Supplementary document1. Shape memory effect and the characteristics whose performance in polymers.........12. Principle of shape memory effect in polymers...........................................................33. Self-healing SMPCs...................................................................................................54. Electric stimulation.....................................................................................................75. Magnetic stimulation..................................................................................................96. light stimulation........................................................................................................107. Applications in aerospace and aviation....................................................................11Reference..........................................................................................................................19

1. Shape memory effect and the characteristics whose performance in polymers

The shape memory effect is usually achieved by polymer phase transition or

reversible bonds (or molecular switches), which we call phase transition shape

memory polymers and molecular switch shape memory polymers respectively. First,

we discuss phase transition shape memory polymers, which are the most common and

most often appropriate for applications. The polymer mainly consists of a crystalline-

melt state or a glass-rubber-viscous state according to the regularity of its structure

and the speed of cooling. Some polymers also have a liquid crystal state. Due to their

block and branching structure, many copolymers have a number of glass transition

processes. The large deformation of the shape memory polymer generally relies on

the local affine deformation of the network, so the stationary phase must be present,

which limits the available phase transitions to crystalline and glassy transitions.

Compared with other materials such as metals or ceramics, polymers are a type of

complex condensed matter. The boundaries are not obvious, but there are different

physical and chemical properties in different regions, indicating that the use of the

term “phase” is reasonable [1-3]. Similarly, the phase transition of a polymer occurs

over a broader temperature range than that of a relatively simple condensed state such

as metal, which gives shape memory polymers distinct properties from those of shape

memory alloys (such as the temperature memory effect [4-9]). In general, the polymer

phase transition corresponds to locking or unlocking a degree of freedom, and the

source of this ambiguity is the distribution of polymer structure and composition.

The shape memory effect is classified according to the reversibility and the

number of stable states. The reversibility is classified into one-way SME and two-way

SME. So far, two-way SME is reported only in shape memory alloys [10-12].

Therefore, reversibility is not emphasized in shape memory polymers. (There have

been some novel reports of shape memory effects that are often mistaken for two-way

SME, such as in [13,14], which were subsequently clarified.) The number of stable

states is classified into dual-SME and multi-SME, i.e., more than one programmed

shape in a steady-state representation of the shape memory cycle is called multi-SME.

Note that the steady state is temperature dependent, meaning that the shape of the

material will remain constant in a particular temperature range, while small creep

recovery is neglected. In general, these effects are related to the thermodynamic

process and the properties of the materials themselves. Above all, we introduce the

first and most extensive shape memory effect, dual-SME, which can be expressed in

any shape memory material and thus can be used as a standard for judging shape

memory polymers.

Although the principles of the shape memory effect based on different

transitions are different, we take cognizance that their shape memory cycles are

similar. The similarity allows us to use the shape memory cycle of the glass transition

as a representative of various shape memory cycles. A shape memory cycle includes a

programming process and a recovery process. The programming process is the

process of creating a programmed shape. From a mechanical perspective, this is a

process of material storage strain, registered as a yielding process. The process often

requires heating to a temperature above the glass transition temperature range

(generally described by tan δ) followed by the application of an external force, and

use external forces to maintain its shape until it has cooled. This type of programming

is called hot programming, while other methods are known as cold programming. Hot

programming can avoid material damage and irreversible deformation and can

improve the fracture strain and recovery rate relative to cold programming, while cold

programming has important research value in self-healing and self-finishing [15-18].

Any shape other than the original shape is a temporary shape, and in particular, the

shape obtained by programming is called the programmed shape to distinguish it from

other temporary shapes. At the end of the programming process, the external load will

be removed, and the material will return by a small amount toward its original shape,

but this deformation is small, so there is no need to distinguish which is the

programmed shape. The recovery process is generally a no-load or low-load process.

(The external load is not sufficient for reprogramming. The recovery described here is

a concept used for application, so we do not need to define it too strictly.). After

heating to the Tg range, the material gradually releases the stored strain [19,20]. In

general, upon heating to the lower bound of the glass transition temperature range,

shape recovery begins. To achieve a higher recovery rate, it is beneficial to heat to a

high temperature above the Tg range. We call this recovery a hot recovery, while the

alternative is known as cold recovery. In shape memory polymer composites,

considerable cold recovery also occurs at room temperature.

2. Principle of shape memory effect in polymers

For further explanation from the perspective of molecular mechanics, the

shape memory effect in a shape memory polymer-based glass transition is mainly due

to the two-phase structure of the polymer: the stationary phase to maintain the original

shape of the macrostructure and the reversible phase of softening and hardening.

When the temperature is lower than the Tg range, the shape of the reversible phase can

be frozen; otherwise the entropic elasticity drives the polymer network to recover to

the initial state. The stationary phase ensures that the network deformation is affine

without viscous flow due to external forces. In general, physical crosslinking and

chemical crosslinking correspond to thermoplastic shape memory polymers and

thermosetting shape memory polymers, respectively. When the Tg range of the

stationary phase is high, and softening and relaxation do not occur in the temperature

range of use of the material, so the memory of the original shape can be ensured.

When the Tg range of the reversible phase is low, softening and hardening can easily

occur as the temperature changes, and the segment of material has a high deformation

capacity at higher temperatures. When the temperature is low, the polymer network is

in a low-energy state, the segment cannot rotate freely, and the polymer is generally

elastic at the macroscopic scale. When the temperature gradually increases to the Tg

range, the segment rotation is unlocked, and then the polymer is highly elastic on the

macroscopic scale, with a high deformation capacity. When the temperature is

lowered again to below the Tg range, the segment does not return to the low-entropy

state as the segment rotation locks, so the material shows a temporary shape on the

macroscopic scale. The origin of the shape memory effect described above has been

supported by recent experiments [21-26].

Using a well-designed polymer with a novel programming process, we can

achieve a variety of new shape memory effects, such as multi-shape memory effect

(multi-SME) [27-33] and temperature memory effect (TME) [6,34-36]. Some

polymers have multiple available transition temperatures [27-29], such as two glass

transition temperatures, usually achieved by the copolymerization of two polymer

segments. In two-transition temperature programming, during gradual warming for

the recovery process, the material can be observed to switch among three shapes

(including two temporary shapes and one original shape). This effect is called a triple

shape memory effect (trip-SME). Similarly, a multi-shape memory effect may be

defined and achieved. This effect is limited by the distribution of the transition

temperatures of the polymers; the glass transition temperature ranges of different

segments may be coincident with each other, which reduces the fixation rate to a

certain extent. We already know that the glass transition corresponds to the unlocking

of the rotational freedom of the segment, so when the glass transition temperature

range is very wide, the segment rotation is gradually released with increasing

temperature, which makes it possible to program more than one change in the broad

glass transition temperature range. This effect is called the temperature memory

effect. Temperature memory effects with good fixation rate were observed to require

a large difference in temperature for each programming process to avoid interference

with the recovery process.

In addition, there are some novel materials that combine the shape memory

effect with other active movement principles, such as the reversible shape memory

effect [14,37], which introduces a reversible expansive crystal phase in the shape

memory polymer network whose transition temperature (T1) is lower than Ttrans of the

shape memory effect. The material can be reversibly altered in shape using T1, or

undergo programming/recovery using Ttrans. The strain caused by the crystallization is

relatively small, so to obtain a greater deformation, bending is usually selected.

Shape memory effect-based reversible bonds involve reversible chemical

bonds [38-41] or supramolecular interaction [42-48]. In contrast to the polymer phase

transition freezing its own degrees of freedom, the reversible bond is equivalent to

externally introducing a lock of a fixed programmed shape. The elastomeric polymer

is adjusted to the desired shape by external force, and then chemical bonds or

supramolecular interaction is introduced to lock the shape (triggered by reaction

conditions such as heat [38,39], light [42], metal ions [43,49], pH [49,50], redox [51],

or ultrasound [52,53]), which is the programming process of a molecular switch shape

memory cycle. Because the introduced bonds are reversible, the reversible bonds will

be unlocked under appropriate stimulation conditions, and the programmed shape will

undergo recovery. These reversible shape locks generally involve modification to the

polymer network (e.g., the addition of functional groups, grafting, blocks); there is

also the use of doped forms [54-56]. Again, upon the reaction to remove the reversible

bonds, the polymer reverts to high elasticity, and the original shape is recovered.

However, the shape memory effect based on reversible bonds is used less in

applications than that of shape memory polymers based on phase transition due to

cost and practicality.

The stimulation methods for the programming process and recovery process

are varied. In addition to stimulating the shape memory effect based on reversible

bonds, indirect heating can be applied by a functional filler [57-59], which broadens

the application scope of shape memory polymers. Among these stimulations are

electric stimulation, light stimulation, and magnetic stimulation methods, each with its

own unique advantages in the application field, such as precision and non-contact

procedures. See Section 3.2 for details.

3. Self-healing SMPCs

Self-healing materials that rely on physical expansion are generally soft

thermoplastic materials or poorly crosslinked rubber materials whose chain parts

exhibit good expansion ability. In real applications, many thermosetting shape

memory polymers with high crosslinking density form a complex solid network using

chemical crosslinking. The interaction between the surfaces after they are broken

apart is relatively weak, and the speed of expansion of the chain parts is low, which

hinders self-healing. Therefore, the use of adhesive agents that are not part of the

material itself is a promising idea. Through the reasonable design of the expansion of

the adhesive agent, artificial healing could be transformed into self-healing of the

material system. The mixtures mainly include two types of structures, microcapsules

and micro vessels.

Microcapsule adhesive agents are easy to apply to all kinds of polymers,

and the production and mixture of microcapsules are relatively simple. This method

has been widely researched. Microcapsules of healing agents on scales ranging from

nanoscale to microscale have been reported. Ways to mix healing agents also vary

widely and generally include single capsules and multi capsules (divided by the type

of capsule). At least one kind of capsule with healing agents is included, and catalysts

and triggers can be included in the same capsule or in other capsules. The earliest self-

healing material mixed with microcapsules was a single healing agent [60-63].

However, the healing agent tended to lose effectiveness over time. There were two

fundamental solutions to this problem. One of them was to ensure that healing agents

worked only upon encountering a specific external stimulation, such as moisture

[64,65] or light [66,67]. The other was to separate capsules with catalysts or triggers

and ones with healing agents, which required a double capsule system. Both liquid

[68,69] and solid [70,71] catalysts have been used.

On the other hand, in contrast to self-healing, scars are new phases made by

healing agents, which are uneven in properties with the material itself. As a result, in

addition to the healing of mechanical ability, there other kinds of healing agents that

heal electric properties [72-74], surface hydrophobicity [75,76], or even

microstructures [77]. Furthermore, methods of producing hollow polymer fibers from

nanoscale to milliscale for the storage of healing agents to realize self-healing have

been reported [78]. This kind of method could effectively avoid problems with

surface inconsistency in the composition.

Inspired by the circulatory systems of animals and the microtubule

structures of plants, researchers have designed microscopic polymer vessel systems,

which could achieve self-healing after the addition of healing agents. Self-healing

through microscopic vessels is similar to self-healing through microcapsules and can

involve a single vessel network [79-81], a double vessel network [82,83] or even

more vessels. The double vessel network is similar to a double capsule system that

contains at least one healing agent network. In addition, microscopic vessels and

capsules can also realize self-healing in combination [84]. However, healing agents

can be managed more efficiently through vessel structures than through the admixture

of microcapsules. For this kind of self-healing material, building microscopic vessels

is an essential problem. Now solutions include chemical dissolution [79], mixing with

fibers and then dissolving [83,85], and 3D printing [86].

Next, we discuss the effect of shape memory polymers on self-healing

properties, i.e., the spontaneous completion of the splicing process. This can be

achieved by a shape memory polymer matrix or by shape memory polymer fiber

doping. A large number of molecular switch shape memory polymers have been

previously reported to be self-healing, such as Diels-Alder reaction-based shape

memory polymers [39,88] and thermoplastic shape memory polymers [89]. For

general thermosetting shape memory polymers, self-repairing abilities can be obtained

by doping with a thermoplastic polymer such as PCL. Luo et al. reported the addition

of a PCL-based shape memory polymer fiber with electrospinning to epoxy resin to

obtain a self-healing shape memory polymer composite [90,91].

4. Electric stimulation

Electric stimulation is the most convenient and precise of all kinds of

stimulation. The use of current-driven shape has produced a great increase in the

applications of shape memory polymers. Because of their restricted electronic

structure and low conductivity, intrinsically conductive polymers have never been

reported. Conversely, polymers mixed with conductors have many advantages, such

as high efficiency, stability and ease of design. The heat is fed into the polymer

through the current, enabling the endogenous initiate recovery process. Carbon

materials such as graphene, nanotube and nanofiber as well as nanoscale granules of

metal can be used for the conductive doping of polymers.

Cho and Goo et al. first reported shape memory polymers doped with

conductors in 2004 [92]. Carbon nanotubes were added to polyurethane shape

memory polymers, and when the content of carbon black was between 20 to 30 wt.%,

conductivity as high as 1-10 S/cm was realized. However, the overwhelmingly high

doping ratio significantly lowered the shape memory effect. Afterwards, this group

proceeded to report multi-wall carbon nanotube-doped shape memory polystyrene,

with conductivity on the scale of 10^-3 S/cm [93]. Oxidation of the surface of carbon

nanotubes enhanced the ability of the filler to combine with the polymer matrix but,

on the other hand, destroyed the electronic structure of the carbon nanotubes and thus

reduced the conductivity. These early studies demonstrated the idea of doping with

conductors. However, to achieve shape recovery, extremely high voltages had to be

exerted (greater than 40 V) [92].

In 2007, Leng et al. reported a kind of shape memory polymer with the

simultaneous addition of carbon black nanoscale granules and short carbon fibers

[94]. The use of 5 wt% carbon black nanoscale granules and 2 wt% short carbon fiber

gave the material a conductivity of 2.32 S/cm, and the material only required a

voltage of 24 V to initiate shape recovery. Compared with a polymer doped with the

same mass percentage of carbon black and short fiber, this polymer had conductivity

more than 100 times greater, which was due to the conductive three-dimensional

structure constructed by short fiber with the assistance of carbon black. Forming

connecting chain or net structures effectively enhanced the conductivity. Leng et al.

reported a new kind of shape memory polymer composite in 2008 [95,96], which

further added 0.5 vol% of microscale nickel powder to carbon black-doped shape

memory polymer. Using external magnetic fields to arrange the nickel powder in

chains enhanced the conductivity, while without magnetic fields, the doped nickel

powder made a trivial contribution to conductivity. This shape memory polymer

composite could realize perfect shape recovery under voltage as low as 6 V. In 2011,

this group reported that applying an electric field to rearrange the carbon nanotubes of

a shape memory polymer composite material resulted in conductivity that was 100

times greater than that of the composite with unordered carbon nanotubes [97]. Bai

and Fu et al. introduced carbon black nanoparticles with self-networking ability into

the polymer, and the fixation rate and recovery rate were significantly enhanced due

to the resulting co-continuous structure. In addition, with the use of a low doping rate

to obtain a good conductive effect, the material required 80 seconds at 30 V to return

to the original shape [98]. These examples sufficiently illustrated the significant

contributions of ordered conductive networks to conductivity, as shown in Figure 4.1.

Figure 4.1. Why does a shape memory polymer blended with short fibers and carbon

black have better conductivity? Short fibers are electrically conductive

"expressways", and carbon black acts as a bridge between the highways that are

dispersed in insulating polymers. [57]

Guo et al. used a digitally controlled spraying-evaporation deposition

modeling process to deposit carbon nanotubes on shape memory polymer films to

achieve efficient electrical driving of shape memory polymers with resistivity of only

28.7 Ω/sq [99]. The location of the deposition and the number of layers can enable

local recovery and control of the response rate to obtain a rich variety of shapes.

In recent years, shape memory polymer composite materials doped with

conductors has been reported on a large scale. The examples include new structures

and combinations with lower voltages and faster reaction speeds. Wang et al.

chemically crosslinked modified carbon nanotubes with polymer networks and

obtained more even distributions [100]. Cho et al. covered the surface of carbon

nanotubes with polypyrrole to enhance the conductivity of shape memory polymers

and achieved shape memory of more than 90% in 20 seconds at a voltage of 25 V

[101]. In 2010, Leng et al. first reported that the use of carbon nanoscale fiber

together with self-assembled carbon nanoscale paper conferred conductivity on shape

memory polymer, and at a voltage of 16.2 V, the shape recovery was completed in

330 seconds [102]. In 2014, this group used carbon nanoscale fiber and BN self-

assembly to provide conductive doping and achieved 96.7% shape memory in 80

seconds at a voltage of 4.8 V [103]. In 2015, this group further reported a shape

memory polymer doped with graphene, which could achieve 100% shape recovery in

5 seconds at a voltage of 6 V, a far higher value than in previous reports [104].

Additionally, this group designed heat driving by gold electrodes [105]. Experiments

illustrated that annealing could effectively lower the resistance produced by the

remaining stress and defects in microscopic structures.

5. Magnetic stimulation

Magnetic driving is a typical approach to stimulation without direct contact.

The principle is the indirect heating of shape memory polymers by doping with

magnetic materials, such as ferrite and soft magnetic materials. This heating method is

highly suitable for medical instruments such as implanted shape memory polymer

supporters. In 2006, Lendlein et al. added magnetic nanoscale particles to poly(ε-

caprolactone) with shape memory effects and thereby made a new type of shape

memory polymer stimulated by an alternating magnetic field [106]. Further, this

method can be improved in a heating mode which mixes magnetic induction heating

and direct heating [145]. Since the magnetic heating is controllable, the required for

ambient heating is tunable. In this way, the material exhibits its apparent switching

temperature being varied. They also added magnetic materials to shape memory

polymer systems with temperature memory effects and obtained shape memory

polymer nanoscale composites with magnetic memory effects [107]. Leng and

Smoukov et al. reported the magnetic driving of a composite thin membrane made of

Nafion and Fe3O4, with the controllable realization of as many as four programmed

shapes [108]. This magnetic driving was highly controllable. Even when the local

temperature exceeded 80°C, the surface temperature remained near the body

temperature (38-40°C), which offered great potential in medical applications.

To address the problem that doped materials such as the magnetic medium

ferrite oxide had obvious boundaries with the composite and were unevenly

distributed, modifications of the ferrite oxide surface and crosslinking it into the

composite network provided an effective solution. Lendlein et al. covered the surface

of magnetic particles with oligo(u-pentadecalactone) (OPDL), covalently

incorporating nanoscale metal granules into the composite, thereby obtaining even

doping distributions, and achieved precise control of a two-segment shape effect and a

reversible shape memory effect through magnetic fields [109]. Bai et al. (2014)

decorated the surface of super-smooth magnetic iron oxide nanoscale granules

(SPIONs) with norbornene and distributed them within the composite [110]. Then,

using iron oxide as the center, norbornene shape memory chains were created and

crosslinked with the original polymer through the double decomposition of alkene,

and this single-pot reaction effectively enhanced the distribution of the nanoscale

metal granules. Yang et al. (2012) decorated ferriferous nanoscale granules with oleic

acid, which prevented the accumulation of nanoscale metal granules and showed that

surface modification could enhance the connection between nanoscale metal granules

and polymers, thus improving the efficiency of magnetic driving [111]. Simon et al.

furthermore researched doping with nanoscale ferriferous oxide granules decorated

with single and double layers of oleic acid [112]. Leng et al. doped poly(3-

caprolactone) shape memory polymers with multi-wall carbon nanotubes covered

with ferriferous oxide and achieved thermic, electric and magnetic driving within a

united material system. This composite material exhibited good biological

compatibility [113]. Ali et al. used a double-sided Cu-coated polyimide to produce a

magnetic stimulation shape memory polymer drug release mechanism that required

simple heating stimulation [114].

6. light stimulation

At lower wave bands, ultraviolet light has useful chemical effects and

initiates photochemical reactions such as free radical production and pericyclic

reactions. All reported shape memory polymers using photochemical switches are

stimulated by light in this range. The most typical example is a series of shape

memory polymers based on a cinnamic acid molecular switch, which uses a [2+2]

cycloaddition. In 2014, Biyani et al. composited rubber epoxyethane/epichlorohydrin

copolymer (EO-EPI) with nanoscale cellulose crystals derived from benzophenone

(Bp-CNCs). These crystals formed rigid networks in the polymer, crosslinked with

polymer networks in the neighborhood and adopted a fixed shape under ultraviolet

light. This method was easily generalized to all kinds of polymers, replacing the

design of special functional group structures [115].

Gold nanoscale particles as well as gold nanoscale rods have the convenient

ability of absorbing visible light because of plasma resonance on the surface. Zhang et

al. added gold nanoscale particles to poly ethylene oxide and obtained a shape

memory polymer that could be heated by a 532 nm laser. This polymer

simultaneously showed self-healing ability [116]. In 2014, this group reported shape

memory polymers doped with gold nanoscale rods, which could realize effective light

driving shape recovery with only 5 wt% of gold nanoscale rods [117]. By using

ordered gold nanoscale rods, specific polarized light driving could be realized [118].

Similarly, Zhao et al. added gold nanoparticles to a temperature-shaped memory

polymer to produce an optical actuator controlled by temperature and IR light [119].

Tonndorf et al. reported a light-stimulated shape memory polymer yarn that was

doped by a semi-continuous layer-by-layer technique with gold nanoscale rods [120].

This yarn could be used to make photosensitive fabrics.

7. Applications in aerospace and aviation

Because of their stable single-deformation ability, shape memory materials

have attracted broad attention in all kinds of aerospace expansion structures and in

driving machine-based locking-release structures with limited deformation times.

Early studies of variant structures, mainly using shape memory alloys to provide

driving forces, included a series of applications of spatially expanding hinges

[121,122] and variant structures in aircrafts [123-125]. Their relatively large

density（ 6~8 g/cm3）made it difficult to use shape memory alloys as structural

materials, and they were necessarily accompanied by other light materials in the form

of wires or boards, which increased the difficulty of design and introduced the

inconvenience of choosing the accompanying structural materials. On the other hand,

the shape memory effects of the shape memory alloys themselves were not

satisfactory. Therefore, shape memory polymers became a rising star in solving

problems.

By comparing the properties of fiber-strengthened shape memory polymers

to those of shape memory alloys, we directly obtained ideas of the necessary

differences in application design. Because of the low mass, high toughness and large

deformation of polymer-based shape memory composites, they could be made into

matrices of large expansion structures (such as reflective antennas [126]), which

achieved the combination of driving devices and structural materials.

The environment in space is much worse than that in the atmosphere.

Polymers and other organic materials will be more severely eroded than metal or

ceramic materials by atomic oxygen and ultraviolet radiation and will be subjected to

extreme temperature differences, which will lead to loss of mass of polymers,

reduction in their dynamic properties, or even complete loss of function [127-131]. As

a result, shape memory polymers must be carefully selected and tested for

applications in space. For shape memory polymers, these tests should not only

consider conventional factors such as losses in mass and changes in components and

modulus but also the loss of the shape memory ability. It has now been proven that

epoxy resin-based and cyanate ester-based shape memory polymers have good

tolerance of the outer-space environment. Furthermore, because of their outstanding

thermodynamic properties (especially the tolerance of high and low temperature

cycling) and stable chemical properties, polyimide-based shape memory polymers

have been identified as ideal candidates for future applications of shape memory

polymers in space. Similarly, fillers with good space tolerance such as carbon material

and glass fiber must be chosen when making shape memory polymer composites.

When doping with materials with active chemical properties, such as healing agents,

special design considerations are necessary to prevent these active properties from

being influenced by space radiation.

In their shape recovery processes, the dynamic performances of shape

memory polymer composites are adhesive elastic, and thus their motion is a process

of creeping. In the past, structures for deployment in space often used spring hinges

with locking devices, which produced a large impact and probably damaged internal

electronic devices and their mechanical component. With the miniaturization of

satellites, these impacts have become increasingly more unignorable, and thus

developing low-impact hinges has become very important [132]. Shape memory

polymer composites not only offered the advantage of low impact but also integrated

the locking function, offering a broad future in applications. The CTD company first

introduced the idea of shape memory polymer hinges [133,134]. Early hinges used

rectangular sections, but the force of recovery of the hinges was not sufficient with

this design. Several improved designs have been developed, include right and back

arc cross section and parabolic section. Leng et al. proved that the best dynamic

properties were obtained with a circular arc of 120 degrees and back SMPC hinges

with a shape recovery rate greater than 90% in the bending process, which could be

used in driving the expansion of structures such as large solar power battery boards

[132]. However, these hinges faced the problem that two SMPC layers might interfere

with each other, giving rise to serious stretching deformation and causing damage to

the layers or even cracks. Therefore, designing new methods of combining layers is a

future direction of research on SMPC hinges. In addition to hinges, shape memory

polymers can be used to design and construct locking-releasing structures in

aerospace applications [135,136].

Making space expansion trusses out of shape memory polymers is also an

attractive application. Trusses are main components of loading in aircraft. Traditional

metal trusses have the shortcomings of large mass and mechanical complexity. By

comparison, shape memory polymers have the advantages of low mass, large

deformation and simple driving methods, which have already attracted studies from

many perspectives. The CTD company developed the idea that a double hollow space

expansion beam could consist of two rods or three slices of long SMPC layers in the

shape of circular arcs [137]. For larger loads, three-slice structures should be selected.

Afterwards, this company developed improved plans for compressing and piling

layers of circular arc sections, which increased the compression rate and expansion

ability [138]. Leng et al. proposed the idea of constructing expansion beams in

segments, which achieved good recovery effects and offered ideas to make large

spatial expansion structures [139] (Figure 7.1(a)).

Furthermore, there are some other intuitive new expansion structures with

ingenious forms and simple structures that nevertheless can achieve their functions

effectively. Inspired by the spiral compression deformation beam system designed by

the CTD company [140], the CRG company made experimental models of spiral

compression deformation beam systems [141] (Figure 7.1(b)). This idea, inspired by

the CTD company, replaced the original metal material with SMPC, which could

effectively avoid damage to the material. Furthermore, it could perform locking and

expansion using its intrinsic properties and had the advantages of simple structure,

small volume and high reliability. The CTD company designed a kind of SMPC space

expansion beam with a gapped circular section that could easily be curled under

heating, which greatly decreased the necessary storage space [142] (Figure 7.1(c)). By

expansion, this structure could easily be warped to recover the original cylindrical

beam. Using this beam structure, this company designed a kind of flexible solar

power battery array, which had the advantages of low density and low cost and was

easy to produce on a large scale [143] (Figure 7.1(d)).

(a)

(b)

(c) (d)

Figure 7.1. Some space expansion shape memory polymer composite beam structures.

(a) Modular space deployment truss. A variety of trusses can be easily produced by

this unit. (b) SMPC truss similar to climbing rattan. (c) SMPC space expansion beam

with gapped circular section and (d) flexible solar power battery array made by from

this beam. [156,158,164,166]

Compared to shape memory alloys, because of their characteristics of low

density and large deformation, shape memory polymers offer great advantages in

producing structural materials for large-scale deployable structures. SMPCs can also

act as deployable skeleton structures for large-caliber inflatable antennas [144]

(Figure 7.2(a)). At present, several kinds of space reflective mirrors use SMPCs as the

substrate material. Furthermore, SMPCs can serve as the solid structure of large-

caliber inflatable antennas [145] (Figure 7.2(b)). A deployable SMPC antenna could

be used as a reflective surface in the shape of a rotational paraboloid, which can be

compressed and collapsed into an umbrella-shaped structure to save storage space

[146,147]. NASA planned to deploy 35 m antennas in space, which are still

impossible to transport in their original state [148]. The special sunflower-shaped

collapsible structure made by the ILC Dover Company and the Jet Propulsion

Laboratory used SMPC in the rims as supporting structures of the antennas and will

hopefully meet this challenge. Moreover, Hayes et al. made the original shape of an

origami structure antenna using pre-stretched phenylethylene, proving that it was

possible to install planar metal foils into 3D antennas [149]. The ILC Dover Company

and Folded Structures Company jointly designed and constructed an inflating light

secondary space structure, which could be used as an extraterrestrial living cabin

[150] (Figure 7.2(c)). Its supporting skeleton was made of SMPC, and its covering

consisted of a periodic highly compressible origami structure, which provided good

expansion effects in ground tests. Leng et al. used an SMPC truss arm to design and

construct a deployable cubic frame with a high compression rate, which is a basic

structural component to produce a large subsystem for space deployment. [151]

(Figure 7.2(d)). Experiments showed that satisfactory shape recovery could still be

achieved after three repetitions of compression and expansion.

(a)

(b)

(c)

(d)

Figure 7.2. Large deployable shape memory polymer composite structure. (a)

Inflatable antennas based on an SMPC skeleton structure and (b) SMPC-based

reflector substrate. (c,d) Expandable space accommodation. [144,145,150,151]

In the aerospace field, morphing vehicles have always been a topic of great

focus. The ability of aircrafts to adapt to the active or passive environment, such as

flight conditions involving differences in velocity or airflow, is extremely attractive

[152-155]. The outstanding self-adaptive ability of shape memory polymers has made

them a high-priority choice in making morphing vehicles. Lockheed Martin

Aeronautic Company designed an aircraft with foldable wings, which could change its

aerodynamic properties in a wide range to adapt easily to different mission

environments [156-158]. The skin of the foldable part consists of shape memory

polymers, which ensures that the wings maintain a smooth surface upon expansion.

However, pure polymers, with poor dynamic properties, probably cannot be adapted

to the severe and complex environments of flight. Leng et al. developed the idea of

introducing composite elastic fiber into shape memory polymers to obtain good

strengthening effects without sacrificing deformation ability [159]. Furthermore, this

group showed that introducing deformable skins of composite SMPC tubes into soft

silicon rubber made it convenient to heat the materials through cycling airflows

[160,161].

Shape memory polymers are also applied to morphing vehicles as a type of

strong driving device [148,162,163]. In the search for light structural materials, it is

attractive to replace alloys with shape memory polymers as driving devices. Leng et

al. clearly improved the expansion and windbreak abilities of deformable wings by

adding different strengthening states to shape memory polymers, which could achieve

vertical inflation of the wings to provide greater lift [164] (Figure 7.3(a)). Based on

this work, the group further made vertically bendable wings [165]. Additionally, this

group incorporated steel wire springs into shape memory polymers, which provided

dual functions of strengthening and heating and achieved effective deformations of

the wings [166] (Figure 7.3(c)).

(a)

(b)

(c)

Figure 7.3. Application of shape memory polymers and composites in morphing

aircraft. (a) Morphing aircraft with foldable wings. The skin of the deformable part is

made of SMP. (b) The thickness of the wing consisting of SMPC can be changed to

vary the aerodynamic performance. (c) Steel wire springs are incorporated into the

SMP to provide the dual functions of strengthening and heating. [156,158,164,166]

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