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Shining a Light on Tiny Polymer Shapes

    Visiting graduate student studies high-throughput manufacturing of precisely shaped micro particles.

    Ryan Oliver, a visiting graduate student of Associate Professor of Mechanical Engineering A. John Hart, is working on a technique called Maskless Fluidic Lithography that allows creation of unique shapes in a liquid polymer such as PEG-DA by exposing it to patterned ultraviolet light, a process known as photo-polymerization. 

    Oliver Polymer-Shapes
    Examples of micro shapes polymerized by ultraviolet light in polyethylene glycol diacrylate (PEG-DA). SEM image courtesy of Ryan Oliver, Mechanosynthesis Group.

    For example, working with a common biocompatible polymer, polyethylene glycol diacrylate (PEG-DA), Oliver uses a projector as a mask to pattern shapes. Unlike the wafer masks used in semiconductor processing, which are produced as single use items, the integrated projection system allows for rapid change of the pattern.

    Key to the system is a Texas Instruments Digital Micromirror Device (DMD), which can be reconfigured easily by turning on and off micro mirrors to produce a multitude of shapes. The mirrors can turn on and off 32,552 times a second. “Because the mirrors are so fast, we can make decisions very quickly, which is hard to do with a masked system. You would spend several days ordering or fabricating a mask rather than milliseconds if you needed a new pattern," he says. For the polymer processing, the amount of 365-nanometer light being projected can be controlled by the mirrors. Because of the ability to control how long each mirror is switched on during a single second, the projection varies in intensity, which enables formation of two-dimensional or three-dimensional structures. Oliver likens the process to layer-by-layer assembly in a single step.

    The stop flow lithography approach, inspired by research reported by Patrick Doyle's group at MIT, was chosen while Ryan and Professor Hart were at the University of Michigan as a platform for studying the manufacture of large quantities of custom micro-particles. The vision is to use particles that are designed to work together and act as a sensitive biosensor. To realize the vision, Ryan has a goal to produce micro particles from about 250nm to about 100 microns with a library of shapes such as diamonds, triangles, squares and octagons. "We're exploring methods of taking them down to the nano scale, but the current system produces micro particles," Oliver says.


    Visiting graduate student Ryan Oliver with a microscope. Oliver's projects under Associate Professor of Mechanical Engineering A. John Hart include the Robofurnace, an automated system for making carbon nanotube forests and studying their growth, and high-throughput manufacturing of polymer microstructures for biosensing. Photo: Denis Paiste, Materials Processing Center

    “The thing that sets this method apart is one, high throughput, two, flexibility using the DMD chip, and three, the fact that you can control the shape as well as the size of the particles, and possibly the chemistry,” Oliver explains.

    Oliver is studying how to manipulate a collection of polymer particles on a liquid surface in order to assemble them in specific ways. “We needed a platform in order to synthesize micro particles that we could perform self-assembly experiments on because that promises to allow us to build sensors that we can’t build now, that are too complex; they’re made out of too many types of materials to fabricate using traditional manufacturing methods,” Oliver says. “A lot of applications may require control over the shape, the surface finish, the chemistry and the size of micro particles, so we’ve been exploring this as a method toward that end as well as understanding how to improve the shape accuracy while increasing throughput, so that's a future goal."

    Oliver is a visiting graduate student under Associate Professor of Mechanical Engineering  A. John Hart  whom he followed to MIT from the University of Michigan. He also led work on the Robofurnace project, an automated bench top chemical vapor deposition system for growing carbon nanotubes and other nano materials. He hopes to finish his Ph.D. through Michigan in August 2013. His dissertation will focus on a suite of tools for high-throughput polymer micromanufacturing and manipulation, including the direct-write fluidic lithography method.

    Oliver presented his work on polymers at a Materials Research Society meeting and the Enabling Nanofabrication for Rapid Innovation workshop in 2013 but hasn't published his results in a journal yet.

    Such templates polymers can be used for a range of processes from drug delivery to cell culture assays to casting molds. Researchers in Professor Hart's Mechanosynthesis Lab also adapted the UV-light based polymerization to a roll-to-roll system in addition to the microfluidic system.

    One drawback with PEG, which is a hydrogel, is it readily absorbs water, so it can
    swell or change shape in wet environments.

    "Beyond the manufacturing process, we are interested in secondary means to assemble the particles into complex, hierarchical structures, such as those including cells. These assemblies could be very useful for performing high-throughput bioassays or building novel tissue-like structures," Oliver says.

     – Written by Denis Paiste, Materials Processing Center

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