environmentally sustainable fibers from regenerated protein

Reviews

Environmentally Sustainable Fibers from Regenerated Protein

Andrew J. Poole,* Jeffrey S. Church, and Mickey G. Huson

CSIRO Materials Science and Engineering, P.O. Box 21, Belmont, Victoria, 3216, Australia

Received September 21, 2008; Revised Manuscript Received October 17, 2008

Concerns for the environment and consumer demand are driving research into environmentally friendly fibers asreplacements for part of the 38 million tonnes of synthetic fiber produced annually. While much current researchfocuses on cellulosic fibers, we highlight that protein fibers regenerated from waste or byproduct sources shouldalso be considered. Feather keratin and wheat gluten may both be suitable. They are annually renewable,commercially abundant, of consistent quality, and have guaranteed supply. They contain useful amino acids forfiber making, with interchain cross-linking possible via cysteine residues or through the metal-catalyzedphotocrosslinking of tyrosine residues. Previous commercially produced fibers suffered from poor wet strength.Contemporary nanoparticle and cross-linking technology has the potential to overcome this, allowing commercialproduction to resume. This would bring together two existing large production and processing pipelines, agriculturalprotein production and textile processing, to divert potential waste streams into useful products.

Introduction

Concern for the environment, rising oil prices, and the finitenature of oil reserves is driving research into ways to replacepetrochemical products with biobased materials. Targetsinclude bioplastics, films, packaging, building materials, and arange of other products including fibers.1-3 Global fiberproduction in 2005 was 70.6 million tonnes, of which 38 milliontonnes was synthetic, mainly polyester, nylon, and olefin fiber.4

Developing biobased alternatives for even a portion of this offersthe potential of significant environmental benefits.

A further driver comes from consumer demand, with growthof the “eco-friendly” and “organic” markets in textiles (as wellas food and other areas), reflecting the increased interest andpower of consumers. Surveys show environmental compatibilityis increasing as a sales argument,5 as demonstrated by organiccotton fetching a premium price over the nonorganic fiber, eventhough they are physically indistinguishable.6 However, surveysalso warn that consumers will not compromise product perfor-mance to have an eco-friendly product.6,7

While there is no international standard to describe eco-friendly, a fiber made from renewable raw materials, using anenvironmentally friendly and commercially viable process, andhaving triggered biodegradability (i.e., is biodegradable in

composting situations after disposal) or recycling capability canbe considered eco-friendly (Figure 1).

The desire for such products has led to a renaissance in fiberssuch as hemp and the adoption of nontraditional fibers, such asbamboo, for use in apparel.8 Attempts are being made to uselignocellulosic agricultural byproduct such as cornhusks, corn-stalks, and pineapple leaves as alternative sources of cellulosicfibers,9 and at least one regenerated cellulosic product is incommercial production, Lenzing Modal, which is produced frombeech wood in a process described as being in accordance withthe principal of sustainability.10

Another biomaterial that has attracted less attention but isworthy of consideration is agriculturally derived protein.

Fibers of regenerated protein were produced commerciallyin the 1930-50s and by today’s standards they would beconsidered natural, sustainable, renewable, and biodegradable.Casein from milk was used by Courtaulds Ltd. to makeFibrolane and by Snia to make Lanital; groundnut (peanut)protein was used by ICI to make Ardil; Vicara was made bythe Virginia-Carolina Chemical Corporation from zein (cornprotein); and soybean protein fiber was developed by the FordMotor Company.11,12

The regenerated fibers had several qualities typical of the mainprotein fibers, wool and silk; they were soft with excellent drapeand high moisture absorbency. They could be processed onconventional textile machinery and colored with conventional

* To whom correspondence should be addressed. Phone: +61-3-52464000. Fax: +61-3-5246 4057. E-mail: [email protected].

January 2009 Published by the American Chemical Society Volume 10, Number 1

Copyright 2009 by the American Chemical Society

10.1021/bm8010648 Published 2009 by the American Chemical SocietyPublished on Web 11/26/2008

dyes. Superior to wool in some regards, they did not prickle,pill or shrink. They could be produced as staple or filament,crimped or straight, with control over diameter, and dope-dyedif required.13-15

Their drawback was poor mechanical strength when wet. Dryfiber strength was acceptable due to interchain hydrogen bondingbetween protein macromolecules.11 In the wet state, however,the fiber became weak as hydrogen bonding occurred prefer-entially with water molecules and the density of interchaincovalent cross-links was insufficient to impart strength16 (Table1).

These technical issues, rising raw material and productioncosts and the ascent of the petrochemical synthetic fibers, withtheir constant and consistent supply of materials and superiorperformance, caused production of regenerated protein fibersto stop in the late 1950s.

To make a protein fiber for today’s market would requirethe wet-strength problem to be solved. To this end, the advancesmade since the 1950s in cross-linking technology and use ofnanoparticle reinforcing agents have not been applied toregenerated protein fibers, though they offer the potential ofimproving tensile strength.

Further, since the 1950s new protein sources have becomeavailable as agricultural byproducts, with concomitant infra-structure for large-scale production. Keratin from feathers andgluten from wheat are of particular interest.

In this paper we discuss whether with new technical advancesand new protein sources, regenerated protein fibers are likelyto re-emerge as ecologically friendly fibers.

Requirements of Textile Fibers. It is rare that one individualproperty will determine the value of a fiber. Rather, a combina-tion of properties will govern technical and commercial success.For conventional textile applications these relate to20 (1)acceptable tensile strength of around 5 g/denier (574 MPa); (2)acceptable elongation at break (above 10%); (3) reversibleelongation in the range up to 5% strain; (4) modulus of elasticitybetween 30 and 60 g/denier (3443-6887 MPa) conditioned andnot dropping too much in the wet; (5) moisture absorption of2-5%; (6) dyeability, comfort, easy care, and abrasion resistance(though this equally relates to fabric construction and is notalways important); (7) resistance to dissolution and strongswelling in water and moderately strong acids, alkalis and basicsolvents up to temperatures of 100 °C; and (8) without atendency to catch fire or support combustion.

For the fiber to be utilized by industry, the quality andquantity must be consistent over time.6 This requires a reliablesupply chain to be in place with both the quality and quantityof raw material to be consistent over time. There must also besufficient profit available throughout the production pipeline.

Fiber Structure. Fibers are composed of oriented assembliesof linear macromolecules.21 Properties of the assembly can be

improved through choice of polymer, cross-linking and crystal-linity, and the incorporation of reinforcing fillers or nanopar-ticles.20

Polymer. In general, a polymer molecular weight of 10-50kDa will produce good fibers.22 Experience from synthetic fibersshows the optimum varies depending on the polymer. Forexample, the closely related compounds polyhexamethylenefumaride and polyhexamethylene succinamide exhibit goodfibrous properties at 12 and 25 kDa, respectively.23

For proteins, increasing molecular weight is thought toincrease the area of contact between chains,16 but going beyondthe optimum molecular weight does little to improve fiberproperties.23 At very high molecular weight the protein chainscan loop back and forth, which limits fiber strength.16 Unifor-mity of chain length is thought to be a potential advantage.21

Molecules should be linear and consist of residues withoutbulky side groups as these can prevent the close packing ofchains and reduce crystallinity.16 Close packing is also desirableto give shorter covalent cross-link distance.

Amino acids capable of forming interchain cross-links aredesirable. Cysteine residues are particularly useful as they canspontaneously combine to form cystine through formation of acovalent disulphide cross-link. Other desirable residues includetyrosine, glutamic and aspartic acid, arginine, lysine, andserine.24,25

Cross-Linking and Crystallinity. Cross-linking and crystal-linity affect the protein fiber’s tensile strength and otherproperties. Their degree and character are greatly affected bythe amino acid composition of the protein and the processingconditions. Before fiber spinning, the dissolved protein chainsmust be put into an extended, unraveled form (noncovalentinterchain bonds disrupted). The spun fiber is drawn (elongated)to maximize chain alignment, give close packing of chains andto allow regions of crystallinity to develop. Elongation occursabove the glass transition temperature (Tg) as below thistemperature mobility is suppressed.26 The chain alignment leadsto considerable increases in mechanical strength.

Spontaneous interchain cross-linking can occur betweenneighboring chains via suitable residues, particularly cysteine.Additional covalent links can be formed between a range ofamino acid residues using chemical cross-linking agents. Metal-catalyzed photo cross-linking forms interchain dityrosine resi-dues.24 Transglutaminase is capable of cross-linking wool fibersand is used in commercial wool finishing; it forms links betweenlysine and glutamine residues.27 Glutaraldehyde is used in foodapplications and forms covalent links between lysine andtyrosine residues.28,29

An important factor is the distance that cross-linking agentsare able to bridge, and chains must be packed tightly enoughthat at least two reactive sites are available at the requireddistance for cross-link formation.25 Cross-linking is also limitedby the location of suitable amino acid residues along the chain.

Fillers and Nanoparticles. The inclusion of nanoparticles inpolymer systems to produce so-called nanocomposite materialshas been shown to give remarkable increases in tensile propertiesat low nanoparticle addition levels.30 The Toyota group wereone of the first to demonstrate excellent mechanical propertiesin a polymer nanocomposite by reinforcing nylon with clay.31

Layered silicates, such as montmorillonite and hectorite, havestrong interaction between silicate layers and the matrix viahydrogen bonding in nylon 6.32 These results have beenextended to other polar polymers, (e.g., epoxies), though theiruse is not always straightforward and the change in propertiesdepends on many factors such as nanoparticle size, aspect ratio,

Figure 1. Concept of an eco-friendly fiber.

2 Biomacromolecules, Vol. 10, No. 1, 2009 Poole et al.

surface area, and polymer/filler compatibility.32,33 Other proper-ties can also be modified, for instance, the Tg can either increaseor decrease depending on the polymer, nanoparticle, and weightfraction used.34 Biodegradability can be increased as shown bycomposting experiments with polymers such as polylactic acid.32

Clays have high affinity for protein and so it is not surprisingthat good results have been obtained with nanoclay fillers inregenerated protein. Chen and Zang35 adding montmorillonitenanoclay at the 20% level in soy protein sheets increased theYoung’s modulus from 180 to 587 MPa and tensile strengthfrom 8.8 to 15.4 MPa compared to an unmodified control. Thenanoclay was shown to become highly exfoliated with surfacepositive charges on the globulins anchoring into the negativelycharged montmorillonite galleries and good nanoclay dispersionoccurred due to electrostatic interactions and hydrogen bonding.

Yu et al.36 increased the tensile strength and Young’s modulusof soy protein sheets using rectorite nanoclay. Maximumincrease in tensile strength (from 6.8 to 12.9 MPa) was at 12%nanoclay addition while maximum increase in Young’s moduluswas at 16% addition (3.6 times increase to 621 MPa).Simultaneously, the elongation at break decreased from over100% to less than 10% with 16% nanoclay addition.

Huang and Netravali37 increased the tensile properties of soyprotein using nanoclay and flax fibers and cross-linking withglutaraldehyde. Ai et al.38 enhanced soy polymer sheets with4% nano-SiO2, while Chen et al.39 used lignin at 6% withglutaraldehyde to increase Young’s modulus from 8.4 to 23.1MPa and the Tg from 62.5 to 70.4 °C.

Carbon nanotubes offer considerable potential for compositereinforcement due to their remarkable mechanical strength.40

Multiwall CNTs of varying sizes have reinforced soy proteinsheets. The protein chains wrapped and penetrated the CNTs,interacting with both internal and external surfaces. The strengthimparted by the CNTs depended on the protein/matrix interfacetransferring stress to the CNT. CNTs of 10-15 nm diameterincreased Young’s modulus from less than 120 MPa toapproximately 250 MPa at 0.25% addition41 with tensile strengthincreasing from less than 8 to about 12 MPa. It can be assumedthat CNTs with functionalized surfaces may interact morestrongly with the protein and give a greater effect. Nonetheless,CNTs are likely to remain too expensive to use as commodityfillers for some time.

Cellulosic whiskers can be used as reinforcing microfibrilsto create nanocomposites of outstanding properties.42 Thelignocellulosic fibers are derived from annually renewableresources and provide environmental benefits with respect todisposability and renewability. The increase in mechanicalproperties is governed by the aspect ratio of the nanofiber, its

degree of crystallinity, processing methods and matrix structure.The modulus of the native cellulose perfect crystal is estimatedto be 150 GPa,43 while measurements of the highly crystallinebacterial cellulose nanofibers show a Young’s modulus of 78( 17 GPa.44 Cellulose nanofibers have relatively reactivesurfaces,42 making them amenable to covalent bonding.

Protein Sources. The protein used for regenerated fiberproduction must have both the correct polymer characteristicsand the necessary eco-friendly characteristics discussed earlier.Two proteins which appear to meet these criteria are keratinfrom chicken feathers and gluten from wheat.

Feather Keratin. Chicken feathers are probably the mostabundant keratinous material in nature.45 An estimated 5 milliontonnes are produced annually as a waste stream from theproduction of chicken meat, of which over 65 million tonneswas produced worldwide in 2007 (calculated from refs 46 and47). Meat processing occurs on a year-round basis in centralizedlocations, and the collected feathers have minimal value. Theirdisposal represents a significant problem to the poultry farmingindustry, with some used in low-grade animal feeds and theremainder going to landfill, thus making transport the main costof the raw material.

Feather keratins are small proteins, uniform in size, with amolecular weight around 10 kDa. They are rich in cysteine andhydrophobic residues and have a �-sheet conformation.43,45,46

Arai et al.48 sequenced chicken feather keratin and found it tobe 96 residues long containing seven cysteine residues. Thesewere restricted to the terminal regions: six in the N-terminaland one in the C-terminal region. These regions were almostdevoid of the �-structure. The central portion of the moleculewas rich in R-structure and contained a high proportion ofhydrophobic residues. Overall, the molecule was poor in chargedamino acids.

Whole feathers and feather fibers (barbs) are widely studiedfor potential biomaterial applications due to their inherentproperties of strength and chemical resistance.2,3,49,50 Featherfiber strength and modulus are 1.4 and 35.6 g/denier, respec-tively (161 and 4086 MPa, respectively), similar to wool,9

leading to their use in composite extruded fibers made withLDPE, HDPE, and polypropylene,51-53 in composite materialswith wood-MDF,2 with poly(methyl methacrylate),54 in com-pression-molded HDPE,55 and in biobased composite materi-als.56

Solubilized feather keratin is becoming more widely studiedas a source of biopolymer for films,3,57,58 including edible filmsor coatings and50 compostable packaging46 and for inclusionin composite materials.52

Table 1. Physical Properties of Commercial Regenerated Protein Fibers Compared To Natural and Synthetic Fibers

dry wet

fibertenacity

(g/denier)initial modulus

(g/denier)breaking

extension (%)tenacity

(g/denier)initial modulus

(g/denier)breaking

extension (%)

Fibrolane (casein) 1.1 40 63 0.35 2 60Ardil (peanut) 0.8-1.0 30 10-110 0.3 0.5 90Vicara (zein) 1.0 50 28 0.6 15 28soybean (Drackett Co.) 0.6 40 40 0.12 4 40wool, merino 1.6 25 43 1.1 10 57cotton 3.6 30 9 4.0 10 10silk (Bombyx mori) 3.7 120 16 3.4 30 26polyester (Terylene 45/24) 5.3 120 15 5.3 15 120nylon 6 (Grilon 30/7) 5.4 19 31 4.7 19 26polypropylene (Ulstron) 7.4 80 17 7.4 80 17

Where tenacity is the specific force necessary to break the fiber in units of g/denier, where denier is the mass in grams of 9000 meters offiber, measurements being standardized on fiber mass rather than diameter.17 Data are from Farrow,18 except polypropylene from Ford.19

Environmentally Sustainable Protein Fibers Biomacromolecules, Vol. 10, No. 1, 2009 3

Biodegradable and breathable food packaging films have beenproduced from solubilized keratin59 and keratin fiber has beenproduced by extruding at low temperature (120 °C) after mixingwith glycerol, water, and sodium sulfite.60

Regenerated keratin fibers were produced by wet-spinningprotein-detergent complexes in the laboratory during the1940s.61 Dry strength of greater than 3 g/denier (344 MPa) wasachieved; however, wet strength was poor, being 60% lower.Commercial production was not attempted, partly due to thedisorganized supply routes available in the 1930-50s.

In a parallel development, regenerated keratin fibers have beenproduced from wool62 and wool/casein blends63 at laboratoryscale. Wool is similar to feather in some regards, both keratinsbeing highly cross-linked, although wool proteins are hetero-geneous with a generally higher molecular weight (10-55kDa64), higher cysteine content (10.5 mol % for whole merinofiber65 with some sulfur-rich components containing 12-41 mol%66), and a predominantly helical configuration.66 The purewool regenerated fibers were stronger than the protein blendfibers and had a tensile strength of 1 g/denier (115 MPa) and33% extension at break for conditioned samples, with theR-keratin configuration of the native wool being converted to a�-conformation in the regenerated fiber.62,67 The authors sug-gested this made the fibers a usable textile material and thattheir process could form the basis of a commercial process, withthe caveat that quality wool fiber was too expensive to use asa starting material. To overcome this, they suggested using wastewool or other keratin sources such as horns, hooves, or nails asprotein sources. Commercial production was not attempted,possibly in part because there was only a 35% conversionefficiency of native wool to regenerated fiber.62

Current research interest in regenerated feather keratinmaterials has not spread to fibers, though the presence of cross-linking sites, hydrophobic residues, and uniform molecularweight suggest this material should form robust fiber. The maindrawback is the difficult solubilization route and low molecularweight, which may make fiber production more difficult.

Wheat Gluten. Gluten is the principal protein fraction isolatedfrom wheat (and some other grains). It is biodegradable,abundant, and renewable.68 Its availability is likely to increaseas industrial use of wheat increases, such as for biofuelproduction in the European Union and Canada.69

The world production of wheat was 625 million tonnes in2005/06.47,69 The grain contains about 12% proteins, with75-85% of this being gluten proteins. Gluten is defined asthe material that remains when wheat dough is washed toremove starch granules, though in practice the term refers tothe proteins.70 Other constituents in industrially preparedgluten are lipids (3.5-6.8%), minerals (0.5-0.9%), andcarbohydrates (7-16%).71

The gluten proteins contain hundreds of components over awide range of molecular weights, with primary protein chainsranging from 20 to 90 kDa.70 They are subdivided into thegliadins and the glutenins; two groups of roughly equalproportions that can be separated according to their solubilityin alcohol-water solutions (e.g., 60% ethanol): the solublegliadins and the insoluble glutenins.70,72

The gliadins are protein molecules in which disulphidebonding is solely intramolecular.72 They have a molecularweight of 30-80 kDa, are rich in proline and glutamine andhave a low level of amino acids with charged side groups. Theyact as a plasticizer in dough formation, associating with otherchains through hydrophobic interactions and hydrogen bond-ing.71

The glutenins are proteins with primary protein chains linkedtogether via intermolecular disulphide bonds, although intrachainbonds are also present. Molecular weight varies from 500 tomore than 10000 kDa for the cross-linked molecules makingthem among the largest in nature. The primary protein chainsare grouped into the high-molecular-weight glutenin subunits(HMW-GS; 70-90 kDa; 7-13% of total gluten protein) andthe low-molecular-weight glutenin subunits (LMW-GS; 20-45kDa; 19-25% of total gluten proteins, making them predomi-nant).70,72 The reduced subunits have similar solubility inethanol-water as the gliadins.70

HMW-GS contain cysteine residues that allow end-to-end,probably head-to-tail, linkages. This forms a backbone fromwhich the LMW-GS branch. The LMW-GS contain eightcysteine residues, two for interchain bonding, and six forintrachain bonds.70

During bread making, additional intermolecular cross-linksare formed between tyrosine residues.73 Dityrosine links arenaturally occurring in a range of proteins, including the elasticligaments of insects (resilin), elastin, and collagen.74 Trans-glutaminase is capable of cross-linking globulins and HMW-GS.28,29

Gluten has been studied for producing food packaging filmsdue to its ready availability, good film forming properties,potential to make edible packaging, and environmental creden-tials.1,75 Its main drawback has been its high water sensitivity;water acts as a plasticizer leading to poor wet strength andreduced barrier properties.

Gluten films have been found to have a Tg around 38 °Cand so are brittle at room temperature.76 Glycerol is often addedas a plasticizer, making the films soft and pliable.1 Glycerol-plasticized gluten has also been thermo-molded into biodegrad-able plastics.77

Montmorillonite nanoclays added as fillers have been foundto improve tensile properties of gluten films. When added at5% level to films plasticized with glycerol, the nanoclayincreased Young’s modulus from 3.7 to 10.6 MPa.76

Gluten has a structure that is consistent with being able toform highly cross-linked nanocomposite fibers. Residues thatself-cross-link are present as well as residues that cross-link withglutaraldehyde, transglutaminase, or metal-catalyzed photocross-linking, and gluten interacts with montmorillonite nano-clay. However, the first report of gluten fibers produced by awet-spinning technique appeared only recently.78,79 Glutensolution was extruded from a syringe into a coagulating bathof 10% (w/w) sodium sulfate and 10% (w/w) sulfuric acid.The fiber produced had a Young’s modulus of 5 GPa in theconditioned state (21 °C, 65% relative humidity), though thewet results were not reported. The fibers were drawn but didnot develop crystalline regions.

No reports have been found of chemically cross-linkednanocomposite gluten fibers.

A summary of the desired properties for forming protein fiberis compared to feather keratin and wheat gluten in Table 2.

The proteins can be blended to modify their properties or tomaximize their utilization, by diluting one protein with another.When the proteins being blended contain cysteine residues, theywould also be expected to form interchain disulfide cross-linksbetween the different proteins. Barone et al.80 found there wasintimate interaction and cross-linking between blends of featherkeratin and gluten and feather keratin and the cysteine-containingprotein lactalbumin. The authors stated that blending differenttypes of proteins was a convenient method for altering proteinproperties. The “toughness” of thermally processed keratin films


was increased by blending the keratin with gluten in equalproportions, though at the expense of decreasing the film’sstrength and stiffness. Similarly, the properties of keratin andlactalbumin blend films were found to be dependent on theproportion of each protein in the blend, and the authorsconcluded the film mechanical properties were a compromisebetween those of the individual proteins. In contrast, Wormell63

showed little correlation between the mechanical properties ofregenerated protein fibers and the composition of the proteinblend used in their production. Wool keratin and casein werethe proteins used. The tenacity of the fibers appeared to be moreaffected by fiber denier than the protein ratios.63 This highlightsthat while blending may be a convenient way of modifyingprotein properties, and perhaps of maximizing protein utilization,the optimum blend of proteins will need to be found empirically,taking into account fiber processing conditions.

Fiber Production. The first step in fiber production isdissolution of the protein. As the 1930-50s regenerated proteinfibers used globular proteins, dilute alkali solutions weresufficient to swell and dissolve the proteins. Fiber could thenbe produced by extruding into an acid solution.81 The situationis more complex for feather keratin and wheat gluten as theycontain intermolecular covalent bonds (disulphide cross-linksbetween cysteine residues) that must be cleaved while preservingthe covalent bonding of the primary protein chain, as short chainsegments will adversely affect fiber strength.15

Cleaving the disulphide bonds can be achieved by reduction,oxidation, sulfitolysis or oxidative sulfitolysis.82 Reductionreactions are commonly carried out using thiols (R′-SH). A largeexcess of the thiol is required.83 The reaction is reversible andproceeds by two nucleophilic displacement reactions.

R-CH2-S-S-CH2-R+R′S-hR-CH2-S-S-R′+R-CH2-S-

R-CH2-S-S-R′+R′S-hR′S-S-R′+R-CH2-S-

where R represents the peptide backbone. The net reactionresults in the formation of two free thiol groups attached to theprotein chains that can be reformed into disulfide cross-links.

Oxidation of the disulfide bond can be carried out usingperoxide as shown below:

R-CH2-S-S-CH2-R+O22- y\z

[O]2R-CH2-SO3

-

This reaction forms cysteic acid and is not reversible makingit less attractive for protein dissolution.

Sulfitolysis describes the cleavage of the disulfide bond bysulfite:

R-CH2-S-S-CH2-R+ SO32-hR-CH2-S-SO3

-+

R-CH2-S-

It has been suggested that bisulfite is the active species.83 Inany case, the reaction is reversible and gives an S-sulfonateanion and a thiol.

Oxidative sulfitolysis converts the disulfide into two S-sulfonate anions:

R-CH2-S-S-CH2-R+ 2SO32-+H2O98

[O]

2R-CH2-S-SO3-+ 2OH-

This reaction is not reversible.Gluten appears easier to dissolve than feather keratin. Reddy

and Yang78 readily dissolved it using 1% sodium sulfite asreducing agent and 8 M urea as swelling agent.

Keratin is often dissolved using 2-mercaptoethanol and urea,with the reaction cleaving cross-links without damage to theprotein backbone. However, the cost of these reagents wouldprohibit their use for commercial fiber production. An alternativedescribed by Jones and Mecham84 uses the inexpensive andrelatively abundant sodium sulfide.

The reaction of sodium sulfide with keratin is very complex.Sodium sulfide dissociates in water to form a thiol and ahydroxide anion as follows:85

Na2S+H2Of 2Na++HS-+OH-

The hydrosulfide anion reduces the disulphide bond accordingto the reaction:

R-CH2-S-S-CH2-R+HS-fR-CH2-SH+R-CH2-S-S-

In reality, a mixture of the protonated and deprotonated formsof both products, cysteine thiol and perthiocysteine, would bepresent. Both of these products are highly reactive.

The high pH not only disrupts hydrogen bonding, disaggre-gating the protein, but the hydroxide ions can also react withthe disulfide bonds forming dehydroalanine by �-elimination(Scheme 1).

The dehydroalanine residues readily form cross-links byreacting with the amino acid side chains of cysteine and lysineto form lanthionine (Scheme 2) and lysinoalanine (Scheme 3).

These reactions have the potential to provide a mechanismfor cross-linking the proteins after fiber formation, thus improv-ing their physical properties. However, the strong reducingconditions generated by sodium sulfide have the potential todamage the protein backbone,62 although Jones and Meecham84

Table 2. Desired Protein Properties for Forming Fibers Comparedto the Properties of Feather Keratin and Wheat Gluten

propertyoptimum for

fiber productionfeatherkeratin

wheatgluten

molecular weight 10-50 kDa 10 kDa 20-90 kDamolecular

weight rangenarrow narrow broad

crystallinity desired native materialhas crystallites

noncrystalline

cross-linking sites desired yes: cysteine yes: cysteine,tyrosine

linear molecule desired yes often nothydrophobic groups desired yes often notraw material

availabilityreliable supply,

reliable quality5 million

tonnes annuallypotentially

largeenvironmental

credentialseco-friendly byproduct;

low-valueuse or landfill

byproduct;food forhumanconsumption

biocompatibility nontoxic nontoxic nontoxic

Figure 2. Wet spinning process. Protein dope is pumped through aspinneret into a coagulation bath to form fiber. The fiber is drawn,usually between rollers of different speeds, and cross-linked prior todrying and winding up.


supplied some evidence that they managed to cleave the cross-links without causing substantial damage to the protein chain.

Once the protein chains have been dissolved, disaggregatedand elongated, they are spun into fiber using a wet-spinningtechnique (Figure 2). Protein in an efficient solvent (referred toas protein dope) is pumped through a fine spinneret into acoagulation bath that contains a poor solvent, generally a

concentrated salt solution that dehydrates the protein, causingit to come out of solution and form a filament. The filament isthen stretched to increase chain alignment and crystallinity anddecrease the distance between potential cross-linking sites.

Potential for Fiber Manufacture. In the 1930-50s, thechoice of protein was based on economic abundance rather thanstructure. Today a similar compromise between environmentalacceptability, availability and fiber forming properties is likely.

Feather keratin, wheat gluten, and other protein sources (e.g.,zein from corn used for bioethanol production) are eco-friendlyinsofar as they are annually renewable byproducts that are beingunder-utilized as a resource, are produced in substantial quanti-ties, and will continue to be produced in the future.

Producing regenerated protein fiber will connect the existingproduction pipelines for protein production and the textileproduction, marketing, and distribution pipeline (Figure 3), asthe protein fibers will feed into existing textile processesincluding the cotton system.14

Regenerated fibers have not been produced that containnanoclays, cellulose nanofibers or other nanoparticles as rein-forcing fillers, even though these offer improvements in me-chanical strength. Nanoparticles can also improve biodegrad-ability and, in the case of cellulose nanofibers at least, arethemselves renewable. Combined with cross-linking technolo-gies, these offer the potential to produce fibers of acceptablestrength.

If experimental results show the combination of nanoparticlefillers and cross-linking techniques produce fibers of acceptablestrength, the regenerated protein fibers will have all theenvironmental and technical attributes required for success. Aneconomic analysis will then be the final determinant as towhether they are commercially viable.

Market Opportunities and Competitive Advantage. De-mand from consumers for eco-friendly products is growingstronger.5 While organic fibers are meeting part of this need,they are unlikely to be produced in sufficient quantities to meetall the demand for eco-friendly fiber.6 Organic fibers can haveadditional problems, such as organic cotton returning a yieldapproximately 50% lower than conventional cotton and sorequiring more land and water for the same fiber production.Weed control is often by tillage, which can be detrimental tosoil conservation.6

Hemp, organic wool, and recycled petrochemical syntheticfibers are marketed as eco-friendly.5,86 Consumers who buythese materials are generally environmentally conscious.5

Regenerated protein fibers should compete well with organicand other eco-friendly fibers based on their environmentalcredentials. The processing route should allow them to becertified as organic provided organic practices were used in the

Scheme 1

Scheme 2

Scheme 3

Figure 3. Regenerated keratin fiber production would connect twoexisting production pipelines.


production of the protein (i.e., organic chicken or wheat). Theycould similarly be declared free of genetic modification. Theywill be able to be processed on conventional textile machineryand dyed with conventional textile dyes. Potential productmarkets where they may have competitive advantage are in eco-friendly apparel as well as technical and industrial applications.

Conclusion

Regenerated protein fibers are potentially environmentallysustainable, renewable and biodegradable. Two protein sources,feather keratin and wheat gluten, have been considered for theirsuitability to make an eco-friendly regenerated fiber. Both appearto be viable, although low wet strength is likely to beproblematic. The inclusion of nanoparticles and use of cross-linking technologies offer the potential to improve mechanicalstrength to make them fit for use in apparel or technical textileapplications. All elements of a supply chain are in place fortheir production: there is a guaranteed supply of material fromcentralized locations and these materials are inexpensive andconsistent in quality. Once produced, fiber can be processed onconventional textile equipment and use conventional dyes, thusmoving into the existing textile distribution chain.

References and Notes

(1) Hernandez-Munoz, P.; Kanavouras, A.; Ng, P.; Gavara, R. J. Agric.Food Chem. 2003, 7648–7654.

(2) Winandy, J.; Muehl, J.; Micales, J.; Raina, A.; Schmidt, W. Potentialof chicken feather fibre in wood MDF composites. In EcoComp 2003;Queen Mary, University of London: London, 2003.

(3) Yin, J.; Rastogi, S.; Terry, A.; Popescu, C. Biomacromolecules 2007,8, 800–806.

(4) Saurer, The Fiber Year 2005/2006; http://www.textileindia.in/Feb2007/cot.htm (accessed 28 April, 2008).

(5) Anon, Green Textiles in Demand. WSA Performance and Sports Materi-als; World Trades Publishing: Liverpool, England, 2007, pp 30-34.

(6) Thiry, M. If the environment is important. AATCC ReView; AmericanAssociation of Textile Chemists and Colourists: Research TrianglePark, North Carolina, 2007; pp 21-28.

(7) Anon, How green is becoming the new black in the textile chain. WSAPerformance and Sports Materials; World Trades Publishing: Liver-pool, England, 2007, pp 35-37.

(8) Anon, If he hits a home-run I’ll eat my hat. WSA Performance andSports Materials; World Trades Publishing: Liverpool, England, 2007,pp 48-51.

(9) Reddy, N.; Yang, Y. J. Polym. EnViron. 2007, 15, 81–87.(10) Lenzing, http://www.lenzing.com/en/index.jsp (accessed 1 April, 2008).(11) Hearle, J. J. Mater. Sci. 2007, 42, 8010–8019.(12) Moncrieff, R. Man-Made Fibres, 6th ed.; Newness-Butterworths:

London, 1975.(13) Cook, J. Handbook of Textile Fibres II. Man-Made Fibres, 4th ed.;

Merrow Publishing Co., Ltd: England, 1968.(14) Mauersberger, H. Matthew’s Textile Fibers. Their Physical, Micro-

scopic, and Chemical Properties; John Wiley and Sons Inc.: NewYork, 1954.

(15) Press, J. Man-Made Textile Encyclopedia; Textbook Publishers Inc.:London, 1959.

(16) Lundgren, H. AdV. Protein Chem. 1949, 5, 305–351.(17) Morton, W. E.; Hearle, J. W. S. Physical Properties of Textile Fibres;

The Textile Institute: Manchester, 1993.(18) Farrow, B. J. Text. Inst. 1956, 47, T58–T101.(19) Ford, J. E. Fibre Data Summaries; Shirley Institute: Manchester,

England, 1968.(20) Atlas, S. M.; Mark, H. F. Structural principles of fiber-forming polymers.

In Man-Made Fibers. Science and Technology; Mark, H. F., Atlas, S. M.,Cernia, E., Eds.; Interscience Publishers: New York, 1967; Vol. 1.

(21) Hearle, J.; Peters, R. Fibre Structure; Butterworth and Co., Ltd:London, 1963.

(22) Huang, F.; Rha, C. Polym. Eng. Sci. 1974, 14, 81–91.(23) Peters, R. Textile Chemistry. The Chemistry of Fibres; Elsevier

Publishing Company: Amsterdam, 1963; Vol. I.

(24) Elvin, C.; Carr, A.; Huson, M.; Maxwell, J.; Pearson, R.; Vuocolo,T.; Liyou, N.; Wong, D.; Merrit, D.; Dixon, N. Nature 2005, 473,999–1002.

(25) Ziegler, K. Crosslinking and self-crosslinking in keratin fibres. InChemistry of Natural Protein Fibers; Asquith, R., Ed.; Plenum Press:New York, 1977; pp 267-300.

(26) Ziabicki, A. Physical fundamentals of the fiber-spinning processes.In Man-Made Fibers; Mark, H. F.; Atlas, S. M.; Cernia, E., Eds.;Interscience Publishers: New York, 1967; Vol. 1.

(27) Gerrard, J.; Fayle, S.; Wilson, A.; MPNewberryRoss, M.; Kavale, S.J. Food Sci. 1998, 63, 472–475.

(28) Gerrard, J.; Brown, P.; Fayle, S. Food Chem. 2003, 80, 35–43.(29) Gerrard, J.; Brown, P.; Fayle, S. Food Chem. 2003, 80, 45–50.(30) Manias, E.; Touny, A.; Wu, L.; Strawhecker, K.; Lu, B.; Chung, T.

Chem. Mater. 2001, 13, 3516–3523.(31) Fukushima, Y.; Okada, A.; Kawasumi, M.; Kurauchi, T.; Kamigaito,

O. Clay Miner. 1988, 23, 27–34.(32) Sinha Ray, S.; Okamoto, M. Prog. Polym. Sci. 2003, 28, 1539–1641.(33) Holmes, G.; Ford, K. In ACS 16th Annual Technology Conference on

Composite Materials, Blacksburg, Virginia, September 9-12, 2001;American Chemical Society: Washington, DC, 2001.

(34) Jordan, J.; Jacob, K.; Tannenbaum, R.; Sharaf, M.; Jasiuk, I. Mater.Sci. Eng., A 2005, 393, 1–11.

(35) Chen, P.; Zhang, L. Biomacromolecules 2006, 7, 1700–1706.(36) Yu, J.; Cui, G.; Wei, M.; Huang, J. J. Appl. Polym. Sci. 2007, 104,

3367–3377.(37) Huang, X.; Netravali, A. Compos. Sci. Technol. 2007, 67, 2005–2014.(38) Ai, F.; Zheng, H.; Wei, M.; Huang, J. J. Appl. Polym. Sci. 2007, 105,

1597–1604.(39) Chen, P.; Zhang, L.; Peng, S.; Liao, B. J. Appl. Polym. Sci. 2006,

101, 334–341.(40) Coleman, J.; Khan, U.; Blau, W.; Gun’ko, Y. Carbon 2006, 44, 1624–

1652.(41) Zheng, H.; Ai, F.; Wei, M.; Huang, J.; Chang, P. Macromol. Mater.

Eng. 2007, 292, 780–788.(42) Samir, M.; Alloin, F.; Dufresne, A. Biomacromolecules 2005, 6, 612–

626.(43) Sakurada, I.; Nukushina, Y. J. Polym. Sci. 1962, 57, 651–660.(44) Guhados, G.; Wan, W.; Hutter, J. Langmuir 2005, 21, 6642–6646.(45) Onifade, A.; Al-Sane, N.; Al-Musallam, A.; Al-Zarban, S. Bioresour.

Technol. 1998, 66, 1–11.(46) Schrooyen, P.; Dijkstra, P.; Oberthur, R.; Bantjes, A.; Feijen, J. J.

Agric. Food Chem. 2000, 48, 4326–4344.(47) USDA World Supply & Utilisation of Major Crops, Livestock, and

Products, www.ers.usda.gov/publications/agoutlook/aotables/2007/06jun/aotab23.xls (accessed 2 August, 2007).

(48) Arai, K.; Takahashi, R.; Yokote, Y.; Akahane, Y. Eur. J. Biochem.1983, 132, 501–507.

(49) Halford, B. Going beyond feather dusters. Chem. Eng. News 2004,36–39.

(50) Schrooyen, P.; Dijkstra, P.; Oberthur, R.; Bantjes, A.; Feijen, J. J.Agric. Food Chem. 2001, 49, 221–230.

(51) Barone, J. Composites, Part A 2005, 36, 1518–1524.(52) Barone, J.; Schmidt, W. Compos. Sci. Technol. 2005, 65, 173–181.(53) Bullions, T.; Hoffman, D.; Gillespie, R.; Price-O’Brien, J.; Loos, A.

Compos. Sci. Technol. 2006, 66, 102–114.(54) Martınez-Hernandez, A.; Velasco-Santos, C.; Icaza, M. d.; Castano,

V. Polymer 2005, 46, 8233–8238.(55) Barone, J.; Schmidt, W.; Liebner, C. Compos. Sci. Technol. 2005,

65, 683–692.(56) Hong, C.; Wool, R. J. Appl. Polym. Sci. 2005, 95, 1524–1538.(57) Tanabe, T.; Okitsu, N.; Yamauchi, K. Mater. Sci. Eng., C 2004, 24,

441–446.(58) Yamauchi, K.; Yamauchi, A.; Kusunoki, T.; Kohda, A.; Konishi, Y.

J. Biomed. Mater. Res., Part A 1996, 31, 439–444.(59) Martelli, S.; Moore, G.; Paes, S.; Gandolfo, C.; Laurindo, B.

LWTsFood Sci. Technol. 2006, 39, 292–301.(60) Barone, J.; Schmidt, W.; Gregoire, N. J. Appl. Polym. Sci. 2006, 100,

1432–1442.(61) Lundgren, H. Text. Res. J. 1945, 15, 335–353.(62) Happey, F.; Wormell, R. J. Text. Inst. 1949, T855–869.(63) Wormell, R. L. J. Text. Inst. 1948, 39, T219-T224.(64) Gillespie, J. M. The Proteins of Hair and Other Hard R-Keratins. In

Cellular and Molecular Biology of Intermediate Filaments; Goldman,R. D., Steinert, P. M., Eds.; Plenum Press: New York, 1990; pp 95-128.

(65) Bradbury, J. H.; Chapman, G. V.; King, N. L. R. Chemical compositionof the histological components of wool. In Symposium on Fibrous


Proteins Australia 1967; Crewther, W. G., Ed.; Butterworths: Sydney,1967; pp 368-372.

(66) Plowman, J. E. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci.2003, 787, 63–76.

(67) Happey, F. Nature (London, U.K.) 1949, 164, 184–185.(68) Bietz, J.; Lookhart, G. Cereal Foods World 1996, 41, 376–382.(69) FAO, Food Outlook, Global Market Analysis, June 2007. www.fao.org

(accessed, 2 August, 2007).(70) Wieser, H. Food Microbiol. 2007, 24, 115–119.(71) Van Der Borght, A.; Goesaert, H.; Veraverbeke, W.; Delcour, J.

J. Cereal Sci. 2005, 41, 221–237.(72) D’Ovidio, R.; Masci, S. J. Cereal Sci. 2004, 39, 321–339.(73) Tilley, K.; Benjamin, R.; Bagorogoza, K.; Okot-Kotber, B.; Prakash,

O.; Kwen, H. J. Agric. Food Chem. 2001, 49, 2627–2632.(74) Malencik, D.; Sprouse, J.; Swanson, C.; Anderson, S. Anal. Biochem.

1996, 242, 202–213.(75) Tropini, V.; Lens, J.-P.; Mulder, W.; Silvestre, F. Ind. Crops Prod.

2004, 20, 281–289.(76) Tunc, S.; Angellier, H.; Cahyana, Y.; Chalier, P.; Gontard, N.; Gastaldi,

E. J. Membr. Sci. 2007, 289, 159–168.

(77) Sun, S.; Song, Y.; Zheng, Q. Food Hydrocolloids 2007, 21, 1005–1013.(78) Reddy, N.; Yang, Y. Biomacromolecules 2007, 8, 638–643.(79) Yang, Y.; Reddy, N.; Li, Y. United States Patent Application

20060282958, 2006.(80) Barone, J. R.; Dangaran, K.; Schmidt, W. F. J. Agric. Food Chem.

2006, 54, 5393–5399.(81) Harris, M. Handbook of Textile Fibres; Textile Book Publishers, Inc.:

New York, 1954.(82) Asquith, R.; Leon, N. Chemical Reactions of Keratin Fibers; Plenum

Press: New York, 1977; pp 193-265.(83) Maclaren, J.; Milligan, B. Wool Science. The chemical reactiVity of

the wool fibre; Science Press: Marrickville NSW, 1981.(84) Jones, C.; Mecham, D. Arch. Biochem. 1943, 2, 209–223.(85) Sienko, M.; Plane, R. Chemistry; McGraw-Hill Book Company: New

York, 1961.(86) Patagonia, Fabrics: E-Fibers, http://www.patagonia.com (accessed 15

August, 2007).

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