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We present a thiol-ene/methacrylate-based photopolymer capable of creating coplanar physical features (e.g. micro-fluidic channels) and optical index features (e.g. waveguides) using standard mask-based lithography techniques. This new photopolymer consists of two monomer species that polymerize at different rates. By selectively exposing different areas of a device for various amounts of time, we can select the state of the polymer (i.e. liquid, rubbery, or glassy) to create fluid channels or optical index structures such as waveguides. Using only three exposure steps and two masks, we demonstrate an integrated refractometer with a 90° channel-waveguide crossing to illustrate the fabrication process and the ability to create lithographically aligned waveguides across a gap.
©2012 Optical Society of America
1. Introduction
Lab-on-a-chip devices have benefitted from the ability to combine optical features and fluidic features in a common optofluidic device. In many approaches to integrating these features, optical waveguides are formed using complex procedures that involve creating the waveguide from separate materials (e.g. encapsulating optical fibers, patterning silicon, or adding high-index polymer). Tighter integration of optical and fluidic structures with reduced fabrication complexity is advantageous for strong light-liquid interactions (e.g. refractometers) and more complex optical structures (e.g. Bragg holograms). Holographic photopolymers offer a new route to fluidic-photonic device integration in a single material that may reduce the complexity of integrating optical and fluidic features while allowing for the possibility to create more complex optical features.
Holographic photopolymers are solid materials that self-develop index structures in response to optical exposure. In these materials, optical exposure locally consumes monomer, driving in-diffusion of replacement monomer which increases the index of refraction [1,2]. Without any need for subsequent wet processing, these materials are ideal for thick, complex photonic structures such as densely-overlapped holograms [1], narrow-band holographic filters [3], optical waveguides [4,5], and gradient-index lenses [6]. As with other photopolymers, the component monomers are inexpensive and able to encapsulate hybrid components during the initial liquid to solid transition. We expand on the capabilities of current holographic polymers by developing a polymer capable of creating both fluidic channels and index features through sequential UV exposure. The polymer acts as a negative photo-resist that we use to create physical fluid channels along with monolithic index structures in a single polymer layer. We illustrate the processing of this material with a 90° waveguide-channel crossing to implement a refractometer (Fig. 1 ).
Fig. 1 Cartoon of a coplanar microfluidic device that contains a fluid channel and a polymer waveguide crossing each other.
2. About the polymer
2.1 Formulation
The polymer consists of three monomer species that polymerize according to two reactions which occur at different rates (see Section 2.3). The methacrylate, hexanediol dimethacrylate (HDDMA) (50% wt), is a low-index (n = 1.457), chain-growth polymer that gels rapidly to create both the desired physical structures and the matrix for subsequent optical feature fabrication. The thiol-ene, triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetra(3-mercaptopropionate) (PETMP) (16% wt and 34% wt respectively), are high-index monomers (n = 1.510 and n = 1.530 respectively) that cure more slowly in the presence of the methacrylate in a step-growth process.
The methacrylate and thiol-ene monomers do not significantly co-polymerize even though they are both initiated by the same UV photo-initiator, Irgacure 651 (0.10% wt). These three monomer species form a two-stage reactive polymer network that allows us to control the state of the polymer (i.e., liquid, rubbery, or glassy) depending on the amount of UV exposure. Nair et. al. have also demonstrated the ability to control the state of the polymer using a two-stage reactive polymer network, however they utilized orthogonal first and second stage curing mechanisms [7]. Aluminum nitrosophenyl-hydroxylamine (N-PAL) (0.01%wt) is used as an inhibitor to suppress undesired thermal initiation.
2.2 Material properties
The resin and cured polymer are optically clear and the material shows no measurable absorption in the visible range (400 - 800 nm) from UV-Vis spectroscopy measurements. The index of refraction of the liquid monomer resin was measured experimentally to be 1.4861 ± 0.0002 using the Link® Abbe Refractometer Model 2WAJ, which is in reasonable agreement with the calculated Lorentz-Lorenz value of 1.4857. The index of refraction of the gelled polymer and the cured bulk resin are 1.500 ± 002 and 1.529 ± 002 respectively, measured using prism coupling at 633 nm. The refractive index increase δn achieved through exposure and diffusion is ~0.006 above the bulk index. The refractive index of the the bulk material and the acheivable index contrast can be controlled either through selection of different monomers or using different ratios of HDDMA to TATATO and PETMP. As with other similar methacrylate and thiol-ene polymers [8], this material shows good biocompatiblity.
2.3 Polymerization characteristics
Figure 2(a) provides photo-rheology data for the polymer taken at 365 nm with an intensity of 13.7 mW/cm2. The shear modulus of the polymer has two different phases, liquid and solid, with a gradual transition in between. In its liquid phase, the shear modulus starts low at ~3 kPa. After 75 seconds of irradiation the modulus begins to increase. The modulus levels off at ~3 x 105 after 175 seconds of irradiation with the resin reaching a glassy state after ~300 seconds of exposure. The inflection point of the shear modulus increase occurs at ~100 seconds, which corresponds to the experimentally determined gel point of the polymer. Past the gel point and up to ~230 seconds of exposure, the polymer is a rubbery solid capable of allowing diffusion of monomer.
Fig. 2 Polymer [a] shear modulus and [b] methacrylate and ene conversion versus exposure time. Both the rheology data and the conversion data were taken at 365 nm with an intensity of 13.7 mW/cm2.
Figure 2(b), also taken at 365 nm with an intensity of 13.7 mW/cm2, shows the percent conversion of the methacrylate and the ene monomers as a function of time. Initially, the resin is liquid. Upon exposure, the methacrylate monomer converts much more rapidly to polymer than the ene monomer until the mixture gels at ~100 seconds. At this point, the material is a low-modulus solid where ~40% of the methacrylate monomer has been converted to polymer while ~5% of the ene monomer has converted, indicating that the gelled solid is primarily polymethacrylate. The remaining polymerizable material consists of ~60% of the initial methacrylate, ~95% of the initial ene monomers, and sufficient photo-initiator to be photosensitive. For exposures between 100 and 230 seconds, both monomer species convert to polymer at roughly equal rates. In this exposure range, the material is solid but not rigid so that diffusion of monomer is able to occur. After ~300 seconds the polymer is fully cured; both the methacrylate and the ene-monomers have reached their maximum conversion and the material is no longer photosensitive. By using masks to control how long various portions of the polymer are exposed, we can control the state of the polymer (e.g. liquid, rubbery, glassy) and the relative amount of monomer conversion to affect the creation of fluidic or optical features.
3. Device fabrication
3.1 Device preparation
To make a device we first assemble a glass-monomer resin sandwich on an anodized aluminum base whose maximum dimensions are 26 mm x 12 mm x 11 mm. We sandblast the base prior to anodization to create a matte, non-reflective finish that minimizes secondary reflections during UV exposure. We cast the top and bottom of the polymer against fused-quartz glass (n = 1.460) that will serve as the top and bottom cladding of the waveguide providing vertical optical confinement. Index structuring of the polymer provides horizontal optical confinement in the waveguide. The top piece of glass is a 160 μm thick coverslip (Esco Products R412012S1-UV) chosen to minimize diffraction effects of the light passing through the mask. The bottom layer of glass is 1/16 inch thick (Esco Products Q340063) and diced to match the size of the coverslip. The polymer layer is 12 mm x 12 mm x 63.5 μm where the thickness is set by plastic spacers. Although one can set the polymer thickness using spin-coating techniques, a casting method was more conducive to the size and scope of the devices being fabricated [9]. To create optically flat waveguide faces for butt-coupling of the fiber source, Polydimethylsiloxane-coated glass is attached to the sides of the base corresponding to the entrance and exit faces of the waveguide. The coating allows the polymer to release from the coated glass leaving optically flat entrance and exit waveguide faces after the polymer is fully cured. Once we assemble the monomer sandwich, the steps required to make an integrated optofluidic device are shown in Fig. 3 .
Fig. 3 Integrated optofluidic device processing steps. [a] UV light converts the liquid resin to a solid gel where the mask is transparent. [b] Unexposed monomer remains liquid. [c] Uncured monomer is removed using suction and/or a solvent wash. [d] UV light further polymerizes the illuminated areas, causing a monomer gradient in the sample. [e] The exposed polymer is depleted of monomer, resulting in a gradient in the monomer concentration. [f] The sample is held at elevated temperature for several days to allow for monomer to diffusion into the region exposed in step [d] as shown by the arrows. [g] A final flood cure is performed to fix the optical features and consume remaining monomer in the sample. [h] The final device contains a fluid channel and cured polymer with a line of high-index polymer surrounded by a lower-index polymer.
3.2 Device processing
First we illuminate the fluidic-channel mask, a 190.5 μm diameter wire (Fig. 3(a)), for 100 seconds and bring the exposed monomer to the gel point (Fig. 3(b)). The wire used in this demonstration was straight for simplicity in demonstrating the processing of the material; however devices with more complex channel geometries as well as multi-layer devices are also readily achievable with this technique and have been previously demonstrated in similar materials [9,10]. During this first exposure, predominantly methacrylate polymerization converts the liquid to a solid gel where the mask is transparent. Unexposed resin remains liquid and can be removed by suction and/or a methanol solvent wash leaving an empty channel across the device (Fig. 3(c)). Tygon tubing fitted to a vacuum port or a syringe filled with methanol placed against the fluid channel is sufficient to suck or push the liquid resin out of the fluid channel. A wire was more desireable than the equivalent chrome-on-glass mask for this first step because the weight of the mask causes resin to seep between the coverslip and the mask. When exposed to UV light during the first exposure, resin beneath the clear part of the mask and the coverslip becomes gelled. This causes the mask to bond to the coverslip and prevents the removal of the mask in future processing steps. A high quality transparency film mask to reduce the weight of the mask could also be utilized. If we are making a waveguide-only device, we complete step one (Fig. 3(a)) without a mask, thereby flood-curing the entire sample to the gel point with no need for a solvent wash.
To create the index feature (i.e. a waveguide), a chrome-on-soda-lime glass mask with a 63.5 μm transparent line is exposed to light for 130 seconds during a second exposure step (Fig. 3(d)). Local conversion of the thiol-ene monomers depletes thiol-ene monomer concentrations in the illuminated region while adjacent non-illuminated regions remain thiol-ene monomer rich (Fig. 3(e)). The rubbery methacrylate matrix allows diffusion of high-index thiol and ene monomers from the monomer-rich regions to the monomer-depleted regions. This in-diffusion of monomer raises the index of refraction roughly proportional to the transparency of the second mask, implementing an analog, negative-tone response. Due to diffraction, the actual width of the waveguide is roughly 67μm. This pattern blurring leads to a gradient index response at the edges of the waveguide, which is useful in reducing edge scatter in the waveguide. The dimensions of the polymer waveguide were chosen to approximate the dimensions of the fiber from the source. The size and shape of the waveguide is limited by one’s ability to make the mask so that feature sizes down to a few microns should be readily achievable with mask-based lithography techniques. Thus single-mode waveguides are achievable with this material. Smaller feature sizes are possible using direct-write lithography techniques.
Characteristic diffusion coefficients for holographic photo-polymers are on the order of ~0.1 μm2/s at room temperature [1]. Diffusion time across a 100 μm channel is thus on the order of a week or more. This diffusion time can be reduced by approximately a factor of two by elevating the temperature to 80 °C. However, differential thermal expansion can lead to delamination of the polymer from the glass substrate. Therefore, samples in this study were allowed to diffuse in the dark at a maximum of 60 °C for one to four days depending on the scale of the optical structure (Fig. 3(f)).
Finally, Fig. 3(g) shows a flood cure of the entire sample with no mask for 270 seconds to consume all remaining initiator, inhibitor, and monomer, rendering the polymer chemically and optically inert. The final device contains an open channel that fluid can flow through along with a line of increased index of refraction that acts as an optical waveguide (see Fig. 3(h)). Note that since the waveguide is made with a mask that crosses the fluid channel, the waveguide is lithographically aligned across the gap.
3.3 Device analysis
After the device is complete we use a 3-D micrometer stage to butt-couple an FC/PC-connectorized 635 nm fiber source to one end of the waveguide and a photo-detector to the other end of the waveguide. Once aligned, we attach the fiber and detector to the anodized-aluminum base holding the polymer sandwich using tapped holes to lock the components in place. A hole in the anodized base provides access to the microfluidic channel and we adapt a luer-lock syringe to hold an FEP tube (Upchurch Scientific F-242X) that butt-couples to the microfluidic channel. If one desires continuous flow, a syringe pump can be attached to the device instead. Figure 4 shows pictures of the assembled device.
Fig. 4 Images of assembled devices. [a] A finished device is attached to a base, while a FC/PC connector is attached to a 3-D stage to allow alignment of the fiber to the polymer waveguide. A microscope objective re-images the end of the waveguide for analysis. [b] Once the fiber is aligned to the waveguide, the FC/PC connector is disconnected from the stage and attached to the base of the device which allows for portability. [c] The fully assembled device with attached detector, fiber, and syringe with water.
We made two categories of devices (Fig. 5 and Fig. 6 ): waveguide-only devices and refractometers. Total loss for a 12.5 mm x 67 μm x 63.5 μm waveguide-only sample (i.e. output guided power compared to the incident power) was 1.91 dB, which includes coupling loss, material absorption, and Fresnel losses. We estimate the waveguide loss (i.e., output guided power compared to total output power) to be 0.57 dB or approximately 0.46 dB/cm propagation loss. This loss is sufficiently low for typical sizes of microfluidic devices. The refractometer consists of a microfluidic channel with a waveguide across it whose optical throughput depends on the index of the fluid flowing through the channel compared to the index of the waveguide and air gap (Figs. 5(c) and 5(d)). When we introduce a 190.5 μm channel in a waveguide plus channel device, the waveguide loss of the refractometer is 6.0 dB decreasing to 3.9 dB when filled with water. Although not a field-ready device, the performance of our device demonstrates that our material processing technique produced a refractometer whose optical transmission efficiency is modulated by Fresnel reflections and diffraction as evidenced by the reduction in loss when water is added to the channel. With further optimization and calibration with known sources, this device could be implemented as a refractometer.
Fig. 5 Images of functioning devices. [a] A waveguide only device with a butt-coupled SMF-28 fiber attached on the right. The light on the left side of the waveguide travels to a 10X microscope used to magnify the output. The output face of the waveguide is reimaged and shown in [b]. [c, d] Refractometer devices containing a fluid channel (vertical) and a waveguide (horizontal). In [c] the channel contains air. In [d] the channel contains water.
4. Conclusion
In summary, we present a holographic photopolymer capable of creating integrated optofluidic devices. Mask-based photo-lithography enables simple processing of co-planar optical and fluidic features that are tightly integrated together and lithographically aligned. A waveguide-only device demonstrates the ability of the polymer to develop low loss waveguides. This low loss is most likely achieved because of the lack of edge scatter in the waveguide due to the gradient index response of the polymer at the edges of the waveguide, which is typical of holographic photopolymers [11]. A connectorized refractometer device, roughly 1 cm3 in volume, demonstrates the ability of the polymer to create lithographically aligned waveguides across a fluidic channel in a compact planar geometry.
Although we demonstrate a planar optical waveguide formed with a binary chrome mask and UV lamp source, the use of direct-write laser lithography [4] and spatial light modulation (i.e. an analog, programmable mask) can form 3D-gradient index structures not possible with traditional wet processing. More intricate 3D-fluidic features can be achieved by using wax as a sacrificial material in the fluid channel and stacking layers of polymer [9]. Further, the refractive index is tunable within these methacrylate/thiol-ene systems. Thiol-ene stochiometry and the amount of methacrylate will affect not only the gel point and final material properties, but also the refractive index as well. In general, controlling the index of refraction can best be achieved by selecting different monomers with different base refractive indices [12]. Thus the refractive index of this two-stage reactive polymer network, and possibly the material properties as well, can be tuned to address the needs of a particular application. Combining these capabilities along with the ability to encapsulate optical components such as fibers and detectors potentially allows this polymer to provide increased flexibility in creating arbitrary 3D index and/or micro-fluidic structures in integrated optofluidic devices.
Acknowledgments
We gratefully acknowledge financial support of the National Science Foundation (Grant No. ECS-0335765) and the AFOSR MURI program (Grant No. FA9550-09-1-0677). Undergraduate student research support was provided by the University of Colorado Discovery Learning Apprenticeship program, the Carleton College Clinton Ford Physics Research Fund, and the Carleton College Clare Booth Luce Foundation Fellows Program.
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