi-step compression of
a truss structure is shown in Figure 11c(ii). Such printed multi-materials structures were
then combined with triboelectric nanogenerators (TENGs) to fabricate multi-level TENG
devices. Highlighting the merits of sequential deformation, the multilevel TENG generated multilevel electric signals in response to different levels of mechanical force, providing opportunities for both energy harvesting and sensing.
Recently, the high-resolution DIW printing of filler-free epoxy-based SMPs with high
transition temperatures and mechanical strength was reported [121]. In the first step, a
cyanate ester and an epoxy resin were reacted to yield a prepolymer with a high viscosity.
In the second step, 3D printing and photocuring of the prepolymer with acrylate- or vinylfunctional additives resulted in a hard gel, which was thermally cured in the third step,
resulting an IPN structure. DIW through a combination of hybrid pre-polymer formation
and a two-step curing process resulted a complex mesh structure with high transition
temperature (~170 °C) and tensile strength (~70 MPa) along with excellent shape memory
(Rr > 86%, Rf > 95%). Furthermore, the applicability of this 3D printed SMP was demonstrated via the formation of a spiral spring with high modulus and a deformable sealing
ring for easy installation (Figure 11d)
Figure 11. (a) Schematic demonstration of printing of epoxy based SMP IPNs assisted by UV and
thermal curing. (b) A lattice structure with a single layered wall and its SEM image. Reproduced
with permission from reference [120], copyright 2018 Royal Society of Chemistry. (c-i) Sequence of
three dynamic photomasks and the total effective dose pattern by combining all the light dose for
each photomask during photomask assisted DIW printing of graded lattice structures (c-ii) results
of compression test at different stages of deformation for the truss structure. Reproduced with permission from reference [29], copyright 2019 John Wiley & Sons. (d) A high temperature shape
changing of process of a spiral and a sealing ring 3D printed structure. Reproduced with permission
from reference [121], copyright 2019 Elsevier.
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3.2.3. Generation of Porosity during DIW
Direct bubble writing of thiolene-based inks yielded a rapid means of fabricating
foams with narrow cell size distributions in the absence of both surfactant and solvent
[122]. The ink formulation had a viscosity of 0.410 Pa·s and consisted of hexa- and tetrafunctional thiol monomers along with a trifunctional alkene and a photoinitiator, as
shown in Figure 12a. By changing the gas pressure, it was possible to tune the pore size,
foam density, and Young’s modulus. Furthermore, a multi-layered construct with layers
having different pore sizes could also be printed. The pore size distribution of such a construct is shown in (Figure 12b). An easily accessible Tg (~40 °C) of the printed foams was
exploited for the purposes of shape-memory experiments. A three-dimensional printed
flower was programmed and was then shown to recover its original shape in 10 s by heating to 60 °C in water (Figure 12c).
Figure 12. (a) Schematic illustration of the resin and the printing process used, in which air co-flows with
ink to produce a stream of bubbles that are cured on-the-fly after impacting the surface. (b) The top view
of a printed sample. The graph depicts the bubble diameter distribution for a continuously printed multilayer porous solid. Small bubbles: Pair = 21.2 KPa; no bubbles: Pair = 18 kPa; large bubbles: Pair = 22.2 kPa.
(c) Shape memory cycle for a 3D printed flower. The diameter of the flower is 10 cm. Reprinted with
permission from reference [122], copyright 2020, American Chemical Society.
Wu et al. printed porous 3D structures by incorporating two different gas-filled microballoon pore formers with different Tg values into a siloxane matrix with silica particles
(SiO2) [123]. The effects of shell stiffness and the Tg of the microballoons on compressive
behavior and compression set in the printed siloxane based structures were evaluated. In
the case of lower Tg microballoon-filled prints, a greater degree of compression and a
higher recovery ratio were achieved, while in the case of prints containing higher Tg microballoons, reduced compression and a lack of recovery upon reheating were observed.
The potential use of the lower Tg prints in wearable protective padding and cushions with
Tg values optimized to match human body temperature was proposed.
4. Vat-Photopolymerization Based 4D Printing
Vat photopolymerization printing (VPP) utilizes a light source to selectively cure the
surface of a vat filled with a liquid photosensitive resin, thus enabling the formation of a
specified 3D shape [124]. Specifically, a controlled beam of light is used for the localized
irradiation of a photosensitive resin to form or crosslink polymer chains, resulting in the
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generation of a solid construct. Normally, parts printed with VPP technology must be
post-cured following printing to enhance stability. This technique is widely used to print
complex geometries with high accuracy and excellent surface finish. A variety of photosensitive resins have been developed for 4D printing of multicomponent SMP networks
and IPN structures [72]. Nevertheless, the shielding effect of some of the fillers and additives may result in delayed or slow photopolymerization during VPP of multi-material
systems. Commonly used VPP techniques for the 4D printing of SMPs resins such as digital light processing (DLP), stereolithography (SLA), and multiphoton polymerization
(MPP) are discussed here.
4.1. Digital Light Processing (DLP) of trSMPs
The digital light processing (DLP) method employs a digital projector to flash a 2D
image across the entire platform and cures an entire layer of material at once. As DLP
technology is not restricted to the curing of a single spot at a time, it can significantly
reduce printing times. Furthermore, through local variations in light intensity, DLP allows
for inhomogeneous cross-linking within a layer, enabling the formation of gradients in
mechanical and shape-memory properties [125]. DLP based 3D printing technologies
cover a broad spectrum of printing resolutions from 0.6 µm to 90 μm by adjusting the
magnification of the projection lens [69,72].
Photocurable and electroactive shape-memory polymer nanocomposites (SMPCs) based
on a thermosetting poly(ethylene glycol) diacrylate/poly(hydroxyethyl methacrylate) matrix
containing multi-walled carbon nanotubes (MWCNT) were successfully prepared via DLP 3D
printing [71]. The composition of the photocurable (meth)acrylate system was tuned to tailor
the thermomechanical properties of the matrix, whereas the effects of CNTs on the photoreactivity and rheological properties of the formulations were investigated to assess the printability of CNT-containing formulations. When increasing the CNT content from 0.1 to 0.5 wt.%, a
delay in the onset of photopolymerization was observed. This delay in curing time was attributed to the UV shielding effect of the MWCNTs, whose light absorption capability affected
the efficiency of the photoinitiator. The 3D-printed demonstrators displayed high shape fixity
and shape recovery ratios (Rf = 100%, Rr > 95%) and an electrically triggered shape-memory
effect realized via Joule heating. Furthermore, by changing the position of copper electrodes,
sequential recovery of different segments of a printed zig-zag shaped demonstrator could be
achieved (Figure 13a,b).
Zhang et al. used the DLP printing method to produce multiple SMPs within a single
construct using the same precursor [126]. They modulated the intensity of the light to enable the spatio-temporal tuning of material properties, including shape-memory transition temperature, rubbery modulus, and maximum elongation, within a single part. The
photo-sensitive ink consisted of comonomers methyl acrylate (MA, 56.9 mole%) and
isobornyl acrylate (40.3 mole%) with 1,6-hexanediol diacrylate (HDDA, 2.8 mole%) as the
crosslinker and bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide as the photoinitiator.
By digitally controlling the light pattern with different focus time periods, unusual shape
shifting 3D nano-photonic and electronic devices combining softer and more rigid regions
were constructed.
Ge et al. used high-resolution projection microstereolithography (PμSL) to print
multi-material shape-memory structures [127]. PμSL is capable of printing high-resolution (to 0.6 μm) complex 3D architectures covering multiple scales and with multiple materials over a relatively wide printing area (up to ∼90 mm × 50 mm). To print multi-material SMP structures, an automatic material exchange mechanism was integrated into the
PμSL manufacturing system. The photo-sensitive resin consisted of benzyl methacrylate
(BMA) as linear chain extender and poly(ethylene glycol) dimethacrylate (PEGDMA), bisphenol A ethoxylate dimethacrylate (BPA), or di(ethylene glycol) dimethacrylate
(DEGDMA) as crosslinker. Tailored glass transition temperatures and thermo-mechanical
properties were achieved through variations in crosslinker type and concentration. Using
this approach, a multi-material gripper was fabricated whose hinges consisted of an SMP
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and whose tips consisted of a soft material whose modulus could be adjusted based on
the modulus of the target object (Figure 13c,d).
Choong et al. developed SMP/nanosilica composite resins for DLP 4D printing and
evaluated the effects of nanosilica on the printing process and the shape-memory properties of the resultant parts [128]. Curing depth studies showed that SiO2-SMP composites
attained higher curing depths at a faster rate in comparison with the neat SMP. The fast
polymerization rate of the SiO2-SMP composites was attributed to the nanosilica particles
acting as heterogeneous nucleation sites for polymerization. Silica nanoparticles acted as
multifunctional crosslinks enabling excellent dispersion and improved mechanical and
shape memory properties of the composites. The SiO2-SMPs exhibited outstanding shape
memory performance with 100% shape fixity and 90–97% shape recovery. Projection stereolithography fabrication was used to print a flower-shaped sample using the developed
SMP composites. A printing speed of 2.8 mm/min ≈ 168 mm/hr was used to complete the
printing of a flower (76 mm in height) in 27 min [128]. The shape recovery process of an
SMP composite of this type under a hot air gun is shown in Figure 13d.
Figure 13. (a) Schematic demonstration of sequential recovery of a permanent shape (PS) from a
programmed temporary shape (TS), through different recovery stages (RS1, RS2, RS3 and RS4), by
connecting two adjacent electrodes at a time. (b) Actual recovery ratios (Rr) at different time intervals Reproduced with permission from reference [71],copyright 2021 John Wiley & Sons. (c) Schematic demonstration of the fabrication of a multimaterial structure based on PμSL. (d) Demonstration of the transition between the as-printed shape and the temporary shape in multimaterial grippers. Reproduced with permission from reference [127], copyright 2016 Springer Nature. (e) Shape
recovery of 3D printed silica-SMP composites upon exposure to hot air. Reproduced with permission from reference [128], copyright 2020 Elsevier.
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Zhang et al. reported the 4D printing of a photocurable resin to produce chemically
crosslinked SMP systems with self-healing capabilities [129]. These UV cured double-networks were based on the benzyl methacrylate (BMA) monomer and a poly(ethylene glycol)-dimethacrylate (PEGDMA) crosslinker, with linear polycaprolactone (PCL) added to
impart self-healing (SH) capabilities to the 4D printed structures. Three-dimensional
printing by DLP enabled highly deformable networks with remarkable shape-memory
and self-healing capabilities. The effects of PCL concentration on the thermomechanical
behavior, viscosity, and the self-healing capabilities of the SH-SMP systems were investigated. The mechanical properties of a damaged structure could be recovered to nearly
100% when more than 20 wt.% of PCL was present in the SH-SMP system.
4.2. Stereolithography (SLA) of trSMP
In stereolithography (SLA), a spot laser induces localized photopolymerization or
crosslinking, enabling layer-by-layer fabrication of the final product [69]. The layer height
of standard SLA printers ranges from 12 to 150 μm, with the printing speed ranging from
10 to 20 mm/h. Like DLP, the platform in SLA printers is immersed in the resin and moves
parallel to the z-axis, while the laser beam traces the part cross-section and cures the area
according to the CAD model. SLA is used to print high-resolution 3D structures with excellent surface finish [72,130]. There are only a few reports concerning the SLA printing of
trSMP networks using photosensitive resins. For example, a tert-butyl acrylate-co-di(ethylene glycol) diacrylate (tBA-co-DEGDA) network with shape memory properties was
successfully printed by SLA [113]. The wt.% of DEGDA crosslinker and photo-initiators
in the photo-sensitive resin was varied to tailor the Tg of the resulting networks. The
printed structure showed 100% recovery and stability of its shape-memory properties
even after 22 thermo-mechanical cycles. Miao et al. reported the stereolithographic printing of acrylated epoxidized soybean oil to produce smart biomedical scaffolds [131].
Shape memory tests confirmed that the scaffolds could fix a temporary shape at −18 °C
and fully recover their original shapes at human body temperature (37 °C). This indicated
the potential of these biobased printed structures for 4D printing. In another study, a photopolymer composed of polyurethane acrylate, epoxy acrylate, isobornyl acrylate, and
radical photoinitiator was synthesized and used to print 3D objects via the SLA technique.
The printed objects showed high mechanical strength and toughness along with excellent
shape memory performance. Nevertheless, none of these reports deals with multicomponent aspects of SMP, which is the major focus of this review.
4.3. Multi-Photon Polymerization (MPP) of SMPs
Multi-photon polymerization (MPP) is a direct laser writing technology that uses an
ultrafast pulsed laser to generate a very high flux of photons in a small volume over a very
short period of time [132]. Typical systems use green light (λ ~ 515 nm or 532 nm), nearinfrared light (λ ~ 780 nm, 1064 nm, or 1080 nm), or a combination of these wavelengths.
MPP techniques allow for the voxel-by-voxel writing of complex structures with critical
dimensions of micro/nano scale size (with sub-micrometer resolution) into a liquid resin
by scanning a tightly focused femtosecond-pulse laser beam. Unlike other stereolithographic techniques, MPP can freely cure solid polymer in the resin vat, eliminating the
need for material deposition in a layer-by-layer manner [132,133].
Zhang et al. demonstrated the 4D printing of trSMPs at sub-micron length-scales using two-photon lithography [134]. They showed a remarkable relationship between color
transmittance and the nanostructure of 4D printed trSMPs. The SMP photoresist based on
VeroClear (Stratasys Inc., Edina, MN, USA) was used to 4D print submicron-scale grids.
The grids transmitted only a limited range of wavelengths when illuminated by white
light. When the structure was programmed by heating and deforming above its Tg of 80
°C, it achieved an “invisible” state capable of transmitting visible light of all wavelengths.
After reheating above Tg, the nanostructure recovered it original state and again permitted
only a certain range of wavelengths to pass through, as the spectrum of wavelengths was
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scattered differentially by the grid. By varying the printing parameters during additive
manufacturing (laser power, write speed, nominal grid height), different colors could be
easily realized in the parts and switched by the programming process (Figure 14).
Figure 14. Schematic of color and shape change of a constituent nanostructured element of a “disappearing” 3D printed structure composed of a shape memory polymer (SMP). The as-printed
structure with upright grids (left) functions as a structural color filter that transmits only specific
wavelengths of visible light. Deformation of the structures at elevated temperature flattens the
nanostructures (right), rendering the part colorless; it remains in an invisible state after cooling to
room temperature. Heating recovers both the original geometry and color of nanostructures, leading to a submicron demonstration of 4D printing. Reproduced with permission from reference
[134], copyright 2021, Nature Communications.
In another report, De Marco et al. demonstrated the 4D printing of trSMP-based microstructured stents with minimum features of 5 µm by using the MPP technique referred
to as direct laser writing (DLW) [135]. The limitation associated with printing trSMPs at
the microscale were overcome by using an indirect 4D printing approach. In the first step,
high-resolution micromolds were produced by DLW of a photoresist. These micromolds
were then infused with a photo-sensitive trSMP resin, which was cured by UV irradiation,
followed by dissolution of the photoresist-based micromold to yield a microstructured
part. Following programmed deformation, the stent-shaped trSMP microstructures recovered their initial shape when the temperature was raised. The printing of microstructured
parts could be helpful in the fabrication of soft microrobots to solve medical issues.
5. Material Jetting (MJT) 4D Printing
Material jetting (MJT) is another commonly applied 3D printing technique that can
be used for the single-step fabrication of 4D multi-material objects [38]. In this technique,
droplets of photopolymer material are jetted onto the build platform and each layer is
immediately cured by UV light [76,136]. The first ever MJT printers were introduced by
Objet Geometries Ltd. (Rehovot, Israel), which later merged with Stratasys Ltd. The nozzles of these printers are able to switch between different materials, including support
materials in the case of complex part geometries. The gel-like support material can be easily removed by hand or water jetting. At this time, MJT printers from Stratasys Ltd. are
probably the most common commercially available jetting printers, with many authors
using them to fabricate 4D multi-material systems based on trSMPs [24,137,138].
Ge et al. used an MJT printer (Objet 260 Connex, Stratasys, Edina, MN, USA) to directly print glassy SMP fibers in an elastomeric matrix [23]. Laminated structures with a
range of fiber orientations could be printed and thermo-mechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted
strips, folded shapes, and complex contoured structures with nonuniform, spatially varying curvature (Figure 15a–h). The strips shown in Figure 15c–e had two layers one on
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top of each other: one with 25% fibers and the other one without. The fibers were oriented
at 0° (Figure 15c), 90° (Figure 15d) and 30° (Figure 15e). The strips in Figure 15f–h had
alternating sections side by side without fibers and with the fibers oriented at 0° (Figure
15f), 90° (Figure 15g), and 30° (Figure 15h). The original flat plate shape was recovered by
heating the material again. The printed active composites could be integrated with other
structural or functional components to create active devices. For example, hinges were
directly printed in a two-layer laminated form: one layer of matrix material and one layer
of an active laminate with a prescribed fiber size and spacing. The hinges were then connected to inactive (rigid) panels. These rigid panels were used as end tabs to apply mechanical loads. The hinges function occurs via a mechanism of programmed strain mismatch (eigenstrain) between the two layers that leads to constant bending curvature over
the hinge region. The authors developed a theoretical model to describe the behavior of
the printed active composite hinges and used it to design several active origami structures,
including a self-assembling box, a pyramid and two origami airplanes (Figure 15i,j) [77].
Figure 15. Active composite laminates obtained by design of laminate architecture. (a) A two-layer
laminate designed with one layer being a lamina with fibers at a prescribed orientation and one
layer being pure matrix material is printed, then heated, stretched, cooled, and released. Upon release of the stress the laminate assumes a complex shape as a function of its architecture. When
reheated it then assumes its original shape, a flat rectangular strip. (b) An actual 3D-printed strip in
its original shape and (c–h) show results of this process with differing fiber architectures. Reproduced with permission from reference [23], copyright 2013 AIP Publishing. (i) Active origami box
and pyramid. The printed flat cross and star shapes assemble themself into the desired box and
pyramid shapes after the programming steps. (j) Active origami airplanes. Flat triangles with three
or five hinges assemble into origami airplanes with different wing arrangements. Reproduced with
permission from reference [77], copyright 2014 IOP Publishing, Ltd.
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Mao et al. utilized the MJT printing method to fabricate complex structures displaying sequential shape recovery [31]. Helical 3D objects with rigid (non-active) sections connected by active hinges on each corner with a radius of 5 mm are shown in Figure 16a.
Two foldable composite strands with hinge structures were printed. The first sample was
composed of nine hinges of the same SMP (Tg = ~55 °C), while the second had seven hinges
composed of different SMPs having Tg values ranging from 32 °C to 65 °C, with inner
hinges having the lowest Tg values and outer hinges having the highest Tg values. After
printing, the structures were deformed into a flat configuration in hot water at TH = 90 °C,
above the Tg of all of the trSMP sections. Then, the samples were cooled to TL = 10 °C, at
which point all the trSMP hinges were in the glassy state. During recovery by heating (Tr
= 90 °C) in sample 1, all hinges were activated simultaneously, leading to incomplete folding (Figure 16b). For sample 2 the inner hinges with lower Tg recovered faster than the
outer hinges, thus enabling sequential activation and yielding unconstrained and rapid
(~7.0 s) folding to realize the targeted configuration (Figure 16b).
Figure 16. Multi-shape memory effects of a printed active composite strip. (a) The schematic of the
helical trSMP component. (b) A series of photographs shows its shape recovery process (1) in the
case of uniformly activated hinge sections, and (2) in the case of sequentially activated hinge sections
[31] (CC BY license). (c) The design and dimensions of the multi-material sample. The enlarged
drawing represents a cross-section of the structure. (d) The original printed sample. The length scale
at the bottom is in mm. (1–4) Shape change of the sample at different temperatures [139] (CC BY
license).
Wu et al. used MJT 3D printing to design and manufacture flat layered composites
consisting of families of digital trSMP fibers with different Tg values embedded in a rubbery matrix [139]. After being programmed into a temporary shape by a simple thermomechanical training sequence, the printed composite was able to change into multiple
shapes by stepwise increases in environmental temperature. For example, the design and
shape changing capability of a two layered composite having SMP fibers with different Tg
values (fiber 1 Tg ~ 57 °C; fiber 2 Tg ~ 38 °C) embedded in a rubbery matrix with a low Tg
(~2 °C) is shown in Figure 16c. The composite strip was stretched to a prescribed strain at
a programming temperature TH that exceeded the Tg of both fibers, then cooled to a fixing
temperature of TL = 0 °C, well below the Tg of both fiber materials. By stepwise heating the
Tg values of the rubbery matrix and the trSMP fibers were exceeded, one after the other,
and a series of intermediate bending shapes was realized as a consequence (Figure 16d).
Here, the amount of deformation was dependent on the thermomechanical properties of
the fibers and the matrix (such as Tg, stiffness and stress relaxation times), as well as the
programming strain and the ambient temperature.
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6. Summary and Outlook
Four-dimensional printing is a modern state of the art technology that has received
growing interest in recent years from the materials science, process technology, and mechanics communities [20,136]. A major part of 4DP is based on the 3D printing of SMPs,
and tremendous progress has been achieved in this context in recent years [38,61,113]. The
4DP of SMP multicomponent systems such as composites, blends, and layered 3D objects
has enabled the fabrication of high precision complex prototypes with unique capabilities
and offering multiple functionalities. These functionalities can be activated through external stimuli such as heat, light, electric current or magnetic fields and may be triggered
simultaneously or in a sequential fashion [26]. The importance of 4D-printed SMPs is apparent based on the growing number of studies and research papers related to this subject
that have appeared in the last decade [36]. This review has highlighted different 3D/4D
printing methods and strategies investigated in recent years to print multicomponent
SMPs formulations. Advanced fabrication methods for the printing of heterogeneous
SMP-based structures have been discussed, including MEX techniques (FDM, DIW), VPP
(SLA, DLP and MPP), and MJT. Up to this point, the materials selection, functional behavior and working principles of 4D printed trSMP multicomponent constructs have been
presented. In this last section, the current limitations and underlying barriers to continued
progress are identified and discussed.
Although most of the printed trSMP-based systems are still in prototype form, they
show strong potential to become functional objects with industrial applications in the near
future [37,60]. Different techniques have been used to print complex, multifunctional geometries impossible to achieve via conventional manufacturing techniques such as extrusion, injection molding or compression molding. These geometries enable complex actuation pathways and mechanical deformations following a predetermined program. Furthermore, the combination of shape memory with self-healing behavior, piezoelectric activity, magnetic sensitivity, electrical conductivity, and structural functionality makes
such multifunctional constructs attractive in a broad range of use cases, including intelligent electronics, aerospace, robotics, biomedical, and day-to-day applications (textiles, ornamentals, and footwear) [27,101,107,108,115]. Multicomponent approaches not only help
to improve the mechanical properties of trSMPs but also allow for the introduction of new
actuation methods such as remote triggering via magnetic field, electric current or light,
thus increasing the versatility of such 4D objects [20,21,26].
Though the studies reviewed here demonstrate that significant progress has been
made related to the 4DP of trSMPs and composites, several challenges remain. For example, FFF, the most used technique for SMP 4DP, results in anisotropic materials and introduces interfaces and internal defects where mechanical stresses concentrate, leading to
delamination after repeated actuation. Likewise, residual stresses arising from temperature gradients during processing can overlap with the recovery stresses generated by the
shape-memory effect, thus making the deformation of the polymer more difficult to predict and control. Furthermore, optimizing the printing parameters for a new formulation
is always tedious and challenging. In the case of softer polymers, filament buckling occurs,
while in the case of reinforced composites, material clogging jams the nozzle; in both
cases, the printing process stops. The issue of filament buckling can be avoided by adding
fillers or blending with other polymers to improve the filament stiffness [24,44]. To avoid
the clogging issue, the filler content can be reduced, flow enhancers can be added, and the
printing parameters can be optimized to enable the smooth flow of thermoplastic material
through the nozzle [87,88]. Furthermore, as FFF deals with highly viscous fluids, the nozzles used are the largest of all those used in the context of AM-based materials deposition
more generally; as a result, poor surface quality is common, and post-treatment is required to improve it [140]. New libraries of SMPs and composites combining improved
properties, excellent printability and retained shape memory response are required to
overcome these challenges and enable the generation of high-quality parts.
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The adaptability of DIW to various materials makes it the second most commonly
used MEX technique for 4DP. Various trSMP multicomponent systems, including functional composites and IPNs, have been printed using DIW to produce smart 4D objects
[111,115]. Here, moderate ink viscosities and shear-thinning behavior are required [111].
DIW has been used to print a range of polymer systems, including both thermoset and
thermoplastic composites [115,116]. To enable the printing of thermosets, time consuming
post-processing steps are often required, however, while in the case of thermoplastic systems, the polymer must be dissolved in a volatile solvent which should evaporate at just
the right rate during the printing process. The latter (solution) method has enabled the
printing of multi-material trSMP parts with magnetic, electric, and piezoelectric functionalities [28]. Nevertheless, DIW has relatively low printing speeds since parts are built by
depositing strands of materials, and like FFF, it produces 3D structures with low surface
quality due to the size of the nozzles used [111]. Furthermore, DIW, like FFF, is often used
to print trSMPs with amorphous switching domains, which only enable a one-way shapememory effect. The use of trSMP with crystallizable switching domains would enable reversible bidirectional SME. However, an alignment of polymer chains would be required
to achieve well-oriented crystalline domains [141]. Such type of polymer chain orientations has been used for 4D printing of liquid crystalline elastomers to enable reversible
movements in these elastomers, where the drawing force imposed by the movement of
the nozzle in the extruded printing process was able to align the mesogen units along the
specific printing path [142].
In contrast with MEX techniques, VPP techniques are restricted to photosensitive
systems, and printing is carried out layer-by-layer using some type of light source [69].
Though the printing of complex 4D objects based on trSMP multicomponent systems such
as composites containing carbon and silica nanoparticles has been reported, VPP is inherently more suitable for single phase materials with photo-sensitive moieties [72]. In the
case of composites, fillers concentrations must be optimized to avoid shielding effects that
could interfere with photoinduced polymerization or crosslinking [71]. Likewise, while
VPP has enabled the multi-material printing of complex microstructured trSMP-based
parts using different vats filled with different photosensitive formulations, switching between them reduces the speed of an already slow process (thanks to its high resolution)
still further [125,143]. Furthermore, it is inherently difficult to incorporate crystalline segments because oligomers based on such segments are solid at ambient temperature. One
way to make these oligomers printable is to heat the vat above the melting point of the
crystalline segments. Nevertheless, controlling unwanted thermal polymerization can be
a challenge. Finally, while VPP enables the printing of both macro- and microstructured
smart 4D objects with high levels of precision and surface finish, there remains a need for
a greater variety of photosensitive formulations yielding materials with broader ranges of
properties/offering new functionalities.
Compared to MEX and VPP, MJT is very well suited for manufacturing 3D objects
with very fine features based on different materials [75]. Different compositions can be
readily placed in different locations, bringing additional functionalities. MJT printers are
normally equipped with multiple high-resolution nozzles that jet different photopolymers
from different reservoirs [38]. Various multi-material trSMP based 3D objects such as
smart hinges and layered architectures have been printed via MJT and can be made to
show complex self-folding and shape-memory behavior including sequential transformations [20,23,31,77]. However, MJT printers are expensive, and materials libraries for
such printers are limited [76]. Nevertheless, it is believed that, with the rapid development
of MJT printing, more photopolymers will become available, and the implementation of
new design concepts and methodologies will be possible.
In summary, the 4D printing of trSMP-based systems continues to expand, with
many significant achievements reported related to the 3DP of smart multifunctional materials and objects. However, some challenges remain to be overcome, and some research
areas would benefit from additional attention. For example, the 3DP of trSMP systems
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with bidirectional shape changing capability, temperature-resistant SMPs acceptable for
space/aerospace applications, and systems capable of realizing high strain energy densities all represent decisive areas where further development is needed to move beyond labscale prototypes and more fully realize the promise of such systems. In this context, multidisciplinary research and development will be crucial to realize the 3DP of novel SMP
systems with complex molecular architectures for a range of impactful real-world applications. Finally, one crucial aspect of achieving real-world applications that must be elaborated is the long-term stability of 4DP parts and self-deploying structures. Exposure to
environmental fluctuations and aging processes can erode the initially printed properties
or generate other unwanted changes that will limit their response and functionality.
Author Contributions: Conceptualization: M.Y.R. and J.G.-G.; methodology: M.Y.R. and J.G.-G.;
data collection: M.Y.R., J.G.-G. and D.F.S.; data analysis: M.Y.R. and J.G.-G.; writing of manuscript:
M.Y.R. and J.G.-G.; figure arrangement: M.Y.R. and J.G.-G.; correction of manuscript: M.Y.R., J.G.G., G.M., D.R., D.F.S. and S.W.; supervision: D.F.S. and S.W. All authors have read and agreed to
the published version of the manuscript.
Funding: Not applicable. MDPI waived the APC for the editorial services of J.G.-G.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data is contained within the article.
Conflicts of Interest: The authors declare no conflicts of interest.
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