This article was published in 1999 and has not been updated or revised. P O LYM ERS AN D PEO PLE n April 1997 Dr. Frank Baker, an emerge n c y straddle the boundary between living and nonliving— medicine specialist from the Chicago area, took like Frank Baker’s artificial skin—are beginning to sug- I pa r t in a clinical trial to test a form of arti f i c i a l gest exciting possibilities for improving human hea lth. ski n for t r ea t i n g in su li n -d epen d en t di a bet i cs whose t i ssu e Behind these developments lies more than 150 years ha d been degra ded by the secondar y effects of chronic high of pr og r ess in polymer r es e a rch by hu ndreds of scientists blood sugar. Baker, who has had diabetes for more than as well as more than a century of res e a r ch i n cell biology four decades, wa s in danger of losing a foot because of and or gan transplants. As described in the following ha r d-to-heal skin ulcers. For him the trial results were ar ticle, which highlights the work of only a few of many close to mir a culou s: the la bor a tor y- g r own skin d idn ’t res e a rchers, the pa th to recen t a dva n ces in moder n med- just cover and protect his wound, it released chemicals ical treatment began with the investigations of scientists that caused his own tissue to grow back much faster. in t e r ested in a better ba sic u ndersta ndin g of chemistr y As Baker put it, the artificial skin “saved my foot.” and biology. The material that worked this medical wonder was synthesized from polymers, long molecular chains for me d by the chemical bonding of many small molecules of one Sor t i n g O u t N a t u re or more types. Most people are probably more familiar with polymers in the form of the plastics that make up Humanity has a long history of trying to under- such everyday products as plastic food containers, bubble stand the substance and structure of the physical wrap packing, and videotape. But polymers also are world around us, whether by simple observation or found everyw h e r e in n a tu re. Wood, a n ima l a n d veg- experimental manipulation. In ancient Greece, for eta ble fiber s, bone, a n d hor n a re polymer s, for exa m ple, example, Aristotle concluded that all materials were as is the deoxyribonucleic acid (DNA) inside the cell made up of combinations of only four elements: air, nucleus and the membrane that separates one cell from earth, fire, and water. During the Middle Ages, an o t h e r . Indeed, when the alchemists tried in vain to polymer industr y bega n in the convert common metals into nineteenth century, it made gold. By the late eighteenth materials that we r e der ived century, chemists had begun fr om n a t u r a l po l y m e r s — ar tificial celluloid from plant cellulose, for instance. Artificial skin grown in the Eventually the industry began laboratory on scaffolding made syn t hesi zi n g n ew m a t er i a ls, of lon g cha in molecu les ca lled su ch a s n ylon , t ha t r ep l a c e d polymers can help heal the wounds of pa tients with ulcers natural materials and were ca u sed by poor blood cir cu la - made without natural pre- tion. (Photo courtesy of cu r sor s. Toda y pr od u ct s t ha t Organogenesis Inc.) N A T I O N A L A C A D E M Y O F S C I E N C E S synthesizing and breaking down chemicals in an effort to determine their fundamental components. Early in the nineteenth century, English chemist John Dalton, Launching the Polymer observing that chemicals would combine only in Industry specific ratios, concluded that matter was made of indivisible “atoms” (a concept first proposed by the In 1870, four years before the structure of the Greek philosopher Democritus in about 400 BC). carbon atom was elucidated, American inventor John Nineteenth-century chemists also determined that Wesley H yatt won a contest to find a material for it was possible to synthesize so-called organic com- billiard balls to replace ivory—then as now in short pounds, once believed to be made only in living orga n - supply. Hyatt’s prize-winning contribution was cellu - is m s , from inorganic chemicals. loid, based on cellulose, a polymer that is the basic Even as chemists pursued their investigations into structural material of plant cell walls. It was the start the nature of nature, inventors were creating new of the polymer industry. Hyatt treated cellulose materials by treating natural substances with various nitrate, or guncotton—an explosive material made chemicals at elevated temperatures and pressures. by exposing cotton plant fibers to nitric and sulfuric In 1839, American inventor Charles Goodyear discov- acids—with alcohol and camphor. What he got was a ered a technique, which he called vulcanization, for hard, shiny material that could be molded when hot. manipulating the properties of the sap from rubber Cheap and uniform in consistency, this new material trees by treating it with heat and sulfur. The process did indeed replace ivory in billiard balls. Occasionally converted a gummy, springy material used mainly to though, when the celluloid billiard balls collided, they erase (“rub out”) into a dry, tough, elastic material created a small detonation like a firecracker because that would make automobile tires possible—and of the explosive nature of cellulose nitrate, which eventually a transportation revolution. is related to trinitrotoluene (TNT) in composition. Investigators working at the theoretical level were Celluloid also replaced horn in combs, found wide equally productive, arriving at a series of independent use in housewares, and was made into the first flexible realizations that would eventually lay the foundation photographic film. In 1887, Count Hilaire de for the polymer industry. In 1858, German chemist Chardonnet created a related product when he Friedrich Kekulé developed the framework for under- learned to spin cellulose nitrate into Chardonnet silk, standing the struc t u r e of organic molecules when he the first synthetic fiber to enter production and a showed that a carbon atom can form chemical bonds forerunner of rayon, nylon, and Dacron. with up to four other atoms and that multiple carbon Both celluloid and Chardonnet silk were polymers atoms can join together to create long chains—a dis- c reated by altering natural polymers. The first tru l y co v er y also made at about the same time by Scottish synthetic polymer did not come along until 1909, chemist Archibald S. Couper. Then, in 1874, Jacobus when American inventor Leo Baekeland treated phe- van’t Hoff of the Netherlands and Joseph Le Bel of nol, or carbolic acid, another derivative of coal tar, France independently suggested that the carbon atom’s with the pre s e rvative formaldehyde under heat and four bonds are arranged so that they point at the cor- p re s s u re. His product, Bakelite, was hard, immune ners of a tetrahedron, or pyramid. Since carbon atoms to harsh chemicals, electrically insulating, and heat ar e the framework for natural and artificial polymers, the resistant—characteristics that made it useful for a two discoveries would in time furnish a three - d i m e n - myriad of household goods and electrical part s . sional pi c t u r e of the molecular struc t u r e of polymers. Soon Bakelite was being used to make tools, machines, and cookingware. A key discovery in the late 19th century was that the ca r bon a tom ca n for m bonds with up to four ot her a t oms, ea ch ma r kin g Science Explains Polymers on e cor n er of a t et r a he - dron, or pyramid. Many The rapid success of Bakelite sparked a flurry ca r bon a toms ca n bon d, of synthesis investigations and innovations in both forming long molecular cha in s, or polymer s, in America and Europe. As the financial stakes rose, possible combinations that the hit-or-miss garage inventor’s approach that had number in the billions. dominated the industry gave way to more systematic 2 BEYON D DISCOVERY This article was published in 1999 and has not been updated or revised. efforts. No longer content simply to tinker with raw In 1920, German organic chemist materials and various processing conditions, scientists Hermann Staudinger proposed began basic research designed to understand the mol- that the unusual strength and ela sticity of polymer s wa s du e to ecular structure of polymers. their great length and high molec- In 1920, the German chemist Hermann Staudinger, ular weight. (Photo of Hermann fascinated by the seemingly unique properties of poly- Staudinger courtesy of Institute mers, began investigating their behavior and chemical for Macromolecular Chemistr y, characteristics. Staudinger’s research suggested that Freiburg, Germany); (Diagram of a polymer , in t his ca se, polyet h - polymers are composed of long chain molecules ylene strands, courtesy of Biografx) of many identical or closely related chemical units.