Browsing by Author "Krishnan, VK"

Now showing 1 - 20 of 42
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    A comparison of efficiency of two photoinitiators for polymerization of light-cure dental composite resins
    (JOURNAL OF APPLIED POLYMER SCIENCE, 2008) Lizymol, PP; Krishnan, VK
    The purpose of the study was to compare the effect of two photoinitiators, (-)camphorquinone (CQ) and 1-phenyl-1,2-propanedione (PPD) on curing performance of light-cure dental composite resins. Bisphenol Aglycidyl methacrylate (BisGMA)/triethylene glycol dimethacrylate (TEGDMA) monomer mixture was used as the resin matrix. The resin matrix was mixed with CQ and/or PPD along with 0.25% of 4-(dimethyl amino) phenethyl alcohol (DMAPEA) catalyst. The effect of photoinitiator on curing performance was evaluated and compared in terms of properties such as depth of cure, diametral tensile strength (DTS), flexural strength (FS),, flexural. modulus (FM), vickers hardness number (VHN), water sorption (WS), and solubility of cured composite. Statistical evaluation using Analysis of Variance (single factor) showed that the photosensitization efficiency of CQ and PPD are comparable. However, their combination showed synergistic effect for properties such as DTS and solubility. (c) 2007 Wiley Periodicals, Inc.
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    A study of the diluent effect on the properties of urethane based dental composites
    (JOURNAL OF POLYMER MATERIALS, 2003) Lizymol, PP; Krishnan, VK
    Low molecular weight monomers such as triethylene glycol dimethacrylate (TEGDMA) and tetraethylene glycol diacrylate (TEGDA) have been conventionally used as diluents in combination with highly viscous dimethacrylate organic resins such as Bisphenol A - Glycidyl methacrylate (bis-GMA) in dental composite formulations to facilitate easier mixing and for providing improved properties. In this study a new low molecular weight dimethacrylate 1,3 bis (methactyloxy) 2-hydroxy propane [MHP] has been synthesized and used in dental composites. Glycidyl methacrylate was reacted with methacrylic acid in presence of a base catalyst, the product of which was subsequently used as a precursor for preparing two urethane tetramethacrylate resins by reacting with hexamethylene diisocyanate [HMDI] and toluene diisocyanate [TDI]. Dental composite pastes prepared using these two urethane resins had MHP or TEGDMA as diluents and dimethyl aminophenethyl alcohol [DMAPEA] or dimethyl p-toluidine [DMPT] as activators and their properties were compared. Comparisons were made in terms of compressive strength (CS), diametral tensile strength (DTS) and Vickers microhardness (MH). Composites containing HMDI showed improved mechanical properties compared to those prepared using TDI. Composites containing MHP showed comparable properties with those incorporated with TEGDMA. SEM studies showed evidence for uniform mixing of the filler in the urethane tetramethacrylate resin thinned with MHP.
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    Aging Effects of Dental Restorative Materials upon Surface Hardness
    (JOURNAL OF POLYMER MATERIALS, 2009) Lizymol, PP; Krishnan, VK
    This study was conducted to compare the effect of one-year storage of one indigenously developed and seven commercially available dental restorative materials on surface microhardness (VHN) and the use of hardness value as an indirect measure of degree of conversion and degradation in aqueous medium. Four composite pastes (Spectrum- TPH, Z-100, P -60 and indigenously developed Ormo 48, one compomer (F- 2000) and three glass ionomers (Fuji II LC, Fuji II CC and Ketac Molar) were either self or light cured and stored in distilled water at 37 +/- 1 degrees C and Vickers microhardness was measured at intervals of 1 hour, 1 and 7 days, 1, 3, 6, 9 and 12 months. A minimum of 6 measurements was carried out on both surfaces. Mean and standard deviations were calculated. Statistical evaluation was done by means of one-way analysis of variance (ANOVA). Degradation and monomer conversion were correlated with the support of Fourier Transform Raman spectra of cured and uncured paste. A significant difference in hardness was noticed for each material over storage time. Among the materials tested, composites exhibited the highest hardness values throughout the study. The values reached a maximum after 24 hours and showed a significant decline over a period of 1 year. Compomer had the maximum hardness after seven days. However in case of FUJI glass ionomers, no significant change in hardness was noticed even after 1 year whereas Ketac Molar showed a significant increase in hardness with time. FT Raman spectra of photo cured organically modified ceramics composite showed residual unsaturation (1610 cm(-1) peak). The results indicate that composites and compomers show significant decrease in hardness with time whereas glass ionomers tend to show either significant (P< 0.05) increase or no change on storage.
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    Aging studies of a radiopaque light-cured dental composite in food-simulating liquids
    (JOURNAL OF APPLIED POLYMER SCIENCE, 1998) Krishnan, VK; Yamuna, V
    Sorption behavior and changes in physical properties of an indigenously developed radiopaque light-cured dental composite upon aging in citrate buffer (pH: 4.0), 50% ethyl alcohol and saline for a total period of 80 days, have been studied. Water uptake into the composite reaches saturation within 14 days in all media, whereas solubility values show an upward trend even after storage for 80 days. Solubility in alcohol medium is found to be high compared to saline and citrate media. Compressive strength and diametral tensile strength measurements tend to increase initially and then decrease gradually with time due to degradation of the composite upon storage. However, this reduction Is not appreciable, as indicated by the strength values after 80 days, which are higher than the mandatory 220 MPa for CS and 34 MPa for DTS. Microhardness increases with time in all three cases, although alcohol medium tends to soften the material, resulting in an initial sharp decrease. (C) 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1153-1158, 1998.
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    Aqueous based Synthesis of CdSe/ZnS Q-dots: Study on Luminescence Properties and Cytotoxicity
    (PROCEEDING OF INTERNATIONAL CONFERENCE ON RECENT TRENDS IN APPLIED PHYSICS & MATERIAL SCIENCE (RAM 2013), 2013) Painuly, D; Bhatt, A; Krishnan, VK
    Present study aims to modify the thioacid capped CdSe Quantum-dots (Q-dots) surface by ZnS coating by direct synthesis in aqueous medium. CS formation was confirmed by red shift as well as enhancement in the luminescence peak compared to bare Q-dots. Effects of processing parameters during the shell preparation such as core concentration and sulphur concentration on the luminescence properties of CS have been studied. Processing parameters have been optimized at maximum luminescence efficiency. Cytocompatibility behavior was found to be better for CS compared to their bare Q-dots counterpart after evaluation. Cytotoxicity of CS has been further evaluated by changing the sulphur concentration and after aging for 8 days.
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    Biological evaluation of a new organically modified ceramic-based dental restorative resin
    (JOURNAL OF APPLIED POLYMER SCIENCE, 2012) Lizymol, PP; Mohanan, PV; Sabareeswaran, A; Krishnan, VK
    Material tissue interactions of a newly developed organically modified ceramic (ormocer)-based dental restorative composite based on a resin containing mixture of alkoxides of silicone and calcium with polymerizable methacrylate end groups are discussed in this study. Admira, a commercially available ormocer was used as control. A solgel process has been used to synthesize organically modified ceramic resins from liquid precursors, which allow one to produce inorganicorganic hybrid polymer materials, which can be functionalized to optimize their physical and chemical properties. Besides, the resulting material properties have been significantly modified by further technological processing such as photochemical curing of the materials by incorporating dimethacrylate groups as organically polymerizable units. Intracutaneous (intradermal) irritation test is found not to elicit any gross signs of tissue reaction. The results of maximization test for delayed hypersensitivity also did not show any adverse skin reaction during the induction or challenge period. Histological analysis after 1, 4, and 12 weeks of subcutaneous implantation of restorative composite resin on the dorsal surface on one side of the spinal column of Wister rats is also found not to reveal any inflammatory response after the implantation. Granuloma and material debris is found to be absent in all three periods of implantation in both test and control. No necrosis was observed around the implanted materials, which were found to be encapsulated by fibrous connective tissue consisting of predominantly fibroblasts and inflammatory cells. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
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    Biomaterials: An overview
    (NATIONAL MEDICAL JOURNAL OF INDIA, 1999) Valiathan, MS; Krishnan, VK
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    Contribution of fibroblasts to the mechanical stability of in vitro engineered dermal-like tissue through extra cellular matrix deposition
    (Bio Research Open Access, 2014-10) Nair, RP; Joseph, J; Harikrishnan, VS; Krishnan, VK; Krishnan, LK
    Tissue-engineered skin with mechanical and biological properties that match the native tissue could be a valuable graft to treat non-healing chronic wounds. Fibroblasts grown on a suitable biodegradable scaffold are a feasible strategy for the development of a dermal substitute above which epithelialization may occur naturally. Cell growth and phenotype maintenance are crucial to ensure the functional status of engineered tissue. In this study, an electrospun biodegradable polymer scaffold composed of a terpolymer PLGC [poly(lactide-glycolide- caprolactone)] with appropriate mechanical strength was used as a scaffold so that undesirable contraction of the wound could be prevented when it was implanted. To enhance cell growth, synthetic PLGC was incorporated with a fibrin-based biomimetic composite. The efficacy of the hybrid scaffold was evaluated by comparing it with bare PLGC in terms of fibroblast growth potential, extracellular matrix (ECM) deposition, polymer degradation, and mechanical strength. A significant increase was observed in fibroblast attachment, proliferation, and deposition of ECM proteins such as collagen and elastin in the hybrid scaffold. After growing fibroblasts for 20 d and 40 d, immunochemical staining of the decellularized scaffolds showed deposition of insoluble collagen and elastin on the hybrid scaffold but not on the bare scaffold. The loss of mechanical strength consequent to in vitro polymer degradation seemed to be balanced owing to the ECMdeposition. Thus, tensile strength and elongation were better when cells were grown on the hybrid scaffold rather than the bare samples immersed in culture medium. Similar patterns of in vivo and in vitro degradation were observed during subcutaneous implantation and fibroblast culture, respectively. We therefore postulate that a hybrid scaffold comprising PLGC and fibrin is a potential candidate for the engineering of dermal tissue to be used in the regeneration of chronic wounds.
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    Cytocompatibility Evaluation of the Mercaptoethanol capped CdSe Quantum Dots and CdSe/ZnS Core/Shell
    (J Biomedical Nanotechnology, 2013-03) Painuly, D; Bhatt, A; Krishnan, VK
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    Development and evaluation of a single solution bonding agent as a dental adhesive
    (JOURNAL OF POLYMER MATERIALS, 2005) Krishnan, VK; Lizymol, PP; Kumari, TV; Rauf, TA; Thomas, MM
    Development of a dentine bonding agent based on a tetramethacrylate resin with free carboxyl groups and its evaluation are described in this paper. Pyromellitic dianhydride was reacted with glycerol dimethactylate at 42-45 degrees C for 5 h in presence of an amine catalyst to produce a colorless and viscous pyromellitic anhydride-glycerol dimethacrylate adduct [PMGDM]. The resin was characterized using refractive index, infra red and NMR spectroscopy and HPLC chromatographic techniques. The resin was used to formulate a single solution bonding agent [SSBA] using acetone as the solvent, 2-hydroxyethyl methacrylate [HEMA] as the diluent, camphorquinone [CO] as the photoinitiator and dimethylamino phenethyl alcohol [DMAPEA] as the accelerator. Shear and tensile bond strengths of composites polymerized by photoinitiation adhered on dentinal and metallic surfaces using SSBA and an imported control Single Bond were evaluated and compared. Cytotoxicity evaluation by the MTT assay of SSBA and Single Bond on L-929 mammalian cell lines demonstrated metabolic activity of 1.7% of cells in contact with SSBA extract and 1.5% of cells in contact with Single Bond extract compared to 100% for cells alone. Shear and tensile strength evaluation at different intervals of time for the newly developed bonding agent stored at 22 +/- 2 degrees C upto 70 days showed no statistically significant variation in the strength values with time.
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    Development and evaluation of properties of a bioactive cement
    (JOURNAL OF POLYMER MATERIALS, 1998) Divakaran, C; Krishnan, VK
    The properties of a visible light cured bioactive cement developed using an organic resin binder bisphenol A-glycidyl methacrylate (BIS-GMA) and a fluoroaluminocalcium silicate glass have been studied. Bisphenol A-glycidyl methacrylate was synthesized by reacting diglycidyl ether of bisphenol A and methacrylic acid and characterized using infra red spectroscopy and high performance liquid chromatography. It was found to form a paste when incorporated with glass filler powder of 1 mu m particle size and could be cured into a hard cement when exposed to a light source of 440-480 nm wavelength for about 30-60 sec. Compressive strength, diametral tensile strength, transverse strength and Vickers microhardness have been measured for paste formulations containing 200 220 240 and 260 phr glass filler and compared. The concentration of photoinitiator has also been varied and its effect upon the properties studied.
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    Effect of amine activators on the properties of chemical cured dental composites
    (JOURNAL OF BIOMATERIALS APPLICATIONS, 1997)
    The purpose of this study was to evaluate the reactivity and the effect of concentration of three tertiary amines upon the mechanical properties of a chemical curing dental composite. Chemical cured composite pastes were prepared by keeping peroxide concentration constant at 1 wt% (by weight of resin mixture) and by varying the amine/peroxide molar ratio from 0.25 to 1.5. Composite samples were prepared for all three amine pastes aged for 1, 15, 30, 45, and 60 d stored at 8, 22, and 37 degrees C. The loss in activity of the tertiary amine with time was measured. Changes in compressive strength, diametral tensile strength, and microhardness were also measured. A sharp decrease in working and setting times corresponding to increased activity was noticed with an increased amine content. The activity was found to vary in the order N,N-dimethyl p-toluidine (DMPT) > 2-(4-dimethylaminophenyl) ethanol (DMAPEA) > N,Ndiethanol p-toluidine (DEFT). DMPT is found to be more temperature sensitive than DMAPEA and DEFT. However, DEFT is found to provide better storage stability out of all three amines tested. Each amine was found to possess optimum concentrations at which the mechanical properties showed maximum values. DEFT is preferred for long-term storage stability in chemical-cured dental composites where aging tends to reduce the activity of the amine.
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    Effect of diluent upon the properties of a visible-light-cured dental composite
    (JOURNAL OF MATERIALS SCIENCE-MATERIALS IN MEDICINE, 1997)
    Four long-chain aliphatic organic diluents, triethylene glycol dimethacrylate (TEGDMA), ethylene glycol dimethacrylate (EDMA), triethyleneglycol diacrylate (TEGDA) and tetraethylene glycol diacrylate (TTEGDA) were used as diluents for preparing a visible-light-cured dental composite. Cured samples of pastes prepared using the above diluents were stored separately for a period of 60 days in distilled water at 37 degrees C and tested for changes in compressive strength, diametral tensile strength and microhardness at specific intervals of 1, 7, 14, 21, 28 and 60 days. Samples prepared using TEGDMA showed better strength values initially compared with the composite specimens prepared using the rest of the diluents although they are found to deteriorate slowly upon storage in distilled water up to 60 days. Samples prepared using EDMA showed retention in properties whereas samples prepared using TEGDA and TTEGDA showed large deviations in strength values during the first 28 days, did not deteriorate further and were found either to show improvement upon ageing or to remain constant. Sorption values seem to reach a maximum for all composites and then to decrease further after 14 days whereas solubility is found to increase with time.
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    Effect of initiator concentration, exposure time and particle size of the filler upon the mechanical properties of a light-curing radiopaque dental composite
    (JOURNAL OF ORAL REHABILITATION, 1998) Krishnan, VK; Yamuna, V
    Concentration of camphorquinone initiator, exposure time of the light source and particle size of a radiopaque glass filler have been varied for an indigenously developed light-curing dental composite and the changes in the microhardness, compressive strength and diametral tensile strength studied. Higher initiator concentration and exposure times are found to improve the microhardness values while a concentration above 0.25% does not signify any drastic improvement in compressive and diametral strength. Changes in properties are found to be statistically significant at low initiator concentrations. A filler particle size around 1 mu m is found to give better properties compared with larger sizes.
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    Effect of Pal of filler particles on the physical properties of a dental composite
    (INDIAN JOURNAL OF ENGINEERING AND MATERIALS SCIENCES, 1998) Manu, S; Usha, K; Jyothi, S; Krishnan, VK; Pal, SN
    Use of silane treated fillers in chemical curing Bisphenol A-Glycidyl methacrylate (Bis-GMA) based dental composites has been found to improve the properties appreciably. The nature and concentration of the silane used is found to be critical in deciding the final properties. However, water sorption does not seem to be, considerably affected by varying silane concentration. Morphology of the fractured surface indicates that composites containing 3-(trimethoxysilyl) propyl methacrylate (TSPM) treated fillers provide, better filler-matrix compatibility resulting in better strength characteristics.
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    Effect of polyacid on the properties of polyalkenoate cements
    (CURRENT SCIENCE, 1996) Krishnan, VK; Sreekumar, J
    Four polyacids were prepared by reacting acrylic acid with itaconic and maleic acids at different ratios and these were treated with a fluoroalumino calcium silicate glass to form polyalkenoate cements. The effect of working and setting times of the glass ionomer upon variation of the polyacid content in the liquid has been studied. Mechanical properties such as compressive strength and diametral tensile strength were found to increase sharply upon increase in polyacid concentration, The monomer ratio in the polyacid was also found to be critical in determining the final properties of the dental material.
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    Effect of resin matrix ratio, storage medium, and time upon the physical properties of a radiopaque dental composite
    (JOURNAL OF BIOMATERIALS APPLICATIONS, 2000)
    Three light curing composite pastes with varying resin matrix ratios [bisphenol A-glycidyl methacrylate (BIS-GMA)/urethane tetramethacrylate (UTMA) 25:75, BIS-GMA/UTMA 50:50, and BIS-GMA/UTMA 75:25 w/w%] were prepared in combination with a radiopaque glass powder and camphorquinone photoinitiator: Cured samples were aged at 37 degrees C in three food simulating media such as citrate buffer (pH 4.0), PBS buffer (pH 7.4), and 75% ethyl alcohol. Samples were withdrawn at specific intervals of 1, 15, 30, 45, and 60 days and tested for changes in mechanical properties, sorption, and solubility characteristics. Statistical calculations revealed significant changes in compressive strength (CS) for composites depending on the resin matrix ratio and type of medium used for aging. While diametral tensile strength (DTS) was affected adversely in citrate medium for composites with higher urethane content, samples stored in alcohol medium showed deterioration of transverse strength (TS) and microhardness (VMH) for all composites studied. Increase in BIS-GMA content in the resin matrix and storage in alcohol medium resulted in higher sorption and solubility values and lower microhardness.
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    Effect of tartaric acid on the properties of polyalkenoate cements
    (JOURNAL OF POLYMER MATERIALS, 1996) Krishnan, VK; Sreekumar, J
    Four polyacids were prepared by reacting acrylic acid with itaconic and maleic acids at different ratios and these were treated with a fluoroalumino calcium silicate glass to form polyalkenoate cements. The effect of working time and setting time of the glass ionomer upon addition of a chelating agent (+) tartaric acid into the polyacid has been studied. Mechanical properties such as compressive strength and diametral tensile strength were found to improve-upon addition of 5% by weight of the chelating agent. The monomer ratio in the polyacid was also found to be critical in determining the final properties of the dental material.
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    Effect of tertiary amine accelerators on the physical properties of visible light cured dental composites
    (JOURNAL OF POLYMER MATERIALS, 1996) Manu, S; Krishnan, VK; Pal, SN
    The effect of changing the tertiary amine accelerator on the physical properties of visible light cured dental composites is described in this paper. Three aromatic and two aliphatic tertiary amines were used in this study. The diametral tensile strength and Vickers microhardness of the cured specimens were determined. The presence of highly active aromatic amines like dimethyl p-toluidine (DMPT) and dimethyl amino phenethyl alcohol (DMAPEA) gave better properties for the cured composite compared to diethanol p-toluidine (DEFT).
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    Evaluation of a light cured dental composite containing a radiopaque glass filler
    (CURRENT SCIENCE, 1997) Krishnan, VK; Yamuna, V
    Light curing dental composite pastes were prepared using an indigenously developed Bisphenol A-glycidyl methacrylate (BIS-GMA) resin and a commercially available radiopaque glass filler of different particle sizes. SEM examination of the virgin filler and composite surface revealed agglomeration of particles in virgin state and fine encapsulation of the filler particles in resin matrix of composite. Variation of the filler content shows that an amount of 220 phr (parts per hundred of resin mixture) of the radiopaque glass filler of larger grain size or 250 phr of the smaller grain size incorporated into the paste tends to improve the compressive strength and diametral tensile strength considerably compared to composite containing conventional quartz fillers.