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Polymer Science and Engineering: The Shifting Research Frontiers 2 Advanced Technology Applications New classes of polymeric materials with unique applications are being introduced. In many cases, the properties and their usage were discovered only recently. This chapter covers two areas: (1) health, medicine, and biotechnology, a rapidly developing domain based largely on known materials but moving to designed and engineered polymers, and (2) information and communications, an emerging field for polymers significantly based on their electronic properties. These two areas are attracting a great deal of attention, particularly among researchers who are not traditional specialists in polymer science. The growing importance of these fields makes the interdisciplinary aspect of polymer research abundantly clear. HEALTH, MEDICINE, AND BIOTECHNOLOGY Polymers play a major role in all aspects of biological processes. In fact, it is legitimate to proclaim that polymers are the molecular basis of life. The genetically inherited information required for the growth and health of living systems is encoded in the macromolecule deoxyribonucleic acid (DNA), the backbone of which forms the famous double helix. The molecular genetic code uses only four purine and pyrimidine bases to dictate the structure of the proteins that make up so much of living systems. DNA directs the assembly of about 20 amino acids in complex sequences that become the proteins. These proteins are polypeptide polymers that differ from one another only in the sequence of their constituent amino acids. All enzymes, which control the reaction rates in biological systems, are proteins. Collagen proteins form fibers and connective tissue
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Polymer Science and Engineering: The Shifting Research Frontiers found in tendons, cartilage, blood vessels, skin, and bone. Elastin, an elastic substance found in ligaments and in the walls of blood vessels, is also a protein. Other polymers such as polysaccharides are also important. They make up chains of sugar units present as a major constituent in all connective tissue. Ribonucleic acid (RNA) molecules also carry information and can serve protein-like functions. Thus informational, chemical, mechanical, and other properties of living systems find their origin in the molecular structure of their component polymers. Medicine, as a biological science, therefore must be dependent on the nature of polymers. Bandages and dressings are dominated by polymers in modern practice. Molds and impressions of teeth, dentures and denture bases, adhesives, and fillings are polymer based. Sutures, which were made of cat gut for over 2,000 years, are now made of synthetic polymers. Hard and soft lenses required after cataract surgery, artificial corneas, and other ocular materials are all polymers. Orthopedic implants, artificial organs, heart valves, vascular grafts, hernia mesh, and artificial arms, legs, hands, and feet all depend critically on polymeric materials. Similarly, catheters, syringes, diapers, blood bags, and many other trappings of modern medicine depend heavily on polymeric materials. Most of these items arrive in sterile form, packaged in polymers. Significant quantities of polymers are used in medical devices, consumable medical products, and the packaging for medical products. The most common products are devices such as catheters and intravenous lines, nearly 100 million of which are used annually in the United States. Because medical products use functional rather than structural polymers and their value is not related to the volume they occupy, medical products should be quantified on the basis of number of functional units rather than in terms of pounds of polymers. Polymers are natural allies of medicine because living tissue is composed substantially of polymers. As our understanding of the processes of life advances, and our ability to tailor synthetic polymer structures to specific chores matures, the power of medicine will grow dramatically. The opportunities for collaborative programs involving materials scientists and medical researchers and practitioners are unlimited. Few, if any, areas of research offer more obvious benefits to society. Medical devices generally entail intimate contact with living tissue. Organisms are extremely sensitive to the presence of foreign substances and are aggressive in repelling an invading object or agent. To date, empirical means have provided considerable progress in finding materials that are less offensive to living organisms. Polyesters, polyamides, polyethylene, polycarbonate, polyurethanes, silicones, fluorocarbons, and other familiar polymers have been employed successfully in medical applications. Establishing the factors controlling the biocompatibility of these materials is a difficult process that has been only partially defined. Materials experimentation in medical applications has always called for courage as well as technical know-how, but in this litigious era the problems are amplified. Even so, progress continues on a broad front.
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Polymer Science and Engineering: The Shifting Research Frontiers Advances in any technology depend critically on research. Currently, there is a paucity of fundamental information on interactions between synthetic materials and the biological medium, partly because of the complex mechanisms involved and the fact that the human body is a most hostile environment. Hence, there is a need for research that will generate the fundamental information necessary to design materials that will be compatible with human tissue and perform the required functions. In particular, there is a need for better understanding of surface interactions and relationships between physical properties of a polymer and biological events, such as clot formation in blood. A persuasive argument can be made that biomaterials development is poised for a revolutionary change. For quite some time, an important objective of biomaterials research has been a search for "inert" materials that elicit minimal tissue response. But nothing about organisms is static. As the processes of cellular signaling and differentiation become more thoroughly understood, it is likely that new polymers will be engineered to manipulate these processes in positive, productive ways. The emerging science of tissue engineering, for example, will depend directly on the development of new biologically active polymeric matrices to guide the controlled generation (or regeneration) of skin, cartilage, liver, and neuronal tissue. Enzymes, semisynthetic enzymes, and genetic engineering provide a revolutionary opportunity in the production of novel materials for medical uses. The challenges are great; the rewards are greater. The potential economic and societal impact of polymers designed for use in health care, biotechnology, and agriculture is enormous. Almost everyone is already in contact with biomaterials. Hence, polymers are positioned to play a vital role in improving the quality of life, enhancing longevity, and reducing the cost of health care. Polymers in Health Applications Implants and Medical Devices Development of medical implants has been limited by many factors. The synthetic nondegradable materials needed in such products as orthopedic joints, heart valves, vascular prostheses, heart pacemakers, neurostimulators, and ophthalmic and cochlear implants must meet many technical requirements, including being stable and biocompatible in the host environment for moderate to long lifetimes. However, the fact that most of the polymers currently used in implants were not initially designed for medical use means that those polymers may not meet such requirements. This carries with it inherent risks, such as those dramatically brought to light in the course of recent litigation concerning silicone breast implants. Also, toxic breakdown products have been reported for certain polyurethanes under consideration for an artificial heart pump design. Development of new techniques for screening and testing the biological response of
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Polymer Science and Engineering: The Shifting Research Frontiers candidate materials is clearly a priority matter. By and large, however, empirical testing has found material implants to be remarkably successful. Each implant application calls for a specific set of properties. A key property required in implants exposed to blood is nonthrombogenicity; that is, the material must not cause clotting. Polymers being considered for vascular prostheses include poly(ethylene terephthalate) fibers, expanded polytetrafluoroethylene foams, segmented porous polyurethanes, and microporous silicone rubber. Surface treatments include hydrophilic coatings, seeded endothelial cells, immobilized heparin (an anticoagulent), and a garlic extract. The diversity of candidates is impressive. Although none of these materials are completely satisfactory, good blood flow has been maintained for many years in some cases. Polymers also play a major role in devices used to oxygenate blood. They must operate without blood damage. Silicone rubber and polypropylene have been used successfully in both solid and microporous forms. These materials, in microporous form, are widely used during cardiopulmonary bypass surgery, where blood exposure is relatively short term. For long-term exposure, solid membranes are used. Again, surface treatments, such as immobilized albumin, are providing promising results. Synthetic, biomimetic phospholipid membranes are under development as coatings that render surfaces compatible with blood. Inclusion of the phosphorylcholine headgroup is thought to be a most promising approach, and it has been employed on poly(vinyl chloride), polyethylene, polypropylene, and other polymers. The phosphorylcholine group can also be added as a plasticizer in polyurethanes and other polymers. Artificial kidney machines employ polymeric hollow fibers to purify blood by hemodialysis. Cellophane (regenerated cellulose) was introduced early on, and Cuprophan, a form of regenerated cellulose that has been strengthened by cuprammonium solution treatment, remains the material of choice, although many other polymers have been tried. Many factors are involved, including treatment of the dialyzer for reuse and avoidance of removal of desirable factors from the blood. It seems likely that synthetic polymers will eventually come into use, although to date they do not have the proper combination of properties. Dental materials are dominated by polymers to an increasing extent. Impression materials are made of silicone and polysulfide elastomers that cure rapidly in the mouth and maintain their shape. Denture bases are made from polymers based on poly(methyl methacrylate) (PMMA) that are cross-linked through a free radical process. Fillings that match the teeth in appearance are composed of highly filled difunctional methacrylates that are cured by exposure to blue light. Silane-coated ceramic fillers provide the visual match and the hardness and durability required. The use of photocuring relieves the dentist of the need to work within the limited time allowed by amalgam fillings. The composite has been engineered to minimize contraction during cure, an extremely important aspect of any filling material. Polymers also play a central role in
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Polymer Science and Engineering: The Shifting Research Frontiers dental adhesives. Further advances in dental materials can be expected as polymer systems are designed and engineered to satisfy the complex needs of the area. The vignette "Dental Composites" further describes the role of polymers in dentistry. The general field of load-bearing implants involves metals, ceramics, and polymers, and the field has advanced rapidly in recent years. Hip replacements DENTAL COMPOSITES 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. For perhaps 100 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 1950s 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. 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.
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Polymer Science and Engineering: The Shifting Research Frontiers have become common, and satisfactory performance is experienced for decades. 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 1940s. 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. The polymers currently in use are copolymers of vinyl pyrrolidone with poly-HEMA or PMMA. 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. Diagnostics 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. The development of automated clinical analyzer (ACA) systems, for example, relied on the heat sealing and good optical properties of the ionomeric polymer called Surlyn®. Hence, the availability of relatively inexpensive polymers will positively influence the development
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Polymer Science and Engineering: The Shifting Research Frontiers of disposable diagnostic test kits. 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. Controlled Drug Release Interest in drug delivery research is increasing for a number of reasons: the need for systems to deliver novel, genetically engineered pharmaceuticals, the need to target delivery of anticancer drugs to specific tumors, the need to develop patentable sustained delivery systems, and the need to increase patient compliance. 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." Success in this area will be rewarded with improved quality of life and longevity. Several drug release technologies have become clinically and commercially important. They can be classified into various categories by their mechanism of release: (1) diffusion-controlled systems, where the drug is released by solution-diffusion through a polymeric membrane or embedded into a polymeric matrix where the matrix controls the rate of delivery from the system; (2) erosion-controlled systems, where the drug release is activated by dissolution of the polymer, disintegration of the polymer, or chemical/biodegradation; or (3) osmotically controlled systems, where the contents are released by the rate of osmotic absorption of water from the environment, thereby displacing the drug from the reservoir. 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
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Polymer Science and Engineering: The Shifting Research Frontiers IMPLANTED POLYMERS FOR DRUG DELIVERY 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. 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. applications. (See the vignette "Seasickness Patches.") Recently, more sophisticated technologies have emerged, such as electrotransport systems, whereby the drug is driven from a reservoir under the influence of an electric field. 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. A number of controlled-release products are on the market in the United
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Polymer Science and Engineering: The Shifting Research Frontiers SEASICKNESS PATCHES 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 72-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. Yet this seemingly simple patch—a glorified sticker/Band-Aid—employs at least six layers of carefully chosen polymers, each of which has a specific function and each of which must be compatible with the neighboring materials. 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. Table 2.1 lists a few of the available products as examples of drug delivery systems, along with the mechanisms of release and polymers used as major components of the rate-controlling element.
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Polymer Science and Engineering: The Shifting Research Frontiers TABLE 2.1 Examples of Currently Available Controlled-release Drug Delivery Systems Product Drug Delivery Route Major Rate-controlling Polymer Mechanism of Release Indications for Use Procardia XL® Nifedipine Oral Cellulose acetate Osmotic Hypertension and angina Duragesic® Fentanyl Transdermal Ethylene vinyl acetate Diffusional Chronic pain Proventil/Repetabs® Albuterol Oral Acacia/carnauba wax Erosional Asthma Estraderm® Estradiol Transdermal Ethylene vinyl acetate Diffusional Hormone replacement Norplant® Levonorgestrel Implant Silicone rubber Diffusional Contraception Catapres-TTS® Clonidine Transdermal Polypropylene Diffusional Hypertension Zoladex® Goserelin Implant Polylactide-glycolide Erosional Prostate cancer SOURCE: Compilation of information from Physicians' Desk Reference (1994).
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Polymer Science and Engineering: The Shifting Research Frontiers Biological Polymers Biopolymers Versus Synthetic Polymers The volume of biopolymers in the world far exceeds that of synthetic macromolecules. Biological polymers include DNA, RNA, proteins, carbohydrates, and lipids. 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. For RNA and DNA, there are 4 different monomers (beads of different colors). 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: polymers can be synthesized as homopolymers, chains composed of only a single type of monomer, or simple block copolymers, where the monomers repeat only in the simplest patterns, AAA BBB AAA BBB, or random 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
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Polymer Science and Engineering: The Shifting Research Frontiers industry and then undergo further chemical modification in order to impart necessary properties to the device. 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. Often composites of these polymers with piezoelectric ceramics are used to provide enhanced performance. Polymers that emit light when exposed to ionizing radiation or high-energy particles are used as the active elements in radiation detectors (scintillation detectors). These polymer systems have advantages over liquid scintillation detectors because of their ease of fabrication and ruggedness with comparable sensitivity. Resist Materials For the last decade, the microelectronics industry has been engaged in a race to shrink the dimensions of semiconductor devices. The result of this effort is the continued improvement in the price-to-performance ratio of microelectronic devices and the myriad products that are produced from them (see the vignette "Resists and Micromachines"). The market for silicon hardware will exceed
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Polymer Science and Engineering: The Shifting Research Frontiers RESISTS AND MICROMACHINES Imagine a tiny robot—a micromachine—the size of a red blood cell, swimming through the arteries of a stroke victim until it reaches the blood clot in the victim's brain. The micromachine drills through the clot, restoring blood flow. The parts for such robots might one day be built using the same polymers that are used to stencil the incredibly complex pattern of an integrated circuit onto a silicon chip. These polymers, called resists, react chemically when exposed to ultraviolet light, X-rays, or other energetic electromagnetic radiation. One polymer commonly used as a resist, poly(methyl methacrylate), is better known to most people as Plexiglas. Tiny gears, for example, have already been made. The process starts with a blank wafer of silicon, to which a thin layer of titanium has been applied as a sort of frosting. A layer of resist, as thick as the gear is supposed to be (usually about a few microns thick, or much less than the thickness of a human hair), is applied to the titanium surface. The wafer is then bombarded with X-rays that have passed through a gold mask with many gear-shaped holes cut in it. Wherever the X-rays hit the wafer, the resist molecules become soluble. Wherever the wafer is shielded by the mask, the resist does not react and remains insoluble. Washing the wafer in the solvent mixture leaves a gear-shaped hole in the resist to use as a form. The form is filled by electroplating copper into it—the titanium layer on the wafer is connected to a negatively charged electrode, and the wafer is immersed in a solution of copper ions. The copper deposits itself on the exposed titanium, but not on the resist, which does not conduct electricity. Once the resist is removed by an aggressive solvent or oxygen plasma, the free-standing copper gear remains on the titanium. Dunking the wafer, gears and all, into a bath of hydrofluoric acid dissolves the surface titanium, freeing the gears from the wafer. Employing the pattern-making capability of polymer resists, it is feasible to make metal gears of the order of 1 micrometer in diameter. Living cells are tens of micrometers across, and a small blood vessel is about 50 micrometers in diameter. Thus, fairly elaborate machines appear possible, although other parts and assembly will require a great more development effort. $100B and the market for systems based on silicon will exceed $1,000B in the next few years, opening a highly competitive international market (Sze, 1988). The processes employed to manufacture the silicon hardware are intrinsically dependent on the polymeric materials that are used to define the patterns required for the many layers of circuitry. In a typical process, several hundred steps are required to produce a wafer containing hundreds of chips. About two-thirds of these steps are devoted to pattern formation, a form of lithography. In the process as practiced by the semiconductor industry, the silicon wafer on which the devices (e.g., the individual transistors and other elements) are to be fabricated is coated with a thin film of a material called a resist. Pattern-wise exposure of the resist to radiation of the appropriate wavelength results in a radiation-induced chemical reaction in the resist film, which renders the exposed areas more soluble in some developer solvent (positive tone imaging) or less soluble (negative tone imaging). The pattern is formed by passing the radiation
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Polymer Science and Engineering: The Shifting Research Frontiers through a mask, which is similar to a stencil, which blocks the radiation in areas in which no reaction is desired. The result is a relief image consisting of regions of resist and regions of bare circuit. These relief images in the resist allow the underlying substrate to be processed selectively in those areas where the resist has been removed. The processes involved include etching, metal deposition, ion implantation, and oxidation of silicon. Virtually all production of semiconductor devices is accomplished by exposing the resist film to UV radiation through a projection lens system analogous to the familiar slide projector, although exposure tools shrink the projected image rather than expand the image of the slide. Present systems employ UV radiation with a wavelength of 365 nanometers (nm), but 248-nm systems are being introduced. Electron beam and X-ray radiation offer alternatives for the future. Each change in wavelength and radiation type requires development of new polymeric resist materials. Owing to the high cost of the exposure tools, it is important that the throughput of the machines (e.g., the rate at which they can produce exposed wafers) be as high as possible. The amount of light available at 248 nm is only one-tenth that provided by the older machines operating in the near UV. Therefore, the feasibility of moving to deep UV was entirely dependent on the ability of chemists to develop new generations of polymeric resists that are as much as 100 times as sensitive as resists formerly used. These new resists derive their high sensitivity from exploitation of an acid-catalyzed reaction that converts an insoluble moiety to one that is soluble. Exposure converts a neutral substance into an acid, thereby generating a latent image of the mask. The resulting films are then baked to provide the activation energy necessary to start the catalytic reaction in which the acid generated upon exposure facilitates a reaction of the resist polymer (i.e., at a side group) to convert it to a form that is soluble in the developer. The radiation-created catalyst can convert many polymer groups, giving rise to the "chemical amplification" made necessary by the scarcity of deep-UV photons. Although there are many other factors involved in moving from near-to deep-UV lithography, both chemical and other, development of the chemically amplified polymeric resists was an essential contribution. As the lateral dimensions of devices shrink, the width of the resist images required to define their component structures must shrink also. The thickness of the resist film does not shrink, however, owing to the necessity of being pinhole free and robust in subsequent processing (e.g., ion implantation or etching). Thus, the aspect ratio of the relief structure is increasing and could be as high as five by the end of the decade. This is a very demanding requirement that will require a significant advance in resist technology. One promising approach to the production of high-aspect-ratio imaging at small dimensions is "top surface imaging." In this process the resist film is formulated to be opaque to the exposure radiation, and chemical transformation occurs only on the top surface of the resist. The transformation is designed to
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Polymer Science and Engineering: The Shifting Research Frontiers produce selective reactivity with an organometallic reagent such that only the exposed surface incorporates the organometallic. If silicon is incorporated, subsequent anisotropic oxygen etching of the film results in rapid formation of a thin layer of silicon dioxide in the areas that reacted with the reagent. This thin oxide layer protects the polymer beneath while the unexposed, unprotected polymer areas are etched away by the oxygen plasma. The products of the etching are gaseous and are pumped away. The aspect ratio of the image produced by this process is dependent on the anisotropy of the oxygen etching process. Aspect ratios exceeding five in polymer relief images have been achieved by this method. Although many features of the top surface imaging procedure remain to be worked out, it is a promising method. Some solution must be found that will provide support for the research and development required to produce these materials that are so critical to the continued advance of semiconductor technology. Sematech, the U.S. (industry-government supported) consortium is investing at some level in resist development. A few U.S. companies are investing "in house" or in collaboration with resist vendors. It will be interesting to see how this conflict between the demand for small volumes of highly sophisticated specialty polymers and the high cost of developing such materials plays out over the next few years. Compact Disk Technology Compact disks have emerged as the dominant recording medium for the musical entertainment field. Information is recorded as a series of pits on radial or concentric tracks that extend from the inner to the outer diameter of the disk. The pits are typically 0.25 µm deep, 0.5 µm wide, and up to 3.5 µm long. The information is read by means of a laser beam that is reflected when it falls on the flat of the disk but is almost entirely deflected when it falls on a pit. This digital bit stream is converted to an analog signal to reproduce the music. The disk itself is made of a polymer by means of a process that is technically demanding and economical. The manufacturing sequence consists of encoding the pit pattern onto a glass master. This is accomplished by means of a lithographic process employing a polymer resist and an irradiating laser. The open areas thus formed are etched to form the pits. Nickel is then vacuum deposited, thickened, and formed into a negative "stamper." The stamper is then seated into a mold cavity, and CDs are produced by injection molding of polycarbonate or poly(methyl methacrylate). The molding process is carried out at very high pressures, and dust particles must be avoided. A class-10 cleanroom is usually required to achieve quality replication and long stamper life. Molded-in stress is a major consideration. When polycarbonate flows in the submillimeter thickness of optical disks for several centimeters, there is significant molecular orientation that manifests itself as birefringence or optical distortion.
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Polymer Science and Engineering: The Shifting Research Frontiers Material and process parameters have been refined to control birefringence and maintain replication integrity. New long flow grades of the materials have been developed specifically for the CD market. Polymeric Materials for Photonics Photonics is a technology analogous to electronics in which the photon replaces the electron as the working particle. Many of the applications now accomplished electronically, including transmission, switching, amplification, and modulation, can also be realized using photonics, and there are advantages to be gained by converting to a photon-based technology in some areas. Transmission of light in fiber-optic systems is the direct analogy of electrical transmission in coaxial cable systems. Fiber-optic systems are now in place all over the world, and they handle much of the world's long-distance telephone traffic. The transmission medium of the fibers employed is based on inorganic glasses, but polymers are used for protective coatings and in cabling structures. Polymers can also be made into optical fibers, but the loss is considerably larger than with the inorganic fibers and only short-distance applications are realistic. The main advantage of polymer fibers is their flexibility when made in larger diameters, which are easier to splice. Today, fiber-optic cables are generally terminated at the area substation level, where the optical signal is converted back to an electrical signal for transmission to the customer. This conversion process is necessary because the optical components needed to reach the individual telephone or terminal are not available at sufficiently low cost at this time. What is required to allow fiber to be connected to the home are inexpensive optical switches and amplifiers, which will enable the advantages of broad-band communications to be brought to every subscriber. Polymeric organic materials will play a major role in the realization of optical technology as fiber to the home becomes a reality. Two kinds of optical technology need to be developed and commercialized before the photonics revolution can be fully realized. The first is linear optical technology, which includes not only the long-distance fibers mentioned above, but also shorter fibers and the optical equivalent of printed wiring boards of the electrical domain. These optical circuits can be created today by means of a photolithographic procedure in which lines of high refractive index are formed in thin polymer films by photochemical techniques. The circuit pattern is defined by irradiating a photoresist through a mask. The substrate film bared by development of the resist is then exposed to light, which causes the chemistry, for example, the polymerization of monomers, that gives rise to the increase in refractive index needed to form an optical guide. Nonlinear optical materials will also be required for the manufacture of switches, modulators, and amplifiers, and this technology has not progressed as far as the linear domain. Demonstration-of-principle devices have been fabricated
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Polymer Science and Engineering: The Shifting Research Frontiers that prove the viability of polymers in this application. The necessary switches, amplifiers, and modulators can be made today with inorganic materials, but there is some question whether these realizations can be combined and manufactured in large volume at sufficiently low cost. Organic thin film technologies may fit the economic as well as the technological requirements, but many advances will be required and the outcome is uncertain. A nonlinear polymer in general has two components: the polymer itself and an optically nonlinear molecule (a chromophore) that is either chemically attached to the polymer or dissolved in it. In order for the polymer-chromophore system to be optically nonlinear, the chromophores must be aligned such that on average they are all pointing in the same direction within the polymer matrix. This alignment is accomplished through a process called poling. The polymer is poled by cooling it through the glass transition temperature while it is in a very strong electric field, and the order induced by the field is frozen in. Poled polymeric systems have process and property advantages over their inorganic crystalline competitors. The polymers can be formed into thin films and lithographically patterned, and they can be chemically modified to tailor and improve bulk properties. There are disadvantages as well, in that the orientation in the poled polymer systems tends to decay with time, a problem that can probably be overcome. Polymeric Light-emitting Diodes Recently, light-emitting diodes (LEDs) based on conducting polymers have been achieved in a number of laboratories around the world. The active element is a thin film structure based on a modified poly(phenylene vinylene) (PPV), with a metal film as the electron injector and polyaniline as the hole injector. Various colors have been demonstrated, and the operating characteristics are competitive with inorganic LEDs. Highly flexible devices have been fabricated supported on a poly(ethylene terephthalate) base. The possibility of making large-area displays exists. Much research and development remains to be done. For example, low-work-function metals are required, and they are difficult to passivate. However, the simplicity of fabrication of the laboratory devices, involving spin casting from solution, is promising if the problem of limited device lifetime can be solved. Polymers for Electrophotography One of the major applications of polymers with tailored electronic and optical properties has been in electrophotography for copier, duplicator, and printer applications. In this application an electroactive polymer is used as one component of the light-sensitive element used for creating the latent electrostatic image
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Polymer Science and Engineering: The Shifting Research Frontiers of an original subject. The image source can be light reflected from a document and focused onto the surface of the photoreceptor or a digital file of an original image, which is used to control a laser beam that is scanned over the surface of the photoreceptor. The electrostatic image is rendered visible by dusting the surface of the photoreceptor with an electrostatic powder composed of a pigment-loaded thermoplastic polymer. The latent image can then be transferred to paper by a combination of pressure and electrical bias and then fused to the paper by heating. The photoreceptor itself was the key invention that enabled the development of electrophotography as a commercial success. The original photoreceptor materials were based on selenium and its alloys as well as group II-VI and other semiconductor materials. Because of the poor mechanical properties of selenium and its alloys, photoreceptors had to be fabricated on rigid metallic drums. This, in turn, dictated relatively cumbersome and expensive copier machine architectures. These materials had a number of shortcomings, including degradation of photoconductive properties, instabilities in surface properties leading to incomplete toner transfer, and catastrophic abrasion. Research efforts in several industrial and university research laboratories were successful in identifying polymeric materials that exhibited photoconductivity. The early photoconductive polymers were mainly sensitive to UV light. The copying process, however, requires differential reflectivity from the printed areas of the original document, which is very low for UV light but much higher for visible light sources. The need for visible light sensitivity was therefore apparent. Eventually, the problems associated with spectral sensitivity and a variety of other technological requirements were solved, and it was clear that the polymeric materials could be used advantageously in copier and printer technology. The latent electrostatic image is formed by first depositing a layer of ionic charge from a corona discharge onto the photoreceptor surface. This induces an equal but opposite charge on the metal layer below, resulting in the formation of an electric field within the photoconductor layers. As light passes through the transport layer and is absorbed by the photosensitive pigment layer, the pigment molecules are photoionized with the assistance of the internal electric field to form mobile charge carriers. The negative photogenerated charge in the film drifts under the influence of the electric field to the metal, and the positive charge drifts through the transport layer and neutralizes some of the ionic charge that was deposited on the surface. Since photogeneration will occur only where light strikes the photoreceptor, a pattern of ionic charge corresponding to the original image is formed on the surface. The surface potential associated with this charge distribution is used to attract the toner as described above. Photoreceptor belts have been engineered to exhibit excellent mechanical properties, and this achievement has allowed the design of compact and cost-effective copier and printer architectures. Useful lifetimes, photosensitivities,
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Polymer Science and Engineering: The Shifting Research Frontiers and mechanical durability of photoreceptors have been extended well beyond those of the original selenium-based drums. Photoreceptor wavelength sensitivities have now been extended to the near-infrared so that inexpensive diode lasers and light-emitting diode arrays can be used for digital printing applications. Polymers in Holography A light interference pattern comprising relatively large light intensity variations on a microscopic scale is created where two previously separate light beams from the same laser intersect. A hologram is a physical record of such a pattern and is formed by exposing a photosensitive recording film to the interference pattern. When a hologram is illuminated with one of the two laser beams used in its recording, it produces a light beam that is essentially identical to the other recording beam. Familiar image holograms are usually produced from a simple collimated or diverging light beam, called a reference beam, and a beam formed by scattering light from a complex three-dimensional solid object. When such a hologram is illuminated with the reference beam, it produces a light beam that appears to come from the solid object used in its recording. Holograms can also be made from light beams produced by conventional optical elements such as lenses and mirrors. The resulting holograms, called holographic optical elements (HOEs), perform optical functions of the elements used in their recording. One HOE type, for example, is recorded with a collimated light beam and a light beam that converges to a focal point. When illuminated with the collimated beam, the resulting hologram will produce a focused beam; it acts, therefore, as a holographic lens. HOEs have important advantages over conventional optical elements. They are lightweight and compact and can take the place of heavy and bulky glass elements. They can be made very large or very small. They can replace expensive conventional optics for the production of arbitrarily complex light beams. They can be inexpensively mass produced. There is a wide variety of current and potential applications for holography. The use of holographic three-dimensional images is probably the most familiar application. These images are typically used on credit cards and for product advertisement and promotion. In these applications, holograms add both eye appeal and security. Holographic images are also used in nondestructive testing. Holographic optical elements can be made in large thin films for use in solar lighting control and solar energy collection, and they can be made very small for use in optical communication systems. Narrow-band holographic mirrors may also be useful for laser eye protection. Optical computing, pattern recognition, and very-high-density information storage are other potential applications of holography. Many holographic applications require the high performance that is possible
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Polymer Science and Engineering: The Shifting Research Frontiers only with ''phase" holograms, in which the original interference pattern is recorded as a refractive index variation. Conventional phase hologram recording materials have, unfortunately, limitations that have inhibited the growth of practical holography. Phase hologram recording based on photopolymerization is a relatively recent development, and it promises to overcome important problems of current recording materials. Holographic photopolymer systems comprise, as major components, a film-forming polymer (often called the binder), a photoinitiation system, and a monomer. The polymer binder aids in coating the appropriate substrate and helps to maintain film integrity during holographic exposure and subsequent processing. Properties of the binder can also strongly influence both the shelf life of the coated film and the rates and extents of photochemical reactions that occur during hologram formation. Monomers join in a chain reaction during laser exposure. In fact, the relatively good light sensitivity of photopolymers results from the large number (100 to 1,000) of monomer units that react per absorbed photon. The chemical and physical changes associated with monomer polymerization preserve the interference pattern created during exposure as a corresponding pattern of refractive index variation. Numerous and diverse chemical and physical requirements greatly limit monomer choice. Shelf life and light sensitivity must be balanced. Film clarity and image stability are essential. Large refractive index changes are desirable. The monomer must also be compatible with the other components of the system. The ideal material does not yet exist. There is an excellent chance, however, that continued research with photopolymer systems will produce new holographic recording materials that will make practical many potential applications of holography. Conclusions The foregoing examples illustrate the breadth of application of advanced polymeric materials in applications that are not generally recognized. In these applications, the polymer is in some sense the active element that plays the central role. No other class of materials can rival its range of properties, flexibility in processing, and potential for low cost. Quality of performance is an essential and challenging feature that is being demonstrated by polymeric materials in an impressive array of applications. And the polymer revolution in this arena is just beginning. The importance of polymers in advanced technology is a key factor in the future of materials development, as indicated in the following applications: Polymer dielectrics in electronics offer the basis for the smallest circuits and the highest speed of operation.
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Polymer Science and Engineering: The Shifting Research Frontiers Conducting polymers have been commercialized in rechargeable batteries and offer the greatest promise for high energy storage with low weight. Polymer sensors exist for chemical species, thermal and acoustic radiation, temperature, pressure, humidity, ionizing radiation, electric charge, and more. Buildings can be equipped with a network of optical fibers linking remote locations with a management console. The polymer sensors can be built into the optical fibers to report the presence of toxic gases or to turn off unneeded lights to conserve energy. Implanted sensors can detect the glucose level in blood and call for insulin injections by means of an implanted pump, as needed. Electromagnetic shielding will become increasingly necessary, and conducting polymers offer solutions that are conveniently fabricated in complex shapes. Polymer resists are the basis for the microlithography that makes integrated circuit electronics possible. They are also the basis for the emerging field of micromechanics, which could produce machines smaller than a human cell. High-density information storage is available through compact disk technology, and improved polymers will improve the performance of this medium. In the future, polymer-based holographic devices could revolutionize the storage and manipulation of information. Polymers offer solutions to critical economic problems facing the introduction of photonics, the light analog of electronics. The couplers, splitters, and other elements of photonic "circuit boards" all admit to polymeric solutions that may provide the economic breakthrough needed for the photonic revolution. Broad-band communications can be brought directly to the home and office by polymer or glass fibers, using polymeric photonic circuits. "Smart" windows based on polymeric materials could reflect light when the sun is too bright and transmit light when it is not. The fabrication of liquid crystal display devices for computers and television can be facilitated and the robustness of the product enhanced by the incorporation of conductive, transparent polymer films. Light-emitting diodes based on flexible polymeric films have been fabricated and are likely to find diverse applications in the future. Electrophotography is now based on polymeric photoactive materials, and these have made possible many improvements, such as compact and convenient machine architecture, durability of machines, and long-term print quality. Polymers are now the recording medium of choice for holography in many applications. This technology offers the promise of ultrahigh-density information storage. The field is flourishing, and the future is bright. The United States must participate vigorously in this emerging area, from research to development to
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Polymer Science and Engineering: The Shifting Research Frontiers commercialization. Competition in the field is worldwide and moving rapidly ahead. REFERENCES Physicians' Desk Reference. 1994. 48th ed. Montvale, N.J.: Medical Economics Data. Sze, Simon. 1988. VLSI Technology. 2nd ed. New York: McGraw-Hill Book Co.
Representative terms from entire chapter: