4. Smart Materials

Wholly synthetic materials that mimic biological functions, such as artificial muscles, might eventually be displaced in a biomedical context by tissue engineering, but they are nevertheless likely to play an important role in prosthesis for many years to come. And their field of application is by no means exclusively biomedical. Materials that impart mechanical force in response to some external stimulus have a wealth of applications, ranging from robotics to adaptive optics (astronomical telescopic mirrors that change shape to cancel out the distorting effects of atmospheric turbulence), from vibration control and earthquake protection to loudspeaker technology and noise reduction.

Artificial muscles are examples of so-called smart materials, which furnish the perfect illustration of materials fulfilling active functions, rather than serving as passive fabrics. They are, in a sense, materials that act as machines: a robot arm flexes because of a material that changes its properties, rather than because a system of hydraulic pistons or electrically powered cogs and gears is set in motion. Smart materials that effect some change, whether it be mechanical motion, closing or opening a valve, or throwing an electrical switch, are called actuators. A second broad classification is that of sensor materials, which sense and signal some crucial change in their ambient environment. Sensors coupled to actuators make a formidable technological marriage.

For example, there are efforts to reduce cabin noise in aircraft by coupling acoustic sensors to actuators that vibrate in perfect antipathy to the noise, broadcasting "anti-noise" that cancels out the droning sound imparted by the vibrating aircraft superstructure. The coupling of sensors to actuators virtually defines robotics, where sophisticated information-processing and pattern-recognition software is needed to make sense of the input data from sensors before it can be translated to some appropriate action mediated by actuators. Here there is a move towards making more use of materials properties, rather than information processing, to achieve the required ends. It should in principle be simpler and less demanding on the control algorithms to take advantage of, say, the inherent elasticity and resilience of an artificial muscle in determining limb movement than to equip the robot with many unidirectional "stiff" actuators to achieve the same end.

Most of the artificial muscles (i.e., materials which convert a signal, typically electrical, to mechanical motion) in use today are "hard materials," such as piezoelectric ceramics: ferroelectrics such as barium titanate and lead zirconate-titanate (PZT). As actuators they are used, e.g., in ink printers. Because the conversion works also in reverse-mechanical motion (pressure) excites an electric current-they also serve as sensors for vibration, sound or sonar systems. High-force mechanical displacement, as might be required in the aerospace industry, can be effected by magnetostrictive metal alloys such as Terfenol-D, a blend of terbium, dysprosium and iron that contracts when a magnetic field is applied by an electromagnet.

Figure 4. Terfenol-D is a Magnetostrictive Alloy: it
Contracts when Placed in a Magnetic Field
This is a smart material used to make a kind of artificial muscle

Shape-memory alloys such as Nitinol (an alloy of nickel and titanium), on the other hand, change shape in response to a change in temperature. Nitinol wires have been used in robotic hands that achieve a lightness of touch not easily afforded by electromechanical or hydraulic actuators.

But there is a great deal of interest in making "soft" artificial muscles too. Part of the motivation here is biomedical: actuators made from compliant polymers may be more compatible with soft tissues, and there is also greater scope for engineering organic materials to ensure biocompatibility with the body's chemistry. But polymer-based smart materials could also be more lightweight and cheap to produce-benefits for any kind of engineering.

Many soft smart materials are polymer hydrogels: gels of crosslinked polymers that will swell and shrink reversibly in water. These "volume transitions" can be very abrupt, like freezing or melting transitions, and can be induced in some gels by changes in environmental conditions: temperature, pH, electric fields, light, or the presence of some chemical substance. For example, swelling might be induced entropically through conformational changes of the "free" parts of the polymer chains in the gel, and so is triggered at some temperature threshold. Or the volume change could be of electrostatic origin due to ionization of acidic side-groups on the chains, and so induced by a change in hydrogen-ion osmotic pressure, caused by application of an electric field.

Conformational changes in a hydrogel have been used to create a shape-memory polymer, which can be deformed at room temperature, but will regain its original shape on heating to 50 ĄC. But the mechanical force generated by volume changes in hydrogels is rather modest because of their very softness, and much of the interest in their "smart" behavior stems instead from the changes in permeability of the gel network that these transitions bring about. Drug molecules entrapped inside the gel when it is cross-linked will typically remain there in the collapsed state of the gel but are able to escape when the gel expands. This offers a mechanism for the controlled release of drugs inside the body, stimulated perhaps by non-invasive methods, such as gentle warming or light-induced processes. The drugs could be carried into the body in microscopic particles of the gel, which can be administered orally. Transitions induced by pH changes might be particularly useful in this context, as they could be triggered by the passage of the gel medium into or out of the acidic environment of the stomach. And swelling transitions triggered by a certain biochemical substance could make the release pattern sensitive to pertinent chemical changes in the body. For example, a prototype insulin release medium for diabetes sufferers has been prepared from a hydrogel loaded with both insulin and the enzyme glucose oxidase. The reaction of the enzyme with (high concentrations of) glucose brings about a change in the pH of the solution-and the gel swells, releasing its load of insulin.

Some electrically conducting polymers have also shown potential for use as actuators. The electrochemical insertion or removal of dopant molecules can bring about conformational changes in conducting polypyrrole that are translated into changes of shape and thus mechanical force. And the piezoelectric properties of polyvinylidene fluoride recommend it as a cheap pressure-sensitive medium for computer keyboards.

The key question for the future is: how smart can materials get? In some contexts, this is a question about the magnitude of their responsiveness. There is still a need for mechancial actuators that can combine large force generation with large displacements-a difficult combination, at present often obtained by switching many actuators in parallel. There is still no "artificial muscle" that compares in these respects with real muscle, which achieves large (and rapid) displacements by the interdigitation of the fibrous cells in the sarcomere assembly. For sensor technologies, the question may be one of sensitivity: how to maximize the ratio of output to input signal. Here again biology excels, for example, with the single-photon sensitivity of photoreceptors in the rod cells of the retina. One area of critical technological importance here (and it is one not commonly included explicitly under the banner of smart materials) is the development of highly sensitive read-out heads for magnetic data storage. Higher sensitivity would permit greater read-out speeds and greater storage densities (that is, smaller areal densities per bit). Such improvements are already being attained by replacing the traditional read-out heads, working by electromagnetic induction, with smart materials that alter their resistance in an applied magnetic field. A strong "magnetoresistive" response of this sort is obtained in so-called magnetic multilayers, stacked thin films of magnetically coupled iron or cobalt alternating with thin films of a non-magnetic metal such as chromium or copper. But significant further improvements are promised by a recently-discovered class of oxide ceramics such as lanthanum strontium manganite, which exhibit a much larger magnetoresistive response graphically dubbed colossal magnetoresistance.

next page...