An investigation on multilayer shape memory polymers under finite bending through nonlinear thermo-visco-hyperelasticity

This study presents a semi-analytical solution to describe the behavior of shape memory polymers(SMPs) based on the nonlinear thermo-visco-hyperelasticity which originates from the concepts of internal state variables and rational thermodynamics.This method is developed for the finite bending of multilayers in a dual-shape memory effect(SME) cycle.The layer number and layering order are investigated for two different SMPs and a hyperelastic material.In addition to the semi-analytical solution,the finite element simulation is performed to verify the predicted results,where the outcomes demonstrate the excellent accuracy of the proposed solution for predicting the behavior of the multilayer SMPs.Since this method has a much lower computational cost than the finite element method(FEM),it can be used as an effective tool to analyze the SMP behavior under different conditions,including different materials,different geometries,different layer numbers,and different layer arrangements.

Shape Memory Polymer Composite Booms with Applications in Reel-Type Solar Arrays

Solar arrays are the primary energy source for spacecraft.Although traditional rigid solar arrays improve power supply,the quality increases proportionally.Hence,it is difficult to satisfy the requirements of high-power and low-cost space applications.In this study,a shape-memory polymer composite(SMPC)boom was designed,fabricated,and characterized for flexible reel-type solar arrays.The SMPC boom was fabricated from a smart material,a shape-memory polymer composite,whose mechanical properties were tested.Additionally,a mathematical model of the bending stiffness of the SMPC boom was developed,and the bending and buckling behaviors of the boom were further analyzed using the ABAQUS software.An SMPC boom was fabricated to demonstrate its shape memory characteristics,and the driving force of the booms with varying geometric parameters was investigated.We also designed and manufactured a reel-type solar array based on an SMPC boom and verified its self-deployment capability.The results indicated that the SMPC boom can be used as a deployable unit to roll out flexible solar arrays.

Buckling-controlled two-way shape memory effect in a ring-shaped bilayer

Shape memory polymers(SMPs)usually have a one-way shape memory effect.In this paper,an easy-operating method to realize a two-way shape memory effect was demonstrated in a ring-shaped bilayer structure where the two layers are SMPs with different thermal transition temperatures.By designing specific thermomechanical processes,the mismatched deformation between the two layers leads to a morphology change of ring-shaped bilayer structures from a smooth ring to a gear-like buckling shape under cooling and a reversible recovery to the smooth shape under heating.Such a morphology change is ascribed to occurrence and recovery of thermoelastic buckling.This method was validated by finite element simulation.We experimentally investigated the influence of pre-strain on buckling,and it was found that both the buckling occurrence and recovery temperature vary with pre-strain.Furthermore,considering a ring-shaped SMP-SMP bilayer structure,finite element analysis was conducted to study the influence of film thickness and modulus ratio of two layers on buckling behavior.The results showed that the critical buckling wavelength was greatly influenced by film thickness and modulus ratio.W e made a theoretical analysis that accorded well with the numerical results.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

A Shape-Memory Deployable Subsystem with a Large Folding Ratio in China’s Tianwen-1 Mars Exploration Mission

Once China’s Tianwen-1 Mars probe arrived in a Mars orbit after a seven-month flight in the deep cold space environment,it would be urgently necessary to monitor its state and the surrounding environment.To address this issue,we developed a flexible deployable subsystem based on shape memory polymer composites(SMPC-FDS)with a large folding ratio,which incorporates a camera and two temperature telemetry points for monitoring the local state of the Mars orbiter and the deep space environment.Here,we report on the development,testing,and successful application of the SMPC-FDS.Before reaching its Mars remote-sensing orbit,the SMPC-FDS is designed to be in a folded state with high stiffness;after reaching orbit,it is in a deployed state with a large envelope.The transition from the folded state to the deployed state is achieved by electrically heating the shape memory polymer composites(SMPCs);during this process,the camera on the SMPC-FDS can capture the local state of the orbiter from multiple angles.Moreover,temperature telemetry points on the SMPC-FDS provide feedback on the environment temperature and the temperature change of the SMPCs during the energization process.By simulating a Mars on-orbit space environment,the engineering reliability of the SMPC-FDS was comprehensively verified in terms of the material properties,structural dynamic performance,and thermal vacuum deployment feasibility.Since the launch of Tianwen-1 on 23 July 2020,scientific data on the temperature environment around Tianwen-1 has been successfully acquired from the telemetry points on the SMPCFDS,and the local state of the orbiter has been photographed in orbit,showing the national flag of China fixed on the orbiter.

Switchable shape memory polymer bio-inspired adhesive and its application for unmanned aerial vehicle landing

Controlled and switchable adhesion is commonly observed in biological systems.In recent years,many scholars have focused on making switchable bio-inspired adhesives.However,making a bio-inspired adhesive with high adhesion performance and excellent dynamic switching properties is still a challenge.A Shape Memory Polymer Bio-inspired Adhesive(SMPBA)was successfully developed,well realizing high adhesion(about 337 kPa),relatively low preload(about90 kPa),high adhesion-to-preload ratio(about 3.74),high switching ratio(about 6.74),and easy detachment,which are attributed to the controlled modulus and contact area by regulating temperature and the Shape Memory Effect(SME).Furthermore,SMPBA exhibits adhesion strength of80–337 kPa on various surfaces(silicon,iron,and aluminum)with different roughness(Ra=0.021–10.280)because of the conformal contact,reflecting outstanding surface adaptability.The finite element analysis verifies the bending ability under different temperatures,while the adhesion model analyzes the influence of preload on contact area and adhesion.Furthermore,an Unmanned Aerial Vehicle(UAV)landing device with SMPBA was designed and manufactured to achieve UAV landing on and detaching from various surfaces.This study provides a novel switchable bio-inspired adhesive and UAV landing method.

Theoretical Analysis of the Buckling Behaviors of Inhomogeneous Shape Memory Polymer Composite Laminates Considering Prestrains

The mismatch in thermal expansion coefficients between the fiber-rich and resin-rich regions of a shape memory polymer composite(SMPC)laminate,along with the residual strain during SMPC fabrication,results in buckling deformation of the inhomogeneous laminate.This paper presents a macroscopic model for buckling of an inhomogeneous SMPC laminate under initial biaxial prestrains.Both linear and nonlinear buckling analyses are carried out using the energy method.The influences of prestrain biaxiality,temperature,and ply angle on the buckling wavelength,critical buckling prestrain,and buckling amplitude are calculated.The results demonstrate that the critical buckling wavelength of the SMPC laminate is independent of the prestrain,while the amplitude is almost independent of temperature.In addition,the optimal fiber stacking configuration with the maximum critical buckling prestrains of inhomogeneous SMPC laminates is determined by a genetic algorithm.

Two-Way 4D Printing： A Review on the Reversibility of 3D-Printed Shape Memory Materials

The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional （4D） printing. With the right external stimulus, the need for human interaction, sensors, and batteries will be eliminated, and by using additive manufacturing, more complex devices and parts can be produced. With the current understanding of shape memory mechanisms and with improved design for additive manufacturing, reversibility in 4D printing has recently been proven to be feasible. Conventional one-way 4D printing requires human interaction in the programming （or shapesetting） phase, but reversible 4D printing, or two-way 4D printing, will fully eliminate the need for human interference, as the programming stage is replaced with another stimulus. This allows reversible 4D printed parts to be fully dependent on external stimuli; parts can also be potentially reused after every recovery, or even used in continuous cycles-an aspect that carries industrial appeal. This paper presents a review on the mechanisms of shape memory materials that have led to 4D printing, current findings regarding 4D printing in alloys and polymers, and their respective limitations. The reversibility of shape memory materials and their feasibility to be fabricated using three-dimensional （3D） printing are summarized and critically analyzed. For reversible 4D printing, the methods of 3D printing, mechanisms used for actuation, and strategies to achieve reversibility are also highlighted. Finally, prospective future research directions in reversible 4D printing are suggested.

4D printing of PLA/PCL shape memory composites with controllable sequential deformation

Shape memory polymers(SMPs)are a promising class of materials for biomedical applications due to their favorable mechanical properties,fast response,and good biocompatibility.However,it is difficult to achieve controllable sequential shape change for most SMPs due to their high deformation temperature and the simplex deformation process.Herein,shape memory composites based on polylactic acid(PLA)matrix and semi-crystalline linear polymer polycaprolactone(PCL)are fabricated using 4D printing technology.Compared with pure PLA,with the rise of PCL content,the 4D-printed PLA/PCL composites show decreased glass transition temperature(Tg)from 67.2 to 55.2°C.Through the precise control of the deformation condition,controllable sequential deformation with an outstanding shape memory effect can be achieved for the PLA/PCL shape memory composites.The response time of shape recovery is less than 1.2 s,and the shape fixation/recov-ery rates are above 92%.In order to simulate sequential petal opening and sequential drug releasing effects,a double-layer bionic flower and a drug release device,respectively,are presented by assembling PLA/PCL samples with different PLA/PCL ratios.The results indicate the potential applications of 4D-printed PLA/PCL composites in the field of bio-inspired robotics and biomedical devices.

Spinodal dynamics of metastable glass transition domains in amorphous polymer towards thermomechanically tailorable shape memory effect

Glass transition plays a critical role to determine the dynamic behaviors of amorphous shape memory polymers(SMPs).However,the fundamental relationships between shape memory effect(SME)and dynamic glass transition have not been well understood,even though this topic has been studied for decades.In this study,we apply a mean-square displacement function of Adam-Gibbs(AG)domain size model to explore metastable glass transition between normal glass state and rubbery state of amorphous SMPs,based on both mode-coupling theory and mean-field model.A statistic viscosity equation is formulated to study the dynamic glass transition of metastable AG domains in an amorphous SMP.A dynamically spinodal model is also developed to connect dynamic glass transitions to thermomechanical processes,based on statistic viscosity equation and phase transition model.Furthermore,using the spinodal models,multiple shape memory behaviors have been predicted for amorphous SMPs with dual-,triple-and quadruple-SMEs,resulted from their different routes of themomechanical evolutions.Finally,the proposed models are verified using the experimental data reported in literature.

Adjustable volume and loading release of shape memory polymer microcapsules

Research on microcapsules has been conducted in recent years given trends in miniaturization and novel functionalization.In this work,we designed and prepared a series of unique shape memory polyurethane(SMPU)microcapsules with stimuli-responsive func-tions.The microcapsule has a core-shell structure in which the surface morphology can be adjusted,and it has a certain loadbearing capacity.In addition,the SMPU microcapsule has a stimuliresponsive function for shape memory and solvent response.The temperature of its shape recovery is approximately body tempera-ture,and it can swell to rupture under the stimulation of organic solvents.Thus,the SMPU microcapsule has potential applications in biomedical fields,such as drug release.

Smart Multiple Wetting Control on ZnO Coated Shape Memory Polymer Arrays

Recently,surfaces with intelligent wetting controllability have aroused increased attention.Endowing the surface with stimuli-responsive surface chemistry and tunable surface microstructure can achieve a surface with smart wetting performances.However,almost all existing surfaces only focused on single surface chemistry or micromorphology,thus to achieve smart multiple wetting regulation is still difficult.Herein,we report a ZnO coated shape memory polymer(SMP)surface,and the surface chemistry and micromorphology can be synergistically regulated.ZnO can provide adjustable surface chemistry under UV irradiation,and SMP can offer tunable micromorphology due to its shape memory effect(SME).Based on the combined effect between the above two features,surface wetting performance can be smartly regulated among multiple states.Moreover,due to the excellent controllability of the surface,the application in directional droplet transportation was also demonstrated.This paper offers a new surface with tunability in both surface chemistry and micromorphology,and given the excellent wetting controllability,the surface is believed to be applied in a lot of fields,such as droplet manipulation,fluidic devices and selective catalysis.

A thermoviscoelastic finite deformation constitutive model based on dual relaxation mechanisms for amorphous shape memory polymers

This paper proposes a new thermoviscoelastic finite deformation model incorporating dual relaxation mechanisms to predict the complete thermo-mechanical response of amorphous shape memory polymers.The model is underpinned by the detailed microscopic molecular mechanism and effectively reflects the current understanding of the glass transition phenomenon.Novel evolution rules are obtained from the model to characterize the viscous flow,and a new theory named an internal stress model is introduced and combined with the dual relaxation mechanisms to capture the stress recovery.The rationality of the constitutive model is verified as the theoretical results agree well with the experimental data.Moreover,the constitutive model is further simplified to facilitate engineering applications,and it can roughly capture the characteristics of shape memory polymers.

A shape-reconfigurable,light and magnetic dual-responsive shape-memory micropillar array chip for droplet manipulation

Droplet manipulation on an open surface has great potential in chemical analysis and biomedicine engineering.However,most of the reported platforms designed for the manipulation of water droplets cannot thoroughly solve the problem of droplet evaporation.Herein,we report a shape-reconfigurable micropillar array chip for the manipulation of water droplets,oil droplets and water-in-oil droplets.Water-in-oil droplets provide an enclosed space for water droplets,preventing the evaporation in an open environment.Perfluoropolyether coated on the surface of the chip effectively reduces the droplet movement resistance.The micropillar array chip has light and magnetic dual-response due to the Fe3O4 nanoparticles and the reduced iron powder mixed in the shape-memory polymer.The micropillars irradiated by a near-infrared laser bend under the magnetic force,while the unirradiated micropillars still keep their original shape.In the absence of a magnetic field,when the micropillars in a temporary shape are irradiated by the near-infrared laser to the transition temperature,the micropillars return to their initial shape.In this process,the surface morphology gradient caused by the deformation of the micropillars and the surface tension gradient caused by the temperature change jointly produce the driving force of droplet movement.

Application of interpolated double network model for carbon nanotube composites in electrothermal shape memory behaviors

Multi-wall carbon nanotube filled shape memory polymer composite(MWCNT/SMC)possessed enhanced modulus,strength,and electric conductivity,as well as excellent electrothermal shape memory properties,showing wide design scenarios and engineering application prospects.The thermoelectrically triggered shape memory process contains complex multi-physical mechanisms,especially when coupled with finite deformation rooted on micro-mechanisms.A multi-physical finite deformation model is necessary to get a deep understanding on the coupled electro-thermomechanical properties of electrothermal shape memory composites(ESMCs),beneficial to its design and wide application.Taking into consideration of micro-physical mechanisms of the MWCNTs interacting with double-chain networks,a finite deformation theoretical model is developed in this work based on two superimposed network chains of physically crosslinked network formed among MWCNTs and the chemically crosslinked network.An intact crosslinked chemical network is considered featuring with entropic-hyperelastic properties,superimposed with a physically crosslinked network where percolation theory is based on electric conductivity and electric-heating mechanisms.The model is calibrated by experiments and used for shape recoveries triggered by heating and electric fields.It captures the coupled electro-thermomechanical behavior of ESMCs and provides design guidelines for MWCNTs filled shape memory polymers.

Preliminary Investigation of the Reversible 4D Printing of a Dual-Layer Component

The rapid development of additive manufacturing and advances in shape memory materials have fueled the progress of four-dimensional (4D) printing. With increasing improvements in design, reversible 4D printing or two-way 4D printing has been proven to be feasible. This technology will fully eliminate the need for human interference, as the programming is completely driven by external stimuli, which allows 4D-printed parts to be actuated in multiple cycles. This study proposes a new reversible 4D print- ing actuation method. The swelling of an elastomer and heat are used in the programming stage, and heat is used in the recovery stage. The main focus of this study is on the self-actuated programming step. To attain control over the bending, a simple predictive model has been developed to study the degree of cur- vature. The parameters, temperature, and elastomer thickness have also been studied in order to gain a better understanding of how well the model predicts the curvature. This understanding of the curvature will provide a great degree of control over the reversible 4D-printed structure.

A mini review： Shape memory polymers for biomedica applications

Shape memory polymers （SMPs） are smart materials that can change their shape in a pre-defined manner under a stimulus. The shape memory functionality has gained considerable interest for biomedical applications, which require materials that are biocompatible and sometimes biodegradable. There is a need for SMPs that are prepared from renewable sources to be used as substitutes for conventional SMPs. In this paper, advances in SMPs based on synthetic monomers and bio-compounds are discussed. Materials designed for biomedical applica- tions are highlighted.

World’s first spaceflight on-orbit demonstration of a flexible solar array system based on shape memory polymer composites

With a 10%reversible compressive strain in more than 10 deformation cycles,the shape memory polymer composites(SMPCs)could be used for deployable structure and releasing mechanism.In this paper,without traditional electro-explosive devices or motors/controllers,the deployable SMPC flexible solar array system(SMPC-FSAS)is studied,developed,ground-based tested,and finally on-orbit validated.The epoxy-based SMPC is used for the rolling-out variable-stiffness beams as a structural frame as well as an actuator for the flexible blanket solar array.The releasing mechanism is primarily made of the cyanate-based SMPC,which has a high locking stiffness to withstand 50 g gravitational acceleration and a large unlocking displacement of 10 mm.The systematical mechanical and thermal qualification tests of the SMPC-FSAS flight hardware were performed,including sinusoidal sweeping vibration,shocking,acceleration,thermal equilibrium,thermal vacuum cycling,and thermal cycling test.The locking function of the SMPC releasing mechanisms was in normal when launching aboard the SJ20 Geostationary Satellite on 27 Dec.,2019.The SMPC-FSAS flight hardware successfully unlocked and deployed on 5 Jan.,2020 on geostationary orbit.The triggering signal of limit switches returned to ground at the 139 s upon heating,which indicated the successful unlocking function of SMPC releasing mechanisms.A pair of epoxy-based SMPC rolled variable-stiffness tubes,which clapped the flexible blanket solar array,slowly deployed and finally approached an approximate 100%shape recovery ratio within 60 s upon heating.The study and on-orbit successful validation of the SMPC-FSAS flight hardware could accelerate the related study and associated productions to be used for the next-generation releasing mechanisms as well as space deployable structures,such as new releasing mechanisms with low-shocking,testability and reusability,and ultra-large space deployable solar arrays.

Shape Memory Polymer Fibers:Materials,Structures,and Applications

Shape memory polymer(SMP)is a kind of material that can sense and respond to the changes of the external environment,and its behavior is similar to the intelligent refection of life.Electrospinning,as a versatile and feasible technique,has been used to prepare shape memory polymer fbers(SMPFs)and expand their structures.SMPFs show some advanced features and functions in many felds.In this review,we give a comprehensive overview of SMPFs,including materials,fabrication methods,structures,multifunction,and applications.Firstly,the mechanism and characteristics of SMP are introduced.We then discuss the electrospinning method to form various microstructures,like non-woven fbers,core/shell fbers,hollow fbers and oriented fbers.Afterward,the multiple functions of SMPFs are discussed,such as multi-shape memory efect,reversible shape memory efect and remote actuation of composites.We also focus on some typical applications of SMPFs,including biomedical scafolds,drug carriers,self-healing,smart textiles and sensors,as well as energy harvesting devices.At the end,the challenges and future development directions of SMPFs are proposed.

Recent Progress in Shape Memory Polymers for Biomedical Applications

Shape memory polymers （SMPs） as one type of the most important smart materials have attracted increasing attention due to their promising application in the field of biomedicine, textiles, aerospace et al. Following a brief intoduction of the conception and classification of SMPs, this review is focused on the progress of shape memory polymers for biomedical applications. The progress includes the early researches based on thermo-induced SMPs, the improvement of the stimulus, the development of shape recovery ways and the expansion of the applications in biomedical field. In addition, future perspectives of SMPs in the field of biomedicine are also discussed.