Klaus D. Jandt

Light-emitting diode (LED) polymerisation of dental composites : Flexural properties and polymerisation potential

The clinical performance of light polymerised dental composites is greatly influenced by the quality of the light-curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing of the light output with time. This may result in low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue-light-emitting diode (LED) LCUs have the potential to polymerise dental composites without having the drawbacks of halogen LCUs. Despite the relatively low irradiance of current LED LCUs, their efficiency is close to that of conventional halogen LCUs with more than twice the irradiance. This phenomenon has not been explained fully yet. Hence, more tests of the LED LCUs effectiveness and of the mechanical properties of oral biomaterials processed with LED LCUs need to be carried out. This study investigates the flexural properties of three different composites with three different shades, which were polymerised with either a commercial halogen LCU or an LED LCU, respectively. In most cases no significant differences in flexural strength and modulus between composites polymerised with a halogen LCU or an LED LCU, respectively, were found. A simple model for the curing effectiveness based on the convolution absorption spectrum of the camphorquinone photoinitiator present in composites and the emission spectra of the LCUs is presented.

Atomic force microscopy of biomaterials surfaces and interfaces

Abstract The use of atomic force microscopy (AFM) in biomaterials science and engineering applications has increased rapidly over the last few years. Beyond being merely a tool for measuring surface topography, AFM has made significant contributions to various biomaterials research areas dealing with the structure, properties, dynamics and manipulation of biomaterials surfaces and interfaces. This paper critically reviews methodological approaches and presents aspects of this research. Selected examples presented include micro and nanostructure and properties of biomaterials surfaces, molecular level interactions at biomaterial–biomolecule interfaces, interfaces between biomaterials and mineralised tissues as well as advances of mineralised tissue research. In these areas, AFM is shown to be a useful and versatile tool to study micro and nanostructure, to probe mechanical properties or to investigate dynamic process at biomaterials surfaces and interfaces.

Future perspectives of resin-based dental materials

OBJECTIVE This concise review and outlook paper gives a view of selected potential future developments in the area of resin-based biomaterials with an emphasis on dental composites. METHODS A selection of key publications (1 book, 35 scientific original publications and 1 website source) covering the areas nanotechnology, antimicrobial materials, stimuli responsive materials, self-repairing materials and materials for tissue engineering with direct or indirect relations and/or implications to resin-based dental materials is critically reviewed and discussed. Connections between these fields and their potential for resin-based dental materials are highlighted and put in perspective. RESULTS The need to improve shrinkage properties and wear resistance is obvious for dental composites, and a vast number of attempts have been made to accomplish these aims. Future resin-based materials may be further improved in this respect if, for example nanotechnology is applied. Dental composites may, however, reach a completely new quality by utilizing new trends from materials science, such as introducing nanostructures, antimicrobial properties, stimuli responsive capabilities, the ability to promote tissue regeneration or repair of dental tissues if the composites were able to repair themselves. SIGNIFICANCE This paper shows selected potential future developments in the area of resin-based dental materials, gives basic and industrial researchers in dental materials science, and dental practitioners a glance into the potential future of these materials, and should stimulate discussion about needs and future developments in the area.

Polymerization and light-induced heat of dental composites cured with LED and halogen technology

Most commercial light curing units (LCUs) for dental applications use conventional halogen bulbs. Commercial LCUs using light emitting diodes (LEDs) have recently become established on the market, even though some aspects of their performance have not been fully investigated. Temperature rise of dental composites during the light-induced polymerization is considered to be a potential hazard for the pulp of the tooth. This study, therefore, investigated the temperature rise in three different composites (Z100, Durafill, Solitaire2) in two shades (A2, A4) polymerized for 40s with two LED LCUs (Freelight, custom-made LED LCU prototype) and two halogen LCUs (Trilight, Translux). The Trilight was used in the standard and soft-start mode. The temperature rise within the composites were recorded for 60s with a thermocouple and also observed with a high-resolution infrared (HRIR) camera. The factors LCU (p < 0.0001), composite (p < 0.0001) and shade (p = 0.0014) had statistically significant influences on the temperature rise. All composites cured with the halogen LCUs reached at a depth of 2 mm, a statistically significant higher temperature (p < 0.0001) than those cured with the LED LCUs. Only one composite showed a statistically significant lower temperature rise for the halogen LCUs at the 95% confidence level, when the soft-start mode was used instead of the standard mode. In general, the composites with the lighter shade (A2) reached higher temperatures than the darker shade (A4), if the LED LCUs were used. When the halogen LCUs were used, the situation was reversed, the composites with the darker shade (A4) reaching higher temperatures than the lighter shade (A2). This study showed that a HRIR camera represents a powerful tool for the observation of temperature propagation on small samples. This study also showed that LED LCUs represent a viable alternative to halogen LCUs for the light polymerization of dental composites because of a generally lower temperature increase within the composite.

Second generation LEDs for the polymerization of oral biomaterials

OBJECTIVES New blue, so called second generation light emitting diodes (LEDs) are now available with a high optical power output. These LEDs will potentially find widespread application in commercially available light curing units (LCUs). This study, therefore, investigated the curing performance of a prototype LCU containing one high power LED and a conventional halogen LCU (Polofil). METHODS The performances of the LCUs were evaluated by measuring the Knoop hardness and depth of cure of the composites. Three dental composites were selected (Z100, Admira and Revolcin Flow) in a light (A2) and a dark shade (A3.5 or A4), respectively, and were polymerized for 40 s each. RESULTS The LED prototype (irradiance=901 mW/cm2) achieved a statistically significantly greater (p<0.05) depth of cure than the halogen LCU (irradiance=860 mW/cm2) for all composites. Generally, there was no statistically significant difference in Knoop hardness on the top and bottom of a 2 mm thick disk for the composites Z100 and Admira if polymerized with the LED prototype or halogen LCU. The composite Revolcin Flow, however, showed in general a statistically significant lower Knoop hardness if polymerized with the LED LCU. SIGNIFICANCE The present study shows that second generation LEDs have the potential to replace halogen LCUs if the composites are selected carefully. Furthermore, this study confirmed that the depth of cure test does not discriminate between LCUs performance for composites containing co-initiators, but the Knoop hardness test does.

Photoinitiator dependent composite depth of cure and Knoop hardness with halogen and LED light curing units.

Light curing units (LCUs) are used for the polymerization of dental composites. Recent trends in light curing technology include replacing the halogen LCUs with LCUs using light emitting diodes (LEDs) reducing curing times and varying the LCUs light output within a curing cycle. This study investigated the time dependence of the Knoop hardness and depth of cure of dental composites polymerized with a halogen LCU (Trilight) and two LED LCUs (the commercial Freelight and custom-made LED LCU prototype). The halogen LCU was used in the soft-start (exponential increase of output power) and standard mode. Four dental composites (Z100, Spectrum, Definite, Solitaire2) were selected, two of them (Definite, Solitaire2) contain co-initiators in addition to the standard photoinitiator camphorquinone. The depth of cure obtained with the Trilight in the standard mode was statistically significantly greater (p < 0.05) than that obtained with the LED LCUs for all materials and curing times. The custom made LED LCU prototype (LED63) achieved a statistically significantly greater depth of cure than the commercial LED LCU Freelight for all materials and curing times. There was no statistical difference in Knoop hardness at the 95% confidence level at the surface of the 2 mm thick sample between the LED63 or Trilight (standard mode) for the composite Z100 for all times, and for Spectrum for 20s and 40s curing time. The composites containing co-initiators showed statistically significantly smaller hardness values at the top and bottom of the samples if LED LCUs were used instead of halogen LCUs. The experiment revealed that the depth of cure test does not and the Knoop hardness test does discriminate between LCUs, used for the polymerization of composites containing photoinitiators in addition to camphorquinone.

High power light emitting diode (LED) arrays versus halogen light polymerization of oral biomaterials: Barcol hardness, compressive strength and radiometric properties.

The clinical performance of light polymerized dental composites is greatly influenced by the quality of the light curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing light output with time. This may result in a low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue light emitting diode (LED) LCUs have the potential to polymerize dental composites without having the drawbacks of halogen LCUs. Since these studies were carried out LED technology has advanced significantly and commercial LED LCUs are now becoming available. This study investigates the Barcol hardness as a function of depth, and the compressive strength of dental composites that had been polymerized for 40 or 20s with two high power LED LCU prototypes, a commercial LED LCU, and a commercial halogen LCU. In addition the radiometric properties of the LCUs were characterized. The two high power prototype LED LCUs and the halogen LCU showed a satisfactory and similar hardness-depth performance whereas the hardness of the materials polymerized with the commercial LED LCU rapidly decreased with sample depth and reduced polymerization time (20 s). There were statistically significant differences in the overall compressive strengths of composites polymerized with different LCUs at the 95% significance level (p = 0.0016) with the two high power LED LCU prototypes and the halogen LCU forming a statistically homogenous group. In conclusion, LED LCU polymerization technology can reach the performance level of halogen LCUs. One of the first commercial LED LCUs however lacked the power reserves of the high power LED LCU prototypes.

A brief history of LED photopolymerization

OBJECTIVES The majority of modern resin-based oral restorative biomaterials are cured via photopolymerization processes. A variety of light sources are available for this light curing of dental materials, such as composites or fissure sealants. Quartz-tungsten-halogen (QTH) light curing units (LCUs) have dominated light curing of dental materials for decades and are now almost entirely replaced by modern light emitting diode light curing units (LED LCUs). Exactly 50 years ago, visible LEDs were invented. Nevertheless, it was not before the 1990s that LEDs were seriously considered by scientists or manufactures of commercial LCUs as light sources to photopolymerize dental composites and other dental materials. The objective of this review paper is to give an overview of the scientific development and state-of-the-art of LED photopolymerization of oral biomaterials. METHODS The materials science of LED LCU devices and dental materials photopolymerized with LED LCU, as well as advantages and limits of LED photopolymerization of oral biomaterials, are discussed. This is mainly based on a review of the most frequently cited scientific papers in international peer reviewed journals. The developments of commercial LED LCUs as well as aspects of their clinical use are considered in this review. RESULTS The development of LED LCUs has progressed in steps and was made possible by (i) the invention of visible light emitting diodes 50 years ago; (ii) the introduction of high brightness blue light emitting GaN LEDs in 1994; and (iii) the creation of the first blue LED LCUs for the photopolymerization of oral biomaterials. The proof of concept of LED LCUs had to be demonstrated by the satisfactory performance of resin based restorative dental materials photopolymerized by these devices, before LED photopolymerization was generally accepted. Hallmarks of LED LCUs include a unique light emission spectrum, high curing efficiency, long life, low energy consumption and compact device form factor. SIGNIFICANCE By understanding the physical principles of LEDs, the development of LED LCUs, their strengths and limitations and the specific benefits of LED photopolymerization will be better appreciated.

Surface mediated in situ differentiation of mesenchymal stem cells on gene-functionalized titanium films fabricated by layer-by-layer technique

In this work, multilayered and gene-functionalized titanium films composed of chitosan (Chi) and plasmid DNA (pEGFP-hBMP2, pGB) were employed to investigate the surface mediated in situ differentiation of mesenchymal stem cells (MSCs). The Chi/pGB multilayered structures were fabricated by layer-by-layer (LbL) assembly technique and degraded to release plasmid DNA complexes depending on bilayer numbers over 7 days. Therefore, the differentiation behaviors of MSCs cultured onto Chi/pGB multilayered titanium films surface were investigated. Chi/pGB LbL-modified titanium films show significant higher (p<0.01) transfection efficiency than those of other groups transfected by lipofectamine 2000 regarding the expression of green fluorescent protein (GFP). Reverse transcription-polymerase chain reaction (RT-PCR) assay revealed that MSCs adhered onto Chi/pGB LbL-modified titanium films could still express hBMP2 mRNA over 7 days culture. Compared with control groups, MSCs cultured onto Chi/pGB LbL-modified titanium films display significantly higher (p<0.01 or p<0.05) production levels of alkaline phosphatase (ALP) and osteocalcin over 7 days and 14 days culture, respectively. These results demonstrate that Chi/pGB LbL-modified titanium films are beneficial for sustained in situ inducing osteoprogenitor cells to differentiate into mature osteoblasts over long time. The approach presented here has potential applications in the development of gene-stimulating biomaterials and implant technology.

Ultrasonication as a Method to Study Enamel Demineralisation during Acid Erosion

The aim of this study was to use ultrasonication as a method to measure subsurface demineralisation of enamel. Polished human enamel samples with surface profiles within ±0.3 μm were divided into 6 groups of 10 specimens. The groups of specimens were exposed to 0.3% citric acid (pH 3.2) for 30 min, 1, 2, 3 or 4 h. The depths of the resulting lesions were measured using a profilometer. A control group was stored in water for 4 h. Ultrasonication in water was performed on the specimen groups for 5, 30, 120, 240 and 480 s with profilometric measurements at each time point. The depth of the erosion increased linearly with the exposure time. Most of the additional loss of enamel occurred with the 5–second ultrasonication. The 30–min and 1–hour erosion lesions were further deepened by approximately 1 μm with 5 s of ultrasonication. The 2–, 3– and 4–hour lesions were deepened by 2–4 μm with 5 s of ultrasonication. There were no changes in the control group. It is concluded that ultrasonication removed softened enamel from the surface of the eroded enamel. Ultrasonication together with accurate measurement of lesion depth by profilometry offers a useful method for studying the depth of enamel softening associated with erosion.