Shape-memory polymer

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Shape-memory polymers (SMPs) are polymeric smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape when induced by an external stimulus (trigger), such as temperature change. [1]

Contents

IUPAC definition

Polymer that, after heating and being subjected to a plastic deformation, resumes its original shape when heated above its glass-transition or melting temperature [2]

Note:

  • Crystalline trans-polyisoprene is an example of a shape-memory polymer.

Properties of shape-memory polymers

SMPs can retain two or sometimes three shapes, and the transition between those is often induced by temperature change. In addition to temperature change, the shape change of SMPs can also be triggered by an electric or magnetic field, [3] light [4] or solution. [5] Like polymers in general, SMPs cover a wide range of properties from stable to biodegradable, from soft to hard, and from elastic to rigid, depending on the structural units that constitute the SMP. SMPs include thermoplastic and thermoset (covalently cross-linked) polymeric materials. SMPs are known to be able to store up to three different shapes in memory. [6] SMPs have demonstrated recoverable strains of above 800%. [7]

Two important quantities that are used to describe shape-memory effects are the strain recovery rate (Rr) and strain fixity rate (Rf). The strain recovery rate describes the ability of the material to memorize its permanent shape, while the strain fixity rate describes the ability of switching segments to fix the mechanical deformation.

Result of the cyclic thermomechanical test Wiki thermomechanical test.jpg
Result of the cyclic thermomechanical test

where is the cycle number, is the maximum strain imposed on the material, and and are the strains of the sample in two successive cycles in the stress-free state before yield stress is applied.

Shape-memory effect can be described briefly as the following mathematical model: [8]

where is the glassy modulus, is the rubbery modulus, is viscous flow strain and is strain for .

Triple-shape memory

While most traditional shape-memory polymers can only hold a permanent and temporary shape, recent technological advances have allowed the introduction of triple-shape-memory materials. Much as a traditional double-shape-memory polymer will change from a temporary shape back to a permanent shape at a particular temperature, triple-shape-memory polymers will switch from one temporary shape to another at the first transition temperature, and then back to the permanent shape at another, higher activation temperature. This is usually achieved by combining two double-shape-memory polymers with different glass transition temperatures [9] or when heating a programmed shape-memory polymer first above the glass transition temperature and then above the melting transition temperature of the switching segment. [10] [11]

Description of the thermally induced shape-memory effect

A schematic representation of the shape-memory effect SMProcess.jpg
A schematic representation of the shape-memory effect

Polymers exhibiting a shape-memory effect have both a visible, current (temporary) form and a stored (permanent) form. Once the latter has been manufactured by conventional methods, the material is changed into another, temporary form by processing through heating, deformation, and finally, cooling. The polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus. The secret behind these materials lies in their molecular network structure, which contains at least two separate phases. The phase showing the highest thermal transition, Tperm, is the temperature that must be exceeded to establish the physical crosslinks responsible for the permanent shape. The switching segments, on the other hand, are the segments with the ability to soften past a certain transition temperature (Ttrans) and are responsible for the temporary shape. In some cases this is the glass transition temperature (Tg) and others the melting temperature (Tm). Exceeding Ttrans (while remaining below Tperm) activates the switching by softening these switching segments and thereby allowing the material to resume its original (permanent) form. Below Ttrans, flexibility of the segments is at least partly limited. If Tm is chosen for programming the SMP, strain-induced crystallization of the switching segment can be initiated when it is stretched above Tm and subsequently cooled below Tm. These crystallites form covalent netpoints which prevent the polymer from reforming its usual coiled structure. The hard to soft segment ratio is often between 5/95 and 95/5, but ideally this ratio is between 20/80 and 80/20. [12] The shape-memory polymers are effectively viscoelastic and many models and analysis methods exist.

Thermodynamics of the shape-memory effect

In the amorphous state, polymer chains assume a completely random distribution within the matrix. W represents the probability of a strongly coiled conformation, which is the conformation with maximum entropy, and is the most likely state for an amorphous linear polymer chain. This relationship is represented mathematically by Boltzmann's entropy formula S = k ln W, where S is the entropy and k is the Boltzmann constant.

In the transition from the glassy state to a rubber-elastic state by thermal activation, the rotations around segment bonds become increasingly unimpeded. This allows chains to assume other possibly, energetically equivalent conformations with a small amount of disentangling. As a result, the majority of SMPs will form compact, random coils because this conformation is entropically favored over a stretched conformation. [1]

Polymers in this elastic state with number average molecular weight greater than 20,000 stretch in the direction of an applied external force. If the force is applied for a short time, the entanglement of polymer chains with their neighbors will prevent large movement of the chain and the sample recovers its original conformation upon removal of the force. If the force is applied for a longer period of time, however, a relaxation process takes place whereby a plastic, irreversible deformation of the sample takes place due to the slipping and disentangling of the polymer chains. [1]

To prevent the slipping and flow of polymer chains, cross-linking can be used, both chemical and physical.

Physically crosslinked SMPs

Linear block copolymers

Representative shape-memory polymers in this category are polyurethanes, [13] [14] polyurethanes with ionic or mesogenic components made by prepolymer method. Other block copolymers also show the shape-memory effect, such as, block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran.

Other thermoplastic polymers

A linear, amorphous polynorbornene (Norsorex, developed by CdF Chemie/Nippon Zeon) or organic-inorganic hybrid polymers consisting of polynorbornene units that are partially substituted by polyhedral oligosilsesquioxane (POSS) also have shape-memory effect.

Wiki norbornene.jpg

Another example reported in the literature is a copolymer consisting of polycyclooctene (PCOE) and poly(5-norbornene-exo,exo-2,3-dicarboxylic anhydride) (PNBEDCA), which was synthesized through ring-opening metathesis polymerization (ROMP). Then the obtained copolymer P(COE-co-NBEDCA) was readily modified by grafting reaction of NBEDCA units with polyhedral oligomeric silsesquioxanes (POSS) to afford a functionalized copolymer P(COE-co-NBEDCA-g-POSS). It exhibits shape-memory effect. [15]

Chemically crosslinked SMPs

The main limitation of physically crosslinked polymers for the shape-memory application is irreversible deformation during memory programming due to the creep. The network polymer can be synthesized by either polymerization with multifunctional (3 or more) crosslinker or by subsequent crosslinking of a linear or branched polymer. They form insoluble materials which swell in certain solvents. [1]

Crosslinked polyurethane

This material can be made by using excess diisocyanate or by using a crosslinker such as glycerin, trimethylol propane. Introduction of covalent crosslinking improves in creep, increase in recovery temperature and recovery window. [16]

PEO based crosslinked SMPs

The PEO-PET block copolymers can be crosslinked by using maleic anhydride, glycerin or dimethyl 5-isophthalates as a crosslinking agent. The addition of 1.5 wt% maleic anhydride increased in shape recovery from 35% to 65% and tensile strength from 3 to 5 MPa. [17]

Hard phaseCrosslinkerTr (°C)Rf(5)(%)Rf(5)(%)
PETGlycerol/dimethyl 5-sulfoisophthalate11–3090–9560–70
PETMaleic anhydride8–1391–9360
AA/MAA copolymerN,N'-methylene-bis-acrylamide9099
MAA/N-vinyl-2-pyrrolidoneEthyleneglycol dimethacrylate9099
PMMA/N-vinyl-2-pyrrolidoneEthyleneglycol dimethacrylate45, 10099

Thermoplastic shape-memory

While shape-memory effects are traditionally limited to thermosetting plastics, some thermoplastic polymers, most notably PEEK, can be used as well. [18]

Light-induced SMPs

A schematic representation of reversible LASMP crosslinking Lightinduced.jpg
A schematic representation of reversible LASMP crosslinking

Light-activated shape-memory polymers (LASMP) use processes of photo-crosslinking and photo-cleaving to change Tg. Photo-crosslinking is achieved by using one wavelength of light, while a second wavelength of light reversibly cleaves the photo-crosslinked bonds. The effect achieved is that the material may be reversibly switched between an elastomer and a rigid polymer. Light does not change the temperature, only the cross-linking density within the material. [19] For example, it has been reported that polymers containing cinnamic groups can be fixed into predetermined shapes by UV light illumination (> 260 nm) and then recover their original shape when exposed to UV light of a different wavelength (< 260 nm). [19] Examples of photoresponsive switches include cinnamic acid and cinnamylidene acetic acid.

Electro-active SMPs

The use of electricity to activate the shape-memory effect of polymers is desirable for applications where it would not be possible to use heat and is another active area of research. Some current efforts use conducting SMP composites with carbon nanotubes, [20] short carbon fibers (SCFs), [21] [22] carbon black, [23] or metallic Ni powder. These conducting SMPs are produced by chemically surface-modifying multi-walled carbon nanotubes (MWNTs) in a mixed solvent of nitric acid and sulfuric acid, with the purpose of improving the interfacial bonding between the polymers and the conductive fillers. The shape-memory effect in these types of SMPs have been shown to be dependent on the filler content and the degree of surface modification of the MWNTs, with the surface modified versions exhibiting good energy conversion efficiency and improved mechanical properties.

Another technique being investigated involves the use of surface-modified super-paramagnetic nanoparticles. When introduced into the polymer matrix, remote actuation of shape transitions is possible. An example of this involves the use of oligo (e-caprolactone)dimethacrylate/butyl acrylate composite with between 2 and 12% magnetite nanoparticles. Nickel and hybrid fibers have also been used with some degree of success. [21]

Shape-memory polymers vs. shape-memory alloys

A summary of the major differences between SMPs and SMAs [24]
SMPsSMAs
Density (g/cm3)0.9–1.26–8
Extent of
deformation
up to 800%<8%
Required stress
for deformation (MPa)
1–350–200
Stress generated
upon recovery (MPa)
1–3150–300
Transition
temperatures (°C)
−10..100−10..100
Recovery speed1s –
minutes
<1s
Processing
conditions
<200 °C
low
pressure
>1000 °C
high
pressure
Costs<$10/lb~$250/lb

Shape-memory polymers differ from shape memory alloys (SMAs) [25] by their glass transition or melting transition from a hard to a soft phase which is responsible for the shape-memory effect. In shape-memory alloys martensitic/austenitic transitions are responsible for the shape-memory effect. There are numerous advantages that make SMPs more attractive than shape memory alloys. They have a high capacity for elastic deformation (up to 200% in most cases), much lower cost, lower density, a broad range of application temperatures which can be tailored, easy processing, potential biocompatibility and biodegradability, [24] and probably exhibit superior mechanical properties to those of SMAs. [26]

Applications

Industrial applications

One of the first conceived industrial applications was in robotics where shape-memory (SM) foams were used to provide initial soft pretension in gripping. [27] These SM foams could be subsequently hardened by cooling, making a shape adaptive grip. Since this time, the materials have seen widespread usage in, for example, the building industry (foam which expands with warmth to seal window frames), sports wear (helmets, judo and karate suits) and in some cases with thermochromic additives for ease of thermal profile observation. [28] Polyurethane SMPs are also applied as an autochoke element for engines. [29]

Application in photonics

One field in which SMPs are having a significant impact is photonics. Due to the shape changing capability, SMPs enable the production of functional and responsive photonic gratings. [30] By using modern soft lithography techniques such as replica molding, it is possible to imprint periodic nanostructures, with sizes of the order of magnitude of visible light, onto the surface of shape memory polymeric blocks. As a result of the refractive index periodicity, these systems diffract light. By taking advantage of the polymer's shape memory effect, it is possible to reprogram the lattice parameter of the structure and consequently tune its diffractive behavior. Another application of SMPs in photonics is shape changing random lasers. [31] By doping SMPs with highly scattering particles such as titania it is possible to tune the light transport properties of the composite. Additionally, optical gain may be introduced by adding a molecular dye to the material. By configuring both the amount of scatters and of the organic dye, a light amplification regime may be observed when the composites are optically pumped. Shape memory polymers have also been used in conjunction with nanocellulose to fabricate composites exhibiting both chiroptical properties and thermo-activated shape memory effect. [32]

Medical applications

Most medical applications of SMP have yet to be developed, but devices with SMP are now beginning to hit the market. Recently, this technology has expanded to applications in orthopedic surgery. [18] Additionally, SMPs are now being used in various ophthalmic devices including punctal plugs, glaucoma shunts and intraocular lenses.

Potential medical applications

SMPs are smart materials with potential applications as, e.g., intravenous cannula, [29] self-adjusting orthodontic wires and selectively pliable tools for small scale surgical procedures where currently metal-based shape-memory alloys such as Nitinol are widely used. Another application of SMP in the medical field could be its use in implants: for example minimally invasive, through small incisions or natural orifices, implantation of a device in its small temporary shape. Shape-memory technologies have shown great promise for cardiovascular stents, since they allow a small stent to be inserted along a vein or artery and then expanded to prop it open. [33] After activating the shape memory by temperature increase or mechanical stress, it would assume its permanent shape. Certain classes of shape-memory polymers possess an additional property: biodegradability. This offers the option to develop temporary implants. In the case of biodegradable polymers, after the implant has fulfilled its intended use, e.g. healing/tissue regeneration has occurred, the material degrades into substances which can be eliminated by the body. Thus full functionality would be restored without the necessity for a second surgery to remove the implant. [34] Examples of this development are vascular stents and surgical sutures. When used in surgical sutures, the shape-memory property of SMPs enables wound closure with self-adjusting optimal tension, which avoids tissue damage due to overtightened sutures and does support healing and regeneration. [35] SMPs have also potential for use as compression garments [36] and hands-free door openers, whereby the latter can be produced via so-called 4D printing. [37]

Potential industrial applications

Further potential applications include self-repairing structural components, such as e.g. automobile fenders in which dents are repaired by application of temperature. [38] After an undesired deformation, such as a dent in the fender, these materials "remember" their original shape. Heating them activates their "memory". In the example of the dent, the fender could be repaired with a heat source, such as a hair-dryer. The impact results in a temporary form, which changes back to the original form upon heating—in effect, the plastic repairs itself. SMPs may also be useful in the production of aircraft which would morph during flight. Currently, the Defense Advanced Research Projects Agency DARPA is testing wings which would change shape by 150%. [6]

The realization of a better control over the switching behavior of polymers is seen as key factor to implement new technical concepts. For instance, an accurate setting of the onset temperature of shape recovering can be exploited to tune the release temperature of information stored in a shape memory polymer. This may pave the way for the monitoring of temperature abuses of food or pharmaceuticals. [39]

Recently, a new manufacturing process, mnemosynation, was developed at Georgia Tech to enable mass production of crosslinked SMP devices, which would otherwise be cost-prohibitive using traditional thermoset polymerization techniques. [40] Mnemosynation was named for the Greek goddess of memory, Mnemosyne, and is the controlled imparting of memory on an amorphous thermoplastic materials utilizing radiation-induced covalent crosslinking, much like vulcanization imparts recoverable elastomeric behavior on rubbers using sulfur crosslinks. Mnemosynation combines advances in ionizing radiation and tuning the mechanical properties of SMPs to enable traditional plastics processing (extrusion, blow molding, injection molding, resin transfer molding, etc.) and allows thermoset SMPs in complex geometries. The customizable mechanical properties of traditional SMPs are achievable with high throughput plastics processing techniques to enable mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable glass transition temperatures.

Brand protection and anti-counterfeiting

Shape memory polymers may serve as technology platform for a safe way of information storage and release. [41] Overt anti-counterfeiting labels have been constructed that display a visual symbol or code when exposed to specific chemicals. [42] Multifunctional labels may even make counterfeiting increasingly difficult. [43] [44] Shape memory polymers have already been made into shape memory film by extruder machine, with covert and overt 3D embossed pattern internally, and 3D pattern will be released to be embossed or disappeared in just seconds irreversibly as soon as it is heated; Shape memory film can be used as label substrates or face stock for anti-counterfeiting, brand protection, tamper-evident seals, anti-pilferage seals, etc.

Multifunctional composites

Using shape memory polymers as matrices, multifunctional composite materials can be produced. Such composites can have temperature dependant shape morphing (i.e. shape memory) characteristics. [45] [46] This phenomenon allows these composites to be potentially used to create deployable structures [47] such as booms, [48] hinges, [49] wings [50] [51] etc. While using SMPs can help produce one-way shape morphing structures, it has been reported that using SMPs in combination with shape memory alloys allows creation of more complex shape memory composites that is capable of two-way shape memory deformation. [52]

See also

Related Research Articles

<span class="mw-page-title-main">Polymer</span> Substance composed of macromolecules with repeating structural units

A polymer is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass, relative to small molecule compounds, produces unique physical properties including toughness, high elasticity, viscoelasticity, and a tendency to form amorphous and semicrystalline structures rather than crystals.

<span class="mw-page-title-main">Polyurethane</span> Polymer composed of a chain of organic units joined by carbamate (urethane) links

Polyurethane refers to a class of polymers composed of organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane is produced from a wide range of starting materials. This chemical variety produces polyurethanes with different chemical structures leading to many different applications. These include rigid and flexible foams, and coatings, adhesives, electrical potting compounds, and fibers such as spandex and polyurethane laminate (PUL). Foams are the largest application accounting for 67% of all polyurethane produced in 2016.

<span class="mw-page-title-main">Polyethylene</span> Most common thermoplastic polymer

Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bottles, etc.). As of 2017, over 100 million tonnes of polyethylene resins are being produced annually, accounting for 34% of the total plastics market.

<span class="mw-page-title-main">Composite material</span> Material made from a combination of two or more unlike substances

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions. Composite materials with more than one distinct layer are called composite laminates.

<span class="mw-page-title-main">Young's modulus</span> Mechanical property that measures stiffness of a solid material

Young's modulus is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Young's modulus is defined as the ratio of the stress applied to the object and the resulting axial strain in the linear elastic region of the material.

<span class="mw-page-title-main">Thermosetting polymer</span> Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Polymer chemistry is a sub-discipline of chemistry that focuses on the structures of chemicals, chemical synthesis, and chemical and physical properties of polymers and macromolecules. The principles and methods used within polymer chemistry are also applicable through a wide range of other chemistry sub-disciplines like organic chemistry, analytical chemistry, and physical chemistry. Many materials have polymeric structures, from fully inorganic metals and ceramics to DNA and other biological molecules. However, polymer chemistry is typically related to synthetic and organic compositions. Synthetic polymers are ubiquitous in commercial materials and products in everyday use, such as plastics, and rubbers, and are major components of composite materials. Polymer chemistry can also be included in the broader fields of polymer science or even nanotechnology, both of which can be described as encompassing polymer physics and polymer engineering.

<span class="mw-page-title-main">Creep (deformation)</span> Tendency of a solid material to move slowly or deform permanently under mechanical stress

In materials science, creep is the tendency of a solid material to undergo slow deformation while subject to persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist both shear flow and strain linearly with time when a stress is applied. Elastic materials strain when stretched and immediately return to their original state once the stress is removed.

<span class="mw-page-title-main">Copolymer</span> Polymer derived from more than one species of monomer

In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are sometimes called bipolymers. Those obtained from three and four monomers are called terpolymers and quaterpolymers, respectively. Copolymers can be characterized by a variety of techniques such as NMR spectroscopy and size-exclusion chromatography to determine the molecular size, weight, properties, and composition of the material.

<span class="mw-page-title-main">Elastomer</span> Polymer with rubber-like elastic properties

An elastomer is a polymer with viscoelasticity and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of elastic polymer, is often used interchangeably with rubber, although the latter is preferred when referring to vulcanisates. Each of the monomers which link to form the polymer is usually a compound of several elements among carbon, hydrogen, oxygen and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature, so that considerable molecular reconformation is feasible without breaking of covalent bonds. At ambient temperatures, such rubbers are thus relatively compliant and deformable.

<span class="mw-page-title-main">Hydrogel</span> Soft water-rich polymer gel

A hydrogel is a biphasic material, a mixture of porous, permeable solids and at least 10% by weight or volume of interstitial fluid composed completely or mainly by water. In hydrogels the porous permeable solid is a water insoluble three dimensional network of natural or synthetic polymers and a fluid, having absorbed a large amount of water or biological fluids. These properties underpin several applications, especially in the biomedical area. Many hydrogels are synthetic, but some are derived from nature. The term 'hydrogel' was coined in 1894.

Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers (TPR), are a class of copolymers or a physical mix of polymers that consist of materials with both thermoplastic and elastomeric properties.

<span class="mw-page-title-main">Self-healing material</span> Substances that can repair themselves

Self-healing materials are artificial or synthetically created substances that have the built-in ability to automatically repair damages to themselves without any external diagnosis of the problem or human intervention. Generally, materials will degrade over time due to fatigue, environmental conditions, or damage incurred during operation. Cracks and other types of damage on a microscopic level have been shown to change thermal, electrical, and acoustical properties of materials, and the propagation of cracks can lead to eventual failure of the material. In general, cracks are hard to detect at an early stage, and manual intervention is required for periodic inspections and repairs. In contrast, self-healing materials counter degradation through the initiation of a repair mechanism that responds to the micro-damage. Some self-healing materials are classed as smart structures, and can adapt to various environmental conditions according to their sensing and actuation properties.

Rubber toughening is a process in which rubber nanoparticles are interspersed within a polymer matrix to increase the mechanical robustness, or toughness, of the material. By "toughening" a polymer it is meant that the ability of the polymeric substance to absorb energy and plastically deform without fracture is increased. Considering the significant advantages in mechanical properties that rubber toughening offers, most major thermoplastics are available in rubber-toughened versions; for many engineering applications, material toughness is a deciding factor in final material selection.

<span class="mw-page-title-main">Ferroelectric polymer</span> Group of crystalline polar polymers that are also ferroelectric

Ferroelectric polymers are a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field.

<span class="mw-page-title-main">Soft robotics</span> Subfield of robotics

Soft robotics is a subfield of robotics that concerns the design, control, and fabrication of robots composed of compliant materials, instead of rigid links. In contrast to rigid-bodied robots built from metals, ceramics and hard plastics, the compliance of soft robots can improve their safety when working in close contact with humans.

4-dimensional printing uses the same techniques of 3D printing through computer-programmed deposition of material in successive layers to create a three-dimensional object. However, in 4D printing, the resulting 3D shape is able to morph into different forms in response to environmental stimulus, with the 4th dimension being the time-dependent shape change after the printing. It is therefore a type of programmable matter, wherein after the fabrication process, the printed product reacts with parameters within the environment and changes its form accordingly.

<span class="mw-page-title-main">Inverse vulcanization</span>

Inverse vulcanization is a process that produces polysulfide polymers, which also contain some organic linkers. In contrast, sulfur vulcanization produces material that is predominantly organic but has a small percentage of polysulfide crosslinks.

<span class="mw-page-title-main">Thermally induced shape-memory effect (polymers)</span>

The thermally induced unidirectional shape-shape-memory effect is an effect classified within the new so-called smart materials. Polymers with thermally induced shape-memory effect are new materials, whose applications are recently being studied in different fields of science, communications and entertainment.

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