Hardness evaluation of a dental composite polymerized with experimental LED-based devices Cristina Kurachi a, * , Aparecida M. Tuboy b , Daniel V. Magalha Äes a , Vanderlei S. Bagnato a a Sa Äo Carlos Institute of Physics, University of Sa Äo Paulo, Caixa Postal 369, 13560-970, Sa Äo Carlos, SP, Brazil b MM Optics: Rua Geminiano Costa, 157 13560-050, Sa Äo Carlos, SP, Brazil Received 1 February 2000; revised 15 June 2000; accepted 28 September 2000 Abstract Objective: The main goal of this study was the hardness evaluation of a composite resin cured by ®ve LED (Light Emitting Diodes) based devices and a comparison with a conventional curing unit. The hardness test was used to compare the ef®cacy of both types of light source. Methods: The LED-based devices were made employing an array of LEDs (Nichia Chem. Ind., Japan) emitting light peaked at 470 nm. Composite resin (Z100, shade A3) was cured for 20, 40, 60, 120 and 180 s with each LED-based device and for 40 s with the halogen lamp. The composite samples were prepared with 0.35, 1.25 and 1.8 mm of thickness. Five samples of each set of parameters were done. The hardness evaluation was performed at the non-illuminate surface with three indentations for each sample. Results: All the samples cured by the LED-based devices showed inferior hardness values when compared with the halogen lamp at the typical curing time (40 s). The L6 (device composed of six LEDs) was the most ef®cient one of the LED-based devices. Its obtained irradiance was 79 mW/cm 2 , whereas the halogen lamp irradiance was of 475 mW/cm 2 . For the L6 device here presented, longer exposure times or a thinner resin layer are required to achieve reasonable hardness values. Signi®cance: Besides the difference of irradiance when compared with halogen lamps, LED-based devices show to be a promising alternative curing instrument. Further development in instrumentation may result in devices even more ef®cient than conventional lamps. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Light emitting diode; Composite resin; Cure; Microhardness 1. Introduction Dental composites are an important class of material widely employed in restorative procedures. In recent years, the popularity of aesthetic tooth-colored restorations has promoted a rapidly increasing use of resins. Methods and devices to prepare and cure resins have evolved jointly; passing from chemically cured resins to the modern form of light curing. Because the polymerization of light-cured resins depends mainly on the characteristics and type of the radiation source used, a way to achieve better properties of the ®nal restoration cured is the improvement of the curing unit. In this direction, new types of light sources should be tested to verify their viability for clinical applica- tion. For many years, halogen lamps have been more widely employed than any other device as a practical alternative method to cure resins. Operating with a white halogen bulb ®ltered by a dielectric pass-band ®lter to remove the unde- sirable wavelengths, conventional composite-curing lamps operate in the deep blue region of the spectrum. However, this type of equipment still emits a considerable number of other wavelengths. The spectral impurities of the conven- tional curing-lights deliver several wavelengths that are highly absorbed by dental materials, inducing heating of the tooth and resin during the curing process. Another draw- back in the use of the conventional curing-lights is a decline of irradiance over time due to the bulb and ®lter ageing [1,2]. The most frequently employed composite resins have camphorquinone as a photoinitiator, which is sensitive to light in the blue region of the visible spectrum. According to Nomoto [3], the most ef®cient wavelength is 470 nm and the most adequate wavelength is in the 450±490 nm wave- length range. Any source of light with reasonable irradiance in this region can be employed as a curing device. Never- theless, most of the existent visible-light curing units often fail when faced with the challenges of today's demands for more complex resin restoration. The present design of such devices (lamp, ®lters and light tip) makes it impossible to Dental Materials 17 (2001) 309±315 dental materials 0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S0109-5641(00)00088-9 www.elsevier.com/locate/dental * Corresponding author. E-mail address: cristina@if.sc.usp.br (C. Kurachi).