Spontaneous Shape Change Research Articles

Discontinuous Metric Programming in Liquid Crystalline Elastomers.

Liquid crystalline elastomers (LCEs) are shape-changing materials that exhibit large deformations in response to applied stimuli. Local control of the orientation of LCEs spatially directs the deformation of these materials to realize a spontaneous shape change in response to stimuli. Prior approaches to shape programming in LCEs utilize patterning techniques that involve the detailed inscription of spatially varying nematic fields to produce sheets. These patterned sheets deform into elaborate geometries with complex Gaussian curvatures. Here, we present an alternative approach to realize shape-morphing in LCEs where spatial patterning of the crosslink density locally regulates the material deformation magnitude on either side of a prescribed interface curve. We also present a simple mathematical model describing the behavior of these materials. Further experiments coupled with the mathematical model demonstrate the control of the sign of Gaussian curvature, which is used in combination with heat transfer effects to design LCEs that self-clean as a result of temperature-dependent actuation properties.

Curvature-driven instabilities in thin active shells.

Spontaneous material shape changes, such as swelling, growth or thermal expansion, can be used to trigger dramatic elastic instabilities in thin shells. These instabilities originate in geometric incompatibility between the preferred extrinsic and intrinsic curvature of the shell, which may be modified by active deformations through the thickness and in plane, respectively. Here, we solve the simplest possible model of such instabilities, which assumes the shells are shallow, thin enough to bend but not stretch, and subject to homogeneous preferred curvatures. We consider separately the cases of zero, positive and negative Gauss curvature. We identify two types of supercritical symmetry-breaking instability, in which the shell’s principal curvature spontaneously breaks discrete up/down symmetry and continuous planar isotropy. These are then augmented by inversion instabilities, in which the shell jumps subcritically between up/down broken symmetry states and rotation instabilities, in which the curvatures rotate by 90° between states of broken isotropy without release of energy. Each instability has a thickness-independent threshold value for the preferred extrinsic curvature proportional to the square root of Gauss curvature. Finally, we show that the threshold for the isotropy-breaking instability is the same for deep spherical caps, in good agreement with recently published data.

Interfacial metric mechanics: stitching patterns of shape change in active sheets.

A flat sheet programmed with a planar pattern of spontaneous shape change will morph into a curved surface. Such metric mechanics is seen in growing biological sheets, and may be engineered in actuating soft matter sheets such as phase-changing liquid crystal elastomers (LCEs), swelling gels and inflating baromorphs. Here, we show how to combine multiple patterns in a sheet by stitching regions of different shape changes together piecewise along interfaces. This approach allows simple patterns to be used as building blocks, and enables the design of multi-material or active/passive sheets. We give a general condition for an interface to be geometrically compatible, and explore its consequences for LCE/LCE, gel/gel and active/passive interfaces. In contraction/elongation systems such as LCEs, we find an infinite set of compatible interfaces between any pair of patterns along which the metric is discontinuous, and a finite number across which the metric is continuous. As an example, we find all possible interfaces between pairs of LCE logarithmic spiral patterns. By contrast, in isotropic systems such as swelling gels, only a finite number of continuous interfaces are available, greatly limiting the potential of stitching. In both continuous and discontinuous cases, we find the stitched interfaces generically carry singular Gaussian curvature, leading to intrinsically curved folds in the actuated surface. We give a general expression for the distribution of this curvature, and a more specialized form for interfaces in LCE patterns. The interfaces thus also have rich geometric and mechanical properties in their own right.

Self-organization and shape change by active polarization in nematic droplets

Active forces drive critical biological processes such as spontaneous organization and shape change during cell division. Here, we present a minimal hydrodynamic model leading to a unified description of self-organization and division in nematic droplets through active polarity sorting of cytoskeletal filaments by molecular motors. We find that motors self-organize within droplets while structuring filaments into polarized aster defects. At large activity, motors deform droplets leading to multidroplet chains and droplet division, consistent with experiments on actomyosin tactoids. We predict droplet steady-state phase diagrams that inform programmable shape changes in confined soft materials.

Shear flow-controlled shape memory of polymer resin dispersed liquid crystal elastomer microparticles

Thermomechanically active shape-programmable elastomer microparticles are very promising for development of particulate composites with unique operational properties, which could be attractive for various applications, e.g., in 3D printing. Liquid crystal elastomers are one of the suitable candidate materials due to their remarkable spontaneous shape change response. Performed rheological and thermomechanical tests demonstrate that shear stress can be used to efficiently manipulate the nematic order-driven morphology of liquid crystal elastomer microparticles (μLCEs). Specifically, by exploiting the soft-elasticity character of the material through manipulation of shear amplitude time profile, the nematic director can be oriented along the flow, which results in alignment of microparticles. Using this method, a suspension of well-aligned, thermomechanically elongated monodomain μLCEs can be created by shear stress-assisted cooling. Although the alignment can be lost in absence of persistent flow, it is restored instantaneously on re-application of the flow. The reason for this is the preservation of particle elongation at room temperature in zero flow. This shape memory can be erased by heating the system to the isotropic phase. Our work represents an important step forward in the development of a new generation of shape-programmable materials, which are potentially suitable for additive manufacturing of artefacts with anisotropic physical properties.

Investigation on Spontaneous Shape Change of 4D Printed Starch-Based Purees from Purple Sweet Potatoes As Induced by Microwave Dehydration.

The time evolution of three-dimensional (3D) printed food structures as affected by their composition and postprinting stimulus is an area of research that has recently received increasing attention. In this study, the spontaneous shape change of 3D printed purple sweet potato purees of different formulations as triggered by microwave dehydration was investigated. The rheological properties, water distribution behavior, and dielectric properties of the purees were first studied. Addition of salt reduced the viscosity, storage modulus, loss modulus, and yield stress but increased the relaxation time of the purees. Addition of fructose syrup resulted in opposite results. Addition of both salt and syrup decreased the dielectric constant but increased the dielectric loss of the purees. Increased microwave power and salt content increased the rates of dehydration and deformation but decreased the maximum deformation degree of the printed samples. The syrup also decreased the maximum deformation degree. A desirable deformation pattern could also be achieved by manipulating the infill parameters. Transformation of two-dimensional planar flowers and butterflies into 3D configurations as a result of varying the aforementioned parameters is illustrated. The proposed technique to induce spontaneous shape change of a 3D printed starch-based product should lay a foundation for further application of four-dimensional food printing.

Polyampholyte Hydrogels with pH Modulated Shape Memory and Spontaneous Actuation

AbstractPolyampholyte hydrogels are synthesized by one‐step copolymerization of cationic monomer 3‐(methacryloylamino)propyltrimethylammonium chloride, anionic monomers sodium p‐styrenesulfonate (NaSS), and methacrylic acid (MAA) without chemical crosslinker and adding salts. The hydrogels exhibit pH responsive shape memory behavior; the temporary shape of the hydrogel is formed manually after immersing in NaOH solution and fixed in HCl solution, while the shape recovery occurs by immersing in NaOH again. Most interestingly, the hydrogel shows a spontaneous shape change after the first shape memory cycle. When the recovered hydrogel with a little residual deformation is immersed in HCl again, it twists spontaneously and rapidly to the previous temporary shape. The spontaneous twisting and recovering can be repeated for ten times. Furthermore, the hydrogel swells quickly and is strengthened in HCl, while shrinks and weakens in NaOH during the shape change procedure. This unique synergistic effect of fast swelling, residual helical deformation, and increased strength plays a significant role in the spontaneous shape alternation. This new finding will initiate a new prosperous design for new soft actuators requiring successive actions.

Large spontaneous deformation of uniaxially oriented main-chain liquid–crystalline elastomers composed of BB-n mesogens

ABSTRACTWe observed the spontaneous shape change of a uniaxially oriented liquid–crystalline elastomer composed of smectic main-chain liquid–crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracted by about 650% on heating up to the isotropic phase, the sample length recovered by about 450% on cooling to room temperature in the first heating–cooling process. In contrast, the elastomer exhibited completely reversible deformation in the second heating–cooling process. The shape change occurred with almost no change in the orientational order of mesogens in the temperature region of the liquid–crystalline phase. The mechanism of the spontaneous thermotropic shape change is discussed.

J2240104 表面拡散による形態変化を利用したシリコン基板中への多層空洞の形成([J224-01]『マイクロ・ナノ機械の信頼性』のための材料創製・測定・解析技術 ナノ構造加工・プロセス)

We have investigated the formation of multi-layer void structures in Si(001) substrates, applying spontaneous shape changes of high-aspect ratio hole patterns by high temperature annealing. Multi-layer void structures are formed via closure of the hole opening, pinch-off of the vertically elongated closed voids, and coalescence of laterally arranged voids. We have shown that the observe shape transformation is interpreted as surface-diffusion-driven shape evolution, performing numerical simulations based on continuum theory.

Spontaneous shape changes in polymersomes via polymer/polymer segregation

We demonstrate that nanometre-sized polymer vesicles (polymersomes) assembled from two dissimilar diblock copolymers can undergo shape changes, driven by strong lateral polymer/polymer segregation within the membrane. The two particular block copolymers consisted of identical hydrophobic fragments to stimulate co-assembly, while their hydrophilic segments were either neutrally or negatively charged. It is hypothesized that demixing of the two types of polymer amphiphiles within the bilayer is caused by the different hydrophilic polymer fractions exhibiting intrinsically different interfacial curvatures upon self-assembly. Evidence for spontaneous domain formation came from dynamic light scattering analysis, electron microscopy and a fluorophore/quencher system. Given the vast amount of possible polymeric amphiphiles that can be co-assembled into hybrid aggregates, many more interesting and possibly biomedically relevant materials can be generated once inter-membrane phase separation becomes well-characterised and understood.

Polar Liquid Crystal Elastomers Cross Linked Far from Thermodynamic Phase Transitions: Dislocation Loops in Smectic Clusters

Nematic networks with three different concentrations of polar and nonpolar mesogens and the same concentration of a novel cross-linking agent give rise to unusual liquid single crystal elastomers (LSCEs) that are transparent monodomain nematic networks with smectic clusters. The largest spontaneous length change is observed in the sample with 70 mol% of the polar mesogen which also has the highest glass transition temperature and smectic clusters with a slowly increasing but nearly constant layer spacing on cooling from 90°C to 25°C. X-ray scattering intensity from smectic clusters with layer spacings that monotonically increase on cooling first increases to a maximum atT*~60∘C corresponding to clusters of about 30 layers. BelowT∗, the scattering intensity decreases as the number of layers in a cluster decreases. To account for this surprising nonlinear behavior that correlates with nonlinear features of the networks’ macroscopic spontaneous shape change and birefringence, a model is proposed where dislocations form in the layers atT∗. BelowT∗, more dislocations form to break down the layer structure. The possibility of dislocation formation atT∗independent of mesogenic concentrations is attributed to a conformational change in the crosslinker which is present at the same concentration in the three LSCEs.

A third measure-metastable state in the dynamics of spontaneous shape change in healthy human's white cells.

Human polymorphonuclear leucocytes, PMN, are highly motile cells with average 12-15 µm diameters and prominent, loboid nuclei. They are produced in the bone marrow, are essential for host defense, and are the most populous of white blood cell types. PMN also participate in acute and chronic inflammatory processes, in the regulation of the immune response, in angiogenesis, and interact with tumors. To accommodate these varied functions, their behavior is adaptive, but still definable in terms of a set of behavioral states. PMN morphodynamics have generally involved a non-equilibrium stationary, spheroid Idling state that transitions to an activated, ellipsoid translocating state in response to chemical signals. These two behavioral shape-states, spheroid and ellipsoid, are generally recognized as making up the vocabulary of a healthy PMN. A third, “random” state has occasionally been reported as associated with disease states. I have observed this third, Treadmilling state, in PMN from healthy subjects, the cells demonstrating metastable dynamical behaviors known to anticipate phase transitions in mathematical, physical, and biological systems. For this study, human PMN were microscopically imaged and analyzed as single living cells. I used a microscope with a novel high aperture, cardioid annular condenser with better than 100 nanometer resolution of simultaneous, mixed dark field and intrinsic fluorescent images to record shape changes in 189 living PMNs. Relative radial roundness, R(t), served as a computable order parameter. Comparison of R(t) series of 10 cells in the Idling and 10 in the Treadmilling state reveals the robustness of the “random” appearing Treadmilling state, and the emergence of behaviors observed in the neighborhood of global state transitions, including increased correlation length and variance (divergence), sudden jumps, mixed phases, bimodality, power spectral scaling and temporal slowing. Wavelet transformation of an R(t) series of an Idling to Treadmilling state change, demonstrated behaviors concomitant with the observed transition.

Giant Unilamellar Vesicles with Internalized Poly-(n-Isopropylacrylamide)-Vinyl Ferrocene Copolymer (PNIPAAm-VFC), a New Membrane-Interactive Thermoresponsive Material

It is very challenging to develop artificial systems mimicking components of the biological cell, but offers rewards with respect to a better understanding of cell processes, function and complexity. Giant unilamellar vesicles are micro-sized biomimetic compartments. They possess a simple, very important structural feature of natural cells, the double layer membrane, but are otherwise limited in internal functionality. There are a few existing methods of introducing more complex internal structure into liposomes. Internalizing water soluble polymers like poly-N-isopropylacrylamide (PNIPAAm) or poly-ethylene glycol (PEG) is a suitable method to increase concentration, viscosity, and to achieve compartmentalization, thus approaching more complex artificial cell architectures [1-3].This time we want to present a new type of thermoresponsive polymer, PNIPAAm with co-polymerized vinyl ferrocene (VFc)[4]. The increased hydrophobicity achieved by 3% (mass) ferrocene, promised better dynamic properties, reduced equilibrium compartment size and more homogenous hydrogels. During our investigation, an exceptionally strong interaction between PNIPAam-VFc and the vesicle boundary was observed. Polymer chains are anchored in the bilayer membrane by means of the lipophilic metallocene groups, creating unexpectedly strong attachment points. After increasing the temperature above the lower critical solution temperature where gel formation sets in, numerous lipid nanotubes are pulled from the vesicle, connecting the vesicular membrane with the internal hydrogel compartment surface. This leads to spontaneous shape changes of the vesicle, associated with multiple protrusion formation, followed by rapid coarsening to a single liposome.1. Karlsson, A., et al., Journal of Physical Chemistry B, 2005. 109(4): p. 1609-1617.2. Cans, A.S., M. Andes-Koback, and C.D. Keating, Journal of the American Chemical Society, 2008. 130(23): p. 7400-7406.3. Markstrom, M., et al., Langmuir, 2008. 24(9): p. 5166-5171.4. Nagel, B., A. Warsinke, and M. Katterle, Langmuir, 2007. 23(12): p. 6807-6811.

Spontaneous deformation of main-chain liquid-crystalline elastomers composed of smectic polyesters

We observe the spontaneous shape change of a uniaxially deformed liquid-crystalline elastomer composed of smectic main-chain liquid-crystalline polyesters in a cyclic heating–cooling process. Although the elastomer contracts by about 115% on heating up to the isotropic phase, the sample length recovers by 55% on cooling to room temperature in the first heating–cooling process, and the elastomer exhibits an almost complete reversible deformation in the second heating–cooling process. By a comparison of the results of sample observation with those of X-ray analysis, we recognise that the strain λ was linearly coupled with the orientational order parameter S. In addition, the results of the X-ray analysis imply that a cybotactic nematic state, in which smectic clusters lie scattered in a nematic-like matrix, emerges after exposure to the isotropic phase.

Effect of elastic interactions on coarsening in elastically inhomogeneous multiphase systems

We investigate the effect of interactions between inclusions on the coarsening behavior of elastically inhomogeneous multiphase systems with lattice misfit using a recently introduced two-dimensional multiscale model based on the classical time-dependent density-functional theory. We show that spontaneous shape changes are very efficient in limiting the impact of the interactions on the chemical potential of inclusions. For this reason, the interactions between isolated pairs of inclusions are unable to significantly affect coarsening. At higher volume fraction, the efficiency of shape changes drops and major shifts of the chemical potential are observed; these are shown to be strong enough to counteract capillarity. For example, we show that sufficient confinement by neighboring inclusions causes inverse coarsening and we provide illustrations of this. We also examine the effect of interface elasticity in the presence of interactions. We demonstrate that elasticity induces kinetic faceting and gives rise to oscillations in the chemical potential of the inclusions. This is shown to lead to the stabilization of microstructures composed of inclusions of various sizes. Finally, we propose a simple modified Lifshitz-Slyozov-Wagner model for assessing the effect of interactions on coarsening. The results are shown to be in excellent agreement with experiment.

Orientational order and finite strain in nematic elastomers

Nematic elastomers exhibit large, spontaneous shape changes at the transition from the high-temperature isotropic phase to the low-temperature nematic phase. These finite deformations are studied here in the context of a nonlinear, properly invariant, variational theory that couples the orientational order and elastic deformation. The theory is based on the minimization of a free-energy functional that consists of two contributions: a nematic one due to the interaction of the mesogenic units and an elastic one arising from the stretching of the cross-linked polymer chains. Suitable choices for these two contributions allow for large, reversible, spontaneous shape changes in which the elastic deformation can affect the isotropic-nematic transition temperature. The change in transition temperature as well as the magnitude of the resulting spontaneous deformation is illustrated for various parameter values. The theory includes soft elasticity as a special case but is not restricted to it.

Effects of perturbations on a trained shape memory micro-actuator

The so-called Two-Way-Shape-Memory Effect (TWSME) introduced using a training process has been used to produce a reversible motion in a small actuator. It is usually noted that the TWSME induces a preferred oriented martensite which results, during the reverse transformation, in a spontaneous shape change on cooling. While heating up, the material transforms to austenite and recovers its original shape. However, if the material is mechanically deformed in martensite, in order to modify the martensite variants distribution induced by the TWSME, a two-step transformation occurs: when heating up, the material first transforms into the low-temperature shape, -i.e. the memorized shape induced by the training process- and then to the high-temperature shape. This two-step transformation has been observed in a SMA micro-device. These experiments are presented in details and possible interpretations will be discussed.

Two-way shape memory effect of sputter-deposited Ti-rich Ti–Ni alloy films

Two-way shape memory effect of sputter-deposited Ti-rich Ti–Ni alloy films was investigated. Amorphous films of Ti–48.6at.% Ni alloy obtained by sputtering were crystallized by holding at 743 K for 0.3 ks and then aged under constraint (circular shape) at the same temperature for 1.8 ks. On these specimens, observation of spontaneous shape change during cooling and heating, and also differential scanning calorimetry (DSC) were done. The aged films showed two-way shape memory effect in association with the B2⇔R⇔B19′ transformation.

Two-Way Shape Memory Properties of a Ti-51Ni Single Crystal Including Ti<SUB>3</SUB>Ni<SUB>4</SUB> Precipitates of a Single Variant

A single crystal of a Ti-51Ni (at%) alloy was grown by floating zone method, and Ti 3 Ni 4 precipitates of a single variant were formed coherently to the matrix by aging treatment under a compressive stress applied along the [111 ] direction. By using this single crystal, two-way (all-round) shape memory property was investigated. The precipitates were lenticular in shape with 30 nm in thickness and 250 nm in diameter, and their volume fraction was about 9% The matrix was expanded elastically about 0.2% on average along the [111] direction. The elastic energy accumulated in the matrix was estimated to be about 0.3 MJ/m 3 ; it was about 1.5% of the latent heat of the B2 → R transformation. The thermally induced R-phase was composed of a single variant, and the spontaneous strain along the [111] direction was about 0.8% of expansion. The maximum work we could gain from the spontaneous shape change was estimated to be about 0.17 MJ/m 3 .

Strain effects on the macroscopic behaviour and martensite morphology in shape-memory CuZnAl alloys

Shape-memory alloys exhibit super-elasticity when they are deformed at a temperature less than the transformation temperature. The specimen remains in the deformed shape on releasing the strain at this temperature but recovers the undeformed original shape on heating to temperatures over the reverse transformation temperature. As a result of their ability to generate spontaneous shape changes on thermal cycling, these alloys can convert heat directly into mechanical work due to the substantial recovery stresses generated during the martensite-to-parent transformation. These alloys are very sensitive to thermal effects and are influenced strongly by both quenching and post-quench heat-treatments. The effects of external stress on the character of the macroscopic shape in two shape-memory CuZnAl alloys are investigated. The effects of post-quench heat-treatment on martensite morphology were observed by means of optical microscopy. Thermal hysteresis occurring during the heating and cooling processes is attributed to internal friction. Furthermore, parallel-sided martensite plates which can be grown almost in one dimension have been observed in the post-quench heat-treated specimens. It is suggested that the equilibrium phase (α-fcc) formed in the aged specimens causes the loss of shape memory.