New study shows how carbonated water can be used to tune properties of hydrogels for various uses

Low-methoxy (LM) pectin gels have gained momentum in tissue engineering and drug delivery in recent years due to their excellent biocompatibility. LM pectin forms stable hydrogels by forming supramolecular assemblies with egg-box structures in the presence of divalent ions like Ca²⁺, generally provided by a calcium chloride (CaCl₂) solution or calcium carbonate (CaCO₃). In particular, the use of CaCO₃ enables the preparation of LM pectin gels with controlled shape. Previous research has shown that minute changes in hydrogel synthesis precursor parameters can majorly impact the hydrogel properties and their applications. For instance, decreasing the pH or acidifying the precursor accelerates the gelation rate and increases the mechanical strength of LM pectin hydrogels. CO₂ is a popular acidic agent because it can decrease pH and be easily removed from the system after gelation. However, despite CO₂ being a promising acidic agent, a precise understanding of its effects on hydrogel synthesis and properties is poorly understood.

A group of scientists from Japan, led by Mr. Ryota Teshima, a Master's student from Tokyo University of Science (TUS), decided to fill this gap. To do so, they prepared LM pectin/CaCO₃ hydrogels using the standard method and under controlled thermodynamic conditions with carbonated water as the source of CO₂. The research team also included Dr. Shigehito Osawa (currently at Tokyo Women’s Medical University), and Dr. Yayoi Kawano, Prof. Takehisa Hanawa, Prof. Akihiko Kikuchi, and Prof. Hidenori Otsuka from TUS. Their research was made available online on February 16, 2023, and published in volume 8, issue 8 of ACS Omega on February 28, 2023.

Explaining the reasoning behind this approach, Mr. Teshima says, "We chose carbonated water as the acidic agent for gelation because conventional methods would require high-pressure conditions to ensure proper supply of CO₂ to the hydrogel precursor. Instead, using carbonated water allowed us to supply carbonate ions and CO₂ to the hydrogel mixture and trigger gelation under ambient pressure and temperature conditions."

The team simultaneously prepared two LM pectin/CaCO₃ hydrogels—one with added carbonated water and the other without. They observed that the one with carbonated water underwent faster gelation and showed increased mechanical strength compared to the one without. These improvements could arise from the lower pH of carbonated water enhancing the solubility of CaCO_3, which further increases the availability of free Ca²⁺ ions. More Ca²⁺ means better cross-linking between pectin polymers, quicker gelation, and more robust hydrogels.

After the hydrogel formation, carbonated water/CO₂ was volatilized from the surface in contact with the atmosphere. The team found that post-CO₂ escape, the pH of the final hydrogel was higher than the hydrogel prepared without carbonated water. The team suggested this could be because many carboxy groups were consumed during cross-linking. with Ca²⁺ and the acidic component, CO₂, was completely volatilized from the hydrogels after gelation. The team further prepared aerogels by freeze-drying the hydrogels. They found highly ordered networks of elongated porosity in those prepared through the carbonated water route, suggesting that the CO₂ from carbonated water could induce intrinsic structural changes in the hydrogels. The researchers also prepared hydrogels with different amounts of CO₂ in the carbonated water to demonstrate that the pH and strength of LM pectin hydrogels can be tuned in this manner. The insights from this study can prove helpful when fabricating functional hydrogels with tailored properties.

This research contributes to the United Nations' sustainable development goals (SDGs), as the LM pectin was primarily derived from food waste, such as apple and citrus fruits. "Every year, 2.5 billion tons of food is wasted worldwide, which includes vegetables and fruits that are discarded without ever reaching the market because of quality issues. We believe that the promising results of our study will encourage other researchers to use items thrown out as food waste as raw materials for their scientific ventures," concludes Mr. Teshima.

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Reference                    

DOI: https://doi.org/10.1021/acsomega.2c07552

About The Tokyo University of Science
Tokyo University of Science (TUS) is a well-known and respected university, and the largest science-specialized private research university in Japan, with four campuses in central Tokyo and its suburbs and in Hokkaido. Established in 1881, the university has continually contributed to Japan's development in science through inculcating a love for science in researchers, technicians, and educators.

With a mission of "Creating science and technology for the harmonious development of nature, human beings, and society," TUS has undertaken a wide range of research from basic to applied science. TUS has embraced a multidisciplinary approach to research and has undertaken intensive study in some of today's most vital fields. TUS is a meritocracy where the best in science is recognized and nurtured. It is the only private university in Japan that has produced a Nobel Prize winner and the only private university in Asia to produce Nobel Prize winners within the natural sciences field.

Website: https://www.tus.ac.jp/en/mediarelations/

About Ryota Teshima from the Tokyo University of Science
Mr. Ryota Teshima is a Master's student at the Tokyo University of Science. Even as a young researcher, he has already published three research papers in science education and the development of hydrogels for biomaterials. Mr. Teshima's academic journey has been outstanding, and he has received several awards. He has also been selected as a fourth generation member of the Masason Foundation created by SoftBank Group Representative Masayoshi Son.

Funding information
This study was supported by support grants for research activities from the Masason Foundation.