Some natural and some synthetic polymers have molecular things in common. Building on those is one way toward an eventual arsenal of plastics that will biodegrade or photo-degrade.

After and before. The difference between these scanning electron micrographs of polycaprolactone before (right) and after two months in the ground show the effects of burial on this degradable plastic.

Protein, starch, cellulose, the nucleic acids and lignin—in fact all the polymers that occur naturally in

living things—are capable of being degraded biologically. They have been around long enough for microorganisms to have evolved that can use them as food. This is a good thing; otherwise we would long ago have been buried under the accumulated organic debris of 3.5 billion years of living and dying.

Synthetic polymers, however, are another matter. The plastics, synthetic fibers and films produced by modern chemical technology to suit precisely the needs of industrial applications are, with only a few exceptions, nonbiodegradable; their molecular structures are just enough different from those of natural polymers that few enzymes have the proper configuration to break manufac­tured polymeric macromolecules into

smaller pieces. This is also not such a bad idea; it keeps the telephone from dis­integrating on your desk, the plastic milk container from coming undone in your refrigerator, your car's steering wheel from coming apart in your hands or your polyester shirt from shredding off your back.

Nevertheless, for many agricultural, medical and ecological uses, a synthetic polymer that could be injection molded into solid shapes, spun into fibers or extruded, and would dissolve or other­wise degrade under preselected con­ditions, would also be a fine thing to have. Natural polymers cannot generally be adapted to these industrial requirements; when heated, they decompose before they melt. And if the natural polymers are modified to improve their processability they usually become more expensive, and in some cases they become non-

MOSAIC January/February 1978 29

biodegradable. That is the bind in which polymer

chemists involved with this problem find themselves. As a consequence, more of their attention is being directed toward the development of new kinds of polymers that are degradable than to the search for ways to improve the physical properties of the many natural, degrad­able polymers.

The dilemma posed to modern society by the proliferation of synthetic ma­terials is not a simple one to resolve. It is, if one looks below its surface, not even an easy one to define. It is almost the prototypical on-the-one-hand/then-on-the-other-hand type of problem with which our society, both dependent on and anxious about technological fixes, appears to be confounding itself.

On the one hand, the volume (although not the weight) of plastics and fibers produced in the United States now exceeds that of metals produced for all applications. On the other hand, plastics account for no more than about five percent of the material in municipal wastes. On the one hand, it is argued, our main disposal problem is not with municipal wastes but with roadside litter. On the other hand, less than one percent of plastic packaging materials becomes litter (most gets into garbage and trash cans). On the one hand, plastics are almost indest ruct ible in landfill ope ra t ions . O n the o ther hand, degradability in landfills, unless or until the large molecules can be redesigned around specific problems, is not a,n advantage: It delays the future use of the landfill for a stable building site. And the rotting plastic, again unless especially designed, would further aggravate the problem of contaminants leaching into nearby water supplies. On the basis of existing technology, in fact, this is now the position of the U.S. Environmental Protection Agency, which has said plastics in sanitary landfill should be inert.

It would be facetious to maintain that, if the price were right, plastic materials could be developed that would both compete with natural materials and be suitably biodegradable. Nevertheless, almost uniformly among those working in the field of degradable plastics, the consensus is that the problems—and the forces that dictate the direction of the principal research efforts—are principally economic.

The three major polymer packaging materials that account for 90 percent of the plastics found in municipal waste, for

example—polyethylene (38 percent), poly (vinyl chloride) (31 percent) and polystyrene (21 percent)—all have com­petition from natural alternatives. They remain popular not necessarily or only because they do not degrade, but also because they are competitively priced with comparable nonsynthetic materials and may exceed their performance in such characteristics as water retention or gas permeability.

Talk about a future "great revolution" in degradable materials for packaging may have its place, says polymer chemist James E. Potts, senior research scientist at the Union Carbide Corporation, Bound Brook, New Jersey, "but it's not going to happen 'til the price is right. And it may n e v e r h a p p e n It h a s n ' t been demonstrated that the public cares enough to pay more." William J. Bailey, professor of organic chemistry at the University of Maryland and long an active research participant in the field of biodegradable polymers, agrees. "There are lots of uses in agriculture and in medicine for biodegradable polymers," says Bailey, "but until they become less expensive to develop and until there is more demand, there will be few general ecological uses."

Special applications

So in recent years the emphasis in biodegradable polymer research has shifted away from potential consumer packaging materials and toward more specialized applications where the need is perhaps even greater and the cost of the material is not the limiting factor. Here optimism runs strong, and the search for new kinds of biodegradable polymers with specific physical properties con­tinues at a steady pace.

An example of one such polymer that has been in use for some time is a synthetic biodegradable suture made of polyglycolate and marketed by the Davis and Geek Division of American Cyan-amid. Sutures made of catgut (from the intestines of sheep, not cats) are composed of protein and are thus biodegradable. But they produce an allergic reaction in some patients. Nylon has been used as a substitute, particularly for suturing external wounds. But nylon is not degradable, so the sutures do have to be removed in a separate procedure. The polyglycolate sutures have nylon's strength and nonallergenic quality, but they are absorbed by the body in three to six months and they do not have to be removed. What would be the ideal for

surgery would be a whole selection of synthetic degradable sutures that would bioabsorb at different rates for use in different kinds of operations.

Another application, in an advanced experimental stage at Union Carbide, is the use of biodegradable plastic con­tainers for growing young trees. Injection molding of the biodegradable polymer polycaprolactone produces a bullet-shaped container in which a seed is planted. The seedlings are raised in a greenhouse and later planted mechan­ically, containers and all. The plastic helps retain moisture and protects the young seedling from attack during its first year, after which the plastic biodegrades in the soil. According to Potts, trees planted this way are now growing in New Mexico, Washington and five southern states. The use of a strong degradable plastic film instead of burlap in wrapping the roots of mature trees when transplanting them should also improve the retention of moisture and nutrients by the tree, as well as reduce the shock of transplantation.

Another potential agricultural use of sheets of degradable plastic is as mulch. For growing some crops, the use of an opaque plastic sheet to cover the ground is becoming increasingly popular. The plastic retains moisture, suppresses weeds, eliminates the need for cultivation and often lengthens the growing season. The problem is what to do with the plastic when finished. Often it's hauled down to the end of the field and burned, but that causes bad air pollution and in some states is illegal. But if the plastic is pho-todegradable or biodegradable, or a combination of the two, it fragments into small pieces and then can safely be plowed under. This application of degradable plastics may someday gain wide favor. (This use, however, is not without its hazards; at least one small company has been found legally liable for the loss of a farmer's crop when the film mulch it supplied photodegraded too quickly. The company had to pay for the lost crop.)

These possible future uses will depend on the ability to design and fabricate economically competitive polymers that incorporate components that are suscep­tible to breakdown and that, at the same time, have physical properties suitable for each specific use. Only four or five years ago little was known about the subject, but work since then has made a modest beginning in gaining a basic understan­ding at the laboratory level of what must be done to match the biodegradability of natural polymers.

30 MOSAIC January/February 1978

Apart from the lack of enzymes in nature to attack synthetic polymers, the resistance of conventional plastics to microorganisms is primarily due to two factors, J. E. Guillet of the University of Toronto points out. One is the low surface area of molded objects and low permeability of plastic films. The second is the high molecular weight of the plastic material. A polymer is a large molecule of high molecular weight derived from a number of smaller units linked together in a chain, like the beads of a necklace.

This arrangement produces a molecule that, although large, can pack to produce a surface area relatively low in relation to its molecular weight. Also, micro­organisms, in some cases, tend to attack the ends of molecules, and the number of ends is inversely proportional to the molecular weight. With such long-chain molecules, there just aren't very many ends available. To make plastics degradable, Guillet points out, it is necessary, first, to break them down into very small particles with a large surface area and, second, to reduce their molec­ular weight.

Tests of a wide range of commercially available or easily accessible polymers, reported in 1973 by Potts and his colleagues, found that only the aliphatic polyesters and polyester urethanes show­ed any reasonable degree of biodegrad-ability. (The two previously mentioned polymeric materials that are commer­cially used for the biodegradability, polyglycolate and polycaprolactone, are both aliphatic polyesters.)

Mimicking nature

It should not really be surprising that a polyester can be biodegradable in some synthetic forms. As Bailey notes, one polyester, poly- (beta-hydroxybutyrate), is a naturally occurring material that many bacteria and fungi use for energy storage in the same way that animals use fat.

Polymer scientists searching for new biodegradable materials thus widened the search to other polymers that were similar to those occurring in nature. One such class is the polyamides, of which the nylons are the best known synthetic example. Nylons are very closely related structurally to proteins. Both contain amide groups.

In the search for another class of biodegradable polymers, the polyamides have many attractive features. They contain nitrogen, which many micro­organisms need for growth (fertilizer added to compost piles accelerates decom­

position). And the naturally occurring polyamides, the proteins, are incredibly abundant, have numerous linkages and are, of course, biodegradable. Further­more, the polyamides are hydrophilic and have reasonably high melting points even at relatively low molecular weights.

Bailey, at his University of Maryland laboratories, has explored in depth a way, in effect, of mimicking microorganisms' characteristics so that their natural enzymes will behave toward his new polymers as if they were proteins. He has done so by chemically combining alpha amino acids, the building blocks of proteins, with various kinds of synthetic

Evidence. Fungus growth on a biodegradable polymer at the University of Connecticut,

polyamides (nylons). The result is a hybrid of an alpha amino acid and nylon. One example is the hybrid formed between polymers of the simple amino acid glycine and nylon-6; it is highly crystalline and has a melting point about halfway between that of its two parent polymers.

Most important of all, it is bio­degradable. The enzyme, in effect, treats the material as if it were a protein, because it contains an alpha amino acid. Bailey's copolyamide, in fact, contains two different amide bonds, both of which are similar (but not identical) to the peptide bond of proteins. The hope was that the similarity would enable the enzymes from bacteria and fungi to cleave the polymer to small fragments that the micro­organisms could use as food. Indeed, culture tests do show that the material is readily consumed by both fungi and bacteria as food. The fungus Aspergillus niger completely degrades the copolymer in about three weeks; bacteria like Flavobacterium and Alcaligenes also degrade it.

As a result of such work, Bailey was able to announce that "a new class of biodegradable polymers has been dis­covered which have very attractive physical properties and apparently have the versatility in structure so that a large variety of materials with a wide range of physical and chemical properties can be prepared."

The work of incorporating alpha amino acids into synthetic polymers is con­tinuing at Maryland. Most recently, Bailey says, he has been trying to cross a polyester (which has a cleavable bond) with polystyrene. This work is just getting under way.

MOSAIC January/February 1978 31

interdisciplinary team. University of Connec­ticut scientists in the degradable plastics search include (from left) J. P. Bell, a chemical engineer; S. J. Huang, a synthetic-polymer chemist, and J. R. Knox, a physical biochemist.

A team approach

The broadest and most diverse program of studies into biodegradability is being carried out by a polymer research group at the University of Connecticut at Storrs. There, scientists from various fields and from different departments of the univer­sity have banded together informally through the university's Institute of Materials Science for a broad-gauged, comprehensive research assault on the phenomena of biodegradability and the synthesis and testing of biodegradable polymers.

The group, supported by the National Science Foundation, includes James R. Knox, a physical biochemist specializing in enzyme studies; James P. Bell, a chemical engineer; Samuel J. Huang, a synthetic polymer chemist (on a one-year sabbatical during 1977-78 at the IBM Research Laboratories in San Jose, California); J. A. Cameron, a micro­biologist, and Anthony P. Simonelli, a pharmaceutical scientist working on drug applications. Other members include Jay A. Pavlisko and Mark Roby.

They have managed to make and test, with varying degrees of success, no fewer than two dozen polymers that seem to show biodegradability, with lifetimes ranging from two months to two days. "We have made different classes of polymers that degrade at different rates," says Huang. "We are now in the position of looking at the detailed physical proper­ties and applications of these polymers."

The work is still only in the laboratory stage, but it has come a long way from three and a half years ago when, as Huang says, "very little was known about biodegradable polymers," and very few polymers were known to degrade. "Now we can say that it is possible to make a synthetic biodegradable polymer that will degrade at the rate you want and have all kinds of needed physical properties for fabricating it into films, fibers or even controlled-release drug systems."

The work is so far what Knox calls "a benchtop chemistry accomplishment." No vast sheets of biodegradable plastic ready for commercial use await the visitor to Knox and Bell's laboratories on the campus amid the rolling green hills of central Connecticut. Most of the testing work is with small amounts of synthe­sized powder, crystal or fiber. The problem of future general application is still one of economics. The specialized uses envisioned won't be so limited by the economic question, but, as Bell says, to bring any new polymer into development

requires a formidable amount of work. Early in the group's effort it became

apparent that useful biodegradable polymers will have to come from new synthetic approaches designed to incor­porate biodegradable structural units into polymer chains having desirable physical properties. Since then the basic idea has not changed: It is still, as Knox says, "to in t roduce separable bonds in to polymers"; putting it another way, the effort is to "design a polymer to fit the enzyme."

Spoon-feeding the enzymes

Enzymes are proteins that function as catalysts in the vast array of chemical reactions necessary for the processes of life. They are remarkably specific. In Knox's words, "They're very picky." The enzyme known as catalase, for example, breaks down hydrogen peroxide and nothing else. It is this specificity that makes enzymes so valuable; they bring about only the chemical reaction they are supposed to. And because a biodegrada-tion reaction has to be carried out through the action of some enzyme, it is this specificity that has to be addressed in designing polymers that are biode­gradable.

What accounts for the remarkable specificity of an enzyme? Briefly, the structure of any particular enzyme molecule is such that it fits around a molecule of the substance on which it acts in just such a way that the vulnerable bond of the substance can be cleaved. Each enzyme, in other words, has a surface made to order for the surface of its substrate, the substance on which it acts. The enzyme holds the molecule to be attacked in just the right position for the vulnerable bond to be chemically separated, say by the action of an in­coming water molecule.

The mammalian enzyme chymotryp-sin, for example, attacks only an amide bond that has a particular substituent, a phenylalanine group, adjacent to the bond. This results in a lock-and-key fit so that the amide bond is positioned for attack from a water molecule.

Similarly, the enzyme trypsin wants a positively charged substituent at the position next to the amide bond because it itself has a negative charge in the analogous location.

And the enzyme elastase wants something very small in the substituent position because its configuration is such that it has a small "trap door" so that only a side product that is very small can be held by the enzyme.

Through such basic knowledge of biodegradation applied to polymers, Knox, Bell, Huang and their colleagues have shown that introduction of proper substituents to polymers that contain enzyme-vulnerable amide or ester groups does improve the biodegradability of the molecules. The several dozen biodegrad­able compounds they have made tend to have long, intricate names (or in some cases no name as yet), but according to Huang they fall into four categories: poly (amide-esters), poly(amide-ure-thanes), poly(amide-enamines) and poly (hydroxy-esters).

Summarizing the first few years of their research efforts, the Connecticut team states: "Our results show that it is feasible to design and synthesize biodegradable polymers, having a variety of structural units and properties, from easily available starting materials. The potential use of these materials in manufacturing slow-release drugs, insec­ticides and fertilizers should be in­teresting."

Knox anticipates with enthusiasm a future time when there will be plastics designed to be not only biodegradable but also recyclable. He envisions large, es­pecially designed "enzyme reactors" in which plastic wastes would be en-zymatically reconverted back to their original compounds, separated, purified and recombined into new polymers again.

Right now the economics in no way come close to making such a situation feasible, but Knox believes conditions could change and that such enzyme-reactor recycling of plastics could become economically feasible 25 years from now. Union Carbide's Potts, in contrast, sees formidable obstacles—not only economic, but also chemical—to such back-to-the-chemical-beginning kind of plastics recycling. It is at least 50 years away, he says, and he considers that for now a better kind of "second use" is simply to burn plastic and other municipal wastes in power plants for the production of energy.

Drug delivery

The virtues of plastics recycling may be debatable, but a possible future use of biodegradable plastics in the controlled release of drugs is generating quiet excitement. The Connecticut group is a leader in the search. The need is great. "We're really in the dark ages when it comes to drug delivery systems," says Simonelli, the group's member from the university's School of Pharmacy. Drugs

32 MOSAIC January/February 1978

Seedlings in plastic. En­casing a seedling in degradable polycaprolac-tone (left) protects it at its most vulnerable. After a while (below) soil organisms find a way to break down the polyster,

that need. The rate could be controlled by the amount of polymeric material sur­rounding particles of the drug. Further­more, as Simonelli notes, an ideal drug delivery system would be one that goes only to the target area within the body and delivers only the amount of drug needed. One possible way would be to

design the carrier polymer so that it could be broken down only by the particular enzymes found in the particular organ of the body the drug is supposed to reach.

Degradable polymers could also be used as skin implants for the long-term con­trolled release of drugs. The drug com­pany, Pfizer Inc., is working on this technique as a way of administering penicillin to cattle while avoiding the environmental disadvantages of putting antibiotics in animal feed. For humans, say, for release of birth control drugs over a considerable period of time, Simonelli proposes that the problems of a surgical skin implant might be circumvented by direct injection of a drug/polymer com­bination. The polymer would be designed so that it would cross-link after injection, forming a solid beneath the skin where it would remain to release the drug over the months to come. A newly developed biodegradable polymer that is a deri­vative of polytartrate seems a good candidate for this use, Simonelli reports.

A similar potential use of bio­degradable polymers in the controlled release of chemical agents could avert some of the environmental problems that accompany the application of insecticides and fertilizers. "If we can physically or chemically bind insecticides or fertilizers into a biodegradable polymer," Huang suggests, "it would release the chemicals onto the crops just as needed. We wouldn't have to use any more fertilizer or insecticide than was absolutely necessary." This work is in the lab­oratory stage.

These medical and environmental uses bring to mind the issue that comes up when any new agents are being con­sidered for use in the body or on crops. Are they possibly carcinogenic? The Connecticut group is not indifferent to the question and the team's micro­biologist, Cameron, has set up a program for screening for mutagenicity and toxicity all of the polymers that the group has developed. "Most of the materials we are working with," says Cameron, "do not appear to be of the class that would cause a problem. . . .The final products tend to look more or less like proteins."

The other approach

An entirely different approach to the subject of biodegradable polymers in­volves taking natural materials that are biodegradable and trying to improve their physical properties so that they can be made into more useful fibers, films or molded plastics. The Connecticut group

MOSAIC January/February 1978 33

now put into the body rapidly reach a peak level in the blood stream and then dissipate down to uselessly low levels. Far better would be a drug that would be released at just the needed rate over a longer period of time.

Biodegradable polymers used as a carrier or matrix for the drug could fulfill

has explored this route with gelatin. Gelatin is an animal protein by-product readily available in quantity and relatively low in cost. It already has many applications, especially in photographic emulsion, but the new goal is to make a material that is water insoluble yet pliable enough to be used as fibers, ribbons or sheets.

The technique used is called cross-linking. Adjacent gelatin molecules are linked together—for example, by linking amine groups on each molecule—to make a stronger polymer. The technique is not new, but the trick is to come up with the right degree of cross-linking. Too many cross-links and the material becomes brittle; too few and it becomes overpliable. The Storrs group has successfully made fibers; the waxy yellow strands sit coiled in vials on a counter in one of the laboratories there. The prob­lem, says Knox, is that the group has had trouble getting repeatable results on tests of the fiber's tensile strength; the strength has varied too much from batch to batch. As a result, Knox says, the work with gelatin has been put aside for the time being, and the group since has devoted most of its efforts to the syn­thetic work or the modification of com­mercial polymers.

Another natural animal material deriv­ed from chicken feathers is being explored with some success as a possible plastic film by Mitchel Shen, professor of chemical engineering at the University of Califor­nia at Berkeley. Chicken feathers are made of keratin, a natural protein polymer, and they are an abundant renewable resource, especially in places like California where there are large numbers of chicken farms. Shen and his students use a chemical treatment to break down the disulfide linkages in the feather material to make it soluble. After the solvent is evaporated and the un­dissolved fibrous material is discarded, the keratin part, which does dissolve, can be fashioned into clear films. The films are thin and reasonably strong. Shen says the material is not yet competitive with commercially available materials, but he believes it has good potential for a broad variety of applications.

Photodegradation

For many applications, a plastic that degrades in the sunlight is desirable. Often, if a material photodegrades to a certain extent, the molecular weight is reduced and the number of molecular chain ends is increased sufficiently that those enzymes that prefer chain ends can

34 MOSAIC January/February 1978

more readily participate in breaking them down.

Over the years, J. E. Guillet of the University of Toronto and his colleagues have shown that the inclusion of a ketone monomer (RR'CO) in small quantities in the backbone of many polymers causes the polymer to become sensitive to ultraviolet light. The R and R' stand for various alkyl and aryl substituents. By changing the nature of these two sub­stituents, the rate of the degradation process can be controlled. The reaction begins when the carbonyl group (C=0) absorbs a quantum of ultraviolet light. An electron localized on the oxygen atom is raised to an excited state. From this excited state, energy must be released, and, as Toronto's Guillet says, there is a good probability that this will be done by a photochemical reaction. What is called the classical "Norrish type II reaction" results in splitting of the main polymer chain.

Guillet has shown this photodegrada­tion to take place when a ketone is structurally incorporated into such polymers as polyethylene, polystyrene, poly(methyl acrylate), aliphatic polyesters and nitrile copolymers. More recently he has shown that it happens with the acrylonitrile fiber-forming polymers.

Using these basic principles and others, often involving the presence of oxygen and the use of certain catalysts, numerous photodegradable plastic compositions have been patented by the University of Toronto and by many companies. The technology of photodegradable plastics is considerably ahead of the technology of biodegradable plastics. Potts estimates that several hundred patents have been issued for photodegradable polyethylene and polystyrene, but, though some are being marketed, he believes most com­panies have taken them out as a defensive move while they sit back and wait to see if the demand develops.

Whether the skeptic's view that the demand won't develop prevails in the coming years remains to be seen. But if conditions change and the need for them intensifies, a large variety of both photodegradable and biodegradable plastic materials will await the public as a result of the new understanding gained by the research of many scientists in recent years. And, whatever the outlook for packaging materials, there seems little doubt that the specialty applications of degradable plastics have a bright future.®

Research reported in this article is supported in part by the Polymers program of the National Science Foundation.