Concluding Remarks

A comprehensive review has been given of the advances in the area of nanocapsules. The outline of the theoretical background, including the energy levels, the quantum effects, the magic effects, the surfaces/interfaces, and the metastable phases in nanocapsules have been represented. The most applicable nanoencapsulation procedures, including physical, chemical, physicochemical, and mechanical techniques, have been introduced. The physical properties, including optical, electronic, magnetic, and dynamic properties, and the biomedical properties, including artificial cell and red blood cell substitutes, drug delivery, and controlled release, have been reviewed for nanocapsules.

Nanocapsules are the matter in intermediate states between bulk and atomic materials which offer a model system for investigation of the dimensionally confined systems in basic research areas. Nanocapsules have physical properties that differ distinctly from those of bulk materials. As a small limited system, three of the physical dimensions of nanocapsules could approach a length-scale characteristic of a physical phenomenon (with different phenomena being characterized by different length scales), and similarly in the time domain. The quantum size effects could contribute evidently to the properties, which originate mainly from the discreteness of the energy levels, and the breakdown of the cyclic condition and the translation symmetry in the limited systems. The effects of the quantum interference, the magic number, the surfaces/interfaces, the core/shell structure, the metastable phases, etc. cannot be neglected in nanocapsules. Depending on the size, the shape, the core/shell structure, etc., the properties of nanocapsules could be very different individually, ranging from metals, insulators, and semiconductors, to optical absorption, magnetic hardening, etc.

Sufficiently small materials could aggregate sensitively to various shapes and nanostructures, which could change the properties of the materials. Spontaneous shape selection occurs in the mesoscopic systems, originating from the ability of finite systems to adjust their shape and structure in order to minimize the free energy of the systems. The small size is beautiful also due to the fact that mesoscopic systems could be designed and constructed artificially which would be so different with those formed naturally. It is clear that the properties of the nanocapsules depend sensitively on the procedures for sample preparation.

The different nanocapsules can be prepared by various methods with different mechanisms, which could control the size, the size distribution, the morphology, phases, and structure of the nanocapsules, the thickness of the shell, the size of the grains, etc. Some methods may be suitable for synthesis of only some typical nanocapsules. The most applicable nanoencapsulation procedures include: physical methods such as heat evaporation, arc discharge, laser/electron beam heating, sputtering, etc.; chemical methods such as chemical vapor deposition, solid-state reactions, sol-gel, precipitation, and hydrothermal, solvent, latex, oxidation, polymerization, processes etc.; physicochemical methods such as coacervation-phase seperation, etc.; mechanical methods such as air suspension, pan coating, spray-drying and congealing, and multiorifice centrifugation.

Nanocapsules are designed to have a therapeutic action to release drugs in the right place or to have functionality to attack and reduce cancerous cells. Although many applications have not been investigated yet, current developments are becoming increasingly more useful in the field of medicine. Nanocapsules have been intensively studied not only because of the possibilities for controlled release, but also because of increased drug efficacy and reduced toxicity after parenteral administration. The nanocapsules have been formulated into a variety of useful dosage forms including oral liquid suspensions, lotions, creams, ointments, powders, capsules, tablets, and injections. Nanoencapsulation has been applied to solve problems in the development of pharmaceutical dosage forms as well as in cosmetics for several purposes including conversion of liquids to solids, separation of incompatible components in a dosage form, taste-masking, reduction of gastrointestinal irritation, protection of the core materials against atmospheric deterioration, enhancement of stability, and controlled release of active ingredients. Their pharmacological effect has been improved or their secondary effects reduced compared with free drugs after administration by oral, parenteral, or ocular routes. The nanocapsules have been found also to be suitable as biodegradable drug carriers for the administration and controlled release of drugs, like insulin, indomethacin, calcitonin, doxorubicine, gangliosides, oligonucleotides, etc.

Another major activity in the application of the nanocapsules has been focused on the topics of artificial cell and red blood cell substitutes. This is beneficial to the formulation of polymeric artificial nanocapsules, which can be made in a reproducible manner having a specific size and shape and in reasonable quantities. Artificial cells for pharmaceutical and therapeutic applications has been expanded up to the higher range of macrocapsules and down to the nanometer range of nanocapsules and even to the macromolec-ular range of cross-linked hemoglobin as blood substitutes. Artificial cells have been prepared by bioencapsulation in the laboratory for medical and biotechnological applications. The three generations of blood substitutes have been developed based on an original idea of a complete artificial red blood cell.

Basic research of the structure and properties of the nanocapsules and related issues (like nanotubes [1036-1046]) underlies future technologies, from nanoscale machines, nanotribological systems, cellular injections, and nanocatal-ysis, to miniaturization of electronic circuitry and novel information storage and retrieval systems. Furthermore, the nanoencapsulation process has been successfully applied to many industries as follows: as controlled-release systems of drugs and bioactive agents in pharmaceuticals; as creams, lotions, and fragrance in cosmetics; microgranules with protective colloids, fertilizers, insecticides, herbicides, microporous capsule walls for controlled release in agriculture; envelope manufacture using microencapsulated glues in adhesives; pigment presscakes, metal phosphate complex microcapsules, encapsulated aluminum flake pigments in pigments; encapsulation of heterogeneous catalysts, spherical alumina-containing particles, dual-walled hard microcapsules, hollow-ceramic-coated spheroid catalyst carriers in catalysts and catalyst carriers; photographic printing, light reflective dye-containing capsules for photographic film; food; paper; plastic; and others, such as microcapsulated liquid lubricants, chemiluminescent warning capsules, encapsulated thermonuclear fuel particles, encapsulated flame retardants, encapsulated liquid explosive compositions, and microcapsulated vulcanizing agents. Due to the length limit of this chapter, we could not have collected all the research work and applications in the fields above. Nevertheless, we are optimistic that nanocapsules as well as nanoencapsulations will be one of the most active fields in nanoscience and nanotechnology. Many novel nanocapsules will be synthesized by many novel procedures which will possess many novel properties that are helpful for a better understanding of the physical phenomena and natural rules in the mesosystems and are applicable in many novel fields and industries.

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