1. What SMPs and LCEs are (the short version)

• An SMP is a polymer whose network with net points switches between “mobile” and “frozen” with temperature: a crosslinked network has net points (which fix the permanent shape) and switching domains that soften at the glass transition temperature Tg or the melting point Tm. The material is deformed while soft, fixed by cooling, and on reheating returns to its original shape through the entropic elasticity of the network. Most change one way Sourced [Ref. 9]
• An LCE is a network that combines liquid-crystal order with rubber elasticity: when the alignment of its units (mesogens) is disturbed by heating or other stimuli, it contracts along the alignment direction and expands across it. When the stimulus stops, it returns: the deformation is reversible Sourced [Ref. 9, 8]
• Read the numbers with their definitions and conditions: shape recovery changes with “which cycle”, Tg with “which measurement method”, and LCE strain with “which alignment, crosslinking and temperature”. For both SMPs and LCEs, long-term life is not well established Sourced [Ref. 1, 9]

2. How an SMP works — a network with net points, and the temperature at which it switches

An SMP has a network joined at net points (chemical crosslinks and the like), and its chains switch between “mobile” and “frozen” with temperature. The review by Hasan et al. describes an SMP as a crosslinked network with permanent net-points and distinct switching domains (for example segments above Tg or Tm). The material is deformed into a temporary shape when the switching domain is soft, fixed upon cooling, and then recovers its original shape upon reheating thanks to the stored entropic elasticity of the network Sourced [Ref. 9].

View the original SVG schematic and exact annotations

Heat is not the only stimulus. The introduction of Staszczak et al. lists heat, light, magnetic fields, solvents, moisture, lasers and microwave heating, says that heat is the most frequent stimulus, and notes that for a thermo-responsive SMP Tg serves as the activation temperature. Below Tg the frozen chain motion gives a high elastic modulus; above Tg the modulus falls dramatically, internal stresses are released and the shape recovers Sourced [Ref. 1]. For light, Lendlein et al. reported polymers containing cinnamic groups that can be deformed and fixed by ultraviolet illumination and can recover their original shape at ambient temperature under ultraviolet light of a different wavelength; the fixed shapes were stable even when heated to 50 °C Sourced [Ref. 3].

The number of temporary shapes can also be designed. Xie notes that known SMPs memorize one or two temporary shapes (dual- and triple-shape memory) and that the maximum number correlates with the number of discrete reversible phase transitions in the polymer. He then reports multi-shape memory in a perfluorosulphonic acid ionomer that has only one broad transition Sourced [Ref. 4].

3. Evaluating SMPs — shape fixity, shape recovery and recovery force

The performance of an SMP is described mainly by three quantities: the shape fixity R_f (how well the temporary shape is held), the shape recovery R_r (how fully the permanent shape returns) and the recovery force (recovery stress) (the force it exerts when trying to return). These are not constants of the material but results of a “thermomechanical cycle” with a defined way of deforming, temperatures and number of cycles.

Staszczak et al. used the following program on a thermoset polyurethane SMP (MP4510; the producer’s stated Tg is 45 °C). (I) Tension at Th = 65 °C (Tg + 20 °C) at a strain rate of 10⁻³ s⁻¹ up to a maximum strain ε_m = 20%; (II) cooling to Tl = 25 °C (Tg − 20 °C) at 7 °C/min while holding the strain, to fix the shape; (III) unloading at Tl; (IV) reheating with no load to Th at 4 °C/min. From the strain after unloading, ε_un, and the strain left after reheating, ε_ir, R_f and R_r are obtained (the definitions of Tobushi and Hayashi) Sourced [Ref. 1].

For five specimens in one cycle, the mean was about 98% for R_f and about 93% for R_r. In three consecutive cycles R_f stayed at about 98%, while R_r was about 90% in the first cycle and above 99% in the third. The authors attribute the good repeatability to the stability of the chemical crosslinks of a thermoset, and describe a “thermomechanical training” effect after the first cycle. They also note that an ideal SMP has R_f and R_r near 100%, but that even a thermoset with chemical crosslinks does not fix or recover perfectly Sourced [Ref. 1].

How Tg was measured

The same paper measured the Tg of the same sample by two methods. The peak of the loss tangent tan δ in dynamic mechanical analysis (DMA; bending, 1 Hz, 2 °C/min) was about 45 °C, in agreement with the producer’s value. In the first heating of differential scanning calorimetry (DSC; about 10 mg, −10 to 210 °C, 10 °C/min) a step change appeared at 34.82 °C, so Tg was about 35 °C. The authors explain that the two values are not contradictory: DMA is a mechanical test and DSC measures the change in heat capacity of a static sample, the heating rates differ, Tg is a temperature range, and its value depends on the method Sourced [Ref. 1]. The DMA storage modulus was E′_g = 1250 MPa in the glassy region and E′_r = 12.1 MPa in the rubbery region, a ratio of about 100 (103). The paper says that a sharp transition from glassy to rubbery needs a difference of at least two orders of magnitude in modulus [Ref. 1].

Recovery force: newtons or megapascals?

Fulati et al. studied the recovery force of blends of four-branched poly(ε-caprolactone)-based SMPs while varying crystallinity and crosslinking density. The mechanical properties varied over a wide range along the crystallinity gradient (stretchability 50.5–1419.5%, toughness 0.62–130.4 MJ/m³) while shape-memory properties were maintained. The recovery force was affected by crosslinking density and correlated with the energy-storage capacity of the SMP. For the long-chain 4b100PCL alone the force was 0.061 N at 50% strain and 0.894 N at 800% strain, and with 30% of short-chain polymer it was 0.162 N at 50% strain (an increase of about 166%) Sourced [Ref. 2].

4. How an LCE works — liquid-crystal order and rubber elasticity

The review by Herbert et al. describes liquid crystalline elastomers (LCEs) as crosslinked polymer networks that combine the elastic properties of rubber with the anisotropic properties of liquid crystals. Their key advantage is that multifunctionality and responsivity can be programmed by patterning the local orientation, and the review surveys approaches to alignment: mechanical, surface-enforced, field-induced and rheological Sourced [Ref. 8]. The review by White and Broer details the thermally and photogenerated macroscale mechanical responses of liquid crystalline polymer networks and elastomers (bending, twisting and buckling) and the control of local topography [Ref. 7].

View the original SVG schematic and exact annotations

Hasan et al. summarize the mechanism as follows. In a monodomain LCE in its nematic state the polymer chains adopt an elongated, prolate-ellipsoid conformation. When a stimulus such as heat disrupts the orientational order of the mesogens, the chains become more disordered and spherical, and macroscopically the material contracts along the director and expands perpendicular to it. When the stimulus is removed, the elastic memory of the crosslinked network re-establishes the original network configuration and mesogen order. The reversible strain can be substantial, with up to 400% reported for thermally responsive LCEs (a cited value), whereas more densely crosslinked glassy liquid crystal networks (LCNs) have actuation strains of 5% or less Sourced [Ref. 9].

Ware et al. reported LCEs in which the director is written in local volume elements (voxels) as small as 0.0005 mm³. The director controls the inherent local mechanical response (55% strain), and flat sheets with a patterned director transform into three-dimensional objects under thermal or chemical stimuli Sourced [Ref. 6].

5. Forming — 3D and 4D printing

SMPs and LCEs attract attention as materials for “4D printing” because the printed part changes shape over time. Hasan et al. distinguish 3D printing, which creates a shape, from 4D printing, which gives a time-dependent, programmable shape change Sourced [Ref. 9].

(1) SMPs: several SMPs in one part from photo-curable resins

Ge et al. reported a way of printing high-resolution (up to a few microns) multimaterial SMP architectures by projection microstereolithography (PμSL). Using a family of photo-curable methacrylate-based copolymer networks, they designed the rubbery modulus, the glass transition temperature and a failure strain of more than 300%, larger than that of any existing printable material. An automated material exchange builds composite architectures, and simulations of local deformation, shape fixity and free recovery rate agreed well with experiments Sourced [Ref. 5].

(2) LCEs: building the alignment in during printing

In LCEs the alignment is fixed during forming. Hasan et al. say that direct ink writing (DIW) and vat photopolymerization are the most widely adopted methods, and that ink chemistry, rheology, curing and printing parameters directly govern mesogen alignment and actuation performance. Thiol-Michael chemistry is important because it gives precise control of the nematic-to-isotropic transition temperature (T_NI), crosslink density and actuation performance, and printing in the mesophase, 10–20 °C below T_NI, gives higher order parameters and larger actuation strains Sourced [Ref. 9].

6. Cycle life and speed — use in soft robots

As far as we could confirm, the long-term life of neither SMPs nor LCEs is well established. Hasan et al. list biocompatibility, long-term durability and manufacturing scalability as challenges for LCEs, and state that lightly crosslinked networks are prone to fatigue and many actuators operate for fewer than 100 cycles (a cited report). Raising the crosslink density suppresses swelling but reduces actuation strain Sourced [Ref. 9].

We set out the examples the same review presents, with their conditions. An electrically driven LCE actuator bent 162° within 30 s and was stable over 50 cycles; a 9.87 g robot driven by it crawled at 0.28 body lengths per minute and lifted 1.5 times its weight (a cited report). In another example the bend was 320° at 2.5 V, with a power consumption of 620 mW/cm². A response time of 20 s and a curvature of 0.25 mm⁻¹ are also reported. The review concludes that improvements are needed in speed, efficiency, fatigue life, integrated power and sensing, and 3D programming Sourced [Ref. 9].

7. Telling SMPs, LCEs and shape memory alloys apart

All three materials produce a change of shape, but their mechanisms and the way their numbers should be read differ. The table lists the values we could confirm with their sources and conditions. They come from different samples and test conditions, so they are not ranked.

All Sourced (statements of each source; “same explainer” refers to another article of this series, where the sources are given). The arrangement of the table is our commentary. Values come from different samples and conditions and cannot be ranked.

8. Limits, and what we could not confirm

(1) Recovery stress, long-term life and output

We could not show, from the primary sources we confirmed, the recovery stress of SMPs (in MPa), the long-term cycle life of SMPs and LCEs, or the output (work) density of LCEs. The recovery force (N) comes only from Fulati et al. and we did not convert it to stress.

(2) How the sources were read

We read the full texts of Staszczak et al. and Hasan et al. For Fulati et al. we read the abstract and part of the results; for Ware et al., Lendlein et al., Xie, White and Broer, and Herbert et al., the abstracts on publisher or bibliographic database pages; and for Ge et al., the abstract only. We have not checked the original papers for the values that Hasan et al. cite (400%, under 100 cycles, the electrically driven examples).

(3) The text of the standards

We have not read the text of ISO 11357-2, ISO 6721-11, ISO 527-1, ISO 37 or JIS K 7121, K 7122, K 7244, K 7161-1, K 6251 and K 7127. The ASTM International website states that AI use is prohibited, so we did not open any ASTM site; ASTM standards are listed only where the number and title could be confirmed in an instrument maker’s page (Hitachi High-Tech) or the FDA database.

(4) Standards specific to SMPs and LCEs

A search of the ISO catalogue (10 October 2026) found no standard for “shape memory polymer”. In a JISC search by title, the only JIS containing the Japanese term for “shape memory” are the six metals standards (H 7xxx), and in the FDA database the standards containing “shape memory” are nickel-titanium ones. We could not confirm any IEC or IEEE standard, or any standard that targets LCEs.

(5) Biocompatibility and uses

The biocompatibility of LCEs is listed as a challenge by Hasan et al. and is outside this article. We say nothing about the safety or effectiveness of SMPs in medical uses.

(6) Product names and companies

MP4510 is the name of the sample of Staszczak et al. We do not rank any product or company.

9. How to read evaluations and standards

The evaluation of SMPs and LCEs divides into (a) switching temperatures (Tg, Tm, T_NI), (b) modulus and tensile properties, (c) shape fixity, shape recovery, recovery force and actuation strain, and (d) behavior under repetition. For (a) and (b), general standards for thermal analysis and mechanical testing of polymers can be used. We could not confirm standards that directly specify (c) and (d), so each paper uses the researchers’ own definitions (such as the formulas of Tobushi and Hayashi) and test program.

Table of corresponding standards

JIS titles are our translations from the Japanese.

Numbers, titles and editions were confirmed as follows: ISO on the ISO catalogue pages; JIS in the JISC database; ASTM in the instrument maker’s list of thermal-analysis standards (Hitachi High-Tech) and in the FDA database of recognized consensus standards; we did not open any ASTM site [Ref. 10, 18, 11, 12, 13, 14, 19, 16, 15]. The ASTM editions are those given in those sources and are not necessarily the latest. The arrangement of the table is our commentary.

The number, title, edition and date of each standard are Sourced (confirmed on the official pages of the standards bodies, the Japanese Industrial Standards Committee, an instrument maker and the FDA). Which standard goes in which cell is our own arrangement (Our calculation). We found no revision or draft in progress, so no row falls under Not yet confirmed.

Cautions when reading standards

1. For quantities without a dedicated standard, read the paper’s definition. Many papers use the definition of Tobushi and Hayashi for shape fixity and shape recovery, but the details of the formula (one cycle or the N-th cycle) can differ from paper to paper. Staszczak et al. themselves use separate formulas for a single cycle and for consecutive cycles [Ref. 1].
2. State the measurement method and heating rate next to a Tg value. ISO 6721-11 specifies methods for determining Tg from a heating scan of dynamic mechanical properties [Ref. 11], and ISO 11357-2 specifies how to determine the glass transition temperature and step height by DSC [Ref. 10]. They measure different quantities and need not give the same value.
3. Storage and humidity before the test lie outside the standards. Because moisture lowers the Tg and shape fixity of an SMP, Staszczak et al. controlled humidity [Ref. 1]. Even if a standard is followed, a value cannot be compared with others unless drying and storage before the test are stated (our commentary).
4. A standard is a “method”, not a guarantee of material performance. Read a manufacturer’s value together with the standard, edition and test conditions.

10. Glossary

11. References (primary sources)

1. Staszczak M. et al. “Characterization of Polyurethane Shape Memory Polymer and Determination of Shape Fixity and Shape Recovery in Subsequent Thermomechanical Cycles”, Polymers 14, 4775 (2022), DOI 10.3390/polym14214775 (open access; we read the full text) — doi.org/10.3390/polym14214775
2. Fulati A. et al. “Influences of Crystallinity and Crosslinking Density on the Shape Recovery Force in Poly(ε-Caprolactone)-Based Shape-Memory Polymer Blends”, Polymers 14, 4740 (2022), DOI 10.3390/polym14214740 (open access) — doi.org/10.3390/polym14214740
3. Lendlein A. et al. “Light-induced shape-memory polymers”, Nature 434, 879–882 (2005), DOI 10.1038/nature03496 — doi.org/10.1038/nature03496
4. Xie T. “Tunable polymer multi-shape memory effect”, Nature 464, 267–270 (2010), DOI 10.1038/nature08863 — doi.org/10.1038/nature08863
5. Ge Q. et al. “Multimaterial 4D Printing with Tailorable Shape Memory Polymers”, Sci. Rep. 6, 31110 (2016), DOI 10.1038/srep31110 (open access) — doi.org/10.1038/srep31110
6. Ware T. H. et al. “Voxelated liquid crystal elastomers”, Science 347, 982–984 (2015), DOI 10.1126/science.1261019 — doi.org/10.1126/science.1261019
7. White T. J., Broer D. J. “Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers”, Nat. Mater. 14, 1087–1098 (2015), DOI 10.1038/nmat4433 — doi.org/10.1038/nmat4433
8. Herbert K. M. et al. “Synthesis and alignment of liquid crystalline elastomers”, Nat. Rev. Mater. 7, 23–38 (2022), DOI 10.1038/s41578-021-00359-z — doi.org/10.1038/s41578-021-00359-z
9. Hasan M., Liu Y. “4D-Printed Liquid Crystal Elastomers: Printing Strategies, Actuation Mechanisms, and Emerging Applications”, J. Compos. Sci. 9, 633 (2025), DOI 10.3390/jcs9110633 (open access; we read the text) — doi.org/10.3390/jcs9110633
10. ISO “ISO 11357-2:2020 Plastics — Differential scanning calorimetry (DSC) — Part 2: Determination of glass transition temperature and step height” (ISO catalogue page) — www.iso.org/standard/77310.html
11. ISO “ISO 6721-11:2019 Plastics — Determination of dynamic mechanical properties — Part 11: Glass transition temperature” (ISO catalogue page) — www.iso.org/standard/74988.html
12. ISO “ISO 527-1:2019 Plastics — Determination of tensile properties — Part 1: General principles” (ISO catalogue page) — www.iso.org/standard/527-1
13. ISO “ISO 37:2024 Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties” (ISO catalogue page) — www.iso.org/standard/86892.html
14. Japanese Industrial Standards Committee (JISC) JISC JIS list for K 7244 (test methods for dynamic mechanical properties of plastics). K 7244, K 7161-1, K 6251, K 7121, K 7122 and K 7127 were confirmed (number, title, dates) on the JISC list and detail pages — www.jisc.go.jp/app/jis/general/GnrJISNumberNameSearchList?show&jisS…
15. U.S. Food and Drug Administration (FDA) “Recognized Consensus Standards: Medical Devices” database, search results for titles containing tension testing (including ASTM D412; page last updated 28 September 2026) — www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfStandards/results.cfm?…
16. Hitachi High-Tech “Standards – ISO/ASTM/JIS/IPC”, list of thermal-analysis standards (instrument maker’s technical page, in Japanese) — www.hitachi-hightech.com/jp/ja/knowledge/analytical-systems/thermal…
17. ISO “ISO 11357-1:2023 Plastics — Differential scanning calorimetry (DSC) — Part 1: General principles” (ISO catalogue page) — www.iso.org/standard/83904.html
18. ISO “ISO 11357-3:2025 Plastics — Differential scanning calorimetry (DSC) — Part 3: Determination of temperature and enthalpy of melting and crystallization” (ISO catalogue page) — www.iso.org/standard/88106.html
19. Japanese Industrial Standards Committee (JISC) JIS search (by number or title). JIS Z 2255, C 2501, K 7121, K 7122, R 1611, H 7801, R 3257, R 1703-1, H 7101 and others were confirmed (number, title, dates of establishment and revision) on the JIS list and detail pages — www.jisc.go.jp/app/jis/general/GnrJISSearch.html
20. U.S. Food and Drug Administration (FDA) “Recognized Consensus Standards: Medical Devices” database, search results for titles containing nickel-titanium (database page last updated 28 September 2026) — www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfStandards/results.cfm?…
21. Romančík J. et al. “Shape Memory Alloy Actuators in Robotics”, Actuators 15, 162 (2026), DOI 10.3390/act15030162 (open access) — doi.org/10.3390/act15030162
22. U.S. FDA, Center for Devices and Radiological Health (CDRH) “Strategy to Estimate Low to High Cycle Fatigue Transition of Nitinol for Fatigue to Fracture Test Planning” (Catalog of Regulatory Science Tools) — cdrh-rst.fda.gov/strategy-estimate-low-high-cycle-fatigue-transitio…

12. Claim-to-source audit

Last updated: 10 October 2026. Sources are limited to primary material (peer-reviewed papers, and the databases of standards bodies, the Japanese Industrial Standards Committee and the FDA, and an instrument maker’s page). We have not read the text of the standards. Because the article includes organization of structure and readings of materials design, these are marked as “commentary” and kept apart from sourced facts. We could not confirm, and therefore do not state, the recovery stress of SMPs, the long-term cycle life of SMPs and LCEs, the output density of LCEs, evaluation standards specific to SMPs and LCEs, or the text and test conditions of ASTM standards. The recovery, strain, cycle-count and Tg values are results that specific papers obtained for specific materials and conditions, and cannot be generalized to other materials or products. We give no medical advice and no ranking of any product or company. Figs. 2 to 5 and 7 are vector drawings and Figs. 1 and 6 are AI-generated images; none of them shows an actual micrograph or a real product.