1. Introduction

The conventional method of hologram replication by embossing is a multi-step process [1]. In the first step, a surface relief hologram is generated on photoresist, silverhalide emulsion or thermoplastic film [2–7]. Relief micro-pattern is further copied (by sputtering and electroforming) into more durable material like nickel. Resulting shim is used as an embossing tool for mass replication of holograms by thermal stamping into plastic foils [8].

The stamping shim has a limited lifetime due to high pressure (1–15 MPa) and temperature (150–250 ⁰C) during embossing [9, 10]. Therefore, it is of interest to find more durable materials, and simplified methods of holographic shim production. It seems that materials used in dentistry are good candidates, since they must imitate outstanding mechanical, chemical and thermal properties of hard dental tissues (dentin and enamel).

Photopolymer composites are extensively used in dental practice as filling material for cavities in tooth tissues. A typical dental composite consists of inorganic filler particles, embedded in a methacrylate based polymer matrix with camphorquinone added as a photo-initiator [11, 12]. Polymerization is done using blue light, corresponding to camphorquinone absorption maximum around 470 nm. Inorganic fillers (quartz, lithium aluminum silicate, glass) give the composite desired mechanical properties, comparable to dental tissues. Filler particles can measure from 0.04 microns up to several microns, depending on the manufacturer and the type of the composite. The resulting compressive strength ranges from 250 to 450 MPa [13].

It should be added that a range of composites were previously used as holographic recording materials [14–16]. They can be mixtures of various polymers (epoxy resin - photopolymer, dye doped LCD, cromophore - polymer matrix). Also, mixtures of inorganic hard particles (titania and silica) with polymers were used as holographic recording materials [17–19]. However, none of these materials was used as a hologram embossing tool.

We have simplified a process of embossing tool production, by direct copying of holographic surface relief grating into a layer of dental composite. Resulting copy is an almost exact replica of original grating (with respect to relief depth, profile and spatial frequency), as verified by atomic force microscopy (AFM). In addition, the composite shim is thermally stable, very hard, durable [20] and, thus, an excellent tool for further mass production of holograms.

Complete process of hologram embossing tool production is explained, starting with original, surface relief hologram, manufacturing and ending with contact copying and photo-polymerization of the dental composite. We present the results concerning depth of profile, spatial frequency, thermal and mechanical properties of selected dental polymers.

2. Process of surface relief hologram replication in layers of dental composite

Pullulan, which is a natural linear polysaccharide [21], was sensitized with ammonium dichromate and used for surface relief hologram recording [22–24]. Dicromate sensitized pullulan (DCP) samples were prepared by mixing 8% aqueous solution of pullulan and 50% ammonium dichromate by weight of pullulan. The solution was coated onto clean glass slides in a horizontal position and the film was dried overnight under normal laboratory conditions. The thickness of the dried DCP layer was approximately 10 µm.

Surface-relief diffraction gratings were formed by the interference of two equal power coherent beams. DCP was exposed with single-frequency 400 mW diode pumped Nd-YAG laser, at 532 nm. He-Ne laser operating at 632.8 nm was used for real-time monitoring of diffraction efficiency. Exposure of DCP film was considered complete when the real-time maximum diffraction efficiency was achieved [23]. The exposed plates were developed with a mixture of 60% isopropanol and 40% water for 2 min and then immersed in the pure isopropanol for 30 s. Five DCP gratings, with spatial frequency ranging from 300–900 lines/mm, were produced in order to study diffraction efficiency and surface grating profile.

Uncured dental composites have a consistency of a thick cream or honey (depending on the type and the manufacturer), which makes them easy to smear on top of the surface relief pullulan grating. A small amount of the composite was placed between holographic grating and a glass slide. By applying pressure on the resulting “sandwich”, the composite was uniformly distributed and air bubbles were squeezed out. We did not take special measures to control the pressure since we experienced no problems with trapped bubbles at all. It is the consequence of the dental polymer structure, which is rather compact and keeps together well.

Polymerization was performed by an array of blue LEDs, with total irradiance of 80 mW/cm². After 2 minutes exposure, composite was fully cured. It is our experience that dental composite does not stick to DCP, enabling simple mechanical separation. Also, composite does not bond to other materials that we tried — dichromated gelatin, nickel, gold, photoresist- and there were no need to use an anti-sticking coating. After separation, negative replica grating was obtained.

Three types of commercial dental composites were chosen for this study: “Helio Progress” (manufactured by Vivadent), “Herculite xrv”, (manufactured by Kerr) and “Degufill ultra” (manufactured by Degussa). Chemical composition details were not available, probably due to proprietary reasons. However, “Helio Progress” and “Degufill ultra” are classified as microfilled dental composites (with particle size 0.04–0.2 µm), while “Herculite xrv” is microhybrid material (with blend of particles ranging from 0.04–5 µm) [11]. Anyway, we found no differences in holographic properties between the three materials.

3. Comparison of holographic properties of the DCP original and the composite copy diffraction grating

Gratings with spatial frequencies of 315, 380, 470, 760 and 870 lines/mm were made and copied into dental composite. Spatial frequencies were chosen in the range corresponding to bandwidth of rainbow holograms [25] and dot image holograms [26], which are mostly used for embossing purposes. Profiles of the DCP original gratings depend on recording geometry and chemical processing. They varied from purely sinusoidal to saw-tooth profile. For the analysis, non-sinusoidal ones were chosen, since the faithfulness of the copying process can be verified.

The diffraction grating profiles of originals and copies were analyzed by AFM and average values of the grating period and its depth were calculated for each scan. For illustration purposes, two surface scans (20×20 µm area) of DCP original and copy (“Helio Progress”) are shown in Fig. 1(a) and 1(b). One dimensional scans are shown, too. The measured average relief depth of the DCP grating is 54 nm and its average period is 1.15 µm, correspondingly, the measured average relief depth of the dental composite grating is 52 nm and its average period is 1.14 µm.

 

Fig. 1. (a) The AFM image of original DCP diffraction grating (top) with period of 1.15 µm and its line profile on a reduced scale (bottom) (b) The AFM image of dental composite (“Helio progress”) diffraction grating copy (top) with period of 1.14 µm and its line profile on a reduced scale (bottom). In both cases the scan size is 20×20 µm.

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Fast Fourier Transforms (FFT) of AFM scans of an original and a copy from Fig. 1 are shown in Fig. 2(a) and 2(b), respectively. Note that FFT of, both, original and a copy are almost identical, meaning that higher harmonics (up to the third) are well preserved. It can be estimated that dental polymer is capable of transferring spatial frequencies up to 2610 lines/mm (three times fundamental frequency of 870 line/mm).

Different surface profiles can be successfully replicated into dental composites. Figure 3 shows a saw-tooth shaped dental polymer grating and its line scan. The measured average relief depth of the dental composite copy (“Herculite xrv”) is 167 nm (155 nm on the DCP original) and the average period is 3.10 µm (3.17 µm on DCP original).

 

Fig. 2. (a) FFT of AFM image of original DCP grating (top) with period of 1.15 µm and its line profile (bottom) (see Fig 1(a)). (b) FFT of AFM image of dental composite diffraction grating copy (top) with period of 1.14 µm and its line profile (bottom) (see Fig 1(b)).

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The relation between the surface profile depth and the spatial frequency is presented in Fig. 4. As expected, the depth decreases with increase of the spatial frequency. It is, however, interesting to note that the depth of the copy is somewhat larger than the original (for spatial frequencies up to 870 line/mm). Also, the spatial frequency of the copy differs from the original, within the statistical error. This disparity can be attributed to volume contraction of composite during polymerization, which is well known in polymer science and in dentistry [27, 28].

Our results show that contraction of the polymer is not isotropic (i.e., it contracts more in one direction compared to an orthogonal one). It can be argued that the speed of polymerization is not uniform and differs in the bulk material compared to ridges of the diffraction grating. Inside ridges, monomer can be polymerized more quickly because there is less monomer supply from the surrounding areas, like in the case of the bulk material. This may result in non-uniform shrinkage of the composite and increased depth of the grating.

 

Fig. 3. (a) The AFM image of dental composite (“Herculite xrv”) diffraction grating with period of 3.10 µm and (b) its line profile on a reduced scale.

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Fig. 4. Average relief depth versus spatial frequency.

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Figure 5 shows the relationship between the diffraction efficiency (measured in reflection) and the spatial frequency, for both the original DCP gratings and the dental composite replica gratings. It can be seen that the maximum diffraction efficiency of the replica gratings are only a few percent less then the maximum diffraction efficiency of the original DCP gratings.

 

Fig. 5. Diffraction efficiency of DCP original and composite copy versus spatial frequency

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In conclusion, obtained results are reproducible and do not vary from composite to composite and also for different AFM scans on the same composite.

4. Mechanical and thermal stability of the dental composite shim

We have performed some preliminary tests of mechanical and thermal stability of dental composites. After heating at 250° C, for one hour, no change of optical properties of dental embossing tool was observed. It was also found that exposing the dental composite embossing tool to ultrasonic waves had no adverse effect i.e. we observed no drop in diffraction efficiency.

Simple initial tests of dental polymer shim as a hot embossing tool were performed on 30 µm thickness metalized plastic foil. Hand press was used — therefore pressure could not be controlled. Also, composite shim was heated to approximately 150 °C. Diffraction gratings were successfully pressed into foil, although the quality was not very good, due to impossibility to tightly control the whole embossing procedure. Anyway, dental embossing shim survived the test without degradation.

To conclude, dental composite survived all the mentioned tests with no drop in diffraction efficiency.

5. Conclusions

The standard holographic shim production technique involves complicated and time consuming procedures like nickel sputtering and electroforming. Method presented in this paper is simpler and faster, while producing the resulting shim of excellent mechanical properties — comparable to ordinary nickel shims. Furthermore, nickel shims are thin, and susceptible to warp (which seems to be the main mechanism of failure), while dental polymer can be made several millimeters thick, making them tougher.

It was found that the original DCP grating and its copy are almost exact replicas of each other in all important aspects. The maximum diffraction efficiency is slightly lower for replica grating compared to the original. On the other hand, there is minor disparity between frequency and relief depth of originals and copies. This result was attributed to polymerization contraction effect.

There is a vast range of dental polymers produced by many manufacturers. Composites vary in chemical polymer composition, photoinitiation system and inorganic filler particle type and size. It seems that any nanofilled dental composite can be used as holographic material. During our research, no significant differences in holographic replication properties between three types of composites were found.

Our future investigation includes copying of diffraction gratings and holograms using hot embossing. Initial tests were performed with promising results. Nevertheless, it was found that all process parameters must be controlled more tightly in order to obtain better replicas. The ultimate test of dental composite shims would be their incorporation into an embossing process on an industrial scale. However, this would require significant changes in existing embossing machines, taking into account the increased thickness of shim (several millimeters vs. several tenths of millimeter). It might be necessary to change commonly used rotary press to linear press. Finally, this will be the subject of our future research.

Holographic properties of dental polymers should be further tested, by copying from other surface relief materials like photoresists, to see if better relief depths and higher spatial frequencies could be transferred. Also, dental polymer should be tested as a direct holographic recording material, since this would enable production of shims without intermediate steps.