Pectin was extracted from citrus pomelo waste, and the effects of calcium ions (Ca²⁺) on the gelation and hydrogels properties were investigated over a pH range of 3.2–8 by using viscoelastic analysis. The gelatinization of Ca²⁺-pectin was examined at concentrations of 0.9, 1.8, 2.4, and 3.6 M of Ca²⁺ in aqueous pectin solutions of 1%, 2%, 3%, and 4%. The gel transition of Ca²⁺-pectin solution to hydrogels was determined by measuring the storage modulus (G’) and loss modulus (G”) under mechanical strain from 0.01 to 100%. In a hydrogel of 3% pectin at Ca²⁺ = 2.4 M, as pH increased to 7, the G’ at 0.01 strain % was 3 × 10⁴ Pa, and 3 × 10³ Pa at pH 5, indicating that the crosslinking weakened at acidic pH. Due to the crosslinking between the calcium ions and the ionized carboxylic acid groups of pectin, the resulting hydrogel became stiff. When the mechanical strain % was in the range of 0.01–1%, G’ was unchanged and G” was an order of magnitude smaller than G’, indicating that the mechanical stress was relieved by the gel. In the range of 1–100%, the gel deformation progressed and both the moduli values were dropped. Collapse from the gel state to the solution state occurred at 1–10 strain %, but the softer hydrogels with G’ of 10³ Pa had a larger strain % than the stiffer hydrogels with G’ of 10⁴ Pa.

Hot spring (HS) waters manifest diverse positive effects on the skin due to their unique chemical compositions. Sodium hyaluronate acid (HA) comprises N-acetylglucosamine and D-glucuronic acid, and distinguishes itself with superior qualities in skin regeneration, providing moisturizing and anti-aging benefits. The combination of HA with HS water is widely applied across ophthalmology, pneumology, nutrition, and cosmetics. This study delved into the application of HA in cosmetology, with a focus on its interaction with HS water and its effects on moisture retention and promoting wound healing. In particular, with the alkaline pH levels of the Yumoto HS, HA molecules may undergo dissociation to be ionized resulting in a negatively charged polymer and interacting with positively charged ions in the HS water through electrostatic interactions. The shifted peaks in the FTIR result and zeta potential shifts to a less negative region in the case of HA-HS compared to HA-DI indicate an ionic interaction between HS water and HA. Moisture tests confirmed the sustained hydration when HA is dissolved in HS water, underscoring its potential to improve skin hydration at certain concentrations, specifically at 0.5% and 1%. Additionally, MTT assay results demonstrated that HS water stimulates the growth of fibroblast cells compared to distilled water, implying its potential beneficial effect in wound healing. These findings suggested the multifaceted benefits of HAHS in skincare, highlighting its role in enhancing skin hydration and potentially accelerating wound healing processes, thus presenting avenues for the development of advanced cosmeceutical formulations.