NEWS ARCHIVE
June 2009 - The goal: competitively priced microbial polysaccharides
A number of polysaccharides are commercially produced by fermentation of renewable resources – for example, xanthan gum, gellan, dextran, and pullulan. 
They generally find use in higher value applications such as thickeners and rheology modifiers in food and medical applications. Pullulan is a water-soluble, colorless material that makes strong films with high adhesion and oxygen barrier properties. In addition, it is edible
and sold in food applications, such as protective coatings and packaging. A large market is also in capsules for pharmaceuticals. Chemically, pullulan is a polymer of glucose repeat units with α-(1,4) linkages, much like starch, but with a portion of 1,6-linkages. This is enough to disrupt the regular structure and render the polymer with properties that produce amorphous, flexible films and greater water solubility. Thus it has very attractive physical and mechanical
properties. However the cost to produce it is still significantly greater than similar polymeric materials produced from petrochemical sources. However, with a driving force to production of more sustainable and environmentally friendly materials, efforts continue to improve the production cost of pullulan. These improvements should be applicable to other microbial polysaccharides and help their overall commercial development and broader utility.
In a recent publication, researchers described improvements to the pullulan production process. Key challenges in the production process lie in the product isolation after the fermentation is complete - in particular, separation of proteins, color bodies, and other impurities. Currently proteins are separated by selective precipitation of the polysaccharide from an aqueous solution by dilution with ethanol. Much but not all of the protein contaminant remains in solution. Color is conventionally removed by carbon absorption although separation of the resulting carbon from the viscous solution is difficult and leads to product losses. New improvements which optimized the recovery of pullulan have been demonstrated and involve separating cell mass by centrifugation, heating the broth at 80 C to denature and precipitate protein, decolorization by treatment with hydrogen peroxide, concentration by vacuum distillation of water, and then ethanol precipitation. The final pullulan product was obtained with a high recovery and in higher purities than commercial material (“Downstream processing of pullulan from fermentation broth,” S Wu, Z Jin, JM Kim, Q Tong, and H Chen, Carbohydrate Polymers 2009, 77, 750-753 [abstract]).
February 2009 - Biodegradable, polysaccharide based scale inhibitor for industrial water treatment
The average person is largely unaware of the industrial water treatment business. Yet it is very 
important
for the efficient workings of industry today. Water is used in many industrial operations, for example in cooling towers or
geothermal systems for heat transfer, or in oilfield operations to maximize recovery of oil. The tanks, pipes, heat exchangers,
etc. all must be kept clean of fouling by inorganic and organic materials which is a continual challenge. One class of
chemicals employed for this purpose are scale inhibitors. They work by inhibiting crystal formation and deposition on equipment
surfaces. Commercial products are phosphonates and low molecular weight polyacrylates and polymaleates. Although effective,
they are lacking in biodegradability which is desirable considering that the environmental fate of these materials often is
discharge into water bodies.
This is where polysaccharides come in ... water soluble, polymeric, biodegradable, economical, and as a bonus, produced from renewable resources. By suitable modification of the polysaccharide structure, the required scale inhibition properties can be achieved. As described in a recent patent application (“Polysaccharide based scale inhibitor,” S Kesavan, G Woodward, F Decampo (Rhodia Inc), US Patent Application 20080277620, 2008 [full text] the galactomannan polysaccharides, such as guar gum, were converted into very effective inhibitors of calcium carbonate and barium sulfate scale formation. A combination of carboxyalkylation and partial depolymerization was employed to generate a structure that effectively inhibited formation of mineral scales and also was stable to oxidizing biocides and heat that are encountered in industrial applications. Optimum (100%) calcium carbonate scale inhibition was observed for carboxymethyl guar with a degree of substitution of 1.6, weight average molecular weight of 20,000 g/mol, and 25 ppm concentration.
October 2008 - Using the polysaccharide triple helix to produce functional one-dimensional nanocomposites
We all are aware of the elegant and powerful double-stranded helix structure of DNA which is the basis of
the life reproduction process. Another helix structure with amazing properties is found in some polysaccharide
materials and it is being exploited to generate new composite materials for nanotechnology. β-1,3-Glucans,
as found in biologically produced and commercially available polysaccharides such as curdlan, schizophyllan,
and scleroglucan, exist in triple stranded helix structures. The interior of the helix structure is hydrophobic
and the exterior is hydrophilic. This structure might be considered to be an extended version of a cyclodextrin.
The triple helix structure is favored in aqueous mixtures but it can be denatured to a random coil structure by
dissolution in polar aprotic solvents. The helix can be reassembled by exchanging the solvent back to water and
it is this reversable process which has been recently employed to wrap the polysaccharide around other materials
which are hydrophobic and fibrous (one dimensional) and thus modify their properties. In addition, this
“sugar-coating” should improve biocompatibility. 
Also note that
the β-1,3-glucan structure is stable in the human system due to a lack of enzymes available to degrade it
and offers utility in medical/health applications.
Carbon nanotubes have very impressive mechanical properties and electrical conductivity and are at the forefront of new materials in nanotechnology. Processing and use of carbon nanotubes is limited by their hydrophobicity and agglomeration. A hydrophilic character can be incorporated onto the surfaces of the nanosized fibers by co-assembling, or wrapping, them with a β-1,3-glucan. Although chemical derivatization methods can also render them water soluble, mechanical and physical properties such as conductivity are adversely affected by the covalent structural changes. Modification by noncovalent means has minimal effect on the original nanotube properties. Thus, a polysaccharide-carbon nanotube complex is formed and water solubility is achieved by treating an aqueous suspension of nanotubes with a solution of a β-1,3-glucan such as schizophyllan in DMSO. The schizophyllan backbone was further derivatized to append groups that can interact with a high specificity with other biological molecules. This design is proposed for use as a sensor in a recent patent application (“Polysaccharide-Carbon Nanotube Complex,” M Mizu, S Shinkai, T Hasegawa, M Numata, T Fujisawa, K Sakurai, US Patent Application 20080242854, 2008 [full text]). A similar approach has been used by these authors to wrap polyaniline and polythiophene to produce nanofibers with good conductivity. Most recently, carbon nanotube fibers have been wrapped by β-1,3-glucan which had been derivatized with either cationic or anionic functional groups. Combination of these two complementary materials yielded self-assembled hierarchical architectures of different shapes depending on their ratio (“Creation of Hierarchical Carbon Nanotube Assemblies through Alternative Packing of Complementary Semi-Artificial β-1,3-Glucan/Carbon Nanotube Composites,” M Numata, K Sugikawa, K Kaneko, and S Shinkai, Chemistry - A European Journal 2008, 14, 2398 – 2404 [abstract]).
Although these discoveries are very far from large scale commerciallization, they illustrate a unique property (the reversible helix formation and incorporation of hydrophobic structures into its interior) of readily available β-1,3-glucan polysaccharides that may prove useful in other applications.
August 2008 - Controlled-release systems based on a hydrophilic matrix
“I want it all and I want it now!” - That seems to be the motto of modern society. However, in certain cases
it is best to have a product delivered slowly over time. Too much of a good thing can have unintended consequences. This is
true in pharmaceutical applications and in applications involving herbicides, biocides, personal care products, fragrances,
even toilet cleaning products, where overuse of the product can be 
harmful, expensive, or produce some
other undesirable sensory effects. Thus, controlled, or sustained, release technology has evolved. Polymeric materials
play an important part in these products due to their physical and mechanical properties which facilitate the holding and
then releasing of active ingredients in a controlled fashion. Biobased, polysaccharide materials, such as cellulose ethers,
have proven to be especially useful and a large commercial market exists and continues to grow. Cellulose ethers are produced
in many structural variations: different alkyl ether substituents (methyl, hydroxypropyl, hydroxyethyl, ethyl, and
carboxymethyl being the most commercially important) in homogeneous or mixed compositions, of varying degrees of substitution
and molecular weights. Although the most important materials are semi-synthetic (produced by derivatization of cellulose with
alkyl halides or epoxides), a large fraction of the material nevertheless is derived from naturally occurring, renewable
cellulose.
In applications where an active ingredient is to be released from a tablet formulation into an aqueous (biological or industrial) system, a hydrophilic, water-soluble polymer matrix is very effective. Key examples of these polymers are hydroxypropyl methylcellulose (HPMC, hypromellose) and hydroxypropylcellulose (HPC). The postulated mechanism of release involves absorption of water from the environment onto the surface of the dry tablet followed by slow diffusion of water into the tablet. The hydrophilic polymer swells and rapidly forms a highly viscous, hydrogel layer on the surface of the tablet. This layer slows the rate of further diffusion of water into the tablet and helps maintains its mechanical integrity. With time the active ingredient diffuses through the gel layer and is released into the environment. At the surface of the tablet, the water soluble polymer slowly dissolves, or erodes, and thus the active ingredient is released by a combination of the three processes – diffusion, swelling, and erosion. The hydrophilic polymer matrix continues to swell and erode, and the tablet eventually is completely dissolved in the water. The rate of release of the active ingredient is complicated and depends on many factors including the type of water soluble polymer. The following publication provides a more detailed description of the theory, materials, and design of such controlled release formulations: “Using METHOCEL Cellulose Ethers for Controlled Release of Drugs in Hydrophilic Matrix Systems,” The Dow Chemical Company product literature, July 2000 [link]
Recent progress in the understanding of controlled release processes is seen in a study of the diffusion mechanism of active ingredients in cellulose ether-based tablets (“Towards elucidation of the drug release mechanism from compressed hydrophilic matrices made of cellulose ethers. I. Pulse-field-gradient spin-echo NMR study of sodium salicylate diffusivity in swollen hydrogels with respect to polymer matrix physical structure,” C Ferrero, D Massuelle, D Jeannerat and E Doelker, Journal of Controlled Release 2008, 128, 71-79 [abstract]). As a model for one stage of the controlled release process, hydrogels were prepared from commercial HPMC, HPC, and HEC (hydroxyethylcellulose), and water and a low concentration of a model solute, sodium salicylate. The hydrogels were characterized by differential scanning calorimetry, and diffusion of the solute in the different hydrogels was measured by NMR. The authors concluded that the composition of the polymers (alkyl ether substituents and molecular weights) did not affect the diffusion of the solute up to a polymer concentration of 45 wt%. The diffusion rate is apparently determined by the free volume available for the solute to pass through, which is not significantly affected by the substituent compositions in these three polymers or the molecular weights tested. In the real-world controlled release tablet formulations, any differences in release rate must be due to other effects which is the subject of a forthcoming study.
June 2008 - Improving the biocompatibility of synthetic polymers by incorporation of hyaluronic acid
Synthetic polymer chemistry continues to provide us with amazing materials for just about every application imaginable. One of the current frontiers in which synthetic materials are being employed is at the interface with biological systems, that is, to replace damaged or failing tissues in living organisms, especially humans. Often the desired functional properties, such as strength and flexibility have been achieved, however longer term biocompatibility remains to be a challenge. For example, articles made from synthetic polymers which are used in the blood stream, such as vascular grafts, develop thrombosis in which platelets and red blood cells adhere to the unnatural material and form clots. Similarly, contact lenses made from synthetic materials can be fouled by adsorption of proteins that occur in tear fluids and cause discomfort to the wearer. There are many naturally occurring polymers that do not suffer these biocompatibility problems however they are not easily produced, especially in a “tailorable” way with a range of properties or with the facile processability of synthetic materials. Recently, researchers have achieved hybrid materials with the best features of both biological and synthetic polymers. Two examples follow:
Synthetic materials used in the blood stream (e.g., vascular grafts, stents, valves) suffer from adhesion of blood components and clot formation. Some materials which have less thrombosis problems unfortunately have other problems such as not matching the flexibility of surrounding tissues or not fully adhering well to and incorporating into the surrounding endothelial tissue. Previously, some materials have been developed that reduced thrombosis by incorporating an anticoagulant, heparin, either in releasable or surface-bound forms. In a recent report, a biobased polysaccharide which possesses anticoagulant properties, hyaluronic acid [more...] (HA), has been copolymerized into a polyurethane to produce a strong, flexible material with properties that could be varied depending on the level of HA incorporation (“The haemocompatibility of polyurethane-hyaluronic acid copolymers,” F Xu, JC Nacker, WC Crone, and KS Masters, Biomaterials 2008, 29, 150-160 [abstract]). Films of this material exhibited greatly reduced platelet and red blood cell adhesion, yet maintained adhesion of endothelial cells, even at low levels of HA incorporation.
Contact lenses are another synthetic polymeric material that comes in contact with biological fluids. Problems arise when proteins that occur in tears begin to adsorb on the lens surface and result in eye irritation. Current commercial approaches involve incorporation of wetting agents either in releasable or bound form. These wetting agents affect the hydrophilicity of the lens surface and its associated affinity for proteins. In a recent report, researchers successfully incorporated hyaluronic acid into a polyhydroxyethyl methacrylate film both by absorption and optionally by subsequent crosslinking (“Hyaluronic acid containing hydrogels for the reduction of protein adsorption,” M Van Beek, L Jones, and H Sheardown, Biomaterials 2008, 29, 780-789 [abstract]). Both materials showed enhanced hydrophilicities, depending on incorporation levels, but the effect was fleeting with the releasable HA. Bound HA maintained its hydrophilicity and showed decreased levels of tear protein adsorption – lower than commercial products – which suggested that this material indeed has protein repellant properties.
Ultimately, articles used in biological applications may be produced entirely by biological means. But until we are able to meet that awesome challenge, careful engineering of hybrids of biological and synthetic polymers like those described above will provide valuable materials for these needs.
April 2008 - Environmentally friendly aqueous viscosified treatment fluids and breakers for oil drilling
A large market for polysaccharides is in the oil drilling industry.
Seems like an odd combination ... Isn't it ironic that renewable
materials, polysaccharides, are used to facilitate production of a
depleting material, petroleum? What makes polysaccharides so valuable
in this application is the high viscosity of their aqueous 
solutions
and the ability to crosslink them to form shear-thinning gels (high
viscosity when the solution is at rest and low viscosity when being
pumped). Such viscous and visco-elastic treatment fluids are useful in
underground oil well operations for drilling, fracturing, diverting,
and gravel packing. That is, the treatment solutions are effective for
suspension of solids in drilling muds, removal of drill cuttings, and
placement of gravel packs and proppants (particulate materials that
bridge pores in the rock formations to maintain oil flow). The high
viscosity also facilitates transfer of hydraulic pressure throughout
the formation and prevents undesired loss of the fluid into the
formation. Thus, the aqueous polysaccharide solutions provide a
spectrum of engineered well treatment fluids. In addition to these
valuable functional properties, other benefits are derived from the
green features of polysaccharides, such as their renewability, very low
toxicity, and biodegradability. Examples of polysaccharides that are
used in oil drilling are guar gum (and its derivatives, hydroxypropyl
guar and carboxymethyl guar), xanthan gum, scleroglucan,
carboxymethylcellulose, and other galactomannans, such as locust bean
gum. Whereas, the much higher purity grades of these polysaccharides
are used for more high valued products, such as food and personal care,
the lower purity grades are well suited for the oil industry and thus
allow full usage of all materials from polysaccharide production.
In many of the applications of well treatment fluids described above, it is also necessary to reduce the viscosity of the fluid when that property is no longer needed prior to oil production. Various methods have been developed to depolymerize the polysaccharide and thus reduce the viscosity by incorporation of “breakers”, such as acids, oxidants, and enzymes. Breakers often require an activator (or catalyst) to increase the viscosity reduction rate. There is a need to improve the efficiency and environmental friendliness of these activators. A recent patent discloses the use of an activator for an oxidation-based breaker which is effective at moderate temperatures (<100 °C) and which is composed of a relatively innocuous Fe(II)-protein complex (“Methods for reducing the viscosity of treatment fluids,” RE Hanes Jr, RW Pauls, DE Griffin, KA Frost, and JM Terracina [Halliburton Energy Services, Inc], US Patent 7,334,640; 2008 [full text]).
I'm sure if we really tried, we could find a polysaccharide that complexed Fe(II) and worked as well as this protein.
February 2008 - Surface modification of cellulose mediated by xyloglucan
Like teammates, xyloglucan and cellulose have complementary abilities and work together to achieve something that neither could do alone. In naturally occurring wood, xyloglucan binds tightly and specifically to cellulose fibers. Cellulose fibers by themselves have very impressive strength, but an even greater composite strength is achieved when the fibers are “glued” together with xyloglucan. Xyloglucan's interaction with cellulose has also been employed in industrial applications to improve the performance of textiles and paper. Recently xyloglucan has been exploited as a vector or anchor to introduce new functional groups to cellulose surfaces and alter its properties under mild and environmentally friendly conditions.
Xyloglucan (XG) is widespread in nature in plants but is most commonly
isolated from tamarind kernel powder (~60% XG) which is produced
commercially on a large scale. XG actually is a group of
polysaccharides defined generally as neutral, unbranched polymers of
glucose with xylose pendent groups. This chemical structure (see
Figure) is very similar to that of cellulose in that they both have the
same poly(b-1,4-glucopyranose) backbone. The xylose substitution
pattern along the backbone varys depending on the
natural source. Certain other monosaccharides are also
typically found attached to the xylose units. Whereas cellulose is
highly crystalline and water-insoluble, XG is readily water soluble.
However when XG is associated with cellulose as in wood, a strong
complex results as evidenced by the fact that XG cannot be extracted
from it with water. The complex can be broken however by extraction
with strong aqueous base. Details of the XG-cellulose relationship are
reviewed in a recent article along with creative new methods which
make use of XG as an anchor to introduce new chemical functional groups
to cellulose surfaces and modify its properties (“Xyloglucan in
cellulose modification,” Q Zhou, MW Rutland,
TT Teeri, and H Brumer, Cellulose 2007, 14, 625-641 [abstract]).
The strong absorptive association between XG and cellulose in aqueous solution provides a very mild and versatile mechanism to append useful functionality to the surface of recalcitrant cellulose fibers. Following are methods which have been recently demonstrated:
- Activation of XG to form carboxyl groups, followed by coupling with proteins (enzyme or antibody). The resulting protein-polysaccharide conjugate was then absorbed onto cellulose powder in water. The gentle binding conditions maintain the biological activity of the protein.
- Derivatization of oligomeric XG (prepared by enzymatic hydrolysis of native XG) by reductive amination to introduce a terminal amine group. Various functional groups (R), such as fluorescent dyes, proteins, thiols, and initiators for graft polymerizations, can be linked via the amine. The functionalized XG-R oligomer was then transferred onto high molecular weight XG via a biomimetic enzymatic transglycosylation reaction and the resulting high mass XG-R finally absorbed onto cellulose. The great utility of this method lies in its aqueous, environmentally friendly processing conditions and ease of attachment of a wide range of functional groups.
- Grafting of hydrophobic polymers onto the hydrophilic cellulose surface was achieved by using the cellulose-anchored XG which was derivatized with graft polymerization initiator as prepared in method 2. Styrenic, acrylic, and aliphatic polyester polymer branchs were grafted and a hydrophobic surface was formed. The method is milder than alternate cellulose grafting methods and the polymer product was also biodegradable by common industrial enzymes.
- Coupling of carboxy-terminated hydrophobic polymer (polystyrene) to amino-XG to form an amphiphilic compatibilizer for the preparation of nanocomposites of cellulose nanofillers in a nonpolar matrix.
Cellulose is a valuable industrial material due to its low cost, sustainable production, biodegradability, and material properties but is not easily modified in its native state. However, XG has the unique ability to mediate surface modification processes to produce new functional properties for cellulose while also maintaining its environmentally friendly features.
December 2007 - Cellulose acetate - an important biobased material in liquid crystal displays
It's very apparent that liquid crystal display (LCD) devices are
popular and
finding their way into more products everyday. Examples are displays
for computers, televisions, appliances, portable devices like
phones, GPS devices, and the original application .... watches. The
advantages of LCDs are light weight, 
compact size and low power
consumption - vast improvements over the previous cathode ray tube
(CRT) displays. LCDs have a complex construction and are a truly
amazing triumph of science and engineering [read more].
Within an LCD, light is transmitted through a liquid crystal cell
which is sandwiched between two polarizing plates. The polarizing
plates are protected by a transparent film which is composed of
cellulose acetate (more specifically cellulose triacetate or
triacetyl cellulose in which all three of the hydroxyl groups on each
glucose repeat unit is fully acetylated). It's ironic that
cellulose acetate, one of the oldest commercial polymers, plays a
crucial part in this high-tech electronics appication.
Cellulose acetate (CA) is produced by the synthetic derivatization of biologically produced cellulose. As such, it is one of the first synthetically modified, biobased polymers and has found wide utility. Acetylation of cellulose causes a dramatic change in properties. Cellulose is hydrophilic, highly crystalline, can't be melt processed due decomposition before its high melting point, and is poorly soluble in common solvents. CA is a hydrophobic, amorphous material that can be dissolved in common solvents or melted, especially after mixing with plasticizers, and can be readily processed into different forms for many applications. Its functional properties include: moisture resistance, optical clarity, high heat resistance (softening temperature), melt processability, high mechanical strength and toughness. These properties have made CA valuable in the following commercial applications: plastic articles (tool handles, eyeglass frames), cigarette filter material, water purification membranes, textile fibers (known as “acetate”), optical films. The latter application, specifically its use in LCDs, is becoming an especially important market for CA and driving increased production of this biobased material. Its combination of high clarity, isotropic transmittance (passes light equally in each direction), good moisture and temperature resistance, low cost, adhesion to high surface-energy polarizing films, and also its renewable raw material source make CA the material of choice.
There is much recent patent activity in the highly competitive LCD market which is being driven by needs for increased performance and lower costs. Many patents describe improvements in the use of CA as a protective, isotropic film for use on the polarizing plate components. Another important performance need is the ability to improve the view of the the display from oblique angles which has been one of the drawbacks of LCDs compared with CRTs. If you are viewing this webpage on a LCD display, this effect should be apparent by looking at the display from different angles. In addition to the properties that have made CA useful as a protective film, it has another potentially useful functional property: when a film is stretched at a temperature near its softening point it develops an optical anisotropy due to the alignment of the polymer molecules. This property can be utilized in an “optical compensation film” to improve the viewing angle of the display. In addition, a recent patent application describes how the anisotropy can be tuned by altering the substitution pattern of the ester groups on the cellulose backbone which exhibit slightly different refractive index contributions. That is, varying the ester groups (acetate, propionate, butyrate, benzoyl), or presumably any other functional group, and their degrees of substitution will affect the optical properties of the polymer film. An equation has been developed which correlates the optical anisotropy with the cellulose ester composition (“Cellulose Compound Film, Optical Compensation Sheet, Polarizing Plate, and Liquid Crystal Display Device,” Y Nozoe, T Omatsu (Fujifilm Corp), US Patent Application 20070259134; 2007 [full text]). It's fair to say that modern polymer science, as in this example, is making its greatest advances today by taking "old" polymers where they have never been before.
November 2007 - If you could dissolve wood in a solvent what could you do with that solution?
Maybe ...
... Regenerate the
wood in a different form that isn't possible by
conventional methods, such as fibers, films, composites, or less
ordered material that could be more easily converted to
glucose for ethanol production.
... Modify
its chemical structure and properties, homogeneously, with
chemical reactions.
... Analyze it to study the
wood's composition without having to
first separate the components.
These are just some of the ideas that are being explored now that wood and other lignocellulosic materials have been found to be soluble in ionic liquids (ILs).
Wood
is primarily composed of lignin and the polysaccharides, cellulose
and hemicellulose. Hemicellulose is a class of relatively complex
polysaccharides, i.e., containing multiple monosaccharide repeat unit
structures and with side chains/branching, and whose structure varies
with plant species. Lignin is a network (crosslinked) polymer with
oxygenated phenylpropane repeat units. Wood, and other
lignocellulosic biomass, are renewable and
CO2-neutral resources, however processing of wood into other
products is
inefficient and often involves hazardous and
environmentally unfriendly processes (e.g., production of paper,
or purified cellulose and
its derivatives). Recently, many biopolymers in
their purified forms have been found to be soluble in highly polar
ionic liquids. These include polysaccharides such as cellulose and
chitin which otherwise are only soluble in certain relatively
hazardous solvents. Ionic compounds which are liquid at less than 100
°C and
have been shown to be powerful solvents
are alkylimidazolium or pyridinium compounds, such as
1-butyl-3-methylimidazolium chloride. They are also touted as green solvents, possessing the
following properties:
- extremely low volatility and flammability
- high thermal stability
- low chemical reactivity
- recyclability by extraction and ion exchange processes
Their high polarities and hydrogen-bonding abilities make ionic liquids good solvents for polysaccharides. The solvating power and green properties of ILs are enabling development of new polysaccharide products [more on ionic liquids].
Wood itself has just recently been found to be soluble in certain ionic liquids (“Dissolution of wood in ionic liquids,” I Kilpelainen, H Xie, A King, M Granstrom, S Heikkinen, DS Argyropoulos, J Agric Food Chem 2007, 55, 9142–9148 [abstract]). Norway spruce sawdust was soluble at 8 wt% in allyl methylimidazolium chloride at 110 °C for 8 h. More compact heterocyclic cations yielded better solubilities. Faster solubilization was obtained with smaller particle sizes. Effective solubilization was evidenced by the ability to completely acetylate the hydroxyl groups of the polysaccharides by reaction of the wood solution with acetic anhydride/pyridine. In addition, this acetylated wood material could be isolated by precipitation from a non-solvent, dissolved in deuterochloroform and characterized by NMR. Thus, valuable new information could be gained about the structure of wood without separation of the components.
Wood which had been dissolved in IL and then precipitated from a nonsolvent with vigorous agitation was largely amorphous as shown by X-ray analysis. This may have practical benefit for cellulose-based bioethanol production. Thus, treatment of IL-processed wood with cellulolytic enzymes afforded 60% of the theoretical amount of glucose as compared to 12% with non-IL processed wood.
The authors did not explain why or how the lignin component in wood dissolves in IL. If lignin is indeed a crosslinked, network polymer, it should not be a soluble material. So, either the lignin structure is not, or only lightly, crosslinked or the crosslinking is chemically reversable under the dissolution conditions. Finally, important issues that still must be addressed for widespread industrial use of ILs are toxicity and cost. In particular, effective processes must be developed for removal of IL residues from products, and recycling of the bulk of the IL solvent.
October 2007 - "Molecular engineering" of alginate's structure leads to improved properties
Alginate is a versatile polysaccharide produced commercially from seaweed (20,000 tons/yr). It is primarily used as a thickener or gelling agent for aqueous mixtures (other applications). It thus affects the flow properties of a solution – its rheology. These properties are valuable in food preparations, pharmaceutical formulations, and specialized medical applications such as cell encapsulation. Alginate forms strong gels in the presence of divalent cations, particularly calcium, by way of ionic crosslinks between the polyanionic alginate chains. Thus, when an aqueous solution containing sodium alginate is mixed with a water-soluble calcium salt, the mixture forms a gel with elastic properties. Alginate's unique gelling properties result from its primary chemical structure. The polymer backbone is composed of two monosaccharide repeating units, mannuronate (M) and guluronate (G), which are isomers differing in configuration at the C-5 position (see figure). They can be present in different ratios and in different sized blocks of M or G units. The G-blocks form especially strong interchain crosslinks with calcium ions and are a large factor in the gel properties. Alginate from different seaweed sources has different M/G ratios and different M and G block lengths and, as commonly occurs with natural products, has additional heterogeneity due to varying growth conditions. Recently, researchers have developed a method to synthesize alginate compositions in a controlled fashion with relatively long G-blocks at varying M/G ratios and an absence of M-blocks. These materials formed gels which showed markedly improved performance over native alginates (“Molecular Engineering as an Approach to Design New Functional Properties of Alginate,” YA Mørch, I Donati, BL Strand, and G Skjåk-Bræk, Biomacromolecules 2007, 8, 2809-2814 [abstract]).
Figure. (Top) Mannuronate (M) and guluronate (G) structures. Note different configurations at C-5. (Bottom) Synthetic conversion of all-M backbone structure to alternating M-G and then G-blocks.
The structure of alginate was optimized to produce gel beads, or capsules, specifically for medical applications (cell encapsulation, tissue engineering) where strength, stability, and controlled, uniform porosity are important. It is very likely that lessons learned in this study will be valuable in alginate's industrial applications as well. Thus, a series of alginate compositions were synthesized with increasing lengths of G-blocks in a very controlled manner (see Figure). Starting from a poly-mannuronate homopolymer obtained from a bacterial source, G-units were introduced by epimerization of every other M-unit using an enzyme catalyst. A second enzyme was then used to generate G-blocks by epimerization of more M units, the final content of the G-blocks depending on the reaction time. This series of alginates with increasing G-block lengths was tested and compared with two commercially available alginates with lower and more heterogeneous G-block content and at a relatively low and high M/G ratio (from Laminaria hyperborea and Macrocystis pyrifera seaweeds, respectively). The new compositions affected calcium ion binding and backbone flexibility between crosslinks. Improvements in elasticity/gel strength, syneresis (shrinkage), capsule stability to treatment with saline solution, and permeability to small globular proteins were achieved relative to the two native alginate materials.
September 2007 - Orally-dissolving strips for delivery of drugs and flavors
Maybe
you've seen them in your drug store – small packages containing thin
strips of material that
dissolve quickly in your mouth and deliver a dose of drug, breath
freshener, or vitamin. The base material is a water
soluble polymer that can be formed into a film of sufficient
strength and
flexibility, and most importantly, is safe to ingest. What
type of materials can do all this? You guessed it ...
polysaccharides. Some examples of film-forming, water-soluble,
ingestible polysaccharides are hydroxypropyl methylcellulose (HPMC),
carboxymethylcellulose (CMC), and pullulan. A listing of other
polysaccharides that are approved for use as food additives
or have “Generally
Regarded As Safe” status is provided by the US Food And Drug
Administration.
Active ingredients, flavors, and colors can be absorbed into the polysaccharide film which provides a convenient means for packaging, dispensing, and ingestion. When contacted with saliva in the mouth, the film dissolves rapidly, releasing the ingredients. This form of delivery offers many advantages, especially for children and elderly people who have difficulty swallowing conventional pills. The films generally only can carry about 50 mg of ingredient per strip which is about the size of a postage stamp, so applications are limited to ingredients of high potency. A recent patent application describes one process for preparation of these materials. A polysaccharide film, cast from aqueous solution and dried, is subsequently sprayed with an aqueous solution of the active ingredient. As the coating dries the ingredient is absorbed into the film (“Films for use as dosage forms,” EZ Nowak [Bioprogress Technologies Ltd], US patent application 20070184099; 2007 [full text]). A recent article has demonstrated that a xanthan gum and konjac glucomannan mixture, two other commercially available ingestible materials, can provide a material that melts at body temperature and may be useful for delivery of absorbed ingredients (‘‘ 'Melt-in-the-mouth' gels from mixtures of xanthan and konjac glucomannan under acidic conditions: A rheological and calorimetric study of the mechanism of synergistic gelation,” AA Agoub, AM Smith, P Giannouli, RK Richardson, ER Morris, Carbohydrate Polymers 2007, 69, 713–724 [abstract]).
Orally-dissolving strips are
rapidly growing in market acceptance and numerous
companies now are involved in their commercialization. The films and
ingredients can also be assembled in different architectures in order
to provide additional valuable features such as controlled release
or larger dosages (for additional commercial examples see the
Bioprogess Technologies website).
The market for drug products employing oral
thin film formulations has been estimated to be $2 billion by 2010
(ref).
July 2007 – A more sustainable process to produce regenerated cellulose
As industry seeks sustainable and more environmentally friendly polymeric materials, cellulose comes to the forefront. That is a result of its abundance in nature, low cost, useful properties, as well as its biodegradability and other green features. Cellulose exhibits good mechanical properties and possesses an affinity for water yet is insoluble in it. Unfortunately, cellulose cannot be melt-processed into the many forms and shapes needed in industrial applications like conventional plastics due to thermal decomposition. However, it can be processed, or regenerated, via solvent-based processes to afford the commercial cellulose fibers such as rayon and films such as cellophane - even filtration membranes. The solvent-based processes used to regenerate cellulose require volatile and/or toxic solvents and relatively complex (costly) operations. Finding alternative solvents for these processes is not a simple matter – the strong interchain forces that give cellulose its unique structure and properties prevent its dissolution in all but the most polar solvents, such as N-methylmorpholine oxide (Lyocell process) or require derivatizing solvents (carbon disulfide, Viscose process).
A new regeneration process which employs an environmentally friendly solvent system consisting of aqueous NaOH and urea offers great potential. The dissolution is carried out at -10 °C and is notably rapid. The optimum solvent composition was 7 wt% NaOH/12% urea and the optimum cellulose concentration for processing was 4 wt%. This solution is subsequently treated with aqueous sulfuric acid/sodium sulfate to regenerate the cellulose solid in various forms. Membranes have been prepared in this way which possessed mean pore sizes of 110 – 1200 nm and water permeabilities of 12 - 43 mL h-1 m-2 mmHg-1 (“Influence of coagulation temperature on pore size and properties of cellulose membranes prepared from NaOH-urea aqueous solutions,” J. Cai, L. Wang, and L. Zhang, Cellulose 2007, 14, 205-215 [abstract]). Similarly, fibers were produced which exhibited properties very similar to those produced by the commercial Lyocell process (“Structure study of cellulose fibers wet-spun from environmentally friendly NaOH/urea aqueous solutions,” X. Chen, C. Burger, F. Wan, J. Zhang, L. Rong, B. S. Hsiao, B. Chu, J. Cai, and L. Zhang, Biomacromolecules 2007, 8, 1918-1926 [abstract]). Overall, this process requires no volatile or toxic solvents, and produces relatively inoccuous waste, uses only inexpensive raw materials, proceeds with simple and fast operations, and affords cellulose materials with excellent properties in many useful forms.
|
The innovative process
improvement described above is an example of application of the green
chemistry principles laid out by the Green
Chemistry Institute of
the American Chemical Society. The twelve principles, listed below, are
meant to
guide the development of safe, environmentally friendly, and truly
sustainable chemical products.
|
June 2007 – An extruded form of xanthan gum rapidly disperses in water
Xanthan gum is one of the most important polysaccharides in commercial use. It is used as a thickener, suspending agent, and emulsion stabilizer in food products, personal care formulations, and oil drilling. It is produced industrially by microbial fermentation and is classified as an extracellular polysaccharide as it is produced in the cell and then released into its surrounding aqueous medium (more on xanthan structure and properties). Its exceptional rheological properties include: high viscosity at low shear rates (when the liquid is barely moving), greatly reduced viscosity at high shear rates (when the liquid is stirred or poured – called shear thinning or pseudoplastic behavior), and low sensitivity to changes in pH and ionic strength. These properties are attributed to the high molecular weight chains which form rigid helical structures in aqueous solution. Shear thinning occurs as relatively weak, noncovalent intermolecular associations and chain entanglement are disrupted (more on rheology of hydrocolloids).
Xanthan gum is produced commercially as a solid powder which must be redissolved in water prior to use in its various applications. Being a high molecular weight polymer, this is a relatively slow process and requires high-shear agitation to achieve homogeneous dispersion. Recently a new method has been developed which produces a form of xanthan gum that is rapidly and homogeneously dispersed in water. In this method a concentrated xanthan-water mixture is extruded in a twin screw extruder at 85 °C, followed by drying and grinding (“Impact of the extrusion process on xanthan gum behavior,” N. M. Sereno, S. E. Hill, and J. Mitchell, Carbohydrate Research 2007, 342, 1333-1342 [abstract]).
Interestingly, aqueous mixtures produced from the extruded xanthan exhibit a “particulate” behavior like a dispersion of crosslinked, highly swollen polyelectrolyte gel particles, in contrast to the molecular solution behavior of non-extruded xanthan. The aqueous dispersions reached their maximum viscosity in less than one minute of mixing (0.75% concentration) which was much faster than nonextruded xanthan. The extruded xanthan also produced a higher final viscosity than the nonextruded material at concentrations greater than 0.1% at various shear rates in pure water. At low shear rates the aqueous mixtures of extruded xanthan did not show the viscosity plateau typical of polymer solutions, but rather was consistent with a dispersion of swollen particles. The viscosity of extruded xanthan dispersions also showed a high sensitivity to salt concentrations unlike the non-extruded material. The particulate structure reverted to a disordered coil structure, in common with that of the nonextruded xanthan, after heating to a temperature above its order/disorder transition.
Generation of the unique network structure under the high shear and high concentration conditions during extrusion was postulated as a sequence of disruption of the initial ordered structure, followed by realignment of the polymer molecules and formation of new intermolecular helical junction zones amid unordered amorphous regions. As the polymer emerges from the extruder and cools, a kinetically stable network structure is obtained. Upon heating in water, this structure reverts to the random coil of non-extruded xanthan and ultimately, after cooling, to the helical molecular solution of non-extruded xanthan. In summary, an intriguing and potentially very useful form of xanthan gum is prepared by extrusion processing which rapidly forms aqueous dispersions that possess unique rheological properties. Its properties can be returned to that of non-extruded xanthan by a heating/cooling cycle.
May 2007 – Hydroxypropyl cellulose films for packaging
A BIG market for plastics is in packaging. Articles and materials such as food must be contained and/or protected during shipment and storage. In general, the plastic in which they are packaged must be flexible, tough, strong, transparent, inexpensive, and sometimes provide other functions such as barrier to oxygen, carbon dioxide, and water vapor. Consider the many types of plastic packaging you encounter during a walk through a grocery store. However, with our growing human population, these materials make an increasing impact on our environment. Because most plastics are based on nonrenewable petrochemical resources and contribute to environmental and health problems over their life-cycle, biobased materials are being developed to alleviate their impact. In general, polysaccharide materials do not make good packaging materials because they are relatively brittle, unless plasticized with additives, and have limited moisture resistance. Regarding the former property, there have been efforts recently to identify and develop polysaccharide-based thermoplastics which are inherently tougher. In addition, since many polysaccharides are also approved for food use, they offer potential value in unconventional applications such as edible films and food coatings.
Some commercially available polysaccharides do exhibit some toughness at ambient conditions, in particular, derivatives such as hydroxypropyl cellulose. Hydroxypropyl cellulose is produced by reaction of cellulose with propylene oxide, a petrochemical, and is thus not entirely renewable (... life is full of compromises). At high degrees of substitution (DS=3, three propylene oxides per glucose repeat unit), hydroxypropyl cellulose is soluble in both water and organic solvents. Its solutions and polymer melts show liquid crystallinity and solid films contain some crystalline order. Like other polysaccharides, it is hydrophilic and its properties are affected by absorbed moisture but notably to a lesser extent, especially at a DS of 3. In the context of packaging applications, researchers have recently showed hydroxypropyl cellulose films do possess a high strength and toughness (“Effect of water content on the fracture behavior of hydroxypropyl cellulose films by the essential work of fracture method,” I. Yakimets, N. Wellner, A.C. Smith, R.H. Wilson, I. Farhat, J. Mitchell, Mechanics of Materials 2007, 39, 500-512 [abstract]).
For you folks that like some details -- The glass transition temperature of hydroxypropyl cellulose (DS 3, Klucel™ LF, manufactured by Hercules, Inc) was 23 deg C at 80 % relative humidity (RH), increasing at lower RHs. Films absorbed 3.3 and 8.5 wt % water at 44 and 70 % RH, respectively, which was less than cellulose, hydroxyethyl cellulose, and hydroxypropyl methylcellulose. Tensile strength and stiffness at ambient temperature decreased with moisture content: yield stress 16 MPa (0 %RH) to 6 MPa (80 %RH); elastic modulus 1.2 GPa (0 %RH) to 0.3 GPa (80 %RH) showing the plasticization effect of water. Ductile behavior was observed even at 0 %RH, while a much greater ductility occurred at water concentration >3.3% (44 %RH). High fracture resistance was also observed at a water concentration >3.3% (44 %RH). The high ductility and toughness is likely a result of the high hydroxypropyl DS which acts as an internal plasticizer and reduces the high energy H-bond intermolecular interactions. This basic study indicates that hydroxypropyl cellulose may find a niche in packaging and edible film applications, especially where moisture resistance is not required. It also provides impetus to develop polysaccharide derivatives made entirely from renewable resources for packaging applications.
April 2007 - Ophthalmic drug delivery based on controlled gelation of guar gum solutions
Polysaccharides have shown great value in drug delivery systems – one key reason is their hydrophilicity – which makes them compatible with the aqueous environments in living things. Water-soluble polysaccharides, or hydrocolloids, have an additional extremely valuable feature – an ability to be crosslinked in aqueous solution. Crosslinking transforms the initial liquid of relatively low viscosity, depending on polymer concentration, to a highly swollen semisolid, or hydrogel, with elastic properties, or points in between with intermediate degrees of visco-elastic properties. Crosslinking can occur under a variety of conditions depending on the particular polysaccharide. In a previous highlight, the crosslinking of alginate was exploited for commercial, non-drug applications. Crosslinkable hydrocolloids are used for delivery of ophthalmic drugs (such as glaucoma treatment). Thus, a low-viscosity, aqueous solution of a drug and polysaccharide is topically administered to the eye where the liquid immediately forms a gel due to different pH and ionic strength conditions. The drug is thereby able to be administered accurately and diffuse from the semisolid hydrogel in a controlled fashion. A recent invention has improved delivery performance (“Ophthalmic compositions containing galactomannan polymers and borate,” B. Asgharian [Alcon Manufacturing, Ltd], US Patent 7,169,767; 2007 [full text]).
Some
ionic polysaccharides can gel simply by a change in pH but often that
involves a relatively large pH change which can be irritating to the
eye. A milder gelation process is achieved by crosslinking of
certain polysaccharides, which contain cis-diol units, with borate
ions. This crosslinking occurs at near-neutral pH and only requires a
change in pH of the initial drug/polysaccharide solution of 0.5 to
1.0 units. Commercially available
polysaccharides which possess the
cis-diol unit and show the desired performance are galactomannans,
polymers of mannose with galactose side groups. Examples of
galactomannans, are guar gum, locust bean gum, and tara gum, all
produced naturally in plants and each possessing different ratios of
galactose to mannose. Softer gel properties are obtained using
partially substituted galactomannan derivatives, e.g., hydroxypropyl
ethers. The mechanism of crosslinking involves formation of borate
ester bonds between the polymer chains. The ester bonds of cis-diols
are much more stable than single hydroxyl groups and afford effective
crosslinks. The extent of crosslinking and thus the strength of the
gel is controlled by the ratio of polysaccharide to borate. Control
of gel strength is not as easily obtained with one-part, ionic
polysaccharide gels. Overall, the borate-crosslinked-guar drug
delivery system produces a clear viscous gel when applied to the eye
from an initial low viscosity, pH-neutral liquid.
Also, note that the same basic borate-guar chemistry is used on a MUCH larger scale in the oil drilling industry to create viscoelastic liquids to improve oil recoveries!
March 2007 – Maintaining the quality of oils in food products during storage with polysaccharide-based antioxidants
Food
is more than an energy and nutrient source that gets us through the
day: from another perspective, it is an industrial product for
mass distribution that delivers energy and nutrients in an
appetizing way. “Industrial” and “appetizing” in the same
sentence? ... Sure, why not? In fact, certain biobased,
polysaccharide additives help make modern foods more appealing and
storage-stable. Water-soluble polymers increase the viscosity and
modify other flow properties of solutions (their rheology) in
different
ways. In food applications, where modification and control
of product properties are important and nontoxicity is essential,
water-soluble polysaccharides (PSs) are employed. These PSs, also
known as hydrocolloids, are of great commercial value as thickeners,
gelling agents, emulsifiers, and emulsion stabilizers, providing
desirable sensory properties for the consumer. Some polysaccharide
hydrocolloids occur naturally in foods and are responsible for the
food's distinctive properties. Think yogurt. Commercial examples
of food hydrocolloids are starch, cellulose derivatives, such as
carboxymethyl cellulose and methyl cellulose, alginate and its
derivatives, carrageenans, gum arabic, guar gum, gum tragacanth,
locust bean gum, other plant gums, xanthan gum, gellan, welan,
rhamnsan, and curdlan. As a result of their different chemical
structures, each polysaccharide can impart unique viscoelastic
properties to aqueous solutions or mixtures.
In addition to their effects on the rheological properties of food, polysaccharide hydrocolloids have recently been found to inhibit air oxidation of lipids (e.g., vegetable oils) in aqueous emulsions which are commonly found in food products. Degradation of taste and flavor results from lipid oxidation. Synthetic small-molecule antioxidant compounds, such as BHT (butylated hydroxytoluene) and TBHQ (t-butylhydroquinone), are frequently used to inhibit oxidation, however there is some concern about their safety, so the ability to use nontoxic PSs is of great interest.
Recent studies have focussed on identification of polysaccharides with optimum antioxidant performance in oil/water emulsions. In a real-world application - Greek salad dressing, composed of olive oil and lemon juice – researchers have found that gum arabic and propylene glycol alginate both afforded antioxidant activity in tests mimicking real world storage conditions. Chemical analysis was supported by consumer testing (smelling for rancidity) (“Oxidative stability of olive oil–lemon juice salad dressings stabilized with polysaccharides,” D. Paraskevopoulou, D. Boskou, A. Paraskevopoulou, Food Chemistry 2007, 101, 1197–1204 [abstract]). A second study also found that oxidation of sunflower oil in emulsions could be inhibited in the presence of polysaccharides. Curdlan was most effective, followed by rhizobium polysaccharide (a developmental product), xanthan gum, and carboxymethylcellulose. Curdlan (poly-b-1,3-glucose) showed half the activity as TBHQ at neutral pH, but was superior at pH 3-5 (“Free-radical scavenging and antioxidative activities of some polysaccharides in emulsions,” Y.F.M. Kishk, H.M.A. Al-Sayed, LWT-Food Science and Technology 2007, 40, 270–277 [abstract]). The mechanism of inhibition of oxidation is not clear, mainly due to the complexity and varying composition of the tested mixtures. However, in these studies certain polysaccharides did show good radical scavenging abilities and inhibition of formation of oxidation products in standard tests. Previous studies have indicated that the antioxidant activity may also result by 1) chelation and deactivation of metal ion impurities, such as iron, which catalyze oxidation, and 2) reduction of oxygen mass transfer at the water-oil interface where amphiphilic PSs concentrate and produce higher localized viscosities. Overall, these studies do indeed demonstrate effective inhibition of oxidation in the presence of polysaccharide additives. A much better understanding of the mechanism is still needed though in order to guide the selection of the best additive for individual applications.
February 2007 – A cleaner world with the help of polysaccharides: mineral scale control and particle dispersion
In everyday life we see crusty mineral scale deposits around water faucets and films or stains on dishware cleaned in automatic dishwashers. On a much larger scale (no pun intended), equipment surfaces that are used in water-containing industrial processes can become fouled by mineral salts. Minerals such as calcium and magnesium carbonate are naturally present in hard water. Other salts such as calcium phosphate are formed by interactions of hard water ions (calcium) with phosphate ions present in cleaning or anti-corrosion ingredients. When changes occur to aqueous solutions of these mineral salts which are near their saturation point, such as temperature increases, water evaporation, or addition of other solutes, the mineral salts can precipitate and form a hard scale on surfaces which is difficult to redissolve due to low solubility. Undesirable effects of the scale are reduced heat transfer, restricted liquid flow, and in the case of consumer cleaning applications, residual films or stains. Examples of situations where scale formation is a very important commercial problem include boilers, cooling water systems, desalination systems, fabric and dishware cleaning, and oil and water wells. Thus, products that can prevent scale formation or improve the dispersability of precipitated solids are of great value. Currently, some synthetic water-soluble polymers which bear polar/ionizable functional groups have been commercialized for this purpose, for example, polyacrylic acids. Many polysaccharides also have some of these structural features but until now have not been as effective. Due to increasing costs associated with the petrochemical based products, there is renewed interest in developing polysaccharides for scale control.
Recently, researchers at National Starch and Chemical Company have discovered that partially oxidized polysaccharides, primarily starchs, which contain relatively low amounts of carboxylic acid and aldehyde functionality were effective at prevention of scale formation and dispersion of solid particles (“Modified Polysaccharides,” K.A. Rodrigues, J.S. Thomaides, A.L. Cimecioglu, M. Crossman, US Patent Application 20070015678; 2007 [full text]). The modified polysaccharides were prepared by a practical and selective oxidation reaction with chlorine bleach catalyzed by TEMPO (tetramethylpiperidinyloxy). A second step to reduce the polysaccharide molecular weight was performed by controlled heating of the oxidized starch with base at elevated temperatures. The resulting modified polysaccharide apparently was very effective at prevention of scale formation in simulated water-based heat transfer systems and anti-encrustation of fabrics during laundry with commercial detergents. Apparently, the modified polysaccharides function by disrupting crystal formation and dispersing of the formed solids. The modified polysaccharides were not effective as calcium ion sequestering agents unlike polyacrylic acid. In addition, the modified polysaccharides were effective at dispersing inorganic (clay, iron oxide, kaolin) and organic particles (carbon black) in water.
January 2007 - Biofilm control with polysaccharides: fight slime with slime!
Wherever
there is water, there is the potential for microbial growth and often
in our human society that becomes a real problem. In particular,
industrial equipment such as heat exchangers and cooling towers can
lose their effectiveness due to excessive
microbial growth on their
surfaces. In addition, medical implants can develop persistent
microbial growths on their surfaces and threaten the health of the
human host. Microorganisms exist in nature predominantly in the form
of these slimy surface-attached colonies, called biofilms, which
provide protection from stress (e.g., desiccation, pH changes),
biocides (chlorine, antibiotics), and host immunological defenses (read more about biofilms).
Recent research indicates that certain naturally occurring
polysaccharides could lead to new products for controlling
biofilms.
In addition to bacterial cells, the biofilm structure is composed of highly hydrated extracellular polymeric substances (EPS) which gives the biofilm, among other things, its adhesive and cohesive properties. That is, EPS is the glue that adheres cells to surfaces and to themselves. The predominant components of EPS are polysaccharides. Their hydrophilic polymeric structure provides a permeable matrix suitable for cell growth in an aqueous environment and substantial mechanical properties.
Most biofilm control strategies function by killing the microorganisms with industrial biocides or antibiotics. Some new, interesting approaches are being developed to control biofilm formation by mechanisms which do not actually kill the microorganisms - for example, surface coatings that prevent adhesion, and use of small “quorom sensing molecule” inhibitors (more on QSIs).
In a similar vein, researchers have recently discovered how some bacteria are able to compete with other species by limiting their competitors' ability to develop biofilms (“Broad-spectrum biofilm inhibition by a secreted bacterial polysaccharide,” J. Valle, S. Da Re, N. Henry, T. Fontaine, D. Balestrino, P. Latour-Lambert, J.-M. Ghigo, Proceedings of the National Academy of Sciences (USA) 2006, 103, 12558-12563 [abstract]). This behavior was attributed to a polysaccharide which was released from the cell surface capsule into the aqueous environment where it interacted with other species of bacteria and inhibited their ability to develop into biofilms without killing them or causing growth defects. Thus, the aqueous phase produced by a pathogenic strain of E. coli was separated from the cell culture and shown to inhibit biofilm formation by numerous other biofilm producing organisms. The active ingredient was isolated and identified as a polyanionic, high molecular weight (500 kDa) copolymer with a backbone repeat unit structure of [galactose - phosphate - glycerol - phosphate] with phosphodiester linkages. Thus the structure is not a polysaccharide by the classical definition (strictly sugar units in the backbone). This material possessed unique surface-active effects and effectively inhibited formation of biofilms, both adhesion on surfaces and aggregation of cells. The details of how this polysaccharide interacts with bacterial cells on the molecular scale was not clear but one wonders whether it interacts directly with the bacterial cells or possibly with the structurally similar EPS polysaccharides of the developing biofilm. In summary, these results suggest a new approach to biofilm control. Now is the time for chemists to get involved with the biologists, perform structure-activity studies on similar polysaccharides or other polymers, and develop the next generation of products for the biofilm control market.
Addendum, January 2007: Also note that chitosan, a commercially available, high molecular weight, amine-functionalized polysaccharide, was also recently shown to provide valuable biofilm control properties (P. Stewart and R. Carlson, Center for Biofilm Engineering, Montana State University). Chitosan is truly impressive in its ever-growing list of useful properties. More on chitosan in previous news highlight.
December 2006 - Inventing with alginate: encapsulation of enzymes in detergents, and removable colored coatings
Alginate is a polysaccharide that is produced commercially from seaweed. Courtesy of its ionic substituents, alginate possesses very unique and useful properties. In particular, sodium alginate is water soluble while its calcium salts are water insoluble, although highly swelled. Conversion between the two salt forms is reversible by any ion exchange process. Aqueous sodium alginate solutions can be mixed with a second component (payload) and then gelled by introduction of calcium ions. This material, either in wet form or after drying, can be utilized for specific purposes with respect to the payload and then at a later time, treated with a compound which exchanges sodium for calcium and the material becomes water soluble again, washed away, and releasing the payload. The calcium alginate composite is also porous and the payload may also be released by diffusion. Thus, calcium alginate can be used to immobilize and protect an active ingredient for storage and release it under subsequent use conditions. Similarly, calcium alginate can hold a component in an insoluble form while in use and then when no longer needed, it can be dissolved away. In these cases, alginate is a delivery agent with valuable material properties due to its polymeric character, such as mechanical strength and adhesion. more alginate background...
Recently, two interesting examples have been reported which demonstrate the practical value of this concept: 1) encapsulation of an enzyme in a detergent formulation, and 2) a removable colored coating.
Enzymes
are important components of commercial laundry detergents however
they are very sensitive and must be protected from the other
aggressive cleaning components and elevated temperatures during
storage. The ability of alginate to encapsulate
substances has been
successfully used to prepare dry, enzyme-containing granules which
survive formulation and storage, and then rapidly release the enzyme
during washing (“Granulation of subtilisin by internal gelation of
alginate microspheres for application in detergent formulation,”
A.W.J. Chan, I. Mazeaud, T. Becker, and R. Neufeld, Enzyme and
Microbial Technology 2006, 38, 265-272 [abstract]).
Enzyme-containing granules were produced by a novel process called
“internal gelation” in which the aqueous components were
emulsified in oil to produce fine droplets. Finely divided calcium
carbonate was also dispersed in the aqueous phase and upon
acidification, yielded water-soluble calcium ions. This caused
gelation of the alginate and formation of beads of controlled size
and shape while incorporating the enzyme and other additives. After
separation of the oil phase and drying, the beads retained about 80%
of the initial enzyme activity. The encapsulation yield, granule
attrition resistance, and dissolution time were optimized by
variation of alginate structure (molecular weight and repeat unit
structure - dependent on commercial source). It is also notable that
the stability of the enzyme in its granulated form was greatly
improved relative to free enzyme. Finally, 90% release of the enzyme
was achieved after 3 minutes in dissolution tests.
In the second example, a non-toxic, biodegradable colored coating which was also removable was developed (“Removable colored coatings based on calcium alginate hydrogels,” M. Kobaslija, D.T. McQuade, Biomacromolecules 2006, 7, 2357-2361 [abstract]). The process was demonstrated by spraying a dye-containing aqueous sodium alginate solution in letter patterns onto synthetic turf which had been treated just prior with a calcium chloride solution. The two components formed a gel and then, after drying, a colored film which adhered strongly to the artificial turf. Only a small amount of dye leached from the coating after treatment with water, however treatment with an aqueous EDTA solution effectively dissolved the coating and it washed away. The inexpensive and environmentally friendly coatings were proposed for consumer and personal care markets.
December 2006 - Optimization of chitosan membrane structures for wound healing applications
Most naturally occurring polysaccharides (PSs) are hydrophilic. That is, they can absorb varying amounts of water or even dissolve in water, depending on their individual compositions. This behavior is due to the chemical structure of PSs which possesses a polar character and an energetically favorable interaction with water which also has a polar structure. This property has been exploited for the production of wound dressing membranes. This application requires a moist climate around the wound, high permeability for water vapor and oxygen, good fluid transport through the membrane, and adequate mechanical properties. Chitosan (poly[b-1,4-glucosamine]) is a polysaccharide which excels in this application and has been commercialized recently (HemCon™, Chitoskin™). It is derived from chitin (poly[b-1,4-N-acetylglucosamine]), a naturally occurring PS found in abundance in shrimp and crab shells. Chitosan is unique among commercially available PSs in that it possesses amino groups along its backbone. This feature confers valuable properties, especially, solubility in acidic aqueous solutions and ease of derivatization and crosslinking. Preparation of chitosan membranes of various structures has been facilitated by exploiting its solubility properties – soluble in acidic aqueous solutions, but insoluble in neutral and basic aqueous solutions. Thus, additives can be effectively incorporated and chemical modifications can be performed in aqueous acidic solutions, and then membranes can be formed upon neutralization/volatilization of the acid. Chitosan also possesses useful biological properties for wound healing (ability to immobilize microorganisms, accelerate wound healing, decrease bleeding, stimulate macrophage activity, and control microbial growth). A recent publication describes research into engineered chitosan membranes in which the flexibility, strength, and permeability were optimized for wound dressings (“Formation and Characterization of Chitosan Membranes”, C. Clasen, T. Wilhelms, and W.-M. Kulicke, Biomacromolecules 2006, 7, 3210-3222 [abstract]).
The first method to produce membranes with improved flexibility and gas permeation involved incorporation of a plasticizer, glycerol (~25 wt%). It disrupts the strong interchain attractions of chitosan allowing the polymer chains to move more freely and small molecules, such as oxygen, to permeate more easily.
Crosslinking of the polymer chains was accomplished by reaction with the difunctional aldehyde, glutaraldehyde. Increasing degrees of crosslinking, controlled by the ratio of the aldehyde to the amine groups of chitosan, produced a greater stiffness and lower permeability. In the presence of glycerol additive, crosslinking improved the break strength of the membrane while maintaining a desirable flexibility.
Permeability of the membrane to gases, such as oxygen and water vapor, was greatly increased by incorporation of pores into the membrane. Pores (0.5-100 mm) were created by addition of either polyethylene glycol or silica to the chitosan during the membrane preparation, followed by dissolution and removal of each additive with the concommitant formation of microporous or macroporous structures, respectively. Permeabilities increased by orders of magnitude, however to maintain sufficient mechanical properties for the application, a cellulose mesh reinforcement was also required.
Combinations of these methods produced membranes with sufficient mechanical properties for wound healing applications and with permeabilities tailored for specific treatments. Wound healing membranes are just one example of the many exciting, new products being developed using chitosan's unique properties.
November 2006 - Inexpensive thermoplastics from sugar beet pulp
Starch
has been successfully commercialized as a thermoplastic material to
address needs for biodegradability and renewability. Starch is a
polysaccharide polymer composed of d-glucose repeat units with a-1,4 linkages. It
contains differing amounts of branching, from low (amylose) to high
(amylopectin) (more
polysaccharide chemistry and properties ...). By
adding a small amount of water (15-20%), starch is plasticized so
that
it may be processed in the melt by extrusion and injection
molding. The resulting plastic articles are strong and stiff though
brittle. These properties may be altered by addition of other
plasticizers, such as glycerol, and other additives in varying
amounts. The properties of the hydrophilic, water-containing plastic
products also are affected by the relative humidity of the
environments in which they are used. Much product development of
starch-based thermoplastics continues today.
An alternative biobased material that is under development is sugar beet pulp (SBP). It is a byproduct of the sugar industry and is produced at an annual volume of 6 megatons in Europe (2002). It is composed mostly of polysaccharides, approximately equal amounts of cellulose, hemicellulose (arabinans and arabinogalactans), and pectin. Considering that it has limited markets and sells for about 0.1 euro or dollar per Kg (2002), it may make an attractive, lower cost substitute for starch. Researchers from the Laboratoire de Chimie Agro-Industrielle at ENSIACET in France have successfully demonstrated that SBP can be processed as a thermoplastic and injected molded much like starch and with very similar properties (“Thermo-mechanical processing of sugar beet pulp. I. Twin-screw extrusion process,” A. Rouilly, J. Jorda, L. Rigal, Carbohydrate Polymers 2006, 66, 81–87 [abstract]).
After sucrose extraction from sugar beets, the biological cell structure remains in the pulp and limits its processability in the melt. However, it can be “destructurized”, that is, solubilized or melted, in a twin screw extruder with application of a sufficient amount of mechanical energy (established by screw element design) and optimization of water content (30%). The resulting product was a homogeneous composite of cellulose microfibers surrounded by a continuous phase of hemicellulose and pectin. This highly destructurized SBP was then successfully injection molded at 130 ºC without additional plasticizer to yield a material that was dense (1.4 g/cm3), strong (tensile strength 18 MPa), stiff and brittle (tensile strain around 1% for a tensile modulus of 2 GPa) – properties much like thermoplastic starch. It contained 37% water-soluble content and disintegrated upon soaking in water. One wonders how many other polysaccharide-rich agricultural byproducts might make useful plastics or contribute useful properties in blends with other plastics.
October 2006 - Improving the economics of ethanol production with co-products
Market demand is increasing for commodity products such as fuels and polymers that are derived from renewable, agricultural resources. Examples are ethanol and polylactic acid. Although economics still favor the alternatives derived from depleting petrochemical sources, biobased materials are becoming more competitive. Two approaches to improve the economics of ethanol production from biomass have been reported recently. In the first, a valuable plastic material, cellulose acetate, was produced from cellulose-containing byproducts from the grain industry. In the second approach, an effective wood adhesive was produced from cellulosic ethanol fermentation residues. Both approaches offer additional revenue streams from biomass derived ethanol production, making the overall (ethanol plus coproduct) process more economical.
Cellulose acetate of various degrees of acetylation has many uses, is currently produced at a rate of 1.5 billion lb/yr globally, and sells for about $1.8/lb. Researchers at the US Department of Agriculture have demonstrated the ability to produce cellulose acetate by acid-catalyzed reaction of relatively crude forms of cellulose-containing agricultural byproducts with acetic anhydride (A. Biswas, B. Saha, J.W. Lawton, R.L. Shogren, J.L. Willett, Carbohydrate Polymers 2006, 64, 134–137 [abstract]). Highest yields were generally observed after a pretreatment of corn fiber, wheat straw, and rice hulls with dilute acid and heat. A maximum of 30% of the initial cellulose content was converted to cellulose triacetate which was attributed to the corresponding amount of amorphous cellulose content. Possibly, process improvements involving a more powerful cellulose solvent would afford higher cellulose acetate yields.
Ruminal cellulolytic bacteria ferment cellulose, hemicelluloses, and pectin to ethanol. During the degradation process, bacteria adhere to the cellulose substrate with the aid of adhesive compounds, adhesins. After separation of the ethanol-containing liquid phase, the fermentation residue (FR) consists of incompletely fermented biomass, adherent bacterial cells, and associated adhesins. Researchers at the University of Wisconsion and the US Department of Agriculture have shown FR to be an effective wood adhesive http://news.cals.wisc.edu/newsDisplay.asp?id=1548 (2006); “Wood adhesives prepared from lucerne fiber fermentation residues of Ruminococcus albus and Clostridium thermocellum,” P.J. Weimer, R.G. Koegel, L.F. Lorenz, C.R. Frihart, W.R. Kenealy, Applied Microbiology and Biotechnology 2005, 66, 635-640 [abstract]). Adhesive strengths obtained with FR derived from purified cellulose were about half that of commercial phenol-formaldehyde (PF) adhesive and lacked moisture resistance. However, FR was most effective when used in mixtures with PF, allowing substitution of up to 73% of PF while maintaining the same high performance even under wet conditions. FR derived from a crude cellulose-containing biomass, lucerne (alfalfa) fiber, was similarly effective in replacing 30% of PF in adhesive mixtures. The adhesive performance of FR was attributed to the major, polysaccharide components. Thus, usage of a relatively toxic, petroleum-based chemical could be reduced by replacement with an economical biobased material. Utilization of the FR product with a value of at least that of ethanol offers an alternative strategy for ethanol production in which high substrate conversion or costly pretreatment operations are unnecessary. Read more about progress in the development of naturally occurring adhesive materials in Biological Adhesives, edited by A.M. Smith and J.A. Callow (Springer, Berlin, 2006).
September 2006 - Bacterial cellulose composites
Cellulose
is the polymer produced in the greatest quantity in nature. It is
primarily produced in plants, such as trees and cotton, but also in
certain bacteria. Bacterially produced cellulose (BC) is an emerging
industrial material with remarkable properties. BC is produced
extracellularly by common, nonpathogenic bacteria in the form of a
thick, highly hydrated, rubbery film, or pellicle. For many years it
has been produced commercially in the form of a flavored gel food
product, “nata de coco”. BC has properties that distinguish it
from plant cellulose. It is produced without the lignin and
hemicellulose that accompany plant cellulose and can be readily
obtained in a very pure form by a simple washing process. The
resulting BC is a highly hydrated (>99% water) network of highly
crystalline, ribbonlike structures (60 nm 
wide)
which are each
composed of microfibrils (2x6 nm cross-section). The structure is
very porous and readily permits the infusion of solutes and solids.
This property of the native, hydrated material along with the
desirable mechanical properties of the subsequently dried material
have recently been exploited to produce unique composite materials.
Examples include composites containing metal particles for fuel
cells, hydroxyapatite for orthopedic biomaterials, and carbon
nanotubes for electrically conductive materials. Once dried, BC loses
its ability to rehydrate to its initial high water content. The dried
film has a very high surface area (200 times that of plant cellulose)
and outstanding mechanical properties, such as stiffness which
approachs that of aluminum (Young's modulus of 30 GPa, measured
isotropically across surface of plane). Thus, its high purity, high
crystallinity, and ultrafine network structure afford remarkable
mechanical properties that have found applications in speaker
diaphragm membranes, resin composites, paper, textiles, tires, wound
healing membranes, and artificial skin and blood vessels.
Examples – Metal-containing bacterial cellulose film for fuel cell application: A BC film containing metallic palladium particles was produced by dipping the native BC pellicle in a palladium ion solution, followed by in situ reduction. The resulting hydrogel was then dried to produce a film that was successfully utilized to produce a fuel cell (“Metallization of bacterial cellulose for electrical and electronic device manufacture,” B.R. Evans, H.M. O'Neill, V.M. Jansen, J. Woodward, US Patent 6,986,963; 2006 [full text]).
Hydroxyapatite-bacterial cellulose composite biomaterial: Calcium-deficient hydroxyapatite was successfully grown in a BC matrix to produce a homogeneous composite containing 50-90% apatite in a biomimetic structure that may be valuable as an orthopedic biomaterial (“Biomimetic synthesis of calcium deficient hydroxyapatite in a natural hydrogel,” S.A. Hutchens, R.S. Benson, B.R. Evans, H.M. O'Neill, C.J. Rawn, Biomaterials 2006, 27, 4661-4670 [abstract]).
Bacterial cellulose-thermoset resin composite: BC was used to prepare a phenol-formaldehyde-based thermoset composite. Sheets (1 mm thick) were prepared from dried BC films and impregnated with resin (up to 20 wt%). After curing by heating under pressure, the sheets showed 30 GPa Young's modulus which was substantially higher than corresponding composites based on microfibrillar cellulose from plants. The better mechanical properties were attributed to the extremely fine structure and optimal network alignment obtained under pressure (“Bacterial cellulose: the ultimate nano-scalar cellulose morphology for the production of high-strength composites,” A.N. Nakagaito, S. Iwamoto, H. Yano, Applied Physics A – Materials Science and Processing 2005, 80, 93-97 [abstract]).
August 2006 - Oxidation chemistry: from bacteria to beer bottles
Research in the following two reports exploits a characteristic chemical property of polysaccharides, free radical oxidation.
a) Process to reduce the molecular weight of bacterial exopolysaccharides
Seaweed is a source of commercially valuable water-soluble, biodegradable polysaccharides, such as alginate and carrageenan. Recently, marine microorganisms have also been found to be a rich source of these compounds. Many interesting properties have been discovered owing to their unique structures. The marine microbial polysaccharides possess very high molecular weights (106 g/mol). Derivatives with lower molecular weights are expected to have different properties, especially biological activities and rheological properties. A recent article demonstrates a method to partially depolymerize the polysaccharide, reducing its molecular weight, in an industrially practical and reproducible process (“Free radical depolymerization with metallic catalysts of an exopolysaccharide produced by a bacterium isolated from a deep-sea hydrothermal vent polychaete annelid,” A.-C.Petit, N.Noiret, C.Sinquin, J.Ratiskol, J.Guezennec, S.Colliec-Jouault, Carbohydrate Polymers 2006, 64, 597-602 [abstract]). Thus, a metal catalyzed oxidation using aqueous hydrogen peroxide (Fenton conditions) at 50 °C produced molecular weights (weight average) of 20,000-100,000 g/mol depending on the ratio of oxidant and experimental conditions from an initial molecular weight of 3 x 106 g/mol. More oxidant gave lower molecular weights. Best results were obtained with copper(II) salt catalysts and by maintaining a low peroxide concentration by continuous addition. Only small changes in the monosaccharide repeat unit composition occurred during the depolymerization reaction, implying nonselective degradation of the glycosidic linkages.
b) Light-catalyzed oxygen-scavenging films for packaging
Exposure to oxygen can be detrimental to flavor and other product attributes during storage. Polymer-based oxygen scavengers are of much commercial interest partly due to their mechanical properties and some commercial products have been already been successfully developed (e.g., Oxbar™). An oxygen scavenging system has recently been developed which consisted of a blend of ethyl cellulose, a commercially available plastic, and nanocrystalline titania, a commercially available semiconductor photocatalyst (2:1) (“Demonstration of a novel, flexible, photocatalytic oxygen-scavenging polymer film,” A.Mills, G.Doyle, A.M.Peiro, J.Durrant, J. Photochem. Photobiol. A: Chem. 2006, 177, 328-331 [abstract].) Its effectiveness as an oxygen scavenger was demonstrated by preparing a coating of it on the inside of a glass bottle, adding a luminescent oxygen indicator, filling with oxygen, and sealing. Upon irradiation with ultraviolet light (UVA), a decrease in oxygen was observed. Incorporation of a mild reducing agent, triethanolamine (3 wt%), in the film accelerated the rate of oxygen reduction (2.5 times higher) and oxygen concentration was non-detected (<1%) in 14 h. Calculations indicated a similar efficency as the commercial oxygen scavengers. Ideally this oxygen scavenging system would be employed as an internal layer in a multilayer plastic laminate with the outer layer separating the scavenger film from the product. Ultimately a photocatalyst which is visible light-driven may be more practical in commercial applications.
July 2006 - Salt tolerant aqueous thickener
Water soluble polymers provide valuable thickening and
rheology modifying properties for personal care
products, paints, and construction materials. Synthetic, ionic polymers
are often employed, however the
viscosity of their solutions is reduced in the presence of other ionic
additives. Neutral polysaccharides are not as powerful thickeners
however they are not
greatly affected by salts. Considerable research is being performed to
develop an understanding of the effect of structural
changes on the performance of thickeners. Recently researchers at Kao
Corporation (Japan) have shown excellent thickening and emulsification
performance with a
remarkable salt tolerance using a novel amphiphilic polysaccharide
("Salt Tolerance of
an Aqueous Solution of a Novel Amphiphilic Polysaccharide Derivative",
K. Kawakami, T. Ihara, T.
Nishioka, T. Kitsuki, and Y. Suzuki, Langmuir 2006, 22,
3337-3343 [abstract]).
This hydrophobically and hydrophilically modified hydroxyethyl
cellulose (HHM-HEC) was prepared by derivatization of commercially
available
hydroxyethyl cellulose (molecular weight 106 Da) with
stearyl ether (0.0036 units per glucose repeat unit) and sulfonate
functional groups (0.11
units per glucose unit) (see Figure).
HHM-HEC structure, x=0.0036, y=0.89, z=0.11
Hydrophobically modified water
soluble polymers form three dimensional networks through weak
intermolecular association of the hydrophobic groups. Thus, HHM-HEC
provided much greater viscosities than the corresponding unmodified
HEC. Interestingly, the gels became more elastic and thixotropic as
salt was added. Such properties are extremely useful in cosmetic
formulations containing salts. In non-hydrophobically modified
polyelectrolytes, addition of salts normally cause shielding of the
intramolecular ionic interactions on the polymer and contraction of
the random coil structure with a resulting loss of viscosity. By
comparisons of the HHM-HEC system with the corresponding HEC compounds
which were either hydrophobically or
hydrophilically derivatized, the researchers
showed that the ionic coil contraction effect in the presence of salts
is countered by an
increase in the hydrophobic association resulting in a stronger gel.
July 2006 - Electrospinning of polysaccharide fibers from room temperature ionic liquid solutions
Due to strong intermolecular attraction and crystallinity, many polysaccharides have limited solubility or are insoluble in water and organic solvents. Recently, room temperature ionic liquids (RTILs) have been found to be very effective solvents for polysaccharides and this is now enabling developments in solvent-based processes. RTILs are commonly quaternary ammonium salts which are non-volatile, thermally stable, and liquid at room temperature. Solvent properties and other physical properties can be optimized by modification of the cation and anion composition. Researchers have now found that RTIL solutions of cellulose can been employed for preparation of fibers by electrospinning ("Preparation of Biopolymer Fibers by Electrospinning from Room Temperature Ionic Liquids", G. Viswanathan, S. Murugesan, V. Pushparaj, O. Nalamasu, P.M. Ajayan, and R.J. Linhardt, Biomacromolecules 2006, 7, 415-418 [abstract]). The electrospinning method has been used to produce micron and nanometer-sized fibers for numerous applications. Solvents that are typically used for electrospinning of other polymers are removed from the fibers by evaporation. Since RTILs have extremely low volatilities, they must be removed by solvent extraction. Thus, a 10 wt% solution of cellulose in the RTIL, butylmethylimidazolium chloride, was electrospun with a voltage of 20 kV and the resulting fibers were collected in a bath of ethanol which extracted the RTIL from the cellulose fibers. Scanning electron microscopy showed smooth surfaced, highly branched, micron-sized fibers. RTILs can also be used to dissolve heparin, a polysaccharide which possesses anti-blood clotting activity. The electrospinning method was successfully employed to produce cellulose/heparin composite fibers that showed anti-clotting activity.