Subject shape-memory

If it is not dissolved or trapped, an embolism moving from the lower extremities can be life-threatening. People afflicted with phlebitis are particularly susceptible to this problem. A shape-memory trap has been devised that, when deployed in the vena cava, is like a multileaved mesh that traps a traveling embolism, retaining it until medication can dissolve it. Introduced in a folded form by a catheter, the mesh is prevented from deploying by subjecting it to a flow of cold saline water. Once in place, it is released from the catheter and, warmed by body heat, opens into its final shape (11). 

Given the mixed results in the literature, it is difficult to know just how caffeine does affect memory. To some extent, the differential effects may depend on the memory assessment method (recall or recognition) and the time frame (immediate or delayed). Gender differences may also cloud the picture, as discussed above. Even when these differences are taken into account, however, unexplained discrepancies remain. One partial explanation may be that the differential effects of caffeine are a function of the subject s memory load. For example, Anderson65 found that caffeine enhanced low load memory tasks but was detrimental in high load tasks. This could be due to the increased arousal induced by the high load task, which, in the presence of caffeine, could produce overarousal. The drop in arousal output as the subject crossed the peak of the inverted U-shaped function could cause the memory deficits observed in some studies. 

Shape memory polymers are defined by their ability to store and recover strains when subjected to a particular thermo-mechanical cycle. Shape-memory polymers can recover their original shape by being heated above their transition temperature, which are defined by different phases in the materials. In particular, the shape-... 

Some of these binary and ternary phases have been studied to a larger extent because of special physical and/or mechanical properties. Of particular interest are the Cu phases CuZn, Cu j +1, 3,Zn i 2.3,Al, and (Cu,Ni)3Al, on which the Cu-Zn-Al and Cu-Al-Ni shape memory alloys are based and which are the subject of the following sections. In addition, the Cu-Au phases CU3AU and CuAu and the Cu-Sn phases Cu3Sn and Cu Snj will be addressed, which are important constituents of Cu-Au alloys and amalgams for dental restorative applications. 

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. 

Zhu et al. studied a biocompatible shape memory polymer blend based on poly(e-caprolactone) (PCL) and polymethylvinylsiloxane (PVMS). Pure PCL was subject to scission rather than cross-linking under irradiahon. In the presence of a small amoimt of PVMS (<20 wt%), both polymers are miscible in the amorphous phase and the radiation cross-linking of PCL is enhanced. Mechanical properties were improved, and a strong shape memory behavior was achieved. Above the melhng point of PCL, the blend exhibited a rubber-like state and could be deformed. The switch temperature was the melting temperature of PCL. With 5 to 15 wt% of PVMS and under 100 kGy y-irradiation, the deformation fixation ratio and the deformation recovery ratio were 100%. 

Once an SMP device is implanted within the body and fully activated, the device ceases to be shape-memory and should have the properties of a typical polymer-based device and are subject to all the same long-term performance concerns. Obviously, long-term biocompatibility and carcinogenicity are a concern of implantable polymeric materials however, mechanical properties of polymers with respect to water absorption and biodegradation will be discussed for the remainder of this chapter. 

In the previous section, water absorption was described as a method to activate the shape-memory effect. However, all polymeric materials are subject to some level of water uptake, which can drastically change the mechanical properties of the polymer. For example, the mechanical properties of the Mitsubishi Thermoplastic SMP, MM5510, are reduced after soaking in water for 72 h (Fig. 12). The figure illustrates that the elastic modulus, yield strength, and toughness aU decrease after exposure to water uptake. 

Shape memory alloys are metallic materials that demonstrate the ability to remember and to return to their original, cold forged shape when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature, they return and hold on to their shape prior to the deformation. Thin film SMAs have been recognised as a new type of promising material for MEMS systems and especially for microactuators due to... 

Chapter 1 briefly introduces the mechanisms of the shape memoiy effect and shape memoiy polymers, summarizing key research in the subject. It also introduces one of the most important shape memory polymers, shape memory polyurethane, and its unique properties. Since a melting transition temperature (T ) or a glass transition temperature (T can act as the switch transition for shape memory polyurethane, it can be called either a T -type or a T -type shape memoiy polyurethane. Chapters 2 and 3 discuss T -type and T -type shape memoiy polymers. The effects of deformation temperature, deformation amplitude, shape fixing temperature and pre-deformation are discussed in terms of their influences on shape memoiy properties. 

Malucelli and co-workers succeeded in preparing shape memory main-chain nematic coatings, by photopolymerizing liquid crystalline elastomers eventually subjected to uniaxial stress. To this aim, a mesogenic diglycidyl-terminated... 

This chapter begins with a general consideration of the crystallographic features of martensitic transformations. The principles are general, and thus detailed descriptions of the crystal struaures and substructures for individual alloy systems such as Ni-Al versus Cu-Sn are avoided. A brief survey of shape-memory phenomena within the framework of martensite crystallography is presented this subject and the various martensite crystal structures are presented in detail in Chapter 26 by Schetky in Volume 2. Martensitic transformations and shape-memory phenomena are common to many... 

A relatively new gronp of metals that exhibit an interesting (and practical) phenomenon are the shape-memory alloys (or SMAs). One of these materials, after being deformed, has the ability to return to its predeformed size and shape upon being subjected to an appropriate heat treatment—that is, the material remembers its previous size/shape. Deformation normally is carried out at a relatively low temperature, whereas shape memory occurs upon heating. Materials that have been found to be capable of recovering significant amounts of deformation (i.e., strain) are nickel-titanium alloys (Nitinol, is their trade name) and some copper-base alloys (Cu-Zn-Al and Cu-Al-Ni alloys). 

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