No abstract

Dynamic rheological measurements performed during the solution cross-linking of DNA (about 2000 base pairs long) at a concentration of 9.3% w/v show formation of strong to weak DNA hydrogels depending on the concentration of the cross-linker ethylene glycol diglycidyl ether (EGDE). At a cross-linker content of 10 wt % or above (with respect to DNA), the elastic modulus G′ of DNA hydrogels is more than 2 orders of magnitude larger than the viscous modulus G″, and both moduli are essentially independent of frequency over the range 10 -2 -10 1 Hz. The value obtained for G′ (10 3 Pa) is of the same order of magnitude as the elastic modulus for chemical gels or cross-linked biopolymer gels. At lower cross-linker contents (below 10 wt %), weak DNA hydrogels exhibiting frequency-dependent moduli were obtained. Thermal behavior of DNA gels and DNA solutions was investigated by heating the samples above the DNA melting temperature (87.5 °C) and subsequently cooling down to 25 °C. At high cross-linker contents, no significant changes in the dynamic moduli were observed. At low cross-linker contents, however, a significant increase in the dynamic moduli was observed during both heating and cooling. The results were explained with the partial dissociation of the double helix into flexible single strand fragments during heating so that the number of entanglements increases. On cooling back, the dissociated strands cannot reorganize to form the initial double-stranded conformation so that the hydrogen bonds formed act as physical junction zones in addition to the chemical cross-links formed by EGDE. The heating-cooling cycles of DNA solutions produce physical gels exhibiting an elastic modulus in the order of megapascals. Thermoreversible DNA hydrogels were also obtained due to the transition between semidilute and dilute regimes of the same DNA solution depending on the conformation of the DNA chains.

Our study shows that magnesium ions which have so far been considered as non-gelling ions for alginate do induce alginate gelation. Rheology is used to examine effects of alginate chemical composition as well as alginate and magnesium ion concentration. Gelation in this system occurs at ca. 5-10 times higher concentration of ions than reported for calcium-based gels. Alginate network formation with magnesium ions is very slow and is typically accomplished within 2-3 hours. Gelation with magnesium ions is also strongly dependent on alginate chemical composition as the presence of long guluronic units privileges faster gel formation.

Horseradish peroxidase (HRP) can be used for the enzymatic cross‐linking of thiol‐functionalized polymers under mild conditions to form hydrogels and nanogels without the need for added H2O2. Cells can be embedded in the hydrogels and proteins can be entrapped and released from the nanogels. These gels are fully degradable under mild and cytocompatible reductive conditions.

Food packaging is a multidisciplinary area that encompasses food science and engineering, microbiology, as well as chemistry, and ignited tremendous interest in maintaining the freshness and quality of foods and their raw materials from oxidation and microbial spoilage. With the advances in the packaging industry, they could be engineered as easy-to-open, resealable, active, as well as intelligent with the incorporation of sensory elements while offering desired barrier properties against oxygen and water vapor. In this regard, the use of the electrospinning approach allows producing nanofibrous packaging materials with large surface-to-volume ratios and enables the higher loading of active agents into packaging materials. Electrospun packaging materials have been produced from various polymers (i.e., synthetic and natural) and their (nano)composites, and were mainly exploited for the encapsulation of active agents for their use as active food packaging materials. The electrospinning process was also used for the deposition of electrospun fibers on films to enhance their performance (e.g., as reinforcement material, or to enhance barrier properties). They could be even engineered to provide nutraceuticals to food, or antioxidant, antimicrobial or antifungal protection to the packaged food. In this article, first, introductory descriptions of food packaging, barrier properties, and electrospinning are given. Afterward, active and intelligent food packaging materials are briefly discussed, and the use of electrospinning for the fabrication of active food packaging materials is elaborated. Particular interest has been given to the polymer-type used in the production of electrospun fibers and active properties of the resultant packaging materials (e.g., antioxidant, antibacterial, antifungal). Finally, this review paper concludes with a summary and future outlook towards the development of electrospun food packaging materials.

DNA hydrogels with a wide range of tunable properties are desirable for applications to make use the characteristics of DNA. This study describes formation conditions of DNA hydrogels using ethylene glycol diglycidyl ether (EGDE) cross-linker and N,N,N',N'-tetramethylethylenediamine (TEMED) catalyst under various reaction conditions. Rheological measurements indicate that the cross-linking of DNA in semidilute solutions proceeds by alternate gel-sol and sol-gel transitions due to two antagonistic effects of EGDE-TEMED pair; the one destroying the physical bonds (denaturation), the other creating chemical bonds (cross-linking). The viscoelastic properties of the hydrogels and the conformation of DNA network chains could be tuned by adjusting the synthesis parameters. Increasing concentration of DNA at the gel preparation stabilizes its structure so that double stranded (ds-) DNA hydrogels form. The average distance between the effective cross-links in single stranded DNA gel is much larger than that in ds-DNA gel making the former gel stable in aqueous solutions. Creep-recovery tests show that heating a semidilute solution of DNA above the DNA melting temperature (87.5 degrees C) and subsequently cooling down to 25 degrees C increases the elastic response of the solution and produces an elastic DNA mesh. DNA hydrogel undergoes a volume phase transition in aqueous solutions of polyethylene glycol's at which the gel changes about 5 times its volume.

Electrospun nanofibers have sparked tremendous attention in drug delivery since they can offer high specific surface area, tailored release of drugs, controlled surface chemistry for preferred protein adsorption, and tunable porosity. Several functional motifs were incorporated into electrospun nanofibers to greatly expand their drug loading capacity or to provide the sustained release of the embedded drug molecules. In this regard, cyclodextrins (CyD) are considered as ideal drug carrier molecules as they are natural, edible, and biocompatible compounds with a truncated cone-shape with a relatively hydrophobic cavity interior for complexation with hydrophobic drugs and a hydrophilic exterior to increase the water-solubility of drugs. Further, the formation of CyD-drug inclusion complexes can protect drug molecules from physiological degradation, or elimination and thus increases the stability and bioavailability of drugs, of which the release takes place with time, accompanied by fiber degradation. In this review, we summarize studies related to CyD-functional electrospun nanofibers for drug delivery applications. The review begins with an introductory description of electrospinning; the structure, properties, and toxicology of CyD; and CyD-drug complexation. Thereafter, the release of various drug molecules from CyD-functional electrospun nanofibers is provided in subsequent sections. The review concludes with a summary and outlook on material strategies.

Three-dimensional (3D) bioprinting enables the controlled fabrication of complex constructs for tissue engineering applications and has been actively explored in recent years. However, its progress has been limited by the existing difficulties in the development of bioinks with suitable biocompatibility and mechanical properties and at the same time adaptability to the process. Herein, we describe the engineering of a nanocomposite agarose bioink with tailored properties using 2D nanosilicate additives. The suitability of agarose for 3D bioprinting has been debated due to its bioinert nature and compatibility with extrusion-based bioprinting. Nanosilicates were used to tailor the flow behavior of agarose solutions, and detailed rheological characterization of different bioink formulations was performed to elucidate the effect of nanosilicates on the flow behavior and gelation of agarose bioinks. The proper selection of nanosilicate concentration resulted in extrusion 3D printed structures with high shape fidelity and structural integrity. Moreover, the influence of addition of nanosilicates on the bioactivity of agarose was studied, and nanocomposite bioinks showed significant improvement in metabolic activity of encapsulated cells. The bioactivity of the nanocomposite bioinks was also evaluated through a cell spreading assay; the charged surfaces of nanosilicates resulted in full spreading and elongation of fibroblasts, and the extent of change in morphology of cells was found to be directly correlated with the nanosilicate concentration. Our findings suggested that engineered agarose-nanosilicate bioinks can be exploited as a new generation of hydrogel bioinks for extrusion 3D bioprinting with tunable flow properties and bioactivity.