America leads the world in the biosciences, thanks largely to almost 40 years of major support for fundamental research by the federal government and the absence of socialized medicine. This research into biotechnology provides the basis for revolutions in health care, agriculture, food processing, environmental improvement, and natural resource utilitization. The new technologies that continue to be made possible by advances in the biosciences, and particularly in microbiology, are being applied to the search for solutions to some of the world's most pressing problems. These technologies are, in addition, creating new industries and spurring economic growth within the United States. The areas in which biotechnology has had a significant impact include (1) medical products such as pharmaceuticals and diagnostics; (2) chemicals, including both fine or specialty chemicals and commodity chemicals; (3) agricultural products such as chemicals and biologicals, plants and seeds, and improved animal breeds; and (4) food and animal feed products, including additives and supplements, and flavors and fragrances. The commercialization of developments in biotechnology has been a tremendous opportunity for chemical engineers, because they bring their traditional process engineering principles to bear on problems directly associated with the life sciences. As with other career specializations in chemical engineering, this particular focus on biology and engineering has led to the creation of new disciplines within chemical engineering. Through additional coursework and research in physiology, biochemistry, and microbiology, this type of chemical engineer has the capability to bring innovative and economic solutions to problems in health care delivery and in the large-scale implementation of advances in molecular biology. Bio-oriented chemical engineers focus on areas ranging from molecular and cellular biological systems (a discipline known as biochemical engineering) to organ and whole-body systems (a discipline known as biomedical engineering, to be discussed in the next topic). Biochemical engineers are concerned on the engineering problems associated with adapting "new" biology to the commercial production of therapeutic, diagnostic, chemical, or food products. Biomedical engineers apply the tools of chemical engineering modeling and analysis to study the function and response of organs and body systems; to explain the mechanism(s) for the transport of substances within the body, and to design artificial organs, artificial tissues, and prostheses. In this section we'll discuss two very broad areas in which biochemical engineers contribute significantly to the commerical application of microbiological principles: human health and biochemical synthesis. As we will soon see, these two areas blur together when synthesis techniques are applied to the production of compounds of importance to human health, but they can be discussed separately because of their origins and impact. It's also important to note that the lines between biochemical and biomedical engineering are less clearly defined when principles of biochemical engineering are applied to address issues of concern to biomedical engineers. For example, in the general area of diagnostic devices, the biochemical engineer may utilize an antibody to design a system that monitors the level of a specific compound in the body, but it is the biomedical engineer who often determines how the body produced that compound and why a particular antibody is sensitive to its presence. While fundamental discoveries in these areas are typically made by natural scientists—biochemists, microbiologists, chemists—or medical or veterinary researchers, it is the chemical engineer who plays a major role in transforming the basic research results into practical products. Chemical engineers have been instrumental in designing processes for the safe and economical production of extremely complex therapeutic and diagnostic agents (e.g., insulin and hepatitis-B surface antigen), food products (e.g., high-fructose corn syrup), or chemicals (e.g., ethanol).

An important area of human health in which chemical engineers play a role is the design and manufacture of diagnostic systems and devices. Molecular biologists have discovered or created a variety of enzymes and monoclonal antibodies that are capable of detecting a wide range of diseases, disorders, and genetic defects. Chemical engineers then work to incorporate these materials into devices and systems that are fast, inexpensive, accurate, and not susceptible to error on the part of the person carrying out the test. For example, although an enzyme-linked immunosorbant assay (ELISA) exists for detecting the antibodies to cytomegalovirus (CMV) in blood samples, it cannot be reliably used in practice to follow the course of a new CMV infection. The error introduced into the test by having different operators perform it on each new blood sample in the series is sufficient to render highly questionable the interpretation of trends in the series, particularly if changes in the magnitude of the result are small. It is important to be able to follow trends in CMV antibodies because CMV infections can be life-threatening to individuals with compromised immune systems, and congenital CMV infections are the single largest cause of birth defects. Chemical engineering research leading to the design of devices and systems that are fast and "accurate" includes the following: development of selectively adsorbent, functionalized porous media to which immunoreagents can be affixed and that are amenable to speedy optical assay after contact with body fluids; design of fluid-containing substrates that allow small volumes of test fluids to contact reagents efficiently and with highly reproducible assay response; and design of flexible manufacturing systems to make the wide variety of expensive monoclonal antibodies needed for diagnostic test kits. Chemical engineers at several pharmaceutical firms are using hollow fiber reactors to grow monoclonal antibody-producing hybridomas in an in vitro batch process. Research on reactor design to optimize the production of monoclonal antibodies is having a significant impact on the future development, economy, and use of diagnostic tests. What is a monoclonal antibody? The specific immune system (in other words, the sum total of all the lymphocytes, which are the cells responsible for the body's ability to distinguish and react to foreign substances) can recognize virtually any complex molecule that nature or science has devised. The total number of different receptors available on all of the lymphocytes cannot be measured, but most informed estimates place the total number at a minimum of at least 100,000,000. After a lymphocyte encounters an antigen that it can recognize, the cell is stimulated to multiply, and the population of lymphocytes bearing that particular receptor increases. It has been observed that, when one type of lymphocyte, known as a "B cell", is activated by an antigen, the B cell multiplies to form a clone of plasma cells, each secreting the identical immunoglobulin, i.e., antibody. It is such an immunoglobulin—derived from the descendants of a single B cell—that constitutes a monoclonal antibody. Immunoglobulins all have the same basic molecular structure, consisting of four polypeptide chains (i.e., chains of amino acids linked together by chemical bonds known as peptide bonds). Two of the chains, which are identical in any given immunoglobulin molecule, are heavy (H) chains; the other two are identical light (L) chains. The terms "heavy" and "light" simply mean larger and smaller, but they were used in early studies of immunoglobulin structure and have stuck. Each chain is manufactured separately and is coded for by different genes, but the four chains become joined in the final immunoglobulin molecule to form a flexible Y-shape as illustrated below. The antibody response to a natural infection or an active immunization, however, is polyclonal. In other words, it involves many B-cell clones, each of which recognizes a different antigenic determinant and secretes a different immunoglobulin. There is, however, a condition in which the blood serum may contain an astonishingly high concentration of a single immunoglobulin. This results from multiple myeloma, a type of cancer in which a single B cell proliferates to form a tumorous clone of antibody-secreting cells. The immunoglobulins made by myelomas are monoclonal, but although they must be antibodies capable of combining with some antigen, there is usually nothing to indicate what this antigen might be. Myelomas are not uncommon in species other than humans, especially in mice and rats, and they can be made to occur frequently in certain laboratory strains by injecting the experimental animal with mineral oil, which acts as a mild irritant and causes B-cell proliferation. It is possible to obtain hybrid cells, termed hybridomas, that grow like a myeloma but make a chosen, identifiable monoclonal antibody. Thanks to hybridomas, researchers can obtain monoclonal antibodies that recognize individual antigenic sites on almost any molecule, ranging from drugs and hormones to microbial antigens and cell receptors. The exquisite specificity of monoclonal antibodies and their availability in quantity has made it possible to devise sensitive assays for an enormous range of biologically important substances and to distinguish cells from one another by identifying previously unknown marker molecules on their surfaces. For example, monoclonal antibodies that react with cancer antigens can be used to identify cancer cells in tissue samples. Moreover, if short-lived radioactive atoms are added to these antibodies and they are then administered in tiny quantities to a patient, they become attached exclusively to the cancer tissue. By means of instruments that detect the radioactivity, physicians can locate the cancerous sites without surgical intervention. Monoclonal antibodies have also been used experimentally to deliver cytotoxic drugs or radiation to cancer cells. Although the preparation of monoclonal antibodies with hybridomas derived from rat or mouse cells has become routine practice, it has not proved so easy to obtain human hybridomas. This is partly because most human myeloma cells do not grow well in culture, and those that do so have not produced stable hybridomas. If, however, human B cells isolated from blood are infected by the Epstein-Barr virus (the agent that causes infectious mononucleosis), they can be propagated in culture and continue to secrete immunoglobulin. Very few of them are likely to be making an antibody with a desired specificity, even in a subject who has been immunized; but in some instances immunologists have succeeded in identifying and selecting those cells that secrete the wanted immunoglobulin. These can be grown in culture as single clones that secrete a monoclonal antibody. Researchers have used this process to obtain human monoclonal antibodies against the Rh antigen.

The biological activity of many of the next generation of compounds needed to prevent disease (e.g., vaccines) or to cure it (e.g., drugs) depends on precisely designed three-dimensional configurations. These configurations can be most easily created by synthesizing the compounds biologically or from biologically derived precursors, using cells that have been altered through recombinant DNA techniques. The manufacture of these compounds, examples of which are listed in the table below, involve new challenges for chemical engineers. For processes involving bacteria or yeast as product sources, the manufacture of molecules with the correct three-dimensional configuration can require additional steps to modify or retold the proteins. Processes involving plant and mammalian tissue cultures as product sources will require new types of reactors capable of growing the specialized cells, control procedures and sensors tailored for biological processing, and extremely special and gentle purification procedures to ensure that products of adequate purity can be produced without chemical change or loss of configuration. These are formidable engineering problems. Chemical engineers, long involved in the manufacture of antibiotics, peptides, and simple proteins, have significant experience to apply to these problems. Therapeutic agent Antigens Interferons Tissue plasminogen activators Human growth hormone Neuroactive peptides Regulatory peptides Lymphokines Human serum albumin Gamma globulin Antihemophilic factors Monoclonal antibodies Action Stimulate antibody response Regulate cellular response to viral infection    and cancer proliferation Stop thrombosis by dissolving blood clots. Reverse hypopituitarism in children Mimic the body's pain-controlling peptides Stimulate regrowth of bone and cartilage Modulate immune reactions Treat physical trauma Prevent infections Treat hereditary bleeding disorders Provide site-specific diagnostics    and drug delivery Providing new modes of delivering drugs presents almost as important an opportunity as providing new ways of making them. The standard practice of periodically administering drug doses can lead to initial concentrations in the body that may be sufficiently high to induce undesirable side effects. And later, as the drug is metabolized or eliminated, its concentration can drop below the effective level. This effect is illustrated in the simple figure below.

When a medicine table is taken or an injection given, sharp fluctuations of drug levels in the body can result. Chemical engineers are working on ways to deliver drugs that maintain a steady, effective level of the drug in the body.

This problem is particularly important with drugs that are metabolized or eliminated rapidly from the body and with drugs that have a narrow therapeutic range (the span between the therapeutically effective and the toxic concentrations). The optimal pharmacological effect can sometimes be attained by establishing and maintaining a steady-state concentration of the drug or by time-sequencing its administration. The controlled release of short-half-life drugs over a long period of time can be effected by administering the drug through low-flow pumps, as a mixture of capsules that disintegrate at different rates, or in pouches inserted under the eyelid or taped to the skin. Transdermal (through the skin) are currently used to deliver a number of different drugs, including nitroglycerin, which helps prevent angina pain in the heart. Chemical engineers have been instrumental in designing and manufacturing polymers that are capable of such controlled release over long periods of time. Another approach to delivering drugs is to target the administration of a drug to a specific site in the body. This might be accomplished by coupling a drug to an antibody that has been cloned to attack a specific receptor at the disease site. This approach would make possible, for example, the selective exposure of tumor-bearing tissues to high concentrations of toxic drugs. Chemical engineers help to produce such targeted drugs and to explain the kinetics (rates) of monoclonal antibody transport through the body to target sites. Other areas in therapeutics that provide opportunities for interdisciplinary collaboration include the design of special-purpose pumps and catheters, sterile implants that allow access from outside the body to veins and body organs, and imaging techniques for monitoring drug levels. Efforts by chemical engineers to provide improved data acquisition and quantitative modeling of pharmacokinetics can lead to the design of better drug administration procedures and better timing to maximize the delivery of drugs to the organs that need them while minimizing the exposure of other organs. What is modeling? Modeling is the use of mathematical equations to describe the physical and/or chemical behavior of a system. The intent is that a model will accurately predict how a system will repond to a specified set of inputs, obviating the need to carry out an experiment to measure that same response. Whenever possible, models are based on first-principles approaches, which means that the equations governing a system's behavior are derived from a fundamental standpoint. The work of theoretical physicists, chemists, biologists, and engineers this past half-century has led to numerous first-principles models that describe everything from how electrons distribute themselves in an atom to how a fish swims to how the universe expands (or doesn't, depending on your favorite physicist). When a fundamental basis is absent or rather, the system is too complex to derive a governing set of equations, we often rely on empiricism to fill in gaps. For example, the rate at which a bacterium metabolizes a specific pollutant may depend on a large sequence of events too complicated for us to unravel at this time, but what we observe in practice is that the rate is proportional to the sqare of the pollutant's concentration. So, in that case we have "derived" a mathematical expression relating the desired quantity (rate) to the variable on which it depends (concentration). A model equation can be as simple as which says that the force F acting on a body is equal to its mass m times its acceleration a. Or, as complicated as the equation known as the Shrödinger equation, which describes how sub-atomic particles behave in quantum mechanics. Whether a model of a system is first-principles or empirical, whether it's one simple equation or a large system of hideously complicated equations, we can take advantage of the predictive capability of that model to help explain the unknown and to explore the predicted response of a system to different operating conditions. Chemical engineers have helped develop and commercialize a large number of compounds used for preventing and curing diseases. Here we'll look at two examples, one a blood clot dissolving agent cloned from mammalian cells, and the other a cancer-fighting drug produced in plant cell cultures. tPA Many serious health problems result from abnormally located blood clots: heart attacks (clots in coronary arteries), pulmonary embolism (clots in the lungs), and peripheral arterial occlusion and deep vein thrombosis (clots in the limbs). Each year heart attacks afflict over a million people in the United States, and almost half of them die as a direct result. In the past, only two treatments have been available for breaking down blood clots: streptokinase and urokinase. Both treatments lack specificity for clots, so they can cuase a general breakdown of the hemostatic system, sometimes leading to generalized bleeding. Just over ten years ago a superior therapy was approved for use by the federal government, tissue-type plasminogen activator (tPA). This naturally occurring enzyme dissolves blood clots as part of the normal healing process. By administering relatively large quantities of this compound, clot breakdown time can be shortened from about a week to under an hour. Normal circulating levels of tPA are very low, so that to accomplish this dramatic clot breakdown one would need the amount of tPA contained in 50,000 liters of blood. This is clearly not practical. Instead, the tPA molecule has been cloned and expressed in mammalian cells so that it can be produced in quantity. Using cells from mammals, rather than bacteria, results in a product molecule that has the same folding, internal bonding, and cot of sugar residues as the natural tPA protein. Producing the many pounds of tPA necessary to satisfy the world's therapeutic needs requires the special skills possessed by today's biochemical engineers. Sophisticated engineering of the fermentation vessels, culturing conditions, and media compositions is required to culture thousands of gallons of mammalian cells. In addition, new extremes of purity must be achieved to assure the safety of progeins derived from mammalian cells. The cost of the starting materials and the capacity constraints of the equipment require that yields from each fermentation be as high as possible. Cost is still a key factor in the acceptance of tPA in hospitals and clinics. Continued research is necessary to find more economical processes for production of this material. What is fermentation? Fermentation is invariably associated with the use of yeast to convert sugar to ethanol, but the process is much broader and more inclusive that this. Fermentation now denotes the enzyme-catalyzed, energy-yielding pathway in cells by which fuel molecules such as glucose are broken down anaerobically (in the absence of oxygen). In most cells the enzymes occur in the soluble portion of the cytoplasm. The reactions leading to the formation of pyruvate thus are common to sugar transformation in muscle, yeasts, some bacteria, and plants. One product of the pathway is always the energy-rich compound adenosine triphosphate (atp). The other product, pyruvate, can undergo various transformations, depending on the cell type and the availability of oxygen. Industrial fermentation processes begin with suitable microorganisms and specified conditions; e.g., careful adjustment of nutrient concentration. The products are of many types: alcohol, glycerol, and carbon dioxide from yeast fermentation of various sugars; butyl alcohol, acetone, lactic acid, monosodium glutamate, and acetic acid from various bacteria; citric acid, gluconic acid, and small amounts of antibiotics, vitamin B12, and riboflavin (vitamin B2) from mold fermentation. An enzyme is a substance that acts as a catalyst in living organisms, regulating the rate at which chemical reactions proceed without itself being altered in the process. The biological processes that occur within all living organisms are chemical reactions, and most are regulated by enzymes. Without enzymes, many of these reactions would not take place at a perceptible rate. Enzymes catalyze all aspects of cell metabolism. This includes the digestion of food, in which large nutrient molecules (such as proteins, carbohydrates, and fats) are broken down into smaller molecules; the conservation and transformation of chemical energy; and the construction of cellular macromolecules from smaller precursors. Many inherited human diseases, such as albinism, result from a deficiency of a particular enzyme. Taxol The compound taxol is regarded as a promising treatment for a variety of cancers, including those of the ovary, breast, and lung. Taxol occurs naturally in one plant, in the bark of the Pacific yew tree. The Pacific yew is a slow-growing plant found only in old-growth forests of the Pacific Northwest. To treat one cancer patient requires 60 pounds of yew tree bark, the equivalent of three, one hundred year old trees. At first, obtaining taxol in quantity had been expected to require the cutting and processing of thousands of trees, leading to concern about destruction of yew forests. The shortage in supply set off a worldwide race among organic chemists to obtain the molecule from other sources, and also among microbiologists to determine the synthesis pathway of taxol within the yew tree. The taxol molecule, shown below, is large and complex, built from an unusual system of four rings extremely difficult to re-create in the laboratory. But knowing the route to its formation within the cells of the yew tree would possibly enable production of that compound using biosynthesis. The successful synthesis of taxol by two groups of organic chemists in 1994 has had no impact on the commercial supply of this drug, which is no longer scarce thanks to the efforts of the microbiology and chemical engineering communities. Taxol is now being made in a semisynthetic process from chemical precursors collected from yew needles and twigs, which can be harvested without killing trees. The taxol-producing cells are extracted from the needles and twigs, and are grown under carefully controlled conditions. These plant cell cultures may be grown in individual containers, such as the glass tubes shown below, or in larger vat-like systems. Plant cell tissue culture offers numerous advantages over field-grown material including reliable production from a renewable source and the possibility of enhanced productivity due to metabolic manipulation through strategies as precursor feeding, elicitation of enzyme systems, or metabolic engineering. Cell culture of plants often takes place in individual containers cared for by hand. Chemical engineering processing techniques have been used to greatly increase the size of the culture systems and to automate the process.

By manipulating the genetic machinery of the cell, it is possible to cause most cellular systems to produce many different chemicals and virtually any biochemical material. Unfortunately, the growth of cellular systems—particularly in tissue cultures—is constrained by end-product inhibition and repression. In other words, as more of the desired product is produced by biochemical reaction pathways, the rate of production decreases until the effective rate is close to zero. Further, in some cases the end product is toxic to the cells producing it, so its final concentration must be kept below a threshold. For these reasons it is difficult to produce end products in high concentration. Additionally, cells are always grown in aqueous solution, so biochemicals produced by cellular routes must have intrinsically high value in order for the cost of recovery from dilute aqueous solution to be minimized. Thus, most biochemicals of commercial interest to be produced by biotechnology will be high-value products such as enzymes, biopolymers, or metabolic cofactors. In general, their potency is so high that only small quantities will be needed. Accordingly, the challenge to chemical engineers in producing these products is not so much in process scale-up but rather in obtaining high process yield and minimal process losses. Enzymes are an important class of biochemicals; they are the catalysts needed in the chemical reaction cycles of living systems, and they execute their catalytic role with exquisite chemical precision. Enzymes have great potential in synthetic chemistry because they can effect stereospecific reactions, avoiding the production of an unwanted isomer of a complex molecule. (Isomers are substances that have identical molecular formulas but different molecular structures or configurations, and hence different properties. The substances that make up a set of isomers differ only in the arrangement of their component atoms. The most common type of stereoisomer is two molecules that are the mirror image of one another.) Currently, many of the enzymes used in industrial processing (e.g., those used to convert starch into sugar or milk into cheese) are derived from microbial sources because they are beyond the practical reach of current synthetic chemical technology. Biotechnology offers the potential, through cellular genetic control, for making enzymes—not only those that are now used industrially but also others for new uses in synthetic chemistry. The synthesis and processing of these complex molecules require conditions that will maintain their specific three-dimensional structures. One challenge for chemical engineers will be to develop processes that can meet the rigorous requirements for optimally producing and recovering enzymes. Another challenge is to understand the chemical transformations that enzymes catalyze. The goal is to determine how these transformations can be used or tailored through changes in enzyme structure to produce compounds that are difficult or costly to produce by traditional or synthetic chemistry. Addressing these challenges has brought the chemical engineer into close contact with biochemists and synthetic chemists. By no means all biochemical synthesis processes are used for the production of small quantities of high-value products, however. As shown in the table below, different microorganisms will produce different chemical compounds from the fermentation of glucose. One such process is the production of ethanol (C2H5OH) from corn silk using an anaerobic bacterial fermentation process. Another commercial process involving biotechnology is the production of high-fructose corn syrup. Today's sugared sodas—Coke, Pepsi, Sprite, etc.—are sweetened with high-fructose corn syrup (HFCS), a substitute for the expensive natural sugar that is extracted from sugar cane and beets. HFCS, produced by an enzymatic reaction, is an example of the successful application of chemical engineering principles to biochemical synthesis. So successful in fact, that the sale of HFCS is a multi-billion dollar business in the United States each year. To make HFCS a commerical reality two separate bioprocesses had to be developed, scaled up in size, and brought on line in a manufacturing plant. The first bioprocess was a fermentation to manufacture the necessary enzyme. The second process used that enzyme to convert dextrose (also known as glucose) to HFCS. The early involvement of chemical engineers in the design of these processes and their interaction with biologists, was a key to the success of these two endeavors. The fermentation for making isomerase enzyme is relatively fast and can be carried out in a number of process configurations. Basic to all of these configurations are the problems of maintaining sterility and containment; engineering heat and mass transfer; controlling levels of oxygen and carbon dioxide in the fermentation solution; and regulating temperature, pressure, and concentrations of dextrose. The solution must be mixed carefully during fermentation because damage to the cells by agitation can either mechanically kill the microorganisms that produce the enzyme or complicate the recovery of the enzyme from the fermentation broth. The conversion of dextrose syrups to HFCS requires two additional steps. The first is the rigorous purification of the dextrose to remove any contaminants that could inactivate the isomerase enzyme. Once purified in a six-step process the dextrose is passed down a column containing the isomerase enzyme isolated on a carrier. Enzyme concentrations ("loadings") of over 10 million active units of enzyme per cubic foot are common in this process. The isomerization process can be conducted in either of two ways, depending on demand. During periods of high demand dextrose is passed through the column as quickly as possible, resulting in higher column temperature and shorter enzyme life. The cost of having to generate new enzyme is offset by increased throughput. When demand is lower, the dextrose is passed through the column more slowly, resulting in longer enzyme life. The record time in which HFCS was developed and brought to high levels of production and sales is testament to the power and versatility of chemical engineering principles. Chemical Ethanol Butanol Adipic acid Methyl ethyl ketone Glycerol Citric acid Microorganism(s) Saccharomyces cerevisiae    Zymomonas mobilis Clostridium acetobutylicum Pseudomonas Klebsiella pneumoniae Saccharomyces cerevisiae    Dunaliella Aspergillus niger

The challenges for chemical engineers in biotechnology can be described on a continuum from microscale through mesocale to macroscale. At either end of this spectrum are highly interdisciplinary research topics that require modeling and analytical tools currently used by chemical engineers in other contexts. The important mesoseale challenges of bioprocessing will require chemical engineering expertise in reaction engineering, process design and control, and separations. Models for fundamental biological interactions The living microbial, animal, or plant cell can be viewed as a chemical plant of microscopic size. It can extract raw materials from its environment and use them to replicate itself as well as to synthesize myriad valuable products that can be stored in the cell or excreted. This microscopic chemical plant contains its own power station, which operates with admirably high efficiency. It also contains its own sophisticated control system, which maintains appropriate balances of mass and energy fluxes through the links of its internal reaction network. Cell membranes are not simply passive containers for the cell's contents. Rather, they are highly organized, dynamic, and structurally complex biological systems that regulate the transfer of specific chemicals through the cell wall. One important constituent of cell membranes is a class of molecules—the phospholipids—that spontaneously form two-layer films in a number of geometries. Many of the important physical properties of cell membranes, such as two-dimensional diffusion and differentiation between the inside and the outside of a tube or sphere, can be studied with these spontaneously formed structures. We can develop accurate quantitative models that simulate how cells respond to various environmental changes, we can better utilize the chemical synthesis capabilities of cells. Steps toward this goal are being taken. Models of the common gut bacterium Escherichia coli have been developed from mechanisms of sub-cellular processes discovered or postulated by molecular biologists. These models have progressed to the point where they can be used with experiments to discriminate among postulated mechanisms for control of subcellular processes. Some of the most promising potential applications of biotechnology involve animal or plant cells. Models for these organisms, which have greater internal complexity as well as more demanding environmental requirements than simple cells, are not yet available. It will probably be necessary to incorporate the structure of functional subunits of the cell (organelles) into models for complex cells in addition to the chemical structure that is used in bacterial cell models. Cellular reactions are subject to the limitations imposed by the laws of thermodynamics, by diffusion, and by reaction kinetics. Chemical engineers are familiar with the techniques for designing mathematical models that involve these parameters and should be able to make major contributions to the development of cellular models. The development of reliable models hinges on acquiring accurate data bases on enzymes, biologically important proteins, and cellular systems. The data should include physical properties, transport properties, chemical properties, and reaction rate information. Biological surfaces and interfaces Many biological reactions and processes occur at phase boundaries and are thus controlled by surface interactions. Examples include such highly efficient processes as selective transport of ions across membranes, antibody-antigen interactions, cellular protein synthesis, and nerve impulse transmission. Progress in achieving similar efficiencies in engineered enzyme processes, bioseparations, and information transmission can be aided by acquiring more sophisticated knowledge of biochemical processes at interfaces. With this knowledge, such products as synthetic antibodies for human and animal antigens, or synthetic membranes that can serve as artificial red blood cells or transport barriers, can be developed. Surface interactions play an important role in the ability of certain animal cells to grow and produce the desired bioproducts. An understanding of the dynamics of cell surface interactions in these "anchorage-dependent" cells (cells that function well only when attached to a surface) are needed, for example, to improve the design of bioreactors for growing animal cells. Bioprocessing Three major intellectual frontiers for chemical engineers in bioprocessing are the design of bioreactors for the culture of plant and animal cells, the development of control systems along with the needed biosensors and analytical instruments, and the development of processes for separating and purifying products. A critical component in each of these three research areas is the need to relate the microscale to the mesoscale. Much of the early work in applying recombinant DNA technology to the production of bioactive substances has used microbial cell species such as bacteria, yeasts, and molds. These microbes are fairly easy to manipulate genetically and are hardy under adverse conditions. Unfortunately, animal or plant proteins produced by clones of microbial cells often lack the critical three-dimensional structure that is formed when the same proteins are produced by animal and plant cells. For this reason, these proteins may not be biologically active even though they have the correct sequence of amino acids. One important future area of biotechnology lies in using plant and animal cells in place of microbial cells. The large-scale use of plant and animal cells in tissue culture raises important problems in the design and operation of bioreactors. One problem mentioned earlier is that certain animal cells are anchorage-dependent. Also, plant and animal cells are easily ruptured by mechanical shear. Bioreactors for handling such cells must be designed so that the contents of the reactor can be mixed without disrupting the cells. A similar problem exists in the design of systems to transfer the cells from one vessel to another. Plant cells tend to aggregate, and large aggregates pose problems in maintaining a supply of nutrients to all cells and in removing wastes. The development of bioreactors for plant cells will require an understanding of limitations on mass transfer in such aggregates. Some bioreactor systems must be completely protected from microbial contamination, meaning that not a single alien bacterium or virus particle can be allowed to penetrate the system. Reliable and economical systems need to be developed to achieve this level of contamination prevention. Along with the need for prevention is the need to be able to detect contamination at a level of a few microorganisms in a hundred kiloliters of medium. This degree of detection is not yet achievable. Research has continued to improve the crude detection methods that were in use a decade ago. Most industrial bioprocesses are now operated in a batch mode, which means that every thing is mixed together and the process then allowed to proceed to the desired point, much as when something is cooked in a pot on a stove. Batch processing is the method of choice for small-scale production, and it has the advantage that the equipment can be used for intermittent production of more than one product. An intriguing possiblity is that chemicals and biochemicals will be produced by biotechnology on a large-scale, continouous basis. Thus, rather than the fill-stir-cook-empy method characterized by batch systems, the necessary ingredients would be fed continuously to a bioreactor, and products continuously removed. The engineering problems involved in converting from batch to continuous biological processing are not trivial, however. Continuous processing of biological systems places stringent demands on equipment design, instrumentation, and operation for maintaining aseptic conditions and biological containment. One indication of these difficulties is the fact that, although processes for fermenting natural materials to produce beer predate written history, beer is still brewed and aged in batches. Attempts to use a continuous process to manufacture a product as well understood as beer have not produced a beverage with acceptable taste. A complication in bioprocessing is separating the desired product from the system. Cell culture bioreactors produce a dilute mixture of cells in an aqueous medium. Recovery of the product proteins from these cells may require disruption of the cells. This creates a host of problems. Cell walls and organelles must be removed. Proteins must be concentrated from a highly dilute solution that is mostly composed of water and other small molecules. The desired proteins must be separated from other macromolecules with similar physical properties. For biologically active proteins, separations must not only be specific for the target proteins, but also gentle enough to prevent denaturation and loss of biological activity and suitable for large-scale operation. Solving these problems requires generic research on highly selective separations, as well as on the problems of concentrating materials from very dilute solutions. Pursuing these opportunities has resulted in a better understanding of separation processes now used for the large-scale purification of proteins (e.g., precipitation and process chromatography). It may also result in novel separations involving aspects of techniques such as chromatography, membrane separation, fractionation in electric and gravitational fields, immunoadsorption, extraction with supercritical fluids, two-phase aqueous solution extraction, and separation by use of microemulsions. The development of such new separations is crucial to the development of industrial biotechnology. Another approach to separation problems lies in the development of modified organisms that produce the target proteins in high yield and concentration, thus reducing the time and cost of separating the proteins from large amounts of water. This is an area where early involvement of chemical engineers in designing geneticafly engineered organisms has been valuable. With their insights into the requirements of downstream processing of biologically synthesized substances, chemical engineers are valuable members of an interdisciplinary team of molecular biologists and biochemists seeking to tailor the genetic code of cells. Process monitoring and control Continuous and detailed knowledge of processing conditions is necessary for the control and optimization of bioprocessing operations. Because of containment and contamination problems, this knowledge must often be obtained without sampling the process stream, that is, it's necessary to measure the system without actually touching it. Currently, conditions such as temperature, pressure, and acidity can be measured rapidly and accurately. It is more difficult to monitor the concentrations of the chemical species in the reaction medium, to say nothing of monitoring the cell density (number per volume) and intracellular concentrations of hundreds of compounds. The development of rapid, accurate, and noninvasive online measurement sensors and instruments is a high-priority goal in the commercialization of biotechnology. Some of these instruments are building on analytical techniques used in other disciplines, and are generally divided into two groups: in situ (in the system) and ex situ (outside the system). Some of these approaches are discussed in the figure below.

Several approaches to developing analytical instrumentation for bioreactors are shown here. (1) Gases being fed to the bioreactor must be analyzed to determine their flow rate and composition. These data, when combined with similar measurements on the gases exiting from the bioreactor (2), provide information on oxygen uptake and carbon dioxide evolution in the bioreactor. (3) Various sensors may be placed in the bioreactor; properties that might be measured include temperature, pressure, acidity, dissolved oxygen, and liquid feed rates. A number of sensors are under development to measure glucose, ethanol, important ions, and a number of biomolecules. A number of additional techniques can be used to determine the viscosity of the mixture and its optical clarity.