Blue LEDs for Curing
Robin.W.Mills1, Klaus D. Jandt 2*
Department of Oral and Dental Science
University of Bristol
Lower Maudlin Street
Bristol, BS1 2LY UK
We report the construction of a blue LED light unit suitable for curing polymers, such as those used in dental composite filling materials. This unit makes use of blue gallium nitride LEDs as the source of visible blue flux.
Currently, most sources of visible blue flux applied in dentistry use tungsten filament halogen lamps that incorporate a blue filter to produce light in the 400-500 nm region. This light is typically directed down a waveguide such as a fused glass bundle. The end of this waveguide is placed adjacent to the soft uncured composite filling material which has been placed into a prepared tooth cavity. For fillings, a transparent plastic matrix strip is used to hold the soft filling in place during the curing process.
A typical dental composite consists of a 1:1 mixture (wt./wt.) of bis-phenol-2 bis(2-hydroxypropyl) methacrylate, (Bis GMA), and tri(ethylene glycol) dimethacrylate, (TEGDMA) monomers. In addition, this mixture contains the camphoroquinone photoinitiator and a tertiary amine as the reducing agent. The fillers which give the composite the desired properties such as hardness are typically silica particles. These particles can measure 0.04 microns in the case of microfilled composites, or have sizes of 1-3 mm and 0.04 mm in so-called hybrid composites, both having different mechanical properties.
Blue light is used to excite the camphoroquinone photoinitiator which has an absorption peak at 468 nm. This in turn stimulates the production of free radicals from the tertiary amine, causing polymerisation and hardening of the polymer composite.
The main problem of conventional halogen units is that the lamp, filter and reflector degrade with time. The heat generated by halogen lamps adversely affects filters and reflectors over time. Filters can undergo blistering, while reflectors discolour. This leads to a decrease in blue flux and reduction in curing effectiveness. The great deal of heat produced by halogen curing lamps requires intensive fan cooling, which in turn may disperse any bacterial aerosol present in the patients mouth. Other methods used for curing dental filling materials utilise xenon and LASER light sources, but these commercially available units are costly, and at present, inefficient converters of electrical power into visible blue flux. These sources use relatively large amounts of power and generate much heat. In the case of LASERs, stringent safety precautions in the surgery are required.
Blue LEDs, in contrast to these conventional sources, offer a very long lasting and relatively stable output of visible blue flux. They do not require filters, thus avoiding any attenuation of power output due to degradation. LEDs are more efficient converters of electrical power into visible blue flux, and do not generate the large quantities of heat associated with halogen lamps. Much of the spectral radiant intensity of many blue LEDs lies in the 468 nm region peak absorption of the photoinitiator, and therefore produces an almost ideal band width of the light that is required.
Until recently, blue LEDs have had too little radiometric power to be considered for polymerising the components used in dental composites. The breakthrough by Nakamura  has now meant that these components are worth considering. One of the authors  previously suggested using blue LEDs for curing dental composites.
Commercial halogen curing lamps aim to produce flux predominantly in the 400-500 nm range. Caughman et al. , have recommended a minimum irradiance of 280-300 mWcm-2 as being the minimum to effectively cure dental materials. This power density can be achieved at the present stage of development with LEDs running in a continuous wave mode. It requires the construction of an array of semiconductor chips mounted with their output being further concentrated by optical means such as an optical taper.
|Figure 1. Schematic Diagram of a typical setup using a tungsten halogen curing lamp for curing dental composites.|
By using this simple method, we have reduced power consumption by approximately 95% compared with some halogen sources. This reduced power consumption required for the LED source, still produces enough visible blue flux to polymerise a similar depth of filling material to that of some halogen curing lights. This level of efficiency and low power consumption will allow several types of battery technology to be considered to power the LED array, in addition to conventional power sources. The potential for inexpensive and long lasting dental curing light sources appears very promising.
|Figure 2. Schematic Diagram of an LED dental curing source.|
To match natural tooth colour closely, composite filling materials are supplied in a range of colour shades, with the shade A2 lying between the lightest and darkest shades available. It has been shown that darker shades of dental material attenuate the curing radiation to a greater extent than lighter shades. This may sometimes mean that either an increased irradiance or increased exposure time is required, to achieve a sufficient depth of cure of the composite. This depth of cure is a measure of the curing efficiency. The monomers in uncured composites in deep cavities may irritate the pulp (low biocompatibility). Furthermore, undercured composite restorations are likely to fail after a short period of time.
The graph in figure 3 shows the results of a curing experiment in which samples of composite in a round stainless steel mould 6 mm deep x 4 mm diameter. The composite was loaded into the mould and a thin transparent matrix was applied to the top and base of the mould, with the excess material extruded by squeezing between 2 microscope slides. The slides were then removed and the mould placed on a black background with the tip of the radiation guide applied to the transparent matrix strip over the top of the mould aperture. 10 samples were cured with the diode unit and this was repeated using a halogen unit for another 10 samples. The halogen unit was adjusted to give an irradiance of 300 mWcm-2 over the area of the 8 mm diameter fused glass bundle light guide. The depth of cure was recorded by inverting the mould onto a flat surface and lowering a penetrometer needle slowly into the uncured material until it reaches the cured portion of the composite filling. The digital depth gauge, which is zeroed before each measurement, can then be read to give a direct recording of the cure depth to the nearest 0.01mm. This penetrometer method of measuring depth of cure has been described previously . It can be seen in the graph shown above that the samples cured with the LED unit cured a mean depth of 4.00 mm compared with a mean depth of 3.82 mm for the halogen source.
|Figure 3. Graph showing the depth of cure of Spectrum
A2* composite using a halogen and LED source.
* Dentsply DeTrey D-78467, Konstanz.
This result indicates that blue LED arrays are capable of curing a similar depth of dental composite to a halogen unit, in this laboratory experiment. Further mechanical testing is required to determine the properties of composites cured in this way. Further analysis of the spectra is also required, to compare the two sources of visible blue flux and how this relates to curing effectiveness.
The blue light emitting semiconductor field is being driven by the quest for long-life blue laser diodes for increased CD ROM storage potential and improved display technologies. Improvements in blue LED construction are likely to progress rapidly as a by-product of this laser research work by rival companies and universities. As blue LEDs become even more efficient, smaller and better LED curing lights will be possible for the dentist. The authors expect a pocket torch-like device to be available for surgery as well as mobile use in the future.
It is likely that some blue LED manufacturers will recognise the dental and adhesives curing markets potential. The possibility of purpose made LEDs specifically tailored for curing applications could make an exciting development. Collimated LED packages  , would be one example of a desirable modification to reduce the cosine losses in tapered waveguides. If the angle of the light launched into the taper can be made relatively narrow, the exit angle at the narrow end will be correspondingly reduced, with the benefit of an increased irradiance. The challenge of manufacturing these collimated LED packages without a significant loss of radiometric output in the optics, however, does not appear to be an easy task.
LEDs, unlike halogen lamps, lend themselves to being driven by a pulsed supply. Pulsing can raise the level of power delivered, although the recommended duty ratio lengthens the time necessary to deliver the same quantity of energy as continuous wave mode. Pulsing, however, could be used either in isolation or in combination with continuous wave mode, to modify the characteristics of the cured filling material. For example, an initial pulse producing a high irradiance would cure deeper parts of the filling, followed by a lower intensity of continuous wave operation for the superficial layers. The complex permutations of these possibilities are sure to be the subject of further research in the future to search for the ultimate parameters to achieve an optimally cured dental filling.
The authors believe that the impressive progress in the blue light emitting semiconductor field has opened up not only an exciting future for storage and display technology, but also a totally new future for the way that dentists and other workers photoinitiate polymerisation. They are confident of producing LED dental curing devices with levels of irradiance much greater than those already achieved.
1. Nakamura S, Senoh M, Mukai T, P-GaN/N-InGaN/N-GaN Double-Heterostructure Blue-Light-Emitting Diodes. Japanese Journal of Applied Physics, Letter 1993;32:L8-L11
2. Mills RW, Blue light emitting diodes - an alternative method of light curing. British Dental Journal, Letter 1995;178:169
3. Caughman WF, Rueggeberg FA, Curtis JW Jr., Clinical guidelines for photocuring restorative resins. Journal of the American Dental Association 1995; 126: 1280-1286
4. Harrington E, Wilson H.J., Depth of cure of radiation-activated materials - effect of mould material and cavity size. Journal of Dentistry 1993;21:305-311
5. Wilcken S, Packaging holds the key to new LEDs success. Photonics Spectra November 1996
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