Shape memory properties

Fig. 3 Image frames captured from a video of an EGC hydrogel scaffold (a) prior to, (b) during, and (c) immediately following compression, illustrating its elastic shape-memory properties...

Also, PVA hydrogels evidenced a very good behaviour in contact with skin and other tissues, mucosa, or blood. PVA exhibits a bioadhesive nature, shape-memory properties, avoid the protein adsorption onto the gel surface and is biocompatible. 

Figure 21.1. Stents are used to open arteries of the heart blocked by atherosclerotic plaques (A) a balloon and stent are placed across the plaque (B) the balloon is expanded, leaving the stent to prop open the artery (C) restenosis is the process wherein scar tissue builds up around the stent, again causing a flow restriction. A balloon is required for stainless steel, whereas a nitinol stent will expand on its own, due to the shape memory property of nitinol. (From Ref. 11, with permission.)...

A synthetic approach to poly(ester-urethanes) was recently published by Xue et al. [48]. Glycerol was employed as a trifunctional initiator in the enzymatic ROP of CL. The three-arm PCL triol was then reacted with methylene-diphenyl diisocyanate (MDI) and hexanediol to yield a three-arm PCL-based poly(ester-urethane) with shape-memory properties. 

As is the case for smart materials, intelligent textiles have the property to respond to their environment, sometimes in a clearly perceptible way, but sometimes at the molecular level, completely invisible to the observer. They cover a wide range of technologies, from materials with shape-memory properties or sensing and actuating properties, to entire systems based on information technology5. 

Cross-linked PUs with shape memory properties were prepared by Galia, Meier et al. using linear polyols synthesized by ADMET [140]. In this work, ADMET of a 10-undecenoic acid-derived a,co-diene containing a hydroxyl group was performed in the presence of 0.1 mol% of C4. 10-Undecenol was used as chain stopper, and the mixture of oligomers and diols (from 10-undecenol SM) obtained was cross-linked with MDI. The PUs obtained displayed outstanding values of strain fixity and recovery. 

Next-generation metallic biomaterials include porous titanium alloys and porous CoCrMo with elastic moduli that more closely mimic that of human bone nickel-titanium alloys with shape-memory properties for dental braces and medical staples rare earth magnets such as the NdFeB family for dental fixatives and titanium alloys or stainless steel coated with hydroxyapatite for improved bioactivity for bone replacement. The corrosion resistance, biocompatibility, and mechanical properties of many of these materials still must be optimized for example, the toxicity and carcinogenic nature of nickel released from NiTi alloys is a concern. ... 

Hydrogels are another class of polymers with shape memory properties. They are cross-linked polymers with a hydrophilic portion that has high affinity for water and a hydrophobic part that can be controlled by temperature variations. The cross-linked part is responsible for setting the permanent shape at an elevated temperature, whilst the hydrophobic part assumes a secondary shape at a specific or critical temperature. Heating above these temperatures completes the recovery (Liu et al., 2007). 

Mather, P.T., Liu, C., 2011. Blends of Amorphous and SemicrystaUine Polymers Having Shape Memory Properties. Patent, PCX number PCT/US2003/032329. 

Zhang, F., Zhang, Z., Liu, Y., Leng, J., 2014. Shape memory properties of electrospun Nafion nanofibers. Fibers and Polymers 15 (3), 534—539. 

One interesting alloy of titanium and nickel, called Nitinol, exhibits shape-memory properties. Below a particular temperature (the transformation temperature), the crystal structure of the alloy is such that it can be plastically deformed (martensitic). As the alloy is heated, the crystal structure alters to one that is more ordered and rigid (austenitic), and the deformed metal reverts to its original shape. This effect has been exploited in a number of devices, including a stent (a device used to hold open passageways such as arteries). The stent is placed inside a small-diameter catheter for insertion into the body, where it expands on being warmed to bod y temperature. 

Features Good shape memory properties Improved properties on phase 1 Body temp, responsive Fixity under lower temp. Elasticity under common. Larger shrinkage Stable tensile modulus Multifu notions. 

Y. Feng, M. Behl, S. Kelch, A. Lendlein, Biodegradable multiblock copolymers based on oUgodepsipeptides having shape-memory properties, Macromol. Biosci. 9 (2009) 45-54. 

Diisocyanate is often used in the chain extension reactions of biopolymers such as PEA, PCL, and their copolymers [97-101]. The combination of hard segment and soft segment may confer the resulting polyurethanes with shape-memory property [100,101]. Polyurethane is an important type of elastomeric polymer for biomedical applications [1,9,11]. Chain extension or cross-linking by diisocyanate can be adapted to many -OH-terminated or H-containing polymers or prepolymers [102]. The convenience of the urethane chemistry has made it into a very popular way of polymer chain extension method in biomaterial designs. 

P. Ping, W.S. Wang, X.S. Chen, X.B. Jing, Poly(e-caprolactone) [119] polyurethane and its shape-memory property. Biomacromolecules... 

CNTs have been successfully used to aid in the remote recovery of stored energy in a shape-memory thermoplastic polyurethane (TPU) matrix (Figure 2.3). Pristine TPU exhibits relatively modest shape-memory properties. However, by adding MWNTs, the shape-memory properties are greatly... 

Figure 2.3 Shape-memory properties of MWNT-TPU composites, (a) Stretched (800%) 1 wt% MWNT-TPU composite ribbon, tied into a loose knot and heated at 55 °C. The knot closes on strain recovery, (b) Strain recovery and curling of the 1 wt% MWNT-TPU composite ribbon upon IR irradiation within 5 s. (c) Comparison of the stress recovery before (left) and after (right) remote actuation by IR irradiation. Neat TPU (M) bends and does not recover. In contrast, the 1 wt% MWNT-TPU composite (PCN) contracts on exposure to IR irradiation (arrow indicates moving direction), (d) Electrically stimulated stress recovery of a 16.7 wt% MWNT-TPU composite. Reprinted by permission from Macmillan Publishers Ltd. ...

G.-M. Lin, G.-X. Sul, R. Yang, Mechanical and shape-memory properties of polyamide/maleated polyethylene/linear low-density polyethylene blend. J. Appl. Polym. Sci. 126, 350-357 (2012)... 

It is not very often that two polymers are not mixed at a molecular level but are immiscible. Because the immiscible blends have a phase-separated structure, their physical properties including shape memory properties are influenced by phase morphology as well as the nature and relative amount of each phase. For the immiscible blend based SMP, in principle, one component acts as a reversible phase and the other component acts as a stationary phase. 

Li et al. reported that immiscible high-density polyethylene (HDPE)/ poly(ethylene terephthalate) (PET) blends, prepared by means of melt extrusion with ethylene-butyl acrylate-glycidyl methacrylate (EBAGMA) terpoly-mer as a reactive compatibilizer, can exhibit shape memory effects [32]. They observed that the compatibilized blends showed improved shape memory effects along with better mechanical properties as compared to the simple binary blends. In the blend, HDPE acts as a reversible phase, and the response temperature in the shape recovery process is determined by of HDPE. The shape-recovery ratio of the 90/10/5 HDPE/PET/EBAGMA blend reached nearly 100%. Similar behavior was observed for immiscible HDPE/ nylon 6 blends [33]. The addition of maleated polyethylene-octene copolymer (POE-g-MAH) increases compatibility and phase-interfacial adhesion between HDPE and nylon 6, and shape memory property was improved. The shape recovery rate of HDPE/nylon 6/POE-g-MAH (80/20/10) blend is 96.5% when the stretch ratio is 75%. 

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