Recent Advances in Biopolymers and Biomedical Materials

Jun 1, 2008 | Contact Author | By: Robert Y. Lochhead, Stephen F. Foster, Ashley L. Cox, Margaret B. Lochhead, Vipul Padman and Emily A. Hoff, University of Southern Mississippi
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Title: Recent Advances in Biopolymers and Biomedical Materials
filmstripsx aloe vera pectinx hydrogelsx mucoadhesive gelsx controlled deliveryx nanofibersx
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Keywords: filmstrips | aloe vera pectin | hydrogels | mucoadhesive gels | controlled delivery | nanofibers

Abstract: This overview of recent literature provides a snapshot of research trends in biopolymers and biomedical polymers. To improve product efficacy, the personal care industry often looks to other sectors for innovative materials and ideas. Here, the authors encourage readers to innovate by technology transfer and by gaining a better understanding of biopolymers.

In this overview of recently published scientific literature, the authors provide a snapshot of current research trends in biopolymers and biomedical polymers. During experimentation to improve product benefits, the personal care industry eagerly formulates with new and different materials and ideas. Sometimes these new materials are generated within the industry while other times, they are borrowed from other sectors. In reviewing the current scientific literature, the authors hope to encourage readers to innovate by technology transfer and by gaining a better understanding of biopolymers at a time when they are escalating in importance. 

Renewable Resources

Rising oil prices and environmental concerns have focused public interest and scientific research on renewable material sources produced from agriculture and biological stock. The drive within the scientific community is to meet the current thermal and mechanical properties of standard petroleum materials with that of materials produced from agricultural sources such as corn or soybeans. With this in mind, a recent scientific publication in biomacromolecules1 focused on analyzing the material properties as well as phase separation in copolymer blends of poly(3-hydroxybutyrate) (PHB), with poly(L-lactic acid) (PLA), and poly(ε-caprolactone) (PCL). 

PHB is a polymer that is synthesized from bacteria and PLA can be obtained from agricultural resources such as corn. The polymers PLA and PCL were blended with PHB in varying weight ratios of 15% to 85%. This publication compared the copolymer blends of PHB/PLA to PHB/PCL in both mechanical properties as well as phase behavior. The mechanical properties were determined by using a miniaturized stretching machine at a 10% strain/min setting. FT-IR imaging was used to detect phase separation in the cast films by using calibration curves. 

Both of these copolymer blends have similar mechanical properties with PHB content less than approximately 40%, resulting in elongation to break percentages >100%. It is interesting that for PHB/PCL copolymers, the elongation to break did not result in values of 100% until the PHB content was below 35%. PHB/PLA copolymer possesses higher mechanical deformation properties than PHB/PCL copolymers. Analyzing the copolymers for phase separation behavior reveals that both copolymer systems become phase-separated at weight concentrations of approximately 50/50. 

For the PHB/PLA composition range, phase separation occurred from 45% to 65% PHB weight values. The miscibility gap in the PHB/PCL copolymers occurred over a smaller range of approximately 45% to 55% PHB weight values. The mechanical property values measured from the copolymers demonstrated that substitution of a petroleum-based polymer, PCL, with a polymer produced from an agricultural source, can yield similar or better mechanical properties.

Micropackaging of Aroma, Flavor in Filmstrips

Packages for cosmetic products are ideally designed to contain the product and prevent losses due to evaporation; they should also protect the product from degradation by the intrusion of external contaminants such as oxygen or microorganisms. There currently is an emerging trend toward the delivery of fragrance and flavors from filmstrips, which is expected to be boosted by restrictions on liquids and gels by airport security. Edible packaging is used to provide thin, protective layers that keep ingredients such as flavors and fragrances localized and prevent them from intermingling before the point of use. These barrier films can be made from hydrocolloids, i.e., proteins and polysaccharides. 

The requirements of a film for this purpose are good film cohesion, good mechanical properties, impermeability to target fragrances and flavors, and good water-barrier properties. The films can also be in the form of a “solid” emulsion in which lipid-encapsulated flavor and fragrance are dispersed in a hydrocolloid film. Crystallization of the lipid waxes in such films decreases water transfer and, if the lipid globules are small, the water permeability deceases when the lipid concentration in the film is close to 30%.2

Edible films made of iota(i)-carageenan offer the advantages of good mechanical properties, emulsion stabilization and decreased oxygen permeability; this biopolymer also has recently been evaluated as a hydrocolloid film-former for edible aroma emulsion filmsa.3

Carrageenans are sulfated polysaccharides obtained by extraction from seaweed (see Figure 1). 

There are three major classes, namely kappa, iota and lambda carrageenan. This classification is based on the number and position of sulfate groups on the disaccharide repeat unit of the polysaccharide with kappa, iota and lambda carageenans being 20%, 33% and 41%, respectively. The ready availability and reasonable cost of the carrageenans has resulted in their wide use in the food industry as thickeners and texturizers. i-Carrageenan is soluble in hot water and gels below 50°C . The gel structure is a network in which the junction zones are aggregated double-helix coils. When cast, carrageenan produces a dense film of helices 1.39 nm apart. 

Emulsion films were prepared by the inventor by solubilizing the fragrance n-hexanal in a mixture of glyceryl mono- and di-acetates and beeswax. This aroma fat-blend was homogenized into an i- carrageenan solution and the mixture was cast as a film. The aroma compound n-hexanal changed the structure of the carrageenan gel, raising the sol-gel transition temperature from 50°C to 85°C, and the water-vapor permeability increased while oxygen permeability was unchanged. These changes were attributed to plasticization of the film by n-hexanal, and wavelength shifts in FT-IR of the films indicated that the aldehyde group of the n-hexanal interacted with the –OH or sulfate groups of the i-carrageenan molecules. 

This plasticization by n-hexanal apparently increased the mesh-size of the film sufficiently to allow the intrusion of water molecules but not enough to allow permeation of the larger oxygen molecules. Thus, oxidation of the fragrance would be slowed or prevented by the film’s gel network, but the ready intrusion of water would favor the release of aroma on demand.

Polysaccharide Gels: Pectin From a New Source

Like carrageenan, pectin is another polysaccharide that is commonly used as a gellant and texturizer in the food industry. The properties of pectin extracted from aloe vera have recently been disclosed.4

Pectins are anionic polysaccharides consisting primarily of 1,4 α-D galacturonic acid repeat units with 1,2 rhamnose branch points (see Figure 2).

Similarly to alginates, pectins gel in the presence of calcium ions. In these gelled structures, the calcium ions are located specifically between the polyelectrolyte chains like eggs in an egg carton.5–7 By measuring the intrinsic viscosity as a function of ionic strength, it was shown that aloe vera pectin has an inherently stiff polymer backbone; the intrinsic viscosity is an indication of a polymer’s molecular hydrodynamic volume in solution. 

Zeta potential measurements showed that, in aqueous solution, the potential on the pectin molecule is high and the polymer molecules interact strongly to the water and generally avoid each other. However, above a sodium chloride concentration of only 0.1M, the potential drops to a level that allows the polymer molecules to interact with each other, and if the concentration is above the polymer’s critical overlap concentration, a gel network can be established. This is shown by the “scaling” of the electrolyte solution’s intrinsic viscosity with pectin concentration. 

The exponent of 10.9 is much larger than the theoretical value of 4. In this concentration regime, hydrophobic junction zones between the pectin molecules have been detected by the fluorescent probe 8-anilino naphthalenesulfonic acid, confirming that the gel structure is a hydrophilic polymer network linked by hydrophobic junction zones. 

It is interesting though, that, even in the absence of salt and above the critical overlap concentration, the intrinsic viscosity rises faster with concentration than would be predicted (8.6) for a random coil polymer, indicating that even when the polymer molecules effectively repel one another, the flow of the system is constrained. In the presence of calcium the gel had a higher shear storage modulus than loss modulus, indicating that it was more solid-like than liquid-like. Moreover, aloe vera pectin at only 0.20% w/w exhibits almost the same values of elastic modulus as other pectins tested at higher concentrations(2.0% w/w) and higher calcium concentrations (10mM).

Polymers for Tissue Engineering

In medical sciences, tissue engineering has become a highly researched area focusing on the repair of damaged cells and restoration of function to damaged areas of the human body.8 One key area that has inherent difficulties is that of nerve repair, since nerve damage is notorious for being difficult if not impossible to heal. 

Scientists have begun researching new “smart” materials, or materials that perform a specific function upon external stimulus such as light, electric currents, pH, ionic strength and temperature, and have used these stimuli triggers to develop materials that could be useful in tissue engineering.

In a recent article submission,9 researchers synthesized an electroactive copolymer (PLAAP) using PLA and aniline pentamer (AP) with the goal of creating a copolymer that could be used as a nerve repair scaffold material. The conjugated ring backbone of the AP allows the polymer to be electroactive—a property that has been demonstrated as useful in proliferation or differentiation of various cell types. 

This copolymer is soluble in relatively inexpensive solvents such as THF and CHCl3 and is capable of forming films in either. The contact angles of various films formed in these solvents are relatively independent based on film formation in solvents THF and CHCl3. The contact angle of the films are approximately 90 degrees but after the introduction of camphor sulfonic acid to emeraldine PLAAP, the contact angle lowers to almost 50 degrees. This contributes to the copolymer the right surface properties for biocompatibility of rat neuronal pheochromocytoma PC-12 cells, and results in the cells being able to spread across the surface of the PLAAP copolymer.

According to the study, two substrates were seeded with PC-12 cells, a PLAAP substrate and a tissue-culture-treated polystyrene (TCPS), and an electrical stimulus of 0.1V was applied across them using platinum microwire electrodes for 1 hr every day. Two more substrates were seeded, but not electrically stimulated so as to provide a contrasting sample group. It was shown that after four days, neurite extension occurred only in the PLAAP substrate stimulated by electrical stimulus, and demonstrated that the electroactive PLAAP copolymer can accelerate the differentiation of nerve cells through electrical stimulus. This study proved that PLAAP copolymers can be modified so that they are biocompatible with PC-12 cells and then electrically stimulated to differentiate the nerve cells. The copolymer solubility in inexpensive solvents allows relative ease of processing for practical application, and the electroactivity of the copolymer provides the capability for cellular adhesion, growth and proliferation. 

Scaffolds are artificial extracellular matrices used in tissue engineering to integrate tissues and organs, and to provide a structure for cells to propagate, migrate and differentiate and become integrated with intrinsic body tissues. Collagen is the most abundant protein in vertebrates and Type I collagen is the most widely used scaffold material because it is abundant, biocompatible, bioabsorbable and it is not antigenic.10 Monomeric collagen self-assembles to form fibrils in vitro under physiological conditions, the fibrils confer enhanced mechanical properties and biological stability, and the properties are further improved by cross-linking. 

Collagen biomaterials are usually produced by sequential processing in which fibrils are allowed to form. They subsequently are cross-linked;11 however, the mechanical properties of scaffolds formed by this process are inadequate for tissue engineering of hard tissues. Simultaneous processing in which the fibrils are grown and simultaneously cross-linked produces collagen gels with improved mechanical properties.12  

The structure of these collagen gels, cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, was recently studied by scanning electron microscopy and atomic force microscopy and it was discovered that, despite their improved mechanical properties, simultaneously processed collagen gels had fewer fibrils than those that had been sequentially processed. AFM showed that nonfibrous collagen filled the spaces between the fibrils in the simultaneously processed gels. It was postulated that the improved properties resulted from cross-linking of the fibrils with nonfibrous collagen.13

Hydrogels are promising scaffolds for cell adhesion, spreading and proliferation, and also for the repair and regeneration of tissues and organs.14, 15 In order to be used as cell-growth scaffolds for tissue engineering, hydrogels must be biocompatible, must have the correct mechanical properties and pore size, and must be compatible with surrounding living cells.16 pH sensitive hydrogel copolymers of cross-linked acrylic acid, e-caprolactone and 2-hydroxyethylmethacrylate showed good biocompatibility, good biodegradation by the enzyme lipase, and good cell adhesion.17 

However, natural physiological tissue regeneration during wound healing involves the simultaneous appearance of multiple growth factors in controlled amounts.18 Conventional inert hydrogels are severely limited in their capability to deliver multiple proteins because these gels cannot distinguish between proteins of similar size, nor can they govern the release rate of a smaller protein and simultaneously maintain a constant rate of release of a larger protein. In order to mimic the natural process, layer-by-layer polymeric composites capable of the simultaneous, but individually controlled, release of two or more proteins have been developed.19 

Despite many studies, the effect of charge density on multilayer growth is still unclear. For example, very thick films can be prepared from weak polyelectrolytes at high salt concentrations. The exponential growth of these films has been attributed to diffusion of the polyelectrolytes in and out of the multilayer during the B process of layer deposition.20 This knowledge gap prompted a recent study of the formation of multilayer adsorption of alternating layers of chitosan and pectin as a function of degree of acetylates of pectin. During the process of chitosan deposition, it was discovered that the chitosan diffused in and out of the film, and as the fresh become more charged and more water-soluble, this effect was magnified. Due to the excess of positive charge, the interaction between pectin and chitosan did not lead to 1:1 stoichiometric complexes.21 

In addition to these fundamental difficulties, the layer-by-layer composites are fabricated by a series of complex steps; thus, a simpler approach is desirable. Ideally this simpler approach would be monolithic inert hydrogels—however, the tuned-free, diffusive delivery of different actives from an inert hydrogel is almost impossible.

As a consequence of these shortcomings of inert hydrogels, polymer networks with specific protein affinity sites have been developed to release target molecules at a rate determined by the association constant of the site-target molecule couple rather than the passive diffusion of the target molecule through a matrix.22 Recently, a proof of concept study of monolithic affinity hydrogels for controlled protein release was reported.23 

These hydrogels are prepared by copolymerizing poly(ethylene glycol) diacrylate [PEGDA] with glycidylmethacrylate-iminodiacetic acid [GMIDA]. The metal-binding capabilities of iminodiacetic acid [IDA] have been used extensively for protein purification, and this nondegradable hydrogel uses the ligand of IDA capability to tune the simultaneous delivery of two proteins: histidine-tagged green fluorescent protein and lysozyme. 

For example, the affinity-binding of histidine-tagged proteins and GMIDA ligands is mediated by metal ions such as nickel, but nickel at low concentrations does not significantly affect lysozyme release from the hydrogel. Use of this dual-release mechanism allows the release to be tuned for each protein separately. As a corollary, complete and immediate release of the bound protein can be achieved by merely adding a metal chelating agent such as EDTA. Perhaps well-designed hydrogels such as these will someday trickle beneficial agents from topically applied compositions. 

It is interesting that Chen et al. reported that bovine fetal aorta endothelial cells could spread and proliferate on poly(acrylic acid) without any cell adhesive protein modification.24 Another shortcoming of in vitro cell growth scaffolds is that they are irreversible physical gels of materials extracted from cells or tissue, chemically cross-linked gels or nanofiber networks, and it is difficult to remove cells from the matrices of these existing gels without damaging the cells in the process. In order to address this issue, thermoresponsive hydrogels have been developed as matrices that would allow facile harvesting of intact cells for implantation.25 

The advent of living free-radical polymerization has made possible the synthesis of precise molecularly designed block copolymers for this purpose. Amphipathic BAB block copolymers in which the A block is hydrophilic and the B block is thermoresponsively hydrophobic are preferred for this application. A recent study reported that the synthesis and characterization of such amphipathic molecules having a poly(N-isopropylacrylamide) and a hydrophilic inner a block of poly(N,N-dimethylacryamide).25 Poly(N-isopropylacrylamide) was chosen because its lower critical solution temperature (LCST) of 32°C is close to the human physiological temperature (37°C); this polymer is soluble in water below 32°C and it phase-separates when the solution temperature is raised above this threshold. 

For this block copolymer, the B-blocks become relatively hydrophobic and this causes them to aggregate above the LCST to form the core of polymer micelles. When the concentration is low, the micelles are isolated in solution and the composition is liquid-like; however, above a critical concentration, a network structure forms with the micelle cores becoming junction zones in the gel and the rheology becomes more solid-like; the elastic modulus becomes larger than the loss modulus. 

The formation of the gel was shown on a molecular level in this case by measuring the diffusion coefficient of water by pulsed field gradient NMR and by contour microscopy using the fluorescent probe 8-anilino-1-naphthalenesulfonic acid. The apparent diffusion coefficient was high at short times, but it dropped with time of observation and plateaued at 400 milliseconds, indicating that the water was probably being bounded by a gel network. The hypothesis of a gel network was further supported by measurement of the fluorescence observed by confocal microscopy. The fluorescence intensified at elevated temperatures and the image showed large hydrophobic domains separated by tens of micrometers. These are assigned as the micelle core junction zones in the gel. This type of thermoreversible gel could form the basis of a topical treatment that could be removed by merely rinsing with cold water (see Figure 3).

Mucoadhesive Gels

The emergence of antibiotic-resistant pathogens has the potential of driving civilization back to the pre-penicillin era, when relatively minor infections were life-threatening. In such an environment, even the administration of injections could compromise the body’s natural barrier and allow the ingress of undesirable bacteria. In order to meet this challenge, the pharmaceutical industry is directing research attention to more efficient and advanced drug excipients, controlled-release systems and bioadhesives. This is because of lower adminstration and dosage frequencies, longer retention times and the possibility of site specific drug delivery. One attractive route is via the mucus membranes of, for example, the nasal or buccal cavity. Mucus membranes are relatively thin and well served with blood vessels; this makes them attractive targets for penetration of drugs into the vascular system.

The polymer poly(acrylic) acid has been heavily researched for bioadhesion and mucoadhesion application. Poly(acrylic) acid contains a high number of carboxylic groups, providing a criticial characteristic of bioadhesion through hydrogen bonding (see Figure 4).

Organogels of a dispersion of poly(acrylic acid), nonaqueous solvent and a model antimicrobial agent have been formed with the application of an antimicrobial implant in the buccal cavity in mind. The proposed mechanism of mucoadhesion for these organogels is initial secondary interactions between the poly(acrylic) acid and the nonaqueous solvent, and once introduced into the oral cavity, interactions between poly(acrylic) acid and mucin. 

The rheological properties, specifically pseudoplastic flow, of a mucoadhesive gel are critical to the success of drug delivery because of application ease and manufacturing ease. The quick structural recovery of pseudoplastic materials also aids in retention of the gel in the oral cavity. Because of swallowing and a constant flow of saliva, retention time of a mucoadhesive gel in the buccal cavity poses a major challenge. Correlations between viscoelastic properties and mucoadhesion were found in the poly(acrylic) acid organogels where highly structured gels exhibited high mucoadhesion, because of molecular polymer chain entanglements. As the concentration of poly(acrylic) acid increased in the bioactive organogels, loss modulus, storage modulus, dynamic viscosity, compressibility, mucoadhesion and drug release rate also increased. Choice of nonaqueous solvent also affected the physiochemical parameters. The organogel with 5–10% w/w poly(acrylic) acid and the nonaqueous solvent PEG 400 maintained the most potential as an antimicrobial implant.26

Controlled Delivery from Micellar Aggregates and Vesicles

While micelle-aggregated gels are being developed for tissue engineering, the need for better chemotherapeutic agents is driving the reach for precise tailoring of the interactions between hydrophilic polymers and vesicles. In this context, it has recently been shown that polyaspartylhydrazide copolymers, containing butyl moieties and/or (carboxypropyl)trimethylammonium moieties, bind to the surface of phosphatidylcholine/cholesterol vesicles, and this causes the vesicles to bind more strongly to cancer cells to increase the effectiveness of chemotherapy.27

Since the 1960s it has been known that cholesterol fills the voids in the palisade layer of lecithin monolayers and the scientific community understands that one of the roles of cholesterol in cells is to strengthen the phospholipid structure of the cell membranes. Recently, it has been shown that hydrophobic matching between the cholesterol and fatty acid is required for the formation of lamellar liquid ordered phases.28 

It has also been demonstrated by nearest neighbor recognition methods that cholesterol-rich fluid bilayers have an ordered bilayer structure in which free cholesterol and longer chain phospholipid homodimer co-exist in equilibrium with a stoichiometric complex composed of one molecule of cholesterol for every two molecules of longer chain phospholipid homodimer.29 It is not surprising, therefore, that attempts have been made to decorate hydrophilic polymers with cholesterol mesogens to enhance their interaction with lipid membranes and vesicles. Through studies, Langmuir showed that for a homopolymer formed from cholesterol acrylamidobutyrate (CAB), the preferred configuration of the cholesterol rings is to lie flat at the air-water interface (see Figure 5).30

The inference to be drawn from this finding is that presumably the cholesterol rings of such a homopolymer would also lie flat at the surface of a lipid bilayer. If this happened, the cholesterol rings would be sterically restricted from insertion between the lipid molecules of the bilayer and the resulting polymer-lipid interaction would be weak. A copolymer of CAB and 2-acrylamido-2-methylpropanesulfonic acid AMPS gave a similar surface configuration of the cholesterol when the mole ratio of CAB:AMPS was 15:85. However, a copolymer containing 10 mol% CAB gave a close-packed surface structure with the cholesterol rings being organized side-by-side with a vertical orientation to the surface. The inference in this case is that this copolymer would be ideal for interacting with, and perhaps stabilizing, phospholipid bilayers (see Figure 6).

The precision with which these molecules have to be designed to fit their function is demonstrated by the finding that when the cholesterol mesogens content of the copolymer was reduced to 5 mol%, the polymer tended to form bilayers and micelle assemblies. Similarly, the introduction of cholesterol mesogens to carboxymethylchitosan caused this polymer to form nanomicelles that could function as injectable drug carriers.31 

Nanofiber Materials

Scaffolds for tissue engineering can be formed by the self-assembly of peptides into b-pleated sheets32–34 and it has been shown that configurational and conformational organization of peptides can lead to self assembly into complex hierarchical structures such as nanotubes,35 nanofibers36 and helical fibrils.37 An interesting development has been the demonstration that b-sheet-forming wheel-like synthetic peptides self assemble into nanofibers in phosphate buffer and rod-like structures in acid and alkaline media.38 The nanofiber structures were clearly visible in transmission electron micrographs of specimens stained with phosphotungstic acid or uranyl acetate. An emphatic increase of fluorescence on binding with thioflavin T showed that the nanofibers self-assemble as b-pleated sheets and increase in fluorescence upon binding 8-anilino-1-naphthalenesulfonic acid indicated that the outside of the nanofibers were hydrophilic (see Figure 7 and Figure 8).

Modified microcrystalline celluloses are used extensively as rheology modifiers in personal care and also in foods. The rheology conferred by these materials is attributed to the formation of composite network structures between cellulose microcrystalline fibrils and soluble polymers. 

A recent study on the interaction between cellulose nanofibers may shed some light on exactly what is happening between the crystalline microfibrils when they form higher order structures.39 The nature of the surface of cellulose nanocrytals differs depending upon the method of preparation. Thus, hydrolysis with sulfuric acid might be expected to yield surfaces that contain sulfate groups. The effect of surface charge density on the structuring of cellulose nanocrystal films was probed using quartz crystal microbalance with dissipation and atomic force microbalance measurements. 

Cellulose nanofibrils were prepared by mechanically disintegrating bleached sulfite pulp by a microfluidizer. Low-charge density nanofibrils were pretreated by enzymatic hydrolysis and mechanical disintegration, whereas high charge density nanofibrils were carboxymethylated. The cellulose nanofibers were spin-coated on a silica wafer. Interestingly, the carboxymethylated anionically charged nanofibrils gave good homogeneous surface coverage that can be attributed to hydrogen-bonding, whereas the low-charge density nanofibrils did not adhere readily to the silica surface. 

Surface coverage was improved by treatment with polyvinylamine, titanium dioxide or 3-aminopropyltrimethoxysilane, implying that these low charge density nanofibrils carry a negative charge. Quartz crystal microbalance measurements in aqueous media showed that low-charge density nanofibril films were relatively insensitive to changes in electrolyte concentration and pH, but high change density nanofibril films swelled and de-swelled dramatically under the influence of such changes. 

Electrolyte addition caused the films to imbibe water; much more for the high charge density nanofibers. This enhanced swelling was attributed to a Donnan effect; increased electrolyte concentration causes an increase in the pH inside the film, leading to dissociation of more carboxyl groups that in turn causes polyelectrolyte swelling. At high electrolyte concentrations, the water uptake decreases again and this can be explained by electrochemical attraction of counter ions to the nanofibrils being favored over the chemical potential-driven diffusion of counterions into the surrounding aqueous environment. The resulting proximity of the counterions shields the nanofibril charges and causes de-swelling of the film. 

Atomic force microscope measurements of the interfibril interaction were conducted by attaching a cellulose sphere to the tip of the probe and then carefully measuring force as a function of distance between the nanofibril film and the cellulose sphere. These measurements in water at pH 8 indicated the presence of electrical double-layer repulsion, consistent with DLVO theory. However, when the ionic strength was increased, the forces did not decay to the extent expected by double-layer repulsion. This indicates that there is steric repulsion, for both low-charge density and high-charge density nanofibrils and this may also indicate the presence of anchored soluble polymers, presumably hemicelluloses, at their surface. 

Impurities in polysaccharides often diffuse to the surface, making it difficult to measure the true surface energy. Chitosan and cellulose fall under the same class of materials but they differ chemically since OH groups in the cellulose are replaced by NH2 groups. The dispersive components of the surface energy are comparable within the two polymers, but the polar component is higher in cellulose than chitosan.40–42 

In one study, chitosan surface energies were compared with N-acetyl-D-glucosamine (GlcNAc) before and after purification and extraction processes. These processes removed nonpolar impurities present in chitosan; then samples in the form of pellets and films were obtained. Before purification chitosan showed low values of the polar component of surface-free energy compared to GlcNAc, but after purification there was a dramatic increase. For the purified pellets, increase in the polar component confirmed that nonpolar impurities caused the decrease in the polar component of chitosan. The polar component was independent of the degree of deacetylation (DDA). 

For films, the polar component was lower compared to GlcNAc, as the residual impurities in films migrated to the surface and caused a decrease in the surface energy. Scratching the film surface spontaneously increased the surface energy. Thus, the impurities present in the commercially available chitosan were responsible for low polar component of surface energies.43 

These latest findings indicate that the industry’s understanding of the interactions between nanofibrils is far from complete and more fundamental studies could yield rich rewards in the design of new, improved rheology modifiers and nanofiber composites.

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Figure 1. Carrageenan

Figure 1. Carrageenan 

Figure 2. Depiction of pectin structure

 Figure 2. Depiction of pectin structure

Figure 3. Thermoresponsive block

Figure 3. Thermoresponsive block 

Figure 4. Poly(acrylic) acid

Figure 4. Poly(acrylic) acid 

Figure 5. When present in the polymer

 Figure 5. When present in the polymer

Figure 6. At 10 mole%

Figure 6. At 10 mole% 

Figure 7. A ß-sheet-forming wheel peptide

Figure 7. A ß-sheet-forming wheel peptide

Figure 8. Wheel-like peptides

Figure 8. Wheel-like peptides

Lochhead: Recent Advances in Biopolymers footnotes

 a The Gelsite polymer is a product and registered trademark of Delsite Biotechnologies, Irving, Tex., USA.

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