Ulrich Salz

New developments of polymeric dental composites

The currently used commercial restorative composites contain a mixture of various cross-linking dimethacrylates, glass- and/or silicon dioxide fillers, and a photoinitiator system. They are cured by irradiation with visible light. New developments of polymeric composites for restorative filling materials are mainly focused on the reduction of polymerization shrinkage, and improvement of biocompatibility, wear resistance and processing properties. This can be partially achieved by using new tailor-made monomers and optimized filler particles. This review describes the polymeric chemical aspects of the application of new monomers, e.g. cyclic monomers, liquid-crystalline monomers, ormocers, branched monomers, compomers or Bis-GMA analogues or substitutes for restorative composites. In addition, the contribution of new filler-technologies for the improvement of restorative composites is discussed.

Radiopaque dental adhesives: Dispersion of flame-made Ta2O5/SiO2 nanoparticles in methacrylic matrices

OBJECTIVES Radiopaque dental adhesive (DA) of low viscosity were made by forming stable suspensions of weakly agglomerated Ta2O5/SiO2 nanoparticles with primary particle size of about 10nm. METHODS The particles were prepared by one-step flame-spray pyrolysis. Particles were functionalized with gamma-methacryloxypropyltrimethoxysilane (MPS) and dispersed in a methacrylic monomer matrix by centrifugal mixing and ultrasonication. Particle size distributions were analyzed by X-ray disc centrifugation of suspensions and TEM analysis of cured sample composites, while average primary particle size was obtained by N2 adsorption and X-ray diffraction. RESULTS The dispersion method affected the aggregate size distribution of both untreated and surface functionalized particles in these suspensions. The influence of particle content on suspension viscosity, aggregate size distribution and that of Ta2O5 content on radiopacity was investigated. The shear bond strength of such radiopaque particle-containing adhesives on enamel and dentin was comparable to that of the particle-free reference adhesive. CONCLUSIONS Flame-made Ta2O5/SiO2 nanoparticles can be introduced readily into dental adhesives as they form quite stable suspensions. Viscosity stayed low even after adding radiopaque particles up to 20 wt.%. The resulting composites had radiopacity comparable to that of enamel facilitating their distinction from marginal gaps. Bond strength was not significantly influenced by the presence of particles in the adhesive.

Impact of Functional Satellite Groups on the Antimicrobial Activity and Hemocompatibility of Telechelic Poly(2-methyloxazoline)s

Polyoxazolines with a biocidal quarternary ammonium end-group are potent biocides. Interestingly, the antimicrobial activity of the whole macromolecule is controlled by the nature of the group at the distal end. These nonreactive groups are usually introduced via the initiator. Here we present a study with a series of polymethyloxazolines with varying satellite groups introduced upon termination of the polymerization reaction. This allowed us to introduce a series of functional satellites, including hydroxy, primary amino, and double-bond-containing groups. The resulting telechelic polyoxazolines were explored regarding their antimicrobial activity and toxicity. It was found that the functional satellite groups greatly controlled the minimal inhibitory concentrations against the bacteria Staphylococcus aureus and Escherichia coli in a range of 10 to 2500 ppm. Surprisingly, the satellite groups also controlled the hemotoxicity but in a different way than the antimicrobial efficiency.

Supramolecular Cross-Links in Poly(alkyl methacrylate) Copolymers and Their Impact on the Mechanical and Reversible Adhesive Properties

Hydrogen-bonded, side-chain-functionalized supramolecular poly(alkyl methacrylate)s were investigated as light- and temperature-responsive reversible adhesives that are useful for bonding and debonding on demand applications. Here, 2-hydroxyethyl methacrylate (HEMA) was functionalized with 2-ureido-4[1H]pyrimidinone (UPy) via a hexamethylenediisocyanate (HMDI) linker, to create a monomer (UPy-HMDI-HEMA) that serves to form supramolecular cross-links by way of forming quadruple hydrogen bonded dimers. UPy-HMDI-HEMA was copolymerized with either hexyl methacrylate or butyl methacrylate to create copolymers comprising 2.5, 5, or 10 mol % of the cross-linker. The mechanical properties of all (co)polymers were investigated with stress-strain experiments and dynamic mechanical analysis. Furthermore, the adhesive properties were studied at temperatures between 20 and 60 °C by testing single lap joints formed with stainless steel substrates. It was found that increasing the concentration of the UPy-HMDI-HEMA cross-linker leads to improved mechanical and adhesive properties at elevated temperatures. Concurrently, the reversibility of the bond formation remained unaffected, where rebonded samples displayed the same adhesive strength as regularly bonded samples. Debonding on demand abilities were also tested exemplarily for one copolymer, which for light-induced debonding experiments was blended with a UV-absorber that served as light-heat converter. Single lap joints were subjected to a constant force and heated or irradiated with UV light until debonding occurred. The necessary debonding temperature was comparable for direct heating and UV irradiation and varied between 28 and 82 °C, depending on the applied force. The latter also influenced the debonding time, which under the chosen conditions ranged from 30 s to 12 min.

Telechelic Poly(2-oxazoline)s with a Biocidal and a Polymerizable Terminal as Collagenase Inhibiting Additive for Long-Term Active Antimicrobial Dental Materials

Dental repair materials face the problem that the dentin below the composite fillings is actively decomposed by secondary caries and extracellular proteases. To address this problem, poly(2-methyloxazoline) with a biocidal and a polymerizable terminal was explored as additive for a commercial dental adhesive. 2.5 wt% of the additive rendered the adhesive contact-active against Streptococcus mutans and washing with water for 101 d did not diminish this effect. The adhesive with 5 wt% additive kills S. mutans cells in the tubuli of bovine dentin. Further, the additive inhibits bacterial collagenase at 0.5 wt% and reduces activity of MMP-9. Human MMPs bound to dentin are inhibited by 96% in a medium with 5 wt% additive. Moreover, no adverse effect on the enamel/dentine shear bond strength was detected.

Reaction behaviour of monomeric β-ketoesters

SummaryThe addition of 2-acetoacetoxyethyl methacrylate (AAEMA) to aromatic isocyanate such as phenyl isocyanate or tolylene 2,4-diisocyanate, and the Knoevenagel condensation of AAEMA with aromatic aldehydes, respectively, yield polymerizable products. These monomers were characterized by elemental analyses, IR, 1H NMR and partially by 13C NMR spectroscopy. The radical polymerization of synthesized AAEMA-isocyanates adducts forms polymeric blocked isocyanates. The Knoevenagel condensate of AAEMA with benzaldehyde is radically polymerizable and tends to crosslink during its homopolymerization.

Ultrahigh-Aspect Ratio Microfiber-Furs as Plant-Surface Mimics Derived from Teeth

IO N The biomimetic of micro-, nanoand hierarchically structured hairy surfaces of plant origins is in focus of current research, inter alia, because of their extraordinary water-droplet and airbubble storage capability. [ 1–5 ] The latter is based on the great fl exibility of high-aspect-ratio fi laments (trichomes) and thus such surfaces show a typical Cassie-Baxter behavior, [ 6 , 7 ] i.e., water droplets are attached to the surface of air cushions and air pockets are entrapped underneath water. Particularly, the remarkable air-retaining properties make such surfaces economically and ecologically attractive for applications, in which a reduced drag of fl oating units (e.g. ships) in fl uid fl ow is desirable. [ 4 ] Typical examples of such hairy plant surfaces with aspect ratios of 100:1 and higher are the tops of lady’s mantle ( Alchemilla vulgaris ) leaves [ 5 ] and the undersides of Corokia cotoneaster (fam. Argophyllaceae ) leaves. Figure 1 shows a macro image of a Corokia cotoneaster leaf underside covered with water droplets. So far, it is not possible to exactly reproduce such natural hairy surfaces, particularly with respect to a high hair density of less than 10 μ m inter-fi lament distances and aspect ratio of more than 100:1. While the generation of microand nanohairs is readily achievable thanks to advanced spinning technologies such as electro spinning, these fi bers cannot easily be attached end-on in the required high densities. [ 8 ]

Synthesis and polymerization of unsaturated derivatives of thymol

SummaryThe preparation of methacrylic and p-styrenesulfonic acid esters of the antibacterial agent thymol (2-isopropyl-5-methylphenol) and their characterization by IR, 1H and 13C NMR spectroscopy is described. The monomers are radically polymerizable and give polymers in which thymol is covalently attached to the polymer backbone by ester links.