surface of the glass particles, as well as the adjacent tooth substrate, thus precipitating their outer layers but also neutralising itself. as the ph of the aqueous solution rises, the polyacrylic acid begins to ionise, and becoming negatively charged it sets up a diffusion gradient and helps draw cations out of the glass and dentine. the alkalinity also induces the polymers to dissociate, increasing the viscosity of the aqueous solution. the second phase is gelation, where as the ph continues to rise and the concentration of the ions in solution to increase, a critical point is reached and insoluble polyacrylates begin to precipitate. these polyanions have carboxylate groups whereby cations bind them, especially ca2+ in this early phase, as it is the most readily available ion, crosslinking into calcium polyacrylate chains that begin to form a gel matrix, resulting in the initial hard set, within five minutes. crosslinking, h bonds and physical entanglement of the chains are responsible for gelation. during this phase, the gic is still vulnerable and must be protected from moisture. if contamination occurs, the chains will degrade and the gic lose its strength and optical properties. conversely, dehydration early on will crack the cement and make the surface porous. over the next twenty four hours maturation occurs. the less stable calcium polyacrylate chains are progressively replaced by aluminium polyacrylate, allowing the calcium to join the fluoride and phosphate and diffuse into the tooth substrate, forming polysalts, which progressively hydrate to yield a physically stronger matrix. the incorporation of fluoride delays the reaction, increasing the working time. other factors are the temperature of the cement, and the powder to liquid ratio – more powder or heat speeding up the reaction. gics have good adhesive relations with tooth substrates, uniquely chemically bonding to dentine and, to a lesser extend, to enamel. during initial dissolution, both the glass particles and the hydroxyapatite structure are affected, and thus as the acid is buffered the matrix reforms, chemically welded together at the interface into a calcium phosphate polyalkenoate bond. in addition, the polymer chains are incorporated into both, weaving cross links, and in dentine the collagen fibres also contribute, both linking physically and h-bonding to the gic salt precipitates. there is also microretention from porosities occurring in the hydroxyapatite. works employing non-destructive neutron scattering and terahertz (thz) spectroscopy have evidenced that gic's developing fracture toughness during setting is related to interfacial thz dynamics, changing atomic cohesion and fluctuating interfacial configurations. setting of gics is non-monotonic, characterised by abrupt features, including a glass–polymer coupling point, an early setting point, where decreasing toughness unexpectedly recovers, followed by stress-induced weakening of interfaces. subsequently, toughness declines asymptotically to long-term fracture test values. glass ionomer cement as a permanent material fluoride release and remineralisation the pattern of fluoride release from glass ionomer cement is characterised by an initial rapid release of appreciable amounts of fluoride, followed by a taper in the release rate over time. an initial fluoride “burst” effect is desirable to reduce the viability of remaining bacteria in the inner carious dentin, hence, inducing enamel or dentin remineralization. the constant fluoride release during the following days are attributed to the fluoride ability to diffuse through cement pores and fractures. thus, continuous small amounts of fluoride surrounding the teeth reduces demineralization of the tooth tissues. a study by chau et al. shows a negative correlation between acidogenicity of the biofilm and the fluoride release by gic, suggestive that enough fluoride release may decrease the virulence of cariogenic biofilms. in addition, ngo et al. (2006) studied the interaction between demineralised dentine and fuji ix gp which includes a strontium – containing glass as opposed to the more conventional calcium-based glass in other gics. a substantial amount of both strontium and fluoride ions was found to cross the interface into the partially demineralised dentine affected by caries. this promoted mineral depositions in these areas where calcium ion levels were low. hence, this study supports the idea of glass ionomers contributing directly to remineralisation of carious dentine, provided that good seal is achieved with intimate contact between the gic and partly demineralised dentine. this, then raises a question, “is glass ionomer cement a suitable material for permanent restorations?” due to the desirable effects of fluoride release by glass ionomer cement. glass ionomer cement in primary teeth numerous studies and reviews have been published with respect to gic used in primary teeth restorations. findings of a systematic review and meta-analysis suggested that conventional glass ionomers were not recommended for class ii restorations in primary molars. this material showed poor anatomical form and marginal integrity, and composite restorations were shown to be more successful than gic when good moisture control could be achieved. resin modified glass ionomer cements (rmgic) were developed to overcome the limitations of the conventional glass ionomer as a restorative material. a systematic review supports the use of rmgic in small to moderate sized class ii cavities, as they are able to withstand the occlusal forces on primary molars for at least one year. with their desirable fluoride releasing effect, rmgic may be considered for class i and class ii restorations of primary molars in high caries risk population. glass ionomer cement in permanent teeth with regard to permanent teeth, there is insufficient evidence to support the use of rmgic as long term restorations in permanent teeth. despite the low number of randomised control trials, a meta- analysis review by bezerra et al. reported significantly fewer carious lesions on the margins of glass ionomer restorations in permanent teeth after six years as compared to amalgam restorations. in addition, adhesive ability and longevity of gic from a clinical standpoint can be best studied with restoration of non- carious cervical lesions. a systematic review shows gic has higher retention rates than resin composite in follow up periods of up to 5 years. unfortunately, reviews for class ii restorations in permanent teeth with glass ionomer cement are scarce with high bias or short study periods. however, a study of the compressive strength and the fluoride release was done on 15 commercial fluoride- releasing restorative materials. a negative linear correlation was found between the compressive strength and fluoride release (r2=0.7741), i.e., restorative materials with high fluoride release have lower mechanical properties.