Paper BI-TuP3
Stimuli-responsive Thin Films made from Highly Methoxylated Citrus Pectin

Tuesday, October 23, 2018, 6:30 pm, Room Hall B

We have used high-methoxyl citrus pectin polysaccharides to fabricate ultra-thin responsive coatings to be potentially implemented as elements of stimuli-responsive systems with diverse applications in drug delivery, tissue engineering, biomedicine, and etc.

Pectin is composed of a backbone chain structure of D-galacturonic acid units linked by α-1,4-glycosidic bonds. The carboxyl groups present in a polygalacturonic acid chain may exist as charged carboxyl acid groups or esterified with methyl groups. The ratio of methyl esters per total number of carboxyl groups is defined as the degree of esterification (DE). Low-methoxyl pectin (DE<50%) can be cross-linked in the presence of divalent ions such as Ca²⁺ ions. High-methoxyl pectin exhibits weak affinity for Ca²⁺ cations due to the lower charged carboxyl group content challenging their Ca²⁺-induced cross-linking.

Herein, thin coatings of high-methoxyl citrus pectin were fabricated as surface-attached hydrogel networks by spin-casting solutions of pectin onto a solid surface followed by Ca²⁺ induced crosslinking. The cross-linking was performed by introducing the cross-linker (Ca²⁺) in a poor solvent for pectin to eliminate water in the cross-linking process. Cross-linking the coatings in a non-solvent ensures that the coatings remain intact by preventing dilution of the pectin chains. However, the Ca²⁺ ions freely diffused into and cross-linked the pectin to form robust coatings. Using this strategy, pectins with up to 70% esterification were cross-linked. Generally, high-methoxyl pectins do not cross-link in the presence of calcium unless at high pectin concentrations.

The coatings prepared in this manner demonstrated a volume-phase transition induced by temperature. The responses of coatings were assessed by characterizing their swelling behaviors using ellipsometry and ATR-FTIR to provide insights into the nature of the transition. Our findings show that at temperatures below approximately 35 ˚C, the coatings were hydrophilic. At higher temperatures, the coatings expelled water and collapsed giving rise to distinctive de-swelling profiles. The hydrophilic/hydrophobic transition was driven by dehydration of methoxyl groups whereas water remained bound to the carboxylate groups. It was also observed that the response of the coatings can be tuned by adjusting temperature, degree of cross-linking, and pH of the surroundings to induce a desired response. Our finds show that thin films of the high-methoxyl pectin polysaccharides can be employed for establishing responsive surfaces with tunable responses suitable for the pharmaceutical and biotechnology industries.