Abstract:

Smart materials are a specialized class of advanced materials capable of detecting and responding to external stimuli such as stress, temperature, magnetic or electric fields, pH, and light. Unlike conventional materials, they can adapt their behavior in a reversible and predictable way, which makes them highly valuable in modern engineering. This review presents an in-depth overview of smart materials, highlighting their defining characteristics, classifications, and functional mechanisms. It also examines established systems such as shape memory alloys, piezoelectric, and magnetostrictive compounds, alongside emerging candidates including aero-gels, titanium foams, and self-healing polymers. Applications in healthcare, aerospace, construction, robotics, and energy sectors are discussed in detail. Finally, the paper outlines current limitations, including cost and durability, and emphasizes ongoing research trends that could accelerate their integration into mainstream technologies.

1. Introduction

The history of smart materials dates back to the 1960s, when scientists began studying materials that changed shape in response to external stimuli. Since then, many advances have been made in this field, thanks in part to advances in nanotechnology and materials engineering research. Looking ahead, the future of smart materials appears exciting. Advancements in nanotechnology, 3D printing, and material science will pave the way for further innovation and commercialization of smart materials. The main aim to develop smart devices is to minimize waste and increase efficiency in the use of natural resources. That is why smart materials play a key role in the circular economy. These materials are designed to be durable, efficient and resilient, making them ideal for making products that can be effectively reused or manufactured. Smart Materials Market size was valued at USD 79.51 Billion in 2023 and is poised to grow from USD 90.24 Billion in 2024 to USD 248.54 Billion by 2032, growing at a compound annual growth rate of 13.5% during the forecast period (2025-2032). Though smart material industry is expanding, there are several challenges. The main disadvantages of smart materials include high production costs, limited availability, and complex manufacturing processes. They can also have issues with reliability and performance degradation over time, and their use may raise concerns about environmental impact, handling complexity, and cybersecurity risks in integrated systems. 

Human evolution is linked to the manipulation of the environment. Since the first hominid to use a stone as a tool — or a bone according to the iconic scene from 2001: A Space Odyssey —, we have come to recognise this as materials science. This discipline uses physics, chemistry and engineering to study how materials are formed and what their physical properties are, as well as to discover and develop new materials, such as smart materials in order to find new uses applicable to any sector. Human development has always been closely linked with the materials available at given time [1]. The shift from the Stone Age to the Bronze and Iron Ages illustrates how discoveries of new materials revolutionized Societies [2]. In the 20th century, polymers, semiconductors and composites transformed technology, giving rise to today’s advanced materials. Among them, smart materials represent a rapidly growing domain. In the modern era, the discovery of polymers, composites, and semiconductors dramatically expanded technological possibilities, laying the foundation for contemporary industries ranging from electronics to aerospace. Today, innovation increasingly depends on a newer class of advance substances such as smart materials. Conventional materials typically provide strength, stability, or insulation but remain inactive once put into service. In contrast, smart materials are designed to interact with the surroundings, adjusting their properties in response to changes such as temperature, pressure, stress, light or electromagnetic fields [2, 3]. These materials are distinguished by their ability to sense environmental changes and, adapt accordingly. Unlike traditional materials that remain passive, smart materials can undergo functional modifications such as changes in shape, stiffness, color or electrical conductivity. This capacity positions them at the intersection of Mechanics, Electronics, and Biotechnology. With increasing demands for sustainability, automation, and high-performance systems, smart materials are becoming integral to fields like Biomedical Engineering, Aerospace, Civil infrastructure, and Energy harvesting.

The idea of Smart/Intelligent structures has been adopted from nature, where all the living organisms possess stimulus-response capabilities. However, the smart materials are much inferior to the living beings since their level of intelligence is much primitive. The development of smart materials began in 1880 when Pierre and Jacques Curie first identified the direct piezoelectric effect, which enables materials to produce electric charges in response to mechanical stress [4]. In 1930, the shape memory effect was observed in gold–cadmium alloys, followed shortly by the discovery of their pseudoelastic behaviour in 1932, demonstrating the ability to return to their original shape after significant deformation [5]. During 1962–1963, the development of Nickel-Titanium alloys (Nitinol) at the U.S. Naval Ordnance Laboratory introduced materials with practical shape-memory and superelastic properties. Research in the 1980s and 1990s extended into shape memory polymers, electroactive polymers, and other responsive materials, which eventually became commercially available [6]. By 1990, shape memory polymers offered lighter and more versatile alternatives to metal alloys. The 2000s brought advances in electro active polymers, artificial muscles, smart textiles and adaptive coatings. Also, in the recent era the emergence of self-healing polymers, wearable smart materials, enhanced the functionality and practical advances of smart materials. Currently, research is focused on materials that react to complex stimuli, adaptive surfaces and systems capable of real-time responses, with applications spanning robotics, medical devices, and sustainable technologies [7].

This review aims to provide a comprehensive overview of smart materials, including their basic principles, classification, key types, and practical applications, while also examining the associated challenges and potential future directions. It emphasizes the increasing importance of smart materials and their role in innovation across multiple fields.

2. Definition and characteristics of Smart Materials

Smart materials can be broadly described as engineered substance capable of responding autonomously to environmental stimuli in a controlled fashion. In practice, they often act as both sensors and actuators. The essential features of smart materials include responsiveness, rapid reaction, built-in-actuation, specificity and reversibility. Smart materials are commonly classified into two categories: Active or Passive. Active materials have the inherent ability to transduce energy, while passive do not. Passive materials do not change structure but may transmit signals for example: optical fiber, and active systems, which alter their properties or structure in response to external stimuli for example: piezoelectric ceramics, shape memory alloys etc. 

Three basic components of smart systems or materials: A smart system is generally built from three fundamental components as represented in Fig. 1. The first is the sensing element, which detects variations in the surrounding environment, such as force, light, temperature, or chemical composition. The second is the actuating element, which produces a physical or functional response once a change has been identified. The third is the control or processing unit, which interprets the information provided by the sensors and directs the actuators to respond in a precise manner. Collectively, these components such as sensor, actuator, and processor enable smart systems to operate adaptively and efficiently.

Fig. 1 Smart materials components.

Piezoelectric materials are materials that produce a voltage when stress is applied. Since this effect also applies in a reverse manner, a voltage across the sample will produce stress within sample. Suitably designed structures made from these materials can, therefore, be made that bend, expand or contract when a voltage is applied. Shape-memory alloys and shape-memory polymers are materials in which large deformation can be induced and recovered through temperature changes or stress changes (pseudoelasticity). The shape memory effect results due to respectively martensitic phase change and induced elasticity at higher temperatures. A common example is nitinol. Photovoltaic materials or optoelectronics convert light to electrical current. Electroactive polymers (EAPs) change their volume by voltage or electric fields. Magneto striction is a property of ferro magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The effect was first identified in 1842 by James Joule when observing a sample of iron. Ex: Fe, Co, Terfenol– D. Magnetostrictive materials exhibit a change in shape under the influence of magnetic field and also exhibit a change in their magnetization under the influence of mechanical stress. Magnetic shape memory alloys are materials that change their shape in response to a significant change in the magnetic field. Smart inorganic polymers show tunable and responsive properties. pH-sensitive polymers are materials that change in volume when the pH of the surrounding medium changes. Temperature-responsive polymers are materials which undergo changes upon temperature. Halochromic materials are commonly used materials that change their colour as a result of changing acidity which finds application in paints that can change colour to indicate corrosion in the metal underneath them. Chromogenic systems change colour in response to electrical, optical or thermal changes. These include electrochromic materials, which change their colour or opacity on the application of a voltage (e.g., liquid crystal displays), thermochromic materials change in colour depending on their temperature, and photochromic materials, which change colour in response to light—for example, light-sensitive sunglasses that darken when exposed to bright sunlight. Ferrofluids are magnetic 

fluids (affected by magnets and magnetic fields). Photomechanical materials change shape under exposure to light. Polycaprolactone (polymorph) can be molded by immersion in hot water. Self-healing materials have the intrinsic ability to repair damage due to normal usage, thus expanding the material's lifetime. Dielectric elastomers are smart material systems which produce large strains (up to 500%) under the influence of an external electric field. Magnetocaloric materials are compounds that undergo a reversible change in temperature upon exposure to a changing magnetic field. Thermoelectric materials are used to build devices that convert temperature differences into electricity and vice versa. Chemoresponsive materials change size or volume under the influence of external chemical or biological compound. The potential uses of Graphene are unlimited: Batteries with more autonomy, Cheaper photovoltaic solar cells faster computers, flexible electronic devices, more resistant buildings, bionic limbs, etc. Magneto striction is a property of ferro magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The effect was first identified in 1842 by James Joule when observing a sample of iron. Ex: Fe, Co, Terfenol– D.

3. Classification of Smart Materials

Smart materials classified based on the external stimuli are discussed in the following sections:

3.1 Shape Memory Alloys (SMAs):

This class of materials can be deformed and returned to their original shape when exposed to a specific stimulus, such as heat. There are 2 phases of SMAs , the transition between these phases is the basic principle for smartness of these materials. This property of regaining the original shape, is known as the shape memory effect, is due to a temperature-dependent crystalline structure change, also called a martensitic transformation.

Fig. 2: The phase transformation in SMA               

Fig.3: Stress-Strain curves for Ti-50.6Ni alloy

The phases involved in an SMA are Austenite (stable at higher temperature) which is a parent phase and Martensite (stable at lower temperature) with tetragonal or monocline structure. An example of tensile tests for the Ti-50.6Ni alloy at different and constant temperatures are considered to explain the SMA behaviour. At lower temperatures, it is evident the shape memory effect: after an initial linear stage a plateau of stress is evident, followed by residual strain while unloading. After that, the shape recovery on heating starts at As (Austenite start temperature) and finishes at Af (Austenite finish temperature). At temperatures higher than Af, austenite is the only stable phase. Tensile behaviour exhibits a loading path with a plateau at higher stress than that in the martensitic phase. During unloading, the strain is fully recovered and the classic flag-type diagram is evidenced, without the necessity of further heating. The higher the temperature, the higher the level of stress for the plateau [8].

SMA in Robotics: A clever use of muscle wire and a micro-controller circuit is a robotic hand. A robotic hand has - stretched muscle wires attached to the base of each finger. When current is applied to the muscle wire it contracts to its natural length by pulling on the ordinary wire. The micro-controller is programmed to give five of the outputs with switch on and off options. This makes the fingers of the hand move.

3.2 Piezoelectric Materials

These materials produce an electrical charge when subjected to mechanical stress, or conversely, deform when an electric field is applied.

Fig.4: Response of Piezoelectric material

There are two types of piezoelectric effects, namely, the direct effect and inverse effect. In the direct piezoelectric effect, a material is polarized and produces voltage under an applied tensile or compressive stress. In the inverse effect, the application of electric potential induces mechanical displacement in the material. Further, there is distribution of ions in the crystalline structure of the material and the presence or absence of symmetry. There are 32 crystallographic classes, out of which 21 are non-centrosymmetric (lacking center of symmetry) and 20 of them exhibit direct piezoelectricity; the 21st being the cubic class.

Square-shaped structures: when such materials are subjected to compressive stress, the equivalent center of charge is still at the same point, hence there is no change in polarization.

2D hexagonal structures: when stress is applied, a change is triggered in the center of charge of the cations and anions that induces a change in polarization.

Fig.5: The response of square and hexagonal shaped structures.

In the materials where the molecular dipoles are randomly oriented within their crystal structure, an important operation called “poling” is done to obtain an effective piezoelectric response. Poling involves re-orientation of the molecular dipoles in a material by exerting a high electric field at high temperature, followed by subsequent cooling keeping the same electric field to sustain the orientation state. The two well-known methods of poling are:

a) Electrode poling: A high voltage is applied to the piezoelectric material in electrode poling by pressing conductive electrodes on two sides of the material.

b) Corona poling:  An electric field in the range of 5–100 MV m–1 is usually applied. In the corona poling process, a needle with high conductivity is maintained at extremely high voltage (8–20 kV) and is located on a grid at a lower voltage (0.2–3 kV). The piezoelectric material is situated below the grid and is kept in an atmosphere of dry air or inert gas. Due to ionization around the corona tip, the gas molecules get accelerated toward the piezoelectric material surface. The bottom side of the material is covered with an electrode, which is in contact with a hot substrate to achieve better control over poling. [9]

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Fig.6: (a) Electrode poling (b) corona poling.

Piezoelectric materials find applications as, gas lighters or portable sparkers. Piezo elements are used in music for acoustic instruments. They are inserted in stringed instruments such as guitar, violin or Mandoline. The dynamic deformation/vibration of the cords is converted into a small alternating voltage. Due to their good biocompatibility, high energy conversion efficiency, high biological activity, and stability, they show a bright prospect in future medical applications. Under the action of external force, piezoelectric materials are able to release electrons and holes, intervene in the living body to promote the occurrence of certain reactions, or provide a continuous power source for human devices to achieve medical purposes, such as tissue repair, drug release, cancer treatment, and biosensing. Piezoelectric biomedicine technology provides a more efficient and convenient means, improving the quality of life of patients.

3.3 Magneto-rheological Fluids (MRF)

They are field-responsive intelligent materials that refer to the composite fluid/polymer prepared by homogeneously dispersing the magnetizable medium into a non-magnetic matrix [10]. These fluids or materials change their stiffness or viscosity when exposed to a magnetic field [11]. This smart fluid consists of 20-40 % Iron particles, suspended in mineral oil, synthetic oil, water or glycol. These are viscoelastic in nature, light in weight with controllable modulus and excellent sound-absorption. At present, research on such materials is mostly concerned with enhancing the performance of base carrier liquid or achieve controllability and stability.

Fig. 7: Response of MR Fluid.

Constituents of MR Fluids:

The composition of MR materials includes magnetizable medium, base carrier liquid, and additives. The magnetizable medium is usually selected from carbonyl iron powders (CIPs) with high magnetic permeability and saturation magnetization strength. The MR effect causes the magnetizable medium to follow a directional arrangement along the magnetic field to form a magnetic chain, manifested by a change in the physical state and apparent viscosity. The magnetic chains are the main part in realizing the regulation of the structural system, and the magnetic field contributes significantly to the modulation of magneto-static stress.

MR fluid: The base carrier liquid for MR fluid is typically silicone oil, mineral oil, or water, which makes them exhibit great fluidity in the absence of a magnetic field. MR fluid responds immediately and reversibly to a magnetic field by converting from a free-flowing Newtonian liquid to a non-Newtonian liquid with solid or solid-like properties. MR grease: The introduction of a magnetizable medium slightly reduces the initial viscosity of the grease, but most MR grease is in a solid-like state at room temperature. Due to the unique composition and microstructure of the grease, MR grease does not require additional additives and exhibits superior MR effect. MR elastomer: MR elastomer is a kind of rubber-like material without sedimentation of the magnetizable medium, which means that the main role of additives is to enhance the MR effect. In MR elastomers, it has been confirmed that the small-sized particles as fillers can enhance the performance and improve the effect of polymer elastomer, and the interfacial interaction between magnetic particles and polymer elastomer. MR gel:  It is a kind of composite gel with a magnetizable medium suspended in a polymer gel matrix and can be classified as liquid-like (the solvent content is less than 10 wt%) or solid-like (the solvent content exceeds 25 wt%). MR gel is an intermediate between MR fluid and MR elastomer.

3.4 Electro-rheological Fluids (ERF)

Electro-rheological fluids are dispersions of fine hygroscopic particles in a hydrophobic non-conducting medium. The rheological properties, flow and deformation behavior of these fluids, in response to a stress, strongly depend upon the electric field strength imposed.

Fig.8: Response of ER Fluids.

The materials are typically fluids in the absence of an electric field but under constant shear stress at high enough fields, the materials can solidify into viscoelastic solids. In their solid state, various properties of the solid such as shear and tensile strengths and damping capacity, internal friction, and the ability to adsorb energy under impact are strongly dependent on the electric field. When subjected to the external electric field, the electro rheological fluids exhibit a large reversible change in the colloidal suspension rheological properties. Also, the response time of the ER fluid is very quick so that the band width is thick [12]. Materials suited for the dispersed phase include corn starch, various clays, silica gel, talcum powder, and various polymers. The fluid phase consists of a very wide range of liquids or grease which have the common properties of high electrical resistivity and hydrophobicity. This requirement severely limited the potential use of these materials. Various other types of additives, called activators, have been reported and are commonly incorporated into the mixtures, including various surfactants, to enhance the effect and to increase the stability of the dispersions.

The ER fluids operate as follows:

a) Valve Mode where the electrodes are mounted and fixed and vibrational control is achieved by controlling the motion of the flow,

b) Shear Mode where the vibrational control is achieved by varying the shear force keeping one electrode fixed while the other rotates freely,

c)Squeeze Mode where the space between the electrodes is changed which presses the ER fluid.

3.5 Emerging smart materials

Aerogels

Aerogels are made by substituting air for the solvent in a gel networks meshes, during the aeration procedure. Aerogels have a high porosity and are extremely light in weight. Aerogels are composed of 95 % air or gas by volume. Aerogels are emerging as smart materials with their tunable properties and capacity to respond to stimuli, opening doors to diverse applications like drug delivery systems, wearable sensors, and shape-memory devices. Their unique nano-scale structure, characterized by high porosity and surface area, allows for functionalization that creates responsive behaviors, making them valuable for energy storage, biomedical engineering, and advanced electronics. Porous structure of Aerogels, causing low weight and an elevated specific surface area, is a crucial and distinctive quality for food packaging. Due to its unique one-dimensional structure and diverse chemical composition, nano-cellulose offers substantial advantages in terms of efficiency for energy storage material. Graphene-based aerogels (GBAs) are commonly employed in gas sensors, which are essential for protecting the atmospheric environment from the effects of pollution [13].

Titanium Foams

The adaptable porosity and mechanical properties of Titanium foam is recognized as a smart material and is further engineered for specific functions like orthopedic implants, energy absorption in safety components, or highly efficient fuel cell electrodes. The blend of various properties like high strength, low stiffness (when foamed), biocompatibility, corrosion resistance, and large surface area makes it suitable candidate to work within specific environments, making it a versatile material for advanced applications in aerospace, biomedicine, and filtration.

Metamaterials have opened unprecedented avenues for controlling wave propagation, mechanical response, and dynamic behaviour. Some of these metamaterials are capable of reconfigurability, in the sense that they can be modulated in response to external stimuli. Whether the application is passive or reconfigurable, the vast majority of reported research on metamaterials has been at the nano- and micro-scales, attributable to fabrication costs and capabilities, and to the desire to study and leverage enhanced properties at smaller scales.

PAM-actuated Robots With the increasing integration of robots into various fields such as industry, agriculture, specialized industries, and home services, research on human–robot coexistence and collaboration have become more urgent, which is crucial for enhancing the operational capabilities of robots and ensuring personal safety. As a kind of soft actuator with smart structures, pneumatic artificial muscles (PAMs), known for their high power-to-weight ratio, ease of installation, clean energy usage, and resistance to electromagnetic interference, can form a series of PAM-actuated robots combined with rigid robotic architecture, endowing them with the ability to coordinate rigidity and flexibility. Recent advancements in the modelling and control methods of PAM-actuated robots have significantly improved their practical application value. 

3.6. Magnetic materials: Ferrites as smart materials in Energy storage and Photocatalytic applications

The rapid depletion of fossil fuels and the rising global energy demand have intensified the pursuit of sustainable and efficient energy storage technologies. Among various alternatives, supercapacitors have emerged as promising devices due to their high power density and energy density, which fills the gap between traditional capacitors and batteries. Their performance characteristics are often represented using Ragone plot, which compares energy and power density across different energy storage systems.

Supercapacitors, also known as ultra-capacitors, are widely used in electric vehicles, memory backup systems, industrial power management, and renewable energy integration [14, 15, 16, 17]. Based on their charge storage mechanisms, they are classified as Electric Double-Layer Capacitors (EDLCs) and Pseudo Capacitors. EDLCs store energy through ion adsorption at the electrode-electrolyte interface, while pseudo capacitors rely on fast and reversible redox reactions [18, 19, 20]. A typical supercapacitor consists of electrodes, a separator, and current collector, where the electrode material largely determines its performance. Metal oxides such as MnO2, NiO, RuO2, VO2 and CoO2 have demonstrated high capacitance, although materials like RuO2 are limited by high cost and toxicity [21, 22, 23, 24]. Consequently, there is growing interest in developing cost-effective, environmentally benign, and high-performance electrode materials [25,26]. In this context, ferrites have attracted significant attention as smart multifunctional materials due to their unique electrical, magnetic and catalytic properties. They serve as promising electrode materials for supercapacitors and are also utilized in dye degradation, photocatalytic applications. Their tuneable electronic structure, chemical stability, and ability to form flexible and miniaturized nanocomposites make them ideal candidates for next-generation energy and environmental technologies. The materials prepared with ferrites and its composites are incorporating with metal complexes have been advanced for development of various technological applications [27-30]. Copper ferrite is recognized as a highly versatile member of the ferrite family, showing great potential in waste water purification, dye degradation, photo catalysis and electrochemical energy applications. This versatility arises from strong structural integrity and chemical stability and favourable electrochemical as well as catalytic characteristics.

In my research group, focus is on developing smart ferrite materials for supercapacitor energy storage applications. Efforts are also being made to understand the photocatalytic activity of nano ferrites for water splitting [31-34].

4. Applications of Smart materials

Smart switches & actuators (NiTi-long life), Safety device, fuse, alarms (CuZnAl-reliability), Artificial limbs, blood vessels & muscles (SM Polyurethane -bio compatibility), Adhesive tapes/bands (time bound adhesive property/painless removal/healing property), Food packaging industry-wrappers (adoptability), Smart spoons (temperature sensitive polymers), Smart nose & tongue (recognition characteristics), Smart clothes (Adaptive to temperature changes). Biomedical field: Smart sutures are thermoplastic polymers having shape memory and biodegradable properties, which optimizes its shape accordingly. Smart antimicrobial peptides are used for targeted microbe killing. Aerospace: Aerospace engineering is being modernized with the use of smart materials, which improve performance, efficiency, and safety. Embedded piezoelectric sensors are used to detect stress and damage in real-time, enhancing safety by predicting potential failures. SMAs and electrostrictive materials are employed to improve performance, enhance fuel efficiency, and reduce drag during different flight phases. Magneto-rheological dampers are used to control vibrations, enhance passenger comfort and maintain structural integrity. Construction Engineering: Smart bricks are used to detect in-situ conditions like force, stress, temperature, tilt, moisture. Bright bricks may contain photovoltaic materials applied to their external surface to gather solar energy. 3D-printed intelligent bricks reduce material waste by 30–50% compared to traditional bricklaying methods and hence enhances precision and efficiency. Artificial Intelligence: Smart materials provide excellent sensitivity and adaptability, enhancing the detailing of AI systems. Materials like Graphene are employed in creating advanced hardware that can mimic brain-like chips. Sustainable energy solutions: Smart materials can be used in the manufacture of a wide variety of products, from batteries to solar panels. Piezoelectric materials can convert mechanical vibrations into electrical energy, while thermoelectric materials can generate power from temperature gradients, hence, can be utilized in energy production.

5. Challenges and Research Directions

Smart materials with smartness are successful at laboratory scale, but face challenges and complexities by the existing industries and large-scale implementation. The integration of new technology equipment with the existing ones and the compatibility of existing devices to the new devices causes various problems in the implementation of smart material technologies. Research is being carried out to use the ER fluid with surfactants and additives to broaden their applicability and make them user-friendly. Rubber-based MRFs results in relatively low MR effect. The use of composites as a matrix instead of rubber in order to get an ultra-soft MR is need. Additionally, research efforts must focus on expanding the range of stimuli-responsive properties and exploring new combinations of materials.

6. Conclusions

The dumb, conventional materials of the past are now becoming smart. Smart materials are the next generation of materials that have a potential to impact different fields including science, engineering, medicine, and automotive industry. They will have significant impact on civilization. The technology of smart materials is an interdisciplinary, emerging field. A number of technical, peer-reviewed journals are dedicated to publishing information on smart materials.  Three of such are: Smart Materials and Structures, Smart Material Research, and Journal of Intelligent Material Systems and Structures. Smart materials have replaced various traditional materials using their smartness in various applications. Their unique properties of responding to stimuli, adaptability and diversity has revolutionized the entire material science. With smart materials slowly replacing the conventional materials, the industries will soon transform potentially into smart industries. With further research and development, smart materials will become an integral part of our life with enhanced efficiency and sustainability.

Acknowledgment: The Author is grateful to Prof. B. G. Mulimani, former Vice-Chancellor, Gulbarga University, Kalaburagi and B.L.D.E. University, Vijayapura and Mentor, for his encouragement to write this review article. The work and contributions of various authors cited in the reference is greatly acknowledged. The assistance of research scholars, Dr. Prabhakar, Mr. Ramling and Mrs. Anum in the preparation of this article is also acknowledged.