Narendra Reddy

Cytocompatible cross-linking of electrospun zein fibers for the development of water-stable tissue engineering scaffolds.

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 degrees C for up to 15 days. 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.

Potential of plant proteins for medical applications

Various natural and synthetic polymers are being explored to develop biomaterials for tissue engineering and drug delivery. Although proteins are preferable over carbohydrates and synthetic polymers, biomaterials developed from proteins lack the mechanical properties and/or biocompatibilities required for medical applications. Plant proteins are widely available, have low potential to be immunogenic and can be made into fibers, films, hydrogels and micro- and nano-particles for medical applications. Studies, mostly with zein, have demonstrated the potential of using plant proteins for tissue engineering and drug delivery. Although other plant proteins such as wheat gluten and soyproteins have also shown biocompatibility using in vitro studies, fabricating biomaterials such as nano-fibers and nano-particles from soy and wheat proteins offers considerable challenges.

Properties and potential applications of natural cellulose fibers from the bark of cotton stalks

Natural cellulose fibers have been obtained from the bark of cotton stalks and the fibers have been used to develop composites. Cotton stalks are rich in cellulose and account for up to 3 times the quantity of cotton fiber produced per acre. Currently, cotton stalks have limited use and are mostly burned on the ground. Natural cellulose fibers obtained from cotton stalks are composed of approximately 79% cellulose and 13.7% lignin. The fibers have breaking tenacity of 2.9 g per denier and breaking elongation of 3% and modulus of 144 g per denier, between that of cotton and linen. Polypropylene composites reinforced with cotton stalk fibers have flexural, tensile and impact resistance properties similar to jute fiber reinforced polypropylene composites. Utilizing cotton stalks as a source for natural cellulose fibers provides an opportunity to increase the income from cotton crops and make cotton crops more competitive to the biofuel crops.

Crosslinking biopolymers for biomedical applications

Biomaterials made from proteins, polysaccharides, and synthetic biopolymers are preferred but lack the mechanical properties and stability in aqueous environments necessary for medical applications. Crosslinking improves the properties of the biomaterials, but most crosslinkers either cause undesirable changes to the functionality of the biopolymers or result in cytotoxicity. Glutaraldehyde, the most widely used crosslinking agent, is difficult to handle and contradictory views have been presented on the cytotoxicity of glutaraldehyde-crosslinked materials. Recently, poly(carboxylic acids) that can crosslink in both dry and wet conditions have been shown to provide the desired improvements in tensile properties, increase in stability under aqueous conditions, and also promote cell attachment and proliferation. Green chemicals and newer crosslinking approaches are necessary to obtain biopolymeric materials with properties desired for medical applications.

Graft Polymerization of Native Chicken Feathers for Thermoplastic Applications

Inexpensive and biodegradable thermoplastics were developed through graft polymerization of native chicken feather with methyl acrylate as a potential substitute for petroleum products. Poultry feathers are available in large quantities at a low price. However, natural chicken feathers have poor thermoplasticity, cannot be used to develop thermoplastic products, have very limited industrial applications, and are often considered as solid wastes. In this research, the effects of graft polymerization conditions, such as molar ratio of NaHSO(3) to K(2)S(2)O(8), initiator and monomer concentrations, pH, temperature and time of polymerization, on grafting parameters, that is, the conversion of monomer to polymer, grafting percentage, and grafting efficiency were evaluated. Methyl acrylate was found to be successfully grafted onto functional groups on the surfaces of the chicken feathers, and optimal graft polymerization conditions were also obtained. The feather-g-poly(methyl acrylate) developed showed good thermoplasticity, and feather films had substantially higher tensile properties than soy protein isolate and starch acetate films.

Hollow nanoparticles from zein for potential medical applications

Hollow nanoparticles from corn storage protein zein, with average diameters as small as 65 nm and capable of loading a large amount of drug and penetrating into the cell cytoplasm, have been developed for potential drug delivery applications. As an important protein co-product of corn-based ethanol, zein is biocompatible and has been proved to be useful for medical applications through in vitro and in vivo evaluations. Zein can overcome the limitations of inorganic or metal nanoparticles that tend to accumulate in the organs and tissues and is therefore preferable for drug delivery applications. However, it has been observed that only small proteins and peptides are able to penetrate into cells and zein with a molecular weight of 14–44 kDa may not be able to enter into the cells. In this research, hollow zein nanoparticles have been developed and the potential of the hollow zein nanoparticles to load drugs and enter the cell cytoplasm was investigated. Hollow zein nanoparticles developed in this research were capable of loading as high as 369 mg g−1 of the drug metformin at an equilibrium concentration of 3 g L−1. Metformin in hollow zein nanoparticles showed a more sustained and controlled release profile than that in solid zein nanoparticles. Hollow zein nanoparticles were found to be able to enter the fibroblast cells 1 hour after incubation. The biocompatibility, nano-scale diameters, potential for loading a large amount of drugs and the ability to penetrate into cells make hollow zein nanoparticles ideal candidates for carrying various payloads for intracellular drug delivery.

Alkali-catalyzed low temperature wet crosslinking of plant proteins using carboxylic acids.

We report the development of a new method of alkali‐catalyzed low temperature wet crosslinking of plant proteins to improve their breaking tenacity without using high temperatures or phosphorus‐containing catalysts used in conventional poly(carboxylic acid) crosslinking of cellulose and proteins. Carboxylic acids are preferred over aldehyde‐containing crosslinkers for crosslinking proteins and cellulose because of their low toxicity and cost and ability to improve the desired properties of the materials. However, current knowledge in carboxylic acid crosslinking of proteins and cellulose requires the use of carboxylic acids with at least three carboxylic groups, toxic phosphorous‐containing catalysts and curing at high temperatures (150–185°C). The use of high temperatures and low pH in conventional carboxylic acid crosslinking has been reported to cause substantial strength loss and/or undesired changes in the properties of the crosslinked materials. In this research, gliadin, soyprotein, and zein fibers have been crosslinked with malic acid, citric acid, and butanetetracarboxylic acid to improve the tenacity of the fibers without using high temperatures and phosphorus‐containing catalysts. The new method of wet crosslinking using carboxylic acids containing two or more carboxylic groups will be useful to crosslink proteins for various industrial applications.

Natural cellulose fibers from soybean straw.

This paper reports the development of natural cellulose technical fibers from soybean straw with properties similar to the natural cellulose fibers in current use. About 220 million tons of soybean straw available in the world every year could complement the byproducts of other major food crops as inexpensive, abundant and annually renewable sources for natural cellulose fibers. Using the agricultural byproducts as sources for fibers could help to address the concerns on the future price and availability of both the natural and synthetic fibers in current use and also help to add value to the food crops. A simple alkaline extraction was used to obtain technical fibers from soybean straw and the composition, structure and properties of the fibers was studied. Technical fibers obtained from soybean straw have high cellulose content (85%) but low% crystallinity (47%). The technical fibers have breaking tenacity (2.7 g/den) and breaking elongation (3.9%) higher than those of fibers obtained from wheat straw and sorghum stalk and leaves but lower than that of cotton. Overall, the structure and properties of the technical fibers obtained from soybean straw indicates that the fibers could be suitable for use in textile, composite and other industrial applications.

Water-stable electrospun collagen fibers from a non-toxic solvent and crosslinking system

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.

Wet cross-linking gliadin fibers with citric acid and a quantitative relationship between cross-linking conditions and mechanical properties.

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.