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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 605–617 Available online at www.sciencedirect.com jo ur nal home p ag e: www.intl.elsevierhealth.com/journals/dema Review A brief history of LED photopolymerization Klaus D. Jandt a,b,, Robin W. Mills c,∗∗ a Faculty of Physics and Astronomy, Friedrich-Schiller-University Jena, Löbdergraben 32, D-07751 Jena, Germany b Jena Center for Soft Matter, Friedrich-Schiller-University Jena, Germany c School of Oral and Dental Sciences, University of Bristol, Lower Maudlin Street, Bristol BS1 2LY, United Kingdom a r t i c l e i n f o Article history: Received 26 October 2012 Received in revised form 12 February 2013 Accepted 12 February 2013 Keywords: LED Photopolymerization Light curing Photocuring Quartz-tungsten-halogen Materials science Clinical aspects a b s t r a c t 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 biomate- rials, 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 gener- ally 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. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. Corresponding author at: Faculty of Physics and Astronomy, Friedrich-Schiller-University Jena, Löbdergraben 32, D-07751 Jena, Germany. Tel.: +49 3641 947730; fax: +49 3641 94 77 32. ∗∗ Corresponding author. E-mail addresses: [email protected] (K.D. Jandt), [email protected] (R.W. Mills). 0109-5641/$ see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.02.003
Transcript
Page 1: A Brief History of LED Photopolymerization

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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 605–617

Available online at www.sciencedirect.com

jo ur nal home p ag e: www.int l .e lsev ierhea l th .com/ journa ls /dema

eview

brief history of LED photopolymerization

laus D. Jandta,b,∗, Robin W. Mills c,∗∗

Faculty of Physics and Astronomy, Friedrich-Schiller-University Jena, Löbdergraben 32, D-07751 Jena, GermanyJena Center for Soft Matter, Friedrich-Schiller-University Jena, GermanySchool of Oral and Dental Sciences, University of Bristol, Lower Maudlin Street, Bristol BS1 2LY, United Kingdom

r t i c l e i n f o

rticle history:

eceived 26 October 2012

eceived in revised form

2 February 2013

ccepted 12 February 2013

eywords:

ED

hotopolymerization

ight curing

hotocuring

uartz-tungsten-halogen

aterials science

linical aspects

a b s t r a c t

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 biomate-

rials, 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 gener-

ally 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.

© 2013 Academy

∗ Corresponding author at: Faculty of Physics and Astronomy, Friedrich-el.: +49 3641 947730; fax: +49 3641 94 77 32.∗∗ Corresponding author.

E-mail addresses: [email protected] (K.D. Jandt), R.W.Mills@bristo109-5641/$ – see front matter © 2013 Academy of Dental Materials. Puttp://dx.doi.org/10.1016/j.dental.2013.02.003

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Schiller-University Jena, Löbdergraben 32, D-07751 Jena, Germany.

l.ac.uk (R.W. Mills).blished by Elsevier Ltd. All rights reserved.

Page 2: A Brief History of LED Photopolymerization

606 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 605–617

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6052. Basic physics and technology of LEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6053. Materials science of dental materials photopolymerized with LED LCUs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6084. Evolution of commercial LED LCUs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6115. Clinical aspects of LED LCUs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6136. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

. . . . .

. . . . .

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

“All truth passes through three stages. First, it is ridiculed.Second, it is violently opposed. Third, it is accepted as beingself-evident.”Arthur Schopenhauer

1. Introduction

The introduction of resin-based dental materials near themiddle of the last century was a revolution in restorative den-tistry. Dental composites are esthetically pleasing since theypossess a tooth-like appearance, are stable within the oralenvironment, are relatively easy to handle and set on com-mand via self curing or light curing.

Today, almost all commercial dental composites utilizephotopolymerization reactions initiated by blue visible light.Light curing units (LCUs) based on different physical prin-ciples, such as quartz-tungsten-halogen (QTH) bulbs, laser,plasma arc lights, and light emitting diodes (LEDs) are avail-able. Nevertheless, LED LCUs are currently the standarddevices in most modern dental practices.

In many cases, clinicians using LED LCUs on a daily basisare unaware of the physics and/or history of their develop-ment. This knowledge, however, is essential so that LED LCUscan be used to their full potential and are applied appropri-ately in any particular clinical situation.

In addition, there is currently no scientific review paperfocusing on the LED photopolymerization of available dentalmaterials. This review, therefore, addresses this need and isbased mainly on peer-reviewed and frequently cited researcharticles in international journals available through the Webof Science. This current paper reviews the history of LEDphotopolymerization in the area of oral biomaterials/dentalmaterials. Within this framework, the basic principles of LEDs,the history and evolution of commercial LED LCUs, the mate-rials science of dental materials photopolymerized with LEDLCUs, aspects of commercial LED LCUs as well as their clinicalapplications are discussed.

2. Basic physics and technology of LEDs

LEDs are a part of our daily lives. LED technology is appliedin modern light sources for room lighting, car headlights

and dashboards, traffic lights, state-of-the-art television flatscreens or as LASER LEDs in CD or blue-ray DVD data/videostorage equipment [1]. Compared to conventional lightsources, LEDs are small and energy efficient. Hence, dental

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 614

light curing units (LCUs) based on LEDs are relatively small andcan be battery powered, using high performance nickel–metalhydride (NiMH) or lithium-ion (Li-ion) batteries [2].

Users of dental LCUs are often not fully aware of the physicsof these devices. Because light emission of LED LCUs differsgreatly from that of other, more traditional types of LCUs, it isworth having a closer look at the physical principles of LEDs.This knowledge may not only help to better understand howLEDs work, but it may also contribute to the appropriate useof LED LCUs in clinical practice and to recognize the strengthsand limitations of these devices in daily use.

LEDs are semiconductor-based photonic devices in whichthe elementary particles of light (photons) play the key role.LEDs convert electrical energy into optical radiation [3]. Ithas been known for more than one hundred years that lightcan be generated if an electric current passes through amaterial under bias [3]. This phenomenon is called elec-troluminescence and was discovered in 1907 in the naturalsemiconductor silicon carbide [4].

LEDs emit light under forward biased conditions. To under-stand this, one has to consider the energy states of electrons inthe semiconductor. Through the quantum mechanical inter-action of large numbers of atoms (∼= 1025) and electrons in thesolid state, the electronic states may spilt into very closelyspaced electron states called electron energy bands [5]. Thevalence energy band is associated with the highest energy,occupied with electrons at 0 K. The band with the next higherelectron energy is called the conduction band and also con-tains no electrons at 0 K. The valence and conduction bandsare separated by a band gap in which the Schrödinger equationhas no solution, i.e., no electrons are allowed in this band gapunder normal circumstances. For semiconductors, the typicalband gap energies are generally less than 2 eV [5]. Fig. 1 showsthe band structure of an intrinsic semiconductor.

In intrinsic semiconductors, the electrical behavior is basedon the electronic structure inherent in the pure material,such as silicon [5]. If impurity atoms are introduced to dic-tate the electrical behavior of the semiconductor, it is calledan extrinsic (doped) semiconductor. LEDs use both, n-type andp-type doped extrinsic semiconductors, indicating the major-ity charge carriers are electrons or holes, respectively. Dopinga semiconductor with atoms of the group VA of the peri-odic table creates n-type extrinsic semiconductors, whereasdoping with atoms from the group IIIA of the periodic tablecreates p-type extrinsic semiconductors [5]. The former leads

to new, so-called electron donor states in the band gap justbelow the conduction band, whereas the latter leads to accep-tor states in the band gap just above the valence band.
Page 3: A Brief History of LED Photopolymerization

d e n t a l m a t e r i a l s 2 9

Energy

0

Valen ce Band(Filled)

Conduc�on Band(Empty)

Band GapEg

Fig. 1 – Band energy structure of a typical intrinsicsemiconductor at 0 K. The valence band is filled withelectrons and the conduction band is empty. Both bandsare separated by a band energy gap (typically <2 eV). Forextrinsic (doped) semiconductors there are donor statesjust below the bottom of the conduction band (n-typeextrinsic semiconductors) or acceptor states just above thetop of the valence band (p-type extrinsic semiconductors).As a result, the conductivity of extrinsic semiconductors isgreater than the conductivity of intrinsic semiconductors.

t[t(crom(

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priately referred to as poly-wave LED LCUs.Conventional QTH LCUs cannot compete in this respect

Fifty years ago in 1962, Holonyak and Bevacqua reportedhe emission of coherent visible light from GaAsP junctions6]. This event can be considered to be the birth of LEDs emit-ing visible light [7]. The first visible light LEDs were expensiveapproximately 200 USD per unit) and had very few practi-al applications [7]. The first visible light GaAsP LED emitteded light and had a luminous efficacy of 1.5 lumens per Wattf electrical power consumption (lm W−1) [3]. In the early toid-1980s, these types of LEDs were supplemented by green

GaP:N) and yellow (GaAsP:N) types, respectively.For materials in which electron and hole conduction exists,

he electrical conductivity is strongly dependent on the num-er of electrons and holes [5]. For practical purposes, onlyhese doped semiconductors are suitable as materials forEDs. The LED is a diode having p- and n-doped semi-onductors that are connected by a junction (p–n-junction).his arrangement ideally allows, under a forward biased con-ition, an electric current flowing from the p-doped side

anode) to the n-doped side (cathode), whereas no currentideally infinite resistance) is observed under reverse biasedonditions. This effect is based on interactions of the electri-al field and the different electron and hole concentrations,espectively, at the p–n-junction [3].

In doped semiconductor devices, such as LEDs (Fig. 2), elec-rons and holes (also called defect electrons) contribute to thelectric current. Under a forward biased condition (electronsntering the p-doped semiconductor side and exiting throughhe n-doped layer), electrons and holes recombine near the–n-junction of the diode and emit photons: an electron inhe conduction band can spontaneously return to an emptytate in the valence band [3], during which a photon is pro-uced as well as heat. The frequency of photon emission is

ontrolled by the chemistry of composition of both the p- and-doped materials.

( 2 0 1 3 ) 605–617 607

The width of the band gap is fixed for a particular semicon-ductor and determines the emitted photon wavelength and,thus, the color of light generated, leading to the characteristicnarrow light emission spectrum of LEDs. For example, a rela-tively wide band gap electron transition results in the emissionof blue light, whereas a narrow band gap electron transitionresults in emission of red light from LEDs. The band gaps insemiconductors can be either direct or indirect. For LEDs, thedirect band gap is most important, because this value influ-ences the ability to emit photons through direct conductionband–valence band transitions. This ability means that pho-ton absorption or emission through band transitions is basedon the k-selection rule, where k is the crystal-momentum (k-vector) [3]. The wave vectors of the valence band wave function(k1) and of the conduction band wave function (k2) must differby the amount of the emitted photon wave vector, consideringthe physical conservation of momentum [3].

In the early 1990s, the standard LCUs in dental practicesand clinics used incandescent QTH light sources in combina-tion with a blue light (band-pass) filter to generate blue visiblelight. Blue light is necessary to initiate the setting reaction(photopolymerization) in most light cured dental materials.For the realization of the radically new approach of LED LCUs[8,9], at least two prerequisites had to be fulfilled. The firstprerequisite was the availability of blue LEDs of proper emis-sion wavelength, the second was that blue LEDs needed tohave sufficient emitted power to cure dental materials withina reasonable time (20–40 s). Although blue LEDs with a suit-able wavelength emission were available from 1971 [7], theirpower emission was much too small to be used for clinicallyrelevant exposure times. This situation changed in 1994 whenNakamura at the Nichia Corporation developed high bright-ness GaN LEDs [10]. From that point in time, the developmentand scientific exploration of LED photopolymerization of oralbiomaterials began.

LEDs have a number of intrinsic advantages which makethem ideally suited for the photopolymerization of oral bio-materials. The typical spectral line width of light for LEDs is5–20 nm [3]. Compared to the emission spectra from all otherLCUs, with the exception of narrow band argon-ion lasers, thisis extremely narrow (Fig. 3). This narrow emission range is themajor advantage of LED LCUs compared to light emitted fromLCUs based on different principles, because photo initiatorspresent in oral biomaterials have light absorption spectra withdistinct maxima. If the wavelength of the LED LCU is chosenin this range, effective and rapid photopolymerization is theresult.

For oral biomaterials containing more than one photoinitiator with different light absorption spectra, LED LCUsemitting multiple wavelengths can be employed [11]. TheseLED LCUs are sometimes called broad band LED LCUs. Thisterm, however, is not correct because the LED emission charac-teristics (see above) apply. Rather than a broad light spectrum,two or more distinct narrow wavelengths bands are emittedfrom this collection of different wavelength LEDs, each with adistinct maximum. Thus, these types of LCUs are more appro-

with LED LCUs because the emission spectrum of the QTHlamp emits a relatively broad visible light spectrum [9], much

Page 4: A Brief History of LED Photopolymerization

608 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 605–617

Fig. 2 – (a) The energy band diagram of a pn+ (heavily n-type doped) junction without any bias. Built-in potential V0

prevents electrons from diffusing from the n+ to the p side. (b) The applied bias reduces V0 and thereby allows electrons todiffuse or be injected into the p-side. Recombination around the junction and within the diffusion length of the electrons inthe p-side leads to photon emission.Courtesy of Professor S. Kasap, University of Saskatchewan, Can

Fig. 3 – Flux and irradiance of a typical QTH LCU and a LEDLCU. The narrow maximum of the light emission of the LEDLCU coincides with the maximum absorption spectrum thecamphorquinone photoinitiator present in oralbiomaterials. Much of the light of the QTH LCU is emittedoutside the maximum absorption spectrum of thecamphorquinone photoinitiator.

Reprinted from [22] with permission from Elsevier.

of which is useless for photopolymerization and is dissipatedas heat. In addition, these lamps require infrared-blockingblue bandpass filters. These filters can deteriorate over timedue to the substantial heat the lamp produces.

ada.

The second major advantage of LEDs for photopolymer-ization is their efficiency. The external quantum efficiency isdefined as the number of photons emitted externally dividedby the number of charge carriers passing through the p–n-junction [3]. Due to the internal properties of the p–n-junctionand to the differences in refractive indices at interfaces ofthe semiconductor and the ambient air, there are losses ofphotons (i.e., they are not emitted from the semiconductor).Three mechanisms reduce the quantity of photons emittedfrom the LED: (1) internal absorption, (2) Fresnel loss and (3)critical angle loss [3]. The Lambertian emission pattern, i.e.,the angular dependence of light emission is controlled by pla-nar, hemispherical or parabolic epoxy encapsulates on the LEDsemiconductor [7].

An important measure of a LED is its luminous efficacy (theratio of luminous flux emitted to electrical power consumed)and is a measure of how well a light source produces visiblelight [12]. For LEDs, the luminous efficacy is typically in theorder of 60 lm W−1. LED luminous efficacies of 150 lm W−1 havebeen attained [13] for white LEDs whereas typical QTH lampspresent in LCUs, have a luminous efficacy of only 25 lm W−1

[14].When comparing commercially available blue LEDs with

conventional QTH lamps in terms of efficiency, the lumi-nous intensity efficiency � is the measure that is most easilyobtained from technical data sheets and this is more usefulthan the external quantum efficiency. The luminous inten-sity is a photometric measure of the optical radiant intensitywhich is weighted by the sensitivity of the human eye, withthe peak at 555 nm. The luminous intensity Iv of a commercialblue LED with an apex angle of 10◦ which consumes 6.4 mWelectrical power is about 9.3 cd [15]. Thus, the luminous inten-sity efficiency for a LED is �LED = 9.3 cd/6.4 mW = 145 cd W−1 ofblue light. A 75 W QTH lamp with the same apex angle of 10◦

yields 7500 cd [16]. The luminous intensity efficiency of a QTHlamp is, therefore, �QTH = 7500 cd/75 W = 100 cd W−1 across thewhole of the visible spectrum.

Another important advantage of LED LCUs is their over-all energy efficiency in terms of energy required for a curecycle. A contemporary LED LCU that has one 5 W LED chip will

Page 5: A Brief History of LED Photopolymerization

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perate approximately 25 min from a fully charged battery,hich may require 10 h at 2.5 W to recharge [17]. Assum-

ng a light curing cycle of 20 s, 75 curing cycles are possiblerom a charged battery and the energy required for this use is5 W h−1, corresponding to 0.34 W h−1 per curing cycle.

In contrast, operating a typical 150 W QTH lamp basedCU, equipped with a 75 W (12 V) QTH source, for the sameime (25 min), the energy required would be approximately2.5 W h−1. This value corresponds to 0.84 W h−1 per 20 s cur-ng cycle: approximately 2.5 times the energy a LED LCU needso perform the same task as can be easily calculated.

LEDs can have a typical lifetime of 100,000 h or more andndergo little degradation of light output over this time, if notver-driven [18]. This level of durability is a distinct advantagehen compared to the characteristics of QTH lamps [9] whichave an effective lifetime of approximately 50 h [19].

. Materials science of dental materialshotopolymerized with LED LCUs

here are several well-known examples in the history of sci-nce that show that change and new ideas are not alwayselcomed enthusiastically [20] and can be even opposed. This

ituation can be particularly the case if this change hap-ens quickly and has the character of a paradigm shift. Toome extent, this circumstance was also the case with thentroduction of LED LCUs to photopolymerization of oral bio-

aterials. The authors recall submitting one of their firstcientific papers reporting the novel concept of LEDs used forhe photopolymerization of oral biomaterials to a renownednternational journal. This manuscript was rejected, based on

referee’s comment essentially stating that one cannot cureental materials with LEDs.

In the early 1990s, the most common LCUs in dentistry wereTH units. Degradation of the QTH lamp, filter aging, con-

amination of the light-guide tips, and several other factors,otentially reduced the light output of these devices over time.evertheless, users of LCUs did generally not monitor the lightutput of these devices over time and the radiometers neededo do so were not available in most dental practices.

Against this background, in 1995 Mills wrote a letter to theritish Dental Journal wondering if LEDs could be used as anlternative light source for the photopolymerization of den-al composites [8]. Shortly before this letter was published,he Nichia Corporation in Japan introduced high intensity blueaN LEDs [10] having an emission peak around 465 nm. The

ormer letter mentioned further a simple photopolymeriza-ion test of one composite sample [8].

The first scientific peer-reviewed article published in annternational journal presenting data on photopolymeriza-ion of three contemporary dental composites with a LEDCUs was published a few years later [9]. In this paper, theypothesis was tested, that a blue LED LCU can produce anqual composite depth of cure as a QTH LCU adjusted toive the minimum effective irradiance accepted at that time:

00 mW cm−2. This paper is a milestone in the history of LEDhotopolymerization and to date, is the most cited article onED photopolymerization of dental composites [9]. The rea-ons for this acknowledgment are threefold. First, the article

( 2 0 1 3 ) 605–617 609

presented a new technological approach for photopolymeriza-tion of oral biomaterials and for the first time a new powerfulLED LCU was introduced in a peer reviewed scientific article[21]. At that time, single blue LED emitters were not strongenough to emit enough light to polymerize dental compos-ites satisfactorily and within a reasonable time. Therefore, thisfirst LED LCU used 25 blue LEDs in an array. Second, the paperpresented the first graph showing simultaneously, the spec-tral flux and spectral irradiance of a conventional QTH LCUand a LED array based LCU. In addition, it was also shownthat the light emitted from the LED LCU correlated well withthe peak of the absorption of the camphorquinone photoini-tiator. Much of the light emitted by the QTH LCU, however,was not effective for photopolymerization in composites usingthe camphorquinone photoinitiator. Third, the paper demon-strated that a LED LCU with an irradiance of 290 mW cm−2

was able to cure three popular commercial dental compositesto statistically significantly greater depths of cure than was aQTH LCU which had a 60% greater irradiance of 455 mW cm−2.Although the light emission characteristics of QTH LCUs andthe light absorption characteristics of the photoinitiator cam-phorquinone were already known, this article again raisedawareness of the characteristics of the light emitted fromdifferent LCUs and their effects on the properties of the pho-topolymerized materials. In addition, this work triggered anew dynamic of research in the area as is evident from thehigh number of citations of this paper in subsequent publishedresearch.

In a further study, the depths of cure and compressivestrengths of a dental composite of two different shades,each polymerized with a QTH LCU or a LED LCU, respec-tively, were compared [22]. The QTH LCU had an irradianceof 755 mW cm−2 whereas the LED LCU delivered 350 mW cm−2

emitted from 27 blue LEDs (Fig. 4). In addition, an advancedcharacterization of the light of both LCUs was performed.Both LCUs were able to produce a depth of cure exceeding therequirements of ISO 4049 and the compressive strength of thecomposites did not statistically significantly differ betweenthe LCUs, despite that fact the LED LCU had less than halfthe irradiance of the QTH LCU. This result was explainedby the light emission spectrum of the LED LCU providingmore radiant energy within the peak spectral range of cam-phorquinone than that measured from the QTH device. Alater study confirmed that camphorquinone has practicallythe highest photopolymerization efficiency when irradiatedwith blue LEDs compared to other photoinitiators, despiteits rather low polymerization quantum yield [23]. This find-ing showed clearly that LED LCUs were more effective in thephotopolymerization of dental composites than were QTHlights.

A subsequent larger study using three composites from dif-ferent manufacturers and three different shades, confirmedthe ability of LED LCUs for effective composite polymerization[24]. This study also presented a first simple physical modelfor the curing effectiveness of the dental LED LCU based onthe convolution of the absorption spectrum of the photoinitia-

tor camphorquinone present in composites and the emissionspectra of LCUs. This work also showed that a comparisonof the LCUs’ total irradiance is not sufficient to anticipatethe performance of LCU in photopolymerization but instead
Page 6: A Brief History of LED Photopolymerization

610 d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 605–617

Fig. 4 – (Top) An LED LCU research prototype by the authors.It contains 27 LEDs and uses a tapered optical light guidewith a tip diameter of 6 mm (total length of the LED LCU is27 cm). (Bottom) LED array used as light source for the LEDLCU shown containing 27 shaped blue LEDs arranged inconcentric rings. The diameter of the LED array is 15 mm.

Fig. 5 – Spectral irradiance of the LCUs (lines) used in thisstudy and the spectral extinction of the photoinitiatorscamphorquinone and Lucirin TPO (lines and symbol). Theabsorption spectrum of camphorquinone (according tomanufacturer, Rahn Zurich, Switzerland (0.1% inmethanol)) shows the good match with the LED emissionspectra whereas the co-initiator Lucirin TPO (according tomanufacturer, BASF Ludwigshafen, Germany (0.1% inethanol)) does not coincide with the emission of the LEDunits. The broad emission spectrum of the halogen unitshows an overlap with the extinction spectrum of theco-initiator at approximately 375–410 nm.

Reprinted from [22] with permission from Elsevier.

a comparison of the mechanical properties of compositesresulting from each type LCUs is more appropriate.

One year later, in 2001, the hardness of a dental compositephotopolymerized with an LED LCU based on six LEDs (irradi-ance 79 mW cm−2) and a QTH LCU (irradiance 475 mW cm−2)confirmed the greater curing potential of LED LCUs [25]. Never-theless, all samples photopolymerized by the LED LCU showedsmaller hardness values compared to the sample cured withthe QTH LCU.

The degree of double bond conversion of composites wasthen found to be significantly influenced by the light sourcetype and by the energy level (energy density), in addition tothe parameters of material and depth [26]. This study usedan LED, a QTH and a plasma arc LCU. When the same lightenergy was applied, the degree of double bond conversion ofthe composites was not different among the different LCUs upto a depth of 2 mm. The differences in degree of double bondconversion, however, were only significant at depths >3 mm.

By the early 2000s, the first commercial LED LCUs becameavailable. Hence, a published report compared the Barcolhardness and the compressive strength of composites pho-topolymerized for 20 s or 40 s with two different LED LCUprototypes (27 and 54 LEDs), a first commercial LED LCU (7LEDs) and a QTH LCU [27]. While composites photopolymer-ized by the two LED LCU prototypes and the QTH LCU showedsimilar hardness and compressive strengths, the correspond-

ing values of composites exposed using the first commercialLED LCU were statistically significantly smaller. The commer-cial LED LCU lacked the power reserves of the other LCUs,

Reprinted from [34] with permission from Elsevier.

especially for shorter curing times (20 s). At this time (2002),commercial LED LCUs were obviously still no match for QTHLCUs in terms of performance. On the other hand, prototypeLED LCUs having larger numbers of LEDs performed as well asQTH LCUs.

Similar results were found in a later study in 2002, whichcompared composite depths of cure with materials having dif-ferent shades, when photopolymerized using commercial LEDLCU (7 LEDs) and an LED LCU prototype (63 LEDs) [28]. Thisstudy also revealed that the commercial LED LCU was pulsedat 12 Hz. In addition, refined spectral power data of the LCUswere presented.

In the same year, a study was published that investi-gated the Knoop hardness and depth of cure profiles offour different dental composites photopolymerized using aQTH LCU adjusted to the same irradiance as a LED LCUprototype (approximately 570 mW cm−2) a QTH LCU and acommercial LED LCU, having irradiance values of (1144 and122 mW cm−2, respectively) [29]. Two of the composites con-tained co-initiators in addition to camphorquinone (Fig. 5).Composites photopolymerized with the commercial LED LCUshowed a statistically significantly lower hardness for allmaterials and exposure times, whereas the LED LCU prototypeperformed as well as the QTH LCUs up to a composite depthof 1.9 mm. For greater composite depths of 3.1 mm, the QTHLCUs performed better than the LED LCU prototype in terms

of composite hardness. There were, however, no statisticallysignificant differences in compressive strengths of compos-ites between the QTH LCUs and the LED LCU prototype. This
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tudy demonstrated three essentials of the state-of-the-art inhotopolymerization at that time: (i) commercial LED LCUsere “not quite there” yet, i.e., the curing potential of com-ercial LED LCUs was less than that of conventional QTH LCU;

ii) a laboratory LED LCU prototype performed as well as highrradiance QTH LCUs for certain composites; (iii) in order toerform as well as QTH LCUs, the LED LCUs needed severalEDs, because the irradiance produced by one single LED wasot sufficient for practical use.

Shrinkage strain kinetics, temperature rise and hardnessalues were compared among different composites pho-opolymerized using two LED LCUs (160 and 320 mW cm−2,espectively) or a QTH LCU (800 mW cm−2) [30]. The LCUs wereperated in continuous or ramped mode. Composite temper-ture rise during photopolymerization was lower for the LEDCU compared to the QTH LCU, whereas the latter LCU pro-uced the fastest increase in polymerization contraction andhe largest shrinkage strains. Ramp curing decreased the con-raction speed.

The mechanical property data of dental composites curedsing LED LCUs had made it clear that the LEDs were indeeduitable for photopolymerization of dental composites. Atten-ion was then focused on other effects of LED LCUs on these

aterials, such as the temperature rise of composites duringhotopolymerization. This aspect was also of special inter-st, because from the late 1990s high irradiance plasma arcCUs became available that had a significant potential toause high temperature increases during curing. Excessiveeat applied to teeth can cause irreversible pulpal trauma,hen the pulp temperature exceeds 42.4 ◦C [31,32]. In addi-

ion, soft start QTH LCUs entered the market near the year000. Therefore, a key milestone study investigated temper-ture rise and propagation in three different composites ofwo different shades that were photocured using two dif-erent LEDs or two QTH LCUs [33]. The results showed thatomposite’s shade influenced temperature increase and thatomposites exposed to QTH LCUs reached significantly higheremperatures compared to those cured using LED LCUs. Anxception was a composite cured using the soft start modef a QTH LCU. Composites of lighter shades reached higheremperature than those of darker shades when cured withED LCUs. For QTH LCUs, the situation was reversed. Further-ore, this study introduced a high-resolution infrared (HRIR)

amera to document heat propagation in composites. Thesendings demonstrated not only that temperatures of compos-

te cured using LEDs were generally significantly lower thann the same composites exposed to QTH LCUs. In addition,he heat propagation was slower and the heat penetrationower in composites exposed using LED LCUs, compared tohose cured with QTH LCUs. Because the temperature gra-ient (�T/�x) is significantly smaller in composites exposedsing LED LCUs, the resulting heat flux is smaller when com-ared with composites cured with QTH LCUs. The more pulpriendly temperature properties of LED LCU curing, withoutaving to compromise in the resulting composite mechanicalroperties, emphasized the superiority of LED technology and

as attributed to the different light emission characteristicsf LEDs compared to the QTH light.

In addition to the standard photoinitiator cam-horquinone, some dental composites contain co-initiators

( 2 0 1 3 ) 605–617 611

that absorb light at shorter wavelengths than cam-phorquinone. To investigate what effect this conditionhas on the mechanical properties of composites cured usingQTH or LED LCUs, the photoinitiator-dependent compositedepth of cure and Knoop hardness using these LCUs wasinvestigated [34] in a milestone study. The depths of curedid not discriminate between LCUs used for photopolymer-ization of composites containing photoinitiators in additionto camphorquinone, but the Knoop hardness tests did. Thestudy concluded that LED LCUs should be used with cautionwhen considered for composites containing co-initiatorsthat absorb light at shorter wavelengths than the LEDs emit.Furthermore, this study showed that a LED LCU prototypedid cure composites to greater depths than a commercialLED LCU, indicating that commercial LED LCUs did not havethe power required for challenging clinical situations at thatpoint in time, whereas the LED LCU prototype did. For thelatter, the LED LCU prototype required 63 blue LEDs arrangedin a sophisticated and compact array. Although this LEDLCU performed well, simpler LED LCU designs seemed moredesirable at that time.

This goal for a simpler LED unit design need was addressedthrough the introduction of second generation LED LCUs [35].This classification of LED LCUs is based on a single high powerLED and allows simpler LED LCU form factors. The perfor-mance of a single LED LCU with an irradiance of 901 W cm−2

was evaluated by measuring the Knoop hardness and depthof cure of two different dental composites and one ormocercomposite [35]. The single LED LCU achieved a significantlygreater depth of cure for all materials than did the QTH LCUwhich had an irradiance of 860 mW cm−2. There was no dif-ference in Knoop hardness observed for one composite orfor the ormocer composite, when cured with either LCU. Theother composite showed a significantly lower Knoop hardnesswhen polymerized with the LED LCU compared to the QTHLCU. Two lessons were learned from this study: second gener-ation LED LCUs have the potential to replace QTH LCUs, if thecomposite is chosen carefully and the depth of cure does notdiscriminate between the LCU performances, whereas Knoophardness testing does. The development of single LED LCUswas, thus, another major milestone in the advancement of thistechnology for dental applications. Although arrays of discreteLEDs are no longer normally used in dental LED LCUs, sucharrays may be utilized in the future in LED-based high powerroom lightening systems [36].

As shown previously, some composites that contain co-initiators in addition to camphorquinone showed lowerhardness when photopolymerized with LED LCU than thesame composites photopolymerized with QTH LCU [34]. Thus,a test was performed to determine if progressive crosslinkingof the resin phase of such composites after photopolymeriza-tion (post cure hardening) would eliminate this effect [37]. Themeasured increase in post-cure hardening, however, was notsufficient to compensate for the difference in hardness of thecomposites. An interesting additional result of this study wasthat the composite Knoop hardness depends on the indenter

load and has the potential to falsify such measurements if notchosen carefully. These results demonstrated that LED LCUshad matured and could more than match the performanceprovided by QTH LCU technology at that time, however, with
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a slight restriction of application to certain composites con-taining co-initiators. The performance of LED LCUs becameclinically satisfactory [38].

Because most manufacturers of direct resin based restor-ative materials do not provide details about the photoinitiatorscontained in their products, it is challenging to choose anoptimal combination of restorative resin and light-curing unit[39]. Narrow blue LED light spectra emission is ideal for pho-topolymerization of camphorquinone containing materials,but when other photoinitiators, such as 1-phenyl-1,2-propanedione (PPD) are present in very light composite shades(high color value) used for repair of bleached teeth [40], thismay not be the case. This situation was addressed by manu-facturing dual-wave (or “poly-wave”) LED LCUs, emitting in theviolet (410 nm) and blue (470 nm) of the light spectrum [40].Third generation, poly-wave LEDs are used in these LED LCUs(see section Evolution of Commercial LED LCUs).

This progress demonstrating the performance of highpower single LED LCUs paved the way for the clinical massapplication of LED LCUs. Nevertheless, more data from com-posites photopolymerized with LED LCUs were needed toconvince the remaining skeptical minds clinging to the useof conventional QTH technology LCUs. Thus, distinctions ofthe time dependence of composite shrinkage between use ofQTH and LED LCUs was tested [41]. In addition, the reliabilityof the light output was tested up to 360 min of duty time, for aprototype LED LCU, a commercial LED LCU and a QTH LCU. Dif-ferent composites showed similar shrinkage behavior whencured using the LED or QTH LCUs, with one exception [41]. Inthe course of this study, the irradiance of the commercial LEDLCU decreased significantly, whereas the emission spectrumwavelength characteristics of the QTH LCU changed dramati-cally. The prototype LED LCU showed almost no change of lightemission spectrum or irradiance over time.

In addition to the power of an LED LCU, further parame-ters are important for its photopolymerization potential. Animportant aspect affecting the LED LCUs irradiance is thediameter of the light guide tip. At a composite’s depth of2 mm, a 4 mm diameter light guide tip produced a signifi-cantly greater composite hardness than did a 8 or a 10 mmtip [42]. When irradiation times were doubled or tripled for8 mm and 10 mm tips, hardness values eventually rose to thelevels of those when using a 4 mm tip. In other words, extend-ing exposure time may compensate for the lower irradiancefound when using larger diameter light guide tips.

It was found later with LED LCUs, that exposing dentaladhesives to a longer time than recommended by the manu-factures of the adhesives led to a decrease of permeability ofthe bonded interfaces [43]. On the other hand, a study inves-tigating the minimal exposure time of different LED LCUs,demonstrated that many composites, when incrementallyexposed to LED light, were polymerized sufficiently at a maxi-mum of 20 s [44]. This finding underlines that LED LCUs of thelatest generation are not only efficient but are also devicescapable of fast photopolymerization.

At the beginning of the second decade of this current cen-

tury, scientific investigation of LED photopolymerization oforal biomaterials is still very active. Topics of this researcharea include investigations of the influence of LED LCUs onthe cytotoxicity of dental adhesives [45], evaluation of the

9 ( 2 0 1 3 ) 605–617

mechanical properties of composites polymerized using LEDsand with other LCUs [46], curing efficiency of high-intensityLED LCUs [47], depths of cure and gel state-to-glass transitionof composites [48], polymerization efficiency of modern LEDLCUs [49], the never-ending quest for the shortest exposuretime possible with LED LCUs [50]. Beyond this, LED lights arebecoming popular in photodynamic therapy in the oral cavity[51].

4. Evolution of commercial LED LCUs

It was not until 2000 that the first commercial LED LCU becameavailable: the LuxOMax LED LCU [46] was a large pen-likecordless battery-powered design using 7 discrete LEDs. Thedevice had a tapered fused glass fiber light guide to concen-trate the light output at the tip. The unit’s irradiance measured116 mW cm−2 [8]. The delay in bringing a commercial LED LCUinto production was due to two main factors: the power outputof the blue LEDs and their cost. The discrete blue LEDs usedin these early experiments in the 1990s were so-called sin-gle chip, epoxy-resin encapsulated, through-hole devices ofeither the standard T-1 (3 mm diameter LED) or T-1 3/4 (5 mmdiameter LED) versions.

In 1995, the 5 mm blue LED available from Nichia, hadan optical power output of 1.2 mW and cost approximately50 USD [52]. In 2012, Nichia [53] offered 5 mm blue LEDs witha power output of 123 mW at an approximate cost of 0.50 USD[54]. This comparison shows how over a period of seventeenyears in one type of LED, its power increased by two ordersof magnitude, while its cost has decreased by two orders ofmagnitude. This data translates to approximately a cost of40 USD mW−1 of blue light output in 1995 to 0.004 USD mW−1

in 2012. The increase in the external quantum efficiency ofLEDs has an advantage for commercial LED LCU designers.As a greater proportion of the electrical energy driving theLED is converted into emitted light energy, the heat sinkingrequirements of the LEDs become less demanding. This aspectis important, as smaller heat sinking requirements mean apotential reduction in the size, weight and cost of commercialLED LCUs.

The first generation of LED LCUs on the market in 2000,such as the previously mentioned LuxOMax LED LCU, wasthus relatively low-powered, compared to their conventionalQTH LCU competitors. This difference led to some disap-pointing early results, when commercial LED LCUs werecompared with commercial QTH LCUs [28]. In 2002, theseearly comparisons of commercial LED LCUs with QTH LCUsources produced some premature negative predictions forthe future of LED LCUs [55]. Later in 2002, however, it wasdemonstrated that an experimental LED LCU could exceedthe performance of a QTH LCU [2].

First generation commercial LED LCUs improved rapidlyand mirrored the advances and efficiency achieved in blueLED semiconductor research and development. In 2001, theEliparTM FreeLight having 19 discrete LEDs became available,

with an irradiance of 400 mW cm−2 [56]. The manufacturerclaimed this output level was equivalent to a conventional800 mW cm−2 QTH LCU, and studies were quoted in their tech-nical product profile [57]. This more powerful first generation
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ED LCU still used a tapered fused glass fiber light guide tourther boost the irradiance.

The second generation of LED LCUs is regarded as a moverom devices that employed multiple discrete LEDs to thosehat contained the more powerful single LEDs, often speciallyesigned for dental curing applications. Examples of these sin-le more powerful LED emitters are the Luxeon LXHL-BRD1n 2004 [58] with an electrical consumption of 1400 mW torovide 140 mW of blue light, to the more recent LED Engin

nc., LZ4-00B200 in 2012 with an electric consumption of160 mW to give 3800 mW of blue light [59]. Thus, in the firstase 10% of the electrical power is converted into blue light,hile in the second case 30% conversion was attained, a 3-fold

ncrease in efficiency within only a time span of 8 years. Anxample of a single LED, second generation LED LCU using

LED was the EliparTM FreeLight 2 [60]. The irradiance ofhis LED LCU was approximately 1000 mW cm−2 and repre-ented an increase of 2.5 times that of its predecessor: theliparTM FreeLight that used 19 discrete LEDs. It should beoted that a tapered waveguide (the so-called “turbo-tip”) wastill employed to boost emitted irradiance and nickel metalydride batteries were used to supply the operating power

61]. Even in 2005, another second generation LED LCU, hav-ng a conventional pistol grip design, the Kerr L. E. DemetronI, still incorporated a fan, which, considering the efficiencyf LEDs, may have indicated a design compromise with theeat sinking component [62]. The L. E. Demetron II LED LCU,

ike others of this era, used the technique of a tapered lightuide to boost irradiance. Other designs of second generationED LCUs, such as the Dentsply SmartLite PS did not utilize aonventional light guide. This unit was a slim, pen-style unitike the LuxOMax, but had the LED source at the tip of thenit, adjacent to the tooth [63]. Dentsply also announced in012, the imminent launch of a newer version, the SmartLiteocus having a lens system designed to allow effective cur-ng for up to 8 mm from the composite [64]. This techniqueas the advantage of less light loss as there is no light guideetween the light source and the composite. This advantage inurn results in a reduction in electrical power and heat genera-ion if all other parameters remain the same. This design alsoets out to overcome the two main disadvantages of taperedight guides: first the light beam is more divergent. This isaused by a progressive increase in the angle of total inter-al reflection as the light propagates through a tapered fiber,esulted in an exaggerated loss of irradiance compared with

more parallel beam as the LCU tip is moved further awayrom the tooth. The second is the smaller light tip area whichequires repeated overlapping exposures to adequately coverhe composite area. More recently, a second generation LEDCU the “EliparTM S10” (manufactured by 3M ESPE) having a0 mm diameter parallel glass fiber light guide capable of pro-ucing an irradiance of 1200 mW cm−2 has become available

65]. This tip delivers not only four times the minimum irradi-nce of 300 mW cm−2 [66,67] required for a conventional QTHCU, but it is known that the spectral output from an LED LCUs more effective than the equivalent QTH LCU of the same

−2

rradiance [68]. This 1200 mW cm irradiance over a 10 mmiameter parallel light guide is a remarkable advance madeossible by the increase in power and reduction of cost ofEDs. An optical power increase of over 50% is required to

( 2 0 1 3 ) 605–617 613

achieve the same irradiance over a 10 mm diameter light guidecompared with the standard 8 mm version.

As mentioned above, these first and second generationcommercial LED LCUs both consist of blue LEDs designed topeak in the absorption spectrum of camphorquinone (CQ),which is the sole photoinitiator in the majority of commercialcomposites. The incorporation of other photoinitiators hasled to the development of third generation commercial “poly-wave” LED LCUs. The first of these was the pen-like Ultralume5 (Ultradent Products Inc., South Jordan, UT, US) in 2003 [69].This comprised two different LED wavelengths, a central blueLED surrounded by 4 violet LEDs at the end of the curing tip.A more sophisticated dual wavelength LED LCU version calledthe bluephase 20i (Ivoclar Vivadent AG, Schaan, Principality ofLiechtenstein) has a maximum irradiance of 2200 mW cm−2

[70,71]. This LED LCU has 3 blue LED emitter elements anda shorter violet LED wavelength emitter mounted on a sub-strate in a square configuration: 1 emitter in each quadrant.The bluephase 20i has a more traditional gun-shape with apistol grip, conventional fused glass fiber light guide, and acooling fan. Another LED LCU, the VALO (Ultradent ProductsInc., South Jordan, UT, US) sought to improve on this stillfurther in 2011 by using 4 emitters in a similar square con-figuration to the bluephase 20i. However, in the VALO, thereare two blue emitters (439 nm) diagonally opposite to eachother and also two different shorter wavelength emitters diag-onally opposite to each other in the other quadrants: one violet(405 nm) and another, longer-wavelength blue (460 nm) [72].The VALO, thus, has 3 emitters of different wavelengths incor-porated. The heat dissipation aspect of this unit is a metal casemachined from solid aluminum rather than having a fan. Thisdesign appears to apply the innovative principle of using onecomponent for two purposes, i.e., the case and the heat sink.The VALO eliminates the light loss of a light guide by hav-ing the emitters at the curing tip. This unit has a maximumclaimed irradiance of a remarkable 3200 mW cm−2. Anotherthird generation LED LCU, the ScanWave (ACTEON GermanyGmbH, Mettmann), uses 4 LED emitters like the bluephase 20iand the VALO. In this case, however, all 4 emitters have dif-ferent wavelengths from each other and can be switched insequence. The maximum irradiance is 2200 mW cm−2 [73]. Thesophisticated programming options of the ScanWave claim tohelp optimize curing while minimizing heat generation withinthe target. Some preliminary results from this unit have beendescribed [74]. In this previous study, it is suggested that theScanWave could be regarded as a first version of a fourth gener-ation LED LCU because of the sophisticated control algorithms.Both the VALO and ScanWave appear to be good examples ofthe state-of-the-art of LED LCU evolution at the time of writ-ing. Fig. 6 shows two examples of contemporary commercialLED LCUs.

The further evolution of commercial LED LCUs is likelyto be confined to fine tuning the devices to ensure consis-tent reliable spectral outputs over the lifetime of the deviceand uniform irradiances over the whole of the emittingtip area. Previous work [75] has demonstrated the lack of

homogeneity at the end of the light guide and multiple com-mercial chip LED sources are imaged at the other end ofthe coherent light guide tip resulting in a differential curingover the target. One possible solution here to achieve more
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Fig. 6 – Two contemporary commercial LED LCUs: (a) is theSmartLite PS (courtesy of DENTSPLY Ltd., UK) and (b) is theEliparTM S10 (courtesy of 3M Espe Dental Products). Twomain approaches in LED LCU design are shown here: theSmartLite PS in which the LED is located at the end of theLCU which is placed near the tooth and the EliparTM S10 inwhich a conventional fused glass fiber light guidetransmits the light to the tip that is placed near the tooth.

wavelength-specific detectors [28].

homogeneity would be to try a non-imaging incoherent lightguide. The higher energy density of lithium-based batterytechnologies compared with earlier nickel metal hydridehas allowed commercial portable LED LCUs with enhancedendurance. Further advances in battery technology wouldimprove on this aspect. It remains to be seen whether thethird generation of commercial LED LCUs will be sustainedwith their more hazardous wavelengths [76]. This factortogether with the more limited penetration of light toward theUV–violet region [77] resulted in curing wavelengths evolvinginto the blue region of the spectrum. It may be that alternativestrategies achieve composites having high color value occurin the future, without having to accept the disadvantages ofshorter wavelengths of light.

5. Clinical aspects of LED LCUs

When commercial LED LCUs became available to dental prac-titioners after 2000, they slowly evolved and became seriouscontenders to challenge the dominance of QTH LCUs in clini-cal dentistry. It is natural for clinicians to want more powerfullight-curing devices to reduce the time taken for a given pro-cedure for patient comfort. LED LCUs can currently achieveirradiance values in excess of 3200 mW cm−2 [72]. This irra-diance value in isolation is only an approximate guide of theclinical composite’s depth of cure. By achieving higher irra-diance values, the depth of cure can be increased slightly, butthe work by Nomoto et al. reminds clinicians that a doubling ofthe irradiance, if all other parameters stay the same, equatesto only an approximate 20% increase in depth of cure [78].

Doubling the irradiance, will however, double the energy deliv-ered to the tooth of the patient. Higher power means higheramounts of heat generated within the tooth and potential pulp

9 ( 2 0 1 3 ) 605–617

damage [79,80]. It is not only the energy absorbed from the LEDLCU that is important but also the exothermic reaction of thepolymerization [33].

Teeth are sensitive to the rate of change of temperature [81]and can be cooled and warmed slowly within reason withoutcausing pain. Rapid temperature changes can cause pain, suchas drinking hot beverages followed immediately by biting intoice cream. If energy is fed into the dental pulp at a greaterrate than it can be dissipated during curing in a clinical situa-tion, then a rise in temperature will occur and irreversible pulpdamage is possible [70]. In clinical situations, where a localanesthetic is used, the protective warning mechanism of painis masked and so the patient cannot indicate when this occurs.Thus, thermal safety should be of primary consideration whenphotopolymerizing composites.

The gains obtained by increasing irradiance to the very highlevels found in the plasma arc LCUs of the 1990s are oftenquestioned. To the clinician, the composite may appear hardwhen tested with a probe but “premature vitrification” of thecomposite can occur giving the impression that adequate cur-ing has occurred.

Clinical studies have demonstrated that LED LCUs are aseffective as QTH LCUs when curing composites [82]. Thiswas also demonstrated in the area of orthodontics [83]. Thedrawback for single wavelength LED LCUs is that they cannotadequately cure composites with photoinitiators lying outsidethe absorption curve of CQ. This issue has been discussedabove and has now been addressed by “poly-wave” third gen-eration LED LCUs [84]: units having two or more differentwavelength semiconductor chips [70]. There is a fundamen-tal exponential correlation between the wavelength of lightand the penetration of the material to be cured. Scattering oflight S is inversely proportional to its wavelength [85]

S ∝(

1�4

)

and a significant factor governing light penetration of a com-posite [86].

A recent study examined 210 LCUs in government clini-cal health establishments [87]. One hundred and twenty ofthese units were QTH lights and 90 were LED LCUs. Nearly68% of the QTH LCUs and 15.6% of the LED LCUs were judgedto have failed to reach the minimum acceptable irradiance of300 mW cm−2 for clinical success. This same value was usedfor both types of LCU even though it has been demonstratedthat LED LCUs are more effective at the same irradiance whenall other parameters remain equal. It has been suggestedhow the clinician can help offset the deterioration in LCUsin the early stages of declining output by compensating forthis decline [88]. To record the reduction in output of an LEDLCU containing more than one wavelength of LED over time, itmay be necessary to have a more sophisticated radiometer ina clinical situation in which more than one detector indicatesthe reduction of the different wavelength LEDs that comprisethe LCU. Such a radiometer could be constructed by using theLEDs that comprise the LED LCU used in reverse function as

For clinical applications, LED LCUs are the current goldstandard for photopolymerization of resin based dental mate-rials. Before using LED LCUs clinically, it can be seen that the

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ain tasks are to check the wavelength compatibility with theomposite to be cured and to monitor the output in line withhe manufacturers’ guidance in combination with the advicebove.

Currently, studies evaluating the clinical performance ofight polymerized dental composites focus almost exclusivelyED LCUs. The inherent characteristics of LEDs mean that bothlinicians and patients should be able to rely on the poten-ial of a consistent and long-lasting light output. This resultill only be true if the LEDs within the LED LCU are heat-

inked correctly and powered at the recommended voltagend current ratings of the LED manufacturers’ datasheets. Ifhe clinician measures a significant deterioration in light out-ut over a relatively short period of time in an LED LCU, itould be an indication that these manufacturing rules haveot been adhered to in the design and construction of theevice. Two main requirements for LCUs in the treatmentf patients are reliability and consistency of performance.ED LCUs offer clinicians the potential to achieve both ofhese.

. Conclusions

he introduction of LED LCUs has revolutionized the pho-opolymerization of oral biomaterials. Until the introductionf LED LCUs, QTH lamp based LCUs were the standard curingevices in most dental practices. Due to the physical charac-eristics of the solid-state light emitting diodes, LED LCUs havelmost entirely replaced QTH LCUs whose inherent problemsnclude a decay of light output over time, blue light filter degra-ation, relatively limited time of life of the QTH source, highnergy consumption, bulky construction, a requirement for

mains electricity supply, and relatively high heat transfero the pulp chamber of the tooth during photopolymeriza-ion. It is, therefore, not surprising that hand-held pencil-style,attery powered, compact LED LCUs are now the standardhotopolymerization devices in most dental practices world-ide.

This paradigm change in photopolymerization develop-ent was made possible mainly by three factors: (i) the

nvention of visible light emitting diodes exactly 50 yearsgo; (ii) the introduction of high brightness blue light emit-ing GaN LEDs in 1994; and (iii) the creation of the firstlue LED LCUs for photopolymerization of oral biomateri-ls. The physical characterization of these LCUs as wellental composites photopolymerized with these LCUs andlucidation of particularities of the LED photopolymeriza-ion process from 1995 to date have been addressed in thiseview.

The LED-based photopolymerization process has becomehe gold standard of curing dental composites at the begin-ing of the 21st century. With efficient high power, single LEDCUs commercially available, it is likely that this trend willot change for quite some time, especially if one considers

he energy efficiency of LEDs, which are the green way toroduce light. The history of LED photopolymerization of bio-aterials is also a good example of how science works and

rogresses.

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Acknowledgement

We thank Dipl.-Phys. Matthias M.L. Arras, FSU Jena, for calcu-lating the luminous intensity values of the LED and the QTHlamp.

e f e r e n c e s

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engineering. An integrated approach. Hoboken: John Wiley& Sons; 2005. p. 478ff.

[6] Holonyak Jr N, Bevacqua SF. Coherent (visible) light emissionfrom Ga(As1−xPx) junctions. Applied Physics Letters1962;1:82–3.

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