Force-Controlled Microcontact Printing using Microassembled Particle Templates Afshin Tafazzoli Chytra Pawashe Metin Sitti Dept. of Mechanical Engineering Dept. of Mechanical Engineering Dept. of Mechanical Engineering Carnegie Mellon University Carnegie Mellon University and the Robotics Institute Pittsburgh, PA 15213 Pittsburgh, PA 15213 Carnegie Mellon University Email: email@example.com Email: firstname.lastname@example.org Pittsburgh, PA 15213 Email: email@example.com Abstract— In this paper, force-controlled microcontact printing force . On the other hand, applying a force to the stamp using microassembly-based particle templates is investigated. against a surface collapses the topography of the spherical Polystyrene microparticles are assembled semi-automatically into patterns on the stamp such that each recessed layer contacts the a desired pattern on a glass substrate using an Atomic Force Microscope nanoprobe installed on a nanopositioning stage. The surface in a stepwise sequence; the greater the applied force, micropattern on glass is sputtered with aluminum and removed the larger the area of the contact area between the sphere and of microparticles by ultrasonic vibration, resulting in a template the surface. with microfeatures corresponding to the microparticles. A soft Conclusively, a force-controlled printing method is advan- lithography method is used to mold elastomeric polymers on the tageous, which can allow the formation of nanoimprints from template, resulting in a stamp. The stamp is inked and printed using a force-controlled system onto a polystyrene substrate. stamps with micro-features by controlling the contact force. Depending on the particle size and contact force, a smaller micro This paper proposes a method for force-controlled contact to nanometer sized pattern can be formed. As the spherical printing of molded elastomer stamps using microassembled patterns on the stamp collapse due to interfacial contact forces, particles as a template. The organization of the paper is as force-controlled microcontact printing is crucial for controlling follows: First, the microcontact printing steps are presented. the size of stamped features. Green ﬂuorescent protein is used as the ink, enabling the use of ﬂuorescent imaging to observe Second, the microrobotic assembly of microparticles for tem- the stamped imprints. Preliminary experiments using 4.5 and 10 plate formation is explained. The soft lithography and molding µm diameter polystyrene particles shows the feasibility of our processes are then introduced, followed by the polymer inking technique. Thus it is possible to realize micro/nanopatterns using and force-controlled printing methods. Finally, experimental assembled microparticle-based stamps in high volumes. results are discussed, and concluded with a summary of the currents and future works. I. I NTRODUCTION II. M ETHOD High volume fabrication of micro/nanoscale patterns has Fig. 1 displays the microcontact printing steps. In Fig. 1(a), been a signiﬁcant challenge in the areas of micro/nanorobotics a two-dimensional (2D) microparticle pattern consisting of and micro/nanofabrication. Precision assembly and manip- four 10 µm and one 25 µm diameter particles is formed ulation at the micro and nanoscale mostly use a single through microrobotic assembly by pushing microparticles with manipulator  or an array of manipulators , resulting an Atomic Force Microscope (AFM) probe. The pattern is then in low-volume and low-speed manufacturing. As a possible sputtered with aluminum to ﬁll the gaps and holes around solution for this issue in supporting industrial high through- microparticles, resulting in a patterned template. The micro- put applications, a micro/nanorobotic approach can be used. particles are removed in an ultrasonic water bath, leaving the Master templates or masks can be fabricated, which can holes on the template where microparticles were previously. be replicated a large number of times using high-volume Polydimethylsiloxane (PDMS) is formed on the template and micro/nanofabrication techniques including molding, contact cured, resulting in a stamp that is peeled from the template; the printing, embossing, and optical lithography. template can be used multiple times to form PDMS stamps. Microrobotic assembly of microscale particles enables the Finally, a layer of Green Fluorescent Protein (GFP) as ink production of complex and precise micropatterns, a primary is deposited on the patterned stamp, as shown in Fig. 1(b). structure for developing templates. The fabrication of complex The inked stamp is then applied to a polystyrene substrate (a patterns of aligned microstructures has required the use of Petri dish); by controlling the time and contact force during multiple applications of lithography . However, the use stamping, microcontact printing is possible. In the following of a single stamp as the patterning element removes the sections, these steps are explained in detail. difﬁculty of aligning separate elastomeric stamps, which can collapse due to interfacial adhesion. The low modulus and low A. Microrobotic Assembly surface energy of the elastomers allow atomic-scale conformal A micromanipulation system is developed that enables the contacts to establish without the application of an external manipulation of microparticles into deﬁned 2-D patterns on a glass substrate. Fig. 2 displays a photo of the manipulation setup. An AFM probe is attached to a 3-DOF nanopositioning stage (Queensgate NPS-XYZ-100A, 100×100×15 µm3 range, ±5 nm precision); the probe tip points upwards. On a 3-DOF manual positioning stage, a glass slide is mounted (which is the substrate in template formation), and microparticles can be deposited on the underside of this slide (microparticles are initially suspended in a liquid solution, deposited on the substrate, and dried). As a result, the manipulation will occur in an inverted style ; since adhesion forces are dominant at this scale, the microparticles will not fall off the glass slide. An advantage of an inverted setup is that it allows for high-powered, low working-distance lens objectives to be placed over the workspace; a Nikon L200 optical microscope is used with up to a 50× objective, resulting in up to 500× magniﬁcation. In addition, the probe will not occlude the microparticles during manipulation due to the inverted setup, (a) which enhances visual feedback for particle assembly tasks. A framegrabber (Euresys Picolo) in a PC (Intel Pentium-4 1.8GHz in Linux) acquires images from the optical micro- scope’s CCD video camera (MTI DC330) providing visual information for manipulation. The control PC communicates with the nanopositioning system through a digital interface, and can control the positioning stage in real-time. Using the graphical user interface (GUI) on the PC, the operator can CCD move the AFM probe in real-time with a mouse. As a result, the manipulation of microparticles is realized through teleop- Optical Microscope eration. Fig. 3 displays examples of teleoperated microparticle 50X Glass Slide assembly Frame Grabber into user-deﬁned patterns. PC Manual Stage AFM Probe (b) Fine Positioning Stage Fig. 1. Steps of contact printing using the assembled microparticles as a template: (a) Making the template by microrobotic assembly method; (b) Growing a protein layer on the stamp and imprinting it onto a polystyrene Stage Controller Digital I/O Petri dish. (a) (a) 10 µm diameter polystyrene (b) 10 µm diameter polystyrene microparticles for writing letters (C- microparticles for writing letters (N- M-U) and a nanoprobe I-L-U). Optical Microscope AFM Probe Glass Slide (c) Cartesian arrangement of 4.5 µm (d) Radial arrangement of 4.5 µm Manual Stage and 25 µm diameter polystyrene diameter polystyrene microparticles Fine Positioning Stage microparticles in two rows. with a 10 µm diameter polystyrene microparticle in the center. Fig. 2. Micromanipulation system photo. (b) Fig. 3. Different microparticle arrangements constructed using the semi- (c) autonomous microassembly technique with a nanoprobe. As manual microrobotic arrangement of microparticles is B. Template Formation not efﬁcient in developing a large number of patterns, an automatic micromanipulation process is implemented . On A template is formed from the microparticle arrangement the workspace, the operator deﬁnes a target conﬁguration that is developed through the microrobotic methods. The of microparticles for the system to realize (using the GUI). template is the basis for the stamp, which is molded over The controller for the arrangement task is divided into three the template; multiple stamps can be developed from a single functional processes: template before the template is defective. A 3 µm uniform layer of aluminum is deposited onto the microparticle ar- 1) Workspace Detection: From the visual feedback of the rangement, which rests on a glass substrate. The thickness of workspace, the Generalized Hough Transform is applied the aluminum layer will change the depth of features on the to detect the locations of the microparticles, which template; 3 µm was chosen to create molds with appropriate are circular objects. The end-effector is not detected sized features. A Perkin Elmer 2400 8L sputtering system is through visual feedback, rather, it is at a known position used, and is operated on low power at a temperature below determined by calibration of the image-frame to the the melting point of polystyrene, which is the composition frame of the nanopositioning stage. of the microparticles. The sputtering rate is approximately 1 2) Task Planning: Microparticles on the workspace are µm/hour, with ten minute breaks after every 1 µm layer to assigned to goals deﬁned by the target conﬁguration. cool the sputtering chamber. A path is determined for a particle to its goal for the pushing task. As it is possible to have obstacles The aluminum layer ﬁlls voids between the microparticles, (i.e. other particles) during a pushing operation, the resulting in a smooth thin layer of aluminum over the glass and Wavefront expansion motion planner is implemented to existing microparticles, as seen in Fig. 5(a). The microparticles generate paths that plan around obstacles. are then removed by submerging the template in an ultrasonic 3) Task Execution: The end-effector, an AFM probe, is water bath; the ultrasonic vibrations force the microparticles automatically moved along the calculated path. As the off the glass substrate. The resulting template consists of end-effector translates, a particle will be pushed along a smooth aluminum layer with depressions in the positions the path. As it is rare that a particle is pushed to previously occupied by microparticles. These holes appear its destination in one push (the end-effector will lose spherical, however ﬂattens at the base, seen in Fig. 5(b). contact with the particle due to positioning inaccuracies), the detection, planning, and execution processes are re- peated until a particle reaches its goal. The arrangement task completes when all particles conform to the target conﬁguration. An example of autonomous pattern arrangement is dis- played in Fig. 4, where a ‘U’ is developed. Overall, this system can autonomously produce microparticle patterns using particles as small as 4.5 µm, with a positioning accuracy of better than 1 µm, in around 1 minute. However, autonomous arrangement is currently limited to particles of the same size, and relies on a relatively clean, pre-conﬁgured and calibrated workspace. (a) (a) Initial conﬁguration. (b) Final conﬁguration. (b) Fig. 4. Fully autonomous microrobotic arrangement of 4.5 µm diameter polystyrene spheres. Circles represent detected particles through microscope Fig. 5. Images of an assembled ‘C’ pattern from a scanning electron image processing, and crosses represent user-deﬁned target locations. Arrange- microscope: (a) template after aluminum sputtering, (b) ﬁnal template after ment occurs in 61 seconds. ultrasonic microparticle removal. C. Stamp Fabrication stamp onto the substrate, resulting in a print corresponding to The stamp is developed from the template using soft lithog- the pattern on the stamp. raphy, which is a method for transferring a structure from one GFP is used in experiments as ‘ink’, and is deposited onto substrate (the patterned template) to another (the stamp) by the patterned PDMS stamp. The GFP is suspended in a diluted using an elastomeric material. PDMS, a soft polymer, is used phosphate buffered saline (PBS) solution; successful printing as the elastomeric material for soft lithography. The PDMS requires this inking solution to have a GFP concentration base (Sylgard 184) and curing agent is mixed in a 10:1 ratio greater than 50 µg/mL. After the PDMS stamp is cleaned by weight. This solution is ﬁrst applied to the template, but not and rinsed in a water-ethanol (80:20) mixture, approximately over the region on the template where the pattern exists. As 1-2 µL of the inking solution is deposited onto the region the mixing process introduces bubbles into the PDMS solution, of the stamp where the micropattern resides. To let the GFP which can result in unwanted features in the ﬁnal stamp, the chemically bond to the stamp, the ink-coated stamp is placed curing process is performed in a vacuum chamber for about for one hour in a Petri dish containing droplets of water, which 30 minutes to de-bubble. The chamber is then tilted such that prevent the evaporation of the ink solution. As GFP is light the PDMS gradually ﬁlls the patterns on the template. Finally, sensitive, the stamp is shielded from light. the PDMS is cured on a hot plate at 100◦ C for 45 minutes After the GFP is bound to the stamp, the remaining solution before it is carefully peeled off the template, resulting in the on the stamp is removed, leaving a layer of GFP bonded to ﬁnal stamp. the microfeatures of the stamp. This layer can be observed Generally, thicker stamps (about 1-2 cm thick) are better under a ﬂuorescent microscope before and after the stamping during the pattern transfer process due to increased rigidity, procedure to ensure that the GFP has been transferred to the however it is harder to peel a thicker PDMS layer off the stamped substrate. It is critical to promptly stamp a surface template during the stamp formation process. In addition, after removing the excess inking solution, as the GFP will dry thicker stamps will increase the likeliness of destroying the out over time resulting in unsuccessful prints (typically within aluminum layer on the template, which can produce several one minute) . Multiple prints can be obtained from a single thinner stamps (less than 1 cm thick). Fig. 6 displays a stamp inked stamp by successively increasing the contact force for developed from the ‘CMU’ pattern in Fig. 3(a) after being each print, which compensates for the successive loss of GFP peeled off the template; it is a mirrored image of the template. on the stamp. The PDMS stamps can be reused for about 50 times before they deform such that they are unusable . E. Force-Controlled Printing 1) Theory: To calculate the deformations of the spherical stamp patterns pressed on the Petri dish substrate, the Johnson- Kendall-Roberts (JKR) contact mechanics model  is used. This model predicts the high elastic deformation of soft and highly adhesive materials accurately. Using this model, a contact radius a of a particle on a ﬂat surface with a normal load of P is given as: Fig. 6. A PDMS stamp developed from a ‘CMU’ pattern observed under an 1/3 optical microscope. R 2 a= P + 3πRω + 6πRωP + (3πRω) (1) K Since the features on the PDMS stamp have low aspect where K is the equivalent modulus of elasticity of the mate- ratios, it is challenging to transfer the patterns onto another rials in contact, R is the radius of the spherical patterns on substrate. Stamps can be made using different liquid polymers; the stamp, and ω is the interfacial adhesion energy between a hard polymer can form a stamp with high aspect ratio fea- the stamp and the Petri dish substrate. For a sphere on a ﬂat tures. However, as the surface chemistry and stamping process surface, K is derived as: of PDMS are well-deﬁned, and soft PDMS has robustness against alignment errors, PDMS is used in these experiments 2 2 −1 4 1 − ν1 1 − ν2 for the stamp. K= + (2) 3 E1 E2 D. Polymer Inking assuming ν1 = 0.5 and E1 = 2 M P a for the PDMS stamp The polymer inking process is analogous to rubber stamping and ν2 = 0.33 and E2 = 3 GP a for the polystyrene Petri with ink. An ‘ink’ is applied to the patterned polymer stamp, dish substrate, K = 3.55 M P a. Depending on surfaces in and is allowed to bond chemically to the stamp. Alternatively, contact and ink used, adhesion energy is varied from 25 to a layer of ink can be spin-coated onto the polymer stamp . 1000 mJ/m2 with a nominal value of 100 mJ/m2 . Nominal Afterwards, the stamp is pressed against a substrate, which spherical radius of the stamp pattern is taken as R = 5 µm. deforms the features on the stamp, and then is removed. This The contact area of the features on the stamp increases process transfers the ink from the protruding features of the by applying higher contact forces. Fig. 7 predicts the contact radius of the spherical patterns as a function of contact load. X-Y Automated Stage Sample Applying about 50 µN contact force on 5 µm patterns can completely collapse the features and result in a full print of the pattern, and it requires less contact force to collapse smaller spherical patterns. In order to attain nanoprints from 2.5 µm features, a small contact force of 1 µN should be applied. The contact radius is also sensitive to the adhesion energies between the surfaces in contact. The protein layer increases the interfacial adhesion energy, however smaller adhesion is Load Cell better for transferring smaller patterns. Fig. 8. Force-controlled system setup and its components. to the microparticle size; the resulting prints increase in size as the contact force increase during stamping, as predicted from theory. Fig. 7. Simulated contact radius between different spherical patterns (2.5- Fig. 9. Microrobotic assembly of 10 µm particles to form a ’C’ pattern. 12.5 µm) on the stamp and the substrate (ω = 100 mJ/m2 ) as a function of contact force. The ﬁnal imprints can have defects due to imperfections in the inking and printing process, and due to variations in the 2) Experiments: The printing step can be realized both shapes of the microparticles during template formation. Inking manually (by manually pressing the stamp onto a substrate) defects occur when the GFP does not form a uniform layer on and automatically. Using a force-controlled automatic system the stamp, and if the GFP dries out. Alignment and levelling (see Fig. 8)  it is possible to control the contact time, contact of the stamp is very important due to the small features of the force, and approach velocity of the stamp during the printing stamp; like in most conventional contact printing applications, process. A load cell with a resolution of 10 µN is used, it is simpler to print larger micro patterns with high aspect and a spherical joint on the apparatus automatically aligns the ratio stamps. stamp with the substrate. Either the stamp or substrate can be As the resolution of the force-controlled system is limited to stationary while the other is moving, and contact forces can 10 µN , it is not feasible to apply smaller microforces, which be recorded from the load cell during the printing procedure. can limit the minimum size of the printed features. In addition, After the print is made, the stamp is automatically separated ﬂuorescent microscopy does not provide an accurate indication from the substrate. of the size of the prints, as the ﬂuorescence exaggerates the III. R ESULTS AND D ISCUSSIONS sizes of the features. An AFM in friction force microscopy imaging mode can be employed to determine the actual size Using the force-controlled system, microcontact printing of of the printed features, with nanometer precision. As the a ’C’ pattern is conducted and examined on the substrate. 10 tribological properties of the substrate changes on the locations µm polystyrene particles are initially assembled to form a ’C’ of the printed proteins, an AFM probe will deﬂect differently pattern shown in Fig. 9. A 3 µm layer of aluminum is sputtered as it translates over the substrate, which can infer the size of on the pattern and the particles are removed, resulting in a the printed features. template. A thick PDMS stamp is formed from the template, and is inked with GFP. The resulting stamp is printed onto a IV. C ONCLUSION Petri dish substrate with varying contact forces, and observed In this study, templates formed from microrobotic assem- using ﬂuorescent microscopy (see Fig. 10). According to the bly are used for microcontact printing of ﬂuorescent pro- effective contact radius, the imprints can be similar or smaller teins. Prints that are smaller than the size of the assembled microparticles are formed, using force-controlled printing. The spherical geometry of the microparticles creates varying contact areas during printing, depending on the applied force. Finally the protein is transferred from the stamp to the surface during the printing process, resulting in a nano/microprint. The advantage of this microcontact printing method is that high-volume template fabrication can be created through automatic methods, and prints smaller than the features of the stamp can be realized. Further research includes using harder polymers during the stamping process to improve the reliability of the stamp. In addition, the process of applying the GFP on the stamp to form a uniform protein monolayer will be investigated. Higher resolution force-controlled stamping (a) (b) will be explored to attain smaller prints, which can be veriﬁed using frictional force imaging in an AFM. ACKNOWLEDGMENT The authors would like to thank Philip Leduc and Chao- Min Cheng from the Cellular Biomechanics Laboratory at Carnegie Mellon University for collaboration, for contact printing subfeatures from various stamps, and for providing ﬂuorescent microscopy. R EFERENCES  A. Bernard, J. P. Renault, B. Michel, H. R. Bosshard, and E. 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Force-controlled microcontact printing of the ’C’ pattern on the Petri dish substrate by increasing contact force; right diagrams are 3D ﬂuorescent intensities.
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