The purpose of this paper is to provide a review of recent systematic and comprehensive advancement in electrospun polymer fiber and their composites with shape memory property.
The nanofiber manufacture technique is initially reviewed. Then, the influence of electrospinning parameters and actuation method has been discussed. Finally, the study concludes with a brief review of recent development in potential applications.
Shape memory polymer (SMP) nanofibers are a type of smart materials which can change shape under external stimuli (e.g. temperature, electricity, magnetism, solvent). In general, such SMP nanofibers could be easily fabricated by mature electrospinning technique. The nanofiber morphology is mainly affected by the electrospinning parameters, including applied voltage, tip-to-collector distance, viscosity of solution, humidity and molecular weight. For actuation method, most SMP nanofibers and their composites can change their shapes in response to heat, magnetic field or solvent, while few can be driven by electricity. Compared with the block SMPs, electrospun SMP nanofibers’ mat with porosity and low mechanical property have a wide potential application field including tissue engineering, drug delivery, filtration, catalysis.
This paper provides a detailed review of shape memory nanofibers: fabrication, actuation and potential application, in the near future.
Yao, Y., Xu, Y., Wang, B., Yin, W. and Lu, H. (2018), "Recent development in electrospun polymer fiber and their composites with shape memory property: a review", Pigment & Resin Technology, Vol. 47 No. 1, pp. 47-54. https://doi.org/10.1108/PRT-04-2017-0039Download as .RIS
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Copyright © 2018, Emerald Publishing Limited
As we know, shape memory polymers (SMPs) are smart materials that can switch from a temporary to permanent shape upon appropriate stimulation, such as temperature, electricity, light, magnetism, water, pH, specific ions or enzyme (Meng and Hu, 2009; Lu et al., 2015a, b, c, d, e; Dong et al., 2015a, b; Wang et al., 2016; Wang et al., 2017; Dong et al., 2015a). Applications of SMPs span various areas of everyday life because of their low cost, easy processing and good biocompatibility. Such applications can be found in, for example, smart fabrics (Hu et al., 2002; Mondal and Hu, 2006), morphing structures (Liu et al., 2014; Lu et al., 2014; Lu et al., 2016b), smart medical devices (Torbati et al., 2014), implants for minimally invasive surgery (Lendlein and Langer, 2002; Metcalfe et al., 2003), security labels and information dissemination (Lu et al., 2015a, b, c, d, e), heat-shrinkable tubes for electronics or films for packaging (Campbell et al., 2005; Luo et al., 2016) or self-disassembling mobile phones (Hussein and Harrison, 2004).
According to the forms, SMP productions are generally divided into three categories: block, foam and fiber. In the past decades, the investigations of SMPs mainly focus on block SMP at macroscopy sale. By comparing with block and foam, polymer nanofibers have some unique properties such as a large surface area-to-volume ratio, the potential to incorporate active chemistry, filtration properties, high permeability, layer thinness and low basis weight (Ramakrishna et al., 2006) which have attracted more researchers’ attention. SMP nanofibers combine the shape memory property and nanofiber feature, which will enhance their performance and expend their potential application fields. For example, shape memory polyurethane (SMPU) microfiber membrane affords much quicker and sharper shape recovery as compared to the bulk SMPU film when heated in water bath (Zhang et al., 2011). The quicker shape recovery of the microfiber SMPU film is attributed to the higher surface area of a microfiber film which increases the absorption efficiency of heat flow and causes quick diffusion of water. Meanwhile, the microsize is another important factor for effecting shape memory properties of SMP fiber. Polymer chains on the material surface have higher mobility because of less constraint from a neighbor. The volume fraction of such polymer chains is higher in a microsized SMP. That is why the transition temperature of microfiber and thin-film SMP is lower than that of block SMP. SMP nanofibers provide a potential way to improve the shape recovery speed without a change in the chemical composition.
Several excellent papers review the recent development in the materials’ design, stimulus methods and applications of SMPs (Zhao et al., 2015; Lu et al., 2013; Hu, et al., 2012). This review mainly presents recent systematic and comprehensive advancement in electrospun polymer fiber and their composites with shape memory property. Besides, parameters during electrospinning progress are discussed to obtain nanofibers of desired morphology and diameters. The shape memory performance of electrospun polymer fiber and their composites has been demonstrated. The content of this work covers one-way, two-way and multiple-shape memory effect (SME), as well as various actuation methods. Finally, the potential applications of electrospun SMP nanofibers have been exhibited.
2. Fabrication method for nanofiber
2.1 Fiber preparation methods
Currently, there are many kinds of polymer fiber preparation technologies such as wet spinning, dry spinning, melt spinning, gel spinning and electrospinning. The spinning process could be easily described as force the prepared spinning liquid through the spinneret to form fiber that will be solidified by decreasing temperature or removing solvent. The spinning liquid could be prepared by simply melting thermoplastic materials or solvent dissolution. Compared with two conventional spinning techniques, fibers produced via electrospinning have a smaller diameter and a higher surface area. In this case, electrospinning as a mature nanofiber/microfiber manufacturing technology with wide applications has gained tremendous attention. Many kinds of organic/inorganic microfibers could be produced from their sol–gel solutions based on an electric field.
At present, there are mainly two kinds of electrospinning setups, namely, needle-assisted devices and needle-free devices, as shown in Figure 1. The common needle-assisted setup for electrospinning usually consists of a spinneret (typically a hypodermic syringe needle) connected to a high-voltage direct current power supply, a syringe pump and a grounded collector. When a sufficiently high voltage (5 to 50 kV) is applied to the solution, electrostatic repulsion within the charged solution counteracts the surface tension and a jet erupts from the tip of the spinneret at a critical voltage. By comparison, for the needle-free electrospinning, Taylor cones could be created from a thin film of a polymer solution based on free liquid surface electrospinning from a rotating electrode.
The electrospinning process is governed by many parameters including applied voltage, tip-to-collector distance, viscosity of solution, humidity and molecular weight. The morphology of electrospun fibers is significantly affected by each of these parameters:
In most instances, a higher voltage causes a stronger electric field and higher stretching stress, resulting in deduction in the fiber diameter and rapid evaporation of solvent. However, there is more polymer ejection with an increase in the applied voltage, which may facilitate the formation of a larger diameter or bead defects (Bhardwaj and Kundu, 2010). In the case of poly ε-caprolactone (PCL) nanofibers, average diameters of fibers increased with increasing applied voltage from 10 to 16 kV and decreased from 18 to 20 kV, as shown in Figure 2. After parameter optimization, the results at 18 kV give the best smooth fibrous configuration with narrow diameter distribution (Yao et al., 2015).
The distance between the tip and the collector mostly affects fiber dryness and the strength of the electric field. Thus, a certain distance will be required to give the fibers sufficient time to dry before reaching the collector. On the other hand, with the distance increasing, the average diameters of the nanofiber increase because of the decrease in the effective voltage. But, if distances are either too close or too far, then beads will be observed (Bhardwaj and Kundu, 2010).
Solution viscosity also plays an important role in determining the fiber size and morphology. It has been found that a minimum viscosity is required to form fibers instead of drops, and a maximum viscosity is required to control and maintain the flow of a polymer solution (Deitzel et al., 2001). Thus, there is a requirement of optimal viscosity to produce fibers, although it will obviously vary depending on the polymer/solvent system used, as shown in Figure 3.
The humidity and molecular weight have been investigated as two factors to affect the surface feature of electrospun fibers. Casper et al. (2004). reported that the diameter, number and distribution of the pores increased with an increase in humidity. Pores on the surface of fibers became evident as electrospun fibers in an atmosphere with more than 30 per cent relative humidity. The higher molecular weight of polystyrene (PS) leads to larger pores that are less uniform in shape and size. The influences of molecular weight on pore formation could be because of phase separation or viscosity. The phenomenon of pores formation on electrospun fibers could be explained by the combination of both phase separation and breadth figure formation owing to a much more complex electrospinning process. Breadth figures occur because of evaporative cooling during the electrospinning process in moist air. Water from the air condenses on the surface of the fiber as a result of the cooling surface of jet owing to solvent evaporation. Pores formation occurs as the water droplets leave an imprint as the fiber dries. Meanwhile, thermally induced phase separation could be used as another explanation for pore formation. During the electrospinning process, pore formation occurs as the temperature approaches the bimodal temperature induced by solvent evaporation and continues to grow until a crystallization temperature is reached. As porous surface features can serve to capture nanoparticles, thus, releasing drug molecules or increasing the surface area; the ability to manipulate their proper size will allow fibers to meet the needs for specific uses, such as tissue engineering, drug delivery and filtration.
3. Shape memory performance
3.1 Development progress of electrospun SMP nanofibers
Generally, a polymer with SME usually satisfies two structural requirements: one is the soft segments that relate to a low temperature of glass or melting transition, and the other is the hard segments that ensure the SMP forms a stable network structure. At room temperature, an SMP typically has its permanent shape. Upon heating at a temperature higher than its switch transition temperature, Ttran (glass transition temperature Tg or melting temperature Tm), the SMP is easily subject to deformation when an external force is applied. After the temperature drops below the Ttran, the deformation will be maintained owing to the freezing of the molecular chain that stores entropic energy in the system (Zhao et al., 2015). The entropic energy will be released when heated above the Ttrans again, leading to the SMP recovering its permanent shape (Lu et al., 2015a, b, c, d, e). This is the simple mechanism explained for a thermally induced one-way SMP. In this case, Hu et al. succeeded in developing SMPU nanofibers (Zhuo et al., 2008). The prepared solution for electrospinning contained PCL based on SMPU resin with 75 per cent soft segment content and 4,000 soft segment length. The reversible phase transformation of the soft segment is responsible for the SME. Finally, the differential scanning calorimetry curves also verified that the electrospun polyurethane nanofibers would meet the structure requirements of segmented polymer that have SME. The electrospun SMPU nanofibers showed good shape memory properties with 98 per cent shape recovery rate and 80 per cent shape fixity rate after several cycles. Matsumoto et al. explored the shape memory capability of electrospun non-woven fabrics prepared from polyesterurethanes containing poly ω-pentadecalactone (PPDL) hard segments (Matsumoto et al., 2012). The rod-like structures of PPDL which acted as physical crosslinks below the melting temperature of PPDL could not be obtained by other processes (i.e. cast-coated and spin-coated films). Therefore, the author mentioned that the arrangement and reorganization of crystalline structures of SMPs could be influenced by the variation of the characteristic dimension (e.g. fiber diameter) of semi-crystalline structures during the programming and recovery process, leading to a variation in the shape memory capability. Meanwhile, the SMP property of spun fiber could be enhanced by incorporated nanofiber during spun process. For example, Li et al. manufactured a class of biodegradable polymer nanofibers based on the electrospinning technique, using chemically crosslinked PCL as the matrix and Fe3O4 nanoparticles as the reinforced filler (Li et al., 2014). Fe3O4 nanoparticles uniformly dispersed in fibers, and thus, have no effect on the diameters and the surface morphology of the nanofibers. But the shape memory property of nanofibers improved with the incorporation of the Fe3O4 filler, because Fe3O4 hindered the movement of the matrix polymer chain that plays a role of physical crosslinks. On the other hand, in addition to directly electrospun SMP membrane, non-shape memory membrane could also be incorporated with the polymer to achieve the SME. Yao et al. proposed that shape memory composites could be easily manufactured based on thermosetting of resin/rubber and electrospun thermoplastic membrane (Yao et al., 2016). In this system, electrospun fibrous mat of PCL is treated as a “hard segment” to help fix the temporary shape and elastic matrix (electro-active polymer, EAP) serves as a “soft segment”, which also provides an actuation force by converting electrical energy to dynamic energy, as shown in Figure 4. PCL will melt as the stimulus temperature exceeds its melting temperature which is treated as a switch transition temperature of the shape memory system. Meanwhile, voltage is applied on EAP to cause deformation of the composite. When the composite reaches a desired shape, the heating source can be removed but the voltage is still applied to keep the deformed shape. After cooling to a temperature below the transitional temperature of the composite, the temporary shape is fixed with removal of the voltage. Upon heating, the composite recovers to its original shape with the release of frozen strain. That is, the design of the “two-way” shape memory composites which exempt this material from mechanical load in one-way SMPs and from training in traditional two-way SMPs.
Most reported electrospun SMP nanofibers exhibit one-way SME (1WS-ME) (Matsumoto et al., 2012; Zhuo et al., 2008). But, with in-depth research, multiple-shape memory (the number of temporary shapes can be programmed in a single-shape memory cycle) has been reported recently (Bellin et al., 2006; Behl et al., 2009; Luo and Mather, 2010; Xie et al., 2009; Dong et al., 2015a, b; Yu et al., 2014; Yu et al., 2012). Zhang et al. prepared quintuple-shape memory membrane based on Nafion and PEO (spinning aid) (Zhang et al., 2013). Nafion had a broad transition temperature which was required for multi-shape capability, but it should be noted that electrospun membranes without post-treatment did not have an ability to memorize shapes. Because an annealed spun mat will create a chemical bonding instead of a weak physical bonding to form a stable network, which is of benefit for conduction of stress during the shape recovery process. This was the key to induce shape memory behaviors.
Most research studies mentioned in above sections concerned thermally responsive shape memory materials. To expend the electrospun SMP stimulus technique, the general method is to introduce the multi-functional nanomaterials filler to achieve multiple-stimulus responsive SMP composite nanofiber (Luo et al., 2014; Lu et al., 2016a, b; Wang et al., 2017), shown in Figure 5. For example, Zhou et al. fabricated one class of biodegradable electrospun polymer composite nanofibers (Gong et al., 2012). The chemically crosslinked PCL was served as the matrix and multiwalled carbon nanotubes (MWNTs) as the reinforced filler coated with Fe3O4 nanoparticles playing as a magnetism responsive source. The results indicated that the spun composite fibers were able to be triggered by both hot water and alternating magnetic field.
But for chemical-responsive SMP, the general approach to achieve the SME is to soften/dissolve its transition component instead of heating to a temperature above the transition temperature compared with thermal-responsive SMP. Thus, a lower transition temperature will be obtained by means of softening, swelling or dissolving processes. On the other hand, the electrospun mat with porous feature is favorite to solvent absorption with fast diffusion which may benefit for solvent stimuli. For example, the lignin-based materials prepared by Dallmeyer et al. are clearly moisture-responsive, capable of changing shape in response to moisture and regaining their original shape with the removal of the moisture stimulus (Dallmeyer et al., 2013). However, the shape memory behavior is slightly different compared with that of other moisture-responsive SMPs. Many are thermo-active shape memory materials that are programmed upon heating and then recover their shape gradually after exposure to humidity for a prolonged period or after being immersed completely in water (Chen et al., 2009; Du and Zhang, 2010; Huang et al., 2010; Lu et al., 2015a, b, c, d, e). Although it is not clear whether the materials could memorize other “programmed” shapes except curling, the ability to rapidly activate changes in shape (30-60 s) and the relatively rapid recovery (60-120s) is an interesting aspect of these lignin-based materials. Yao et al. fabricated a type of hybrid membrane by hybridizing PCL and polyethylene oxide (PEO) nanofibers via an electrospinning technique (Yao et al., 2015). The shape memory property of crosslinked PCL was achieved by free radical reaction under ultraviolet (UV) irradiation. PEO with remarkable hydrophilic property has been chosen to obtain water-sensitive composites. In wet condition, melted PEO would move to the PCL layer and stack on its surface, which definitely increases the crosslinked density of the PCL layer, and may lead to the higher recovery speed and lower responsive temperature.
As we know, SMPs have attracted much attention because of their good biocompatibility, easy control of the switch transition temperature, lightly modification and multifunction. For SMP nanofibers, it is a novel kind of smart fibrous material with benefits of high porosity, interconnectivity, microscale interstitial space, biocompatibility and a large surface-to-volume ratio. Thus, electrospun SMP nanofibers have a wide potential application field including tissue engineering, drug delivery, filtration, catalysis. Take tissue engineering for example, the electrospun SMP nanofiber mat is suitable for using as smart scaffolds because of its high porosity and shape memory feature, which could control cultured cell growth via shape changing. So far, a number of biopolymers and biodegradable polymers have been electrospun as scaffolds for engineering bone, blood vessels, nerves, skin and muscle (Chong et al., 2007; Bao et al., 2014). It is anticipated that SMP scaffolds will also be used for commercial production of biomedical tissue. Tseng et al. proposed that a thermo-responsive SMP scaffold which is named strain-aligned scaffold was prepared by uniaxial stretching of an electrospun scaffold (Tseng et al., 2013). It has been demonstrated that human adipose-derived stem cells (hASCs) cultured on the themo-responsive SMP scaffold are capable of changing shape and internal architecture, showing normal morphology and good proliferation. In this project, static control scaffolds of either randomly oriented fibers or aligned fibers are also prepared. Results showed that cells preferentially aligned along the fiber direction of the strain-aligned scaffold before transition and lost preferential alignment after transition. It demonstrated that shape-memory-actuated decrease in fiber alignment can control cell morphological behavior, making SMP scaffolds potentially useful as a platform for the study of mechanobiological stimuli. In consequence, scaffolds with shape memory performance are further expected to facilitate cell–scaffold construct preparation, mimicry of dynamic biological processes and scaffold delivery for minimally invasive surgery, which will make a breakthrough in the near future.
In the pharmaceutical industry, electrospun non-woven fabrics are promising tools as supports or carriers for controlled drug delivery because of their high specific surface areas. Owing to the diversity of materials suitable for use with electrospinning, a lot of drugs can be delivered including anticancer drugs, antibiotics, DNA and proteins. There are a number of different drug-loading methods: coatings, embedded drug and encapsulated drug (Sill and von Recum, 2008). The core-shell nanofibers with SME developed by coaxial electrospinning provided one possible way to encapsulate drugs and therapeutics (Zhuo et al., 2010). It was electrospun from the core solution of polycaprolactone-based SMPU (CLSMPU) and shell solution of pyridine shape memory polyurethane (PySMPU). In addition to excellent SME, the core-shell nanofibers also showed excellent antibacterial activity against both gram-negative bacteria and gram-positive bacteria, which resulted from the PySMPU shell materials and the high surface area per unit mass of nanofibers. Thus, the core shell nanofibers incorporating both shape memory functionality and antibacterial property could be used for drug delivery.
Furthermore, some multifunctional SMP composites such as the hybrid membrane electrospun from PCL and PEO have been constructed (Yao et al., 2015), which could be potentially used in medical fields including scaffolds, bandage and embolic sponges, shown in Figure 6. In this case, high porosity of hydrophobic PCL layer with shape memory property could permit air transfer and change pore size in PCL layer, which is capability to keep wounds dry, and hydrophilic PEO fiber could be mixed with medicine, which is capable of dropping medicine in the wound when PEO absorbed the wound secretion; both are biocompatible.
Because of porosity feature, electrospun SMP nanofiber mats have a potential application in smart filtration field. Ahn et al. successfully fabricated SMPU nanofibers with two-way SME via electrospinning, referring to an ability to change macroscopic shapes reversibly (Ahn et al., 2011). It was found that SMPU membranes for the hard-segment concentrations of 33 per cent were prepared, and their pore sizes were reversibly changed ranging from about 150 to 440 nm during the net two way-shape memory (2W-SM) behavior. In this case, the SMPU membrane has the potential to be used as a smart membrane to selectively separate substances according to pore sizes by simply controlling its temperature.
In addition to applications described previously, non-SMP nanofibers have been identified for usage of high-performance polymer batteries, smart textile, defensive facemasks and water pollution control (Hu et al., 2012; Ramakrishna et al., 2006). SMP nanofibers have more potential in these fields.
In the past several years, we have witnessed significant advances in electrospun polymer fiber technology and their composites with shape memory property. This paper presents a brief overview of the various aspects of SMP nanofibers, including the fabrication methods, development progress, new shape memory behaviors (multiple-SME and 2W-SME) and actuation methods. This review also covers some of the main applications of electrospun SMP nanofibers. Electrospinning is an easy and mature method to obtain SMP fibers at microscale/nanoscale. The diversity of fibers’ morphology is mainly affected by the electrospinning parameters including applied voltage, tip-to-collector distance, viscosity of solution, humidity and molecular weight. For the actuation method, most SMP nanofibers can change their shapes in response to heat, magnetic field or solvent, while few can be driven by electricity. This is because most polymers have poor electro-conductivity and it is not easy to uniformly distribute the electro-conductivity filler as a connect network in the polymer mat. Compared with the block SMPs, SMP nanofibers mat shows low mechanical properties, but such an SMP mat and its composites have the benefit of being light in weight, being porous, having a high surface area, having layer thinness, being biocompatible, being easy processible and being multi-functional. Because of these merits, they could play an active role in many promising applications, especially in scaffold and medical field.
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This research was supported by National Natural Science Foundation of China [Grant No.11402066 and 11772108].