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Multifunctional biomedical materials capable of integrating optical functions open up promising new possibilities for the application of photosensitive materials. For example, they are highly desirable for advanced intraocular lens (IOL) implants. For this purpose, we propose hydrogels, based on poly(ethylene glycol) (PEG) prepolymers, which are photochemically crosslinkable and thereby patternable. Various photoinitiators are used and investigated spectroscopically; those with high sensitivity in the optical region of the spectrum are advantageous. Hydrogel films have been obtained, which are applicable for light-based patterning and, hence, for functionalization of both surface and volume: It is shown that a local change in optical properties can be induced in special hydrogel films by photochemical crosslinking. Such a local light-induced material response forms the basis for volume holographic patterning. Cytocompatibility of hydrogels and compositions is evaluated via cytotoxicity tests. Exploiting the interrelationship between structure and function is highly relevant for biomedical materials with multifunctionality.
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
Arina Tyushina
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
Rahima Rahman
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
Cigdem Babalik
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
Zhenfang Zhang
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
Marga C. Lensen*
Nanopatterned Biomaterials (Secr. C 1), Department of Chemistry, Technische Universität Berlin, Berlin, Germany
*Address all correspondence to: tina@physik.tu-berlin.de and marga@lensenlab.de
1. Introduction
Multifunctional biomedical materials capable of integrating optical functions are highly desirable for many applications, such as advanced intra-ocular lens (IOL) implants [1].
IOLs are needed in the case of cataracts; the irreversible turbidity of the natural lens of the eye, which is one of the most common causes of global blindness and which can only be treated by replacing the clouded lens with an artificial IOL implant (see Figure 1).
Figure 1.
Schematic cross section of the human eye (top) and exemplary images of the eye with cataract, i.e., pathological turbidity of the crystalline lens (bottom left) and intraocular lens implant (bottom right) [2]. Photos via Wikimedia Commons.
Intraocular lens implantations have been performed since 1949. The first artificial lenses were made of polymethyl methacrylate (PMMA). Since the beginning of the 1980s, soft lenses made of silicone have been implanted in a folded state. An as-yet unsolved problem is the preservation or restoration of the accommodation ability, i.e., the dynamic adjustment of the refractive power of the eye by tension and relaxation of the ciliary muscle and zonular fibers. The lens contributes only one-third of the total refractive power of the eye. Most of the refractive power (about 40 of 60 diopters) is generated by the cornea alone. However, accommodation of the lens can change the refractive power by about 10 diopters, or up to 20%.
To counteract the loss of accommodative power due to IOLs, bifocal lenses were introduced in the early 1990s. These simultaneously form a distance image and a near image on the fovea (the area of the sharpest vision of the retina). Disadvantages that have to be accepted for this are photic phenomena such as significantly poorer contrast vision, increased glare sensitivity, poorer imaging quality, as well as an increased stray light component (halo) around the respective primary focal point (see also Figure 2, below) [5]. Intraocular lenses of the latest generation—ultraflat, bifocal IOLs with diffractive-refractive optics—have reduced the manifestation of photic phenomena.
Figure 2.
Beam paths of a diffractive and a refractive multifocal intraocular lens (modified from [3], reprinted with permission from [4], Copyright 2008 SLACK Incorporated).
Among the state-of-the-art IOLs are modern foldable hydrogel lenses [6]. Persistent problems with IOLs include postoperative calcification [7] and secondary cataract [8]. Such postoperative opacification occurs in vivo through interaction with the biological environment. The underlying processes are still not well understood.
This is where the idea of volume holographic structuring comes into play.
Volume holography is a very promising field of application for photo-responsive polymers: diffractive structures are induced by a spatially modulated holographic exposure [9]. Holographic elements such as diffractive structures can accommodate classical optical functions, while at the same time being extremely flat in shape and low in weight. This gives rise to a great potential for replacing classical refractive optical systems or extending them with new functionalities. In any case, the prerequisite is the availability of suitable, photo-patternable materials that can exhibit function through structure. An example of such diffractive structures with classical optical function includes holographic lenses (HL), which owe their optical functionality, i.e. focusing the light onto the retina, to their micro−/nano-structure (see Figure 3).
Figure 3.
Holographic lens (HL) on gold-coated substrate. Scale bars: 200 µm (left) and 20 µm (right). Reprinted with permission from [10]. Copyright 2015 American Chemical Society.
Prospective IOLs, based on multifunctional biomedical material with integrated optical functionality, could fulfill their function—i.e., to focus the light onto the retina (see Figures 1 and 2)—with an optically structured volume [2]. As a result, the shape and surface of the IOL would remain free and available for other purposes. This leaves subsequent surface modifications optional to achieve specific interactions with the biological environment. To realize this, we propose a strategy that integrates optical structuring of the volume, combined with specific modification of the surface. Therefore, volume holographic structuring can be applied for the integration of three-dimensional optical structures with specific functionality in terms of diffractive properties. The optical functionality of an IOL with integrated holographic lens as a diffractive element consists in focusing the light onto the retina. Multiple holographic elements may be combined in stacks, so that the functionality of the individual elements overlap. The selectivity of a stack then results in a superposition of Bragg selectivity of the individual elements [11]. Thus, for example, multiple foci can be combined by multiplexing. Bifocal holographic lenses can also be created by combining different phenomena [12]. Azo compounds [13, 14] are suitable for this purpose, for example, and the formation mechanisms here are due to light-induced anisotropy and surface relief gratings.
In the case of modern intraocular lenses, diffractive structures are already being used to improve the optical functionality of the implant [15]. Figure 2 shows the advantage of diffractive optics over purely refractive multifocal IOLs.
However, at the current stage of development, only surface structuring has been used. In optically structuring the volume of the IOL, whether in addition or instead of surface patterning, lies a hitherto untapped opportunity with great potential. But new light-sensitive biomaterials are needed to exploit this potential.
Advanced IOL implants are just an example of why new materials for volume holography are the subject of active research [16, 17].
The versatile applications of volume holographic materials are numerous and range from recording media, holographic data storage, self-written waveguides and wavelength-selective devices, to solar energy concentrators and diffractive elements for biomedical applications [1, 9, 18, 19]. The long list of potential applications is driving the need for suitable materials for volume holography.
Most photosensitive materials are designed either for volume or for surface patterning. However, some systems have been described for hybrid volume/surface gratings, where a periodic modulation of the surface is observed in addition to a volume phase grating [20, 21, 22]. Such dual grating structures are of great interest for many applications and offer special opportunities for a deeper understanding of the underlying grating formation mechanisms [23].
As a volume holographic material, photosensitive polymers represent a particularly interesting group among stimuli-responsive polymeric materials, characterized by their ability to be used in a noninvasive and easily controllable manner [24]. Light as a stimulus enables optical patterning by applying volume holography as a single-step method to fabricate diffractive 3D micro- and nanostructures [25].
The formation of holographic gratings in polymers is determined by the interplay of polymerization and diffusion, where the chemical gradient is always the driving force: If a component is consumed (e.g. by bonding and crosslinking) or changed (e.g. by isomerization), a concentration gradient is created. This results in diffusion of the consumed or changed components and is accompanied by a material transport, which creates the permanent grating [9].
While light-induced polymerization-diffusion processes enable the structuring of any optically transparent, photosensitive material that exhibits sufficient mobility in the sense of diffusion, permanent structures can only be achieved if the diffusing species are incorporated into a network, e.g., a chemically crosslinked gel.
Employing photosensitive hydrogels combines the advantages of hydrogels and light; the hydrated structure of the hydrogel makes them favorable as a bio-compatible material, and the use of light as a patterning tool allows dynamic control of their properties. The combination of these advantages makes these systems attractive for biomedical applications [26].
Hydrogels, e.g., based on poly(ethylene glycol) (PEG), are generally highly interesting for application as scaffolds for tissue engineering. They are cytocompatible and intrinsically resistant to protein and cell adhesion, and they can be controlled and functionalized on the molecular level [27]. Numerous applications open up based on explicit control over molecular structure and mechanical properties, such as elasticity and degree of crosslinking, as well as surface morphologies [28].
As biomaterials, hydrogels offer versatile possibilities for (photo)chemical crosslinking and are suitable for the generation of topographic surface structures as well as for three-dimensional, macroporous scaffolds. Acrylate-terminated. PEG macromers undergo chemical crosslinking in the presence of photoinitiators that generate radicals when exposed to light [27]. This makes PEG-DA hydrogels interesting as scaffolds into which desired bioactivity can be tailored via light-induced patterning. In this context, it has already been shown that hydrogels can also be structured photolithographically by exploiting diffusion processes [29]. The latter phenomenon forms the basis for volume holography. However, in addition to the possibilities and advantages, holographic gratings in hydrogels also bring new challenges, for example, with respect to temporal stability—this concerns the storage of time-stable holographic gratings in hydrogel matrices [30].
When a new photosensitive material is designed, the type of photoinitiator (PI) used is key for the specific photo-response of a certain material. The properties of the PI have strong influence on holographic grating formation in the respective material [31]. It also influences how well certain conditions are met, such as resistance to humidity [32]. Eosin-Y (EY) and Erythrosin B (EB) are used as PIs for holographic grating formation in an AA/PVA photopolymer [33]. EY features excellent spectroscopic properties, which makes it suitable for use with light sources in the visible range and safe for living organisms [34]. EB is only suitable for free radical polymerization [33].
Photochromic hydrogels, capable of photoinduced reversible color changes, exhibit excellent reversible conversion behavior, which can also be used for repeatable writing of optical information [35]. Photomechanical hydrogels based on typical molecular photoswitches, such as azobenzene, function as smart materials that respond to light [36].
The azobenzene moieties switch from trans to cis conformation upon exposure to UV light and reverse their conformation upon exposure to the visible light. Also, these gels proved to be thermally responsive as well: thermal cis → trans isomerization occurs spontaneously due to the thermodynamic stability of the trans isomer [37]. Thereby, in case of azobenzene-based light-driven real-time information-transmitting systems, the information is expected to be transmitted at the molecular scale with response times ultimately within the nanosecond or picosecond range [38]. In this process, azo-dye molecules can also be used as diffusing dopants to create holographic gratings [39]. Finally, the use of mutually reactive acrylate functionalities on both the azobenzene derivatives and the prepolymer macromolecules ensures the fixation of the diffraction gratings.
Poly(ethylene glycol) diacrylate (PEG, Mn 575) and 2-hydroxy-40-(2-hydroxyethoxy)-2-methylpropiophenone (photoinitiator (PI)—Irgacure 2959), Erythrosin B and Eosin Y were from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). The chemical structures of the hydrogel components are shown in Figure 4.
Figure 4.
Irgacure 2959, Erythrosin B and Eosin Y are used as photoinitiators (PI); PEG-DA as a precursor for PEG hydrogel [1].
2.1.2 Synthesis of PEG-Based Photopolymer with PIs
PEG-DA was used as a precursor. It was mixed with PI (Irgacure, EB and EY, respectively).
PI concentration was kept between 1% and 6 ppm. The chemical structures of different PIs are shown in Figure 4. For good mixing of both substances, the mixture was sonicated for around 30 min. At first, the mixture was converted in a cuvette and measured with a UV–Vis spectrometer to obtain spectra before crosslinking. Then, the mixture was dispensed on a glass slide and covered with a thin glass cover slip to achieve a flat and thin hydrogel sample. The glass-sandwich was placed under a UV-light source (366 nm) for 60 min and the glass coverslip was peeled off. A flat, thin standalone hydrogel film was received and also prepared for UV–Vis measurement. For this purpose, the hydrogel was placed on a thin glass coverslip and measured with a UV–Vis spectrometer to obtain spectra after crosslinking.
2.2 Functionalized 8-arm PEG-OA
A composition based on an 8-arm star-shaped PEG with acrylate groups (8PEG-OA) (octa-acrylate) was chosen for the preparation of the hydrogel film, and an azo compound (azobenzene-functionalized acrylates) was added [37]. The components and their chemical structures can be found in Figure 5.
Figure 5.
Components of functionalized 8-arm PEG.
This mixture was placed between two glass slides with the addition of photoinitiator (2%), ammonia (30%), and water (100%). Ammonia acts as a crosslinking agent and induces the gel formation [40]. Ammonia as a crosslinker for gel formation is a great tool to make gels that do not disintegrate and which can be finally fixed by photochemical crosslinking of residual acrylate groups (i.e., those who have not been consumed by reaction with ammonia). Subsequently, the sample was further photochemically crosslinked under ultraviolet light (UV).
PEG-DA was mixed with PI (Irgacure, EB, and EY, respectively). Films were prepared by photocrosslinking with 366 nm for 1 h. In terms of optical transparency, mechanical integrity, flexibility, and stability, the new gels compare well with other gels based on PEG-DA [41, 42, 43].
For PEG-DA with Irgacure, EB and EY as PI, a certain minimum concentration of PI was needed to make the gel. The optimal concentration for the different PIs is shown in Table 1. We found a suitable PI concentration for producing the new hydrogels with PEG-DA and different PIs to be 0.025% for Irgacure, 0.1% for EB and 0.5% for EY, respectively.
Photoinitiator (PI)
PI Concentration to Make Gel with PEG-DA
Irgacure 2959
0.025%
EB
0.1%
EY
0.5%
Table 1.
PI concentration to make gel for the different PIs [1].
3.2 UV: VIS spectra
Figure 6 shows the UV–Vis spectra before and after crosslinking for the novel PIs. In general, for all new PEG-DA hydrogels, we find the absorption band of the dye in the polymer matrix shifted somewhat toward higher wavelengths compared to the pure PIs [44, 45], while crosslinking tends to cause a small shift toward lower wavelengths again, as already substantiated in the literature [46].
Figure 6.
UV–Vis spectra for the novel PIs (EB and EY) with PEG-DA before and after crosslinking, respectively. The hydrogels with new PIs (EB and EY) exhibit good spectroscopic properties, so they can be used with light sources in the visible range [1].
While Irgacure, which is an often employed and suitable photoinitiator for bio-materials research, has an absorption maximum in the UV/Vis-spectrum around 300 nm [44], the novel dyes under investigation display a strong absorption of visible light with wavelengths up to 550 nm (see Figure 6).
The major advantage of the new PIs (EB and EY) over the standard Irgacure is their high sensitivity in the optical range of the spectrum, which enables optical patterning—e.g., by volume holography.
For further optimization, the respective optical response must be determined depending on the composition. In some cases (such as with an organic cationic ring-opening polymerization system), competing effects can be observed with respect to the contribution to the formation of the optical grating [47]. It is also known that optical shrinkage can have a significant effect on the formation of gratings [48] and that the amount of PEG in composition affects film shrinkage, as well as its optical properties [49]. In addition, we have found that photoinitiators can also contribute to light-induced changes in optical properties and subsequent pattern formation [50].
3.3 Cytotoxicity tests
A live/dead staining assay has been used to study the cell viability after incubation with PI EB and EY and also EB 0.1% with PEG-DA before and after crosslinking for 24 h. In the live/dead staining assay, dead cells turn up red, while living cells turn up green when observed with a fluorescence microscope.
Figure 7 shows fluorescent images of cell tests by live/dead cell assay after being incubated for 24 h with cell line L929. Cell tests shown in Figure 7 demonstrate cytocompatibility of PEG-DA hydrogels and PIs, confirming its suitability for biomedical applications.
Figure 7.
Fluorescence microscopy images of cells test by live/dead cell assay with L929 after incubated for 24 h. (a) Control cells; (b) EB; (c) EY; (d) EB 0.1% with PEG-DA; (e) optical micrograph of cross-linked EB/PEG-DA hydrogel; (f) fluorescence image of cross-linked EB/PEG-DA hydrogel. Scale bar depicts 50 μm [1].
3.4 Local change in optical properties
The refractive index of the functionalized hydrogel film based on 8-arm PEG was determined by ellipsometry. Results are shown in Table 2.
Refractive Index
Sample A (without Azo)
Sample B (0.3% Azo)
Sample C (0.6% Azo)
Unexposed
1.417 ± 0.006
1.423 ± 0.002
1.429 ± 0.002
UV exposed
1.423 ± 0.008
1.418 ± 0.002
1.422 ± 0.002
Contrast
−0.005
−0.007
Table 2.
Elypsometrically determined local refractive index n of the hydrogel layers [51].
Comparison of exposed and unexposed areas shows a decrease in refractive index due to UV exposure of the order of 10−3 in case of functionalized hydrogel. In addition, a dependence of the contrast on the azo content was found: An azo content of 0.3% results in a contrast of 0.005. A higher azo content of 0.6% shows a higher contrast of 0.007.
The change in optical properties found can be attributed to transformation processes in the molecular structure. UV exposure causes the material to crosslink, whereby double bonds are converted to single bonds. Single bonds have a lower polarizability compared to double bonds. The transformation should therefore be accompanied by a reduction of the refractive index. On the other hand, the crosslinking arguably causes an increase in the density of the material, which is linked to an increase in the refractive index. Experimentally, a decrease in refractive index was observed in the course of UV exposure-induced crosslinking. Thus, the increase in polarizability seems to outweigh the increase in density of the material.
Altogether, the decrease in refractive index found over the course of UV irradiation-induced crosslinking can be attributed to the increase in polarizability and seems to outweigh the increase in density of the material.
As we know, an inducible refractive index contrast of the order of 10−3 allows a diffraction efficiency close to 100% for 200 μm thick layers [9]. It should be noted that the experiments performed here to determine the refractive index contrast do not even involve diffusion processes. Thus, the contrast is not due to material transport, but only due to changes in molecular structure. Since azo components are involved here, trans→cis isomerization is also a possibility for the contrast-effective change in molecular structure [52]. However, by integrating polymerization-diffusion processes in the form of holographic patterning, a much higher contrast can therefore be expected.
A fundamental prerequisite for the development of a multifunctional optical material is a better understanding of the processes underlying optical patterning, such as the interplay of polymerization and diffusion in holographic gratings.
The general approach is to consider the holographic grating formation as a consequence of photopolymerization and mass transport processes: local polymerization is induced by a light pattern projected into the photosensitive medium. Polymerization proportional to the light intensity results in the induction of a chemical gradient, followed by monomer diffusion and subsequent polymerization. The resultant grating is achieved as a periodic modulation of optical properties, corresponding to the recording light pattern [9].
Among the various effects that can contribute to grating formation via a local change in optical properties are: basically, the exposure-induced change in refractive index due to crosslinking of acrylate groups and the change in density due to crosslinking. In the case of azo components, the cis/trans isomerization causes local changes in molecular structures or configurations, in connection with changes in polarizability with direct effects on the refractive index [52]. In addition, cis-trans isomerization can also contribute to a change in density [53]. Then the important aspect of photo-triggered mass transport in the cause of component diffusion is to be considered: both with respect to diffusion of hydrogel components PEG [29] and with respect to diffusion of the azo-dye [39].
4.2 Generation of one-dimensional diffractive surface gratings
The functionalized hydrogel layers were UV-crosslinked under a photomask (1D modulated; lattice constant 20 μm). In this process, diffractive surface gratings were formed. Figure 8 shows a layer with optical functionality, as well as the corresponding surface characteristics (topography profile from atomic force microscopy studies) of the patterned hydrogel.
Figure 8.
Diffractive structures in hydrogel: hydrogel film with optical functionality (left) and corresponding surface topography (right) [51].
It can be assumed that the diffusion of material plays a substantial role in the surface relief formation: the simultaneous formation of surface relief gratings and volume holographic phase gratings by holographic interference exposure in epoxy-based material can be explained based on photo-triggered mass transport, resulting from the light-induced polymerization–diffusion process in the course of volume hologram formation [23].
4.3 Anticipated grating formation in functionalized hydrogel
The anticipated grating formation in the functionalized hydrogel is schematically shown in Figure 9.
Figure 9.
Anticipated grating formation in functionalized hydrogel [51].
In the first step, the mixture forms a gel by reaction of 8PEG and crosslinker (ammonia), leaving uncrosslinked functional groups (acrylate groups). In the second step, the structured exposure is performed by a photomask (or, if volume holographic structuring is used, by an interference pattern). The photoinitiator is consumed, with photoinduced attachment of the dopant (here, azobenzene-functionalized acrylates). In addition, trans→cis isomerization occurs under UV irradiation. The third step, diffusion of uncrosslinked dopant (and possibly - depending on the exposure dose - also crosslinking and isomerization of the diffused components), forms the permanent optical grating.
It was shown that a local change in optical properties can be induced in special functionalized hydrogel films by photochemical crosslinking, while we found an overall decrease in refractive index over the course of UV irradiation-induced crosslinking.
The strategy of combining optical structuring of the volume and specific modification of the surface is of particular interest for the design of advanced intraocular lens (IOL) implants: based on a multifunctional biomedical material with integrated optical functionality and following the principle of function by structure, such a new type of IOL should achieve improved functionality [2].
In addition to enhanced functionality, moving optical functionality from the surface into the volume of the IOL implant brings other advantages such as the free interface for specific interaction with the biological environment. Since the existing problems with conventional IOLs, such as postoperative clouding, emerge in vivo from interactions with the biological environment, they could be better solved with free surface IOLs.
The next step toward such a multifunctional optical material is to better understand the processes underlaying optical structuring, e.g., the interplay of polymerization and diffusion in the case of holographic gratings. Here, the general approach is to understand holographic grating formation as a consequence of photopolymerization and mass transport processes: local polymerization is induced by a light pattern projected into the photosensitive medium. Polymerization proportional to the light intensity creates a chemical gradient, resulting in monomer diffusion and subsequent polymerization. The final grating is a periodic modulation of optical properties, following the recording light pattern [9].
An exciting question is now, what role the individual components play in the formation of optical structures in cases of PEG-DA hydrogels with EB and EY. And furthermore, it would have to be clarified if other additives—such as crosslinker or dopant, e.g., in the form of azobenzene-functionalized acrylates or gold nanoparticles— may show a positive effect on the formation of optical patterns.
We have created one-dimensional diffractive surface gratings, using a photomask. Such structured hydrogel layers show optical functionality, and the related surface topography was visualized in the atomic force microscope. The observed optical functionality may be caused by the measured surface grating, or additionally by a possibly generated correlated volume phase grating. Which contribution each of the two have, will have to be clarified by further investigations, preferably by the generation of holographic gratings in hydrogel layers and exact analysis of their corresponding diffraction properties. Then the material response can be determined (and optimized in connection with adapted material composition) from the analysis of the refractive index contrast as a function of the exposure conditions.
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Written By
Tina Sabel-Grau, Arina Tyushina, Rahima Rahman, Cigdem Babalik, Zhenfang Zhang and Marga C. Lensen
Submitted: 18 July 2022Reviewed: 23 September 2022Published: 02 November 2022
Open access peer-reviewed chapter
