Chemical interactions in composites of gellan gum and bioactive glass: self-crosslinking and in vitro dissolution

We investigated the interactions between the organic-inorganic phases in composites and the impact on in vitro dissolution. The composite consists of a hydrogel-forming polysaccharide gellan gum (GG, organic phase) and a borosilicate bioactive glass (BAG, inorganic phase). The BAG loading in the gellan gum matrix varied from 10 to 50 wt%. While mixing GG and BAG, the ions released from BAG microparticles crosslinked with the carboxylate anions of GG. The nature of the crosslinking was assessed, and its impact on mechanical properties, swelling ratio, and enzymatic degradation profile upon immersion for up to 2 weeks was studied. Loading up to 30 wt% of BAG in GG caused an increase in mechanical properties associated with an increasing crosslinking density. At higher BAG loading, excess divalent ions and percolation of particles led to a decrease in the fracture strength and compressive modulus. Upon immersion, a decrease in the composite mechanical properties was attributed to the dissolution of the BAG and the loosening of the glass/matrix interface. The enzymatic degradation of the composites was inhibited at higher BAG loadings (40 and 50 wt%) even when the specimen was immersed for 48 h in PBS buffer with lysozyme. During in vitro dissolution in both SBF and PBS, the ions released from the glass led to the precipitation of hydroxyapatite already at day 7. In conclusion, we thoroughly discussed the in vitro stability of the GG/BAG composite and established the maximum BAG loading to enhance the GG crosslinking and mechanical properties. Based on this study, 30, 40, and 50 wt% of BAG in GG will be further investigated in an in vitro cell culture study.

Caries Management-The Role of Surface Interactions in De- and Remineralization-Processes

BACKGROUND

Bioadhesion and surface interactions on enamel are of essential relevance for initiation, progression and prevention of caries and erosions. Salivary proteins on and within initial carious and erosive lesions can facilitate or aggravate de- and remineralization. This applies for the pellicle layer, the subsurface pellicle and for proteins within initial carious lesions. Little is known about these proteinaceous structures related to initial caries and erosion. Accordingly, there is a considerable demand for an understanding of the underlying processes occurring at the interface between the tooth surface and the oral cavity in order to develop novel agents that limit and modulate caries and erosion. Objectives and findings: The present paper depicts the current knowledge of the processes occurring at the interface of the tooth surface and the oral fluids. Proteinaceous layers on dental hard tissues can prevent or aggravate demineralization processes, whereas proteins within initial erosive or carious lesions might hinder remineralization considerably and restrict the entry of ions into lesions.

CONCLUSIONS

Despite the fact that organic-inorganic surface interactions are of essential relevance for de- and remineralization processes at the tooth surface, there is limited knowledge on these clinically relevant phenomena. Accordingly, intensive research is necessary to develop new approaches in preventive dentistry.

Gas Cell Infrared and Attenuated Total Reflection Infrared Spectroscopic Studies for Organic-Inorganic Interactions in Adsorption of Fulvic Acid on the Goethite Surface Generating Carbon Dioxide

The generation of carbon dioxide (CO₂) from Nordic fulvic acid (FA) solution in the presence of goethite (α-FeOOH) was observed in FA-goethite interaction experiments at 25-80 ℃. CO₂ generation processes observed by gas cell infrared (IR) spectroscopy indicated two steps: the zeroth order slower CO₂ generation from FA solution commonly occurring in the heating experiments of the FA in the presence and absence of goethite (activation energy: 16-19 kJ mol^-1), and the first order faster CO₂ generation from FA solution with goethite (activation energy: 14 kJ mol^-1). This CO₂ generation from FA is possibly related to redox reactions between FA and goethite. In situ attenuated total reflection infrared (ATR-IR) spectroscopic measurements indicated rapid increases with time in IR bands due to COOH and COO^- of FA on the goethite surface. These are considered to be due to adsorption of FA on the goethite surface possibly driven by electrostatic attraction between the positively charged goethite surface and negatively charged deprotonated carboxylates (COO^-) in FA. Changes in concentration of the FA adsorbed on the goethite surface were well reproduced by the second order reaction model giving an activation energy around 13 kJ mol^-1. This process was faster than the CO₂ generation and was not its rate-determining step. The CO₂ generation from FA solution with goethite is faster than the experimental thermal decoloration of stable structures of Nordic FA in our previous report possibly due to partial degradations of redox-sensitive labile structures in FA.