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Inkjet Printing of Soft, Stretchable Optical Waveguides through the Photopolymerization of High-Profile Linear Patterns Aleksandra Samusjew, Markus Kratzer, Andreas Moser, Christian Teichert, Krzysztof Konrad Krawczyk, and Thomas Griesser ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13272 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

Inkjet Printing of Soft, Stretchable Optical Waveguides through the Photopolymerization of High-Profile Linear Patterns Aleksandra Samusjew1, Markus Kratzer2, Andreas Moser3, Christian Teichert2, Krzysztof K. Krawczyk*,1, Thomas Griesser*,1 1

Chair of Chemistry of Polymeric Materials & Christian Doppler Laboratory for Functional and

Polymer Based Ink-Jet Inks, University of Leoben, Otto-Glöckel-Strasse 2, 8700 Leoben, Austria 2

Institute of Physics, University of Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria

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Chair of Materials Science and Testing of Polymers, University of Leoben, Otto-Glöckel-Strasse 2,

8700 Leoben, Austria

Keywords: inkjet printing, stretchable, waveguides, wetting, dewetting, contact angle hysteresis

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ABSTRACT: Optical waveguides have been fabricated via photopolymerization of stable, inkjet-printed patterns. In order to obtain high-profile lines, the properties of both the ink and the substrate were adjusted. We prove that suitable patterns, with contact angles close to 90˚, can be printed by using not fully cured, ‘sticky’ PDMS as a substrate. In addition, we propose a simple sliding-drop experiment to show the crucial difference in how the ink dewets the ‘sticky’ and the fully cured substrate, which is otherwise difficult to demonstrate. The light attenuation vs. strain curve of the obtained waveguides was determined experimentally, and was found to be almost linear within the measured strain range. INTRODUCTION

Photonic systems have distinct advantages over classical electronics, such as lack of electromagnetic interference, high sensitivity and negligible heat dissipation, but also the fact that light can trigger a variety of responses in chemical systems, either in the form of photochemical reactions or physical phenomena (e.g. fluorescence). Amongst the basic elements of photonic devices are optical waveguides, which enable information transfer via light propagation by exploiting the phenomenon of total internal reflection. Waveguides can be used e.g. in sensing1,2,3 and to transport light through tissues4,5 which in future may turn out necessary to unlock the full potential of photopharmacology.6 In the recent years, an increased demand for stretchable and biocompatible devices has emerged along with the development of soft robotics7 and biophotonics.4,5,8,9 Stretchable devices can adapt to curved or moving surfaces and can be incorporated into on-body sensing systems. Due to their intrinsic softness they also lower the risk of mechanical damage when put into direct contact with delicate tissues. While most commonly used waveguides have the diameter of a few hundred micrometers, the development of lab-on-a-chip devices and general miniaturization call for versatile 2 ACS Paragon Plus Environment

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manufacturing techniques, which enable fast prototyping of micrometre-size optical elements. Missine et al.10 introduced the concept of stretchable optical waveguides made from PDMS. Fabrication via micromoulding followed by capillary filling enabled the fabrication of about 50 µm × 50 µm channels. Similar waveguides were implemented in a sensor capable of detecting alcohol vapors.11 The drawback of those systems is that the fabrication method does not allow fast prototyping. Woods et al.12 developed flexible, optically transparent and thermally stable waveguides using two-photon absorption lithography. The curing material was based on a silanol-terminated polysiloxane crosslinked with a photopolymerizable acryloxy silane. Lewis et al.13 prepared waveguides using direct ink writing, using a coreshell print head. The core of the waveguide was made from OrmoClear® and the cladding from a stiff hydrogel shell. The role of the cladding was mainly to maintain a circular cross section of the core after deposition. After photocuring, the supporting shell could be removed resulting in free standing waveguides. The same group also described biocompatible optical waveguides made from silk protein, fabricated via direct ink writing.14 To achieve proper solidification of the silk filaments, the ink had to be deposited into a coagulation reservoir, thus requiring a rather complicated printing setup, not to mention the troublesome preparation of the ink itself. The obtained waveguides showed flexibility, but no significant stretchability. Importantly, it was shown that a circular cross section is not a necessary requirement to achieve good light transmission, even for microscopic waveguides. The goal of this work was to design and fabricate small, stretchable, photocured waveguides by using inkjet printing. In order to print waveguides, the ink, once deposited on the substrate, should result in a stable pattern with a possibly high contact angle and the curing should not result in significant deformation of the pattern. The cured material should

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have a possibly high refractive index. The ink should be easy to prepare, preferably from nontoxic components, and be stable at least for one week at room temperature. Inkjet printing has become a standard laboratory technique, due to reasonable costs and easy operation. An inkjet-printing strategy towards optical waveguides was previously proposed in the literature, but resulted in relatively flat patterns, unsuitable for efficient light transport.15,16 Liska et al.17 previously utilized commercially available acrylates and tailored photoelastomers from the Genomer series (Rahn AG) to successfully 3D-print biocompatible structures with mechanical properties matching those of natural blood vessels i.e. high strength and stretchability. Adding to that the good optical transparency of polyurethane acrylate inks, Genomer-based inks were considered good candidates for printing stretchable optical waveguides. EXPERIMENTAL Prepration of the ink. Genomer* 4267 aliphatic urethane acrylate resin (Rahn AG), ethylene glycol vinyl ether (EGVE; TCI), phenoxyethyl acrylate (PhEA; TCI) and Irgacure® TPO-L (BASF) were all used as received. In a glass vial 200 mg of Genomer 4267, 252 mg (2,85 mmol) of EGVE, 548 mg (2,86 mmol) of PhEA and 15 mg (0,047 mmol) of Irgacure® TPO-L photoinitiator were stirred magnetically until a homogenous liquid was obtained. Special attention must be paid, since even minimal exposure to white light may initiate the radical reaction. Characterization of the ink. The viscosity of the ink was measured on a MCR-102 (Anton-Paar). The contact angle measurements were performed using a drop shape analyzer (DSA 100, Krüss). The surface tension was calculated from the results of a pendant drop method, measured on the same device.


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Preparation of the PDMS for substrate and cladding. Sylgard® 184 silicone elastomer kit was purchased from Dow Corning. Parts A and B of the elastomer kit were mixed in a 10:1 ratio. To simplify the degassing process, the liquid PDMS was mixed with 10% (v/v) of n-hexane. The PDMS was poured in a plastic petri dish, in order to obtain a substrate thickness of approximately 1.0 mm, and degassed at room temperature in a desiccator for at least 10 min. The samples were cured for exactly 50 min in an oven preheated to 60 ˚C. After cooling down to room temperature the substrate is ready for ink application. At room temperature, the substrate keeps the desired properties for at least 45 min, although this period can be extended to up to several days if the substrate is refrigerated. The patterns were always printed on the top side of the PDMS, which was exposed to air during curing. For the experiments with surfactants, a desired amount of surfactant was added to component A of the PDMS to obtain a homogenous solution. After adding the curing component B the resin was processed as described above for the pure PDMS samples. Corona treatment was performed on a device with a movable treater electrode (Ahlbrandt System, TG 3001). The power was set to 300 W and the air gap was set to 3.0 mm. The treater electrode was passed twice over the sample substrate at a rate of 0.15 m/s. Plasma treatment was performed on a Plasmalab 80 Plus (Oxford instruments), at a pressure of 40 mTorr. The power was set to 100 W and the gas flow (O2) was 50 mL/min. The samples were treated for 5 s. Refractive indices were determined for λ=589 nm, using an Abbe bench-top refractometer (Fisher Scientific™).



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Sliding drop experiment. A drop of ink (10.0 µL) was placed on the tested substrate, which was then inclined at 45˚. The total length of the sliding pathway was 50 mm (measured from the middle of the drop). Images were taken with a camera at fixed time intervals. Preparation of the waveguides via inkjet printing. The lines were printed with a Dimatix® DMP-2831 (FujiFilm®). The printhead was modified with a cap which allowed the mounting of a UV lamp, so that the distance between the lamp and the substrate was 2.0 mm. Structures were printed with a DMC-11610 cartridge (max. drop volume 10 pL), with all 16 nozzles at a temperature of 50 ˚C. The cartridge angle was set to 4.5˚, and the drop spacing for both X and Y was set to 20 µm. The description of the waveform is provided in the SI (Figure S1). To obtain a stable, continuous line the linewidth must be set to at least 4Y. Lines with the highest profile were obtained by printing 3 additional 1Y lines, centered in the middle of the first line. For photocuring, the printed lines were irradiated with a spot UV lamp (OmniCure S1000). The output of the lamp was characterized with an EIT Power Puck II test set. The printed lines were irradiated approximately 1 s after deposition of the last layer at 2% intensity (1.510 mW/cm2). Then the samples were exposed for 2 min at 10% intensity (17.5±0.5 mW/cm2), followed by irradiation for 2 min at 20% (32.5±0,54 mW/cm2) intensity from a distance of 2.5 cm. The cured lines could be cladded by casting with freshly prepared silicone, followed by degassing and curing. To prepare free standing waveguides the PDMS substrate was cured at 60 ˚C for additional 20 min. The waveguides could be then gently lifted off the PDMS surface with tweezers. Mechanical testing. The mechanical testing and fatigue measurements were performed on a Zwick-Roell strength testing device. Sample dimensions and detailed description of the tensile tests can be found in the SI (Table S1). For the mechanical testing of the printed


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waveguides a small amount of dye (Rhodamine 6G, 0.03% (w/w)) was added to the ink composition to increase contrast. Measurement of transmittance loss upon strain. We have used a setup depicted in Figure S2. In brief, a representative waveguide was spanned between two pads of PDMS, which were then placed carefully on two platforms of the strain device and positioned under the microscope. The tip of the waveguide was maintained at a fixed position during the experiment. On the other platform of the strain device, a commercial LED light was mounted, able to move freely together with the linear movement of the platform. The LED was positioned in order to maximize the intensity at the front face of the waveguide. The mirror image of the tip of the waveguide was observed with a microscope. A screw was used to stepwise increase the distance between the platforms of the strain device, straining the part of the waveguide which was standing free. In our measurement, the starting length was 6.0 mm and the distance was increased in 0.5 mm steps. For each step, a photograph of the front face of the waveguide was taken. The relative intensity was determined by evaluating crosssectional profiles along the long axis of the lens-shaped front face (tip-to-tip), using the ZEN 2012 software from Zeiss. For every data point, the intensity of the background originating from the scattered light was subtracted from the intensity measured in the cross section of the waveguide. The relative intensity values were plotted vs. strain. Measurement of transmittance loss upon bending. The waveguiding properties of bent, free standing waveguides were tested for 3 different bending radii. The setup used for this measurement is schematically depicted in Figure S3. In brief, one end of the waveguide was fixed on a pad of PDMS with a fitted mirror and mounted under the microscope. The other end of the waveguide was placed on a PDMS pad with a fixed LED light. The waveguide was bend around three different bending cylinders (radii: 7.70, 4.00 and 1.15 mm) by placing the



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latter PDMS pad in various positions at equal distance from the bending cylinder, each of the positions corresponding to a given bending angle, which was determined with a protractor. The intensity of the cross-sectional profiles recorded with the microscope´s camera was determined as illustrated in the SI (Figure S4). For each bending radius, 3 cycles of bending and release were performed. RESULTS AND DISCUSSION

A schematic of the inkjet-printing approach for manufacturing optical waveguides is shown in Figure 1, and is similar to the previously reported strategy.15,16 An ink is applied with an inkjet printer to give a stable linear pattern on the surface of an elastic substrate. After photocuring, the printed structures may be covered with a layer of cladding material.

Figure 1. The inkjet-printing strategy towards stretchable waveguides. Ink composition and characterization We have decided to employ polyurethane diacrylate resins as elastic precursors, and a few different resins have been tested. The best results were achieved for samples based on Genomer* 4267 (for details see SI, Table S2). Liska et al.17 used hydroxyethyl acrylate (HEA) as a reactive diluent for polyurethane acrylate resins. In spite of its excellent polymerization properties, HEA is toxic and irritant. As a substituent, we tested ethylene glycol vinyl ether (EGVE), which is also a protic, polar monomer of similar molecular weight, but is nontoxic. The attempt to use pure EGVE has 8 ACS Paragon Plus Environment

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failed, because the vinyl ether proved not to be reactive enough in homopolymerization, resulting in an elastomer with poor mechanical properties. It was hypothesized that acrylates will copolymerize well with vinyl ethers at a molar ratio of at least 1:1.18 After screening a few systems, it was found that using a 1:1 (mol/mol) mixture of EGVE and 2-phenoxyethyl acrylate (PhEA) not only improved the mechanical properties of the core elastomer but also increased its refractive index and improved the printing properties of the ink (Table S3). An optimized ink composition contained 20% (w/w) of Genomer 4267 and 80% (w/w) of a mixture of PhEA:EGVE at a molar ratio of 1:1 (Table S4). As a photoinitiator we have used 1.5% (w/w) of Irgacure® TPO-L (for details on the screening of the initiator, see SI, Table S5) To make an ink printable, its viscosity and surface tension need to be adjusted to the printing device (see SI, S-8). In our case, those values should be in the ranges of 4-10 mPa·s and 27-32 mN/m2, respectively. At room temperature, our ink had a viscosity of 11±0.5 mPa·s and a surface tension of 30±0.5 mN/m2 - high enough to result in some clogging of the 10 µm printing nozzles. An increase in the printing temperature reduced the viscosity to 6.4±0.1 mPa·s, which was sufficient to result in a very homogenous drop deposition, and clogging was not observed even for long operation times. This was confirmed by the images from the dropwatcher (Figure S5). In contrast to the previous reports, no extra solvents are employed in our ink composition, which eliminates the problems caused by the ‘coffee-drop effect’.15 The refractive index of the cured core material was found to be significantly higher than the refractive index of the PDMS cladding (ncore=1.505, ncladding=1.409). Adjusting the surface of the substrate



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As a substrate and cladding material, we have chosen PDMS (Sylgard® 184, Dow Corning) due to its relatively low refractive index, good mechanical properties, biocompatibility and availability. The polyurethane ink has a polar character, which results in the formation of a line of separate drops after deposition on a hydrophobic PDMS surface (Figure 2a). A few strategies can be employed to decrease the hydrophobic character of PDMS.19 Corona and plasma treatment, both have a negative impact on the mechanical properties of PDMS. It was also proven very difficult to adjust the surface properties with those methods. The surface oxidation level after corona treatment was too high, resulting in very broad lines (Figure S6b). In case of oxygen plasma treatment, additionally some fluctuation of the contact line was observed (Figure S7b). The obtained flat patterns, once photochemically cured, attained a cracked, inhomogenous structure (Figure S7c). Furthermore, the surface treatment was characterized by low reproducibility, and the treated substrate showed inhomogeneous hydrophobic recovery (Figure S8). The addition of surfactants typically resulted in obtaining discontinuous lines, or lines with a very low contact angle (Table S6). We have found that printing on partially cured, ‘sticky’ PDMS gave very good, repeatable results (Figure 2b,c). There is a narrow range of curing degree which is optimal (50 min at 60° C in the exemplary case). Too soft PDMS is partially dissolved by the ink, resulting in undesired U-shaped cross sections (Figure 2d,e), and too long curing times result in obtaining discontinuous lines.


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Figure 2. (a) printed patterns, which collapsed into separate droplets on completely cured PDMS; (b)&(c) exemplary patterns printed on ‘sticky’ PDMS cured for 50 min at 60 ˚C, top view and cross section, respectively; (d)&(e) patterns printed on soft PDMS, cured for 40 min at 60 ˚C, top view and cross section, respectively. We hypothesize that for the exemplary case there must be an interface layer, at which the ink and the PDMS chains are interacting and creating a phase which accounts for the excellent stability (‘pinning’) of the contact line. To compare the ‘sticky’ and fully cured surfaces, contact angle measurements were performed. Interestingly, the static contact angle was almost independent from the degree of curing, as well for the ink as for water droplets (Table 1). Schiaffino and Sonin previously concluded that a printed liquid line can only be stable when the contact line is arrested or at least shows hysteresis.20 We found, that although there is little difference in the advancing and receding contact angle between the ‘sticky’ and fully cured substrate, the minimum receding angle is achieved somewhat slower on the ‘sticky’



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PDMS. This effect is difficult to measure using a typical setup for contact angle measurement, because of the small drop size, which implies a short ‘receding distance’ (Figure S9).

50 min /60 ˚C PDMS 51.3 ± 1.2 121.4 ± 1.6 102.2 ± 2.0 14.7 ± 0.7

Static contact angle of the ink Contact angle of water Advancing contact angle of the ink Minimum receding contact angle of the ink Time (s) to achieve the minimum receding contact 54 ± 6 angle of the ink

65 min /60 ˚C PDMS 50.6 ± 0.6 116.6 ± 0.9 96.0 ± 2.8 14.4 ± 0.6 39 ± 4

Table 1. Wetting parameters of the “sticky” and fully cured PDMS. It was previously proposed that the sliding of drops on an inclined surface can be used to determine dynamic contact angles, as well as the maximal speed of wetting and dewetting.21,22 Therefore, a simple sliding-drop experiment was performed in order to compare the ‘sticky’ and the completely cured PDMS (Figure 3). Here, the differences were more than significant: a drop of ink (10 µL), was moving 10 times slower on the ‘sticky’ substrate than on the fully cured substrate (1.7 mm/min and 17 mm/min, respectively). Moreover, in the former case the drop left a long line along its path (Figure 3l, left side), while in the latter case only minute pearling was observed (Figure 3l, right side).


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Figure 3. (a)&(b) Setup used for the sliding drop experiment; movement of the ink drop on „sticky“(c-h), and fully cured (i-k) PDMS; (l) comparison: „sticky“ PDMS after 30 min (left) and cured PDMS after 3 min (right). We therefore conclude that printing stable lines with a high profile requires a system with slow wetting and dewetting. If the printed droplets dewet the substrate too fast, high profile patterns are likely to get split into separate droplets. A sliding-drop test is well suited to show the dynamic behavior of an ink on a given substrate. It could be performed as a part of the standard characterization of any new ink-on-substrate system. In contrast to contact angle measurements it provides a better platform to study dynamic wetting models,10,22 and is simple to perform. Printing and photo-curing of the waveguides Patterns with a high profile can be obtained, by first printing a base line, and then depositing additional ink on top of it (Figure 4a-d). We found that good quality patterns require the base line to be printed with a linewidth of at least 4Y, with a drop spacing of



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20 µm. Such settings produce a base line which is approximately 80 µm wide (the diameter of a single drop is about 20 µm). The drop spacing on the X axis has to be set to exactly 20 µm. This is important, since smaller drop spacing along the X axis results in bulging of the liquid pattern (Figure S10), which can be explained23 by the high advancing angle of the ink on the ‘sticky’ substrate (Table 1). For the additional layers, the linewidth had to be reduced in relation to the base line (1Y for a base of 4Y). Those lines should be centered in the middle of the base line, as presented in Figure 4a-d, leading to patterns with increasing contact angle and aspect ratio (Figure 4e-h). Interestingly, the application of additional layers of ink results in a clear decrease of the linewidth, which additionally helps to achieve the desired high profile, with contact angles up to about 85˚ and aspect ratios exceeding 2:5 (compared to about 1:5 reported in the literature;15,16 see (Figure 4e-h and S11). This unexpected ‘line thinning’ was monitored in time (Figure S12). It was found that a printed baseline (4Y) thins to reach about 83% of the original width within 4 min after deposition (Figure S13). The minimal value - 78% of the original width is reached after 11 min from deposition, coinciding with line instability (bulging, thinning, and decomposition into droplets). Since the hydrophobicity of the PDMS stays almost constant, we hypothesize that the thinning of the patterns can also be explained by the dewetting speed. After deposition, the cohesive ink returns to the optimal contact angle from the flatter structure caused by the impact of the droplets on the substrate. For fully cured PDMS the dewetting is fast, so the pattern immediately decomposes into separate droplets. In case of the ‘sticky’ PDMS the dewetting is slow, and the decomposition does not happen until approximately 12 minutes after deposition.


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Figure 4. Printing of the waveguides: (a) deposition of a broad baseline; (b-d) single line fillups leading to the upgrowth of the structure. Cross-sections a of: (e) base line; (f) base line + 1 single line layer; (g) base line + 2 single line layers; (h) base line + 3 single line layers. (i) Top view of a series of printed waveguides. Interestingly, the lines obtained by multilayer printing had a contact angle significantly higher than the static contact angle of drops deposited with a syringe (85˚compared to 51˚, respectively). If more than 3 additional lines are deposited, a bulging starts (Figure S14). It is important to note that a contact angle of 90˚ constitutes the theoretical stability limit for liquid linear patterns, which was previously discussed in the literature.15,20,24 We were only able to print continuous patterns along the axis X. We suppose that this is mainly because of the different time intervals between the deposition of consecutive droplets, which is much longer in the Y axis. Since the contact line of a deposited droplet or bead moves due to dewetting, it is difficult to adjust the optimal drop spacing to obtain continuous



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lines. Moreover, coalescent droplets form beads which can dewet at a different rates depending on their size. Finally, for inclined lines, the drop spacing has to be adjusted for geometrical reasons. The attempts to produce 2D patterns, have therefore failed (Figure S15). We estimate that, to some degree, such patterns could be printed using direct ink writing via droplet jetting. Multilayer lines with a high profile should be cured directly after deposition of the last layer to avoid bulging. It is important to first pin the contact line by using a very low light dose and then gradually increase the dose until the lines are completely cured. Immediately curing the lines with a high dose results in deformation of the printed patterns, due to the exothermic polymerization reaction and polymerization shrinkage. (Figure S16). The cured lines can directly be used as waveguides. They can also be partially, or fully cladded with PDMS. Cladded waveguides can be cut with a blade at room temperature, without affecting their final cross section. To obtain free standing waveguides, the PDMS substrate needs to be fully cured. The waveguides easily detach from the substrate and can be manipulated with tweezers. Mechanical properties and stability of the waveguides We have tested the elongation at break of the bulk core elastomer. The test results are reported in the SI (Figure S17). Interestingly, the elongation at break for bulk core materials was largely independent of the Genomer: reactive diluent ratio and for all tested samples exceeded 70%. The mechanical properties of a printed waveguide were also tested by direct observation of the waveguides under strain. The typical elongation at which break was observed for the printed waveguides was ≥120%, significantly more than for the bulk core material (Figure S18), and slightly lower than for the pure PDMS substrate (143%). This discrepancy can be


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explained by the very smooth contact line and the uniform structure of the printed pattern. Also, importantly, the waveguides did not deform or detach from the substrate during the strain experiments. Aged samples (3 months after preparation) showed no decrese in mechanical performance. The samples were also tested for fatigue over cyclic uniaxial stretching. No viscoelastic deformation was observed (Figure S19). Waveguiding properties The ability to guide light can be directly observed for freestanding waveguides coupled with a light source (Figure 5a-c). To rule out that the bright surface of the cross section of the waveguides (Figure 4e-h) is coming from scattered light, a small amount of a fluorescence dye (Rhodamine 6G, 0.014% (w/w)) was introduced to the ink composition. The end tip of the UV-cured waveguide was then observed under a microscope. The waveguide was irradiated with blue light 3.0 cm away from the tip, in order to excite the fluorophores. The emitted light, which was now generated exclusively inside the waveguide, could be observed at the end of the waveguide through a lens fitted with a longpass filter with a λcut-off=540 nm (Figure 6d).



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Figure 5. (a) Tip of the free standing waveguide; tip of the free standing waveguide with coupled laser light, bright (b) and dark field (c) images, respectively. (d) Cross section of the waveguide doped with Rhodamine 6G and excited with blue light. The loss of transmittance under applied strain was tested in the strain range which does not induce viscoelastic deformation, using a home-build setup (Figure S2). The intensity values at the end of the strained waveguide were plotted vs. strain to give an almost linear fit (Figure 6). From the slope of the linear fit a relative attenuation of about 0.7 dB for every 10% of applied strain could be calculated.


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Figure 6. Curve of light intensity vs. strain for the uncladded printed waveguide. The influence of the bending on the transmittance was also tested using a home-built setup (Figure S3). As expected, no clear influence could be observed for the tested bending radii (Figure 7), which exceed the diameter of the waveguide by 2 orders of magnitude. Nevertheless, the experiment shows, that the degree to which this microscopic waveguides can be manipulated, which includes bending, is enough so that they could be applied e.g. in selectively delivering light to small objects.



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Figure 7. Influence of the bending angle on the transmittance measured for 3 different bending radii. The error bar represents the highest standard deviation measured for any data point. CONCLUSIONS & OUTLOOK

This work is a proof-of-concept that stretchable elastomeric lines with a high profile can be manufactured on a flat substrate by using inkjet-printing as the only patterning technique. As expected, such patterns can be used as waveguides. We managed to use PDMS as a substrate without using any surface treatment or surfactants. Instead, we controlled the wettability by the curing conditions of the PDMS. Careful analysis of the wetting behavior lead us to the conclusion that obtaining a stable linear pattern with a high profile can be attributed to the dynamic, rather than static wetting properties of the ink on the substrate. Especially, the speed of dewetting seems to be crucial, while the value of the contact angles, both advancing and receding, gives little insight. To monitor the speed of dewetting, we carried out a simple sliding-drop experiment. We conclude that sliding drop tests provide more valuable information about the dynamic behavior of the ink, compared to traditional contact angle measurements, and yet are very simple to perform. The described fabrication method allows the preparation of soft, stretchable, waveguides embedded in PDMS and also free standing waveguides. The waveguiding properties under applied strain and bending were tested. A more in-depth characterization of the optical properties is currently underway. We propose that our preparation technique could be applied in the manufacturing of sensors and elastic lab-on-a-chip photonic systems. Furthermore, the free standing waveguides could be used to selectively irradiate micrometre-sized objects with high spatial resolution.


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ASSOCIATED CONTENT

SUPPORTING INFORMATION

The Supporting Information is available free of charge on the ACS Publications website at DOI: Graphical presentation of the printing waveform, the dimentions of the samples used for mechanical testing of the core material, tensile test parameters, schemes of the setups used for the investigation of the waveguiding performance, imaging process for the evaluation of the light attenuation, overview of the tested ink compositions, characteristics of an inkjet printable fluid, images from the dropwatcher, images of the ink behavior on corona and plasma treated substrates, overview of the surfactants tested for the modification of the PDMS, images of drop shapes during the measurements of the dewetting speed, influence of the drop spacing on the formation of the lines, readout of the contact angles of the printed structures, footage of the thinning of the baseline, a chart presenting the relative decrease in the width of the printed baseline, deformation caused by exceeding the maximal amount of ink layers, outcome of the printing in of complex geometries, image of the patterns deformed by overdosed light, results of the tensile test with regards to the Genomer 4267 content, images of the samples used for tensile test of the printed waveguides, images of the samples subjected to cycling stretching. AUTHOR INFORMATION Corresponding Authors Krzysztof K. Krawczyk*: [email protected] Thomas Griesser*: [email protected]

ACKNOWLEDGMENT

Financial support by the Durst Phototechnik GmbH is gratefully acknowledged. Furthermore,



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the authors thank the Christian Doppler research association and the Austrian Ministry of Science, Research and Economy (BMWFW) for financial support. REFERENCES (1) Washburn, A.L.; Bailey, R.C. Photonics-on-a-chip: Recent Advantages in Integrated Waveguides as enabling Detection Elements for Real-World, Lab-on-a-chip Biosensing Applications Analyst 2011, 136, 227-236. (2) Duveneck, G.L.; Pawlak, M.; Neuschäfer, D.; Bär, E.; Budach, W.; Pieles, U.; Ehrat, M. Novel Bioaffinity Sensors for Trace Analysis Based on Luminescence Excitation by Planar Waveguides Sens. Actuators B 1997, 38-39, 88-95. (3) Budach, W.; Abel, A.P.; Bruno, A.E.; Neuschäfer, D. Planar Waveguides as HighPerformance Sensing Platforms for Fluorescence-Based Multiplexed Oligonucleotide Hybridization Assays Anal. Chem. 1999, 71, 3347-3355. (4) Nizamoglu, S.; Gather, M. C.; Humar, M.; Choi, M.; Kim, S.; Kim, K. S.; Hahn, S. K.; Scarcelli, G.; Randolph, M.; Redmond, R. W.; Yun, S. H. Bioabsorbable Polymer Optical Waveguides for Deep-Tissue Photomedicine Nat. Commun. 2016, 7, 10374. (5) Choi, M., Choi, J.W.; Kim, S.; Nizamoglu, S.; Hahn, S.K.; Yun, S.H. Light-Guiding Hydrogels for Cell-Based Sensing and Optogenetic Synthesis in Vivo Nat. Photonics 2013, 7, 987-994. (6) Lerch, M. M.; Hansen, M. J.; van Dam, G. M.; Szymanski, W.; Feringa, B. L. Emerging Targets in Photopharmacology Angew. Chem., Int. Ed. 2016, 55, 10978–10999.


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W. 3D Optical Waveguides Produced

by Two Photon

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