Hydrogels Based on Starch from Various Natural Sources: Synthesis and Characterization

Abstract

Hydrogels based on a graft copolymer of acrylic acid (AA) and acrylamide (AAm) on a backbone of starch, obtained from different natural sources, have been prepared and characterized for potential agricultural use, as soil moisture regulators and nutrient sources for plant nutrition. Five types of starch are used: corn, waxy corn, wheat, potato, and rice starch. Hydrogels are prepared by a solution polymerization method, in the presence of ammonium persulphate (PSA) as initiator and N,N'-methylene-bis-acrylamide (MBA) as a cross-linking agent. Thermal and physico-chemical properties, such as thermogravimetry analysis and differential scanning calorimetry data and swelling properties of hydrogels, are observed. The influences of the cross-linking agent on the swelling ratio (SR) and solubility of prepared hydrogels are studied and optimized. The highest SR observed in water corresponded to non-cross-linked hydrogels prepared with waxy corn starch (780 g water per g dry matter), and the SR of these hydrogels in ammonium sulfate and calcium nitrate solutions are 9.6 and 1.2 g per g dry matter, respectively. Higher thermal stability is observed with cross-linked starch-g-(AA-AAm) hydrogels. The non-cross-linked hydrogels have a higher SR, but also have significant solubility in water.

Triticale Starch-Based pH-Responsive Hydrogel: Synthesis, Characterization, Diffusion & New Perspective of Triticale Crop as Sustainable Source for Stimuli-Response Hydrogels

“…The statistical parameter of 0.955≤R 2 confirmed the correct adjustment of the diffusion behavior by the Ritger & Peppas method, 14 where the linear tendency of the cumulative release in the square root of time is shown in figure 5A. This is also supported by the kinetics reported by Elvira et al 41 and Kalendova et al 11 where the behavior of natural hydrogels based on starch does not follow a linear tendency of release and swelling in a range of pH~3-9.…”

“…4,5 There is much economic interest in hydrogels due to their wide application (15.33 billion USD forecast in 2022 with a 5% annual rate of towards 2026), however, those with stimulus-response to the pH factor are among the most studied in the pharmaceutical area due to their application as oral excipients. 6,7 Considering that 90% of drugs are absorbed at pH~6-7.4, a large number of semi-synthetic hydrogels from various natural sources with stimulus-response to intestinal pH have been reported, [8][9][10][11] however, its synthesis have involved complex techniques that are not very replicable on a large scale, with corrosive catalyst reagents, slightly toxic and poorly degradable synthetic copolymers, which causes a progressive environmental impact. Starch is a drug excipient recognized by the International Pharmaceutical Excipients Council (IPEC), 12 with a wide reactivity reported for its modification by green chemistry, such as its crosslinking with citrate (pKa~3.1, 4.7 and 6.4) by the application of high heat treatment (≥90°C), and which allows the formation of starch-citrate of anionic nature through diester bonds.…”

“…A wide variety of studies on the synthesis of semi-synthetic polymers with stimuli-response to the pH factor have been reported, where their results point to an optimal control release of pH~1.2-2 (10-60%) and pH~7.4 (60-92.6%) within 8-24 h by in vitro gastrointestinal media, however, most of the syntheses of these semi-synthetic materials reported the use of crosslinkers and copolymers with a certain degree of toxicity (methacrylate acid, acrylic, N,N-methylene-bisacrylamide, 2-hydroxyethyl methacrylate, tetraethyl orthosilicate, glutaraldehyde, 2-acrylamido-2-methylpropane sulfonic acid, acryloyl chloride and epichlorohydrin), as well as unconventional and corrosives initiators such as ammonium persulfate, and with low reproducibility on a large scale. [9][10][11]41,56 In comparison, the release of triticale hydrogels is not optimal due to the short release time, but it is more sensitive to intestinal pH 3.5 h (80-96%). It is worth mentioning that this is the first report of triticale starch-based pH-responsive natural hydrogel, however, the synthesis of two potential natural materials (starch/ethylcellulose and pectin/carboxymethylcellulose) were found with slightly complex techniques and non-toxic reagents with resistance to enzymatic hydrolysis and an optimal control release (pH~1.2/≤5% and pH~7.4/25-70%, for 8-12 h).…”

Triticale Starch-Based pH-Responsive Hydrogel: Synthesis, Characterization, Diffusion & New Perspective of Triticale Crop as Sustainable Source for Stimuli-Response Hydrogels

“…The statistical parameter of 0.95≤R 2 confirmed the correct adjustment of the diffusion behavior by the Ritger & Peppas method, 14 where the linear tendency of the cumulative release in the square root of time is shown in figure 5A. This is also supported by the kinetics reported by Elvira et al 41 and Kalendova et al 11 wherein the behavior of natural starch-based hydrogels does not follow a linear tendency of release and swelling over a range of pH~3-9.…”

“…4,5 There is much economic interest in hydrogels due to their wide application (15.33 billion USD forecast in 2022, with a 5% of annual rate towards 2026), and those with stimulus-response to the pH factor are among the most studied in the pharmaceutical area due to their application as oral excipients. 6,7 Considering that 90% of drugs are absorbed at pH~6-7.4, a large number of semi-synthetic hydrogels from various natural sources with stimulus-response to intestinal pH have been reported, [8][9][10][11] however, its synthesis have involved complex techniques that are not very replicable on a large scale, with corrosive catalyst reagents, slightly toxic and poorly degradable synthetic copolymers, which causes a progressive environmental impact. Starch is a drug excipient recognized by the International Pharmaceutical Excipients Council (IPEC), 12 with a wide reactivity reported for its modification by green chemistry, such as its cross-linking with citrate (pKa~3.1, 4.7 and 6.4) by the application of high heat treatments (≥90°C), allowing the formation of starch-citrate due to the diester bonds (anionic nature).…”