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Emerging Nanotechnologies in Dentistry
Materials, Processes, and Applications
By Karthikeyan Subramani, Waqar Ahmed Elsevier
Copyright © 2012 Elsevier Inc.
All rights reserved.
ISBN: 978-1-4557-7857-7
CHAPTER 1
Nanotechnology and the Future of Dentistry
K. Subramani, and W. Ahmed
CONTENTS
1.1 Introduction 1
1.2 Nanotechnology Approaches 2
1.3 Nanotechnology to Nanomanufacturing 3
1.3.1 Top-Down Approach 4
1.3.2 Bottom-Up Approach 6
1.4 Nanodentistry 10
1.5 Future Directions and Conclusions 14
References 14
1.1 INTRODUCTION
Although nanotechnology has been around since the beginning of time, the discovery of nanotechnology has been widely attributed to the American Physicist and Nobel Laureate Dr. Richard Phillips Feynman who presented a paper called
There's Plenty of Room at the Bottom
in December 29, 1959 at the annual meeting of the American Physical Society meeting at California Institute of Technology. Feynman talked about the storage of information on a very small scale, writing and reading in atoms, about miniaturization of the computer, building tiny machines, tiny factories, and electronic circuits with atoms. He stated that "In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction." However, he did not specifically use the term "nanotechnology." The first use of the word "nanotechnology" has been attributed to Taniguchi in a paper published in 1974 "On the Basic Concept of NanoTechnology." Dr. K. Eric Drexler, an MIT graduate, later took Feynman's concept of a billion tiny factories and added the idea that they could make more copies of themselves, via computer control instead of control by a human operator, in his 1986 book Engines of Creation: The Coming Era of Nanotechnology, to popularize the potential of nanotechnology.
Since then several definitions of nanotechnology have evolved. For example, the dictionary definition states that nanotechnology is "the art of manipulating materials on an atomic or molecular scale especially to build microscopic devices." Other definitions include the US government which state that "Nanotechnology is research and technology development at the atomic, molecular, or macromolecular level in the length scale of approximately 1–100 nm range, to provide a fundamental understanding of phenomena and materials at the nanoscale and to create and use structures, devices, and systems that have novel properties and functions because of their small and/or intermediate size." The Japanese have come up with a more focused and succinct definition. "True Nano": as nanotechnology which is expected to cause scientific or technological quantum jumps, or to provide great industrial applications by using phenomena and characteristics peculiar in nano-level.
Regardless of the definition that is used it is evident that the properties of matter are controlled at a scale between 1 and 100 nm. For example, chemical properties take advantage of large surface to volume ratio for catalysis, interfacial and surface chemistry is important in many applications. Mechanical properties involve improved strength hardness in light-weight nanocomposites and nanomaterials, altered bending, compression properties, nanomechanics of molecular structures. Optical properties involve absorption and fluorescence of nanocrystals, single photon phenomena, and photonic band-gap engineering. Fluidic properties give rise to enhanced flow using nanoparticles and nanoscale adsorbed films are also important. Thermal properties give increased thermoelectric performance of nanoscale materials, interfacial thermal resistance important.
1.2 NANOTECHNOLOGY APPROACHES
Numerous approaches have been utilized successfully in nanotechnology and as the technology develops further approaches may emerge. The approaches employed thus far have generally been dictated by the technology available and the background experience of the researchers involved. Nanotechnology is a truly multidisciplinary field involving chemistry, physics, biology, engineering, electronics, social sciences, etc., which need to be integrated together in order to generate the next level of development in nanotechnology (Figure 1.1). Fuel cells, mechanically stronger materials, nanobiological devices, molecular electronics, quantum devices, carbon nanotubes, etc. have been made using nanotechnology. Even social scientists are debating ethical use of nanotechnology.
The two main approaches in order to explain nanotechnology to the general public have been oversimplified and have become known as the "top-down" approach. This involves fabrication of device structures via monolithic processing on the nanoscale. This approach has been used with spectacular success in the semiconductor devices used in consumer electronics. The "bottom-up" approach involves the fabrication of device structures via systematic assembly of atoms, molecules, or other basic units of matter. This is the approach nature uses to repair cells, tissues, and organ systems in living things and indeed for life processes such as protein synthesis. Tools are evolving which will give scientists more control over the synthesis and characterization of novel nanostructures yielding a range of new products in the near future.
1.3 NANOTECHNOLOGY TO NANOMANUFACTURING
A huge amount of research is being carried out internationally and governments and research organizations are spending large amounts of money and human resources into nanotechnology. This has generated interesting scientific output and potential commercial applications, some of which have been translated into products produced on a large scale. However, in order to realize commercial benefits far more from lab scale applications need to be commercialized and for that to happen nanotechnology needs to enter the realm of nanomanufacturing. This involves using the technologies available to produce products on a large scale which is economically viable. Regardless of whether a top-down or bottom-up approach is used a nanomanufacturing/nanofabrication technology should:
• be capable of producing components with nanometer precision,
• be able to create systems from these components,
• be able to produce many systems simultaneously,
• be able to structure in three dimensions,
• be cost-effective.
1.3.1 Top-Down Approach
The most successful industry utilizing the top-down approach is the electronics industry (Figure 1.2).
This industry is utilizing techniques involving a range of technologies such as chemical vapor deposition (CVD), physical vapor deposition (PVD), lithography (photolithography, electron beam, and X-ray lithography), wet and plasma etching, and so on to generate functional structures at the microand nanoscale (Figure 1.3). Evolution and development of these technologies have allowed emergence of numerous electronic products and devices that have enhanced the quality of life throughout the world. The feature sizes have shrunk continuously from about 75 ?m to below 100 nm. This has been achieved by improvements in deposition technology and more importantly due to the development of lithographic techniques and equipment such as X-ray lithography and electron beam lithography.
Techniques such as electron beam lithography, X-ray lithography, and ion beam lithography have advantages in terms of resolution achieved; however, there are disadvantages associated with cost, "optics," and detrimental effects on the substrate. These methods are currently under investigation to improve upon current lithographic process used in the integrated circuits (IC) industry. With continuous developments in these technologies, it is highly likely that the transition from microtechnology to nanotechnology will generate a whole new generation of exciting products and features.
A demonstration of how several techniques can be combined together to form a "nano" wine glass (Figure 1.4). In this example, a focused ion beam and CVD have been employed to produce this striking nanostructure.
The top-down approach is being used to coat various coatings to give improved functionality. For example, vascular stents are being coated using CVD technology with ultrathin diamond-like carbon coatings in order to improve biocompatibility and blood flow (Figure 1.5). Graded a-Si × Cy:H interfacial layers result in greatly reduced cracking, enhanced adhesion.
1.3.2 Bottom-Up Approach
The bottom-up approach involves making nanostructures and devices by arranging atom by atom. The scanning tunnelling microscope (STM) has been used to build nanosized atomic features such as the letters IBM written using xenon atoms on nickel (Figure 1.6). While this is beautiful and exciting it remains that the experiment was carried out under carefully controlled conditions, i.e., liquid helium cooling, high vacuum, and it took something like 24 h to get the letters right. Also the atoms are not bonded to the surface just adsorbed and a small change in temperature or pressure will dislodge them. Since this demonstration, significant advances have been made in nanomanufacturing.
The discovery of the STM's ability to image variations in the density distribution of surface state electrons created in the artists a compulsion to have complete control of not only the atomic landscape, but also the electronic landscape. Here they have positioned 48 iron atoms into a circular ring in order to "corral" some surface state electrons and force them into "quantum" states of the circular structure (Figure 1.7). The ripples in the ring of atoms are the density distribution of a particular set of quantum states of the corral. The artists were delighted to discover that they could predict what goes on in the corral by solving the classic eigenvalue problem in quantum mechanics—a particle in a hard-wall box.
Probably the most publicized material in recent years has been carbon nanotubes. Carbon nanotubes, long, thin cylinders of carbon, were discovered in 1991 by Iijima. These are large macromolecules that are unique for their size, shape, and remarkable physical properties. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. These intriguing structures have sparked much excitement in the recent years and a large amount of research has been dedicated to their understanding. Currently, the physical properties are still being discovered and disputed. What makes it so difficult is that nanotubes have a very broad range of electronic, thermal, and structural properties that change depending on the different kinds of nanotube (defined by its diameter, length, and chirality, or twist). To make things more interesting, besides having a single cylindrical wall (SWNTs), nanotubes can have multiple wall (MWNTs) cylinders inside the other cylinders.
Bower et al. have grown vertically aligned carbon nanotubes using microwave plasma-enhanced CVD system using a thin film cobalt catalyst at 825°C (Figure 1.8). The chamber pressure used was 20 Torr. The plasma was generated using hydrogen which was replaced completely with ammonia and acetylene at a total flow rate of 200 sccm.
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Excerpted from Emerging Nanotechnologies in Dentistry by Karthikeyan Subramani, Waqar Ahmed. Copyright © 2012 Elsevier Inc.. Excerpted by permission of Elsevier.
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