POLYMER NETWORKS IN DENTISTRY

Polymer networks applied in clinical dentistry can be divided into two groups: (i) hard, solid two-and threedimensional crosslinked structur es formed during photo-curing of dental polymeric filling compositions , and (ii) soft, hydrogel-type of networks based on polymeric ionic co mplexes, used for the tightening of microchannels in teeth. The first group is based on crosslinked di- and trifunctional monomers, and on s olid poly(acrylic acid) - inorganic glasses (''glass - ionomer cements '') This group has found wide clinical applications, in spite of many disadvantages such as susceptibility towards hydrolytic, mechanical. b io- and enzymatic degradations, and contents of toxic, allergenic and mutagenic components. The second group, the soft-hydrogel type of netw orks, has been investigated and developed at our institute in order to tighten channels in teeth. The microchannels, with a diameter of 30-2 00 Angstrom in enamel and 1-3 mu m in dentine, are filled with a loose , native biohydrogel of protein origin. Hydrogels have the ability to swell in water of biological fluids present in the oral cavity, and ca n retain a significant fraction of fluid within the structure. Decreas ing pH below 5.5 causes a slow dissolution of the hydroxyapatite cryst al in the walls of the microchannels with a consequent widening of the ir lumens. Metabolites and toxins from microorganisms, which are alway s present in the oral cavity, can penetrate into these enlarged channe ls and cause inflammatory reactions in the underlying pulp tissue. In order to decrease fluid flow and inhibit penetration of microorganisms , but still allow diffusion of ions and water, we have developed and t ested polymeric hydrogels based on poly(acrylic acid) and metal salts, and chitosan, which can be formed directly in the microchannels of de ntal hard tissues.