smart+materials+by+ALaa+M.+KORTAM

Science and technology have made amazing developments in the design of electronics and machinery using standard materials, which do not have particularly special properties (i.e. steel, aluminum, gold). Imagine the range of possibilities, which exist for special materials that have properties scientists can manipulate. Some such materials have the ability to change shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost instantly when near a magnet; these materials are called smart materials. Smart materials have one or more properties that can be dramatically altered. Most everyday materials have physical properties, which cannot be significantly altered; for example if oil is heated it will become a little thinner, whereas a smart material with variable viscosity may turn from a fluid which flows easily to a solid. A variety of smart materials already exist, and are being researched extensively. These include piezoelectric materials, magneto-rheostatic materials, electro-rheostatic materials, and shape memory alloys. Some everyday items are already incorporating smart materials (coffeepots, cars, the International Space Station, eyeglasses) and the number of applications for them is growing steadily. || Each individual type of smart material has a different property which can be significantly altered, such as viscosity, volume, and conductivity. The property that can be altered influences what types of applications the smart material can be used = __**shape memory alloys**__ = Shape memory alloys (SMAs) are metals that "remember" their original shapes. SMAs are useful for such things as actuators which are materials that "change shape, stiffness, position, natural frequency, and other mechanical characteristics in response to temperature or electromagnetic fields" (Rogers, 155). The potential uses for SMAs especially as actuators have broadened the spectrum of many scientific fields. The study of the history and development of SMAs can provide an insight into a material involved in cutting-edge technology. The diverse applications for these metals have made them increasingly important and visible to the world.
 * A smart fluid developed in labs at the Michigan Institute of Technology

History
Nickel-titanium alloys have been found to be the most useful of all SMAs. Other shape memory alloys include [|copper-aluminum-nickel, copper-zinc-aluminum, and iron- manganese-silicon alloys.] (Borden, 67) The generic name for the family of nickel-titanium alloys is Nitinol. In 1961, Nitinol, which stands for Nickel Titanium Naval Ordnance Laboratory, was discovered to possess the unique property of having shape memory. William J. Buehler, a researcher at the Naval Ordnance Laboratory in White Oak, Maryland, was the one to discover this shape memory alloy. The actual discovery of the shape memory property of Nitinol came about by accident. At a laboratory management meeting, a strip of Nitinol was presented that was bent out of shape many times. One of the people present, Dr. David S. Muzzey, heated it with his pipe lighter, and surprisingly, the strip stretched back to its original form. (Kauffman and Mayo, 4)

Crystal Structures
Exactly what made these metals "remember" their original shapes was in question after the discovery of the shape-memory effect. Dr. Frederick E. Wang, an expert in crystal physics, pinpointed the structural changes at the atomic level which contributed to the unique properties these metals have. (Kauffman and Mayo, 4) He found that Nitinol had phase changes while still a solid. These phase changes, known as martensite and austenite, "involve the rearrangement of the position of particles within the crystal structure of the solid" (Kauffman and Mayo, 4). Under the transition temperature, Nitinol is in the martensite phase. The transition temperature varies for different compositions from about -50 � C to 166 � C (Jackson, Wagner, and Wasilewski, 1). In the martensite phase, Nitinol can be bent into various shapes. To fix the "parent shape" (as it is called), the metal must be held in position and heated to about 500 � C. The high temperature "causes the atoms to arrange themselves into the most compact and regular pattern possible" resulting in a rigid cubic arrangement known as the austenite phase (Kauffman and Mayo, 5-6). Above the transition temperature, Nitinol reverts from the martensite to the austenite phase which changes it back into its parent shape. This [|cycle] can be repeated millions of times (Jackson, Wagner, and Wasilewski, 1).

Manufacture
There are various ways to [|manufacture] Nitinol. Current techniques of producing nickel-titanium alloys include vacuum melting techniques such as electron-beam melting, vacuum arc melting or vacuum induction melting. "The cast ingot is press-forged and/or rotary forged prior to rod and wire rolling. Hot working to this point is done at temperatures between 700 � C and 900 � C" (Stoeckel and Yu, 3). There is also a process of cold working of Ni-Ti alloys. The procedure is similar to titanium wire fabrication. Carbide and diamond dies are used in the process to produce wires ranging from 0.075mm to 1.25mm in diameter. (Stoeckel and Yu, 4) Cold working of Nitinol causes "marked changes in the mechanical and physical properties of the alloy" (Jackson, Wagner, and Wasilewski, 21). These processes of the production of Nitinol are described in greater detail in Jackson, Wagner, and Wasilewski's report (15-22).

Properties
The [|properties] of Nitinol are particular to the exact composition of the metal and the way it was processed. The physical properties of Nitinol include a melting point around 1240 � C to 1310 � C, and a density of around 6.5 g/cm� (Jackson, Wagner, and Wasilewski, 23). Various other physical properties tested at different temperatures with various compositions of elements include electrical resitivity, thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity, magnetic susceptibility, and thermal conductivity (Jackson, Wagner, and Wasilewski, 23-55). Mechanical properties tested include hardness, impact toughness, fatigue strength, and machinability (Jackson, Wagner, and Wasilewski, 57-73). The large force generated upon returning to its original shape is a very useful property. Other useful properties of Nitinol are its "excellent damping characteristics at temperatures below the transition temperature range, its corrosion resistance, its nonmagnetic nature, its low density and its high fatigue strength" (Jackson, Wagner, and Wasilewski, 77). Nitinol is also to an extent impact- and heat-resistant (Kauffman and Mayo, 4). These properties translate into many uses for Nitinol.

Applications
Nitinol is being used in a variety of applications. They have been used for military, medical, safety, and robotics applications. The military has been using Nitinol couplers in F-14 fighter planes since the late 1960s. These couplers join hydraulic lines tightly and easily. (Kauffman and Mayo, 6) Many of the current applications of Nitinol have been in the field of [|medicine.] Tweezers to remove foreign objects through small incisions were invented by NASA. [|Anchors] with Nitinol hooks to attach tendons to bone were used for Orel Hershiser's shoulder surgery. Orthodontic wires made out of Nitinol reduces the need to retighten and adjust the wire. These wires also accelerate tooth motion as they revert to their original shapes. Nitinol [|eyeglass frames] can be bent totally out of shape and return to their parent shape upon warming. (Kauffman and Mayo, 6) Nitinol needle [|wire localizers] "used to locate and mark breast tumors so that subsequent surgery can be more exact and less invasive" utilize the metal's shape memory property. Another successful medical application is Nitinol's use as a [|guide] for catheters through blood vessels (Stoeckel and Yu, 9-10). There are examples of SMAs used in safety devices which will save lives in the future. Anti-scalding devices and fire-sprinklers utilizing SMAs are already on the market. The anti-scalding valves can be used in water faucets and shower heads. After a certain temperature, the device automatically shuts off the water flow. The main advantage of Nitinol-based fire sprinklers is the decrease in response time. (Kauffman and Mayo, 7) Nitinol is being used in robotics actuators and micromanipulators to simulate human muscle motion. The main advantage of Nitinol is the smooth, controlled force it exerts upon activation. (Rogers, 156) Other miscellaneous applications of shape memory alloys include use in household appliances, in clothing, and in structures. A [|deep fryer] utilizes the thermal sensitivity by lowering the basket into the oil at the correct temperature. (Falcioni, 114) According to Stoeckel and Yu, "one of the most unique and successful applications is the Ni-Ti underwire brassiere" (11). These bras, which were engineered to be both comfortable and durable, are already extremely successful in Japan (Stoeckel and Yu, 11). Nitinol actuators as engine mounts and suspensions can also control vibration. These actuators can helpful prevent the destruction of such structures as buildings and bridges. (Rogers, 156) Other applications: <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Here are some of the pictures available from this section: <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|European Space Agency]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|SMA INC.]

Future Applications
<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">There are many possible applications for SMAs. Future applications are envisioned to include engines in cars and airplanes and electrical generators utilizing the mechanical energy resulting from the shape transformations. Nitinol with its shape memory property is also envisioned for use as car frames. (Kauffman and Mayo, 7) Other possible automotive applications using SMA springs include engine cooling, carburetor and engine lubrication controls, and the control of a radiator blind ("to reduce the flow of air through the radiator at start-up when the engine is cold and hence to reduce fuel usage and exhaust emissions") (Turner, 299). <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">SMAs are "ideally suited for use as fasteners, seals, connectors, and clamps" in a variety of applications (Borden, 67). Tighter connections and easier and more efficient installations result from the use of shape memory alloys (Borden, 72).

Conclusion
<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">The many uses and applications of shape memory alloys ensure a bright future for these metals. Research is currently carried out at many [|robotics] departments and materials science departments. With the innovative ideas for applications of SMAs and the number of products on the market using SMAs continually growing, advances in the field of shape memory alloys for use in many different fields of study seem very promising. = __**Piezoelectric Materials:**__ =

Introduction: the piezoelectric effect
<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">The piezoelectric effect describes the relation between a mechanical stress and an electrical voltage in solids. <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">It is reversbile: an applied mechanical stress will generate a voltage and an applied voltage will change the shape of the solid by a small amount (up to a 4% change in volume). <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">In physics, the piezoelectric effect can be described as the the link between electrostatics and mechanics. <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">

History
<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">The piezoelectric effect was discovered in 1880 by the Jacques and Pierre Curie brothers. They found out that when a mechanical stress was applied on crystals such as tourmaline, tourmaline, topaz, quartz, Rochelle salt and cane sugar, electrical charges appeared, and this voltage was proportional to the stress. <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">First applications were piezoelectric ultrasonic transducers and soon swinging quartz for standards of frequency (quartz clocks). <span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">An everyday life application example is your car's airbag sensor. The material detects the intensity of the shock and sends an electricla signal which triggers the airbag.

Piezoelectric materials
<span style="color: black; font-family: 'Arial','sans-serif'; font-size: 16px;">The piezoelectric effect occurs only in non conductive materials. Piezoelectric materials can be divided in 2 main groups: crystals and cermaics. The most well-known piezoelectric material is quartz (SiO2). = __**Magnetorheological fluid (MR)**__ = <span style="font-family: 'Arial','sans-serif';">

Magnetorheological Fluid
<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">The other fluid that can reinforce Kevlar armor is **magnetorheological (MR) fluid**. MR fluids are **oils** that are filled with **iron** particles. Often, surfactants surround the particles to protect them and help keep them suspended within the fluid. Typically, the iron particles comprise between 20 and 40 percent of the fluid's volume. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">The particles are tiny, measuring between 3 and 10 microns. However, they have a powerful effect on the fluid's consistency. When exposed to a magnetic field, the particles line up, thickening the fluid dramatically. The term "magnetorheological" comes from this effect. Rheology is a branch of mechanics that focuses on the relationship between force and the way a material changes shape. The force of magnetism can change both the shape and the viscosity of MR fluids. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">The hardening process takes around twenty thousandths of a second. The effect can vary dramatically depending on the composition of the fluid and the size, shape and strength of the magnetic field. For example, MIT researchers started with spherical iron particles, which can slip past one another, even in the presence of the magnetic field. This limits how hard the armor can become, so researchers are studying other particle shapes that may be more effective. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">As with STF, you can see what MR fluids look like using ordinary items. Iron filings mixed with oil create a good representation. When no magnetic field is present, the fluid moves easily. But the influence of a magnet can cause the fluid to become thicker or to take a shape other than that of its container. Sometimes, the difference is very visually dramatic, with the fluid forming distinctive peaks, troughs and other shapes. Artists have even used magnets and MR fluids or similar ferrofluids to create works of art. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">With the right combination of density, particle shape and field strength, MR fluid can change from a liquid to a very thick solid. As with shear-thickening fluid, this change could dramatically increase the strength of a piece of armor. The trick is activating the fluid's change of state. Since magnets large enough to affect an entire suit would be heavy and impractical to carry around, researchers propose creating tiny circuits running throughout the armor. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">

Magnetorheological fluid before and after exposure to a magnetic field
<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Without current flowing through the wires, the armor would remain soft and flexible. But at the flip of the switch, electrons would begin to move through the circuits, creating a magnetic field in the process. This field would cause the armor to stiffen and harden instantly. Flipping the switch back to the off position would stop the current, and the armor would become flexible again. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">In addition to making stronger, lighter, more flexible armor, fabrics treated with shear-thickening and magnetorheological fluids could have other uses as well. For example, such materials could create bomb blankets that are easy to fold and carry and can still protect bystanders from explosion and shrapnel. Treated [|jump boots] could harden on impact or when activated, protecting paratroopers' boots. [|Prison] guards' uniforms could make extensive use of liquid armor technology, especially since the weapons guards are most likely to encounter are blunt objects and homemade blades. <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">However, the technologies do have a few pros and cons. Here's a rundown: <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Neither type of armor is quite ready for battlefield use. STF-treated Kevlar armor could be available by the end of 2007 [Source: [|Business Week] ]. MR fluid may require another five to 10 years of development before it can consistently stop bullets. [Source: [|Science Central] ]. Check out the links on the next page to learn more about military technology, body armor and related topics.

Other Uses for MR Fluids
<span style="font-family: 'Times New Roman','serif'; font-size: 16px;">MR fluids have numerous uses besides strengthening body armor. Their ability to change from liquids to semisolids almost instantly makes them useful for dampening impacts and vibrations in items like: <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Since it can instantly and reversibly change shape, it could also be used to create scrolling Braille displays or reconfigurable molds. = __<span style="font-family: 'Cambria','serif';">Auxetic Materials __ = <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Many people are sceptical about whether auxetic behaviour really exists, because we expect materials to behave like our elastic band and get thinner when stretched. Why is this? Do we base our expectation on a knowledge of the deformation mechanisms within the material? Or on classical elasticity theory? No, the only reason we think like this is down to everyday experience. Even the property relating directly to this behaviour, Poisson's ratio ( n <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">), is defined so that most `normal' materials have a positive value.
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Car [|shock absorbers]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|Washing machines]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|Prosthetic limbs]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|Bridges]
 * == Background == ||
 * <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Pick up an elastic band and stretch it lengthways as if you were going to `ping' it at somebody. Before letting fly, look at the width of your elongated missile - it's thinner than an unstretched band, as you'd expect. Try this with an auxetic elastic band and you'd be in for a surprise. These bizarre materials can actually become fatter when stretched, a phenomenon which is now attracting the practical interest of many materials scientists.

How do Auxetic Materials Work?
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Poisson's ratio is the ratio of the contractile lateral strain to the tensile axial strain for a material stretched axially, and is typically around +0.2 to +0.4. However, when we look into classical elasticity theory we find that the Poisson's ratios of isotropic materials can not only take negative values, but can have a range of negative values twice that of positive ones. <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">A study of the structure of materials, and how it deforms, demonstrates that auxetic properties are entirely feasible. Figure 1 shows a 2D structure consisting of a regular array of rectangular nodules connected by fibrils. Deformation of the structure is by `hinging' of the fibrils. For the `open' geometry shown in figure 1a, the cells elongate along the direction of stretch and contract transversely in response to stretching the network, giving a positive v. However, modify the structure to adopt a `re-entrant' geometry, figure 1b, and the network now undergoes elongation both along and transverse to the direction of applied load. In other words, this is an auxetic structure. || <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">It is clear that auxetic materials have beneficial properties, so the next step is to produce them, and there are as many different routes as there are materials. Some routes rely on transforming non-auxetic materials into an auxetic form (foams), whereas others employ standard techniques but with novel material architecture to achieve the auxetic effect (honeycombs and fibre-reinforced composites). Still others require novel development of existing processing routes for conventional materials to produce auxetic functionality. <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">As an example of this latter scenario, take the processing of auxetic microporous polymers. The key is to produce a three-dimensional version in the polymer of the two-dimensional schematic in figure 1, i.e. to achieve nodules interconnected with fibrils. The route used to achieve this complex microstructure, for UHMWPE, is a batch process based on conventional polymer powder processing techniques of sintering and extrusion, but adapted so that there is only partial melting of the starting powder, which gives rise to fibrillation. An additional initial compaction stage can be used solely to impart some degree of structural integrity to the extrudates. This means the properties of the polymers can be tailored to fit the applications required and, by varying certain processing parameters, to produce everything from structural auxetic polymers with a Young's modulus of 0.2GNm-2, down to very auxetic, low modulus polymers
 * == Production of Auxetic Materials ==
 * == Production of Auxetic Materials ==
 * == Production of Auxetic Materials ==
 * == Production of Auxetic Materials ==

Biomedical Industry
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Key areas of application are seen in the biomedical field. Prosthetic materials, surgical implants, suture/muscle/ligament anchors and a dilator to open up blood vessels during heart surgery are all possible. Another area relates to the use of auxetic materials in piezoelectric sensors and actuators. Auxetic metals could be used as electrodes sandwiching a piezoelectric polymer, or piezoelectric ceramic rods could be embedded within an auxetic polymer matrix. These are expected to increase piezoelectric device sensitivity by at least a factor of two, and possibly by ten or a hundred times. The development of auxetic materials for micro- and nano-mechanical and electromechanical devices is also being investigated.

Filters
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Auxetic foam and honeycomb filters offer enhanced potential for cleaning fouled filters, for tuning the filter effective pore size and shape, and for compensating for the effects of pressure build-up due to fouling. These benefits rely on the pores opening up both along and transverse to the direction of a tensile load applied to an auxetic filter. The pores of a non-auxetic filter open up in the stretching direction but close up in the lateral direction, leading to poorer filter performance, figure 1. However, stretching an auxetic filter improves performance by opening pores in both directions. The effect of stretching on the de-fouling of an auxetic polymeric honeycomb fouled with glass beads has been investigated. For the particular honeycomb studied the value of n <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;"> is dependent on the stretching direction. The studies clearly demonstrate that defouling is enhanced when the filter is loaded in the direction with the largest negative n <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">. <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;"> || == applications ==

Biomedical Industry
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Key areas of application are seen in the biomedical field. Prosthetic materials, surgical implants, suture/muscle/ligament anchors and a dilator to open up blood vessels during heart surgery are all possible. Another area relates to the use of auxetic materials in piezoelectric sensors and actuators. Auxetic metals could be used as electrodes sandwiching a piezoelectric polymer, or piezoelectric ceramic rods could be embedded within an auxetic polymer matrix. These are expected to increase piezoelectric device sensitivity by at least a factor of two, and possibly by ten or a hundred times. The development of auxetic materials for micro- and nano-mechanical and electromechanical devices is also being investigated.

Filters
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Auxetic foam and honeycomb filters offer enhanced potential for cleaning fouled filters, for tuning the filter effective pore size and shape, and for compensating for the effects of pressure build-up due to fouling. These benefits rely on the pores opening up both along and transverse to the direction of a tensile load applied to an auxetic filter. The pores of a non-auxetic filter open up in the stretching direction but close up in the lateral direction, leading to poorer filter performance, figure 1. However, stretching an auxetic filter improves performance by opening pores in both directions. The effect of stretching on the de-fouling of an auxetic polymeric honeycomb fouled with glass beads has been investigated. For the particular honeycomb studied the value of n <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;"> is dependent on the stretching direction. The studies clearly demonstrate that defouling is enhanced when the filter is loaded in the direction with the largest negative n <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">. ||  ||
 * **<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Figure 1. **<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Schematic of particulate de-fouling capabilities of non-auxetic and auxetic materials  ||

Auxetic Fibres
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">The breakthrough development of a continuous process to produce auxetic materials in fibrous form has created the opportunity to apply their unique characteristics in a wide range of applications previously not possible. Fibres can be used in single or multiple filament structures and can be used to produce a woven structure. Typical performance characteristics expected of auxetic fibres and structures are detailed in the table of applications, (table 1), together with a list of the applications in which these characteristics could offer significant benefits. For example, by analogy with the filter de-fouling scenario of figure 4, biomedical fibrous drug-release materials could be made from auxetic fibres. Extending the fibres opens the micropores and a specific dose of drug is released. <span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Advanced auxetic fibres will include multi-filament yarns in which an auxetic filament is wrapped with one or more other yarns, perhaps high stiffness/strength, dyeable or conductive filaments, so that the benefits of the auxetic material are combined with other beneficial properties for smart technical textiles applications. This will lead to the possibility of hierarchical composites displaying auxetic behaviour at more than one lengthscale. Current research on auxetic composites is concentrated on the use of non-auxetic constituents and so benefits due to the auxetic effect occur at a macrostructural level. Employing auxetic fibres as the reinforcement will enable benefits, such as impact energy and acoustic energy absorption, to be achieved at the microstructural level.

Auxetic Fibre Reinforced Composites
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">Auxetic fibre reinforcements should also enhance the failure properties of composites. Fibre pull-out is a major failure mechanism in composites. A unidirectional composite loaded in tension will undergo lateral contraction of both the matrix and fibre materials, leading to failure at the fibre/matrix interface. Auxetic fibres, on the other hand, allow the possibility of maintaining the interface by careful matching of the Poisson's ratios of the matrix and fibre, figure 2. ||  ||

The Future
<span style="color: #333333; font-family: 'Verdana','sans-serif'; font-size: 11px;">So what does the future hold for auxetics? Despite the very significant developments to date we have only scratched the surface of this exciting and multi-disciplinary field. The successful synthesis and development of molecular and multi-functional auxetics represent key opportunities for the future. In addition to leading to materials with extreme properties such as high modulus and strength, these advanced materials will have potential in sensor, drug-release and separations applications. By accepting a negative Poisson's ratio as a positive property we are truly expanding the applications of these fascinating materials = __**General references:**__ =
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">Stanford university Website: []
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">the institute of material website: [|http://www.iom3.org]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">[]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|Shape Memoy Alloy information from Mide Technology Corporation]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|European Space Agency] on [|SMAs]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;"> [|NASA Langley Research Center] on [|SMAs]
 * <span style="font-family: 'Times New Roman','serif'; font-size: 16px;">[|http://www.piezomaterials.com]