Design of an optical system equipped with blue LEDs for the irradiation of Drosophila melanogaster cultures

. A longer exposure to blue light has negative effects on organisms, therefore we present an optical assembly based on a hexagonal design with blue LED light for homogeneous irradiation of cultures of Drosophila melanogaster , which has a variability range of illuminance controlled by the change of local current, useful for the evaluation of various factors under this parameter.


Introduction
Light has a profound influence on mammalian biological health, including human health.The presence of light is essential for vision and is detected by the retina, a specialized light-sensitive tissue layer in the eye.The retina transmits visual information from the optic nerve to the brain, which can affect both our physical and mental conditions (1).In addition to lighting the environment for vision (i.e., image formation), light also regulates nonimage forming processes, such as the circadian cycle, whose most important Zeitgeber is light for synchronization (2).Organisms, from cyanobacteria to humans, show changes in their metabolism, physiology and behaviour, undergoing daily oscillations with 24-hour periods.These oscillations are regulated by the central circadian clock or circadian cycle, which serves to control and orchestrate these daily fluctuations (3).
Drosophila melanogaster is an excellent model for studying how light affects various processes such as hatching, courtship, locomotion activity, reproduction, etc.It is also widely used to study circadian rhythms that are influenced by light (4).In humans, prolonged exposure to blue light has been shown can cause sleep disruption by suppressing melatonin production as well as modulating brain activity, higher cognitive functions and emotional brain responses to auditory tasks, while in Drosophila melanogaster it shortens their lifespan and accelerates aging, induces sleep disorders, etc (5).In addition, this type of light causes irreparable damage to the retinal photoreceptors in both mice and flies (6).
Long-term exposure to LED light generates changes in the circadian cycles of organisms, including humans, from cycle interruption to deregulation of their coupled functions (7).Therefore, the objective of this work was to carry out the optical design of a system for the homogeneous irradiation of Drosophila melanogaster cultures with blue LED light, in which the effect of different study fields of great interest can be observed, ranging from the development of strains, longevity, reproduction, courtship, to expression of stress response genes, brain neurodegeneration, retinal degeneration, evaluation of antioxidant compounds, etc., over time and multiple generations simultaneously.This assembly is cost-effective and easy to manipulate, programmed in a cycle of 12 light hours and 12 dark hours (LD 12:12) to simulate the change of day and night.

Design and construction of optical assembly
An assembly was built to need for manipulating a large number of cultures and populations of Drosophila melanogaster simultaneously under the same conditions of temperature, humidity and illuminance.The setup comprised a hexagonal arrangement of borosicilate flasks, in which a flask is located at each vertex, as well as a blue LED in the center.Thus, each flask receives the illuminance of 3 LEDs around it separated by 10 cm distance (Figure 1).The blue LEDs used in this arrangement are distinguished by a high power (1 Watt) and low cost, with measurements of 5 mm x 6 mm x 19.8 mm.They are equipped with a useful aluminum heat sink to prevent high temperatures that can disturb the development of the cultures being examined.The LED emission spectrum was measured and had a wavelength centered at 457  45 nm, providing an energy per photon of 4.352 x10 -19 .This arrangement allowed us to manage up to 153 flasks of 100 ml on an area of 2.72 m 2 , wich can be organized randomly in the current experiment with various replicates without any problem, allowing to manipulate more than 5 generations up to 5 repetitions of flies.
Each flask has an irradiation field of approximately 0.07 m in length, but due to the culture medium necessary for feeding the flies, the space is reduced to 0.055 m (Figure 2).Therefore, the irradiation angle in which the adult individuals will be found is 33° with respect to the horizontal position of the LED, as well as the 11° belonging to the culture medium where the eggs and larvae will develop in their first stage and the 46° left over from unoccupied space.

Characterization of optical system
The results of characterization of this optical assembly showed that it can achieve illumination values ranging from 0 to 601.12 lux (Figure 3) and power density levels from 0 to 0.05 mW/cm 2 .These values can be locally programmed using a variable power source by current change injected into the system from 0 to 1.05 A. The assembly was connected to a circuit of 95 LEDs completely in parallel, without any voltage change (3 V).Light on and off changes was controlled by a programmed timer.Finally, this assembly was carried out in a dark room, to avoid the filtration of another light source, in addition, the work area where the optical arrangement is located is completely closed.
The illuminance of the light source in the optical assembly was measured at each point at 0.2 A where an average value of 92.14  3.03 lux was obtained, evidencing that the arrangement proposed in this work is homogeneous for the distribution of light in the area in each point where each flask under study is located.

Conclusions
The presented optical design, equipped with blue LED light, allows us to locally control and monitor a large number of cultures and multiple generations of Drosophila melanogaster simultaneously, with the certainty that our blue light source is homogeneously distributed in space at different light illuminance intensities, giving us the opportunity to evaluate different fields of relevant scientific interest.

Figure 1 .
Figure 1.Arrangement for each flask, this setup was distributed in the work area.

Figure 2 .
Figure 2. Irradiation field received by each flask.

Figure 3 .
Figure 3. Measured illuminance vs different LED current values.The illuminance was measured with luxmeter Benetech GM1010.