Ink-Jet Technique Produces Microvessels and Microlenses
Daniel S. Burgess
Researchers at Max Planck Institut für Polymerforschung in Mainz and Universität Siegen, both in Germany, have fabricated microvessels and microlenses using an ink-jet method that deposits droplets of a solvent onto a polymer substrate. The approach may be suitable for applications in micro- and nanochemistry and in the production of optical components for optoelectronics.
Drops of solvent are deposited on the polymer substrate by ink-jet printing (a). Each drop diffuses into the substrate (b), dissolving the surface and deforming it into a concavity as the drop evaporates (c). The cavity that is formed (d) may be used as a microreaction vessel for micro- or nanochemistry. Alternatively, another polymer may be poured over the surface (e), cured and removed to form a planoconvex lens (f). Images reprinted with permission from Elmar Bonaccurso, Hans-Jürgen Butt, Benjamin Hankeln, Brigitte Niesenhaus and Karlheinz Graf, Applied Physics Letters, 86, 124101 (2005). Copyright 2005, American Institute of Physics.
To demonstrate the technique, they used a commercial ink-jet system to deposit one or more drops of toluene on a flat, 1.2-mm-thick polystyrene substrate or on a 1-mm-thick plate of pressed polymer powder. Each drop diffused into the substrate, dissolving it and creating a concavity by a process that involves swelling, mass transport and surface deformation. The depth and internal diameter of the cavity are dependent on the size and number of deposited drops.
The cavity may be used as a microreaction vessel for chemistry applications or as a planoconcave lens, or another material may be poured over the surface, cured and removed to form a planoconvex lens. The scientists demonstrated the latter using a transparent silicone elastomer, polyvinyl alcohol and polyethylene oxide.
An atomic-force-microscope image reveals a concavity produced by the successive deposition of drops of solvent. The depth and internal diameter of the cavity is dependent on the size and number of deposited drops.
Elmar Bonaccurso, a postdoctoral researcher at the institute, said that, although a number of economical fabrication processes are available for the production of microlenses, they either lack flexibility in the control of the size and focal length of the optical elements or cannot yield concave lenses. Soft lithography and other lithographic approaches, for example, can be highly economical for the parallel production of many units of the same element, but a new master must be constructed for each type of lens, so there is a trade-off between flexibility and cost.
The planoconvex lenses created from the cavities display an rms surface roughness comparable to that of the template -- on the order of 6 to 30 nm, with no scratches.
The ink-jet method does carry material requirements, Bonaccurso said. It must employ a solvent for the substrate that evaporates under the desired temperature and pressure. The solvent also must not wet the substrate too much; it must display an initial contact angle of greater than 30° to 40°. Lastly, the material used to produce a convex lens must not contain a solvent for the substrate. All of this, therefore, suggests the use of polymers, he said. Quartz glass, for example, is unsuitable.
Another practical constraint involves the means of producing the solvent droplets at the desired size. Commercial drop-on-demand systems, he explained, are limited to droplet diameters of 10 µm, so that may be expected to represent the smallest limit for fabricated lens diameters.
Thus far, the researchers have been unable to produce concavities with depth-to-diameter ratios of greater than 1:10. Ongoing work involves the examination of alternative solvent/polymer combinations to improve this. They also are pursuing the automated fabrication of two-dimensional lens arrays using a multiple-nozzle drop dispenser and a motorized X-Y table.
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