Thermally-activated Shape Memory Research Articles

Thermally-Activated Shape Memory Behavior of Biodegradable Blends Based on Plasticized PLA and Thermoplastic Starch.

Biodegradable blends based on plasticized poly(lactic acid) PLA and thermoplastic starch (TPS) have been obtained. The influence of the PLA plasticizer as a compatibility agent has been studied by using two different plasticizers such as neat oligomeric lactic acid (OLA) and functionalized with maleic acid (mOLA). In particular, the morphological, thermal, and mechanical properties have been studied as well as the shape memory ability of the melt-processed materials. Therefore, the influence of the interaction between different plasticizers and the PLA matrix as well as the compatibility between the two polymeric phases on the thermally-activated shape memory properties have been studied. It is very interesting to use the same additive able to act as both plasticizer and compatibilizer, decreasing the glass transition temperature of PLA to a temperature close to the physiological one, obtaining a material suitable for potential biomedical applications. In particular, we obtain that OLA-plasticized blend (oPLA/TPS) show very good thermally-activated capability at 45 °C and 50% deformation, while the blend obtained by using maleic OLA (moPLA/TPS) did not show shape memory behavior at 45 °C and 50% deformation. This fact is due to their morphological changes and the loss of two well-distinguished phases, one acting as fixed phase and the other one acting as switching phase to typically obtain shape memory response. Therefore, the thermally-activated shape memory results show that it is very important to make a balance between plasticizer and compatibilizer, considering the need of two well-established phases to obtain shape memory response.

Effect of the Addition of MgO Nanoparticles on the Thermally-Activated Shape Memory Behavior of Plasticized PLA Electrospun Fibers.

In this work, the thermally-activated shape memory behavior of poly(lactic acid)-based electrospun fibers (PLA-based efibers) reinforced with different amounts of magnesium oxide (MgO) nanoparticles (NPs) was studied at different temperatures. In particular, MgO NPs were added at different concentrations, such as 0.1, 0.5, 1 and 3 wt%, with respect to the PLA matrix. The glass-transition temperature of PLA-based efibers was modulated by adding a 20 wt% of oligomer lactic acid as plasticizer. Once the plasticized PLA-based efibers were obtained and basically characterized in term of morphology as well as thermal and mechanical properties, thermo-mechanical cycles were carried out at 60 °C and 45 °C in order to study their thermally-activated shape memory response, demonstrating that their crystalline nature strongly affects their shape memory behavior. Importantly, we found that the plastificant effect in the mechanical response of the reinforced plasticized PLA efibers is balanced with the reinforcing effect of the MgO NPs, obtaining the same mechanical response of neat PLA fibers. Finally, both the strain recovery and strain fixity ratios of each of the plasticized PLA-based efibers were calculated, obtaining excellent thermally-activated shape memory response at 45 °C, demonstrating that 1 wt% MgO nanoparticles was the best concentration for the plasticized system.

Thermally-activated shape memory alloys for retrofitting bridge double-angle connections

Double-angle connections are prone to fatigue cracking in old steel bridges; however, owing to their complexity, there are not many retrofitting solutions for these details. This study proposes a retrofitting system for double-angle connections using iron-based shape memory alloys (Fe-SMAs). An innovative connection test setup equipped with an optical 3D digital image correlation (DIC) measurement system was developed. The retrofitting system included two end-anchorages set on either side of the connection. Fe-SMA strips were inserted inside the anchorages, and then were thermally activated (pre-stressed) to a temperature of 260 °C. Numerical finite element studies were performed to investigate the static behavior of the system. It was found that the activated/prestressed Fe-SMA strips not only apply a reverse positive moment on the connection (versus an undesirable negative fixity moment), but also “absorb” a portion of the negative moment, owing to the added axial stiffness. The proposed SMA-based system offers a versatile and cost-effective technique (with quick and easy installation) for bridge connections, and can significantly reduce the effects from undesirable sources of fatigue cracking.

Study of Thermal-Responsive Poly-L-lactic acid/Nanohydroxyapatite Composite Filament and its 3D Printing

There have been critical needs for the development of biomedical and cost-effective filaments for fused deposition modeling 3D printer. In the present work, we prepared poly-L-lactic acid (PLLA)/10 wt % nanohydroxyapatite (nHA) composite filaments suitable for 3D printing by melt extrusion. Scanning electron microscopy and tensile tests were carried out to examine surface morphology and tensile properties of the materials. Differential scanning calorimetry and thermal gravimetric analysis had been performed in order to assess filaments’ suitability for the 3D printing process, in terms of melting point, degradation temperature and so on. Thermally-activated shape memory response of the composites had also been studied. The printing effects of filaments were tested using a desktop 3D printer. The samples printed in different directions were used to perform compression and bending experiments. PLLA/nHA macroporous bone scaffold fabricated by 3D printing showed shape memory effect retained from PLLA/nHA composite. Even though the bone scaffold we obtained was far from the ultimate application in minimally invasive surgery, possible solutions to print more satisfying scaffold are discussed in the conclusion of this article.

Shape memory effect on electrospun PLA-based fibers tailoring their thermal response

A research on the thermally-activated shape memory behavior of electrospun nanofibers based on polylactic acid (PLA) plasticized with its oligomer (OLA) was conducted with the purpose of obtaining a suitable material to be used in potential biomedical applications. Three different PLA-OLA formulations with 70:30, 80:20 and 90:10 ratios were processed by electrospinning and studied in order to decrease the PLA glass transition temperature to a temperature closer to the human body. For each electrospun formulation mat, the average diameter of electrospun fibers was found to be 757 ± 193 nm for neat PLA and 768 ± 207 nm, 620 ± 121 nm and 476 ± 80 nm for PLA-OLA 90:10, 80:20 and 70:30 ratios, respectively and correlate it with their mechanical response. First of all, the PLA capability to present shape memory behavior even in non-woven electrospun fibers form has been studied at 60 °C. Thus, two values of switching temperatures (45 °C and 40 °C) were selected in order to evaluate their shape memory response, being temperatures close to the human body temperature. The recovery and fixity ratio of PLA-OLA formulations studied here showed suitable shape memory behavior of the electrospun systems with excellent values of strain fixity as well as strain recovery ratios indicating these materials appropriate for potential biomedical application.

Double network shape memory hydrogels activated by near-infrared with high mechanical toughness, nontoxicity, and 3D printability

In this work, near-infrared (NIR)-responsive double network shape memory hydrogels were formed by chemically cross-linking Pluronic F127 diacrylate macromer (F127DA) and physical blending of poly(lactide-co-glycolide) (PLGA) with graphene oxide (GO, an energy convertor to convert NIR irradiation to thermal energy). The hydrogels were manufactured with 3D-printing technology using ultraviolet light polymerization. The morphologies and crystalline structure of the hydrogels were determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The NIR and thermal activated shape memory properties, mechanical toughness, and cytotoxicity were investigated. The shape memory properties were improved by incorporating GO into the hydrogels and the mechanical properties were enhanced by the addition of PLGA, which served as a second network. The cytotoxicity was assessed by the CCK-8 assay, which revealed no cytotoxicity. The nontoxicity, high mechanical toughness (3.45 MPa in the swollen state), and biosafety at the shape recovery temperature (36 ± 1 °C), which was achieved by NIR stimuli, indicates that shape memory hydrogels can be used in biomedical materials safely. The practical potential of the F127DA/PLGA/GO hydrogels was further revealed by their 3D-printing performance, their shape fixity ratio greater than 85%, and their shape recovery time under 300 s. Our results propose that F127DA/PLGA/GO hydrogels will be a promising material in biomedical applications as a drug carrier and an antibacterial scaffold.

High‐Strength, Thermally Activated Shape Memory Hydrogels Based on Hydrogen Bonding between MAAc and NVP

AbstractHerein, a facile and novel method is reported for fabricating tough hydrogen‐bonded hydrogels through the photoinitiated copolymerization of methacrylic acid (MAAc), N‐vinyl pyrrolidone (NVP), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) in an aqueous solution. PEGMA acts as a temporary hydrogen‐bonding shielding monomer to prevent the phase separation of MAAc and NVP in an aqueous solution. During polymerization, the shielding effect of the PEGMA weakens, and the stable MAAc–NVP hydrogen‐bonding domains, strengthened by the hydrophobic α‐methyl moieties of MAAc and the five‐membered N‐heterocycle of NVP, form in situ, endowing the hydrogels with high tensile strength (up to 3.9 MPa), extensibility (up to 600%), and toughness (up to 9 MJ m−3). In particular, due to the dynamic nature and temperature sensitivity of hydrogen bonds, the hydrogels also exhibit a fast 100% self‐recovery ability and thermally activated shape memory properties. Based purely on the “single‐fold” hydrogen‐bonding interactions, a tough supramolecular hydrogel with fascinating properties is successfully developed. These findings may be essential to either the design or practical application of the purely physical hydrogels with high mechanical strength and thermally activated shape memory properties.

Effect of the addition of polyester-grafted-cellulose nanocrystals on the shape memory properties of biodegradable PLA/PCL nanocomposites

In this work the thermally-activated shape memory response of biodegradable nanocomposites based on PLA/PCL blend reinforced with different type of cellulose nanocrystals has been reported, and compared with those of the neat matrix, at the same transition temperature of 55 °C and at the same different deformations, 50%, 100% and 150%. In particular, cellulose nanocrystals have been synthesized and then functionalized by “grafting from” reaction by ring opening polymerization of both PLLA and PCL using the OH groups onto the cellulose nanocrystals surface as initiators for the reaction. The morphology, thermal and mechanical analysis have been performed in order to obtain the parameters for the thermo-mechanical shape memory cycles. Moreover, the addition of the CNC-based nanofillers on the compatibility of PLA-PCL blends in 70:30 proportion has been evaluated. All the biodegradable nanocomposite formulations showed excellent shape memory response, similar to those of the neat matrix, with strain recovery ratio and strain fixity ratio higher than 80% and 90%, respectively. This fact indicates that in this case, the shape memory response of the nanocomposites is mainly controlled by the response of the neat blend and they are slightly influenced by the increase of compatibility between the components of the blend. In addition, all nanocomposite films were fully disintegrated under composting conditions confirming their biodegradable nature, obtaining that the presence of CNC-based nanofillers speeds up the disintegration rate of the nanocomposites in comparison with the pure matrix.

Thermally-activated shape memory effect on biodegradable nanocomposites based on PLA/PCL blend reinforced with hydroxyapatite

In this work, the effect of the addition of different amount of nanosized hydroxyapatite (nHA) on the shape memory behavior of blends based on poly (lactic acid) (PLA) and poly (ε-caprolactone) (PCL) has been studied. In particular PLA/PCL blend with 70 wt % PLA has been reinforced with 0.5, 1 and 3 wt % nHA. Moreover, the relationship between the morphology and the final properties of the nanocomposites has been investigated by field emission scanning electron microscopy, confocal Raman spectroscopy and atomic force microscopy. In particular, PeakForce has been used to study quantitative nanomechanical properties of the multifunctional materials leading to conclusion that nHA increase the phase separation between PLA and PCL as well as act as reinforcements for the PCL-rich phase of the nanocomposites. Furthermore, excellent thermally-activated shape memory response has been obtained for all the nanocomposites at 55 °C. Finally, the disintegration under composting conditions at laboratory scale level was studied in order to confirm the biodegradable character of these nanocomposites. Indeed, these materials are able to be used for biomedical issues as well as for packaging applications where both thermally-activated shape memory effect and biodegradability are requested.

Biodegradable poly(ester-urethane) incorporated with catechin with shape memory and antioxidant activity for food packaging

Biodegradable poly(ester-urethane) (PU) based on a tri-block copolymer of poly(l-lactic acid) and poly(ɛ-caprolactone), with shape-memory behaviour, and further loaded with catechin (Cat), as antioxidant agent, were developed with the dual objective to obtain a smart as well as an active material for food packaging purpose. Well dispersed catechin produced somewhat UV blocking effect, but slightly amber and red tonality. Catechin demonstrated positive interaction with PU matrix enhancing the interfacial adhesion and leading to a reduction in the surface wettability. The active PU composites, with thermally-activated shape memory ability at temperatures close to those required to pack thermally proceeded food (i.e. 40°C), showed increased ability to recover the initial shape of the neat PU matrix. The antioxidant effectiveness of catechin released after the exposition to a fatty food simulant was proven after 10days which represents the worst foreseeable conditions of the intended use. The disintegration process in composting conditions of the active PU-based materials was speeded up due to the catechin incorporation. The PU formulation loaded with low amount of catechin (1wt% or 3wt%) showed enough transparency, enhanced shape memory behaviour, improved water resistance, reduced UV-light transmission, right catechin release and effective antioxidant activity as well as appropriate disintegration in compost. Thus, active PU-Cat composites result very interesting as smart and active materials for biodegradable food packaging applications.

Thermal/Light Dual-Activated Shape Memory Hydrogels Composed of an Agarose/Poly(acrylamide-co-acrylic acid) Interpenetrating Network

In this work, an agarose/poly(acrylamide-co-acrylic acid) interpenetrating network hydrogel is prepared by the photopolymerization of acrylamide, acrylic acid, and N,N′-methylenebisacrylamide within agarose network at its gel state. The as-prepared hydrogel exhibits two independent kinds of shape memory effects. The thermal-activated shape memory behavior is attributed to the reversible coil-helix transformation of agarose, within the permanently cross-linked poly(acrylamide-co-acrylic acid) network; the light triggered shape recovery is driven by the dissociation of the complexes between carboxyl groups and Fe(III) ions, due to the photoreduction of Fe(III) to Fe(II) in the presence of citric acid. The versatile and simple strategy as presented here should be beneficial for the design of dual-activated shape memory hydrogels and foster their use in a number of fields such as biomaterials, soft robotics, smart actuators, and sensors.

A Novel Thermal-activated Shape Memory Penile Prosthesis: Comparative Mechanical Testing

To compare a novel nickel-titanium (Ni-Ti) shape memory alloy (SMA) penile prosthesis of our own design with commercially available prostheses using a format similar to mechanical testing done at major penile prosthesis manufacturers. We evaluated the mechanical parameters of commercially available penile prostheses and used this information to guide the development of the Ni-Ti-based physiological penile prosthesis that expands and becomes erect with a small amount of heat applied. A penile prosthesis consisting of an exoskeleton of temperature-tuned Nitinol was designed and prototyped. Mechanical testing was performed in a model of penile buckling, penile lateral deviation, and original penile shape recovery commonly used by penile prosthesis manufacturers for testing. Our SMA penile prosthesis demonstrated useful mechanical characteristics, including rigidity to buckling when activated similar to an inflatable penile prosthesis (2.62 kgf SMA vs 1.42 kgf inflatable penile prosthesis vs 6.45 kgf for a malleable prosthesis). The Ni-Ti also became more pliable when deactivated within acceptable mechanical ranges of existing devices. It could be repeatedly cycled and generate a restorative force to become erect. An SMA-based penile prosthesis represents a promising new technology in the treatment of erectile dysfunction. We demonstrated that an Ni-Ti-based prosthesis can produce the mechanical forces necessary for producing a simulated erection without the need for a pump or reservoir, comparable with existing prostheses.

Fatigue behavior of a thermally-activated NiTiNb SMA-FRP patch

This paper presents the details of an experimental study that was conducted to characterize the fatigue behavior of a thermally-activated shape memory alloy (SMA)/carbon fiber reinforced polymer (CFRP) patch that can be used to repair cracked steel members. A total of 14 thermally-activated patches were fabricated and tested to evaluate the stability of the prestress under fatigue loading. The parameters considered in this study are the prestress level in the nickel–titanium–niobium SMA wires and the applied force range. An empirical model to predict the degradation of the prestress is also presented. The results indicate that patches for which the maximum applied loads in a fatigue cycle did not cause debonding of the SMA wires from the CFRP sustained two million loading cycles with less than 20% degradation of the prestress.

The Use of Dynamic Systems Based on Shape Memory Alloys for the Treatment of Neuromuscular Disorders

Shape Memory Alloys (SMA) are very versatile materials that display a number of unusual and interesting properties, in particular, pseudoelasticity, thermally-activated shape memory, mechanical and thermal hysteresis. All of these characteristics can be thought of as sources of inspiration in the development of dynamic systems for mechanical or mechatronic applications. This paper will focus on several results obtained in the field of Neuromuscular Rehabilitation and Neurology. The need to customise functions, to make devices lightweight, portable and suitable to be interfaced with modern diagnostic instrumentation, and a drive towards the development of technologies appropriate for home-based therapies are the reasons why employing SMA in the design of biomedical systems for the treatment of neuromuscular disorders can show all the advantages of solid-state actuation and sensing.

Thermally-activated shape memory behaviour of bionanocomposites reinforced with cellulose nanocrystals

Bionanocomposites with thermally-activated shape memory ability have been designed based on a synthesized poly(ester-urethane) matrix reinforced with both neat and functionalized cellulose nanocrystals. The functionalization of the cellulose nanocrystals was performed by grafting poly(l-lactic acid) (PLLA) chains onto their surface. The matrix has a block copolymer structure of two biodegradable and biocompatible polymers, poly(e-caprolactone) (PCL) and PLLA. This research is focused on the effects of cellulose nanofillers on the thermally-activated shape memory response of the neat matrix confirming that the bionanocomposites are able to show shape memory effects at 35 °C, close to the human body temperature, making these materials good candidates for biomedical applications. Three thermo-mechanical cycles at 50 % of deformation were performed in order to check the thermally-activated shape memory ability of the bionanocomposites and to determine the shape memory parameters, namely the strain fixity (Rf), and the strain recovery (Rr) ratio. Both bionanocomposites, with neat and functionalized cellulose nanocrystals, present excellent shape memory behaviour maintaining the recovery behaviour at values of about 90 % as measured previously for the pure matrix, indicating that the addition of the nanofiller maintains the good ability to recover the initial shape of the matrix. The cellulose nanofillers clearly improve the ability of the polymer to fix the temporary shape. In fact, the bionanocomposites show Rf at about 90 %. Moreover, bionanocomposites reinforced with the functionalized cellulose nanocrystals maintain constant their performance during all the thermo-mechanical cycles thus confirming that the improvement in the shape memory behaviour can be mainly attributed to the increase of the interactions between the functionalized cellulose nanocrystals with the polymeric matrix.

Mechanically Tough, Thermally Activated Shape Memory Hydrogels.

The shape memory behavior of a series of strong, tough hybrid hydrogels prepared by covalently cross-linking quad-polymers of N,N-dimethylacrylamide (DMA), 2-(N-ethylperfluoro-octanesulfonamido) ethyl methacrylate (FOSM), hydroxyethyl acrylate, and 2-cinnamoyloxyethyl acrylate was investigated. The hybrid hydrogels, which had physical and covalent cross-links, contained ∼60-70% water, were relatively soft and elastic, and exhibited high mechanical strength, extensibility, and fracture toughness. The temporary network was derived from glassy nanodomains due to microphase separation of the FOSM species. The switching temperature for shape memory was the glass transition temperature of the nanodomains. Some creep relaxation occurred in the fixed shape due to viscoelastic effects of the nanodomain cross-links, but shape fixing efficiencies of 84-88% were achieved for the fixed shape after 24 h at 10 °C. Shape recovery to the permanent shape was achieved by reheating the hydrogel to 65 °C and was essentially quantitative.

Tuning surface micropattern features using a shape memory functional polymer

Living cells can alter their shapes and functions in response to physical cues at the cell–substrate interface, and consequently the temporal variations of the substrate surface are thought to be very important in governing cell behaviors. In this study, a thermally-activated shape memory function was employed to tune the dynamic change of the surface topography features on a polymer nanocomposite substrate. The polymer nanocomposite is composed of chemically cross-linked poly(ε-caprolactone) with allyl alcohol and Fe3O4 nanoparticles. The regular arrays of surface micropillar patterns were fabricated through thermal embossing micro-imprint lithography. They exhibited excellent shape memory performance with a recovery from a temporary shape at 34 °C to a permanent shape at 41 °C. The static and the dynamic surface micropillars as well as a smooth flat surface were used to study the behaviors of rat bone marrow mesenchymal stem cells. During this process, the surface micropillars switched from a temporal zero degree overwhelmed shape at 32 °C gradually to a permanent 90 degree standing shape at 41 °C due to the shape memory recovery. The results showed that the dynamic changes of the surface topography transition and the resultant force could effectively regulate cell adhesion, spread and alignment compared to the static surface micropillars and the smooth flat surface.

Smart Radially Folding Structures

In this paper, we present novel methods for exploiting passive and active radially folding mechanisms for reactive and dynamic structures. These enable the application of radially folding structures in domains including fluidics, medical stents, and auxetic materials. A compact form of elastic deployment utilizing linkage strain energy is proposed using beam theory analysis. Elastic strain energy is also shown to produce bistable folding behavior, with two low energy states at full contraction and full expansion, and a bistable switching point at some intermediate position. Polymeric smart materials are investigated for driving active folding. These materials can be readily exploited through the features of the folding structure including its ability to resolve 1-D, 2-D, and 3-D actuation strains into a more effective single degree-of-freedom linear, areal, volumetric or rotational output. The elastic and solid-state nature of many polymeric smart materials means they can implement elastic deployment and bistability. A thermally-activated shape memory polymer is shown to fold a 4-segment structure from expanded to contracted states. Experimental testing of an 8-segment dielectric elastomer actuator prototype demonstrates that radially folding structures can resolve large biaxial planar strains generated by dielectric elastomers into a single linear or rotational output stroke.

Development of the Co-Ni-AI ferromagnetic shape memory alloys

Farromagnetic shape memory alloys have been developed in the Co-Ni-Al ternary system with β + γ two-phase structure. The β(B2) phase of the Co-33Ni-27Al and Co-34Ni-28Al alloys exhibits both paramagnetic/ferromagnetic transition and a thermoelastic martensitic transformation from β to β' (L1 0 ). The microstructure of both alloys shows the primary p phase surrounded by the y (Al) phase which brings about the drastic effect in the improvement of the hot workability. By annealing in the two-phase region after solution treatment, the y phase precipitates along grain boundaries in the β phase, besides in the β grain. The alloys annealed at 1273 K showed a good cold workability and a thermal activated shape memory effect. These ferromagnetic and ductile shape memory alloys are very promising for one of the new smart materials.