Mangala P. Patel

Polymerization shrinkage of methacrylate esters

The polymerization shrinkage of a range of poly(n-alkyl methacrylates) in the range C1 to C16, some of their isomers and the polymers of cyclic and heterocyclic methacrylates have been measured using densitometry. The percentage volume shrinkage decreases with the size of the substituent side group. This proved to reflect the fact that the change in molar volume on polymerizing a methacrylate ester is reasonably constant at 22 cc/mol irrespective of the geometry of the substituent group. However, the glass transition temperature of the polymer depends very critically on the geometry of the side group. Hence one method for the development of low shrinkage glassy polymers is to investigate methacrylate esters of large molar volume, but with side group geometry that results in a high glass transition temperature. Polymerization shrinkage can be used to determine the degree of conversion of a polymer, using the value of 22 cc/mol as the change in molar volume.

Chlorhexidine release from room temperature polymerising methacrylate systems

A series of different methacrylate monomers (with either 1 or 2.5% dimethyl-p-toluidine, DMPT) was gelled with poly(ethyl methacrylate) powder (containing benzoyl peroxide) thus forming a room temperature curing system. When doped with 5.625% chlorhexidine diacetate the release from the tetrahydrofurfuryl methacrylate-based samples was considerably greater than that from other methacrylate monomers. This seems to be due to the formation of channels in the polymer. Nuclear magnetic resonance spectroscopy, of solutions that the samples were immersed in, showed chlorhexidine was indeed being released from the polymer. It also showed that doping the polymer with chlorhexidine hindered the polymerisation, resulting in a higher level of residual monomer and low molecular weight components being leached from the polymer. The DMPT also affected the polymerisation with greater leaching from the 2.5% DMPT sample.

A polymeric system for the intra-oral delivery of an anti-fungal agent.

Oral candidal infections are often persistent and intractable and thus the aim of this study was to develop a polymeric sustained release device to improve the topical treatment of these infections. A self curing system based on poly(ethyl methacrylate) and tetrahydrofurfuryl methacrylate (PEM/THFM) was used with chlorhexidine diacetate (CX) added at levels between 0 and 12% w/w. Water uptake by the device was assessed gravimetrically and CX release measured by UV spectrometry. Anti candidal activity was established by culturing azole sensitive and resistant strains of Candida albicans in the presence of the polymeric delivery device with and without CX. Candidal growth was measured by turbidimetry or surviving colony-forming unit (CFU) formation. There was an initial high release of CX over 24 h followed by a slow diffusion up to 7 days. CX inhibited candidal growth and survival markedly in vitro, with the test samples showing less than 0.5 x 10(-7) CFU/ml compared to controls (3-4 x 10(-7) CFU/ml). These results indicate the potential of a chlorhexidine containing PEM/THFM polymeric system in the treatment of persistent candidal infections.

The relationship between Shore hardness of elastomeric dental materials and Young's modulus

OBJECTIVES Hardness of elastomers can be directly related to Youngs modulus, a relationship that was investigated in detail by Gent in a paper in 1958. The aim of this study was to test this relationship for 13 dental elastomers (12 silicone and 1 polyether) using the equation derived by Gent and one from BS 903 (1950) that accounts for departures at low values. METHODS The dental elastomers were subjected to tensile testing and Shore A scale hardness measurements. Youngs moduli were calculated from the hardness values using the Gent equation and the BS 903 equation. These calculated values were then compared with values derived experimentally from the tensile tests. RESULTS Hardness values were in the range 30.2 (+/-0.5)-62.9 (+/-0.8) with the corresponding calculated modulus values in the range 1.1-4.1MPa and 0.9-4.3MPa for the Gent and modified equations, respectively. Youngs modulus values derived from the tensile data were in the range 0.8 (+/-0.3)-4.1 (+/-0.3)MPa, showing good agreement with those calculated from the hardness values. Providing viscoelastic creep is minimal during the duration of the test, there is a reasonably well-defined relationship between Shore hardness and Youngs modulus in the hardness range studied. SIGNIFICANCE Simple, non-destructive hardness measurements can be used to determine Youngs modulus values. Such values are needed in any calculations of stress distributions in soft lining materials, e.g. by FEA.

The regeneration of articular cartilage using a new polymer system

A polymer system based on room temperature polymerising poly (ethylmethacrylate) polymer powder and tetrahydrofurfuryl monomer has been investigated as a biomaterial for encouraging articular cartilage repair. This heterocyclic methacrylate polymer system swells slightly in situ and thus provides a good interface with subchondral bone resulting in mechanical stability with favourable uptake kinetics. Another feature of this polymer system is that it exhibits high water uptake which leads to absorption of the surrounding tissue fluid and matrix proteins, including growth factors; this may encourage the formation of new cartilage. Three weeks after implantation the tissue overgrowth contained cartilage components: chondrocytes, collagen type II, chondroitin 4-sulphate and chondroitin 6-sulphate. In addition numerous chondrocyte clones were observed at the edge of the defect and in the newly repaired tissue. By six weeks a superficial articulating surface was continuous with the normal articular cartilage with underlying tissue which showed some evidence of endochondral ossification. By nine weeks the surface covering of new cartilage had a widened and an irregular zone of calcified cartilage with thickened subchondral bone was present. At eight months the resurfaced cartilage remained intact above a remodelled subchondral bone end plate.

The effect of two hydrophilic monomers on the water uptake of a heterocyclic methacrylate system

The room temperature polymerising system poly(ethyl methacrylate) (PEM)/tetrahydrofurfuryl methcrylate (THFM) has been modified by replacing some of the THFM by hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPM), respectively. In both cases, the equilibrium uptake of the parent system is reduced substantially, in spite of the hydrophilic nature of these monomers. The effect is less with HPM. Corresponding to these decreases in uptake are substantial increases in the diffusion coefficients. This points to changes from a cluster-dominated process, to a more continuum-based process in the dual monomer systems. Addition of chicken serum albumin to these systems increases water uptake. At higher levels of HEMA addition, there is a substantial increase in polymerisation exotherm.

Heterocyclic methacrylates for clinical applications–further studies of water sorption

The room temperature polymerizing system comprising poly(ethyl methacrylate)-tetra hydrofurfuryl methacrylate (PEM/THFMA) has potential in orthopaedic and dental applications, and earlier work has shown it to have unusual water absorption characteristics. This aspect has been studied in further detail, by studying the water absorption behaviour from some biological solutions, and the effect of the addition of an antibiotic (gentamicin). For comparison purposes, a parallel system whereby tetrahydrofuryl methacrylate was replaced by hydroxyethyl methacrylate (PEM/HEMA), was studied. In the case of PEM/THFMA, water uptake was substantially reduced when absorption was carried out from solutions (from about 30% in water to about 1.5% in solutions of higher concentrations), and the corresponding diffusion coefficient increased (by a factor of several hundred). The addition of gentamicin increased uptake, but the extent of increase also decreased in solutions. It was concluded that uptake was related to the osmolarity of the external solution, and also on the presence of osmotic sites within the polymer; hence the uptake process appears to be governed by chemical potential considerations. At the higher uptakes, there was evidence of water clusters. In marked contrast, the uptake by the PEM/HEMA system was independent of the osmolarity of the external solutions, presumably due to the hydrophilic nature of HEMA.

Heterocyclic methacrylate-based drug release polymer system

A heterocyclic methacrylate polymer system, developed originally as a low shrinkage polymer system, has been investigated as a drug release polymer and as a biomaterial for encouraging bone or cartilage regeneration. The system is based on poly (ethyl methacrylate) polymer powder mixed with tetrahydrofurfuryl methacrylate monomer and polymerized at room temperature (PEM/THFM). Promising results have been obtained with this biomaterial, and hence its water uptake properties were investigated in detall, in order to throw some light on the release processes that are involved in vivo and in vitro. Water soluble large molecule analogues were incorporated into the system; these additives increased the water uptake of the system. Isobornyl methacrylate was used as a diluent for the monomer to further reduce the water uptake of the system. In all cases the uptake kinetics did not obey simple diffusion theory, the process being very prolonged and complex.

The water uptake of poly(tetrahydrofurfuryl methacrylate)

Poly(tetrahydrofurfuryl methacrylate) possesses some unique characteristics with respect to its biocompatibility and behaviour in water. The water uptake is high (>70%) and very slow (over 3 yr), but the material remains rigid throughout the process. The mechanism behind the uptake is in two stages; an initial Fickian stage, then as the matrix approaches saturation (about 3 wt%) a second clustering mechanism takes over. The rate of uptake of the second stage of the uptake is controlled by creep (or stress relaxation), and the chemical potential driving the uptake from clustering of the furfuryl rings of the polymer. If clustering or the creep is prevented (by appropriate co-polymerisation) the polymer behaves in an ideal, Fickian manner.

Fluoride ion release from two methacrylate polymer systems

The release of fluoride ions from two room-temperature polymerising systems containing sodium and potassium fluoride, respectively, has been studied. The polymer systems comprised poly(ethyl methacrylate) powder (PEM), with tetrahydrofurfuryl methacrylate (THFM), and n-butyl methacrylate (nBM), respectively. The water uptake of these systems was drastically increased by the presence of fluorides, the increase being much higher with the PEM/THFM system. In both cases, uptake was a monotonic function of the molarity of the fluoride added. The uptake process was in general non-Fickian. However, for all systems, the fluoride release process was Fickian, and diffusion coefficients could be calculated. The amount of fluoride released, and the diffusion coefficients obtained, appeared unrelated to the extent of water uptake of the parent polymers.