The concept of 'intelligent' or 'smart' materials was first noted with metal alloys in the late 1930s, was practically demonstrated about ten years later, and was commercially utilised in 1965. Although the chemo-mechanical potential of gels was demonstrated as early as 195 Os by changes in pH, it was not until the mid 1970s when systematic studies on the intelligent polymeric gels were initiated. Intelligent fibrous materials were first patented in Japan for silk yarn with shape-memory and for thermo chromic fabrics that changed colour with changes in temperature. The development of intelligent materials, Le. materials that usually have a dramatic and reversible response to one or more external stimuli, is critically reviewed, with emphasis on intelligent fibrous materials.

Initial development and concepts were made with polymeric gels followed by intelligent fibrous materials. This review covers,
The salient features of the polymeric gels as they apply to this concept
Characteristics study on intelligent fibrous materials
Opportunities for future development and applications of these intelligent polymers and fibrous materials.

Salient features of intelligent polymeric gels

The intelligent attributes of polymeric gels were discovered and verified by Tanaka who showed in 1975 that small changes concentration or temperature caused the gels to abruptly swell to many times their original dimensions or to collapse into a compact mass. After that discovery, and particularly in the last decade, there have been extensive basic and applied investigations on the intelligent characteristics of polymers. An informative and fairly comprehensive list of external or environmental stimuli that cause intelligent behaviour in polymers is given in Table 1.

The physical stimuli can be changes in temperature, exposure to radiation in the electromagnetic spectrum, electrical fields, applied stress/strain and solvents. The chemical stimuli may be changes in pH, electrolytes or salts, chemical and biological agents. Although not listed, biological stimuli, such as various bacteria, fungi, algae and viruses, could also be effective with appropriate conditions and materials. As shown in Table 2, these physical. chemical and/or biological stimuli can also cause changes in intelligent polymers related to phase, shape, optical and mechanical behaviour, electric fields, surface energies, reaction and permeation rates and recognition. Many of these stimuli and characteristics are also seen relevant to intelligent fibrous materials.

Polymer gel phase transition

The principles and applications of intelligent polymers have been reviewed and discussed in several scientific and trade journals over the past twenty years. Polyacrylamide gels that have been cross-linked with bis-acrylamide groups and partially hydrolysed to carboxyl groups typify materials that are sensitive to changes in solvent and temperature. Abrupt collapse and reverse reswelling phenomena of these gels are associated with phase transitions that can be characterised by a three-dimensional phase diagram. This phase diagram has variables, such as solvent concentration, temperature, ionisation and gel volume that are related to positive and negative osmotic pressures and a critical end point. In addition to ionic interactions between polymer chains, there can also be hydrophobic interactions and interactions due to hydrogen bonding and Van der Wall's forces that are involved in polymer gel phase transitions and accompany dramatic changes to external stimuli.

Application

It contracts to permit desired liquids and dissolved substances to flow through a membrane and expand to stop the flow when the pores shrink.
Within the last five years, gels swollen with Ferro fluids also can be made to create fluids that undergo changes in viscosity with variations in magnetic field. These gels can be employed as vibration dampers, molding systems and remotely activated clutching mechanism.
Modified gels also can be tailored for controlled insulin delivery, whereby they are responsible to glucose by incorporation of appropriate enzymes that lower pH conversion of glucose into gluconic acid.

There are numerous examples of practical applications of polymeric gels that have been prepared or fabricated to react to a variety of external stimuli. Gels, which expand and contract with varying electrical fields, have been suggested as general utility chemical values.

Shape-memory polymer gels based on the interpenetration of only part of one gel network with another gel network (N -isopropylacrylamide NIPA) with polyacrylamide (PAAM) have been synthesised. They have been shown to be responsive to specific external stimuli, such as changes in temperature. Depending on the temperature and placement of the modulated NIPA gel relative to the PAAM gel. a variety of shapes and dimensional changes can be achieved that are more complex and subtle han those attainable with conventional polymeric gels. It is envisioned that other existing and new polymer and macromolecular structures will be used as intelligent materials for a variety of applications.

Intelligent fibrous materials

The concept of 'intelligent or smart materials' for textiles or fibrous substrates was first recognised and demonstrated in 1979 with the development of a shape-memory silk yarn. There are a couple of other developments that occurred in Japan in the late 1980s that can be classified as processes for production of intelligent fibrous materials.

Smart clothing

The micro-encapsulation of zirconium carbide into polyester or polyamide fibres produces a fabric or material that absorbs solar visible radiation and converts it into heat that is released inside a garment made from it. The released heat and radiant heat from the body cannot escape outside and thus warms the wearer, even under cold conditions.

The disadvantage is that there must be sufficient sunlight to activate this system and it is only one-way memory system, i.e. it does not reverse at high temperatures to cool the fibres and the wearer.

Thermo chromic fibre polymer

The other development was the incorporation of compounds into polymeric fibres that exhibited thermochromism. Thus, a variety of intelligent fabrics and garments were produced that exhibited reversible changes in colour when temperature differences of 5 C or greater occurred, These systems were operable at temperatures as low as -40C and as high as 80C.

Smart materials

Reported highlights of a symposium held at an American Chemical Society meeting in 1995 on polymeric smart materials included elastomeric composites with embedded magnetic particles that undergo changes in modulus when subjected to external magnetic fields.

Application

They can be used for noise or vibration reduction in machinery and transportation and for miniaturised fibre optic sensors that contain intelligent dyes or enzymes whose fluorescence changes with changes in concentration of specific chemical entities or conditions. such as glucose, sodium and pH and they also can be used for m'any chemical and biological applications.

Phase changing materials

Polyethylene glycol is a Phase. Change Material (PCM) that has high latent heat, which is stored and released, respectively, in the vicinity of its melting and crystallisation temperatures. When the polyethylene glycol is cross linked and insolublised with tetra functional agents, in the presence of fibrous polymers, it is chemically and/or physically bound to the fibres, retains much of its PCM behaviour, and thus forms an intelligent textile material with a thermal memory that is reversible with changes in the evironmental temperature.

Measurement of temperature adaptability

This temperature adaptability is unique and has been measured and .verified by different scanning calorimetry and infrared thermography (changes in surface temperature). The use of different molecular weights of polyethylene glycol allows one to choose the environmental temperature at which the modified fabrics have optimum thermal storage to provide cooling at surface temperature of the modified fabrics after 30 minutes exposure to heat source was 20-28P lower than a comparable untreated fabric exposed in the same manner. The thermal adaptability of these fabrics containing cross-linked polyols is attributed not only to the latent heat stored and cooled, but also to the hydrophilic nature of the cross-linked polymer on the fibre surface.

Reversible shape-memory

Another intelligent attribute of textiles containing cross-linked polyols reversible shape-memory, which can be activated by proper choice of polar solvent and fabric construction. This reversible shape-memory helps in designing of an intelligent bandage comprised of disposable and non disposable components, which could be wrapped around parts of the body such as fingers, arms, and legs to stop bleeding.

A similar approach was used to design a fabric bandage with metal threads having shape-memory, which was activated by changes in temperature. However, electric current and/or heat would have to be applied to activate the bandage, whereas the absence or presence of body fluids would be more advantageous in this regard.

Other recent patents and publications on intelligent fibrous materials also appear to be primarily focused only upon shape-memory effects. Some examples include:

The preparation of polyester fabrics with durable press properties by cross-linking polycaprolactone and acrylate monomers with high-energy radiation
Preparation of smart fibrous composites (carbon and glass) as sensors for preventing fatal fracture by changes in their conductivity and insulating properties

For example, clothing that generates solar power, fabrics that beep if you risk athletic injury and bed sheets that monitor your heartbeat and physiological health.

Reversible shape-memory helps in designing of an intelligent bandage comprised of disposable and non-disposable components.

At its simplest, intelligent polymers are plastic strands that can carry electricity, altering their conductivity in response to stretching, heating or sunlight. By weaving these into clothing, and measuring changes in the current passing through them, any number of new applications are possible. The first prototype so far has been the 'knee sleeve,' a training device tested last year on Australian professional athletes to reduce knee injuries. It fits over the knee like an open-ended sock. When the fabric is stretched, indicating a harmful movement of the knee, the altered electrical charge within the sleeve's polymers triggers a detachable buzzer. That tells the athlete that he has got bad habits and risks Anterior Cruciate Ligament (ACL) damage, according to Ms Julie Steele, a biomechanics researcher at the University of Wollongong, where the device was designed. Other potential uses could include textiles such as bed sheets that constantly monitor a user's heartbeat, outdoor clothing that change insulation and waterproofing properties in response to temperature and humidity, and clothing that converts sunlight to energy.

Manufacture of intelligent fibrous materials

There are varieties of ways in which intelligent fibrous materials could be developed and evaluated. They are:

The design of a fibrous substrate with a response to only one external stimulus, such as change in temperature, applied stress, solvents or pH. These intelligent fibrous materials could also be designed to respond in the same manner to more than one external stimulus, such as the aforementioned fabrics containing cross-linked polyols, which thermally buffer changes in temperature but reversibly contract and expand when exposed to polar solvents.

Developing materials according to function, primarily as a sensor or as an actuatOJ. A sensor function could be designed so that the smart materials are responsive in a one-way, irreversible mode (materials irreversibly transmit desired signal for safety) or reversible mode.

Actuators would be intelligent materials that would undergo rheological changes to repair damage in another part of the structure or biomedical device by releasing enzymes or drugs into the body with changes in the pore sizes and accessibility of the modified fibrous substrate.

According to the end-uses of the intelligent fibrous materials. Military applications could be repair of uniforms, tents, tyres and other fibrous and electromeric structures, camouflage using thermo chromism, temperature-adaptable fabrics and photo-chromic effects, and carbon fibre composites in aircraft and other military applications to mini mise or reduce vibrations that lead to functional failure.

Intelligent fibrous materials have only begun to be explored for their practical applications, compared to polymer gels.

Biomedical and health-care applications would be the design of smart bandages, materials that could be used in burn and wound therapy, intracorporeal medical devices that adapt to changes in body metabolism and function, and delivery of a variety of drugs and enzymes to patients in a sustained and highly adaptive manner.

There are also a variety of agricultural, horticultural and consumer applications that could exploit the unique characteristics of smart textiles materials by their selective response to external stimuli.

Display device

Conjugated Polymer Electro Luminescence Device (CPELD) consists of organic thin layers that are essentially insulating materials. The operating mechanisms involve injection of electrons and holes from cathode and anode to the organic emitter layer, and hole/electron recombination that generates light emission. A typical configuration of a display device and the yellowish red colour emitted from

the polymer with its photo luminescence intensity are shown. The next step in this research is to integrate the computer with powerful algorithm to this device to control the intensity and the emitted colour using different polymeric layers (Figs. 1 and 2).

Intelligent polymers to assist the hearing-impaired

The University of Wollongong's Intelligent Polymer Research Institute's (IPRI) and Co-operative Research Center (CRC) for Cochlear Implant and Hearing Aid Innovation's, mission is to improve communication for millions of adults and children with hearing-loss in Australia and worldwide. The CRC brings innovative interdisciplinary research leading to new hearing technology devices and clinical procedures, which are nothing but the specially developed intelligent polymers, which will have a direct benefit in improved devices for people with hearing impairment.

The use of intelligent polymers raises the potential for enhanced interfacing of cochlear implants with a hearing-impaired person's own neural system, greatly increasing communication benefits.

Future developments and opportunities

Intelligent fibrous materials have only begun to be explored for their practical applications, compared to polymer gels. Fibrous substrates can serve as intermediate types of materials in strength and flexibility, in contrast to polymeric gels that are very flexible but usually have little strength or molecular integrity. Moreover, attachment of an appropriate polymeric gel or other polymeric entity to a fibrous surface should afford intelligent materials with good mechanical integrity but suitable sensitivity to external stimuli. It is envisioned that over the next decade or two, too many new products based on intelligent polymers and fibrous materials will be produced and commercialised. Smart fibres, fabrics and clothing starts with a review of the background to smart technology and goes on to cover a wide range of the material science and fibre science aspects of the technology including:

Electrically active polymeric materials and the applications of non-ionic polymer gel and elastomers for artificial muscles
Thermally sensitive fibres and fabrics
Cross-linked polyol fibrous substrates stimuli-responsive interpene. trating polymer network hydrogel
Permeation control through stimuliresponsive polymer membranes
Optical fibre sensors, hollow fibre membranes for gas separation
Integrating fibre-formed components into textile structures
Wearable electronic and photonic technologies
Adaptive and Responsive Textile Structures (ARTS) and
Biomedical applications including the applications of scaffolds in tissue engineering

Conclusion

Intelligent polymers are thus going to be a promising field for the future. Already, western countries are adopting these strategies. There is no doubt that with present facilities and technologies India can also excel in the field of intelligent polymers and their derivatives by knowing the intricacies in its manufacture and applications.

References

1. Tatsuya H and Philips G 0, New Fibres, Woodhead Publishing Limited (UK), 1997 (Second edition), pp
12,46117,121,131,152,165,184,193

2. Ravi Kumar M N V and Outta P K.
Synthetic Fibres, 27 (I), 1998, pp 15-18

3. John P 0' Brien and Aneja A P, Review of Progress in Coloration, Volume 29,1999, p 1

4. Kawata T, Kanebo Gohesen Ltd,
Chemical Fibres International, 48121, 1998, pp 81-82

5. Aneja A P, Textile Asia, October 1998 pp 36-38

6. Rocha I, Textile Month, March 1998, p 32.

D Gopalakrishnan, KG Mythili and Nitha K Kumar
Sardar Vallabhai Patel Institute of Textile Management,
Coimbatore 641 004
E-mail: dgk_psgtech@yahoo.co.in
Department of Textile Technology, PSG College of Technology,
Coimbatore 641 004