Chitosan: an integrative biomaterial for lab-on-a-chip devices

Chitosan is a naturally derived polymer with applications in a variety of industrial and biomedical fields. Recently, it has emerged as a promising material for biological functionalization of microelectromechanical systems (bioMEMS). Due to its unique chemical properties and film forming ability, chitosan serves as a matrix for the assembly of biomolecules, cells, nanoparticles, and other substances. The addition of these components to bioMEMS devices enables them to perform functions such as specific biorecognition, enzymatic catalysis, and controlled drug release. The chitosan film can be integrated in the device by several methods compatible with standard microfabrication technology, including solution casting, spin casting, electrodeposition, and nanoimprinting. This article surveys the usage of chitosan in bioMEMS to date. We discuss the common methods for fabrication, modification, and characterization of chitosan films, and we review a number of demonstrated chitosan-based microdevices. We also highlight the advantages of chitosan over some other functionalization materials for micro-scale devices.

Transglutaminase crosslinked gelatin as a tissue engineering scaffold

Gelatin is one of the most commonly used biomaterials for creating cellular scaffolds due to its innocuous nature. In order to create stable gelatin hydrogels at physiological temperatures (37 degrees C), chemical crosslinking agents such as glutaraldehyde are typically used. To circumvent potential problems with residual amounts of these crosslinkers in vivo and create scaffolds that are both physiologically robust and biocompatible, a microbial transglutaminase (mTG) was used in this study to enzymatically crosslink gelatin solutions. HEK293 cells encapsulated in mTG-crosslinked gelatin proliferated at a rate of 0.03 day(-1). When released via proteolytic degradation with trypsin, the cells were able to recolonize tissue culture flasks, suggesting that cells for therapeutic purposes could be delivered in vivo using an mTG-crosslinked gelatin construct. Upon submersion in a saline solution at 37 degrees C, the mTG-crosslinked gelatin exhibited no mass loss, within experimental error, indicating that the material is thermally stable. The proteolytic degradation rate of mTG-crosslinked gelatin at RT was slightly faster than that of thermally-cooled (physically-crosslinked) gelatin. Thermally-cooled gelatin that was subsequently crosslinked with mTG resulted in hydrogels that were more resistant to proteolysis. Degradation rates were found to be tunable with gelatin content, an attribute that may be useful for either long-time cell encapsulation or time-released regenerative cell delivery. Further investigation showed that proteolytic degradation was controlled by surface erosion.

Transglutaminase crosslinked gelatin as a tissue engineering scaffold

Gelatin is one of the most commonly used biomaterials for creating cellular scaffolds due to its innocuous nature. In order to create stable gelatin hydrogels at physiological temperatures (37 degrees C), chemical crosslinking agents such as glutaraldehyde are typically used. To circumvent potential problems with residual amounts of these crosslinkers in vivo and create scaffolds that are both physiologically robust and biocompatible, a microbial transglutaminase (mTG) was used in this study to enzymatically crosslink gelatin solutions. HEK293 cells encapsulated in mTG-crosslinked gelatin proliferated at a rate of 0.03 day(-1). When released via proteolytic degradation with trypsin, the cells were able to recolonize tissue culture flasks, suggesting that cells for therapeutic purposes could be delivered in vivo using an mTG-crosslinked gelatin construct. Upon submersion in a saline solution at 37 degrees C, the mTG-crosslinked gelatin exhibited no mass loss, within experimental error, indicating that the material is thermally stable. The proteolytic degradation rate of mTG-crosslinked gelatin at RT was slightly faster than that of thermally-cooled (physically-crosslinked) gelatin. Thermally-cooled gelatin that was subsequently crosslinked with mTG resulted in hydrogels that were more resistant to proteolysis. Degradation rates were found to be tunable with gelatin content, an attribute that may be useful for either long-time cell encapsulation or time-released regenerative cell delivery. Further investigation showed that proteolytic degradation was controlled by surface erosion.

Combinatorial screening for enzyme-mediated coupling. Tyrosinase-catalyzed coupling to create protein--chitosan conjugates

In nature, tyrosinase-generated o-quinones are commonly involved in processes that lead to functional biomaterials. These biomaterials are chemically complex and have been difficult to analyze. Furthermore, the cascade of reactions involving o-quinones is poorly understood, and it has been difficult to mimic ex vivo for materials processing. We report the use of a combinatorial approach to learn how tyrosinase and low molecular weight phenolic precursors can be used to generate biologically active protein-polysaccharide conjugates. Specifically, we screened various phenolic coupling precursors and various reaction conditions for the coupling of proteins onto the polysaccharide chitosan. Several natural phenols were identified as appropriate precursors for the coupling of polyhistidine tagged organophosphorus hydrolase (His-OPH) onto chitosan films. OPH activity was retained upon coupling and subsequent studies indicated that the histidine tag was not necessary for coupling. Using conditions identified for His-OPH coupling, we observed that various biologically active proteins (cytochrome c, OPH, and His-CAT) could be coupled onto chitosan films. The glycosylated protein horseradish peroxidase was not effectively coupled onto chitosan under the conditions studied. In all cases studied, we observed that coupling required a phenolic precursor, suggesting that tyrosinase is unable to couple by reaction with surface tyrosyl residues of the target protein. In conclusion, this study illustrates a combinatorial approach for the "discovery" of conditions to couple biologically active proteins onto chitosan through natural, quinone-based processes.

Chitosan based water-resistant adhesive. Analogy to mussel glue

Using analogies from nature, we investigated the possibility that tyrosinase-catalyzed reactions of 3,4-dihydroxyphenethylamine (dopamine) could confer water-resistant adhesive properties to semidilute solutions of the polysaccharide chitosan. Rheological measurements showed that the tyrosinase-catalyzed, and subsequent uncatalyzed, reactions lead to substantial increases in the viscosity of the chitosan solutions. Samples from these high-viscosity modified-chitosans were spread onto dry glass slides, the slides were lapped and clipped together either in air or after being submerged in water, and the bound slides were held under water for several hours. Adhesive shear strengths of over 400 kPa were observed for these modified chitosan samples, while control chitosan solutions conferred no adhesive strength (i.e., the glass slides separated in the absence of measurable forces). High viscosities and water-resistant adhesive strengths were also observed when semidilute chitosan solutions were treated with the known cross-linking agent, glutaraldehyde. Further studies indicate a relationship between the increased viscosities and water-resistant adhesion. These results demonstrate that the renewable biopolymer chitosan can be converted into a water-resistant adhesive.

Enzymatic coupling of phenol vapors onto chitosan

Phenols are important industrial chemicals, and because they can be volatile, also appear as air pollutants. We examined the potential of tyrosinase to react with the volatile phenol p-cresol. Three lines of evidence support the conclusion that volatile phenols react with tyrosinase and are coupled (i.e., chemisorbed) onto chitosan films. First, phenol-trapping studies indicated that p-cresol can be removed from vapors if the vapors are contacted with tyrosinase-coated chitosan films. Second, the ultraviolet absorbance of tyrosinase-coated chitosan films changes dramatically when they are contacted with cresol-containing vapors, whereas control films are unaffected by contacting with cresol vapors. Third, pressure measurements indicate that tyrosinase-coated chitosan films only react with cresol vapors if the oxygen cosubstrate is present. Additional studies demonstrate the potential of tyrosinase-coated chitosan films/membranes for the detection and removal of phenol vapors.

Enzyme conjugation to the polysaccharide chitosan: smart biocatalysts and biocatalytic hydrogels

Laccase from Coriolopsis gallica was conjugated to the renewable biopolymer chitosan using carbodiimide chemistry. The laccase-chitosan conjugate was observed to offer three unique properties. First, the laccase-chitosan conjugate displayed pH-responsive behavior such that the conjugate was soluble and active under acidic conditions, but precipitated when the pH was raised toward neutrality. Second, the laccase-chitosan conjugate was more stable than free laccase at extreme pHs. At pH 1, the inactivation rate constant (k(in)) for the soluble laccase-chitosan conjugate was 20-fold less than that for free laccase. At pH 13, k(in) for the insoluble laccase-chitosan conjugate was nearly 3-fold less than that for free laccase. Finally, the laccase-chitosan conjugate could be cross-linked under mild conditions to create biocatalytic hydrogels. Potential benefits for enzyme-chitosan conjugates are discussed.

Enzymatic grafting of hexyloxyphenol onto chitosan to alter surface and rheological properties

An enzymatic method to graft hexyloxyphenol onto the biopolymer chitosan was studied. The method employs tyrosinase to convert the phenol into a reactive o-quinone, which undergoes subsequent nonenzymatic reaction with chitosan. Reactions were conducted under heterogeneous conditions using chitosan films and also under homogeneous conditions using aqueous methanolic mixtures capable of dissolving both hexyloxyphenol and chitosan. Tyrosinase was shown to catalyze the oxidation of hexyloxyphenol in such aqueous methanolic solutions. Chemical evidence for covalent grafting onto chitosan was provided by three independent spectroscopic approaches. Specifically, enzymatic modification resulted in (1) the appearance of broad absorbance in the 350-nm region of the UV/vis spectra for chitosan films; (2) changes in the NH bending and stretching regions of chitosan's IR spectra; and (3) a base-soluble material with (1)H-NMR signals characteristic of both chitosan and the alkyl groups of hexyloxyphenol. Hexyloxyphenol modification resulted in dramatic changes in chitosan's functional properties. On the basis of contact angle measurements, heterogeneous modification of a chitosan film yielded a hydrophobic surface. Homogeneously modified chitosan offered rheological properties characteristic of associating water-soluble polymers.

Enzymatic grafting of a natural product onto chitosan to confer water solubility under basic conditions

Chitosan is a natural biopolymer whose rich amine functionality confers water solubility at low pH. At higher pH's (greater than 6. 5), the amines are deprotonated and chitosan is insoluble. To attain water solubility under basic conditions we enzymatically grafted the hydrophilic compound chlorogenic acid onto chitosan. Despite its name, chlorogenic acid is a nonchlorinated phenolic natural product that has carboxylic acid and hydroxyl functionality. The enzyme in this study was tyrosinase, which converts a wide range of phenolic substrates into electrophilic o-quinones. The o-quinones are freely diffusible and can undergo reaction with the nucleophilic amino groups of chitosan. Using slightly acidic conditions (pH = 6), it was possible to modify chitosan under homogeneous conditions. When the amount of chlorogenic acid used in the modification reaction exceeded 30% relative to chitosan's amino groups, the modified chitosan was observed to be soluble under both acidic and basic conditions, and to have a pH window of insolubility at near neutral pH. 1H NMR spectra confirmed that chitosan was chemically modified, although the degree of modification was low. Copyright 1999 John Wiley & Sons, Inc.

Plant cell biodegradation of a xenobiotic nitrate ester, nitroglycerin

The ability of plants to metabolize the xenobiotic nitrate ester, glycerol trinitrate (GTN, nitroglycerin), was examined using cultured plant cells and plant cell extracts. Intact cells rapidly degrade GTN with the initial formation of glycerol dinitrate (GDN) and the later formation of glycerol mononitrate (GMN). A material balance analysis of these intermediates indicates little, if any, formation of reduced, conjugated or cell-bound carbonaceous metabolites. Cell extracts were shown to be capable of degrading GTN with the simultaneous formation of GDN in stoichiometric amounts. The intermediates observed, and the timing of their appearance, are consistent with a sequential denitration pathway that has been reported for the microbial degradation of nitrate esters. The degradative activities of plant cells are only tenfold less than those reported for bacterial GTN degradation. These results suggests that plants may serve a direct degradative function for the phytoremediation of sites contaminated by organic nitrate esters.

Biological denitration of propylene glycol dinitrate by Bacillus sp. ATCC 51912

In previous studies, bacterial cultures were isolated that had the ability to degrade the nitrate ester glyceryl trinitrate (i.e., nitroglycerin). The goal of the present study was to examine the ability of resting cells and cell-free extracts of the isolate Bacillus sp. ATCC 51912 to degrade the more recalcitrant nitrate ester propylene glycol dinitrate (PGDN). It was observed that the PGDN-denitrating activity was expressed during growth even when cells were cultured in the absence of nitrate esters. This indicates that nitrate esters are not required for expression of denitration activity. Using cell-free extracts, PGDN was observed to be sequentially denitrated to propylene glycol mononitrate (PGMN) and propylene glycol with the second denitration step proceeding more slowly than the first. Also it was observed that dialysis of the cell-free extracts did not affect denitration activity indicating that regenerable cofactors [e.g., NAD(P)H or ATP] are not required for denitration.

Denitration of glycerol trinitrate by resting cells and cell extracts of Bacillus thuringiensis/cereus and Enterobacter agglomerans

A number of microorganisms were selected from soil and sediment samples which were known to have been previously exposed to nitrate ester contaminants. The two most effective bacteria for transforming glycerol trinitrate (GTN) were identified as Bacillus thuringiensis/cereus and Enterobacter agglomerans. For both isolates, denitration activities were expressed constitutively and GTN was not required for induction. Dialysis of cell extracts from both isolates did not affect denitration, which indicates that dissociable and depletable cofactors are not required for denitration. With thin-layer chromatography and high-performance liquid chromatography, the denitration pathway for both isolates was shown to be a sequential denitration of GTN to glycerol dinitrate isomers, glycerol mononitrate isomers, and ultimately to glycerol. GTN was observed to be completely converted to glycerol during a long-term incubation of cell extracts.

Tyrosinase Reaction and Subsequent Chitosan Adsorption for Selective Removal of a Contaminant from a Fermentation Recycle Stream

In the industrial production of penicillin V, the phenoxyacetate precursor is added to the fermentor to direct biosynthesis. When used for producing semisynthetic penicillins, the penicillin V is often hydrolyzed to 6-aminopenicillanic acid with the regeneration of the phenoxyacetate precursor. To reduce raw-material as well as waste-disposal costs, it is desirable to recycle the phenoxyacetate precursor. Unfortunately, the recycle stream is generally contaminated by the p -hydroxylated derivative of this precursor. We examined a two-step approach to eliminate this contaminant. In the first step the tyrosinase enzyme was used to selectively convert the p -hydroxyphenoxyacetate contaminant to a reactive intermediate—presumably its quinone. In the second step, the tyrosinase-generated reactive intermediate was allowed to react with and strongly bind to chitosan. In contrast, the phenoxyacetate precursor was neither oxidized by tyrosinase nor bound to chitosan. When concentrated phenoxyacetate solutions were tested, the combination of tyrosinase and chitosan effectively converted low levels of the p -hydroxyphenoxyacetate contaminant and removed its products from solution, while the concentration of the phenoxyacetate precursor was unaffected.

Bioprocess development to improve foreign protein production from recombinant Streptomyces

Bioprocessing strategies to improve production of the heterologous protein parathion hydrolase from recombinant Streptomyces lividans were investigated. Initial limitations to increased production were overcome by using large amounts of nutrients and feeding these nutrients throughout the fermentation. Batch addition of such large amounts of nutrients resulted in byproduct acid accumulation. Our data suggest that byproducts resulted from incomplete utilization of peptide medium ingredients and not from an overflow of glucose catabolism. Over extended fed-batch operation, oxygen transfer became limiting and these limitations were overcome by sparging oxygen-enriched gas. When cultivation was continued past about 90 h, we observed that despite nutrient feeding and oxygen enrichment enzyme activities no longer increased. Our results show that during such late cultivation periods the rates of enzyme synthesis and deactivation became balanced. If synthesis is prevented, either by a nutritional limitation or by the addition of the protein synthesis inhibitor chloramphenicol, enzyme activities were observed to decrease. Since deactivation rate constants in these experiments were similar to those observed in cell-free studies, and because extracellular protease activities were not detected in our fermentation, it appears that deactivation results from the inherent instability of the parathion hydrolase enzyme.

Plant cell growth under different levels of oxygen and carbon dioxide

An experimental system, in which gases of known composition were passed through flasks, was used to systematically study the effects of oxygen and carbon dioxide on plant cell growth. As expected, oxygen limiting conditions resulted in suppressed growth of Catharanthus roseus cultures. Oxygen limitations did not alter the amount of cell mass produced per gram of sugar consumed which suggests that the production of fermentative metabolites was limited. Varying levels of carbon dioxide were observed to have no effect on the growth rates of either C. roseus or Daucus carota cultures. The amount of C. roseus cell mass generated per gram of sugar consumed appeared to be slightly increased at higher carbon dioxide levels.

Ammonium effects on streptonigrin biosynthesis byStreptomyces flocculus

A defined medium containing glucose and ammonium as the sole carbon and nitrogen sources was developed to support growth and streptonigrin production. In this defined medium, increased initial levels of ammonium resulted in increased growth suggesting that nitrogen is the growth limiting nutrient. In some cases, increased initial ammonium levels resulted in decreased specific streptonigrin productivity, suggesting that nitrogen regulatory mechanisms may adversely affect streptonigrin biosynthesis. This suggestion that nitrogen regulation adversely affects antibiotic biosynthesis is further supported by results from two studies in which the ammonium supply to the cells was controlled. In the first study, streptonigrin productivity and final titer were enhanced by the addition of an ammonium trapping agent. In the second experiment, when ammonium chloride was fed slowly throughout the course of cultivation, the production phase was lengthened and the maximum antibiotic concentration was enhanced compared to the batch controls containing either the same initial or the same total ammonium chloride levels. Although our results indicate streptonigrin production may be subject to nitrogen regulatory mechanisms, the effect of nitrogen on streptonigrin production cannot be strictly correlated to the extracellular ammonium concentration. In fact, we observed that when ammonium was depleted from the medium, streptonigrin production ceased.

Improved production of heterologous protein from Streptomyces lividans

Protein-secreting procaryotic host organisms are currently being sought as alternatives to Escherichia coli for recombinant processing. In this study we examined how manipulation of the cultivation conditions can enhance heterologous protein production by Streptomyces lividans. The recombinant S. lividans used in this study expressed and excreted a Flavobacterium enzyme capable of hydrolyzing organophosphates. Initial shake-flask studies demonstrated that supplementing Luria-Bertani medium with moderate amounts of glucose (30 g/l), led to improved enzyme production. In fermentor studies with controlled pH, a further twofold increase in production was observed when glucose was fed continuously as compared to batch cultivation. This improved production in the glucose-fed culture may be related to a reduced accumulation of acids. Continuous feeding of both glucose and tryptone led to a further sixfold increase in production. In addition to enhancing production 25-fold, the efficiency of enzyme production and the specific activity of the excreted enzyme were also improved by glucose and tryptone feeding. These results demonstrate that in addition to genetic manipulations, optimization of cultivation conditions can lead to significant improvements in the production of heterologous proteins from Streptomyces.

Genetic engineering approach to toxic waste management: case study for organophosphate waste treatment

Currently, there has been limited use of genetic engineering for waste treatment. In this work, we are developing a procedure for the in situ treatment of toxic organophosphate wastes using the enzyme parathion hydrolase. Since this strategy is based on the use of an enzyme and not viable microorganisms, recombinant DNA technology could be used without the problems associated with releasing genetically altered microorganisms into the environment. The gene coding for parathion hydrolase was cloned into a Streptomyces lividans, and this transformed bacterium was observed to express and excrete this enzyme. Subsequently, fermentation conditions were developed to enhance enzyme production, and this fermentation was scaled-up to the pilot scale. The cell-free culture fluid (i.e., a nonpurified enzyme solution) was observed to be capable of effectively hydrolyzing organophosphate compounds under laboratory and simulated in situ conditions.