Strong,Tailored, Biocompatible Shape‐Memory Polymer Networks

Shape‐memory polymers are a class of smart materials that have recently been used in intelligent biomedical devices and industrial applications for their ability to change shape under a predetermined stimulus. In this study, photopolymerized thermoset shape‐memory networks with tailored thermomechanics are evaluated to link polymer structure to recovery behavior. Methyl methacrylate (MMA) and poly(ethylene glycol) dimethacrylate (PEGDMA) are copolymerized to create networks with independently adjusted glass transition temperatures (T_g) and rubbery modulus values ranging from 56 to 92 °C and 9.3 to 23.0 MPa, respectively. Free‐strain recovery under isothermal and transient temperature conditions is highly influenced by the T_g of the networks, while the rubbery moduli of the networks has a negligible effect on this response. The magnitude of stress generation of fixed‐strain recovery correlates with network rubbery moduli, while fixed‐strain recovery under isothermal conditions shows a complex evolution for varying T_g. The results are intended to help aid in future shape‐memory device design and the MMA‐co‐PEGDMA network is presented as a possible high strength shape‐memory biomaterial.     

Temperature‐Memory Effect of Copolyesterurethanes and their Application Potential in Minimally Invasive Medical Technologies

Minimally invasive surgery often requires devices that can change their geometry or shape when placed inside the body. Here, the potential of thermoplastic temperature‐memory polymers (TMP) for the design of intelligent devices, which can be programmed by the clinician to individually adapt their shifting geometry and their response temperature T_sw to the patient's needs, is explored. Poly(ω‐pentadecalactone) as hard segments and poly(?‐caprolactone) segments acting as crystallizable controlling units for the temperature‐memory effect (TME) are chosen to form multiblock copolymers PDLCL. These components are selected according to their thermal properties and their good biocompatibility. Response temperatures obtained under stress‐free and constant strain recovery can be systematically adjusted by variation of the deformation temperature in a temperature range from 32 °C to 65 °C, which is the relevant temperature range for medical applications. The working principle of TMP based instruments for minimally invasive surgical procedures is successfully demonstrated using three temperature‐memory catheter concepts: individually programmable TM‐catheter, an in‐situ programmable TM‐catheter, and an intelligent drainage catheter for gastroenterology.     

Deformable,Programmable, and Shape‐Memorizing Micro‐Optics

The use of shape memory polymers is demonstrated for deformable, programmable, and shape‐memorizing micro‐optical devices. A semi‐crystalline shape memory elastomer, crosslinked poly(ethylene‐co‐vinyl acetate), is used to prepare various micro‐optic components, ranging from microlens and microprism arrays to diffraction gratings and holograms. The precise replication of surface features at the micro‐ and nanoscale and the formation of crosslinked shape memory polymer networks can be achieved in a single step via compression molding. Further deformation via hot pressing or stretching of micro‐optics formed in this manner allows manipulation of the microscopic surface features, and thus the corresponding optical properties. Due to the shape memory effect, the original surface structures and the optical properties can be recovered and the devices be reprogrammed, with excellent reversibility in the optical properties. Furthermore, arrays of transparent resistive microheaters can be integrated with deformed micro‐optical devices to selectively trigger the recovery of surface features in a spatially programmable manner, thereby providing additional capabilities in user‐definable optics.     

Self‐Healing Shape Memory PUPCL Copolymer with High Cycle Life

New polyurethane‐based polycaprolactone copolymer networks, with shape recovery properties, are presented here. Once deformed at ambient temperature, they show 100% shape fixation until heated above the melting point, where they recover the initial shape within 22 s. In contrast to current shape memory materials, the new materials do not require deformation at elevated temperature. The stable polymer structure of polyurethane yields a copolymer network that has strength of 10 MPa with an elongation at break of 35%. The copolymer networks are self‐healing at a slightly elevated temperature (70 °C) without any external force, which is required for existing self‐healing materials. This allows for the new materials to have a long life of repeated healing cycles. The presented copolymers show features that are promising for applications as temperature sensors and activating elements.     

Copolymer Networks Based on Poly(ω‐pentadecalactone) and Poly(ϵ‐caprolactone)Segments as a Versatile Triple‐Shape Polymer System

Thermo‐sensitive triple‐shape polymers can perform two consecutive shape changes in response to heat. These shape changes correspond to the recovery of two different deformations in reverse order, which were programmed previously at elevated temperature levels (T_mid and T_high) by the application of external stress. Recently, an AB copolymer network was described, which surprisingly exhibited a triple‐shape effect despite being programmed with only one deformation at T_high. Here it is explored whether a copolymer network system can be designed that enables a one‐step deformation process at ambient temperature (cold drawing) as a novel, gentle, and easy‐to‐handle triple‐shape‐creation procedure, in addition to the procedures reported to date, which generally involve deformation(s) at elevated temperature(s). A copolymer‐network system with two crystallizable polyester segments is synthesized and characterized, fulfilling two crucial criteria. These materials can be deformed at ambient temperature by cold drawing and show, even at T_high, which is above the melting points of both switching domains, elongation at break of up to 250%. Copolymer networks with PCL contents of 75 and 50 wt% show a triple‐shape effect after cold drawing with shape‐fixity ratios between 65% and 80% and a total‐shape‐recovery ratio above 97%. Furthermore, in these copolymer networks, the triple‐shape effect can be obtained after a one‐step deformation at T_high. Independent of the temperature at which the deformation is applied (ambient temperature or T_high), copolymer networks that have the same compositions show similar switching temperatures and proportioning of the recovery in two steps. The two‐step programming procedure enables a triple‐shape effect in copolymer networks for an even broader range of compositions. This versatile triple‐shape‐material system based on tailored building blocks is an interesting candidate material for applications in fixation systems or disassembling systems.     

Programmable Macroporous Photonic Crystals Enabled by Swelling‐Induced All‐Room‐Temperature Shape Memory Effects

This study reports unconventional, all‐room‐temperature shape memory (SM) effects using templated macroporous shape memory polymer (SMP) photonic crystals comprising a glassy copolymer with high‐glass transition temperature. “Cold” programming of permanent periodic structures into temporary disordered configurations can be achieved by slowly evaporating various swelling solvents (e.g., ethanol) imbibed in the interconnecting macropores. The deformed macropores can be instantaneously recovered to the permanent geometry by exposing it to vapors and liquids of swelling solvents. By contrast, nonswelling solvents (e.g., hexane) cannot trigger “cold” programming and SM recovery. Extensive experimental and theoretical investigations reveal that the dynamics of swelling‐induced plasticizing effects caused by fast diffusion of solvent molecules into the walls of macropores with nanoscopic thickness dominate both “cold” programming and recovery processes. Importantly, the striking color changes associated with the reversible SM transitions enable novel chromogenic sensors for selectively detecting trace amounts of swelling analytes mixed in nonswelling solvents. Using ethanol–hexane solutions as proof‐of‐concept mixtures, the ethanol detection limit of 150 ppm has been demonstrated. Besides reusable sensors, which can find important applications in environmental monitoring and petroleum process/product control, the programmable SMP photonic crystals possessing high mechanical strengths and all‐room‐temperature processability can provide vast opportunities in developing reconfigurable/rewritable nanooptical devices.     

Self‐Healing Polyurethanes with Shape Recovery

Two new thermoresponsive self‐healing polyurethanes (1DA1T and 1.5DA1T) based on the Diels–Alder (DA) reaction between furan and maleimide moieties are developed that use the shape‐memory effect to bring crack faces into intimate contact such that healing can take place. Unlike other self‐healing polymers, these polymers do not require external forces to close cracks but rather they use the shape‐memory effect to autonomously close the crack. Both polyurethanes have a stable polymer structure and comparable mechanical properties to commercial epoxies. A differential scanning calorimeter is employed to check the glass transition temperature of the polymers as well as the DA and retro‐DA (rDA) reaction temperatures. These DA and rDA reactions are confirmed with variable‐temperature proton nuclear magnetic resonance. Healing efficiency is calculated using a measurement of the failure load from compact tension testing. The results show that the shape‐memory effect can replace external forces to close two crack surfaces and the DA reaction can be repeatedly employed to heal the cracks.     

A Printable Optical Time‐Temperature Integrator Based on Shape Memory in a Chiral Nematic Polymer Network

An optical and irreversible temperature sensor (e.g., a time‐temperature integrator) is reported based on a mechanically embossed chiral‐nematic polymer network. The polymer consists of a chemical and a physical (hydrogen‐bonded) network and has a reflection band in the visible wavelength range. The sensors are produced by mechanical embossing at elevated temperatures. A relative large compressive deformation (up to 10%) is obtained inducing a shift to shorter wavelength of the reflection band (>30 nm). After embossing, a temperature sensor is obtained that exhibits an irreversible optical response. A permanent color shift to longer wavelengths (red) is observed upon heating of the polymer material to temperatures above the glass transition temperature. It is illustrated that the observed permanent color shift is related to shape memory in the polymer material. The films can be printed on a foil, thus showing that these sensors are potentially interesting as time‐temperature integrators for applications in food and pharmaceutical products.     

Novel Photoinduced Recovery of OFET Memories Based on Ambipolar Polymer Electret for Photorecorder Application

A new design concept for novel photoresponsive flash organic field‐effect transistor (OFET) memory is demonstrated by employing the carbazoledioxazine polymer (Poly CD) as an electret. Photoactive electrets that can absorb the light effectively rather than photoactive semiconductors are proposed by the “photoinduced recovery” mechanism in the literature; however, the correlation between the chemical structure and photoresponsive electrical performances is ambiguous. In this study, it is reported for the first time that the OFET memory with trapped charges can be optically recovered by a polymer electret and the working mechanism can be explained by the structural design. The highly planar Poly CD electret exhibits photoluminescence quenching in film states, resulting in the generation of sufficient excitons to eliminate trapped charges under light excitation. Additionally, the Poly CD electret with coplanar donor–acceptor moieties is suitable for both p‐channel and n‐channel semiconductors. For p‐type memory devices, a large memory window (82 V) and stable nonvolatile retention performance with high ON/OFF ratio could be obtained. The memories also display good switching reliability for voltage‐programming and light‐erasing cycles. This study provides useful information for the development of polymer‐based photoresponsive flash OFET memories and demonstrates the practical applications of photorecorder and photosensitive smart tag.     

High‐Strain Shape‐Memory Polymers

Shape‐memory polymers (SMPs) are self‐adjusting, smart materials in which shape changes can be accurately controlled at specific, tailored temperatures. In this study, the glass transition temperature (T_g) is adjusted between 28 and 55 °C through synthesis of copolymers of methyl acrylate (MA), methyl methacrylate (MMA), and isobornyl acrylate (IBoA). Acrylate compositions with both crosslinker densities and photoinitiator concentrations optimized at fractions of a mole percent demonstrate fully recoverable strains at 807% for a T_g of 28 °C, at 663% for a T_g of 37 °C, and at 553% for a T_g of 55 °C. A new compound, 4,4′‐di(acryloyloxy)benzil (referred to hereafter as Xini) in which both polymerizable and initiating functionalities are incorporated in the same molecule, was synthesized and polymerized into acrylate shape‐memory polymers, which were thermomechanically characterized yielding fully recoverable strains above 500%. The materials synthesized in this work were compared to an industry standard thermoplastic SMP, Mitsubishi's MM5510, which showed failure strains of similar magnitude, but without full shape recovery: residual strain after a single shape‐memory cycle caused large‐scale disfiguration. The materials in this study are intended to enable future applications where both recoverable high‐strain capacity and the ability to accurately and independently position T_g are required.     

A Shape Memory High‐Voltage Supercapacitor with Asymmetric Organic Electrolytes for Driving an Integrated NO2 Gas Sensor

A high‐voltage supercapacitor with shape memory for driving an integrated NO₂ gas sensor is fabricated using a Norland Optical Adhesive 63 polymer substrate, which can recover the original shape after deformation by short‐time heating. The supercapacitor consists of multiwalled carbon nanotube electrodes and organic electrolyte. By using organic electrolyte consisting of adiponitrile, acetonitrile, and dimethyl carbonate in an optimized volume ratio of 1:1:1, a high operation voltage of 2 V is obtained. Furthermore, asymmetric electrolytes with different redox additives of hydroquinone and 1,4‐dihydroxyanthraquinone to the anode and cathode, respectively, enhance both capacitance and energy density by ≈40 times compared to those of supercapacitor without redox additives. The fabricated supercapacitor on the Norland Optical Adhesive 63 polymer substrate retains 95.8% of its initial capacitance after 1000 repetitive bending cycles at a bending radius of 3.8 mm. Furthermore, the folded supercapacitor recovers its shape upon heating at 70 °C for 20 s. In addition, 90% of the initial capacitance is retained even after the 20th shape recovery from folding. The fabricated supercapacitor is used to drive integrated NO₂ gas sensor on the same Norland Optical Adhesive 63 substrate attached onto skin to detect NO₂ gas, regardless of deformation due to elbow movement.     

掺杂元素对Ni-Mn-Ga合金结构与性能的影响

Ni-Mn-Ga合金是一种新型的智能材料,兼具铁磁性和形状记忆效应的双重优点,以对磁场的快速响应和大的宏观可恢复应变的特点有望成为传感器和驱动器的候选材料.通过掺入金属元素可以弥补该合金马氏体相变温度低,脆性大的不足.综述了掺杂元素对合金结构、相变温度、力学性能、形状记忆效应的影响,并提出了需要深入研究的问题.     

Magnetic Memory Effect of Nanocomposites

The magnetic memory effect (MME) is the ability of magneto‐sensitive materials to remember the magnetic field strength (H_def), at which they were deformed recently. They respond close to H_def either by recovering their initial shape at a switching magnetic field strength H_sw under stress‐free conditions or by building up stress with a peak maximum at H_σ,max under constant strain conditions. This paper explores whether such a MME can be created for polymer‐based nanocomposites. The concept is based on temperature‐memory polymers (TMP) as matrix, in which silica coated iron(III)oxide nanoparticles (mNP) are dispersed. The MME was explored in a cyclic magneto‐mechanical test, in which the nanocomposite sample was elongated to ?_m while being exposed to an alternating magnetic field at H_def. The magnetic memory was read out by determining H_σ,max or H_sw. A linear correlation between H_σ,max (or H_sw) and H_def in a range from 15 to 23 kA m^?1 at a fixed frequency of f = 258 kHz is observed and demonstrates the excellent magnetic memory properties of the investigated nanocomposites containing either crystallizable or amorphous, vitrifiable domains as controlling units. The deformation ?m at H_def can be fixed with an accuracy of more than 72% and the initial shape can be recovered almost completely by more than 86%. The MME allows the design of magnetically programmable devices such as switches or mechanical manipulators.     

Polyampholyte Hydrogels with pH Modulated Shape Memory and Spontaneous Actuation

Polyampholyte 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.     

Rapid Near‐Infrared Light Responsive Shape Memory Polymer Hybrids and Novel Chiral Actuators Based on Photothermal W18O49 Nanowires

Photoresponsive actuators are built by introducing oligo(ethylene glycol) (OEG)‐modified W₁₈O₄₉ nanowires into cross‐linked polyethylene glycol diacrylate (cPEGDA) polymer matrices. Due to the good compatibility, OEG‐W₁₈O₄₉ NWs disperse well and increase the crystallinity of cPEGDA matrices even in high loading concentrations (4.0 wt%). The cPEGDA/W₁₈O₄₉ nanocomposites show efficient photothermal transition and rapid shape memory behaviors. They can raise the local temperature to 160 °C in only 8.5 s and recover the initial shape within 10 s. Making use of the broad and strong absorption property of W₁₈O₄₉, the cPEGDA/W₁₈O₄₉ NW actuators respond to both ultraviolet and near‐infrared light and make contraction and bending motions. Furthermore, by utilizing oriented chain segments of the crystalline polymer and vector sum of shape recovery forces, the cPEGDA/W₁₈O₄₉ NW hybrid actuators exhibit stable helical deformation (right‐handed and left‐handed).     

Nanoimprint Lithography and the Role of Viscoelasticity in the Generation of Residual Stress in Model Polystyrene Patterns

Understanding polymer deformation during the nanoimprinting process is key to achieving robust polymer nanostructures. Information regarding this process can be extracted from monitoring the decay of the imprinted polymer patterns during thermal annealing. In the present work, the effect of both the molar mass and the imprinting temperature on the pattern decay behavior during thermal annealing is investigated. Previously, it was found that the decay rate is fastest for a highly entangled polymer due to the elastic recovery caused by the residual stress created during the imprinting process. The present paper demonstrates that this residual stress level can be modified through control of the imprinting temperature. These results are contrasted with those for an unentangled polymer over a similar range of imprinting temperatures, where it is found that the pattern decay is controlled by simple Newtonian flow. In particular, the pattern decay is well described by surface‐tension‐driven viscous flow, and no imprinting‐temperature effect is observed during thermal annealing. It is shown that the stability of the film against pattern decay can be optimized for moderately entangled polymer films. This effect is attributed to the competition between the effect of increased viscosity with increasing molar mass and increased residual stresses with entanglements. These observations provide guidance for the optimization of imprinting process in terms of selection of molar mass and processing temperatures.     

Programmed Shape‐Morphing Scaffolds Enabling Facile 3D Endothelialization

Rapid formation of a confluent endothelial monolayer is the key to the success of small‐diameter vascular grafts, which is significantly important for treating dangerous and even sometimes deadly vascular disorders. However, the difficulty to homogenously locate endothelial cells onto the lumen of small‐diameter tubular scaffolds makes 3D endothelialization greatly challenging. Here, novel shape‐morphing scaffolds enabling programmed deformation from planar shapes to small‐diameter tubular shapes are designed and developed by combining biocompatible shape memory polymer and electrospun nanofibrous membrane. Endothelial cells can be conveniently seeded and attached on the 2D surface of the scaffolds and subsequently self‐rolled into 3D organization at physiological temperature. Endothelial cell responses and functions are varied on the shape‐morphing scaffolds with different nanofibrous electrospun membranes as the inner layer, arisen from the inducement of scaffolds with different morphological, physical, and biochemical characteristics. Owing to excellent properties of the nanofibrous membrane fabricated by the coelectrospinning of poly‐ε‐caprolactone (PCL) and gelatin methacrylate (GelMA), the shape‐morphing scaffolds with a nanofibrous PCL/GelMA inner layer support desirable homogeneous endothelial cell attachment as well as the rapid formation of biomimetic cell–scaffold interaction and cell–cell interaction under the 3D cell culture condition, therefore offering a visible approach for facile 3D endothelialization.     

Temperature‐Induced Switchable Adhesion using Nickel–Titanium–Polydimethylsiloxane Hybrid Surfaces

A switchable dry adhesive based on a nickel–titanium (NiTi) shape‐memory alloy with an adhesive silicone rubber surface has been developed. Although several studies investigate micropatterned, bioinspired adhesive surfaces, very few focus on reversible adhesion. The system here is based on the indentation‐induced two‐way shape‐memory effect in NiTi alloys. NiTi is trained by mechanical deformation through indentation and grinding to elicit a temperature‐induced switchable topography with protrusions at high temperature and a flat surface at low temperature. The trained surfaces are coated with either a smooth or a patterned adhesive polydimethylsiloxane (PDMS) layer, resulting in a temperature‐induced switchable surface, used for dry adhesion. Adhesion tests show that the temperature‐induced topographical change of the NiTi influences the adhesive performance of the hybrid system. For samples with a smooth PDMS layer the transition from flat to structured state reduces adhesion by 56%, and for samples with a micropatterned PDMS layer adhesion is switchable by nearly 100%. Both hybrid systems reveal strong reversibility related to the NiTi martensitic phase transformation, allowing repeated switching between an adhesive and a nonadhesive state. These effects have been discussed in terms of reversible changes in contact area and varying tilt angles of the pillars with respect to the substrate surface.     

Oligocrystalline Shape Memory Alloys

Copper‐based shape memory alloys (SMAs) exhibit excellent shape memory properties in single crystalline form. However, when they are polycrystalline, their shape memory properties are severely compromised by brittle fracture arising from transformation strain incompatibility at grain boundaries and triple junctions. Oligocrystalline shape memory alloys (oSMAs) are microstructurally designed SMA structures in which the total surface area exceeds the total grain boundary area, and triple junctions can even be completely absent. Here it is shown how an oligocrystalline structure provides a means of achieving single crystal‐like SMA properties without being limited by constraints of single crystal processing. Additionally, the formation of oSMAs typically involves the reduction of the size scale of specimens, and sample size effects begin to emerge. Recent findings on a size effect on the martensitic transformation in oSMAs are compared and a new regime of heat transfer associated with the transformation heat evolution in these alloys is discussed. New results on unassisted two‐way shape memory and the effect of loading rate in oSMAs are also reported.     

Deformation behavior of solid polymer during hot embossing process

Though hot embossing is a well known technique for the fabrication of polymer based micro-device, the deformation behavior of solid polymer during hot embossing process is not investigated clearly. In this paper, the deformation behavior of solid polymer was observed by two methods, synchronous observation and asynchronous analysis. A finite element simulation and a phenomenological model were used to evaluate the deformation behavior of solid polymer during hot embossing. Results showed that the deformation of solid polymer during embossing process included two stages. One was a stress concentration and strain hardening stage, which occurred during the heating and applying pressure process. The “swallowtails” induced by incomplete filling generate at this stage. The other was a stress relaxation and deformation recovery stage, which occurred during the remaining temperature and pressure process. The “swallowtails” were eliminated at this stage. The second stage was significant for improving replication precision, but it had not been reported before.