Formation of diffraction gratings in optically patternable hydrogel films [original]

Received: 12 July 2022 Revised: 25 August 2022 Accepted: 12 September 2022

DOI: 10.1002/nano.202200154

RESEARCH ARTICLE

Formation of diffraction gratings in optically patternable

hydrogel films

Tina Sabel-Grau Zhenfang Zhang Rahima Rahman Marga C. Lensen

Nanopatterned Biomaterials (Secr. C 1),

Department of Chemistry, Technische

Universität Berlin, Berlin, Germany

Correspondence

Tina Sabel-Grau and Marga C. Lensen,

Nanopatterned Biomaterials (Secr. C 1),

Department of Chemistry, Technische

Universität Berlin, Straße des 17. Juni 115,

10623 Berlin, Germany.

Email: [email protected];

Marga@lensenlab.de

Abstract

Local light-induced material response forms the basis for volume holographic

patterning of light-responsive materials. This enables applications as multifunc-

tional biomedical materials capable of integrating optical functions, such as for

advanced intraocular lens (IOL) implants. Therefore, hydrogel films based on 8-

arm PEG and azobenzene-functionalized acrylates are prepared. A local change

in optical properties is induced by UV exposure: a decrease in refractive index is

detected by ellipsometric examination. One-dimensional diffractive surface grat-

ings, generated using a photomask, show optical functionality and are visualized

in the atomic force microscope. The results show that a local change in opti-

cal properties can be induced in functionalized hydrogel films by photochemical

crosslinking, making them suitable for volume holographic patterning.

KEYWORDS

diffraction gratings, hydrogels, light-responsive materials, multifunctional biomedical bioma-

terials, photo curing, photopolymers, volume holography

1 INTRODUCTION

Multifunctional biomedical materials capable of integrat-

ing optical functions are highly desirable for many applica-

tions, such as advanced intraocular lens (IOL) implants.[1]

In the case of intraocular lenses, diffractive structures

are already being used at the current stage of development

to improve the optical functionality of the implant.[2]So

far, only surface structuring has been used. Optically struc-

turing the volume of the IOL – whether in addition or

instead of surface patterning – is an as yet unexploited,

but nevertheless great advancement. However, new light-

sensitive biomaterials are needed to exploit this potential.

Advanced IOL implants are only one example why

new materials for volume holography are constantly being

developed and researched with relentless intensity.[3,4]

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the

original work is properly cited.

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.[5–7]Such dual grating

structures are of exceptional interest for many applications

and offer special opportunities for a deeper understanding

of the underlying grating formation mechanisms.[8]

As a volume holographic material, photosensitive poly-

mers represent a particularly interesting group among

stimuli-responsive polymeric materials, characterized by

their ability to be used in a non-invasive and eas-

ily controllable manner.[9]Light as a stimulus enables

optical patterning by applying volume holography as a

single-step method to fabricate diffractive 3D micro- and

nanostructures.[10]

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1584 SABEL-GRAU et al.

The formation of holographic gratings in polymers

is determined by the interplay of polymerization and

diffusion – where the chemical gradient is always the driv-

ing 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 accompa-

nied by a material transport, which creates the permanent

grating.[11]

While light-induced polymerization-diffusion processes

enable the structuring of any optically transparent, pho-

tosensitive 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.

Employing photosensitive hydrogels combines the

advantages of hydrogels and light; the hydrated structure

of the hydrogel makes them favorable as a bio-friendly

material, and the use of light as a patterning tool allows

dynamic control of their properties. Combination of these

advantages makes these systems attractive for biomedical

applications.[12]

Hydrogels, for example, 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.[13]Numerous applications open up based on explicit

control over molecular structure and mechanical proper-

ties, such as elasticity and degree of crosslinking, as well

as surface morphologies.[14]

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. In addition,

hydrogels can also be structured photolithographically

relying on diffusion processes.[15]The latter phenomenon

forms the basis for volume holography.

Photochromic hydrogels exhibit excellent reversible

conversion behavior, which can also be used for repeatable

writing of optical information.[16]Photomechanical hydro-

gels based on typical molecular photoswitches – such as

azobenzene – function as smart materials that respond to

light.[17]

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 sponta-

neously due to the thermodynamic stability of the

trans isomer.[18]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.[19]In this pro-

cess, azo-dye molecules can also be used as diffus-

ing dopants to create holographic gratings.[20]Subwave-

length surface relief and refractive index gratings can

be recorded in azobenzene-containing liquid-crystalline

polymer films.[21]Multiresponsive supramolecular liquid-

crystalline polymer organogels are suitable for the inscrip-

tion of holographic gratings with switching behavior.[22]

Recent work with functionalized hydrogels based on

8-arm PEG has shown that the mechanical and physic-

ochemical properties can be controlled by the degree

of crosslinking, with uncrosslinked functional groups

remaining after gel formation.[23]The cytocompatibil-

ity of the material was positively evaluated. Such a

(bio)chemically functionalized photosensitive gel is partic-

ularly suitable for applications with specific biointeraction

requirements.

2 RESULTS AND DISCUSSION:

LIGHT-INDUCED STRUCTURING OF THE

HYDROGEL LAYERS

2.1 Local change in optical properties

The refractive index of the film was determined by ellipso-

metric measurement. Results are shown in Table 1.

A comparison of exposed and unexposed areas shows

a decrease in refractive index due to UV exposure of the

order of 10−3in case of functionalized hydrogel. In addi-

tion, 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 crosslink-

ing. Thus, the increase in polarizability seems to outweigh

the increase in density of the material.

2.2 Generation of one-dimensional

diffractive surface gratings

The functionalized hydrogel layers were UV-crosslinked

under a photomask (1D modulated; lattice constant

SABEL-GRAU et al. 1585

TABLE 1 Elypsometrically determined local refractive index n of the hydrogel layers

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

FIGURE 1 Diffractive structures in hydrogel: hydrogel film with optical functionality (left) and corresponding surface topography (right)

20 µm). In this process, diffractive surface gratings were

formed. Figure 1shows a layer with optical functionality,

as well as the corresponding surface characteristics (topog-

raphy profile from atomic force microscopy studies) of the

patterned hydrogel. In general, detailed analysis of SRG

topography provides a good opportunity to gain insight

into the grating formation processes.[8]

2.3 Grating formation in functionalized

hydrogel

Among the various effects that can contribute to grat-

ing 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 compo-

nents, 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.[24]In addition, cis→trans isomerization

can also contribute to a change in density.[25]Then the

important aspect of photo-triggered mass migration in the

cause of component diffusion is to be considered: both with

respect to diffusion of hydrogel components PEG[15]and

with respect to diffusion of the azo-dye.[20]

The anticipated grating formation in functionalized

hydrogel is schematically shown in Figure 2.

In a 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 interfer-

ence pattern). The photoinitiator is consumed, with pho-

toinduced attachment of the dopant (here, azobenzene-

functionalized acrylates). In addition, trans→cis isomer-

ization occurs under UV irradiation. The third step, dif-

fusion of uncrosslinked dopant (and possibly – depending

on the exposure dose – also crosslinking and isomerization

of the diffused components), forms the permanent optical

grating.

3 CONCLUSION

It was shown that a local change in optical properties

can be induced in special functionalized hydrogel films

by photochemical crosslinking. A compound containing

1586 SABEL-GRAU et al.

FIGURE 2 Anticipated grating formation in functionalized hydrogel

8-PEG and azo was used, with the advantage of multiple

crosslinking points that allow the azo units to form the gel.

We found a decrease in refractive index over the course

of UV irradiation-induced crosslinking, which can be

attributed to the increase in polarizability, and which

seems to outweigh the increase in density of the material.

As we know, an inducible refractive index contrast of

the order of 10−3allows a diffraction efficiency close to

100% for 200 µm thick layers.[11]It should be noted that

the experiments performed here to determine the refrac-

tive 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 compo-

nents are involved here, trans→cis isomerization is also

a possibility for the contrast-effective change in molecu-

lar structure.[24]However, by integrating polymerization-

diffusion processes in the form of holographic patterning,

a much higher contrast can therefore be expected.

One-dimensional diffractive surface gratings were cre-

ated using a photomask. Such structured hydrogel layers

show optical functionality, and the related surface topog-

raphy 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 con-

tribution each of the two have, must be elucidated by

further investigations, preferably by holographic exposure

and exact analysis of the diffraction properties.

4EXPERIMENTAL SECTION –

MATERIALS AND METHODS

A composition based on an 8-arm star-shaped PEG with

acrylate groups (8PEG) was chosen for the preparation of

the hydrogel film, and an azo compound (azobenzene-

functionalized acrylates) was added.[18]

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.[23]After gel formation with

NH3, the gels were exposed to UV light (366 nm) for 5–7

minutes. The cured gels were peeled off and the samples

were placed on clean glass slide. The solvent was evap-

orated and further dried until constant weight. The film

thickness was 100 µm. The refractive index of the film was

then determined ellipsometrically.

ACKNOWLEDGMENTS

This research was funded by Deutsche Forschungsge-

meinschaft (DFG, German Research Foundation), grant

number SA 2990/1-1.

SABEL-GRAU et al. 1587

Open access funding enabled and organized by Projekt

DEAL.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are

available from the corresponding author upon reasonable

request.

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How to cite this article: T. Sabel-Grau, Z. Zhang,

R. Rahman, M. Lensen, Nano Select.2022,3, 1583.

https://doi.org/10.1002/nano.202200154

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