GOODYEAR MEDAL ADDRESS: BIOLOGICAL AND GEOGRAPHICAL DIVERSIFICATION OF THE NATURAL RUBBER SUPPLY—A CORNISH CAREER CRUSADE
ABSTRACT
I was honored by the award of the 2024 Charles Goodyear Medal 42 yr after I received my PhD in plant biology and thank Drs Judit Puskas and Adel Halasa for their nominations and support over several yr as well as the selection committee. As the second woman and first biologist to receive this recognition, I will describe some of the basic and applied research that led me to my admiration of plants and subsequent focus on the biological and geographical diversification of the global natural rubber (NR) supply. My focus across federal, industrial, and academic appointments has continuously been on making research meaningful, and my basic research has always been tethered to strongly applied research translation and goals. Achieving domestic rubber production means that the entire value chain from seed to product must be validated, and every part presented and still presents specific research and development challenges. Fortunately, core scientific and engineering principles are constant and readily adaptable to new areas. Many people were happy to advise me in areas in which I had no formal training. My innovative research has led to 10 start-up companies, about 36 issued or pending patents, many with student inventors, over 320 published papers, and an H-index of 51. Working in NR is not only very interesting but has left me with an unshakable belief that my research is meaningful. Failure is not an option because the consequences of relying on a single clonal tree species for this critical material could be unimaginably bad if the rubber tree crop collapses and no alternatives have previously been established at a rapidly scalable level.
DEDICATION
To my father, who never gave up.
PERSONAL HISTORY
I grew up in Beccles, a small East Anglian market town of about 10 000 people, in the Waveney Valley, Northeast Suffolk, England (opposite Wales). I tell people that it is 10 miles west of the easternmost point of Britain and that, if you jump off the country there, you could swim to Holland. Beccles is first mentioned in the written record in the 7th century, served as a lookout post to warn of incoming Viking ships, and was named in the 1086 Domesday Book of William, The Conqueror. Beccles was granted its Borough Charter in 1584 by Queen Elizabeth I and for many yr was a flourishing Anglian river and fishing port. One of its most famous sons was Sir John Leman, Chairman of the Fishmongers Guild and Lord Mayor of London. Sir John endowed the Beccles Grammar School (high school) upon his death in 1631. Horatio Nelson’s parents were married in St Michael’s church, Beccles, in 1758, although he was born in a Norfolk Parish just across the river. Dr Dorothy Crowfoot Hodgins, the second female Nobel Prize winner for Chemistry (after Marie Curie) attended Beccles Grammar School.
I am about as middle a child as you can get. I have both an elder brother and sister and a younger sister and brother, so if my parents needed to take a couple of representative offspring to an event, it was my elder siblings. I was never the baby of either gender to be spoiled, so I grew up a tad competitive. Overachievement came early when I was potty trained with my elder sister. I also developed my nonlocal British accent from BBC Radio 4, which my mom played constantly in the kitchen as she prepared countless meals and baked goods while I was corralled in the corner.
My father Arthur James Cornish was a scholarship grammar schoolboy who grew up on a farm just outside the village of Wetherden, while my mother Beatrice Eudora Cornish (neé White) was the youngest child of the Vicar of Haughley, a neighboring Suffolk village. After serving in the Royal Artillery (armored cars) during WWII, during which he became disabled, my parents married. As the youngest of three sons (he had elder and younger sisters as well), the family farm was never going to be his, and he worked his way up to manager of the Beccles laundry, a very useful position with all the soiled clothes from five babies/children. However, due to his service and subsequent surgeries, his lung function was very poor; he went from being an extremely athletic young man to one who could not ever run or feel well again, burdened with the prospect of dying before 40. Despite this devastating blow, he did not give up or become embittered; he believed that you must play the hand you are dealt. He was forced to stop work due to ill health when his children ranged from 4 to 12 yr old, but he went on to grow all our vegetables on abandoned railway land and converted the house to rental flats to create income. We all grew up under the specter of his imminent demise but his drive and will to live was an enduring example to his family. He finally died at the age of 80. My siblings consider that I am the closest to my father in terms of drive, perseverance, and unwillingness to give up. This is probably a combination of his example and our mutual middle child syndrome.
My father was also very keen that his children went to university, and so he managed to send us to local inexpensive private primary (elementary) schools. I attended primary school in the original building of the grammar school (Figure 1, bottom center). The school motto was “I am, I can, I ought, I will,” and I still relate to that ethic. I also first appreciated how cool plants were while at that school. They do everything animals can but have to do it in one place; they cannot run away when things get tough. The stellar education delivered by that Parents’ National Education Union school allowed the first four of us to pass the state 11-plus exam and gain places at the Sir John Leman Grammar School; the top 20% of children attended this academically focused school, whereas the lower 80% went to what then was called a secondary modern school targeted to more applied careers. That school system was abolished 3 yr later and converted, nationally, to the comprehensive system, so my younger brother never took the 11-plus exam. Nonetheless, the grammar school motto was “Disce aut dicede,” “learn or leave” (Figure 1, bottom center), and I ended up with 10 O-levels and A-levels in biology, chemistry, and physics. The school was particularly good in the sciences, and the science wing was named after our famous Nobel Prize winner Dr Crowfoot-Hodkins. While there, I was what was considered a well-rounded student; I was captain of the netball team (and played for the county), on the tennis and hockey teams, was in the orchestra (in my last year, as concert master) and several choirs, and won both the biology and music prizes. From there, I gained a place at the University of Birmingham, a “red brick university,” meaning it was long established and old enough to have been built of brick rather than concrete (Figure 1, right panel). It was quite a shock to move from a small town to a city of over 2 million. This university’s motto is “Per ardua ad alta,” “through hard work to the heights.” My BSc (first class honors) was in biological sciences, specializing in plant biology and genetics, with a minor in biochemistry, and I won the John Humphreys Prize for plant biology. I continued at this same university for my doctorate in plant stress physiology.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
After graduating with my PhD, when I was 24, I became part of Maggie Thatcher’s brain drain. When she was Prime Minister, there was a huge cutback in research and development (R&D) in the UK. As an example, while I was a graduate student, the University of Birmingham had 63 faculty vacancies, but all but three were frozen. One of the three was in my department. It had 425 qualified applicants, and the ones that made the shortlist were scholars who had done at least one top-notch overseas postdoc. So I thought that I had better go do a top-notch overseas postdoc myself. That decision took me to the Department of Energy Plant Research Laboratory at Michigan State University, where I investigated abscisic acid metabolism in relation to water stress and recovery under Jan Zeevaart, and from there, I went to Cornell to investigate the electrophysiology of the amino acid:proton cotransport system in developing legumes, a key component of assimilate partitioning, under Roger Spanswick. I then moved to Arizona, where I worked on cotton and sugarcane photosynthesis and the molecular biology of rubber biosynthesis in guayule. Natural rubber (NR) had finally crossed my career path.1,2
Fortuitously, this stint in Arizona coincided with Goodyear’s successful lobbying of the U.S. Department of Agriculture (USDA) to set up a new project on the biotechnological production of NR (cis-1,4-polyisoprene), ostensibly so they could work with the USDA Agricultural Research Service (ARS) in a collaborative effort under a Cooperative Research and Development Agreement. The original premise was to find the enzymes and genes that make NR and then express them in corn or yeast so that the U.S. would have a readily scalable source of rubber in the event of rubber tree crop collapse. Today, we are still looking for these genes. Due to a complete lack of American citizens with experience in rubber biochemistry and genetics, I was hired in 1989 by the USDA, before I naturalized, and led the program for over 15 yr. Interestingly, Goodyear canceled their entire plant-based research program a few months after I started up the project. They wanted ARS to take over this research area, not collaborate with them on it. Nevertheless, they did then transfer all their biological materials and equipment to my USDA program, which was a big help. A few years later, they sold all their rubber trees and completely exited the biological side of rubber production.
WHY DO WE NEED DOMESTIC RUBBER PRODUCTION?
dependency on foreign sources
The USA is entirely dependent upon NR and latex (NRL) harvested from contiguous small holdings and plantations of the tropical Brazilian, or para, rubber tree (Hevea brasiliensis) largely grown in SE Asia. NR is a unique elastomeric polymer essential to the manufacturer of about 50 000 products spanning all sectors of the U.S. economy. It is a critical raw material for defense, medicine, commerce, and transportation and cannot be replaced by synthetic materials.3 Global demand has constantly increased since 1900 (Figure 2), largely driven by the transportation sector, and is expected to double as Africa and other regions fully develop. Back in 2004, the History Channel Modern Marvels series did a special on the importance of NR. They (not me) also pointed out that “our four most important natural resources are air, water, petroleum, and rubber.” Most people would guess air and water, a few would think of petroleum, which is after all a natural resource, but only those working in rubber would list rubber as number four. Most people think that tires are completely synthetic because they are black (due to the carbon black reinforcing filler). In 2019, Kal Penn narrated a documentary on Prime Video highlighting the continued importance of NR to the global economy.4
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
supply threat posed by rubber tree diseases
It is truly remarkable that a single species is used for commercial production of such a critical agricultural material, essential in ∼50 000 different products. All other agricultural materials, such as protein, starch, oil, and fiber have multiple sources.5,6 The consequences of relying on a single clonal tree species for this critical material could be unimaginably bad if the rubber tree crop collapses. Fortunately, at least 2500 plant species make NR, and several have been proven as viable replacements or substitutes for tropical NR.7 It is imperative that at least one or two of these are established to meet projected increased demand and at a rapidly scalable level, before any significant collapse of the Hevea rubber tree crop.3 Just 10% of U.S. demand being supplied by alternative sources would be enough to protect supplies and stabilize prices and could be rapidly scaled up in response to a supply shortfall.
Is the looming prospect of rubber tree crop collapse something we should worry about and, more importantly, act upon? As a plant biologist, I am very aware of the nature of tree species collapses, and many of these have reached the general public. Elm trees were mostly destroyed by Dutch elm disease, spread by elm bark beetles in the 1960s and 1970s,8 and these were not genetically identical trees grown less than 3 m apart like tropical rubber trees. More recently, as examples, we have seen papaya collapse (all commercial papayas in Hawaii are now genetically modified),9 and major species declines caused by emerald ash borers,10 live oak disease,11 pine beetles,12 Panama disease of Cavendish bananas,13 and citrus greening disease,14 all of which have destroyed vast swaths of trees.
The risk of crop collapse increases as the genetic diversity of the species under cultivation decreases.15 Clonal production of identical germplasm can have devastating consequences. The Irish potato famine is a well-known example of this. Potatoes originate in the South American Andes and are very diverse phenotypically and genetically16 (Figure 3, top left panel). Genetic diversity provides the species with an impressive ability to fight new diseases. A few types may be susceptible and be killed, but most would survive (Figure 3, top center panels). This is not the case in clonal domesticated potatoes (Figure 3, lower panels), all of which died from an uncontrollable Phytophthora infestans outbreak. In Ireland, the abrupt loss of its staple food supply caused 1 million deaths from starvation and the emigration of 2 million people to Europe and the U.S. Ireland’s population dropped by about 25% between 1845 and 1852.17
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
This type of species collapse could happen to Hevea clonal rubber tree production areas in Asia and Africa. The industry was forewarned of this danger in 2019 when two leaf blights apparently jumped over from oil palm to Hevea in Thailand. In only 6 months, Neofusicoccus and Pestalotiopsis diseases18 spread across 500 000 hectares and seven rubber producing countries. 2020 saw a 10% drop in production (see Figure 2), in other words, 1.4 million metric tons, more than the U.S. imports.19 The only reason the industry did not panic and this outbreak was controlled was because COVID-19 became a pandemic in 2020, closing down the tire factories and generally restricting movement, which greatly reduced disease spread and immediate rubber demand. Many other serious diseases infect Hevea rubber trees as well.20 It is important to note that neither of these 2019 serious diseases were by organisms as bad as South American leaf blight (SALB, Pseudocercospora ulie), a fatal rubber tree pathogen endemic to South America that prevents large-scale rubber production there.21,22 The fungal spores that propagate this disease only live about 5 days and cannot survive an ocean crossing by ship. Until 2022, no direct flights were permitted between endemic SALB regions and Southeast Asia to prevent the accidental introduction of SALB into the main rubber-producing region of the world. The establishment of direct flights in October of 2022, probably in response to the trade talks between China and Brazil at that time, has undoubtedly increased the chance of SALB reaching Asia and rapidly spreading through the more than 13 million hectares of susceptible trees that currently produce 90% of the world’s rubber supply.
why are there no alternative rubber crops?
So why don’t we already have multiple alternative rubber crops in production? Multiple NR sources would provide security and offer new opportunities to rubber chemists and product developers because each NR is slightly different. Unfortunately, both business, social and technical obstacles to commercially viable rubber diversity exist.
Business Impediments. —
The predominant business reason impeding diversification is that harvesting Hevea rubber is both cheap and simple; it is achieved by poorly paid workers tapping the trees by hand. This translates to cheap rubber in the marketplace. The leading alternatives, guayule (GNR, Parthenium argentatum) and rubber dandelion (TNR, Taraxacum kok-saghyz), cannot be harvested this way, and someone must pay to build extraction plants. Also, on an introductory small scale, the alternative NRs (GNR and TNR) would be much more expensive than tropical NR (Figure 4) and be limited to small markets. Many proof-of-concept products have been manufactured, including various types of all-important tires, most recently by Cooper (GNR) in Ohio and Continental (TNR) in Germany, but these products have not led to a pathway to scale. Also, industrial and federal funds historically have been awarded in times of severe shortage (e.g., WWII) or high prices (e.g., the oil embargo), and such global events are clearly reflected in the supply-and-demand plot in Figure 2. However, once a crisis is over, funds dry up, and it is all to do again when the next crisis bites. Thus, any pathway to scale must be firmly based on commercial viability in normal economic times. As said, on a small production scale, alternative NRs are much more expensive than commodity Hevea NR (HNR; Figure 4). As high-yield sustainable crops and extraction plants are introduced, economies of scale will occur and reduce costs. At the same time, as NR demand continues to rise, the price of HNR will increase. Eventually, a cross-over will occur. In the meantime, the cost gap between HNR and the alternatives must be addressed by coproduct valorization and by targeting the alternatives to premium niche markets with high profit margins where they can provide a performance and/or marketing advantage of some kind.23 Also, direct competition with the most expensive synthetic rubbers, such as synthetic polyisoprene,21 should be fully explored.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Technical Impediments. —
Technical impediments include the need for robust high rubber content cultivars that can be grown across a wide range of locations and environments or which have been tailored to specific conditions, efficient extraction and purification processes, maximization of coproduct value, and demonstration of specialty products specifically developed to enter low-volume, high-margin, markets. Only through such markets can a pathway to scale be designed and implemented. Products must be selected that can be profitably produced from 10, 100, 1000, or 10 000 acres of alternative rubber crops and support eventual expansion to the hundreds of thousands or millions of acres (with associated processing infrastructure) needed by the transportation industry. My career has been devoted primarily to overcoming these technical impediments.
Societal Impediments. —
Although disease is arguably the most serious threat to the U.S. rubber supply, it is not the only threat. Climate change, extreme weather events, politics, economics, and the extremely long supply chain between Southeast Asia and the USA all can disrupt supplies. China controls more than 80% of the total supply. Global deforestation moratoria prevent new acreage through clear cutting rain forest.24 The European requirement for Forest Stewardship Council certification intended to ensure responsible forest management requires that the path of products is traced from their origin through the supply chain to the end user which is impossible for much of the Hevea rubber crop, restricting import into European markets and increasing prices.25 Too little money reaches the small holders to sustain healthy rubber production and quality of life. The rapid disease spread in 2019 across seven countries is thought to have been exacerbated by the trees not having been properly fertilized or maintained for many years, so they were already unhealthy when the blights struck. Also, acreage is progressively lost to easier oil palm cultivation and urban development.26
However we accomplish biological and geographical diversification of the NR supply, it is too dangerous to wait for a rubber apocalypse. A world without NR, which could happen, is an unacceptable risk. A breakdown of transportation from loss of tire rubber could cause urban famine and manifold other dire consequences.
NR SCIENCE: R&D
alternative nr crops and production systems
Three complementary rubber-producing species can pretty much cover the globe: Hevea rubber trees in the tropics, guayule in semiarid temperate regions with mild winters, and rubber dandelion in temperate zones with snowy and/or severe winters (Figure 5).3 Mountain gum (Scorzonera tau-saghyz) is another snowy region prospect.27 Hevea is the slowest species to establish; it is cloned by grafting excised buds onto seedling root stocks, and new trees cannot be tapped until they are 6–7 yr old,28 and this process is still done by hand (Figure 6). Guayule needs to be grown for 1.5–2 yr before first harvest but then can be pollarded (cut branches are processed) while leaving a stump which then grows new branches for a repeat harvest the following year.29 This can be done multiple times, making guayule a perennial crop. Rubber dandelion is unique in that multiple production systems can be used (Figure 6), including field, greenhouse, and hydroponics.30–33 Rubber dandelion can be farmed as an annual crop (1 harvest/yr) or as a greenhouse crop (2 harvests/yr). In both cases, the crop must be replanted after each harvest because the rubber is produced in the roots.34 Field production is challenging because waves of aggressive weeds can easily outcompete the dandelion crop. Remarkably, rubber dandelion also can be grown hydroponically with root harvests every 5 weeks because the roots rapidly regrow on the same plants. The hydroponic system forms a high-throughput phenotyping system because the roots can be harvested without killing the mother plant and assayed for rubber content while the roots are regrowing. This will allow high rubber content plants to be identified and moved into the plant breeding program for cultivar development. Unlike rubber trees, GNR, TNR, and their NRL forms (GNRL and TNRL, respectively) are harvested mechanically and chemically, not by hand tapping.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
guayule and interspecific comparisons
My first research on guayule was at Arizona State University, funded by the tail end of the oil embargo federal funds dispersed by the Guayule Administrative Management Committee spanning the 1980s. While there, I developed the rubber transferase (RTase) enzyme assay2 and performed extensive work on the rubber particle-bound proteins of guayule.1,2,35,36 As mentioned earlier, I was then recruited by the USDA-ARS (by Dr Nelson Goodman) to lead the domestic rubber project at the Western Regional Research Center in Albany, CA. Between 1989 and 2003, I was the sole scientist in the federal government charged with the biotechnological development of NR-producing crops, a distinction I was delighted to relinquish in 2003 when I was able to hire a second permanent scientist (Dr Colleen McMahan, a rubber chemist) to join the group. The project was (and still is) intended to lead to domestic commercial production of NR, a vast and strategic agricultural raw material vital to the security of the U.S.3
I had no intention of turning guayule into a domestic crop at that time; this had been attempted in the 1980s [in response to rapid price rises of synthetic rubber caused by the oil embargo (Figure 2)] with the intensive effort of many people and a focus on tires. The solvent extraction process used produced good bulk rubber but was expensive, and when oil prices fell, guayule rubber failed in economic competition. However, in that research, it was proven that high-performance airplane and car tires could be manufactured and brought the guayule crop to a point that it could be efficiently farmed if the economics of rubber production ever became favorable.
regulation of rubber biosynthesis
Biochemistry. —
From the beginning of my research at USDA, I included guayule as a model system in the search for RTase as an example of a plant with generalized rubber production in parenchyma cells rather than in complex laticifers. Thus, it has far fewer proteins and genes associated with rubber production than NRL-producing species.37 I then made direct comparisons between rubber producing species as distantly related to each other as possible but with complementary characteristics: H. brasiliensis, the commercial tropical rubber tree that makes high molecular weight rubber in laticifers; Ficus elastica, the India rubber tree that makes low molecular weight rubber in laticifers; and Parthenium argentatum, guayule, that makes high molecular weight rubber in parenchyma cells.37 Enzymatically active rubber particles can be purified from all three, and they polymerize rubber in similar biochemical processes from an allylic pyrophosphate initiator [usually the C-15 farnesyl pyrophosphate (FPP) in vivo] and the nonallylic isopentenyl pyrophosphate (IPP) monomer, with divalent magnesium ions (Mg2+) as an essential activator (Figure 7).38–40 Initiation rate of new molecules (Figure 7, left panel) and their polymerization from IPP (Figure 7, center panel) are both strongly dependent upon substrate and activator concentration.41–43 Both Hevea and guayule require 8 mM Mg2+ for maximum activity, but Ficus requires 10× this amount.44 The binding affinity (expressed as Km, which is inversely correlated to binding affinity) of the monomer is strongly affected by Mg2+ concentration as well, with KmIPP decreasing 100× in Hevea and 10× in guayule when Mg2+ is raised from 4 to 8 mM, indicating a significant structural shift in the RTase active site.42 Since each rubber molecule only contains a single initiator, mean molecular weight of newly synthesized polymers can be calculated from the ratio of 3H-FPP to 14C-IPP (Figure 7, right panel). RTases usually take approximately 2 h to make high molecular rubber (>1 Mg/mol, about 15 000 monomers).45 The more limited the supply of initiator, the higher the molecular weight of the rubber formed (Figure 7). There is extensive literature around the regulation of rubber biosynthesis that underpins efforts to increase yield through genetic engineering.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
However, the rubber the disparate species make differs with respect to molecular weight, polydispersity, branching, gel and oligomer content, viscosity, and crystallite formation under strain.33,46–53 The protein and lipid composition of the rubber particle membranes also proved to be highly species specific.54 Perhaps surprisingly, because of the similarity in the way they all synthesize rubber,7 the rubber particle-bound proteins, which include the membrane-bound RTase complex, are also dissimilar. Interspecific immunochemical tests demonstrated that the rubber particle-bound proteins of these three species share only very few epitopes and so have very little in common in their surface structure.55–57 Extensive testing on guayule NRL (GNRL) found no relationship with Type I Hevea NRL (HNRL) allergies in human, rabbit, and murine systems.58–62 In contrast, TNR particles and TNRL do share several epitopes with Hevea.63
Molecular Biology. —
Biotechnological targets are identified from biochemical characterization and by identifying production limitations in the different production systems. Overall, rubber yield per acre must be maximized while maintaining desired polymer quality. This is achieved through maximization of rubber concentration and the amount of harvestable rubber-containing tissue. These parameters are affected by gene expression, environment, and management practices.
It was only quite recently that the genetic resources needed to directly modify alternative rubber crops have become available, developed primarily in the U.S., China, and Japan.64–71 Three metabolic engineering approaches can be used to increase rubber synthesis. Carbon flux into rubber can be increased by overexpressing genes for substrate synthesis,72–74 or editing out nonessential competing pathways,66,75 with the caveat that feedback regulation may counter any increased synthesis.73 As this does seem to occur, a complementary approach aims to metabolically balance the side effects of rubber synthesis by removing toxic protons and pyrophosphates, released at each condensation reaction as the polymer grows, from the RTase environment. These first two approaches have doubled guayule rubber yield in some greenhouse-grown transgenic guayule (unpublished). The final approach is to increase the catalytic efficiency of the RTase complex and/or increase the number of RTase complexes. However, this approach is hindered by the complexity of the RTase complex (Figure 8) and a lack of complete understanding of its structure and operation.76 To date, except in a single case of the guayule RTase,77 reconstitution assays intended to identify key subunits have only made rubber in the presence of enzymatically active rubber particles, and so their roles have not been unequivocally identified.78
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
high-margin, low-volume niches
Technoeconomic and Market Analysis. —
Alternative rubber production has been subjected to several technoeconomic (TEA) and life cycle analyses.79–82 Unsurprisingly, TEAs all corroborate that small-scale rubber production cannot compete with commodity-scale Hevea production. Thus, coupling TEA to market analyses (TEMAs) is essential to identify appropriate high-margin, low-volume products to target for market entry.
Until recently, it has been known that plants make rubber (cis-1,4-polyisoprene) that differs among rubber producing species.37 However, it was thought that, within a species, the rubber each plant made was quite constant, with species-specific molecular weight, branching, viscosity, gel content, and composition. For example, HNR and TNR make branched rubber which contains gel. In contrast, unmodified GNR is unbranched, does not contain gel, but forms larger crystallites under strain than TNR or HNR.46–48 As mentioned above, in laticiferous species, the laticifer appears to maintain an environment that supports production of a reproducible type of NR. Guayule, which does not have laticifers, has adapted its RTase to directly control rubber molecular weight and structure. However, analysis of transgenic triploid and tetraploid lines of guayule showed that expression of genes intended to increase yield by increased substrate synthesis or by improved metabolic balancing also altered the molecular weight, macromolecular structure, and composition of the rubber produced by the rubber particles (unpublished). This finding opens a world of possibilities to product developers and the potential for cultivars tailored to specific product segments, bringing even more focus to existing and potential, small-volume, high-margin products.
type i nrl allergy—an opportunity for guayule
Rubber modification is not the only way to commercialize an alternative rubber in the presence of a commodity analog. Intrinsic properties can also be leveraged. In 1991, my team and I became aware of a serious public health problem that, in turn, became a new opportunity for domestic rubber production. I will describe this in some detail to illustrate the complexity of the R&D translation to commercialization when a new feedstock also must be produced to supply an unmet need.
As discussed in a review article83 and citations therein that support this section, a series of fatal anaphylactic reactions were traced to contamination of NRL gloves with high levels of soluble NRL proteins (Figure 9), and the Food and Drug Administration (FDA) issued a medical alert to physicians. NRL is a cytoplasm which contains the rubber particles in suspension, but the cytoplasmic phase is filled with proteins and other cellular constituents (Figure 9). Type I NRL allergy indirectly arose because of the acquired immune deficiency syndrome (AIDS) epidemic which began in the U.S. in 1981. In 1985, the Centers for Disease Control and Prevention (CDC) introduced universal precautions, which included the widespread use of gloves to prevent the spread of bloodborne pathogens such as human immunodeficiency virus. Glove demand had been approximately 2 billion/yr before AIDS, and the U.S. made enough gloves internally for the domestic market. This action by the CDC jumped demand from 2 to 30 billion/yr. In response, many inexperienced glove manufacturers sprang up in Southeast Asia and the Far East. Until this time, gloves and most other dipped goods were leached at the gel stage to reduce the levels of soluble contaminants. The residual rubber particle-bound and hydrophobic proteins that remained entrained in the glove matrix were not sufficient (too low a dose) to induce sensitization. Unfortunately, leached and unleached gloves look the same, and the new manufacturers did not recognize what they could not see, so these new manufacturers ignored the leaching step and marketed gloves with very high levels of soluble protein contained within them. Then many existing manufacturers eliminated their leaching steps to maintain price competitiveness. Simple washing of a fully cured glove does not remove these proteins (nor other residual chemicals), but human saliva and bodily fluids do efficiently extract them, flushing the human immune system with potent allergens. Matters were made worse by the prevalence of powdered gloves. While in the box or package, the proteins in the gloves were sufficiently mobile to coat the powder granules which then puffed into the air during glove donning and removal by healthcare workers. These healthcare workers breathed them in continuously during their work shifts, and the proteins were extracted by their lung mucosa in sensitizing doses. In the early 1990s, the U.S. lost 10–15% of its healthcare workforce as workers were forced out of their careers after becoming sensitized to NRL proteins. Some multiple surgery populations, like spina bifida children, reached levels as high as 70% clinical Type I NRL allergy. This led to NRL (HNRL) gloves and other products being banned from some healthcare facilities. Most unfortunately, there was no putting the genie back into the lamp because, when people became allergic to the high amounts of soluble protein, their immune systems reacted against the entire protein profile, including the rubber particle-bound proteins which alone were insufficient to sensitize humans (Figure 10).62 Purified rubber particle bound proteins appeared even more involved with Type I NRL allergy than the soluble protein culprits. This meant that well-leached gloves could not be used safely by people with clinical, symptomatic Type I NRL allergy.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
The questions then were, (i) could HNRL be treated in some way to prevent new cases or to protect the existing Type I sensitized population, and (ii) could something else be used to avoid the allergy? Clearly, a return to well-leached gloves would prevent new cases but would not be safe for use by sensitized people (Figure 10).62 It takes much more protein to sensitize someone than it does to trigger a reaction in an already sensitized person. Thus, avoidance was the only path forward. Synthetic gloves were introduced with lower performance requirements than NRL under the premise that any glove is better than no glove. Once the industry came to a full understanding of the problem, strict standards on extractable protein content were set, and standardized protein quantification methods were instituted [ASTM D 571284 and ASTM D 649985].
Most NRL products in the U.S. are now properly leached (with the notable exception of dental dams manufactured by Hygienic Corporation and distributed by Hygienic, Patterson Dental and Coletene-Whaledent),86 and these are safe to use by people without Type I NRL allergy. However, about 30 million Americans have IgE antibodies against NRL proteins,87 and it is impossible to predict when an additional exposure might tip an individual over the edge into clinical symptoms. Also, because the residual chemicals cause skin irritation and allergic dermatitis, some patients mistake these contact reactions for the much more dangerous Type I NRL allergy.83
By 2020, demand had risen to 300 billion gloves/yr, and the COVID-19 pandemic transiently doubled this demand, now resettling to 400–450 billion/yr. Serious concerns arose again about declining glove quality because of the unprecedented demand increase.88–90 In 2023, the global gloves market size was valued at $24.65 billion in 2023 and is projected to reach $50.58 billion by 2031, with a CAGR of 9.4% during the forecast period.91
gnrl
My team had already shown that guayule rubber particle proteins have very little immunochemical commonality with HNRL proteins, suggesting that GNRL might be a suitable source of NRL for the medical products market (Figure 10). I proposed using alternative rubber-producing crops to combat Type I NRL allergy at the first International Latex Conference, held in 1991 in Baltimore, and received skeptical interest. This conference also allowed me to meet with medical and industrial representatives concerned with this medical emergency and make professional contacts that stood me in very good stead later and when we proved that GNRL products were safe for use by Type I NRL protein-sensitized people.
Allergy Safety. —
Initially, proof of concept was required before we could move the project forward. Was GNRL really safe, especially for people with clinical Type I NRL allergy? If it was, we potentially could commercialize our first domestic NRL crop without first cloning the rubber biosynthesis genes.
We went on to prove that GNRL proteins do not cross-react with antibodies against HNRL proteins, using a commercial rabbit polyclonal antibody test kit (Figure 10). Critical to continuing progress, once we communicated these results to contacts I had made at the 1991 Latex Allergy Conference, we were able to arrange in vitro and clinical trials at four medical facilities across the nation.58–62,92 Also, we sent samples to interested scientists at National Institutes of Health and FDA for additional evaluation,93 with a view to paving the way for future product approvals and product acceptance from regulators.
These human trials confirmed that GNRL proteins do not cross-react with Hevea Type I NRL allergy. We also showed that GNRL has very little protein compared with HNRL94; most of what it does have is not allergenic and is unlikely to sensitize people in its turn (Figure 11).
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
GNRL Commercialization. —
At this point, I split the research program into two. One half continued the ongoing search for the proteins and genes key to rubber biosynthesis. The other half focused on the commercialization of guayule as a source of allergy-safe NRL for medical and consumer products.
Putting high-performance, safe NRL products into hospital stockrooms could only be accomplished by attracting interest from American companies. The exhaustive search for an industrial partner with the capacity to produce GNRL and products presented imposing challenges. The industrial partner needed to be prepared to support a new farming industry, establish a new commercial extraction process, and produce safe medical products, with or without other strategic partnerships. Companies motivated by the quarterly profit report proved unwilling to take on the relatively long-term development project required, even though many farmers expressed interest in cultivating the new crop, and many NRL product manufacturers were interested in using the new NRL. I want to emphasize the immense assistance I had in this search from the USDA-ARS Office of Technology Transfer (OTT) and from the Information Service (IS). Press releases from IS, beginning with our initial allergenicity results, led to articles on radio, television, and in popular and business magazines, which in turn led to many promising business contacts (including our final exclusive licensee). OTT’s assistance, especially that of Dr Ruxton Villet and the late Mr Bruce Kinzel (under Dr William Tallant at that time) in promoting guayule hypoallergenic NRL at business expos and tech transfer conferences, was also extremely valuable. Over the next 5 yr, I discussed the commercialization project with over 70 companies and had detailed, in-depth discussions with 30 companies taking a serious in-depth look at the project. Many of these companies retain an interest in GNRL and hope to become customers of any company producing it.
Overcoming Technical Barriers. —
The early industrial discussions made it clear that we must perform additional research to address the commercialization barriers perceived by industry. Initially, the most important were the feasibility of scale-up and the absolute necessity for exclusive intellectual property.
Now, guayule does not naturally produce its rubber in a NRL form; if you cut into a stem, it does not leak milky fluid, although resin droplets will slowly leak from severed resin vessels (Figure 12, left panel). However, it does produce rubber particles in individual cells (Figure 12, right panel). GNRL production entails a mechanized process that extracts and purifies the rubber particles while maintaining them in aqueous suspension. We had used a batch process to generate our earlier NRL samples. The initial scale-up work to a continuous process was done on a lab scale and allowed us to file and obtain a U.S. patent98 on the process. Again, the active participation of OTT was essential, Dr Villet and Mrs Janelle Graetor, my patent advisor, being the pivotal players. However, it became clear that scale-up to a field pilot scale also would be needed before U.S. business would seriously consider the enterprise.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
This essential step was facilitated by the existing players in guayule research. We had been collaborating with ARS and university scientists, from the 1980s effort, throughout the project on various aspects, and had regularly attended the appropriate conferences to keep abreast of their research and to inform them of our progress. It was important to get these people behind the development of guayule hypoallergenic NRL. We were awarded Cooperative State Research, Education, and Extension Service funds through Mrs Carmela Bailey, a strong supporter of GNRL research, to fund a scale-up of the extraction process in Arizona with active assistance of scientists and engineers from the U.S. Water Conservation Lab in Phoenix and the University of Arizona, Tucson. In this effort, we extracted NRL from over 5000 lbs of fresh shrub, proving to industry that the patented process could be scaled up (Figure 13).
Overcoming Business Barriers. —
Industrial interest reactivated sufficiently to identify the next set of barriers.
Was GNRL economically viable?
Could high-performance medical and consumer products be made from it?
This is where we got even luckier. OTT had been aware of the need for an ARS cost engineer to assist technology transfer efforts for some time and had strongly promoted such a hire. The Eastern Regional Research Center (ERRC) hired the first USDA-ARS Cost Engineer Dr Andy McAloon, and OTT had smoothed the way at ERRC so that they were willing for Andy to do his first ARS cost analysis on the GNRL extraction process. This analysis did demonstrate the probable economic viability of GNRL produced from the advanced lines from the ARS and University of Arizona plant breeding programs.
By this point, Hercules Corporation in California had entered serious negotiations for an exclusive patent license, and we had retained several gallons of GNRL produced from the pilot process for them to use for prototype manufacture and testing, basically in a good-faith gesture on our part. Hercules, with very little warning, went through a massive internal reorganization, and the new CEO wanted nothing to do with guayule. However, the division with which we had been negotiating, and as a good-faith gesture on their part for future relationships between the company and ARS, awarded us a $20 000 grant to pay for the prototype work they were no longer permitted to do. We then contracted with the Akron Rubber Development Lab, where Dr Harry Bader developed the first successful formulation and made a wide range of GNRL medical products that met or exceeded ASTM performance standards.85,99–101 The excellence of GNRL examination glove and condom films as barriers to the transmission of human viruses was demonstrated through a collaboration with Dr David Lytle at the FDA.93
A company now applied for an exclusive patent license to the issued process patent98 and the pending (stronger) product by process patent.102 The USDA’s intent to grant an exclusive license was announced. Five companies promptly competed for it. A different company won the license, but after a round of appeals, the license finally went to Yulex Corporation, the original applicant, in 1997, led by Mr Dan Swiger. The company incorporated under the name Yulex, which I had coined as the material name for GNRL, but we were still not done.
The U.S. Patent Office initially balked at issuing the stronger NRL product by process patent. Under Janelle Graetor’s guidance, I met the Patent Officer’s requirements to recreate the guayule rubber extraction methods described in six earlier patents (back to 1933) and proved that the NRL rubber from the patented GNRL process was distinctly different. I was only able to do this because of other contacts I had made within ARS. At an international workshop organized by Dr Villet (who had moved to the Office of International Programs), I had met Dr J. L. Willet, one of the research leaders at the National Center for Agricultural Utilization Research, Peoria, IL. One of the earlier patents had originated there, and I had no access to necessary extrusion equipment at my location vital to the recreation of that patent. J. L. was happy to assist and had my guayule shrub extruded through the actual extruder used for that particular patent. He sent the extruded shrub back to me, and I was able to finish up the method. I accompanied Janelle to present the data to the U.S. Patent Officer, who then issued the strong product patent in 1998,102 strengthening Yulex’s proprietary position.
After the licensing of the process and product patents, guayule researchers in several states worked collaboratively to support the introduction of guayule as a real crop on the basis of hypoallergenic NRL production.59,103–105 The prospects of GNRL production allowed us to compete successfully for grants, and Yulex obtained sufficient investment to build a pilot plant in Arizona. During the 1990s and early 2000s, over $8 million of competitive grant funds, from various agencies, were shared among researchers dedicated to domestic production of rubber-producing crops.74,106–120 In total, interdisciplinary scientists and engineers from 23 different national and international sites collaborated to bring GNRL to the brink of commercialization in an archetypical example of convergent research.
In 2004, Yulex Inc asked me to join the company because their investor threatened to withdraw funding if I did not move over to provide the company much needed scientific credibility. I agreed primarily to avoid having to restart the commercialization effort from the beginning. In the first 3 yr, I brought in a distribution partner, greatly strengthened the intelligent property portfolio, worked with industry to produce many different NRL products (Figure 14), obtained an FDA 510(k) premarket approval for an exam glove, wrote and gained approval for ASTM D 1076 Category 4 NRL,95 guided the demonstration plant build, and helped position the company for sale. One of the Big 6 biotech companies did 6 months of due diligence and valued the company at $110 million, up from $10 million when I joined, and they directly attributed $90 million of the increase directly to my efforts. However, the company was not acquired at that time. Yulex failed to transform into an effective operating company, and I resigned in 2010 to join the Ohio State University (OSU) as Eminent Ohio Research Scholar, Bioemergent Materials, to develop rubber dandelion while continuing guayule product R&D. Two years later, Yulex Inc went bankrupt.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
GNRL remains the best of any natural or synthetic elastomeric material currently known, but GNRL commercialization was all to do again. I set up a new start-up company EnergyEne as an OSU spin-off. While at OSU, we developed a range of dipped products using a xanthate-based accelerator system, from Robinson Brothers, Bromwich, UK, to prevent contact reactions to residual chemicals in NRL products. GNRL products became circumallergenic because they now can avoid both contact and systemic allergic reactions.23
dandelion nrl
Rubber dandelion has been largely investigated as a source of rubber for tires, and its potential as a source of NRL (TNRL) has been ignored until very recently. TNR seems to be quite like HNR and is close to a drop-in replacement material in compression-molded composites, so it seemed likely that the TNRL would perform like HNRL. NRL was extracted and purified from several batches of greenhouse-grown plants (Figure 15).33 The particle distribution is monomodal, and mean diameter (1.2 µm) is slightly above HNRL (1.0 µm) and slightly below GNRL (1.4 µm). Gel content ranges from 12 to 40%. TNRL is very pure, and the rubber is high molecular weight and of low polydispersity (Table I). As commonly observed in guayule, the NRL rubber is of higher molecular weight (Mw) and lower polydispersity (D) than the solid form. This is because the extracted NRL only contains healthy rubber particles, whereas the solid rubber also contains coagulated rubber that has had time to degrade. Also, remarkably, the TNRL had achieved an Mw of almost 2 Mg/mol in only 3 months, a much shorter time to surpass 1 Mg/mol than either guayule (at least 1 yr) or Hevea (7 yr).
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
When TNRL was compounded with an optimized xanthate-based accelerator and curing package, cast films were soft, strong, and with a very high elongation to break (Figure 16, left panel). The TNRL behaved more like a GNRL film than an HNRL one. A glove was made from the compounded TNRL (Figure 16, right panel).121
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Currently, the prospects of TNRL commercialization are low because the NRL yield is poor. Unlike Hevea and guayule rubber particles, most of those made in rubber dandelion roots coagulate into solid rubber during the life of the plants (Figure 17).30,34 Genetic approaches can be taken to try to prevent in vivo coagulation.
Citation: Rubber Chemistry and Technology 98, 2; 10.5254/rct.24.00054
Also, the amount of protein associated with the TNRL rubber particles is less than in HNRL, but both rubber particle-bound and NRL proteins cross-react with Type I NRL allergy.64 Proper leaching of the TNRL products during manufacture would make such products safe for use except by people who already have Type I NRL allergy.
processing
Of course, however much a plant makes endogenously, no rubber or NRL is available for manufacturing unless it can be efficiently extracted from the plant. A variety of methods have been used over the years, but they essentially come down to water- or organic solvent-based extractions, which are extensively described in peer reviewed and patent literature.33,122–127
product development
Since around 50 000 products are made from or with NR or NRL, it can be hard to select a target market. Nonetheless, as discussed earlier, many prototypes have been successfully made, usually with industrial partners. This collaborative model is effective because it leverages their complementary skills. For example, a major U.S. catheter company worked with my industrial team and successfully developed the guayule Foley urinary catheter. The company understood the properties they needed for each component, while the GNRL R&D team knew how to generate these properties through different formulations. Similar approaches have led to many types of gloves, balloons, condoms, tubing, foams, an endotracheal tube cuff, and sundry other articles.93,128–131 The findings of my research also revealed significant quality issues in the nitrile glove market.88,89 Solid rubber articles that also have been successfully prototyped include tires, bushings, vibration isolators, and hoses.
sustainability
In addition to profitability as alternative rubber crops establish and expand, sustainability becomes extremely important. Just replacement of U.S. raw rubber imports with GNRL would require 600 processing plants, each producing 2500 Mt GNRL (dwt)/yr. Residual bagasse is very energy rich because it still contains the resin fraction as well as lignin and residual GNR (Table II). On a small scale, bagasse can be compressed to fuel pellets, likely after being cut with a lower-energy feedstock. However, on a larger scale, pyrolysis132–134 yields a bio-oil with an energy content unmatched by other biomass derived oils (Table II). The conversion efficiency is 60%, and the oil is only 16% oxygen (lower than other bio-oils), 0.0136% ash (because dicotyledonous plants do not accumulate silica), and zero sulfur.132 The bio-oil can be converted to advanced fuels at a 50% efficiency.
Guayule is not commonly considered an energy crop because it is so well known as a rubber and resin crop. However, this attitude should be revised. In 2022, Americans used 135.73 billion gallons of gasoline, including 134.55 billion gallons of finished motor gasoline (about 368.63 million gallons/day) and about 0.19 billion gallons of finished aviation gasoline.135 As a coproduct of GNRL production, 17 million acres of guayule could coproduce enough transportation fuel to supply all our airplanes and half of our motor fuels requirements (Table III). This scale of acreage is not huge for the U.S., and ample land is available in the semiarid Southwest. For comparison, in 2024, corn, soybean, wheat, and cotton covered 91, 86, 47, and 12 million acres, respectively.136
Industrial sustainability has many facets that can be explored. In my research group, we have developed semireinforcing fillers from wastes and determined how much carbon black137–139 or silica140,141 they can replace in filled products like tires. In general, these fillers enhance mixability and save processing energy. We have also explored chemical modification142 to allow NRs to encroach on traditional synthetic rubber markets and have developed various processing improvements.126,127,143 Also, liquid rubbers can replace petroleum-derived naphthenic processing aids,144 simultaneously improving product performance while reducing mix viscosity and saving significant mixing energy.
PATH FORWARD
A great deal is now known about how to move into the era of domestic rubber production, and a new model of the RTase has just been published.145 The federal government has begun stepping up grant funding, most recently by National Science Foundation (NSF) funding of a Generation-4 NSF engineering research center (ERC) headquartered at the OSU Wooster campus146,147 (announced on August 21, 2024). The ERC is called Transformation of American Rubber through Domestic Innovation for Supply Security (TARDISS) and is led by Dr Judit Puskas (PI), the first woman to be awarded the Goodyear Medal, Dr Ajay Shah (Center Director), and Dr Katrina Cornish (Scientific Advisor). The ERC will integrate engineering with biology and other science disciplines via convergent research thrusts in bioengineering, crop engineering, and NRL/rubber engineering and work closely with the commercial sector from growers to manufacturers.
The key missing piece remains commercial scale extraction facilities. This is where federal government funding is still needed. Unpublished TEAs indicate that $100 million would comfortably design and construct the first NRL/biofuel biorefinery (2500 dry Mt GNRL/yr) and support the associated guayule acreage. In contrast, estimates for rubber dandelion suggest that a CEA production facility producing roots containing 100 Mt dry TNR/yr would cost about $120 million without an extraction plant. This is much more expensive than guayule but is the fastest method for producing NR if Hevea ever abruptly collapses. As soon as a commercial scale extraction facility or biorefinery is in operation and running profitably, they can and will be proliferated by reinvesting excess revenue and/or accessing conventional bank loans. The output of each processing plant must be closely tied to TEMA to match volume with the highest-margin products so that expansion can be scaled without dependency on government subsidies. The federal government could also encourage farmers by rewarding water conservation and colocation of GNRL extraction facilities with cotton gins in the semiarid Southwest or rubber dandelion utilization of fallow or contaminated lands across the country not suited to food production. Supplying 10% of U.S. demand from alternative sources would protect our supply chain, stabilize global prices, and could be rapidly scaled up in response to a supply shortfall.
My hometown, Beccles, Suffolk, England, and my schools.
Global supply and demand for NR from 1900 and projected to 2050.
Domesticated potatoes are cultivated from eyes (seed potatoes), not from actual seed, so that desirable characteristics can be maintained. Thus, the potatoes within any specific cultivar are genetically identical because each cultivar is a clone. A single clone was farmed across Ireland as a staple food and when the potato blight arose it rapidly spread and destroyed the entire susceptible crop. Genetically diverse potatoes have much more inherent resistant to disease and only a few are likely to die when infected.
Schema of the relative production cost of NRs with scale.
Approximate global production or potential production regions of the leading rubber-producing crops. Rubber trees are restricted to tropical regions (purple), guayule to semiarid regions (yellow), and rubber dandelion and mountain gum to temperate regions with regular rainfall and snowy winters (blue). Opposing seasons can help ensure continuity of supplies.
Hevea plantation and tapper (left), field grown guayule (top center), and rubber dandelion both in the field (top right) and grown hydroponically (bottom right). Dandelion and guayule are processed mechanically (center shows a view of the Ohio State University pilot plant).
The effect of initiator (FPP in this case) and activator (Mg2+) concentration on initiation rate, polymerization rate, and rubber molecular weight in enzymatically active rubber particles purified from guayule. The experiments were done in 375 µM IPP (the IPP Km).
A schema of the rubber particle-bound RT-ase complex. Small species-specific peptides bind IPP and FPP substrates and deliver them to a cis-prenyl transferase held in place by an integral binding protein. The complex components are held by a dimeric scaffold that forms a channel to the interior. As catalysis occurs, the growing polymer is elongated into the rubber particle interior through the channel. Other protein components seem more related to RT-ase stability protecting the growing polymer from premature release of low molecular weight rubber.
Transmission electron micrograph of NRL in a laticifer of Hevea brasiliensis, with amounts and sources of protein that may persist in the final product indicated.
ELISA (D6499) of the cross reactivity of NRL and rubber particle proteins against rabbit polyclonal antibodies raised against proteins extracted from unleached medical gloves.
Hevea versus GNRL protein profiles as measured by different ASTM standard methods. Purified GNRL contains less than 1% of HNRL proteins (as seen in the silver stained SDS-PAGE gel on the right). Most (90%) of the trace protein (allene oxide synthase) in GNRL is immunogenically inert. The remaining 10% of protein do not invoke antigenic or allergenic responses. ASTM standards are included by number.84,85,95–97
Left panel: Guayule stem with a section of stembark excised. NRL is not visible. Guayule does have resin vessels, and a droplet of resin is apparent. Right panel: Scanning electron micrograph of rows of parenchyma cells filled with discrete rubber particles. Photos by Delilah Wood.
The first scale up of GNRL extraction beyond lab scale at the University of Arizona with Dr. Wayne Coates. Cornish (foreground) is wearing overalls.
Some GNRL products, including the first balloon animal made from GNRL in the history of the world.
TNRL was purified from greenhouse-grown plants. Left panel: Plants about 2 weeks before root harvest. Left middle panel: Cut roots showing NRL bleeding from the severed laticifers. Right middle panel: Purified TNRL. Rubber (TNR) made by drying the NRL is very pure (right panel).
Left panel: Stress-strain curves for TNRL cast films and HNRL and GNRL dipped glove films. Right panel: The first TNRL dipped glove.
Left panel: Unlike in guayule or Hevea, most of the rubber particles produced in dandelion laticifers coagulate into sold rubber while the plants are still alive, so NRL yields are quite low. Right panel: (a) Rubber is initially formed by rubber particles, but as the cells age, (b) the particles agglomerate and (c) then coagulate into large masses of solid rubber.
Contributor Notes