In the case of the incorporation of nanomaterials into products, several generations of changes to manufacturing can be anticipated . Current products in the marketplace today typically fall into the "1st generation," where relatively minor modifications to existing processing equipment were needed to incorporate nanomaterials into the product For example, surface coatings of nanofibers and nanowhiskers have been used for improved filtration and for the "nano-pants" fabric made by Nano-Tex  . More than 20 years ago, Toyota incorporated clay nanoparticles into polymer resins to create automotive body panels with improved strength, toughness, and dimensional stability  . These types of nanocomposite products are still fabricated using conventional injection molding, extrusion, and cast film processes, but additional compounding steps or other modifications to the processes were made to create a well-dispersed nanofiller  . As greater understanding is achieved, more advanced processes and products are developed
The following generational designations have been described on several occasions by M .C . Roco, who is recognized as one of the key architects of the National Nanotechnology Initiative (NNI) A more detailed presentation can be found in a chapter by Roco reviewing the history of the NNI, its evolution over the past decade, and the future prospects for this technology and its impact on society  Additional information emphasizing aspects related to manufacturing at the nanoscale appears in a report issued by the National Nanotechnology Coordination Office  .
• The "1st generation" products (2000+): represented primarily by passive nanostructures. The majority of products that are already commercialized fall into this category, where the nanoscale element (e g , nanoparticle, nanoclay platelet, nanotube) is incorporated into a matrix material for coatings, films, and composites, or is part of a bulk nanostructured material. The processes for fabricating the target nanomaterials discussed in this book, as well as the products incorporating these nanoparticles represent the first generation of nanoproducts
• The "2nd generation" products (2005+): represented by active nanostruc-tures. In these structures, the nanoscale element is the functional structure, as in the case of nanospheres and nanostructured materials for drug delivery The materials are functional in that they respond to some external stimuli such as pH or temperature to release the stored drug at a controlled rate Other examples include sensors and actuators, transistors, and other electronics, where individual nanowires serve to provide the switching or amplifying mechanism
• The "3rd generation" products (2010+): represented by three-dimensional nanosystems and multi-scale architectures, expanding beyond the two-dimensional layer-by-layer approach currently used in microelectronics. These systems will be manufactured using various directed self-assembly methods such as bio-assembly (e g , using DNA and viruses as templates), electrical and chemical template-guided assembly
• The "4th generation" products (2015+): represented by truly heterogeneous molecular nanosystems. In these products, multi-functionality and control of function will be achieved at the molecular level
Common to all four generations of product development are three stages where exposure to nanomaterials is the most significant In general, nanomaterials such as carbon nanotubes and silver nanoparticles can be relatively expensive, so companies will want to reduce waste as much as possible Nevertheless, exposure and entry into the waste stream can occur: (1) during fabrication of the nanomaterial; (2) during storage and handling of the nanomaterial, including during incorporation of the nanomaterial into another material, structure, or device; and (3) during material removal or failure upon further processing or disposal of the product Once the nanomaterial is incorporated into a bulk material (e g , a carbon nanotube bonded within a polymer matrix), the concern is the same as that for the bulk material and is not related to the nanoscale dimensions or properties . Prior to embedment or in the case of release at end of life disposal, the unique properties of nanomaterials do have a very different effect The most obvious case is that of worker exposure With particles roughly 1/1000th the diameter of chopped glass fibers, the concern is that filtration and ventilation regulations are not effective The behavior also is not monotonic with size Some properties may actually make it easier to filter or collect any stray nanomaterials . For example, the Brownian motion of nanoparticles results in a more tortuous travel path that may make capture easier. Similarly, the high reactivity of the surface-dominated particles can lead to a greater ease of collection; for example, nanoparticles tend to agglomerate into much larger clusters, making them easier to detect and filter
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