Application of Tot ’ hema Eosin Sensitized Gelatin Film for Adaptive Microlenses

BRANKA D. MURIĆ, University of Belgrade, Original scientific paper Institute of Physics, Belgrade UDC: 617.7-76 DEJAN V. PANTELIĆ, University of Belgrade, DOI: 10.5937/tehnika1706787M Institute of Physics, Belgrade DARKO M. VASILJEVIĆ, University of Belgrade, Institute of Physics, Belgrade SVETLANA N. SAVIĆ-ŠEVIĆ, University of Belgrade, Institute of Physics, Belgrade BRANISLAV M. JELENKOVIĆ, University of Belgrade, Institute of Physics, Belgrade


INTRODUCTION
Optical lenses are widely used in science, industry, and daily life.Microlenses are lenses with dimensions smaller than 1mm.It can be used, either individually or as microlens arrays, in a various applications such as: wavefront sensors, medicine, quantum computer research and so on [1][2][3][4][5][6][7].
Development of tunable lenses with variable focal lengths is very important for different applications.It can be used in eyeglasses for vision correction, zooming devices in photocameras and integrated in many Author's address: Branka Murić, University of Belgrade, Institute of Physics, Belgrade, Pregrevica 118 e-mail: muric@ipb.ac.rsPaper received: 01.12.2017.Paper accepted: 08.12.2017.electrooptical systems.Also, adapive microlenses is significant for tunable photonic waveguides, miniature optical sensing, electronic display, and widezoom cell phone [21][22][23][24].Today, in many cases is necessary to use adaptive microoptical devices, primarily microlenses.In most optical devices, microlenses have an important role in a focusing, imaging, detection, etc. Different variants of adaptive microlenses arrays were proposed [25,26].
Gelatin is a biocompatible and biodegradable polymer extensively used in food, pharmaceutical, biophysics and biomedical fields.Poor mechanical properties of gelatin film can be improved by crosslinking with various chemical agents such as formaldehyde, epoxy compounds, genipin and glutaraldehyde (GTA) [27][28][29].
Previously, by modifying the gelatin with tot'hema solution, we improved the mechanical properties of the brittle film of pure gelatin.So we get a film based on gelatin doped with tot'hema and sensitized with eosin dye [30][31][32][33], (denoted as tot'hema-eosin sensitized gelatin, abbreviated as TESG. The TESG film is easy to prepare, low cost nontoxic, and became stretchable by adding tot'hema solution.On the film microlenses can be formed by direct laser writing.Our main intention was to use TESG, as biocompatible, thermally stable, soft and elastic material, for tunable microlens fabrication.The microlenses with different diameter and depth were produced on a TESG layer using Nd:YAG laser light (2 nd harmonic wavelength of 532 nm).
In this paper, TESG microlenses focal length tunability was obtained by applying controlled strain.The produced adaptive concave TESG microlenses (individual or microlens array) are suitable for numerous applications.Also, they can be copied onto polydimethylsiloxane (PDMS) and used as convex lenses.The shape change of TESG microlenses (and consequently the focal length) was reversibly changed as the applied strain is inside the elastic limit, that be presented in our future work.

Film preparation
All chemical components used for the film preparation are easily available, cheap and nontoxic.Tot'hema (Laboratoire Innotech International-France) -the trade name of a mixture of iron gluconate (equivalent to 50 mg iron), manganese gluconate (equivalent to 1.33 mg manganese), copper gluconate (equivalent to 0.7 mg copper) and excipients.It is a drinkable solution frequently used to treat anemia.
TESG film was prepared by the gravity-settling method pouring 2 ml of TESG solution onto a leveled and cleaned microscope glasses slide covered with a very clean thermoplastic foil.The net result is a film, which is highly absorptive in the green part of the spectrum, permanently soft and elastic.Dried film can be easily removed from the foil.It can be peeled from one substrate and placed on another (plane or curved).The film was dumbbell-shaped specimens cut by the brass mold made according to the standards for elastic materials (ASTM D412).

Microlens fabrication
If TESG film irradiated with laser radiation in the green part of spectrum, (direct, focused or unfocused) laser beam produces lens like-dips.We have shown that concave microlenses can be produced by direct laser writing using second harmonic Nd:YAG laser (wavelength of 532 nm).Microlens formation was followed by the creation of a diffraction picture on a diffuse screen, with millimeter scale, placed behind the film.This pattern was recorded by a CCD camera.The used experimental setup is shown in Figure 1.The laser power of 60 mW and exposure time of 20 s was used in experiment.

Figure 1 -Experimental setup for microlens fabrication
Film thickness Film thickness was measured with a digital micrometer with 0.01 mm accuracy.Six thickness measurements were taken for each TESG film, and the averages were taken as the result.

Water content of TESG film
To estimate varying of water content during the film dehydration, the TESG films were weighed (mw), dried at ambient temperature (20±2 o C) for different time, and weighted (md) again.Water content (or moisture content) was determined as the percentage of initial TESG film weight lost during drying and reported on a wet basis i.e: %   = 100(  −   )/  Triplicate measurements of water content were done for each film, and an average was taken as the result.

Swelling of TESG film
TESG films were weighted in air-dried conditions (Wd).Afterward, they were immersed in a physiological saline solution, containing 9 g/l sodium chloride, for different time periods.Wet samples were wiped with filter paper to remove excess of liquid and reweighted (Ww).The amount of adsorbed water was calculated: Stress−strain measurements of TESG films were determined in a tensile stress testing machine.Tests were carried out at 25 °C with a strain rate of 20 mm/min, using dumbbell-shaped film (see Figure 2) mounted at a specified gauge length (25 mm) into the system.

Stress−Strain Measurements
Using the appropriate software recorded force and corresponding displacement were recalculated into stress and strain.
The average value of five measurements for every film was calculated.The video camera was used to record the tensile responses of all films.

SEM analysis of microlenses
The morphology of produced TESG microlenses were investigated using a high resolution scanning electron microscope equipped with a high brightness Schottky field emission gun (FEGSEM).

RESULTS AND DISCUSSIONS
Variation of water content in TESG films with different tot'hema concentration with drying time is presented in Figure 3.

Figure 4 -Swelling of gelatin films as a function of tot'hema concentrations
The degree of swelling of pure gelatin films stored in physiological solution increases up to about 220 % after 24 h.Tot'hema induces a reduction of swelling.From Figure 4 can be seen that swelling of TESG films exponentially decreases with increasing of tot'hema concentration.We found that swelling improve the film elasticity.
During a laser irradiation, eosin bleaches, thus making TESG film colorless.In this case, as result of dye discolouration under the influence of optical radiation the transparent microlenses are formed, as can be seen in Figure 5.

-Stress versus strain curves of TESG films
The results of uniaxial stress-strain measurements for all examined films are shown in Figure 7.The tensile responses of TESG films were measured up to the breaking point.As can be seen, mechanical TEHNIKA -NOVI MATERIJALI 26 (2017) 6 properties of TESG films strongly depend on the tot'hema concentrations.The extensibility of the films increases, with increasing of tot'hema concentration up to 250%..The Young's modulus, the stress at break and the deformation at break of the films were calculated from the stress-strain curves.We found that Young's modulus and stress at break considerably decrease with increasing of tot'hema concentration.
Further we investigated the microlenses uniaxial stretching.Microlenses fabrication is followed by formation of diffraction picture on the diffuse screen.The recorded diffraction pictures extensions on a screen with a millimeter scale are shown in Figure 8.The relation between the focal length and strain is shown in Figure 9.

Figure 9 -TESG microlens focal length versus strain
The microlens focal length exponentially increases with increasing strain of TESG film.The focal length was reversibly changed as long as the applied strain is inside the elastic limit.It was noticed that there is a linear dependence between the microlens focal length and diffraction pattern width (see Figure 10).Tunable (strain responsive) microlenses were prepared on elastic TESG film.Stretching the film of about 250%, microlenses are uniaxial deformed so that the focal lengths were changed.The microlenses focal lengths from 0.05 mm to 0.2 mm were obtained.As TESG proved to be a good material for adaptive microlensess, we plan to retain sphericity of microlenses in the future and to achieve uniform stretching along both axes.
The TESG microlenses show good optical and imaging properties.The individual concave microlenses can be used.Also, the large-area square or hexagonal close-packed microlenses array can be used for various applications such as: medical laser, optical sensors, light-field cameras, biological structures.Convex microlenses can be produced by coping TESG lenses onto polydimethylsiloxane (PDMS).

Figure 2 -
Figure 2 -TESG films used for tensile testing with dimensions according to the ASTM standards (gauge length 25 mm, width 6 mm)

Figure 3 -
Figure 3 -Variation of water content of TESG films with drying time It can be seen that the water content of each TESG films decreases with drying time up to 48 hour.After drying of two days, there is no additional change in the water content for all TESG films.

Figure 5 -
Figure 5 -Transparent 3x3 microlens array, and row of three microlenses (left) and hexagonal microlens array (right) on the TESG film Figure 6 shows the stretching of microlens formed in the centre of a dumbbell-shaped TESG film.

Figure 6 -
Figure 6 -The TESG microlens on the dumbbellshaped TESG film: 1) unstretched; and 2) stretched The stretching value can be read on the ruler located parallel to film.It can be seen that TESG film stretched from the initial 25 mm to 80 mm.

Figure
Figure 7 -Stress versus strain curves of TESG filmsThe results of uniaxial stress-strain measurements for all examined films are shown in Figure7.The tensile responses of TESG films were measured up to the breaking point.As can be seen, mechanical

Figure 8 -
Figure 8 -Diffraction picture recorded after the TESG microlens was: 1) formed; 2) and 3) stretched in one direction.Millimeter scale is visible.The toric microlenses were produced due to the uniaxial strain.The astigmatic microlenses have two focal lengths along orthogonal directions.For uniform extension (along x and y direction) sphericity of microlenses can be retained.It was shown that produced TESG microlenses change their optical properties, for example focal length, in response to material elasticity.The relation between the focal length and strain is shown in Figure9.

Figure 10 -
Figure 10 -TESG microlens focal length versus diffraction patern widthThe strain responsive, transparent TESG microlenses offer interesting possibilities in tunable optical devices and sensors.Also, closely packed (hexagonal or square) microlenses arrays can be used to mimicking biological structure such as compound eyes.Hexagonal TESG microlenses arrays observed with an electron microscope is shown in Figure11.

Figure 11 -
Figure 11 -Image of hexagonal TESG microlensesarray is recorded using a field-emission gun scanning electron microscope (FEGSM) Using of microlenses for visualization it is possible to obtain high-quality images.The picture of hexagonal TESG microlenses arrays with observed milimeter paper at its centre is shown in Figure12.

Figure 12 -
Figure 12 -Image of hexagonal TESG microlenses array recorded using an optical microscope 5. ACKNOWLEDGEMENTThis paper was written as a part of research on the projects ON 171038 and III 45016 supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia.