Shape memory materials

Shape memory materials are able to remember a shape, and return to it when stimulated, for example, with temperature, magnetic field, electric field, pH-value and UV light. An example of natural shape memory textile material is cotton, which expands when exposed to humidity and shrinks back when dried. Such behavior has not been used for esthetic 

There are many potential applications of shape memory polymers in industrial components like automotive parts, building and constmction products, intelligent packing, implantable medical devices, sensors and actuators, etc. SMPs are used in toys, handgrips of spoons, toothbmshes, razors and kitchen knives, also as an automatic choking device in small-size engines. One of fire most well-known examples of SMP is a clothing application. 

Shape memory polymers can be laminated, coated, foamed, and even straight converted to fibers. There are many possible end uses of these smart textiles. The smart fiber made from the shape memory pol5mier can be applied as stents, and screws for holding bones together. 

Shape memory polymer coated or laminated materials can improve the thermophysiological comfort of surgical protective garments, bedding and incontinence products because of their temperature adaptive moisture management features. 

TABLE 2.3 Some of the Shape Memory Polymers Are Suitable for Textiles Applications 

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Otsuka. K. and C.M. Wayman Shape Memory Materials, Cambridge University Press, New York. NY, 1998. 

Shape-memory materials are those materials that return to a specific shape after being exposed to specific temperatures. In other words, these materials are able to remember their initial shape. This process of changing the shape of the material can be repeated several times. The shape-memory effect has been observed in different materials, such as metallic alloys, ceramics, glasses, polymers and gels. 

Stress-strain curve for a shape memory material. The lower curve is for deformation when the material is entirely martensitic. The deformation occurs by movement of variant boundaries. After all of the material is of one variant, the stress rises rapidly. The upper curve is for the material above its Af temperature. Adapted from a sketch by D. Grummon. 

The above analysis takes the synthesis methods, the performance affected by the dispersion of CNTs, enhanced physical properties and the latest applications of carbon nanotube/polyurethane composites described in literature reports as the reference point. In the interest of brevity, this is not a comprehensive review, however, it goes through numerous research reports and applications which have been learned and described in the recent years. Despite that, there are still many opportunities to synthesize new carbon nano-tube/polyurethane systems and to modify carbon nanotubes with new functional groups. The possibility of producing modern biomedical and shape memory materials in that way makes the challenge of the near future. 

C. T. Liu, H. Kunsmann, K. Otsuka, andM. Wuttigeds, Shape Memory Materials and Phenomena-Frmdamental Aspects and Applications , MRS Symposium Proceedings, Vol. 246, Materials Research Society, Pittsburgh, PA, 1992. 

K. Otsuka and C. M. Wayman, eds, Shape Memory Materials , Cambridge University Press, Cambridge, 1999. 

Temperature-sensitive materials shape-memory materials Temperature-sensitive gels... 

James R. D. and Hane K. R, Martensitic Transformations and Shape Memory Materials, Acta Mater., 4S, 197 (2000). 

R. Kainuma, H. Nakano, K. Oikawa, K. Ishida, T. Nishizawa High Temperature Shape Memory Alloys of Ni-Al Base Systems. In C.T. Liu, M. Wuttig, K. Otsuka et al. Shape-Memory Materials and Phenomena - Fundamental Aspects and Applications. MRS, Pittsburgh (1992) 403-408. 

The idea of teaching materials to remember their past or their modified physical state so that they can get back to it when an external stimuli is applied is fascinating and one that has manifested itself in shape memory materials. 

Applications of shape memory materials in medical textiles... 

Bogue, R., 2009. Shape-memory materials a review of technology and applications. Assembly Automation 29 (3), 214—219 (Emerald Group Publishing Limited). 

As compared to metallic compounds used as shape memory materials, shape memory polymers have low density, high shape recoverability, easy processability, and low cost. Since the discovery by Mitsubishi in 1988, polyurethane SMPs have attracted a great deal of attention due to their unique properties, such as a wide range of shape recovery temperatures (— 30°C to 70°C) and excellent biocompatibility, besides the usual advantages of plastics. A series of shape memory polyurethanes (SPMUs), prepared from polycaprolactone diols (PCL), 1,4-butanediol (BDO) (chain extender), and 4,4 -diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) have recently been introduced [200—202]. 

F.A. Spelman, B.M. Qopton, A. Voie, C.N. Jolly, K. Huynh, J. Boogaard, J.W. Swanson, Cochlear implant with shape memory material and method for implanting the same, US Patent 5800500 (September 1,1998). 

Under specific stimulus, shape memory materials could move from a temporary shape to their original shape. The stimulus could be light, pH, or electric or magnetic field, but the most common shmulus is heat. In this case, a shape memory polymer (SMP) possesses a switch transihon temperature. When the SMP is subject to deformation, its cross-linking structure could store internal stress if it is cooled below this switch temperature. When the polymer is heated above this temperature, it returns to its original shape. Shape memory polymer blends could be achieved using irradiahon. 

Fig. 14 Access to shape-memory materials from photocross-linked metallo-supramolecular polymers. (a) Formation of shape-memory materials using light as a stimulus (a) UV light is absorbed by the metal-ligand complexes and is converted to localized heat, which disrupts the metal complexation (i>) the material can then be deformed (c) removal of the light while the material is deformed allows the metal-ligand complexes to re-form and to lock-in the temporary shape id) additional exposure to UV light allows a return to the permanent shape, (b) Images demonstrating the shape-memory behavior. Reprinted with permission from [274]. Copyright 2011 American Chemical Society...

Besides metal-complexation, hydrogen bonding can also be applied to fix a temporary shape and to create shape-memory materials [275-277]. 

In the literature, the constitutive equation for both the amorphous polymer and crystalline polymer has been well established. Therefore, we can direcdy use these relations to model the amorphous phase and crystalline phase of the SMPFs. We then need to consider the cychc texture change of both subphases because the mechanical behaviors of the individual microconstituents may vary when they are packed in a multiphase material system and a certain deviation in their mechanical responses may exist between the individual and their assembled configurations. Since this is a shape memory material, we also need to model the shape recovery behavior. After that, we can use the above micromechanics relation to assemble the macroscopic constitutive relation. In order to determine the parameters used in the constitutive model, we need to consider the kinematic relations under large deformation. Finally, we will discuss the numerical scheme to solve the coupled equations. 

Figure 7J An ideal crack-closing model by the shape memory material...

Yoshida M, Longer R, Lendlein A, Lahann J (2006) From advanced biomedical coatings to multi-functionalized biomaterials. Polym Rev (Phila) 46 347-375 (Smart and shape memory materials)... 

H. Deka, N. Karak, R. D. Kahta and A. K. Buragohain, Biocompatible hyperbranched polyurethane/multi-waUed carbon nanotube composites as shape memory materials . Carbon, 2010,48, 2013-22. 

Y Y Chu et al. Shape Memory Materials and their Applications, Materials Science Forum, Volumes 394 - 395,2002. 

B. (2008) Novel biodegradable shape memory material based on partial inclusion complex formation between a-cyclodextrin and poly (e-caprolactone). Biomacromolecules, 9 (10), 2573-2577. 

Du, J., Armstrong, S.R., and Baer, E. (2013) Co-extruded multilayer shape memory materials comparing layered and blend architectures. Polymer, 54 (20), 5399-5407. 

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