Article pubs.acs.org/Langmuir
A New Perfluoropolyether-Based Hydrophobic and Chemically Resistant Photoresist Structured by Two-Photon Polymerization Carmela De Marco,*,† Arune Gaidukeviciute,‡ Roman Kiyan,‡ Shane M. Eaton,§ Marinella Levi,† Roberto Osellame,§ Boris N. Chichkov,‡ and Stefano Turri† †
Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy ‡ Laser Zentrum Hannover e.V., Hollerithalle 8, D-30419 Hannover, Germany § Istituto di Fotonica e Nanotecnologie del CNR, Piazza Leonardo da Vinci 32, 20133 Milan, Italy S Supporting Information *
ABSTRACT: Two-photon polymerization technology has been used to fabricate submicrometer three-dimensional (3D) structures using a new polyfunctional perfluoropolyether-based resist, which is a polymer intrinsically hydrophobic and chemically resistant. The fluorinated resist was designed and synthesized in this work and successfully employed to fabricate woodpile structures in various experimental conditions. This is the first demonstration of the capability to fabricate hydrophobic and chemically resistant 3D structures with submicrometer resolution and arbitrary geometry.
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elements.30 The main features of PFPE polymers are a high thermal stability, a low surface energy (8−14 mN/m), a tunable elastic modulus (1−100 MPa),31,32 and antifouling characteristics,32,33 which make them suitable to obtain polymeric materials with high chemical resistance and extremely low wettability.34,35 Moreover, structuring low surface tension polymers through 2PP may be useful to fabricate hydrophobic atomic force microscopy (AFM) tips, required for imaging of hydrophilic surfaces such as biomacromolecules or hydrogels, since the adhesive forces between the standard AFM tips and the surface may affect the quality of topographic imaging. To minimize the adhesion forces between AFM tips and hydrophilic surfaces, hydrophobic self-assembling coatings are normally used, but they are easily removed by the tip interaction with the surface.36 Kim and Muramatsu reported the 2PP fabrication of hydrophobic AFM tips,36 but the contact angle of the photopolymer was about 70°, which was too small to be considered a low wettability surface. In this work, we describe the synthesis and processing of a new PFPE-based hydrophobic and chemically resistant photoresist, successfully structured by 2PP with the fabrication of woodpile structures with an excellent submicrometric resolution. This is in contrast to commercially available linear PFPE resins having dimethacrylate groups, with a degree of functionality too low to be structured efficiently by 2PP. To
INTRODUCTION Two-photon polymerization (2PP) is an innovative technology for three-dimensional (3D) structuring photopolymers at the submicroscale, overcoming the diffraction limit intrinsic to standard lithographies, since it is based on the nonlinear absorption of ultrashort pulsed radiation in transparent photosensitive materials.1−4 So far, the potential applications of this technology span diverse fields, including the production of photonic elements,5−7 the realization of scaffolds for regenerative medicine,8−11 the prototyping of MEMS,12,13 and the fabrication of microfluidic components.14−16 The most popular photoresists for 2PP are negative-tone resins such as those containing acrylic oligomers,17,18 epoxy resins, which are capable of creating structures with high aspect ratios,18,19 and hybrid organic/inorganic sol−gel materials, whose chemical−physical properties can be tuned varying the amount and composition of the inorganic component.20−23 The main advantages of acrylic resins over the other photoresists are their wide commercial availability, their optical transparency, and their ease of processing.17 Unfortunately, the poor chemical resistance of acrylic polymers to organic solvents hinders the development of 3D micrometric structures employable as microfluidic filters and mixers in applications requiring chemically resistant microfluidic elements. Among the various macromolecular families, perfluoropolyether (PFPE) polyurethanes are well known and very suitable to design and realize hydrophobic and chemically resistant materials.24−27 In particular, PFPE-urethane methacrylates are emerging as precursors of solvent-resistant microfluidics28,29 and optofluidic © XXXX American Chemical Society
Received: September 21, 2012 Revised: November 26, 2012
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the best of our knowledge, this is the first example of 3D microfabrication of hydrophobic submicrometric structures. These new PFPE photoresists may be employed for manufacturing solvent-resistant microfluidic elements and hydrophobic AFM tips.
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EXPERIMENTAL METHODS
Chemicals. The PFPE-dimethacrylate (Fluorolink MD700) and the precursor PFPE macrodiol were provided by Solvay−Solexis (Italy). Isophorone diisocyanate (IPDI) and the catalyst dibutyltin dilaurate (DBTDL) were purchased from Sigma−Aldrich (Italy) and used as received. The pentaerythritol triacrylate (PETA, commercial name SR444D) was provided by Sartomer (France). The photoinitiator Lucirin TPO-L was purchased from BASF (Germany). Finally, 3-methacryloxypropyltrimethoxysilane (MAPTMS) was purchased from ABCR (Germany) and used to silanize glass slides. Synthesis and Photopolymerization of the PFPE Photoresist. A two-step synthesis process was followed. In the former, the PFPE macrodiol (molecular weight 1000 g/mol) was end-capped with an IPDI excess to form a NCO-terminated prepolymer (stoichiometric ratio NCO/OH = 2:1). The reaction was carried out at 60 °C in a glass vessel equipped with nitrogen inlet, mechanical stirrer (200 rpm), and condenser. The PFPE-diol was added dropwise to the isocyanate in the presence of DBTDL (0.1% w/w relative to IPDI weight). The reaction was periodically monitored by a Fourier transform IR (FTIR) spectrometer, and it was considered complete when the ratio between the absorbance of NCO groups (∼2260 cm−1) to urethane (1740 cm−1) reached a constant value (about 2 h). In the latter step, PETA was added to fluorinated prepolymer (molar ratio PETA/PFPE−IPDI = 2:1), yielding the final polyfunctional oligomer here referred to as PFPE−IPDI−PETA. The reaction was periodically monitored by FTIR spectroscopy until complete consumption of the NCO groups. The obtained PFPE-based resin was doped with the photoinitiator Lucirin TPO-L (3% w/w), spincoated (1500 rpm) on a previously silanized glass slide, and cured under UV light (λ ∼ 365 nm, 4 mW/ cm2) for 10 min under inert atmosphere. The silanization, needed to improve the adhesion between the resin and the substrate, was carried out exposing glass slides to MAPTMS vapors overnight. The UVcured fluorinated film remains attached to the silanized glass slides after rinsing in ethanol (the solvent used for development). Two-Photon Polymerization Process Parameters. For 2PP structuring, the PFPE-based photocurable material was placed between two 150 μm thick glass slides as shown in Figure 1. The distance
Figure 2. Setup for high-resolution fabrication by 2PP. the high numerical aperture objective, focusing femtosecond laser pulses into the volume of the photoresist. The scanner is mounted on the z axis of the air bearing large-range positioning system (ABL 10100-LN, Aerotech Inc., USA). The CCD camera enables online process monitoring. The sample is mounted on a two-dimensional (2D) air bearing translation stage (ABL 10100-LN, Aerotech Inc., USA). By moving the laser focus three-dimensionally inside the photoresist, using scanner and translational stages, one is able to write complex 3D structures. In order to suppress vibrations, all components of the setup are mounted on a granite frame. Laser patterning is performed with a 100× oil immersion objective (Plan Apochromat, Zeiss, Germany) with a numerical aperture of 1.4. After structuring, the samples are developed in order to remove the nonpolymerized material. For development of the PFPE-based materials, 1-propanol is used. This developer is preferable, since it preserves integrity of submicrometer structures better than ethanol. Characterization. Contact Angle. The contact angle measurements were performed using an optical video contact angle system (OCA-15-plus, Dataphysics). A 1 μL droplet of test liquids was dispensed on the samples using the electronic syringe unit of the instrument equipped with a 500 μL Hamilton syringe. The static contact angle was measured using the sessile drop method with dedicated software (SCA 2.0) determining the contact angle based on the Young−Laplace fitting. Water (Chromasolv water for HPLC, Sigma−Aldrich) and diiodomethane (ReagentPlus 99%, Sigma− Aldrich) were used as test liquids. Electron Microscopy. A scanning electron microscope (SEM) (FEI Quanta 400 FE) was used for imaging of the polymer woodpile structures fabricated by 2PP. For the SEM imaging, polymer structures were covered by a 50 nm thick gold layer. The gold layer was deposited by an E5000 M SEM coater (BIO-RAD, USA).
Figure 1. Experimental layout for 2PP structuring with oil immersion objective (left) and schematic sample view after development (right).
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between the slides was set by a spacer and fixed at 150 μm. Adhesion of the polymerized material to the surface of the bottom glass slide was improved by silanization of the glass substrate. Prior to deposition, the PFPE-based material was filtered with a 0.8 μm syringe filter. The schematic of the setup used for high-resolution 2PP fabrication of 3D polymer structures is shown in Figure 2. The second harmonic of an Yb/glass laser is used as a source of linearly polarized femtosecond pulses. The characteristics of the second harmonic radiation of the Yb/glass femtosecond laser system are the following: 513 nm wavelength, 150 fs pulse duration, 1 MHz repetition rate, and 20 mW laser power. A polarizing cube and a λ/2 waveplate, installed on a motorized rotation stage, are used for energy adjustment of femtosecond pulses. An acousto-optic modulator is employed as an optical shutter. The expanded laser beam is guided through the x−y galvanometer scanner (hurrySCAN-14, Scanlab AG, Germany) into
RESULTS AND DISCUSSION Dimethacrylate PFPE oligomers are commercially available as photocurable resists,37 and accordingly, we prepared a formulation containing a commercial PFPE-urethane dimethacrylate (96% w/w) doped with the photoinitiator Lucirin TPOL (4% w/w). This formulation showed good UV light polymerization but no evidence of 2PP in a wide range of parameters (average power from 0.1 to 6 mW and speed from 100 μm/s to 1 mm/s). It is likely that the overall degree of functionality (number of reactive functions per molecule) of the resin is too low for 2PP structuring technology. In order to improve 2PP structurability of the resist, we increased the degree of functionality of the PFPE. Moreover, we changed B
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methacrylate groups with the more reactive acrylates. Since a PFPE-based product with such features is not commercially available, we designed and synthesized such a photoresist. The synthesis of the new PFPE-based resin consists of two main steps. The structures of the molecules involved in the first part of the reaction (here, referred to as PFPE−IPDI) are shown in Figure 3.
It is known that IPDI is an asymmetric cycloaliphatic diisocyanate, bearing two different isocyanate groups, with one group directly bonded to the cycle (secondary NCO), while the other is bonded to a methylene group (primary NCO). The secondary NCO is more reactive38 because of the minor steric hindrance. Due to the low reactivity of the NCO groups of IPDI toward hydroxyl groups, the use of a catalyst is needed to carry out the reaction between the hydroxyl groups of PFPE and the NCO secondary groups of IPDI. The effect of PFPE segments on some physicochemical properties of the resulting resist was studied. Two photocurable formulations were prepared, with one obtained by mixing the PFPE-based resin with the photoinitiator Lucirin TPO-L (3% w/w), here referred to as PFPE−IPDI−PETA+PI, and the other one obtained by mixing pure PETA with the same PI at the same percentage, here referred to as PETA+PI. The two resists (PFPE−IPDI−PETA+PI and PETA+PI) were spincoated on silanized glass slides and cured under UV light. For each of them, the contact angles (CA) versus water and diidomethane were measured, and the corresponding surface energy dispersive and polar components (γ) were calculated according to Wu’s method39 (Table 1). A clear increase in hydrophobicity was found for the PFPE-based resist, and a consequent sharp decrease was found in the surface energy, which is affected by the reduction of both its contributions from polar (γp) and dispersive forces (γd), suggesting the enrichment of a fluorine-based component on the resist surface as already known for similar systems.26 In addition, the chemical resistance of these two different materials was evaluated by suspending thin, self-supported films in toluene (1 h, room temperature). After these tests, the polymeric films made of PFPE−IPDI−PETA+PI showed a very good chemical resistance and an excellent structural integrity after exposure to the solvent, whereas the films made in PETA +PI were completely broken after a few minutes in contact with the solvent. Fabrication of Two-Photon Polymerized Structures. To find the optimal process parameters window suitable for high-resolution 2PP structuring of the new PFPE resist, an array of woodpile structures (Figure 5a), each measuring 50 μm × 50 μm × 50 μm, was fabricated. The choice of woodpile structures is due to the fact that this is a prototypical layout. Besides being widely used in photonic crystal fabrication, it shows the capability of the material to provide high-resolution features and mechanical stability. The laser power was varied along the x axis of the array, and the structuring speed was varied along the y axis. Properly exposed woodpile structures are located in the area bordered by the two white lines indicated in Figure 5a: at the top-left are underexposed structures, which are only partially cured and almost entirely washed away or strongly distorted during the development, and at the bottom-right, are overexposed structures, showing the material damage due to overheating of the polymer. Damage and 2PP thresholds are shown as curves in the average power− structuring speed parameter space in Figure 5b.
Figure 3. Structures of the molecules involved in the first part of the reaction, PFPE-diol, IPDI, and the catalyst DBTDL, and the chemical structure of the resultant PFPE−IPDI.
The final resin can be represented as a hexafunctional PFPE polyacrylate (Figure 4).
Figure 4. Process for obtaining the PFPE-based resin.
Table 1. Comparison of an Acrylic-Based Resist (PETA+PI) to the Same Resist Modified by Adding PFPE (PETA−IPDI− PETA+PI) type of resist
CA vs H2O
γ (mN/m)
γd (mN/m)
γp (mN/m)
PETA+PI PFPE−IPDI−PETA+PI
67.8° ± 1.2° 104.2° ± 0.8°
51.55 ± 0.87 20.95 ± 0.55
37.50 ± 0.61 17.36 ± 0.4
14.06 ± 0.61 3.60 ± 0.37
C
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Figure 5. (a) SEM image of an array of woodpile structures obtained from the PFPE−IPDI−PETA resist with increasing structuring average laser power from 1.55 mW (left column) to 3.05 mW (right column) (power increasing along the x axis), with a step of 0.075 mW, and structuring speed from 100 μm/s (bottom row) to 1000 μm/s (top row) (speed increasing along the y axis), with a step of 100 μm/s. The white lines separate the properly exposed structures from the underexposed (top-left area) and overexposed damaged (bottom-right area) structures. (b) Graph plotting the process parameters corresponding to the damage threshold (squares) and 2PP threshold (circles).
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CONCLUSIONS In conclusion, a new PFPE-based resin has been synthesized and employed for the successful fabrication of submicrometer 3D structures by 2PP technology. The perfluoropolyether resist, being intrinsically hydrophobic and chemically resistant, can be used to manufacture 3D functional microfluidic elements requiring a good chemical resistance, and to fabricate hydrophobic tips for AFM, optimizing the imaging of wet and biological samples.
SEM images show top views (Figure 6a,b) and tilted views (Figure 6c,d) of a woodpile structure obtained using the
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ASSOCIATED CONTENT
S Supporting Information *
SEM images of a 50 μm × 50 μm × 50 μm woodpile structure of PFPE-based resin obtained by 2PP. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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Figure 6. SEM images of a 50 μm × 50 μm × 50 μm woodpile structure of the PFPE-based resin obtained by 2PP: (a) top view and (b) corresponding magnification; (c) tilted (45°) view and (d) corresponding magnification. The structure is fabricated at a speed of 1 mm/s with an average laser power of 3 mW.
ACKNOWLEDGMENTS We gratefully acknowledge financial support from the European Commission, FP7 project contract no. ICT-2007-224205 (microFLUID, microfabrication of polymeric lab-on-a-chip by ultrafast lasers with integrated optical detection), Fondazione CARIPLO (contract no. 2010-0635), and Deutsche Forschungsgemeinschaft (DFG) grant “Development and fabrication of functional micromechanical and MicroOptoElectroMechanical Systems (MOEMS) by ultra-high resolution 3D multi-photon material processing of new polymer materials”. Solvay−Solexis is acknowledged for kindly providing the perfluoropolyether (PFPE) diol. We thank Dr. K. C. Vishnubhatla and Prof. G. Lanzani at IIT-Milano for the use of their femtosecond laser system, and Renato Bertozzi for technical assistance.
process parameters from the structuring window illustrated in Figure 5, revealing that the perfluoropolyether resist was successfully polymerized. As shown in Figure 6b,d, the fabricated 3D structure contains well-defined submicrometer features. In addition, it can be observed that the structure does not show any shrinkage after development (see ref 23 for more information on this common downside effect). This is an extremely important feature, since it allows faithful production of the desired geometry. Structures with linewidths well below 1 μm were also achieved as the power was reduced toward the threshold for 2PP, but the degree of cross linking of such structures is low, and consequently, some shrinkage and deformations occur (see the Supporting Information, Figure S1).
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