Write a review Rate this item: Preview this item Preview this item. Radiation chemistry of biopolymers Author: English View all editions and formats Rating: Subjects Biopolymers -- Effect of radiation on. More like this Similar Items. Allow this favorite library to be seen by others Keep this favorite library private. Find a copy in the library Finding libraries that hold this item Electronic books Electronic book Additional Physical Format: Document, Internet resource Document Type: Reviews User-contributed reviews Add a review and share your thoughts with other readers.
Add a review and share your thoughts with other readers. Similar Items Related Subjects: Linked Data More info about Linked Data. Primary radiation-chemical processes -- ch. Detection methods for radiolytic products -- ch. Radiation chemistry of water and water solutions -- ch.
Basic regularities of solution radiolysis -- ch. The regularities of radiolysis of aqueous biopolymers and their components -- ch. The problems of radiation chemistry of protein molecules -- ch. Even so, it generally takes about 4 hours to kick in. The adhesive layer is protected before use by a peel-off backing of siliconized polyester. The transdermal patch technology transformed an otherwise unmanageable drug into the most effective motion sickness treatment available, and one good for three days.
Designing and testing the patch required attention to complex issues of drug dosage and behavior as well as the challenge of fabricating a pharmaceutical product in a radically different and untried form. States, and a larger number are in development. Compilation of information from Physicians' Desk Reference The volume of biopolymers in the world far exceeds that of synthetic macromolecules. DNA and RNA are informational polymers encoding biological information , while globular proteins, some RNAs, and carbohydrates serve chemical functions and structural purposes.
In contrast, most synthetic polymers, and fibrous proteins such as collagen which makes up tendon and bone and keratin which makes up hair, nails, and feathers , are structural rather than informational or chemically functional. Structural materials are useful because of their mechanical strength, rigidity, or molecular size, properties that depend on molecular weight, distribution, and monomer type.
In contrast, informational molecules derive their main properties not simply from their size, but from their ability to encode information and function. They are chains of specific sequences of different monomers. For DNA the monomers are the deoxyribonucleic acid bases; for RNA, the ribonucleic acid bases; for proteins, the amino acids; and for carbohydrates or polysaccharides, the sugars. The paradigm in biopolymers is that the sequence of monomers along the chain encodes the information that controls the structure or conformation of the molecule, and the structure encodes the function.
An informational polymer is like a necklace, and the monomers are like the beads.
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Information is encoded in the sequence of bead colors, which in turn controls the sequence of amino acids in proteins. There are 20 different types of amino acid monomers; in the necklace analogy, there are 20 different colors of beads. A globular protein folds into one specific compact structure, depending on the amino acid sequence. This balled-up shape, or structure, is what determines how the protein functions. The folding of the linear structure produces a three-dimensional shape that controls the function of the protein through shape selection.
Except in special cases, synthetic polymer science does not yet have the precision to create specific monomer sequences: But the ability to synthesize specific monomer sequences by a linear process would have extraordinary potential. For example, it is the ability to create specific monomer sequences that distinguishes biological life forms, and the corresponding complex hierarchies of structure and function, from simpler polymeric materials.
Hence one of the most exciting vistas in polymer science is the prospect of creating informational polymers through control of specific monomer sequences. The present state of. This method is limited to preparation of short chains less than 50 amino acid groups and small quantities.
Techniques that allow similar controlled synthesis on a much larger scale would be revolutionary. The study of informational polymers aims to determine the specific shapes of biological polymers at atomic and nanometer resolution, the relationship between structure and function, and how the structure and function arise from the underlying interatomic forces of nature. Because these are the same goals as in the study of synthetic polymers, the topics of biomaterial-related polymer science and engineering cut across all the areas of this report.
A major goal of science is to learn how one molecule binds, recognizes, and interacts with another molecule. If the principles that control the binding and recognition events were understood, we could design activators for biomolecules and drugs, understand biological regulation, and improve separation methods. Major strides are occurring in the following areas: The cellular machinery for motion is complex and varied. For example, some bacteria are propelled by their flagellae, which act like small rotors.
Vertebrate muscle motion depends on the actomyosin system, whose major components are the proteins actin and myosin. The myosin fibers move along the actin fibers, powered by cellular processes involving adenosinetriphosphate ATP. The exact motions of the myosin molecules are not yet understood. The structures of both the actin and the myosin proteins have recently been determined by crystallography. New methods have recently been developed that probe forces and motions, including a mobility assay for watching the motions of muscle and related proteins under the microscope, ''optical tweezers" for measuring forces, and electron spin resonance experiments for detecting conformational changes.
Major advances are happening very rapidly now. To obtain high elasticity and the desirable properties it imparts, polymers are needed that have high chain flexibility and mobility. This need has led both nature and industry to choose polymers with small side chains, little polarity, and a reluctance to crystallize in the undeformed state. Rubberlike elasticity arises from the flexible chains interconnecting the cross-linking of polymer chains. The cross-linking carried out in nature is more sophisticated than the cross-linking used in the production of elastomers in the laboratory.
In biological systems, cross-links are introduced at specific amino acid repeat units and are thus restricted both in their number and in their locations along the chain. Furthermore, they may be carefully positioned spatially as well, by being preceded and succeeded along the chain by rigid alpha-helical sequences.
If we had nature's ability to control network structure, it would be possible for us to design materials with better mechanical properties. For example, many bioelastomers have relatively high efficiencies for storing elastic energy through the precise control of cross-link structure. A desirable advanced material would be an elastomer with low energy loss. Such a material would have the advantages of energy efficiency and fewer problems from degradation resulting from the heat buildup associated with incomplete recovery of elastic energy.
Another desirable advanced material would have high toughness, which may be obtained by exploiting non-Gaussian effects that increase the modulus of an elastomer near its rupture point. Some work on bioelastomers suggests that toughness may be controlled by the average network chain length and the distribution about this average.
There have been attempts to mimic this synthetically by end-linking chains of carefully controlled length distributions, but much more should be done along these lines. Biocomposites are usually composed of an inorganic phase that is reinforced by a polymeric network. The various types of biocomposites found in nature, such as bone, teeth, ivory, and sea shells, differ from synthetic analogs in one or more important respects. First, the hard reinforcing phase in biocomposites is frequently present to a very great extent, in some cases exceeding 96 percent by weight.
Second, the relative amounts of crystallinity, morphology, and crystallite size and distribution are carefully controlled. Moreover, the orientation of crystalline regions is generally fixed, frequently by the use of polymeric templates or epitaxial growth. Third, instead of a continuous homogeneous phase, a gradation of properties in the material is obtained by either continuous changes in chemical composition or physical structure. Finally, larger-scale ordering is often present, for example, in complex laminated structures, with various roles being delegated to the different layers present.
The differences cited above are achieved in biocomposites by nature's use of processing techniques that can be entirely different from those that have been used for synthetic composites. Until recently, in the methods used for synthetic composites, the two or more phases have generally been prepared separately and then combined into the composite structure. Occasionally, some chemistry is involved, but it is, typically, relatively unsophisticated, for example, the curing of resin in a fiberglass composite.
More intelligent approaches are now being used to design materials, particularly those required to have multifunctional uses. In particular, the types of chemical methods that predominate in the construction of biocomposites are being used increasingly by materials scientists. These syntheses are carried out in situ, with either the two phases being generated simultaneously or the second phase being generated within the first. The generation of particles or fibers within a polymer matrix can avoid the difficulties associated with blending agglomerated species into a high-molecular-weight, high-viscosity polymer.
The dispersed phase can be present to much greater extents, and much work could be done on the problem of using the polymeric matrix to control its growth. It may also be possible to avoid geometric problems, such as the alignment of fibrous molecules packed to high densities either because of their response to flow patterns or because of their inherent symmetry. Such anisotropy can be disadvantageous in that it leads to strengthening the material in some directions, but at the cost of weakening it in others. When such molecules are grown within an already formed matrix, however, essentially random isotropic packing can be obtained.
The shell of the macademia nut is an excellent example of this type of reinforcement. In it, bundles of cellulose fibers are present in structures having considerable alignment. The composite is, thus, random and isotropic at larger scale, and this is the source of its celebrated toughness.
Similar arrangements occur in some liquid crystalline polymers, but there is little correlation between the axes of different domains, and nothing has been done yet to mimic this type of composite material. In the case of chemically based methods, the competition between the kinetics of the chemical reactions and the rates of diffusion of reactants and products can also be used to advantage, for example, in the formation of permanent gradients.
This approach is yet another opportunity to exploit nature's ideas. The above exciting areas involve considerable overlap between biomaterials and polymer science. Polymers and biopolymers have a number of common elements, including the problems of understanding molecular conformations as the basis of underlying chemical events, the subtle driving forces, often largely entropic, and considerable overlap in the experimental and theoretical methodologies.
Despite the considerable overlap in problems and methodologies in polymer. Both fields would benefit substantially from more crossover and cross-education. The past half-century has witnessed an explosion in electronics and communications. Our world has been transformed as the transistor-based technologies have given rise to new modes of information storage, processing, and transmission, vital to enhanced productivity, improved health care, and better transportation systems.
These technologies are abundantly evident as supermarket scanners, fax machines, word processors, automatic teller machines, and many other "essentials" of modern life. Silicon and software are legitimately most clearly associated with these advances, but other materials, including polymers, play an essential supporting role, which is growing in importance. Owing to their high performance, manufacturing flexibility, quality, and low cost, polymers are key factors. The role of polymers is predicted not only to increase in quantitative terms, but also, more importantly, to extend into new areas in which polymers have not been employed in the past.
Historically, polymeric materials have been applied mainly as insulators and packaging. These uses often involve substantial quantities of material, for example, several hundred million kilograms for cable production annually, and they will remain important for the long-term future. In these applications, polymers offer ease and economy of manufacture, tough, durable mechanical properties, and excellent dielectric properties i.
Polymers are unlikely to be challenged in these areas. Polyethylene is consistently the material of choice for most communication and power cables, but fluorinated and other polymers are becoming increasingly important for special applications, such as inside wiring where flammability considerations are paramount. Over the last 20 years, polymers and other organic materials have been developed that exhibit electrical and optical properties that were formerly found only in inorganic materials. Polymers have been found that are piezoelectric, conduct electricity electronically, exhibit second-and third-order nonlinear optical behavior, and perform as light-emitting diodes.
Optical wave guides, splitters, combiners, polarizers, switches, and other functional devices have been demonstrated. In addition, lithographic pattern formation by the interaction of polymers with ultraviolet UV light and other forms of radiation has been carried to amazing levels of resolution and practicality and is the basis for fabrication of integrated and printed circuits of all kinds. In this section, some of these more exotic properties of polymers are briefly described.
For many of these materials, applications are only now being developed. It is likely that the new applications will have specialty niche markets, unlike the massive present market of commodity polymers. These products will be sold by function, not weight. Organic polymers play a crucial role as insulating materials in electronics. The most visible applications are in silicon chip encapsulation and in dielectric layers for printed circuit boards PCBs. The polymer employed is usually an epoxy novolac that is highly loaded with silica powder to reduce the coefficient of thermal expansion.
Differences in thermal expansion between chip and encapsulant create large stresses on cooling from mold temperatures and as the temperature of the assembly is cycled in testing and in use. Encapsulation is mainly for mechanical and chemical protection of the chip and the lead frame and thus facilitates handling for automatic assembly. Materials and processes have been developed to a high degree of sophistication. High mechanical strength is achieved with the smallest external dimensions. Printed circuit boards are layered structures of patterned copper connection paths "wires" placed on a polymer substrate.
Polymers employed include epoxies, polyesters, fluoropolymers, and other materials, but glass-reinforced epoxies usually bisphenol-A based are by far the most widely used. Metal patterns are defined photolithographically and plated to the desired thickness, and the layers are then piled up and cured in a press.
Circuits with more than 40 copper layers signal, power, and ground have been produced commercially. Connection to the inner layers is made through ''via" holes that are copper plated. One super-computer was marketed in which all of the electronics was placed on a single multilayer circuit board. The materials and process control requirements are challenging, and the functional end-product is worth a great deal. MCMs represent the leading edge of interconnection technology, and they are used when the time of transit of signals from chip to chip is an important limitation on the processing speed of the electronic system.
The speed of light is the ultimate barrier, and consequently it is essential to employ dielectrics that have the lowest practical dielectric permittivity. This is an area in which polymers offer substantial advantages over inorganic dielectrics. Practically any twentieth-century gadget you can think of, from the cheapest clock-radio to the most expensive mainframe computer, has its electronic guts mounted on printed circuit boards.
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These "boards"—actually fiberglass cloth impregnated with a brominated epoxy polymer resin—got their name because the electronic components on them are wired together by thin copper ribbons deposited directly onto the boards, like ink on paper. The idea that bulky, plastic-clad copper wires could be replaced by ribbons of bare metal on an insulating background was one of the fundamental breakthroughs of the electronics revolution of the s.
Printed circuit board substrates are an example of a "composite material"—a multicomponent material that performs better than the sum of the properties of its individual components. The chemical structures of such a material's components, and their relative proportions, can be tailored to provide just the right set of properties for a given application.
In this case, the material has to be not only lightweight and strong but also an electrical insulator, which rules out the use of metal sheets. The material must also be fracture-resistant, so that it can be cut to shape or drilled without cracking. And the material must be thermally stable—some of the newest, high-technology computer chips give off a lot of heat. The board has to handle such a hot spot without melting.
The board also has to be flame retardant, so that an electrical short does not become a conflagration that wipes out a lot of expensive hardware. In this composite material, the glass-fiber cloth gives the board its lightweight strength, while the brominated epoxy resin eventually becomes a rigid, three-dimensional network that gives the board the necessary stiffness, fracture resistance, and other properties. The manufacturing process starts with a roll of glass-fiber cloth. Carefully adjusted tension rollers feed the cloth at a precisely determined rate through a bath of the resin, which has been dissolved in a solvent.
The resin-impregnated cloth then wends its way over other rollers and through a series of ovens to evaporate the solvent. The heat and a catalyst also ''cure" the resin—promoting the chemical reactions that harden it into a tough, durable solid. Several layers of partially cured cloth can be laminated together before further curing to make an even stronger circuit board. Finally, the cured board, now as stiff as its namesake, is sawn up into the individual circuit boards.
Circuit board substrate materials have evolved over the years. New epoxies are now being used to improve dimensional control. Alternative polymer matrices are used for applications demanding high-temperature performance. Polymers are also being used for the reinforcing fibers themselves. Printed circuit boards, the key interconnection medium for electronics, depend critically on polymers and their composites. By far the most research and development on materials for MCM dielectric layers has gone into polyimides, and most existing applications are based on polymers of this family.
Great strides have been made in achieving the demanding property mix required through careful tailoring of the monomer chemistry. Improved adhesion, lower dielectric constant, reduced sensitivity to moisture, higher thermal stability, and other properties have been improved greatly. The in-plane coefficient of thermal expansion was reduced and adjusted to the range of silicon, metals, and ceramics. Most major electronics companies manufacture MCMs based on polyimides. In spite of the extent of commitment to polyimides, it has proved difficult to achieve all the desired properties in a given composition.
Other polymer dielectrics are in use, and new materials are under consideration. For example, commercial MCMs are manufactured by one electronics systems provider based on a proprietary epoxy-acrylate-triazine polymer that is photodefinable. In spite of the large experience base with the polyimide materials, the newer polymers have advantages and offer attractive alternatives.
All of the candidates are glassy polymers. The dielectric constants may be compared as follows:. In the final analysis, the choice of materials will be based on the sum of property advantages and processing practicality. Polymers offer the lowest dielectric constants and the thinnest "wires. Lithographic processes and associated technologies have advanced to the point that semiconductor device cells and conductor lines i. This is owing to the fact that the propagation of signals through the wiring on the chip and in the module is becoming the dominant limitation on processor cycle time.
The velocity of pulse propagation in these structures is inversely proportional to the square root of the dielectric constant of the medium. Hence, reductions in the dielectric constant translate directly into improvements in processor cycle time, in part because of the speed of propagation. In addition, the distance between signal lines is dictated by noise issues or "cross-talk" that results from induced current in conductors adjacent to active signal lines. A reduction of the. Performance demands on polymers incorporated as permanent parts of the chip structure are even more stringent than the requirements for MCMs and PCBs.
Insulating materials in chip applications must be able to withstand the very high temperatures associated with the processes used to deposit metal lines and to join chips to modules. At a minimum, they must withstand soldering temperatures without any degradation or outgassing. They must have thermal expansion coefficients that are closely matched to that of silicon. Silica meets all of the requirements extremely well, and this would continue to be the material of choice were its dielectric constant not so high.
While much attention has been given to polymers with very low permittivities, there is an increasing need for high-permittivity polymers in capacitor applications. Clearly, organic polymers currently play a critical role as insulators in electronic devices and systems. Continued success in the development of new generations of these critical dielectric materials depends on close interactions between the microelectronics and the chemical communities, a relationship that is not in evidence in the United States.
New partnerships are needed if we are to maintain competitiveness in this vital industry. Organic materials are generally insulators or, in other words, poor conductors of electricity compared with metals and semiconductors. Electrical conductivity in metals and semiconductors arises from the delocalized electrons of the system, and they are best described by "band theory. It has long been known that conjugated systems, that is, linear systems with alternate double and single bonds, should have delocalized electronic states, but it was only in that polyacetylene was shown to exhibit true metallic conductivity.
Earlier, in the s, low-molecular-weight organics had been shown to behave as semiconductors e. Those discoveries stimulated a large amount of research leading to the preparation of many new molecular metals and understanding of the nature of this new class of materials. An organic polymer that possesses the electrical and optical properties of a metal while retaining the mechanical and processing properties of a conventional polymer, is termed an "intrinsically conducting polymer" ICP , more commonly.
The concept of "doping" is the unique, central, underlying, and unifying theme that distinguishes conducting polymers from all other types of polymers. Increases in conductivity of up to 10 orders of magnitude can be readily obtained by doping. Doped polyacetylene approaches the conductivity of copper on a weight basis at room temperature.
The original polymer can be recovered with little or no damage to the backbone chain. The doping and undoping processes, involving dopant counter ions that stabilize the doped state, may be carried out chemically or electrochemically.
By controllably adjusting the doping level, a conductivity anywhere between that of the undoped insulating or semiconducting and that of the fully doped metallic form of the polymer may be obtained. Conducting blends with nonconducting polymers can be made.
This permits the optimization of the best properties of each type of polymer. Polyaniline, the best-known and most fully investigated example, also undergoes doping by a large number of protonic acids, during which the number of electrons associated with the polymer backbone remains unchanged. Appropriate forms and derivatives of many conducting polymers, especially those involving polyaniline and polythiophene, are readily solution processible into freestanding films or can be spun into fibers that even at this relatively early stage of development have tensile strengths approaching those of the aliphatic polyamides.
Blends of a few weight percent of conducting polymers with aromatic polyamides or polyethylene can exhibit conductivities equal to, or even exceeding, the conductivity of the pure conducting polymer while retaining mechanical properties similar to those of the host polymer. In addition, pure conducting polymers and their blends can be oriented by stretching to produce highly anisotropic electrical and optical properties. The thermal, hydrolytic, and oxidative stability of doped forms of pure conducting polymers varies enormously from the n-doped form of polyacetylene, which undergoes instant decomposition in air, to polyaniline, which has sufficient.
The oxidative and hydrolytic stability is significantly increased when the conducting polymer is used in the form of blends with conventional polymers. Clearly, research to improve the stability of conducting polymers is essential to commercial applications in the future. Polyaniline is currently the leading conducting polymer used in technological applications and is commercially available in quantity. Polypyrrole and derivatives of polythiophene and poly phenylene vinylene also have significant potential technological applications.
Rechargeable polyaniline batteries and high-capacity polypyrrole capacitors are in commercial production. Ironically conducting polymers are now being used in batteries and electrochromic displays. Also, new approaches for the synthesis of polymer electrolytes as thin films directly on electrodes via, for example, photopolymerization are needed to complement novel multilayer battery fabrication technology.
Along these lines, a key goal is the design of multifunctional polymers capable of transporting only cations, stabilizing a battery system against overcharging, and exhibiting low reactivity at alkali metal and metal oxide electrodes. Perhaps most important, electrode-polymer electrolyte reactions need to be examined from a fundamental point of view because these represent a major problem for battery cyclability and overall stability. The field of sensors is diverse, reflecting our need to control increasingly complex systems—including environments, processes, equipment, vehicles, and biomedical procedures—that are characterized by high levels of automation.
The key to the success of such automated systems is the measurement technology, which demands rapid, reliable, quantitative measurement of the required control parameters. These parameters include temperature, pressure, humidity, radiation, electric charge or potential, light, shock and acoustic waves, and the concentrations of specific chemicals in any environment, to name just a few.
Obviously, the types of sensors that are applied to such wide-ranging measurements are quite varied in type and principle of operation. Nevertheless, polymers play a significant role as enabling active materials for the design of sensors that are extending current limitations of sensitivity, selectivity, and response time.
A great deal of sensor research and development is focused on tailoring polymeric materials for applications in the chemical and biomedical fields. First, polymers can be functionalized through the incorporation, in their syntheses or. For example, polymers that are modified to bind dyes that respond to blood chemistry oxygen, carbon dioxide, acidity or to immobilize enzymes that produce reactions with substances of biological interest, such as glucose, are used to construct biosensors for in vivo application.
Another polymer property used in such sensors is permeability. The polymer allows diffusive transport of the chemical to the immobilized functionality to enable interaction and subsequent detection of the reaction products. When increased transport kinetics are required, the polymer may be fabricated in a porous state or may be engineered to swell or expand in the medium in which the sensor is immersed, such as water.
In other cases, polymers may be engineered as a controlled-release material, supplying reagents to the surrounding medium for local detection. The polymer properties described here are being used in the development of fiber-optic chemical sensors. These sensors employ dye molecules incorporated into transparent polymers that form either part of the fiber structure or part of an active element, termed the "optrode," located at the terminus of the fiber.
The sensors may incorporate either absorbing or fluorescent dyes for detection of specific chemical species. Light injected into the fiber, at a location remote from the chemical environment being probed, interacts with the dye and is absorbed or produces fluorescence. When a chemical species permeates the polymer and alters the absorption or fluorescence of the dye, the light output of the fiber returning from the optrode is altered in a quantitatively detectable manner.
Chemically modified electrode sensors rely on the measurement of electrical potentials produced by selective electrochemical reactions involving the chemical species to be determined. The development of thin polymer coatings to chemically modify the electrodes is an important topic of research in this field. The polymers are chemically and physically modified to concentrate electroactive sites at the electrode surfaces, to provide large ion and electron mobility, and to ensure a stable environment for the desired electrochemical reactions.
Especially promising areas of investigation include the development of such sensors for determination of specific ions and products of biochemical reactions with enzymes or antibodies immobilized in the polymer film. A related sensor type in the chemical and biomedical fields is the microsensor based on integrated solid-state electronic devices, for example, CHEMFETS. These sensors incorporate chemically sensitive polymer films placed in contact with the gate of a field effect transistor on a transducing silicon chip.
The electrical current output of the device is modulated by the chemical environment at its surface. Polymers used for this purpose must often be deposited and patterned using the standard photolithographic techniques of the semiconductor. Integration of signal processing functions on the sensor chip and on-chip sensor arrays for simultaneous determination of a range of chemical entities are key aspects of the development of this sensor type. These sensors are being applied to analyses ranging from ionic species to gaseous and liquid chemicals and biochemical substances.
Sensors that can be implanted in the body are a major goal. Much effort is being devoted to glucose sensors that would allow insulin pumps to respond to a diabetic person's time-dependent need for this vital hormone. An important extension of the solid-state microsensor makes use of electronically conducting conjugated polymers. The electronic conductivity of these materials is modulated over several orders of magnitude by interaction with a variety of chemicals.
The polymers are deposited on electrodes or solid-state devices by electrochemical polymerization, and dopants are simultaneously incorporated in the polymerization process to enhance conductivity and chemical activity. Sensors of this type have been applied primarily to the detection of gases such as ammonia, nitrogen dioxide, and hydrogen sulfide and ions. Specific polymers, called electrets, have the ability to store electrical charges or to be electrically poled so that they retain a permanent polarization. These polymers can be fabricated into specific structures in which their deformation or movement produces electrical signals that can be resolved.
Electret materials, best exemplified by fluorinated polymers such as poly tetrafluoroethylene , can be fabricated into films, charged, and used to construct condenser-type acoustic transducers electret microphones. Ferroelectric polymers, such as poly vinylidene fluoride , can be poled by applying a strong electric field, and then used to construct acoustic, pressure, or thermal sensors. They are applied in pyroelectric detectors, hydrophones, ultrasonic transducers, shock wave sensors, and tactile sensors for robotics. When you visit your dentist for a new crown or a set of dentures, you may go home with a mouthful of plastic.
Traditionally, crowns for teeth in the back of the mouth, where strength is more important than appearance, have been cast from alloys of mercury with silver or gold. And dentures have been made with porcelain pearly whites rooted in a pink base of an acrylic polymer—a lifelike combination that is rugged enough to chew ice cubes, while the firm plastic base distributes the stresses gently. Fitting these crowns and dentures is a time-consuming process, because they cannot be made to order in your mouth.
But some dental work has to be custom-made on the spot, and that is where nothing but a polymer will do. When a dentist is trying to repair a chipped tooth, say, in the front of the mouth, it is essential that the replacement material not only look like a tooth, but also be capable of being molded in the mouth to what is left of the original tooth. The material must be strong enough to chew with and should seal the tooth's interior from decay-causing bacteria and from hot, cold, or other potentially painful foods.
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For perhaps years, the material used for this purpose was "silicate cement," a composite of glass particles held in an acidic gel matrix. This material worked just fine when brand new but would gradually erode over the years. In the s the technology was developed to allow a polymeric prosthesis to be cast directly onto a properly prepared tooth. These plastics, properly colored, looked just like real teeth and did not decay. Unfortunately, they did have other problems.
Methyl methacrylate, for example, a high-strength polymer used to make Plexiglas, would expand slightly in a mouthful of hot soup and shrink when exposed to ice cream, so that the filling would eventually leak, allowing the tooth to decay underneath it. And the polymerization reaction itself liberated a lot of heat—enough to burn a few unsuspecting patients' tongues! The thermal changes were overcome by incorporating high concentrations of glasslike filler particles into the polymer, but these materials were not strong enough to last long because the glass and the polymer did not stick to each other very well.
The essential step in developing successful dental composites was finding a suitably strong polymer that also adheres to glass.
Radiation chemistry of biopolymers (eBook, ) [theranchhands.com]
Today's composites are based on a dimethacrylate monomer that has side groups dangling off its backbone that are adsorbed onto the surface of the glass particles. And the particles themselves are coated with a coupling agent, such as a silane, that promotes binding. Other "wetting agents" encourage the polymer to seal to the tooth's enamel and the dentin below it. The composite's precursor, a kind of putty that can be applied to the tooth with a trowel-like dental probe, also contains inhibitors that prevent the material from setting prematurely.
Once the composite has been properly sculpted, an ultraviolet light is used to initiate the polymerization. Within minutes, the material gels, and after a few more minutes of reaction time the hardening is complete and the patient can go home. A PMMA material system similar to that employed in the fabrication of denture bases is used to bind metal hip replacements to the femur.
Ultrahigh-molecular-weight polyethylene is used as the hip cup material. Even the metal alloys of the bone replacement are gradually beginning to be replaced by fiber-reinforced composites. Knee replacements have also become much more common and successful in recent years. This general area has made considerable progress based on increased understanding of bone growth processes that aid bonding to the prostheses. This is a huge field that is advancing rapidly as new and superior materials are introduced. In the eye, materials are not brought into contact with blood.
Contact lenses are external to the body, but the materials are maintained in intimate contact with tissue. Glass was used for many years, giving way to PMMA beginning in the s. PMMA is a hard, glassy polymer that is compatible with the surface of the cornea. Soft lenses made of poly 2-hydroxy ethyl methacrylate or simply poly-HEMA hydrogel have become popular more recently, in part because of their high oxygen permeability.
Poly-HEMA will take up water to a high degree and become soft and flexible. Soft contact lenses contain about 70 percent water. Replacement lenses provided following cataract surgery are made of similar polymers. The clouded lens is removed and replaced by a hard lens PMMA or a soft hydrogel lens. The hydrogel lens may be inserted through a smaller incision, but it has a smaller refractive index than that of PMMA, requiring a greater thickness.
An alternative procedure involving injection of a prepolymer liquid into the lens capsule and polymerization in place has been studied. The introduction of new polymer materials continues to make cataract surgery and recovery of sight safer, less distressing, and more effective. Polymers are used in diagnostics either as reagents or as enhancers. Polymeric materials can enhance performance of test materials. They are used as solid supports to bind the material being tested specifically for isolation and detection.
In other uses, they serve a "reporter" role. The bioreagents are generally incorporated into the system through direct attachment via either copolymerization or cross-linking. The resulting aggregate has multiple copies of the reactive signal and thus can influence accuracy, testing time, and automation. The extent of incorporation will affect diffusivity and exchange rates of solutes, nonspecific binding, and overall binding capacity.
Hence, the availability of relatively inexpensive polymers will positively influence the development. Polystyrene, nylon acrylamide, dextrans, and agarose have all been used for attachment of antibodies and antigens. In all of these uses, nonspecific binding has to be minimized because it limits sensitivity and makes interpretation of test results very difficult.
Therefore, the need to understand interfacial biointeractions will continue to be paramount. New materials, such as block copolymers containing polypeptides and segmented poly ether urethanes , have been shown to have specific affinity for proteins. These hybrid materials may prove to be one of the best ways to incorporate both function and structure into the same molecule. For example, it may be possible to incorporate a specific cell-binding segment of a protein into a synthetic polymer, with the latter providing the scaffold and processing capability.
Thus, one can tailor polymers to specific biomolecular and diagnostic functions. Interest in drug delivery research is increasing for a number of reasons: Polymers are essential for all the new delivery systems, including transdermal patches, microspheres, pumps, aerosols, ocular implants, and contraceptive implants. The major disease areas that are expected to benefit from development of new delivery systems include chronic degenerative diseases, such as central nervous system disorders associated with aging, cancer, cardiovascular and respiratory diseases, chemical imbalances, and cellular dysfunction.
Delivery to difficult-to-reach areas such as the brain is desirable, and progress is being made in the area through the use of polyanhydrides, as is discussed in the vignette "Implanted Polymers for Drug Delivery. Several drug release technologies have become clinically and commercially important. They can be classified into various categories by their mechanism of release: Any of these mechanisms can be employed to develop controlled-release delivery systems for oral, transdermal, implant insert, or intravenous administration, although some mechanisms are superior to others for certain.
We have all heard that biodegradable polymers are good for the environment. But they may be good for cancer patients, too. Efforts are now under way to design polymer implants that will slowly degrade inside the human body, releasing cancer-fighting drugs in the process. Such an implant would need several specific properties. It would have to degrade slowly, from its outside surface inward, so that a drug contained throughout the implant would be released in a controlled fashion over time.
The polymer as a whole should repel water, protecting the drug within it—as well as the interior of the implant itself—from dissolving prematurely. But the links between the monomers—the building blocks that make up the polymer—should be water-sensitive so that they will slowly fall apart. Anhydride linkages—formed when two carboxylic-acid-containing molecules join together into a single molecule, creating and expelling a water molecule in the process—are promising candidates, because water molecules readily split the anhydride linkages in the reverse of the process that created them, yet the polymer molecules can still be water-repellent in bulk.
By varying the ratios of the components, surface-eroding polymers lasting from one week to several years have been synthesized. These polymer disks are now being used experimentally as a postoperative treatment for brain cancer. The surgeon implants several polyanhydride disks, each about the size of a quarter, in the same operation in which the brain tumor is removed.
Radiation chemistry of biopolymers
The disk contains powerful cell-killing drugs called nitrosoureas. Nitrosoureas are normally given intravenously, but they are effective in the bloodstream for less than an hour. Unfortunately, nitrosoureas are indiscriminately toxic, and this approach generally damages other organs in the body while killing the cancer cells. But placing the drug in the polymer protects the drug from the body, and the body from the drug. The nitrosourea lasts for approximately the duration of the polymer—in this case, nearly one month. And the eroding disk delivers the drug only to its immediate surroundings, where the cancer cells lurk.
The polymer degradation method of drug delivery is making good progress toward approval by the Food and Drug Administration. See the vignette " Seasickness Patches. Such systems are being developed predominantly for transdermal drug delivery. There are many challenges in designing polymers for controlled-release applications. These polymers must be biocompatible, pure, chemically inert, nontoxic, noncarcinogenic, highly processible, mechanically stable, and sterilizable. The polymers in use today in drug delivery are also mostly borrowed from the chemical industry and in many cases lack the exact required properties.
Novel polymers designed and synthesized to provide optimal properties and characteristics will be required to take full advantage of the emerging technologies described above. The venerable drug scopolamine, found in henbane and deadly nightshade, is perhaps the most effective short-term preventer of motion sickness. Unfortunately, scopolamine does not stay in the blood long. Because the drug must be taken at short intervals, the possibility of accidental overdose—with its side effects of drowsiness, blurred vision, hallucinations, and disorientation—is increased.
This tended to limit the drug's popularity as a seasickness preventative. A way to deliver a constant low dose to the bloodstream for hours on end needed to be found. A polymer-based "transdermal patch" proved to be the answer. The thickness of a playing card and less than three-eighths of an inch in diameter, the patch is applied like an adhesive bandage and does not break the skin. The skin behind the ear is the most permeable, and from there the scopolamine rapidly diffuses into the blood vessels just below the surface. The patch consists of several laminated layers of different polymers, each one designed for a different function.
The topmost layer is a polyester film, colored to match the skin. Adhering to the polyester's underside is a film of vapor-deposited aluminum to protect the drug from sunlight, evaporation, and contamination. Then comes a polymer adhesive that binds the aluminum to the rest of the patch. The next layer, the reservoir, is made of a polyisobutylene skeleton filled with mineral oil that contains a hour supply of the drug in a special skin-permeable formulation.
Between the reservoir and the skin is a polypropylene membrane riddled with microscopic pores. The pores are just the right size to ensure that the drug seeps out at a rate less than it can be absorbed by the most permeable skin. This feature ensures a constant dose rate, regardless of the skin's permeability.
The patch's bottom layer is an adhesive formulation of polyisobutylene and mineral oil. This mineral oil also contains the drug, so that it saturates the skin as soon as the patch is applied and minimizes the time lag before the scopolamine takes effect. Even so, it generally takes about 4 hours to kick in. The adhesive layer is protected before use by a peel-off backing of siliconized polyester. The transdermal patch technology transformed an otherwise unmanageable drug into the most effective motion sickness treatment available, and one good for three days.
Designing and testing the patch required attention to complex issues of drug dosage and behavior as well as the challenge of fabricating a pharmaceutical product in a radically different and untried form. States, and a larger number are in development. Compilation of information from Physicians' Desk Reference The volume of biopolymers in the world far exceeds that of synthetic macromolecules.
DNA and RNA are informational polymers encoding biological information , while globular proteins, some RNAs, and carbohydrates serve chemical functions and structural purposes. In contrast, most synthetic polymers, and fibrous proteins such as collagen which makes up tendon and bone and keratin which makes up hair, nails, and feathers , are structural rather than informational or chemically functional.
Structural materials are useful because of their mechanical strength, rigidity, or molecular size, properties that depend on molecular weight, distribution, and monomer type. In contrast, informational molecules derive their main properties not simply from their size, but from their ability to encode information and function. They are chains of specific sequences of different monomers. For DNA the monomers are the deoxyribonucleic acid bases; for RNA, the ribonucleic acid bases; for proteins, the amino acids; and for carbohydrates or polysaccharides, the sugars.
The paradigm in biopolymers is that the sequence of monomers along the chain encodes the information that controls the structure or conformation of the molecule, and the structure encodes the function. An informational polymer is like a necklace, and the monomers are like the beads.
Information is encoded in the sequence of bead colors, which in turn controls the sequence of amino acids in proteins. There are 20 different types of amino acid monomers; in the necklace analogy, there are 20 different colors of beads. A globular protein folds into one specific compact structure, depending on the amino acid sequence. This balled-up shape, or structure, is what determines how the protein functions. The folding of the linear structure produces a three-dimensional shape that controls the function of the protein through shape selection. Except in special cases, synthetic polymer science does not yet have the precision to create specific monomer sequences: But the ability to synthesize specific monomer sequences by a linear process would have extraordinary potential.
For example, it is the ability to create specific monomer sequences that distinguishes biological life forms, and the corresponding complex hierarchies of structure and function, from simpler polymeric materials. Hence one of the most exciting vistas in polymer science is the prospect of creating informational polymers through control of specific monomer sequences. The present state of. This method is limited to preparation of short chains less than 50 amino acid groups and small quantities. Techniques that allow similar controlled synthesis on a much larger scale would be revolutionary. The study of informational polymers aims to determine the specific shapes of biological polymers at atomic and nanometer resolution, the relationship between structure and function, and how the structure and function arise from the underlying interatomic forces of nature.
Because these are the same goals as in the study of synthetic polymers, the topics of biomaterial-related polymer science and engineering cut across all the areas of this report. A major goal of science is to learn how one molecule binds, recognizes, and interacts with another molecule. If the principles that control the binding and recognition events were understood, we could design activators for biomolecules and drugs, understand biological regulation, and improve separation methods.
Major strides are occurring in the following areas: The cellular machinery for motion is complex and varied. For example, some bacteria are propelled by their flagellae, which act like small rotors. Vertebrate muscle motion depends on the actomyosin system, whose major components are the proteins actin and myosin. The myosin fibers move along the actin fibers, powered by cellular processes involving adenosinetriphosphate ATP. The exact motions of the myosin molecules are not yet understood. The structures of both the actin and the myosin proteins have recently been determined by crystallography.
New methods have recently been developed that probe forces and motions, including a mobility assay for watching the motions of muscle and related proteins under the microscope, ''optical tweezers" for measuring forces, and electron spin resonance experiments for detecting conformational changes. Major advances are happening very rapidly now. To obtain high elasticity and the desirable properties it imparts, polymers are needed that have high chain flexibility and mobility. This need has led both nature and industry to choose polymers with small side chains, little polarity, and a reluctance to crystallize in the undeformed state.
Rubberlike elasticity arises from the flexible chains interconnecting the cross-linking of polymer chains. The cross-linking carried out in nature is more sophisticated than the cross-linking used in the production of elastomers in the laboratory. In biological systems, cross-links are introduced at specific amino acid repeat units and are thus restricted both in their number and in their locations along the chain. Furthermore, they may be carefully positioned spatially as well, by being preceded and succeeded along the chain by rigid alpha-helical sequences.
If we had nature's ability to control network structure, it would be possible for us to design materials with better mechanical properties. For example, many bioelastomers have relatively high efficiencies for storing elastic energy through the precise control of cross-link structure. A desirable advanced material would be an elastomer with low energy loss. Such a material would have the advantages of energy efficiency and fewer problems from degradation resulting from the heat buildup associated with incomplete recovery of elastic energy. Another desirable advanced material would have high toughness, which may be obtained by exploiting non-Gaussian effects that increase the modulus of an elastomer near its rupture point.
Some work on bioelastomers suggests that toughness may be controlled by the average network chain length and the distribution about this average. There have been attempts to mimic this synthetically by end-linking chains of carefully controlled length distributions, but much more should be done along these lines. Biocomposites are usually composed of an inorganic phase that is reinforced by a polymeric network.
The various types of biocomposites found in nature, such as bone, teeth, ivory, and sea shells, differ from synthetic analogs in one or more important respects. First, the hard reinforcing phase in biocomposites is frequently present to a very great extent, in some cases exceeding 96 percent by weight.
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Second, the relative amounts of crystallinity, morphology, and crystallite size and distribution are carefully controlled. Moreover, the orientation of crystalline regions is generally fixed, frequently by the use of polymeric templates or epitaxial growth. Third, instead of a continuous homogeneous phase, a gradation of properties in the material is obtained by either continuous changes in chemical composition or physical structure. Finally, larger-scale ordering is often present, for example, in complex laminated structures, with various roles being delegated to the different layers present.
The differences cited above are achieved in biocomposites by nature's use of processing techniques that can be entirely different from those that have been used for synthetic composites. Until recently, in the methods used for synthetic composites, the two or more phases have generally been prepared separately and then combined into the composite structure. Occasionally, some chemistry is involved, but it is, typically, relatively unsophisticated, for example, the curing of resin in a fiberglass composite.
More intelligent approaches are now being used to design materials, particularly those required to have multifunctional uses. In particular, the types of chemical methods that predominate in the construction of biocomposites are being used increasingly by materials scientists. These syntheses are carried out in situ, with either the two phases being generated simultaneously or the second phase being generated within the first. The generation of particles or fibers within a polymer matrix can avoid the difficulties associated with blending agglomerated species into a high-molecular-weight, high-viscosity polymer.
The dispersed phase can be present to much greater extents, and much work could be done on the problem of using the polymeric matrix to control its growth. It may also be possible to avoid geometric problems, such as the alignment of fibrous molecules packed to high densities either because of their response to flow patterns or because of their inherent symmetry. Such anisotropy can be disadvantageous in that it leads to strengthening the material in some directions, but at the cost of weakening it in others.
When such molecules are grown within an already formed matrix, however, essentially random isotropic packing can be obtained. The shell of the macademia nut is an excellent example of this type of reinforcement. In it, bundles of cellulose fibers are present in structures having considerable alignment.
The composite is, thus, random and isotropic at larger scale, and this is the source of its celebrated toughness. Similar arrangements occur in some liquid crystalline polymers, but there is little correlation between the axes of different domains, and nothing has been done yet to mimic this type of composite material. In the case of chemically based methods, the competition between the kinetics of the chemical reactions and the rates of diffusion of reactants and products can also be used to advantage, for example, in the formation of permanent gradients.
This approach is yet another opportunity to exploit nature's ideas. The above exciting areas involve considerable overlap between biomaterials and polymer science. Polymers and biopolymers have a number of common elements, including the problems of understanding molecular conformations as the basis of underlying chemical events, the subtle driving forces, often largely entropic, and considerable overlap in the experimental and theoretical methodologies.
Despite the considerable overlap in problems and methodologies in polymer. Both fields would benefit substantially from more crossover and cross-education. The past half-century has witnessed an explosion in electronics and communications. Our world has been transformed as the transistor-based technologies have given rise to new modes of information storage, processing, and transmission, vital to enhanced productivity, improved health care, and better transportation systems. These technologies are abundantly evident as supermarket scanners, fax machines, word processors, automatic teller machines, and many other "essentials" of modern life.
Silicon and software are legitimately most clearly associated with these advances, but other materials, including polymers, play an essential supporting role, which is growing in importance. Owing to their high performance, manufacturing flexibility, quality, and low cost, polymers are key factors. The role of polymers is predicted not only to increase in quantitative terms, but also, more importantly, to extend into new areas in which polymers have not been employed in the past. Historically, polymeric materials have been applied mainly as insulators and packaging.
These uses often involve substantial quantities of material, for example, several hundred million kilograms for cable production annually, and they will remain important for the long-term future. In these applications, polymers offer ease and economy of manufacture, tough, durable mechanical properties, and excellent dielectric properties i. Polymers are unlikely to be challenged in these areas. Polyethylene is consistently the material of choice for most communication and power cables, but fluorinated and other polymers are becoming increasingly important for special applications, such as inside wiring where flammability considerations are paramount.
Over the last 20 years, polymers and other organic materials have been developed that exhibit electrical and optical properties that were formerly found only in inorganic materials. Polymers have been found that are piezoelectric, conduct electricity electronically, exhibit second-and third-order nonlinear optical behavior, and perform as light-emitting diodes.
Optical wave guides, splitters, combiners, polarizers, switches, and other functional devices have been demonstrated. In addition, lithographic pattern formation by the interaction of polymers with ultraviolet UV light and other forms of radiation has been carried to amazing levels of resolution and practicality and is the basis for fabrication of integrated and printed circuits of all kinds. In this section, some of these more exotic properties of polymers are briefly described. For many of these materials, applications are only now being developed. It is likely that the new applications will have specialty niche markets, unlike the massive present market of commodity polymers.
These products will be sold by function, not weight. Organic polymers play a crucial role as insulating materials in electronics. The most visible applications are in silicon chip encapsulation and in dielectric layers for printed circuit boards PCBs. The polymer employed is usually an epoxy novolac that is highly loaded with silica powder to reduce the coefficient of thermal expansion.
Differences in thermal expansion between chip and encapsulant create large stresses on cooling from mold temperatures and as the temperature of the assembly is cycled in testing and in use. Encapsulation is mainly for mechanical and chemical protection of the chip and the lead frame and thus facilitates handling for automatic assembly.
Materials and processes have been developed to a high degree of sophistication. High mechanical strength is achieved with the smallest external dimensions. Printed circuit boards are layered structures of patterned copper connection paths "wires" placed on a polymer substrate. Polymers employed include epoxies, polyesters, fluoropolymers, and other materials, but glass-reinforced epoxies usually bisphenol-A based are by far the most widely used. Metal patterns are defined photolithographically and plated to the desired thickness, and the layers are then piled up and cured in a press.
Circuits with more than 40 copper layers signal, power, and ground have been produced commercially. Connection to the inner layers is made through ''via" holes that are copper plated. One super-computer was marketed in which all of the electronics was placed on a single multilayer circuit board. The materials and process control requirements are challenging, and the functional end-product is worth a great deal.
MCMs represent the leading edge of interconnection technology, and they are used when the time of transit of signals from chip to chip is an important limitation on the processing speed of the electronic system. The speed of light is the ultimate barrier, and consequently it is essential to employ dielectrics that have the lowest practical dielectric permittivity.
This is an area in which polymers offer substantial advantages over inorganic dielectrics. Practically any twentieth-century gadget you can think of, from the cheapest clock-radio to the most expensive mainframe computer, has its electronic guts mounted on printed circuit boards. These "boards"—actually fiberglass cloth impregnated with a brominated epoxy polymer resin—got their name because the electronic components on them are wired together by thin copper ribbons deposited directly onto the boards, like ink on paper.
The idea that bulky, plastic-clad copper wires could be replaced by ribbons of bare metal on an insulating background was one of the fundamental breakthroughs of the electronics revolution of the s. Printed circuit board substrates are an example of a "composite material"—a multicomponent material that performs better than the sum of the properties of its individual components.
The chemical structures of such a material's components, and their relative proportions, can be tailored to provide just the right set of properties for a given application. In this case, the material has to be not only lightweight and strong but also an electrical insulator, which rules out the use of metal sheets. The material must also be fracture-resistant, so that it can be cut to shape or drilled without cracking. And the material must be thermally stable—some of the newest, high-technology computer chips give off a lot of heat.
The board has to handle such a hot spot without melting. The board also has to be flame retardant, so that an electrical short does not become a conflagration that wipes out a lot of expensive hardware. In this composite material, the glass-fiber cloth gives the board its lightweight strength, while the brominated epoxy resin eventually becomes a rigid, three-dimensional network that gives the board the necessary stiffness, fracture resistance, and other properties. The manufacturing process starts with a roll of glass-fiber cloth.