Force-Controlled Microcontact Printing using Microassembled by alllona


									        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:                       Pittsburgh, PA 15213

   Abstract— In this paper, force-controlled microcontact printing   force [4]. 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 [8] or an array of manipulators [11], 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 [10]. 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 [9]; 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
                                                                       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
      Glass Slide
                                                                 Frame Grabber into user-defined patterns.
                                                            Manual Stage
                                    AFM Probe
            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-
                                                                             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 [6]. 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
   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

       (a) Initial configuration.              (b) Final configuration.

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) [1]. 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 [7].
                                                                        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 [5] 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)
   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 [3].           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

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) [2] 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.
                                                                                     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.
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             (g)                                      (h)

Fig. 10. Force-controlled microcontact printing of the ’C’ pattern on the Petri
dish substrate by increasing contact force; right diagrams are 3D fluorescent

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