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In this research, controlled delivery of hollow nanoparticles from zein, the corn storage protein, to different organs of mice was achieved via crosslinking using citric acid, a non-toxic polycarboxylic acid derived from starch. Besides, crosslinking significantly enhanced water stability of nanoparticles while preserving their drug loading efficiency. Protein nanoparticles have been widely investigated as vehicles for delivery of therapeutics. However, protein nanoparticles were not stable in physiological conditions, easily cleared by mononuclear phagocyte system (MPS), and thus mainly accumulated and degraded in spleen and liver, the major MPS organs. Effective delivery to major non-MPS organs, such as kidney, was usually difficult to achieve, as well as long resident time of nanoparticles. In this research, hollow zein nanoparticles were chemically crosslinked with citric acid. Controlled delivery and prolonged accumulation of the nanoparticles in kidney, one major non-MPS organ, were achieved. The nanoparticles showed improved stability in aqueous environment at pH 7.4 without affecting the adsorption of 5-FU, a common anticancer drug. In summary, citric acid crosslinked hollow zein nanoparticles could be potential vehicles for controllable delivery of anticancer therapeutics.

Cytocompatible and water-stable ultrafine collagen fibers were electrospun by dissolving collagen in a low corrosive ethanol-water solvent and crosslinked by citric acid (CA) with glycerol as the crosslinking extender. Conventional solvents used for electrospinning of collagen either cause denaturation or contain more than 50% salt potentially leading to poor mechanical properties and water stability of the scaffolds. Collagen scaffolds have to be modified by techniques, such as, crosslinking to overcome the limitations in strength and stability. However, the existing crosslinking methods are either cytotoxic or ineffective. In this research, a benign ethanol-water solvent system and an extender-aided CA crosslinking method were developed. The native collagen conformation was retained after electrospinning, and the dry/wet strengths and water stability of fibers were substantially enhanced after crosslinking. The crosslinked electrospun scaffolds could maintain their fibrous structure for up to 30 days in phosphate-buffered saline at 37°C. Cells exhibited better attachment and growth on the CA crosslinked collagen fibers than on the glutaraldehyde crosslinked scaffolds., (Copyright © 2012 Wiley Periodicals, Inc.)

We demonstrate a novel approach of developing thermoplastic films from feathers by alkaline hydrolysis and crosslinking with citric acid. Unlike previous approaches that used toxic chemicals, complicated and/or expensive methods to develop films, in this research, feathers were hydrolyzed using various concentrations of alkali and the hydrolyzed feathers were compression molded into films using glycerol as plasticizer and crosslinked with citric acid to improve water stability. Alkali hydrolyzed feathers could be compression molded into films with tensile strength of 5.9 MPa and elongation of 31.7% but had poor wet strength. Feather films crosslinked with citric acid had tensile strength of 1.9 MPa and elongation of 24.6% after being in 90% humidity at 21 °C for 24 hours. Alkaline hydrolysis and citric acid crosslinking provides an opportunity to develop inexpensive and biodegradable thermoplastics from the inexpensive, renewable and sustainable poultry feathers., (Copyright © 2012 Elsevier B.V. All rights reserved.)

Regenerated protein fibers developed from casein using alkaline solutions and crosslinked with citric acid have good tensile properties and water stability but were cytotoxic. Casein is obtained as a byproduct during skim milk production but has limited industrial applications. In this research, casein was dissolved using alkaline solutions and the effects of fiber formation conditions on the tensile properties was studied before and after crosslinking with citric acid. Crosslinked casein fibers had tensile strength as high as 110 MPa and breaking elongation of 13.6%. The fibers were also stable in 90°C water between pH 3 and 9 but were hydrolyzed at pH 11. Casein was cytotoxic and did not support the attachment and growth of mouse fibroblasts., (Copyright © 2012 Elsevier B.V. All rights reserved.)

This paper reports the wet cross-linking of gliadin fibers using citric acid without using phosphorus-containing catalysts or high temperatures. Carboxylic acids such as citric acid are inexpensive and nontoxic chemicals preferred for cross-linking proteins and cellulose. However, previous studies have shown that carboxylic acid cross-linked materials experience substantial strength loss and/or yellowing when cross-linked using phosphorus-containing catalysts after drying and curing at high temperatures. In this research, citric acid has been used to cross-link gliadin fibers and the effects of various cross-linking conditions on the breaking tenacity and breaking elongation have been studied. A mathematical relationship that can predict the breaking tenacity of the fibers at various cross-linking conditions has also been developed. This research shows that citric acid in aqueous solutions can cross-link gliadin fibers at low temperatures using alkali as catalyst. The method of cross-linking developed in this research could be useful to cross-link plant proteins for food, fiber, and other applications.

In this research, ultrafine fibrous scaffolds with deep cell infiltration and sufficient water stability have been developed from gelatin, aiming to mimic the extracellular matrices (ECMs) as three dimensional (3D) stromas for soft tissue repair. The ultrafine fibrous scaffolds produced from the current technologies of electrospinning and phase separation are either lack of 3D oriented fibrous structure or too compact to be penetrated by cells. Whilst electrospun scaffolds are able to emulate two dimensional (2D) ECMs, they cannot mimic the 3D ECM stroma. In this work, ultralow concentration phase separation (ULCPS) has been developed to fabricate gelatin scaffolds with 3D randomly oriented ultrafine fibers and loose structures. Besides, a non-toxic citric acid crosslinking system has been established for the ULCPS method. This system could endow the scaffolds with sufficient water stability, while maintain the fibrous structures of scaffolds. Comparing with electrospun scaffolds, the ULCPS scaffolds showed improved cytocompatibility and more importantly, cell infiltration. This research has proved the possibility of using gelatin ULCPS scaffolds as the substitutes of 3D ECMs. [ABSTRACT FROM AUTHOR]

This paper reports the development of electrospun zein fibers with improved water stability and tensile strength for potential drug delivery. The low morphological stability in aqueous environment and poor mechanical properties in dry and wet states have restricted the applications of electrospun protein materials, though these materials possess a unique structure, special adsorption properties, biocompatibility and biodegradability. In this study, the electrospun zein fibers were modified by non-toxic citric acid crosslinking catalyzed by NaOH. An up to 183% enhancement in dry tensile strength and an up to 448% improvement in wet tensile strength were generated. The cross-linked fibers were able to maintain their fibrous structure for 15 days in phosphate-buffered saline at 37°C. Moreover, those cross-linked electrospun zein fibers showed a potential in controlled drug delivery with a 58% drug-loading efficiency and a sustained profile drug release in artificial gastric juice. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports a new method of cross-linking electrospun zein fibers using citric acid as a non-toxic cross-linker to enhance the water stability and cytocompatibility of zein fibers for tissue engineering and other medical applications. The electrospun structure has many advantages over other types of structures and protein-based biomaterials possess unique properties preferred for tissue engineering and other medical applications. However, ultrafine fiber matrices developed from proteins have poor mechanical properties and morphological stability in the aqueous environments required for medical applications. Efforts have been made to improve the water stability of electrospun protein scaffolds using cross-linking and other approaches, but the current methods have major limitations, such as cytotoxicity and low efficiency. In this research electrospun zein fibers were cross-linked with citric acid without using any toxic catalysts. The stability of the cross-linked fibers in phosphate-buffered saline and their ability to support the attachment, spreading and proliferation of mouse fibroblast cells were studied. The cross-linked electrospun fibers retained their ultrafine fibrous structure even after immersion in PBS at 37°C for up to 15days. Citric acid cross-linked electrospun zein scaffolds showed better attachment, spreading and proliferation of fibroblast cells than uncross-linked electrospun zein fibers, cross-linked zein films and electrospun polylactide fibers. [ABSTRACT FROM AUTHOR]

Abstract: This paper shows that citric acid can cross-link starch and improve the tensile strength, thermal stability and decrease the dissolution of starch films in water and formic acid. The poor mechanical properties and water stability of starch have restricted its industrial applications. Although cross-linking is a common approach to improve the properties of starch products, current starch cross-linking methods are either expensive, toxic or do not impart the desired properties to the cross-linked materials. In this research, the possibility of cross-linking starch films using citric acid to improve their strength and stability was examined. Citric acid cross-linked starch films show about 150% higher strength than non-cross-linked films and have strength better than most cross-linked starch and synthetic polymer blended films previously developed. Films cross-linked with 5% citric acid had only 35% loss in weight after being in formic acid for 5h at 50°C whereas the non-cross-linked films dissolved immediately. [Copyright &y& Elsevier]

Abstract: Solution cast films made from peanut proteins extracted from peanut meal and crosslinked with citric acid were found to have good dry and wet tensile properties but poor biocompatibility. Plant proteins are abundantly available, are non-immunogenic and can be used to develop films, fibers, nanoparticles and other types of materials for various applications. Previous attempts on developing films from peanut proteins have used high amounts of glycerol resulting in poor film properties. In this research, peanut proteins were made into films using 3–15% glycerol and the effect of film forming conditions on the tensile properties and thermal behavior was studied. Films were crosslinked with cytocompatible citric acid as the crosslinking agent. Ability of the crosslinked films to be used as substrates for tissue engineering was studied using mouse fibroblast cells. It was found that films developed in this research had much higher tensile strength and modulus than films previously developed from peanut proteins. Crosslinking with citric acid improved the dry and wet strengths of the films without affecting the water vapor permeability. Peanut protein films did not support the attachment and growth of mouse fibroblast cells suggesting that peanut proteins were cytotoxic. [Copyright &y& Elsevier]

For the first time, mechanism and kinetics of citric acid crosslinking of protein in neutral aqueous condition under low temperature are reported. Crosslinking was a conventional method to improve properties of protein products to replace petroleum-derived polymers. However, current crosslinking methods were toxic, expensive or required harsh conditions. Non-toxic carboxylic acid could effectively enhance wet performance properties of multiple protein products under mild conditions, which were critical for preserving the activities during immobilization of enzymes. In this study, more than one carboxyl groups in one citric acid molecule were found capable of reacting with protein in aqueous medium at pH 6.8 under 50 and 75 °C, verifying occurrence of crosslinking. The pseudo second-order reaction had activation energy as low as 24.956 kJ mol −1 , indicating its easy occurrence. Mild crosslinking using citric acid might provide alternative to accelerate low-cost and efficient enzyme immobilization as well as industrial utilization of protein-based products. [ABSTRACT FROM AUTHOR]

Isocratic HPLC was used in conjunction with conductometric titration to quantitatively examine the modification of cotton cellulose by citric acid (CA)/sodium hypophosphite (SHP). CA/SHP had been extensively used as a green crosslinking agent for enhancement of cellulose and other carbohydrate polymers without in-depth understanding of the mechanisms. The current study investigated all identifiable secondary polycarboxylic acids from CA decomposition in the CA/SHP-cellulose system under various curing conditions. It was found that CA decomposition was more sensitive to temperature compared with the desirable esterification reaction. Two crosslinking mechanisms, namely ester crosslinking and SHP crosslinking were responsible for the observed improvement in crease resistance of CA/SHP treated cotton fabrics. An oligomer of citraconic acid (CCA) and/or itaconic acid (IA) was identified as a possible contributor to fabric yellowing. Finally, the crease resistance of fabrics correlated strongly with CA preservation in polyol-added CA/SHP crosslinking systems. The dosage of polyol should be held below an inflexion point to keep the undesirable competition against cellulose minimum. The combination of HPLC and conductometric titration was demonstrated to be useful in studying the CA/SHP-cellulose crosslinking system. The findings have implications for better application of CA/SHP in polysaccharide modifications in general. [ABSTRACT FROM AUTHOR]

A durable press finisher formulabased on citric acid (CA), featuringpolyol extenders, can be made to compare favorably to dimethylol dihydroxyethyleneurea(DMDHEU) or 1,2,3,4-butanetetracarboxylic acid (BTCA) in whitenessand wrinkle recovery performance. Durable press (DP) finishing ofcotton fabrics with CA has long been implicated by inferior resilienceenhancement and appreciable fabric yellowing, which hinder the industrializationof the technique despite the obvious merits of CA being cost-effective,nontoxic, and renewable. In this research, glycerol was used as themodel compound for investigateing the mechanism of polyol’swhitening effect and for obtaining the optimized curing conditions.Then a series of polyols were investigated as extenders for the cross-linkingof cotton with CA. Performance indicators such as the wrinkle recoveryangle (WRA), CIE whiteness index (WI), tear strength (TS) retention,and DP rating of the treated fabrics were correlated with curing conditionsand the structures of the extenders. It was found that the whiteningpower of the extenders is directly related to the number of primaryhydroxyl groups. It was also found that polyols containing both primaryand secondary hydroxyl groups are more effective at enhancing theresilience of treated fabrics. By using xylitol as the extender, DPratings of 3.8–3.9 were imparted to both white and dyed cottonfabrics finished with CA without significant yellowing or discoloration.The addition of the extender was also favorable to the durabilityof the DP effects so that a DP rating of 3.5 was retained after 20washing and tumble-drying cycles. It was shown that by using biobasedextender xylitol two major drawbacks of DP finishing of cotton withCA could be eliminated, presenting an essentially green alternativeto DMDHEU. [ABSTRACT FROM AUTHOR]