Smart and Biomimetic Materials

Smart materials react to stimuli such as heat, light, electricity, or some other factor from their external environment. They include shape-memory alloys and polymers, piezoelectric polymers and ceramics, biomimetic polymers and gels, conductive polymers and controllable fluids, magnetostrictive materials, and chromogenic materials. Smart materials can be used in a smart materials system, such as micromachined electromechanical systems and fiber-optic sensor systems.

Smart materials systems are a late twentieth century design methodology that integrates the actions of sensor, actuator, and control circuit elements in systems that can respond and adapt to changes in their environment or condition in a helpful way. They bestow smartness, or a functionality that enhances the value of materials, technologies, or the final products. Smartness can improve the performance of a system in a way that cannot be achieved using the more established non-smart approaches.

Many other smart materials systems are continually researched and developed, notably by the defense and aerospace industries. The National Aeronautics and Space Administration (NASA), for example, is developing the ultimate smart structure: a shape-changing airplane wing that will reorder and optimize its shape during flight to match the atmospheric conditions or the mission it must carry out.

Shape-Memory Materials. Shape-memory materials behave in a predetermined way when exposed to a particular stimulus. They can exist in different shapes at two or more different temperatures once their transition temperature has been reached. Shape-memory alloys have very different shape-changing characteristics compared with shape-memory polymers.

Devices made from shape-memory alloys can provide force when they are exposed to their transition temperature, as is the case with actuators. However shape- memory polymer devices undergo mechanical property loss while exposed to their transition temperature, as when used to make releasable fasteners.

Shape-memory alloys are much more costly than polymers. It takes time to program a metal alloy that also needs heat treatment at temperatures of hundreds of degrees Celsius, resulting in a maximum deformation of only about 8 percent. Shape-memory polymer systems are being developed that are easier to shape, with more applications than the shape-memory materials already in use, such as the nickel-titanium alloy Nitinol, used to make items such as flexible spectacle frames. The polymers can be programmed into shape in seconds at about 70°C and can undergo major deformations of several hundred percent.

Fiber-Optic Sensors. Fiber-optic sensors are based on optical fibers attached to sensing devices, forming smart systems that may be built into structures such as airplane wings and bridges. Fiber-optic sensor technology is used to measure mechanical properties such as strain, pressure, and temperature. Structures incorporating fiber-optic sensors react to or warn of imminent failure and demonstrate the state of the structure following damage.

Piezoelectric Polymers and Ceramics. Piezoelectric polymers and ceramics are materials that exhibit new properties when they are exposed to an electric current (piezo-electricity). Many polymers, ceramics, and molecules such as water are continuously polarized, with some parts of the molecule being positively charged and other parts negatively charged. By applying an electric field to these materials, the polarized molecules will align themselves with the electric field, producing induced dipoles within the molecular or crystal structure of the material.

Piezoelectric materials are used in acoustic transducers that change acoustic (sound) waves into electric fields and electric fields into acoustic waves. These find an application in devices such as speakers and drums.

Semicrystalline polyvinylidene fluoride (PVDF) has been the only piezoeletric polymer commercially available until recently. Lightweight, flexible and easily produced as sheets or in complicated shapes, its low mechanical and acoustic impedance makes it highly suitable for use in underwater and medical applications. However PVDF has limited temperature use and poor chemical stability in extreme environments. Polyimides may be an alternative as they have excellent thermal, mechanical and dielectric properties combined with high chemical resistance and stability.

Magnetostrictive Materials. Magnetostrictive materials can convert magnetic energy into mechanical energy and also transform mechanical energy into magnetic energy. Magnetostriction is a property of the material that does not lessen over time. These materials expand when exposed to a magnetic field, an effect known as the Joule effect or magnetostriction after James Prescott Joule. In the early 1840s Joule identified the phenomenon when he observed a change in length of an iron sample as its magnetization altered.

This effect is due to the lining up of the magnetic domains in the material with the magnetic field. A change in the size of the width occurs together with the change in length produced by the Joule effect. When the material is stretched or compressed, it undergoes strain and its magnetic energy changes. This is known as a magnetomechanical or Villari effect, commonly used in magnetostrictive sensors.

Magnetorestrictive materials include iron, cobalt, nickel, ferrite, metglass, and terbium alloys (Terfenol-D). They are used in a range of modern devices such as sensors, sonar and ultrasonics, speakers, vibration and noise control, and drills and reaction mass actuators.

Chromogenic Materials and Systems. Chromogenic materials and systems can change their optical properties in response to an electrical, photo, or thermal stimuli. Electrically activated chromogenic systems are used for smart windows and mirrors in the automotive and architectural industry and for low-information content displays. Electrically activated chromogenics can be controlled by the user, unlike photochromic and thermochromic devices which are self-regulating and passive.

Electrochromic materials are chromogenics that change color on electrical stimulation. They are actuator elements that need sensor and control circuits to be added to make the system smart. Reflective hydrides may also be regarded as electrochromics, but they differ in a number of ways from the more commonplace oxide electro-chromics. Originally deposited as a metal, they can be converted to a partially transparent hydride by injection of hydrogen from the gas or solid phase when they switch to a reflective state, which has several potential advantages in terms of energy performance and durability. Transition-metal hydrides have now also been developed.

Liquid crystal windows switch quickly from a transparent state to a diffuse white state; however, they have little control over solar heat gain. Suspended particle displays are also under development. Photochromics darken in sunlight and are therefore mainly used to make sunglasses that darken automatically. Thermotropic materials respond primarily to heat.

Heat-sensitive polymers (thermochromic) are used for children’s toys, tee-shirts, and toothbrushes that change color when touched. These materials have additional functions that are visible in real time and can provide a variety of intelligent responses.

Biomimetic Materials. Biomimetic materials are based on nature’s best designs and attempt to mimic them. Nature solves problems by looking for a solution that works using the minimum amount of energy. Engineers solve problems by searching for an effective solution with the lowest cost. Plants and animals require a great deal of energy to produce the basic materials they need for survival, but they can use almost any shape. Engineers are able to produce a wide variety of materials cheaply, but shapes are often expensive to make.

By copying nature’s designs and shapes, researchers can make more efficient structures that can be used to solve tomorrow’s engineering problems as well as to develop innovative new materials. Biomimetic polymers are being developed based on natural materials; for example, spider’s silk, which is in fact a biopolymer. Biomimetic gels; for example, those based on the sea cucumber, are being researched and may result in a number of biomedical applications.

Microelectromechanical Systems (MEMS). Microelectromechanical systems (MEMS) are miniaturized devices. They may be as small as a silicon semiconductor chip, which is able to integrate sensors, information or signal processing, and control circuits in a single device. A range of MEMS -based devices have been developed including pressure sensors, transducers, transmitters, microrelays, optical attenuators and photonic switch components, and smart security and tagging systems. They are under development for further biomedical applications.

Controllable Fluids. Controllable fluids have properties that depend on an electric or magnetic field. They are a smart technology that may be used instead of piezoelectric transducer-controlled semiactive suspension systems.

Conclusion. Among the more interesting biomedical developments in smart systems in the 1990s were synthetic muscle actuators, which included shape memory alloys, piezoelectrics, and electroactive polymers. A particular example of a smart material used for such an application is IPMC (ion-exchange polymer membrane metallic composites). The synthetic muscles contract or bend when exposed to an electric current and can be made into wires that are as thin as a human hair.