Shape Memory Polymer Fibers Research Articles

Programming-via-spinning: Electrospun shape memory polymer fibers with simultaneous fabrication and programming

Porous shape memory polymer (SMP) scaffolds are promising ‘smart’ materials for potential use in a wide range of biomedical applications. Electrospinning provides an approach to produce fibrous SMP scaffolds to enhance their porosity, mass transfer, and flexibility. Here, we studied the effects of electrospinning parameters (rotating collector rotational speed and solvent) on shape memory and mechanical properties of a biostable thermoplastic polyurethane (PUr) SMP. Scanning electron microscopy confirmed that fiber diameter and tortuosity could be tuned using varied collector rotation speeds and/or solvents. Mechanical properties, including modulus, tensile strength, and ultimate elongation, were tuned independently of chemistry based on variations in fiber architectures. All scaffolds demonstrated shape memory properties. Additionally, due to strains that are trapped in the fibers during the electrospinning process, SMP fibers are programmed into a strained, temporary shape during the fabrication step. These fibers can be immediately triggered to recover to a non-strained primary shape after fabrication to reduce sample preparation time and complexity. As a proof-of-concept, bacterial protease-responsive SMPs were electrospun and exposed to S. aureus in programmed secondary shapes. Upon exposure to bacteria, these SMPs underwent shape recovery, which resulted in reduced bacterial attachment and biofilm formation. These materials could be employed as bacteria-responsive wound dressings in future work. Overall, electrospinning provides a valuable tool for tuning mechanical and shape memory properties independently from chemistry and for programming SMPs during fabrication to enable scale-up of electrospun SMP scaffolds.

Development of shape-memory polymer fiber reinforced epoxy composites for debondable adhesives

The concept of debonding for hybrid structures is promising due to its design and recycling complexities, but conventional debondable adhesives encounter limitations. This study introduced a novel shape-memory polymer fiber (SMPF)–reinforced debondable adhesive applied in the lap shear adhesive joint of aluminum substrates. Following a comprehensive characterization investigation of the SMP including new rememorization test, the SMPF was fabricated in an innovative, easy and cost-effective way. Compared with the conventional adhesive joint, this composite adhesive offered maximum adhesion strength 25% higher at room temperature, conversely, decreased 50% lower strength at high temperature by utilizing shape memory effect. It can be used for joining hybrid structures, which requires simple assembly.

Multifunctional two-way shape memory RGO/ethylene-vinyl acetate composite yarns for electro-driven actuators and high sensitivity strain sensors

Shape memory polymer fibers (SMPFs) coated with conductive layers are attracting increasing attention in soft robots and wearable electronics. However, the thickness and poor stretchability of the conductive layer hinder its applications. Here, the multifunctional two-way shape memory reduced graphene oxide (RGO)/ethylene–vinyl acetate copolymer (EVA) composite yarn is designed following the “swelling-ultrasonicating” and one-step twisting procedures. These RGO/EVA composite fiber conductive paths connected with each other by constructing the helical conductive network. The RGO/EVA composite yarn presents excellent electro-thermal conversion performance and outstanding load-bearing capability, enabling controlled electro-driven reversible strain (more than 10% reversible strain) and high energy density (72.5 J/kg). Additionally, the composite yarn has great stretchability (130% strain), high strain sensing sensitivity (gauge factor of 7.3), large linear working range (up to 70% strain). This work provides an innovative manufacturing strategy for the RGO/EVA composite yarn with two-way shape memory, electro-driven actuation, and strain sensing performance.

Adaptive IR- and Water-Gating Textiles Based on Shape Memory Fibers.

Thermoregulation is an essential function of the human body for adapting to the surrounding temperature. Stimuli-responsive smart textiles can provide effective protection of the human skin temperature from a continuously changing environment. Herein, we develop a smart textile based on shape memory polymer (SMP) fibers for adaptive regulation of IR and water transmission on human skin. An SMP textile is fabricated with hierarchical micro/nanoporous structures to enhance thermal insulation performance, and silver nanowires are coated on one side to provide asymmetric IR reflectivity and hydrophilicity. The porous SMP textile shows great tunability of thermal insulation and asymmetric wettability by deformation and recovery of the shape and structure in response to stimuli. The degree of thermal insulation is controlled by 65.7% of the original value, and the surface temperature of the SMP textile on a hot plate is successfully controlled in the IR images due to adaptive IR reflectivity. Additionally, the directional transportation of water droplets can be switched on/off according to the shape of the SMP textiles, which can be employed for sweat removal from the human skin. This IR- and water-gating smart textile can provide a feasible strategy for protecting the human skin from external environmental changes.

High-performance fibrous artificial muscle based on reversible shape memory UHMWPE

Shape memory polymer fibers (SMPFs) are an important branch of intelligent fibers. Due to their ability to deform under external stimuli, shape memory polymer fibers have been used in intelligent textiles, polymer fillers and medical fields, including surgical sutures, thrombectomy devices, compression stockings, etc. There have been many studies on SMPFs, but almost all reported SMPFs only exhibit a one-way shape memory effect (SME). The deformation process of the one-way SMPFs is irreversible, and the transition from temporary shape to permanent shape cannot be replicated by simply reversing the stimulus. To tackle this problem. We selected ultra-high molecular weight polyethylene (UHMWPE) as the polymer matrix to prepare external stress-free two-way SMPFs. UHMWPE powders are gel-spun and then hot drawn to yield raw UHMWPE fibers. After programming, the UHMWPE fibers possess high tensile strength (338.72 MPa) and Young's modulus (2287.34 MPa), as well as one-way SME, quasi two-way SME and external stress-free reversible two-way SME. The reversible strain of the prepared reversible shape memory UHMWPE fibers is 27.42%. Artificial muscles and actuators can be made from the programmed UHMWPE fibers. The programmed UHMWPE fibers can lift more than 250 times their own weight with ∼10% reversible strain. The programmed UHMWPE fibers are appropriate for industrial mass production since the preparation and programming processes are simple.

Mode shifting shape memory polymer and hydrogel composite fiber actuators for soft robots

Soft robotics is a promising new field offering robot systems that mimic the versatility and complexity of movement and propulsion seen in natural organisms. Such systems require complex actuation including length expansion/contraction, bending and torsion. To date, such actuators have been configured individually. Here, it is shown that composite fibers formed from a shape memory polymer and a thermo-sensitive hydrogel could be configured into any one of the tensile, torsional and flexural actuators. Furthermore, the programmed mode of actuation could be thermally erased and the same fiber re-programmed into a different type of actuator. The fibers were prepared from poly(N-isopropylacrylamide) with polycaprolactone and the fiber composition was tuned to optimize both shape fixity and the degree of actuation. The fibers could be programmed by heating to 60 ℃ and then cooled under tensile, flexural or torsional load. The fiber was then conditioned by immersing in water at room temperature which induced swelling of the hydrogel phase and shape deformation. The pre-programmed shape of the fiber was then restored when the fiber was heated at 35 ℃ in water to de-swell the thermo-sensitive poly(N-isopropylacrylamide) hydrogel. Reversible actuation was observed through multiple cycles of heating and cooling. The optimized fibers generated torsional strokes of 10 turn/m; bending curvature changes of 0.4 mm−1 or tensile strokes of up to 92%. The composite fibers offer a convenient means for generating a variety of different actuation types for soft robotics.

Numerical study of the heating effects of high intensity focused ultrasound on shape memory polymer fiber reinforced self-healing polymer composite

In polymer composites, constrained shape recovery, either by shape memory polymer (SMP) matrix or embedded SMP fibers, is utilized to close wide-opened cracks, followed by molecular scale healing either extrinsically or intrinsically, the so-called close-then-heal strategy. The most popular means to trigger shape recovery and healing is by heating. In this study, the potential for using high intensity focused ultrasound as a trigger for shape memory effect and self-healing of an SMP fiber-reinforced thermosetting polymer composite was explored. The objectives of the study were achieved via a finite element simulation in COMSOL Multiphysics. Simulation results showed that for an 8 mm thick, 18 mm long model composite with 5.8% SMP fiber volume fraction, a temperature rise of up to 5.6 K was obtained in the embedded SMP fiber after 2 s with only 1 s of the insonation. It was found that heat generation occurred at the fiber–matrix interfaces; however, subsequent temperature rise within the embedded fibers was achieved via conventional thermal conduction from the high-temperature interfacial regions into the fibers. The effect of specimen length, thickness, and fiber volume fraction on temperature rising was also evaluated. This study provides a better understanding of using ultrasound to trigger self-healing of SMP fiber-reinforced polymer composites.

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 reflection of life. Electrospinning, as a versatile and feasible technique, has been used to prepare shape memory polymer fibers (SMPFs) and expand their structures. SMPFs show some advanced features and functions in many fields. 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 fibers, core/shell fibers, hollow fibers and oriented fibers. Afterward, the multiple functions of SMPFs are discussed, such as multi-shape memory effect, reversible shape memory effect and remote actuation of composites. We also focus on some typical applications of SMPFs, including biomedical scaffolds, 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.

Influence of uniaxial compression on the shape memory behavior of vitrimer composite embedded with tension‐programmed unidirectional shape memory polymer fibers

AbstractTension programmed shape memory polymer (SMP) fibers have been used as sutures for closing wide‐opened cracks per the close‐then‐heal strategy. However, the composite may be subjected to compression loading during service. These compression loads can reduce the amount of recoverable strain in these pre‐tensioned fibers, limiting their ability to close cracks. The purpose of this study is to investigate the effect of in‐service compression loading on the shape memory effect (SME) of composites consisting of SMP fiber and SMP matrix. To this end, pre‐stretched shape memory Polyethylene Terephthalate (PET) fibers were embedded into a shape memory vitrimer to obtain composite samples with different fiber volume fractions. The SME of both the PET fiber and the vitrimer was investigated. The effect of compression load on the SME of the composite was studied. It is found that, uniaxial compression on the composite along the fiber direction significantly reduced the shrinking ability of the embedded pre‐tensioned SMP fibers. Hence, this is a factor that needs to be considered when designing such types of self‐healing composites.

Excellent triple-shape memory effect and superior recovery stress of ethylene-vinyl acetate copolymer fiber

Shape memory composites are often made from fibers with a shape memory effect. Ethylene-vinyl acetate copolymer (EVA) is a commonly used material in manufacturing shape memory polymer fibers for composites. It is well known that triple-shape memory effect (triple-SME) is important for SM materials to meet multiple temporary shape changes. In this work, the excellent triple-SME of EVA fiber is first reported by regulating EVA fiber crystalline structure transformation through controlling thermo-mechanical coupling conditions. The wide melting transition temperature is observed by dynamic mechanical analysis and differential scanning calorimetry. Thermo-mechanical testing verifies that the EVA fiber has the ability to achieve an excellent triple-SME and tunable reversible SME. Besides, the maximum recovery stress (at iso-strain ε = 5%) is 1.45 MPa when the actuation temperature is 75 °C. The synthesized EVA fiber with an excellent shape memory effect and a superior recovery stress is highly promising for various applications such as deployable space structures and fasteners, artificial muscle and self-finishing smart textiles, etc.

Nanosilica-treated shape memory polymer fibers to strengthen wellbore cement

To attain cement sheath integrity, a tight and ductile microstructure for the cement matrix is desirable. Despite improvements achieved in cement design by employing fiber additives, limitations such as low compressive strength and formation of weak regions may result in fracture initiation points in the cement matrix. In this work we grafted silica nanoparticles on the surface of expandable polymer fibers through a sol-gel process to improve the bonding with the cement matrix and inhibit the formation of weak points in the cement sheath. Considering water-phobic nature of some of these polymeric surfaces, hydrophilic coating of fibers may improve their adhesions to the water-based cement, significantly. The mechanical performance of the samples made of treated fibers are characterized by means of compressive strength, flexural strength, and cyclic loading. Cement samples with nanosilica-treated fibers demonstrated superior results in all mechanical tests in comparison to samples made from untreated fibers. Even at low concentrations, such as 2% by weight, higher flexural strength is observed in the cement with the treated fibers. We also noticed that the flexural strength increases proportionally to the fiber concentration. Therefore, the concentration of additives may be tailored to deter crack propagation in accordance with the other requirements of the cementing operation. The expansive capabilities and rheological properties of the cement are maintained after the surface treatment. The results also demonstrate that the damage to the cement during cyclic loading is reduced when the cement is reinforced with treated fibers that can be interpreted as improved resiliency which is critical in the wells undergone frequent pressure fluctuations during hydraulic fracturing treatments.

Effect of temperature on the programmable helical deformation of a reconfigurable anisotropic soft actuator

Shape reconfiguration is ubiquitous in nature and widely used in many applications such as soft robotics, metamaterials, energy absorption and tissue engineering. Shape reconfigurable soft actuators, due to their ability to adapt and adjust in complex and unpredictable working environment, have been designed by the use of various delicate structures and active materials. However, soft actuators that exhibit reconfigurable helical deformation have not been proposed; they have the advantage of integrating both bending and twisting actuations in one deformation mode. In this work, we present a thermal-induced shape reconfigurable soft actuator that shows reversible actuations with vastly shape differences under thermal stimulus. It exhibits helical deformation at lower temperature and mainly in-plane bending at relatively higher temperature. The reversible shape transition is controlled by a thermal stimulus that changes the anisotropy of the structure, which consists of shape memory polymer fibers embedded in a homogeneous elastic matrix. A theoretical model is proposed based on the minimum potential energy that incorporates the thermomechanical behavior of the shape memory polymer fibers. Experiments are conducted and the results agree well with the theoretical modeling. Using the theoretical model, we establish design principles for reconfigurable soft actuators whose functional response is programmable given the architecture and external stimulus. A six-handed helical soft actuator, constructed to demonstrate its programmable deformation, is utilized to catch a living fish in water.

Fabrication and Evaluation of Shape Memory Polymer Fibers for Application to Artificial Muscles

Fabrication and Evaluation of Shape Memory Polymer Fibers for Application to Artificial Muscles

Design of Ethylene-Vinyl Acetate Copolymer Fiber with Two-Way Shape Memory Effect.

One-dimensional shape memory polymer fibers (SMPFs) have obvious advantages in mechanical properties, dispersion properties, and weavability. In this work, a method for fabricating semi-crystallization ethylene-vinyl acetate copolymer (EVA) fiber with two-way shape memory effect by melt spinning and ultraviolet (UV) curing was developed. Here, the effect of crosslink density on its performance was systematically analyzed by gel fraction measurement, tensile tests, DSC, and TMA analysis. The results showed that the crosslink density and shape memory properties of EVA fiber could be facilely adjusted by controlling UV curing time. The resulting EVA fiber with cylindrical structure had a diameter of 261.86 ± 13.07 μm, and its mechanical strength and elongation at break were 64.46 MPa and 114.33%, respectively. The critical impact of the crosslink density and applied constant stress on the two-way shape memory effect were analyzed. Moreover, the single EVA fiber could lift more than 143 times its own weight and achieve 9% reversible actuation strain. The reversible actuation capability was significantly enhanced by a simple winding design of the single EVA fiber, which provided great potential applications in smart textiles, flexible actuators, and artificial muscles.

Study on 3D printing process of continuous carbon fiber reinforced shape memory polymer composites

In this paper, shape memory polymer (SMP) and continuous carbon fibers (CCF) were utilized as raw materials to manufacture composites by means of 3D printing based on FDM. Various process parameters were systematically studied by the orthogonal experiment to find out the optimal process combination where interfaces and mechanical properties of printed composites specimens were treated as research objectives. The influence of each factor on the performance of the specimen was discussed. The results indicated that the mechanical properties were preferable when the printing temperature was 200°C, the printing speed was 4.5mm/s, the scanning pitch was 1.1 mm, and the ply stacking sequence was 0°. Furthermore, the fiber content had little effect on the shape memory performance of the polymer matrix. The rapid manufacture of shape memory carbon fiber composites has potential use in the field of aerospace.

McKibben artificial muscle realizing variable contraction characteristics using helical shape-memory polymer fibers

McKibben artificial muscles that consist of a rubber tube and a sleeve with knitted fibers have multiple advantages, including flexibility, low cost, and lightweight. Therefore, they are a prospective driving source for devices and robots used in the medical welfare field. The McKibben artificial muscles generate contraction motion by applying pneumatic pressure, and the contraction characteristics depend on initial knitting angle of the fibers. Namely, the contraction characteristics are decided during the fabrication process. Generally, a system with multiple artificial muscles has multiple regulators or valves to drive them with different contraction amount individually. This results in increased size, weight, and cost. To solve this problem, we proposed a novel artificial muscle structure that can change contraction characteristics on site with ease. The actuator has helical shape-memory polymer (SMP) fibers as sleeve fibers. Owing to shape fixity which is a unique function of SMPs, the angle of the helical SMP fibers, that is equivalent to the knitting angle of the sleeve fibers, can be varied even after fabrication, and contraction characteristics can be changed. Different contraction characteristics of a fabricated artificial muscle were confirmed experimentally by changing the angle of the helical SMP fibers.

Injectable Nanoreinforced Shape-Memory Hydrogel System for Regenerating Spinal Cord Tissue from Traumatic Injury.

Traumatic injury in the central nervous system can lead to loss of functional neurons. Transplantation of neural progenitors is a promising therapeutic strategy. However, infusion of dissociated cells often suffers from low viability, uneven cell distribution, and poor in vivo engraftment that could be reinforced by a better cell delivery system. Here, we develop an injectable composite hydrogel system for use as a minimally invasive treatment of spinal cord injury (SCI) using motor neurons (MNs) derived from embryonic stem cells (ESCs). The composite hydrogel is based on a modified gelatin matrix integrated with shape-memory polymer fibers. The gelatin matrix creates a local microenvironment for cell assembly and also acts as a lubricant during injection through a fine catheter. Notably, shape-memory fiber scaffolds are able to recover to maintain the microstructures even after dramatic deformation from injection operation, providing the necessary support and guidance for motor neuron differentiation. We find that the composite hydrogel with an aligned fiber scaffold greatly improves the viability of ESCs and their differentiation toward MNs both in vitro and in vivo. When transplanted to SCI animals by injection, the ESC-loaded composite hydrogels are identified to significantly enhance tissue regeneration and motor function recovery in mice. With this proof-of-concept study, we believe that the injectable composite hydrogel system provides a promising solution for in vivo cell delivery with minimum invasiveness and can be readily extended to other stem-cell-based regenerative treatments.

繊維状形状記憶ポリマーを複合した軸方向繊維強化型ラバーアクチュエータの駆動特性

繊維状形状記憶ポリマーを複合した軸方向繊維強化型ラバーアクチュエータの駆動特性

Artificial muscles made of chiral two-way shape memory polymer fibers

In this work, we demonstrate the unusual improvement of the tensile actuation of hierarchically chiral structured artificial muscle made of two-way shape memory polymer (2W-SMP) fiber. Experimental results show that the chemically cross-linked poly(ethylene-co-vinyl acetate) 2W-SMP fibers possess an average negative coefficient of thermal expansion (NCTE) that is at least one order higher than that of the polyethylene fiber used previously. As expected, the increase in axial thermal contraction of the precursor fiber leads to an increase in the recovered torque (4.4 Nmm) of the chiral fiber and eventually in the tensile actuation of the twisted-then-coiled artificial muscle (67.81±1.82%). A mechanical model based on Castigliano's second theorem is proposed, and the calculated result is consistent with the experimental result (64.17% tensile stroke). The model proves the significance of the NCTE and the recovered torque on tensile actuation of the artificial muscle and can be used as a guidance for the future design.

Multi-shape active composites by 3D printing of digital shape memory polymers.

Recent research using 3D printing to create active structures has added an exciting new dimension to 3D printing technology. After being printed, these active, often composite, materials can change their shape over time; this has been termed as 4D printing. In this paper, we demonstrate the design and manufacture of active composites that can take multiple shapes, depending on the environmental temperature. This is achieved by 3D printing layered composite structures with multiple families of shape memory polymer (SMP) fibers – digital SMPs - with different glass transition temperatures (Tg) to control the transformation of the structure. After a simple single-step thermomechanical programming process, the fiber families can be sequentially activated to bend when the temperature is increased. By tuning the volume fraction of the fibers, bending deformation can be controlled. We develop a theoretical model to predict the deformation behavior for better understanding the phenomena and aiding the design. We also design and print several flat 2D structures that can be programmed to fold and open themselves when subjected to heat. With the advantages of an easy fabrication process and the controllable multi-shape memory effect, the printed SMP composites have a great potential in 4D printing applications.