R Dhamodharan

A novel method of preparing chitosan gels using in situ generated negatively-charged colloidal salts of a variety of metal ions is described. Their potential as scaffolds for tissue-engineering and as recoverable catalysts in aza-Michael addition is demonstrated here. Given their wide range of properties, they have broad scope for applications.

The preparation of porous films (average size variation from 1 to 32 μm) of a 1:1 blend of chitosan with poly(EG-ran-PG) by the controlled evaporation of water from a 2 wt % aqueous acetic acid solution is reported. Interestingly, the blend exhibited porosity that could be tailored from 1 to 32 μm with the temperature of preparation of the blend film. The powder X-ray diffraction, Fourier transform infrared, and differential scanning calorimetry analyses of the films suggested the formation of partially miscible blends. Temperature-induced phase separation of the blend appears to be the mechanism of pore formation. The tensile strength, cytotoxicity, and biocompatibility of the blend films for the growth of mesenchymal stem cells were assessed vis-a-vis chitosan. The 1:1 blend film was observed to lack cytotoxicity and was also viable for the growth of mesenchymal stem cells. The tensile properties of the 1:1 blend were superior to those of the chitosan film. The simple preparation of porous, nontoxic, and biocompatible films could find use as a scaffold in the growth of tissue, and especially bone tissue, in wound dressing, and in filtration if a better control over pore size is achieved.

A facile, rapid, and sustainable synthesis of amorphous, photoluminescent, carbon nanodots in high yields (∼51%), by cogrinding of maleic anhydride and imidazole, is reported for the first time. The heat liberated during the exothermic oligomerization of maleic anhydride, initiated by imidazole in the solid state, was identified to be the cause of carbonization of the reaction mixture. These carbon nanodots showed very low cytotoxicity toward mesenchymal stem cells, even at very high concentrations (1 mg/mL). Having the highest adsorption capacity for ammonia among unmodified carbons reported to date (∼19.3 mg/g), these carbon nanodots could be used for the removal of ammonia from the gas phase. In addition, these carbon nanodots could be used as an ionotropic cross-linker for chitosan, to prepare stable gels, suitable for a wide variety of applications. In contrast to the present solid-state methodologies which involve the use of hazardous chemicals or high temperatures, this solvent-free method of preparing carbon nanodots is sustainable, facile, safe, and inexpensive.

We, for the first time, report a green, solid-state method for synthesizing maleated chitosan, wherein maleation is brought about by stirring chitosan flakes in molten maleic anhydride. In a similar manner, other derivatives like succinated chitosan could also be prepared. Maleated chitosan, given its polyanionic nature in solution form, could be further used to cross-link chitosan polycation through electrostatic complexation. The resulting "all-chitosan" gel, having very low cytotoxicity and a conducive surface for cell attachment, could be used as scaffolds for tissue engineering. In this context it is to be noted that presently, maleation of chitosan has been invariably carried out in solution phase, using harsh, environmentally aggressive solvents such as DMSO, DMF, acetamide and the like.

Mechanical degradation of polymers reported so far, utilize cutting, impact or attrition for size reduction, which is very different from the low-magnitude forces experienced by the polymers during their service life. In this work, we have studied the effect of such low-magnitude forces, on the polymeric materials, using a low-energy rolling compression-type wet-grinder. The rolling compression action produces shear and compression on the polymer, leading to abrasion and resulting in the formation of crazes, micro-cracks and chip-offs, akin to the wearing. Measurements using Raman spectroscopy showed that the shear forces, generated upon grinding, produced strains on the polymer backbone, which upon sufficient build-up, results in chain scission at the points of physical entanglement. These homolytic chain scissions produced “mechano”radicals, which were confirmed by radical-scavenging using DPPH. The ensuing reduction in the molecular weight was further analyzed using GPC, light scattering and viscometry. Surprisingly, XRD measurements showed strain-induced crystallization as well. In order to theoretically validate the studies, a probabilistic model, explaining the “complex” response of the molecular weight distribution and the PDI upon mechanical degradation, has also been presented. Crosslink density function was incorporated to explain the preferential chain scission of the high molecular weight species, leading to a gradual reduction in the average molar mass.

The facile preparation of macroporous, super water absorbing, biocompatible hydrogels of chitosan involving the hydrothermal reaction of a mixture of chitosan (CH), succinic acid (SA) and urea (UR), all of which are sustainable materials, is reported. The structure of the dry CHSAUR was ascertained by CP MAS-SS NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, powder x-ray diffraction analysis (PXRD), and thermogravimetric analysis (TGA). The principle role of UR in the synthesis was identified as the source of ammonia, which increased the pH of the acidic chitosan solution with reaction time, leading to the formation of the insoluble hydrogel of chitosan accompanied by the formation of pores of different sizes and volumes. In addition, a small fraction of urea participated in chemical reaction with the primary hydroxyl groups in the sixth position of the glucosamine repeat units of chitosan resulting in carbamate linkages. The as-prepared hydrogel, following workup and methanol extraction, was found to be chitosan crosslinked with succinic acid through electrostatic interaction. It was macroporous with percentage porosity varying between 49.4% to 64.2%. It also exhibited different extents of water uptake with the maximum of 760 ± 20 g/g being for the one prepared with the weight ratio of 1: 4: 4 of chitosan: succinic acid: urea. The absorption of water is found to arise out of the porosity as well as presence of water attracting chitosan ammonium cation-succinate electrovalent bonds that are formed by the reaction between SA and ammonium cation of the chitosan backbone. The absorption of saline water was relatively poor suggesting that the saline water absorption might be arising largely due to the presence of micropores and specific interaction. The hydrogels exhibited Herschel-Bulkley rheological behavior. The extraction of CHSAUR with 0.1 N NaOH in methanol resulted in the removal of the physical crosslinks, consisting of succinate anions; the presence of chitosan with porous morphology was confirmed additionally by copper (+2) adsorption. In contrast to the widely reported method of preparing microporous chitosan scaffold of cylindrical shape that takes several days to a week, the present method offers a simple means of preparing macroporous chitosan of any shape and size in very large scale with soft foam-like morphology. With its biocompatibility towards mouse fibroblast cells it could find applications in drug delivery, biodegradable super water absorbency and haemostatic applications.

Abstract: A simple and green method for the preparation of nanofibrillated cellulose (NFC) by heating surgical cotton in glycerol is demonstrated as an alternative to the existing mechanical degradation method. The heat treatment of cotton in the presence of 9% w/w sulphuric acid in glycerol (1 M), under relatively milder conditions than those reported in the literature in the absence of glycerol, resulted in the formation of nanocrystalline cellulose (NCC) due to extensive hydrolysis of the amorphous segments. The method reported offers certain unique advantages in the preparation of NFC such as high yield (71%) and much easier post-processing compared to the mechanical degradation method of preparation of NFC. It also offers certain unique advantages in the preparation of NCC such as relatively high yield (56%), the use of lesser quantity of sulphuric acid as well as elimination of the quenching of the reaction through the addition of excess water to the reaction mixture. The residual ‘green solvent’, separated by decantation or centrifugal separation, post-reaction, could be reused for several cycles after filtration with activated carbon. A simple utility of the NCC prepared as reinforcing additive to cement is demonstrated. The addition of 1% (w/w of cement) of NCC and tetraethylorthosilicate modified NCC enhanced the workability of cement mortar and the compressive strength of cured cement composite in sharp contrast to the use of microcrystalline cellulose that required 10% (w/w) for the same enhancement in strength but with poorer workability. Graphical abstract: A sustainable route for preparing NFC through heat treatment in glycerol is reported. In the presence of 1 M (9% w/w) sulphuric acid in glycerol, similar heat treatment resulted in the formation of both NFC and NCC. The residual ‘green solvent’ could be reused for several cycles. The addition of 1% (w/w) of nanocellulose prepared via this method enhanced the workability of cement mortar and the compressive strength of cured cement composite.[Figure not available: see fulltext.].

Biocompatible hydrogels were prepared by mixing aqueous-acidic solution of chitosan with alkali lignin, a major by-product of the paper producing industries, for the first time, by sustainable means. Electrostatic interactions between the phenoxide groups in lignin and the ammonium groups on the chitosan backbone were found to be responsible for the ionotropic cross-linking. These gels were non-toxic to Mesenchymal stem cells, in vitro, and to zebrafish up to 100 μg/ml, in vivo. In addition, these gels provided a conducive surface for cell attachment and proliferation, making it suitable for application as scaffolds in tissue engineering. In presence of the hydrogel, NIH 3T3 mouse fibroblast cells showed good cell migration characteristics suggesting that the gel might be suitable for wound healing application. The chitosan-alkali lignin gelation system was further capable of removing ferric ions from contaminated water by way of complexation and coagulation. Cross-linked films of chitosan and alkali lignin could also be prepared by simply immersing chitosan films into a solution of alkali lignin. Alkali lignin was observed to diffuse into the chitosan “crystal”, forming electrostatic cross-links between the chitosan chains. The choice of lignin, in comparison to the other ionotropic cross-linkers for chitosan, makes the cross-linking system, inexpensive and sustainable.