Reversible Shape Change Research Articles

Sensitively humidity-driven actuator and sensor derived from natural skin system

Precise control over the smart materials exhibiting reversible shape changes in response to environmental stimuli presents a considerable challenge. Here, with a self-assembly strategy by extracting natural materials from pigskin, a single layer bio-inspired, transparent, soft biological film (BF) with the primary characteristics of self-actuation and self-sensing is successfully developed. The self-assembly constructed BF can exchange water and reflect environmental humidity gradients rapidly to activate continuous rotary movement. Temperature which affects the thermal motion of water molecules will induce different orientation movement of the film, and on this basis, a humidity-driven energy transfer motor is developed. More characterizations highlight the behavior mechanism of BF through water exchanging by a hydrogen bonding interaction with the hydrophilic group of amino acids residues on the BF surface. Finally, a wearable, steady and ultrafast-response sensor to detect human breathing, especially for real-time obstructive sleep apnea (OSA) state, is fabricated. This study offers great potential in emerging applications including micro-sensors, switches, soft robots and power source technologies.

The Effect of Ceo2 Addition on Transformation Temperatures and Wear Resistance of Cu-Al-Ni Shape Memory Alloys

SMAs can switch from one crystallographic structure to another in response to temperature or stress stimuli. When SMAs are exposed to mechanical cyclic stress, they can absorb and discharge mechanical energy by experiencing a reversible hysteretic shape change. SMAs are widely used for sensing, actuation, impact absorption, and vibration damping. This work studied the effect of CeO2 addition on the transformation temperature and wear resistance of Cu-Al-Ni SMAs. WhereCeO2 was added at different percent’s 0.5, 1, and 3 wt% to the base alloy, followed by casting and homogenization at 900oC. Some tests were carried out: Differential scanning calorimeter, Optical Microscope, Scanning Electron microscopy, Energy dispersion spectrometer, X-Ray Diffraction, and Wear and Hardness tests. OM and SEM tests reveal that both phases of martensite β and γ are found. Also, the additions of CeO2 show a visible effect on phase formation and transformation temperatures. It was observed that increasing of CeO2 particles in Cu-based SMAs owing to improve interfacial bonding between matrix and reinforcement and also observed that the variants become thicker with increasing in percent. Additions of different percentages of cerium oxide increase the hardness of Cu-Al-Ni SMAs. Due to the addition of CeO2 particles, the sample's wear rate decreases compared to pure SMAs.

Photopolymerization of ceramic/zeolite reinforced photopolymers: Towards 3D/4D printing and gas adsorption applications

Normally, 3D printed objects via photopolymerization usually don’t have fully optimized mechanical properties and functions. In this context, the addition of single filler usually can improve these properties through the access to composite materials. However, the addition of filler inevitably has a negative influence on the light penetration inside the composites strongly reducing the depth of cure. Hence, the fabrications of photo-composites based on a combination of two inorganic fillers (alumina Al2O3 and zeolite LTA-5A) are reported in this work. Compared to pure poly(ethylene glycol) diacrylate (PEGDA) and its other corresponding single filler-based composites, PEGDA with 25 wt% LTA-5A and 25 wt% Al2O3-200 had several improved properties: better depth of cure (DOC), good mechanical properties (the storage modulus, tensile strength, the Young’s modulus). Direct laser write (DLW) was used to manufacture 3D patterns and 3D cross-shaped objects with good resolution. Under the hydrothermal stimuli, a 4D printing behavior (reversible shape change) was observed on the 3D cross-shaped objects. In addition, the 3D object with the condition of 4D printing had the ability of withstand a heavy stuff (the mass difference was more than 100 times). This is the first time to report the fabrication of the double filler-based (50 wt%) composite (ceramic and zeolite) with 4D printing behavior and good mechanical properties. Interestingly, calcinated composites had a certain structure strength and good CO2 adsorption performance. Therefore, this work not only paves the way for the investigation of photo-composites filled with multiple fillers combinations (how to improve the mechanical properties and keep good depth of cure for composites via photopolymerization), but also shows us the potential applications in the field of 3D/4D printing, microlithography and functional composites.

Two-Dimensional Mechanics of Atomically Thin Solids on Water.

Movement of a three-dimensional solid at an air–water interface is strongly influenced by the extrinsic interactions between the solid and the water. The finite thickness and volume of a moving solid causes capillary interactions and water-induced drag. In this Letter, we report the fabrication and dynamical imaging of freely floating MoS2 solids on water, which minimizes such extrinsic effects. For this, we delaminate a synthesized wafer-scale monolayer MoS2 onto a water surface, which shows negligible height difference across water and MoS2. Subsequently patterning by a laser generates arbitrarily shaped MoS2 with negligible in-plane strain. We introduce photoswitchable surfactants to exert a lateral force to floating MoS2 with a spatiotemporal control. Using this platform, we demonstrate a variety of two-dimensional mechanical systems that show reversible shape changes. Our experiment provides a versatile approach for designing and controlling a large array of atomically thin solids on water for intrinsically two-dimensional dynamics and mechanics.

Recent Developments in Shape Memory Elastomers for Biotechnology Applications.

Shape memory elastomers have revolutionised the world since their introduction in the 20th century. The ability to tailor chemical structures to produce a family of materials in wide-ranging forms with versatile properties has propelled them to be ubiquitous. Recent challenges in the end-of-life management of polymeric materials should prompt us to ask, ‘what innovations in polymeric materials can make a strong case for their use as efficient materials?’ The development of smart elastomers that can acquire, convey, or process a stimulus (such as temperature, pressure, electromagnetic field, moisture, and chemical signals) and reply by creating a useful effect, specifically a reversible change in shape, is one such innovation. Here, we present a brief overview of shape memory elastomers (SMEs) and thereafter a review of recent advances in their development. We discuss the complex processing of structure-property relations and how they differ for a range of stimuli-responsive SMEs, self-healing SMEs, thermoplastic SMEs, and antibacterial and antifouling SMEs. Following innovations in SEMs, the SMEs are forecast to have significant potential in biotechnology based on their tailorable physical properties that are suited to a range of different external stimuli.

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers.

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

4D Printed Multifunctional Composites with Cooling‐Rate Mediated Tunable Shape Morphing

AbstractMultifunctional composites can accomplish multiple tasks such as shape morphing, sensing, and load bearing using a single structure. Smart materials including liquid crystal elastomers (LCE) and shape memory polymers (SMP) have long been used as the primary components of multifunctional composites because of their shape and property changes in response to external stimuli. However, LCEs can generate rapid and reversible shape changes but are soft and require a constant temperature to retain their deformed shape; SMPs have favorable mechanical properties but few can achieve reversible actuations. Moreover, both LCEs and SMPs have limited capability for tunable shape morphing. Multi‐material 3D printing of smart materials, also known as 4D printing, has seen significant advances enabling the fabrication of composites with novel functionality. In this work, 4D printing is leveraged to create an LCE‐SMP composite that can achieve not only rapid and reversible shape changes, but also cooling‐rate regulated tunable shape morphing. The latter is achieved by harnessing the distinct time‐dependent thermomechanical properties of LCEs and SMPs. Furthermore, the composite has a high stiffness at low temperature to support heavy loads. The LCE‐SMP composite hence offers a novel approach to achieve tunable shape morphing for future engineering applications.

Ionic liquid-catalyzed synthesis of carbon/polyurethane triboelectric nanocomposites with excellent flame retardancy and oil leak detection

A high-carbon (expandable graphite flakes and carbon black)–loaded polyurethane foam (PUF) composite was prepared using a green and efficient ionic liquid (IL) catalyst. The carbon/PUF composite exhibited excellent flame retardancy and was highly versatile for application in oil leak detection and as triboelectric nanogenerator (TENG). The effect of the IL on the foamability of the carbon/PUF composite was thoroughly investigated. The IL promoted the expansion of the carbon/PUF composite and the microcellular pore formation in it, thereby affording a sponge with low density, excellent flexibility, and compressibility. Owing to the presence of high-carbon fillers, the resultant sponge exhibited excellent flame retardancy by (i) avoiding melt dripping after ignition by a butane torch (∼1400 °C, applied for 20 s), (ii) self-extinguishing after torch removal, and (iii) preventing foam collapse. Additionally, it can pass the UL-94 V-0 rating and the critical fire retardancy metrics, i.e., peak heat release rate, total heat release, and total smoke release, were reduced by 73%, 78%, and 92%, respectively. A flame retardant (FR)-TENG was designed using the carbon/PUF composite. The FR-TENG could be attached to the insole of a shoe, generating an output of 36 and 100 V upon walking and running, respectively. This output was sufficient to concomitantly glow 38 LEDs. Interestingly, owing to the oil-induced reversible shape change, the carbon/PUF composite can be used for the fabrication of high-efficiency oil leak sensors to detect oil spills in pipelines and different water environments. This study offers a new perspective for the development of flame retardant and multifunctional foam-based materials.

A NURBS-based inverse analysis of swelling induced morphing of thin stimuli-responsive polymer gels

Gels are a mixture of cross-linked polymers and solvents, and have been widely studied in recent years for a diverse range of biomedical applications. Because gels can undergo large, reversible shape changes due to swelling, their complex physical response must be modeled by coupling large reversible deformation and mass transport. An ongoing challenge in this field is the ability to capture swelling or residual swelling-induced of such stimuli-responsive gels from initially flat two-dimensional (2D) to three-dimensional (3D) curved shapes. Specifically, because such shape changes typically involve large deformations, shape changes, and the exploitation of elastic instabilities, it remains an open question as to what external stimulus should be prescribed to generate a specific target shape. Therefore, we propose a novel formulation that tackles, using both nonlinear kinematics and material models, the coupling between elasticity and solvent transport using Kirchhoff–Love shell theory discretized using isogeometric analysis (IGA). Second, we propose an inverse methodology that chemomechanically couples large deformation and mass transport to identify the external stimuli prescribed to generate a specific target shape. Our numerical examples demonstrate the capability of identifying the required external stimuli, with the implication that the reconstructed target shapes are accurate, including cases where the shape changes due to swelling involve elastic instabilities or softening. Overall, our study can be used to effectively predict and control the large morphological changes of an important class of stimuli-responsive materials.

A High-Fidelity Preparation Method for Liquid Crystal Elastomer Actuators.

Three-dimensional (3D) structural actuators based on monodomain liquid crystal elastomers (mLCEs) show a wide range of potential applications. A direct ink writing technique has been developed to print LCE structures. It is still a challenge to print high-precision 3D-mLCE actuators. Here, a method of wet 3D printing combined with freeze-drying is proposed. The coagulation bath is designed to restrain the nascent fiber disturbance of the capillary wave and weight by adjusting the ink viscosity and printing speed to control the LC molecular order, enabling uniform (B = 1.02) fibers with a high degree of orientational alignment (S = 0.45) of the mesogens. Furthermore, dynamic disulfide bond formation was used as the cross-linking point, which can allow the LCE network structure to be continuously cured to ensure adjacent layers are effectively bonded and, in combination with freeze-drying, produce the 3D-mLCE actuators of fidelity architecture (98.37 vol %) by printing. The actuators have excellent actuating strain (45.12%), and the dynamic disulfide bond makes them programmable. Finally, a printed bionic starfish and a printed bionic hand can easily grab regular and irregular objects. This work provides a feasible scheme for fabricating complex 3D-mLCEs with reversible changes in shape.

Highly tunable actuation and mechanical properties of 4D-printed nematic liquid crystal elastomers

Liquid crystal elastomers (LCEs) are smart elastomers capable of a reversible shape change triggered by a stimulus such as heat or light. This change in shape is programmed into the material by orienting the mesogen molecules, often through the post-manufacturing alignment of the polymer chains by mechanical stretching. This method limits the possible orientations and necessitates a second curing step to lock the orientation in the structure. Direct ink writing allows for the curing of LCEs directly in an oriented state and for a fine control of the local orientation of the mesogens. The process parameters dictate the actuation and mechanical properties of the 4D-printed object, where time is the fourth dimension. We present in this paper a study of the influence of the printing speed and raster angle on the actuation strain and mechanical properties of the resulting LCE. Complex shape changes can be programmed into specimens that combine variations in printing angles and speeds. We printed two dimensional specimens actuating into helicoids and hinges. Results show the expected influence of printing speed on the level of orientation of the mesogens and on the actuation strain; that is faster printing equals more orientation and actuation strain. The change in raster angle demonstrates the immediate impact of incompatible strains in the structure, as the flat specimen (i.e. a printed rectangle of a couple of layers thick) actuates in a controlled 3D configuration. Finally, the hinge and helicoid specimens show the versatility of 4D-printed LCE systems, where the folding angle of the hinge or the pitch of the helicoid can be programmed through basic geometry, angle, and speed control at the printing stage. This transformative step in the preparation of LCE actuators opens the door to precise customization exploiting the mobility of mesogens on the polymer chains during printing and the flexibility of the direct ink writing process.

Fast-Response Bioinspired Near-Infrared Light-Driven Soft Robot Based on Two-Stage Deformation.

Herein, we report a remotely controlled soft robot employing a photoresponsive nanocomposite synthesized from liquid crystal elastomers (LCEs), high elastic form-stable phase change polymer (HEPCP), and multiwalled carbon nanotubes (MWCNTs). Possessing a two-stage deformation upon exposure to near-infrared (NIR) light, the LCE/HEPCP/MWCNT (LHM) nanocomposite allows the soft robot to exhibit an obvious, fast, and reversible shape change with low detection limitations. In addition to the deformation and bending of the LCE molecular chains itself, the HEPCP in the composite material can also be triggered by a reversible solid-liquid transition due to the temperature rise caused by MWCNTs, which further promotes the change of the LCE. In particular, the proposed photodriven LHM soft robot can bend up to 180° in 2 s upon NIR stimulation (320 mW, distance of 5 cm) and generate recoverable, dramatic, and sensitive deformation to execute various tasks including walking, twisting, and bending. With the capacity of imitating biological behaviors through remote control, the disruptive innovation developed here offers a promising path toward miniaturized untethered robotic systems.

Multi-material stimuli-responsive hydrogels with optically induced actuation

A vision for soft, autonomous materials entails synthesis of multiple senses in multifunctional materials where material response requires sensitivity to external stimuli. Stimuli-responsive hydrogels are of particular interest for optically induced mechanical response due to the ability to transform external stimuli into large, reversible shape change. Specifically, temperature-responsive hydrogels are broadly used and can be designed to achieve deformation through the photothermal effect as a result of surface plasmonic resonance of gold nanoparticles. Here, a multi-material stimuli-responsive hydrogel network with embedded gold nanoparticles is demonstrated in a unit cell pattern with anisotropic swelling behavior in response to visible light. Reversible, anisotropic swelling leads to bending motion that contributes to the development of soft, autonomous materials.

Electrically responsive materials based on dibutyl phathalate plasticized poly(lactic acid) and spherical fullerene

Carbon-based electromechanical actuators, capable of reversible shape changes in response to electrical stimuli, have found many potential utilizations such as robotic artificial muscles, micro-pumps, and sensitive switches. In this work, electroactive materials based on the dibutyl phathalate (DBP) plasticized poly(lactic acid) (PLA) and fullerene (C60) were produced by a simple solvent casting method. The PLA composites exhibited fast and reversible responses under electrical stimulus. The highest storage modulus response was obtained from the 1.0%v/v C60/PLA/DBP at 23.51 × 105 Pa under the 1.5 kV mm−1 electric field. In the bending experiment, the PLA composites bended towards the anode from the attractive force between the negative charges of the induced dipole moments namely the carbonyl groups in PLA and DBP and the π-conjugated electrons of C60 and the positive electrode. The C60/PLA/DBP composite with a small C60 content (0.1%v/v) yielded the maximum bending distance of about 6.0 mm within 10 s and the highest dielectrophoresis force of 1.01 mN at 550 V mm−1. Thus, the electrically responsive PLA composites fabricated here with the short response time and high bending deformation are demonstrated here to be promising biobased materials towards actuator applications.

Direct laser writing of liquid crystal elastomers oriented by a horizontal electric field.

Background: The ability to fabricate components capable of performing actuation in a reliable and controlled manner is one of the main research topics in the field of microelectromechanical systems (MEMS). However, the development of these technologies can be limited in many cases by 2D lithographic techniques employed in the fabrication process. Direct Laser Writing (DLW), a 3D microprinting technique based on two-photon polymerization, can offer novel solutions to prepare, both rapidly and reliably, 3D nano- and microstructures of arbitrary complexity. In addition, the use of functional materials in the printing process can result in the fabrication of smart and responsive devices. Methods: In this study, we present a novel methodology for the printing of 3D actuating microelements comprising Liquid Crystal Elastomers (LCEs) obtained by DLW. The alignment of the mesogens was performed using a static electric field (1.7 V/µm) generated by indium-tin oxide (ITO) electrodes patterned directly on the printing substrates. Results: When exposed to a temperature higher than 50°C, the printed microstructures actuated rapidly and reversibly of about 8% in the direction perpendicular to the director. Conclusions: A novel methodology was developed that allows the printing of directional actuators comprising LCEs via DLW. To impart the necessary alignment of the mesogens, a static electric field was applied before the printing process by making use of flat ITO electrodes present on the printing substrates. The resulting microelements showed a reversible change in shape when heated higher than 50°C.

Direct laser writing of liquid crystal elastomers oriented by a horizontal electric field

Background: The ability to fabricate components capable of performing actuation in a reliable and controlled manner is one of the main research topics in the field of microelectromechanical systems (MEMS). However, the development of these technologies can be limited in many cases by 2D lithographic techniques employed in the fabrication process. Direct Laser Writing (DLW), a 3D microprinting technique based on two-photon polymerization, can offer novel solutions to prepare, both rapidly and reliably, 3D nano- and microstructures of arbitrary complexity. In addition, the use of functional materials in the printing process can result in the fabrication of smart and responsive devices. Methods: In this study, we present a novel methodology for the printing of 3D actuating microelements comprising Liquid Crystal Elastomers (LCEs) obtained by DLW. The alignment of the mesogens was performed using a static electric field (1.7 V/µm) generated by indium-tin oxide (ITO) electrodes patterned directly on the printing substrates. Results: When exposed to a temperature higher than 50°C, the printed microstructures actuated rapidly and reversibly of about 8% in the direction perpendicular to the director. Conclusions: A novel methodology was developed that allows the printing of directional actuators comprising LCEs via DLW. To impart the necessary alignment of the mesogens, a static electric field was applied before the printing process by making use of flat ITO electrodes present on the printing substrates. The resulting microelements showed a reversible change in shape when heated higher than 50 °C.

Liquid Crystalline Hydrogel with Thermally Induced Reversible Shape Change and Water-Triggered Shape Memory.

Liquid crystalline hydrogel (LCH) is synthesized through simultaneous polymerization of hydrophobic and hydrophilic monomers in an oil-in-water emulsion, resulting in phase-separated liquid crystalline network (LCN) embedded in a hydrogel matrix. This material features some properties and functions of both LCN and hydrogel, displaying stable LC phase over repeated hydration and dehydration cycles of the hydrogel matrix. Using mechanically stretched and photocrosslinked LCH, the thermally induced LC-isotropic phase transition in LCN domains can be translated into reversible macroscopic deformation of the LCH. Moreover, the LCH exhibits water absorption-controlled shape memory effect.

Programmable multistability for 3D printed reinforced multifunctional composites with reversible shape change

4D printing empowers 3D printed structures made of hydrogels, liquid crystals or shape memory polymers, with reversible morphing capabilities in response to an external stimulus. To apply reversible shape-change to stiff lightweight materials such as microfiber reinforced polymers, we developed a composite ink that can be printed using direct-ink-writing (DIW), and that exhibits multistability around its glass transition temperature. After curing at room temperature, the flat print thermally morphs into a predefined shape upon heating at an actuation temperature and cooling down. The sample can then reversibly snap between multiple stable shapes when heated above its glass transition temperature thanks to pre-stress-induced multistability. The key that allows thermal morphing and pre-stress multistability is the microstructuring of the 3D printed composites by shear-induced alignment of reinforcing microfibers. This alignment leads to local anisotropy in thermomechanical properties and the build-up of pre-stresses. Furthermore, the ink composition can be tuned to generate shape-dependant reversible functional properties, such as electrical conductivity. Based on finite element modelling and experimental results, the method proposed here can be used for variety of compositions and designs, for applications where stiffness, reconfigurability and shape-dependent functionalities can be exploited.

Microfluidic Obstacle Arrays Induce Large Reversible Shape Change in Red Blood Cells.

Red blood cell (RBC) shape change under static and dynamic shear stress has been a source of interest for at least 50 years. High-speed time-lapse microscopy was used to observe the rate of deformation and relaxation when RBCs are subjected to periodic shear stress and deformation forces as they pass through an obstacle. We show that red blood cells are reversibly deformed and take on characteristic shapes not previously seen in physiological buffers when the maximum shear stress was between 2.2 and 25 Pa (strain rate 2200 to 25,000 s−1). We quantify the rates of RBC deformation and recovery using Kaplan–Meier survival analysis. The time to deformation decreased from 320 to 23 milliseconds with increasing flow rates, but the distance traveled before deformation changed little. Shape recovery, a measure of degree of deformation, takes tens of milliseconds at the lowest flow rates and reached saturation at 2.4 s at a shear stress of 11.2 Pa indicating a maximum degree of deformation was reached. The rates and types of deformation have relevance in red blood cell disorders and in blood cell behavior in microfluidic devices.

Design and 3D printing of paper-based shape memory polymer actuated for soft lightweight fingers

Smart materials and devices that possess multiple functions of sensing and actuation have been investigated over decades as being exploited for soft robots in various applications such as artificial muscles, biomimetic actuators, and surgical tools. One of critical requirements involved is the capability of dramatic change in shape and size to be adapted for different circumstances. Recent progress of the emerging materials has showed many remarkable properties to fabricate them in simple way. This paper reports a fused deposition modeling (FDM) method that can be simply implemented by three-dimensional (3D) printing shape memory polymer (SMP) on paper. Through 3D printing of a SMP filament on paper, the bilayer structures and grippers with a starfish shape were fabricated by exploiting the cost-effective and lightweight materials. By virtue of electrothermal actuation (Joule heating) with an electrical power supply, they showed remarkable and reversible change in shape during 100 cycles of movements. As a result, it demonstrated the capability of control over adaptive shape and size in design and fabrication of soft robots. The FDM-based method in the study is therefore beneficial to extend the flexibility in design and fabrication, as compared to the conventional ones particularly applied for rigid grippers. In the near future, a variety of multi-layer structures and materials fabricated by FDM method and 3D printing materials could be applied for applications such as soft actuators and sensors in versatile conditions.