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A Handbook of Applied Biopolymer Technology
Synthesis, Degradation and Applications
By Sanjay K. Sharma, Ackmez Mudhoo
The Royal Society of ChemistryCopyright © 2011 Royal Society of Chemistry
All rights reserved.
History of Sustainable Bio-based Polymers
TIM A. OSSWALD AND SYLVANA GARCÍA-RODRÍGUEZ
University of Wisconsin-Madison, Department of Mechanical Engineering, 1513 University Avenue, Madison, WI 53706-1572, United States of America
Hidden in an old Art Nouveau townhouse in Essen, Germany, is the world's largest collection of plastic artifacts. This is the private Kölsch Collection, created by the architects Ulrich and Ursula Kölsch, which houses tens of thousands of plastic items, all numbered and placed on shelves from floor to ceiling in every room of the ground floor flat. With every single item perfectly and painstakingly catalogued by Mr and Mrs Kölsch, the collection is the most complete archive of the developments in the plastics industry over a 150-year span. We not only see the development of plastics, but also how this unique material helped in the development and change of design and aesthetics. The Kölsch's collection shows us how, in the first 80 years of the plastics industry, products were made exclusively of biopolymers, most from renewable resources such as cellulose, casein, shellac and ebonite. Here and there, we stumble upon items made of plastic materials that most of us have never even heard of, such as Bois Durci and kopal resin. However, these materials, long dropped from our collective memories, helped shape what is today the plastics industry.
For example, Bois Durci or Hardened Wood, an early plastic that dates back to the 1850s, is a mixture of sawdust (usually of a hardwood such as ebony or rosewood), carbon black, metal particles and blood or egg. The sawdust was mixed with vegetable oils, mineral or metallic fillers, and the blood with a gelatinous substance diluted in water. The dry and wet components were mixed and compressed into finished parts in a steel mold held in a steam-heated screw press. Figure 1.1 shows a French mirror made of this early plastic material by compression molding using a screw press.
Continuing the walk through the Kölsch's art townhouse, we find colorful kopal artifacts brought to us in the Art Deco period between 1921 and 1931 by the EBENA company in Belgium. Also molded in a heated screw press, these items, most of which are beautiful decorative Art Deco bowls, containers, boxes, lamps, radios (Figure 1.2), and clocks, are made of a mixture of wood fiber, pigments and a fossilized tree resin from Congo called kopal. Kopal is found in the ground in the jungles of Africa. It can be considered to be the inland counterpart of amber, which is found in the North and Baltic Seas. The EBENA factory in Wijnegem, Belgium, existed between 1921 and 1931, and was the only manufacturing facility to ever produce kopal products. Since EBENA closed in 1931, the expensive fossilized resins such as kopal and amber have been forgotten as molding materials, except for perhaps a few jewellery applications.
In their collection, it is not difficult to stumble upon items made of casein, a milk protein obtained via acidification or enzymatic action. The resulting curds can be dried, molded, and treated with a hardening agent to yield commercial plastic. Casein had its heyday as a commercial plastic in the early 1900s; the same time period when many other items that are found in the Kölsch's collection were produced.
The recent revival of interest in these materials makes the story of their discovery and development even more relevant today. The great success of petrochemical-based polymers from the Second World War through the present is testament to the versatility, economy, and durability of such synthetic materials. However, the indestructibility that for decades made petrochemical-based synthetics so desirable has increasingly become a liability. Over 25 million tons of plastic entered the municipal solid waste stream in the USA in 2001. This non-biodegradable plastic waste accounted for over 11% of the municipal solid waste in the USA, up from 1% in 1960. Incinerating plastics can cause toxic air pollution; plastic litter is unsightly. Thus there are health, environ- mental, and aesthetic problems with continued use of non-biodegradable petrochemical-based polymers. The expanded use of renewable, biodegradable biopolymers would alleviate the problems associated with disposal of non-biodegradable polymers. In addition, the increasing monetary and political costs of American and European dependence on foreign sources of oil make sustainable, domestically grown resources a desirable alternative.
Renewed interest in biopolymers since the mid-1990s has shown itself in new research into the processing and properties of renewable, biodegradable materials like casein, soy protein, polylactic acid (PLA), polyhydroxyalkanoates (PHA), and other novel materials. This work has demonstrated the opportunity for renewable, biodegradable biopolymers to replace their synthetic counterparts in a variety of applications. But the interest, needs, and materials that are re-emerging are a continuation of a story that began centuries earlier; the opening chapter of this ongoing story is told here.
This chapter presents various biopolymers within a historical perspective in relation to today's needs. We will cover silk, casein, and soy, as well as other materials that have recently been found useful in recent applications such as chitin, collagen, PHA, and PLA.
1.2 Silk: From a Royal Stitch to a Wounded Peasant
According to the Chinese legend, the Empress Si Ling-Chi was drinking a cup of tea under a mulberry tree, when something suddenly fell from the tree and into her cup. As she removed it, she slowly unraveled a thread from what she found was a silkworm cocoon. This 5000-year-old legend marks the discovery of silk, a material that was then processed into fine threads and considered the cloth of the gods. Only royalty was allowed to have it, and anyone who tried to trade silk, silkworms, or mulberry trees was punished with death: silk and its production was kept secret for approximately 3000 years. In the first century A.D., this secret slowly leaked to the outside world, and soon silk became a luxurious fabric throughout Asia and Europe, giving rise to a network of trade routes called the Silk Road.
Silk is a protein polymer (Figure 1.3), whose amino acid composition depends on the producing species. Several lepidoptera larvae are capable of producing silk, but silks produced by spiders and silkworms have been the most studied. The silkworm Bombyx mori has been the most popular species for silk production. Its silk is characterized by fibroin fibers that are held together by a coat of a glue-like protein called sericin (absent in spider silk). This way, composite fibers are arranged and held together to protect the worm inside the cocoon. Silk provides great toughness and strength, as well as very high elasticity. Its resistance to failure under compression makes it comparable to Kevlar.
Silk could be traced back, as a suturing material, to the second century A.D., when Galen of Pergamon wrote De Methodo Medendi. He was well known in the times of the Roman Empire for treating and suturing injured tendons of gladiators. In his work, he stated: "Moreover let ligatures be of a material that does not rot easily like that of those brought from Gaul and sold especially in the Via Sacra ..." (referring to linen or Celtic thread). He continued: "... In many places under Roman rule you can obtain silk, especially in large cities where there are many wealthy women".
In the battlefields of Crécy, northern France, in 1346, cobweb was popular for stopping a wound from bleeding. Its styptic properties (for stopping bleeding) were still popular two centuries later, as reflected in one of Shakespeare's comedies, A Midsummer Night's Dream, where he wrote: "I shall desire you of more acquaintance, good master cobweb. If I cut my finger I shall make bold with you ...".
Around the end of the eighteenth century, it was established that bleeding vessels were better treated by ligatures (tying up the ends of the vessels) than by cautery (burning the ends of the vessels). By this time, waxed thread had been replaced by silk as the material of choice. Philip Syng Physick (1768–1837) was an American who became the first professor of surgery at the University of Pennsylvania. Following the teachings of his mentor, the famous John Hunter, a Scot who became the founder of experimental surgery and surgical pathology, Physick used adhesive leather strips to close a wound. He then noticed that these dissolved in contact with fluids from the wound. He thought this characteristic would be of great advantage in the use of ligatures. This idea was historic, since no one had previously thought of a suture that would be absorbed after performing its function.
In 1867, Joseph Lister, among his great contributions in antisepsis, wrote an article "Observations of ligature of arteries on the antiseptic system". He believed that a silk ligature could be left in the body if bacteria lying within the interstices of the threads could be killed. At that time, ligatures were left long and protruding through the wound to then be pulled out along with the necrosed or dead tissue at the end of the vessel, increasing the risk of a secondary hemorrhage. In his experiments, he started using antiseptic silk ligatures soaked in an aqueous solution of carbolic acid, where he found the ligature was not absorbed after ten months of implantation on the external iliac artery of a 50-year-old woman. These results lead him to explore, in 1868, the use of ox peritoneum and catgut in a carbolic acid solution, seeking an antiseptic absorbable ligature.
In 1881, arguing that carbolized catgut (referring to Lister's mixture of olive oil and carbolic acid) was not an effective antiseptic, Kocher of Berne started a campaign against catgut and in favor of silk. In his rules of surgery, Halsted, who introduced thin rubber gloves in 1890, recommends: "... gentle handling of tissues, meticulous haemostasis, and interrupted silk sutures".
By 1900, however, the catgut industry was firmly established in Germany, using the intestines of sheep, important in their sausage industry. Nevertheless, both catgut and silk were important base materials for the production of sutures for the following 100 years.
By then, the silk industry was large and of great economic importance, as silk was being used for a large variety of consumer goods, from clothing, weavings and stockings for women. However, most of the silk, the raw material for this growing industry, came from Japan and China, a relatively unstable part of the world in the first part of the twentieth century. This prompted the growing Western industries to concentrate on finding replacements for silk, as had been done with natural rubber, through chemistry. The downfall of silk as an industrial material began in 1927, when the DuPont Company hired the chemist Wallace Hume Carothers to run their "pure research laboratories". The exit road for silk was paved by 1938, a year after Carothers's death, when nylon was introduced to the world, primarily as a replacement for silk in hose and stockings and as toothbrush bristles. It is certain that the invention of nylon gravely affected the Japanese trade balance, and in consequence, the overall position of the Japanese industry in world markets at the threshold of the Second World War. The influence of this miracle fiber, that could be produced at the fraction of the cost as its natural counterpart, is indisputable. Allied use of nylon in parachutes during the invasion of Normandy may have played a decisive role in the war's military outcome. The most obvious influence may come from its impact on consumer consumption.
As time progressed, the news for silk turned even more dire; in the 1960s, the use of virgin silk was found to produce an adverse biological response in sutured patients. This was later attributed, in the late 1970s and early 1980s, to sericin in the inner fibroin fibers of the silkworm silk, which was found to cause a type I allergic reaction. Virgin silk was then processed to extract sericin from its fibroin fibers, followed by a coating of wax or silicone to improve material properties and reduce fraying, and received the name of black braided silk (e.g. Perma-Hand). Due to the biocompatibility issues, however, between the 1960s and the 1980s, silk decreased in its popularity as a suturing material. In addition, these events ran in parallel with the development of synthetic biocompatible polymers, based on polyglycolic acid (PGA) (Dexon, Maxon) and polylactic acid (PLA) (Vicryl). These two, together with catgut (Catgut), are classified as biodegradable suture materials, according to the definition of an absorbable suture material by the US Pharmacopeia: one that loses most of its tensile strength within 60 days after being placed below the skin surface. Silk-based sutures, along with other kinds based on braided polyester (Ethibond, Mersilene, Tevdek), nylon (Ethilon) and polypropylene (Prolene, Surgilene), are classified as non-absorbable suturing materials. Some studies on silk, however, have showed its susceptibility to proteolytic degradation and the loss of the majority of its tensile strength in vivo after 1 year of implantation. The braided structure of silk-based sutures increases the risk of infection, but these sutures have great handling and tying capabilities, and therefore it is still used today around eyelids and lips, where incidence of infection is low.
Today, silk-based biomaterials are reviving, accompanied with advances in molecular and genetic manipulations. Its great mechanical properties and degradation characteristics have opened doors in the fabrication of tissue engineering scaffolds, which need ample time to interact with the host tissue before degrading. Some studies have found silk scaffolds comparable to those based on collagen for culturing bone and ligament tissue, as well as fibroblasts and bone marrow stromal cells. The capability of processing silk fibroin into foams, meshes, fibers, and films make silk a promising material for several biomedical applications. Advancements in genetic manipulation and protein tailoring have included spider silk in this array of opportunities, offering even superior mechanical properties when compared to B. mori silk.
1.3 Cellulose: The Quintessential Bio-based Plastic
If we step back to the nineteenth century, another natural polymer, cellulose, in addition to rubber, impacted everyday life. The invention of cellulose plastics, also known as Celluloid, Parkesine, Xylonite, or Ivoride, has been attributed to three people: the Swiss professor Christian Schönbein, the English inventor Alexander Parkes, and the American entrepreneur John Wesley Hyatt.
Christian Friedrich Schönbein, a chemistry professor at the University of Basel, loved to perform chemistry experiments in the kitchen of his home, much to his wife's dismay. Early one morning in the spring of 1845, Schönbein spilled a mixture of nitric and sulfuric acids, part of that day's experiment, on the kitchen counter. He quickly took one of his wife's cotton aprons and wiped the mess up, then rinsing it with water before the acid would damage the cloth. As he hung the apron to dry over the hot stove, it exploded in a loud bang and flame in front of his very eyes. After he recovered from the shock, Schönbein's curiosity led him to impregnate wads of cotton with the acid mixture. Every time, he was able to ignite the mass, leading to an enormous, uncontrollable explosion. He called his invention guncotton. He had invented cellulose nitrate. Guncotton was three times as powerful as gunpowder and did not leave a black cloud after the explosion. Schönbein sold his patent to the Austrian Empire's army, but found no buyers in Prussia, Russia, or France. Finally, he sold his patent to John Taylor, his English agent, who immediately began production of guncotton in England. The production ended when his factory exploded, killing 20 workers. Although there were no buyers, several laboratories did spring up across Europe to investigate guncotton; often blowing up faster than they were being built. In addition to its military applications, Schönbein envisioned other uses for the nitrated cotton mass. He added a solvent or plasticizer made of ether and alcohol and found a way to nitrate the cellulose fibers into a less explosive material which he called kollodium, glue in Greek. He reported to his friend Michael Faraday that this mass "is capable of being shaped into all sorts of things and forms ...". In the spring of 1846, after accidentally cutting himself on the hand, he covered the wound with a thin elastic translucent film made of kollodium. He sold his idea to the English, who for years supplied the world with the first adhesive bandages. In England, there was one person that took particular interest in the Swiss professor's inventions. His name was Alexander Parkes.
Excerpted from A Handbook of Applied Biopolymer Technology by Sanjay K. Sharma, Ackmez Mudhoo. Copyright © 2011 Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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