Optics & Photonics News

Bending 3-D Microshapes with Light

Zina Deretsky, National Science Foundation

A twist on a long-used printing tech- nique allows researchers to use photolithography to make complex microscale 3-D shapes. Researchers in Ryan Hayward and Christian Santangelo’s research groups at the University of Massachusetts, Amherst (U.S.A.), demonstrated using halftone gel lithography with UV light to phototransfer patterns onto flat polymer gel sheets. The gel swells and contracts with changing temperatures. The pattern of dots changes the amount of expansion, resulting in internal stresses that are relieved by bending out of the plane of the sheet (Science 335, 1201). The technique could be used in bioengineering to build devices that change shape.

The polymer gel absorbs water like a sponge, and then expands or contracts depending on temperature. But the ability to expand depends on how strongly cross-linked the polymers are, which, in turn, relies on the dose of UV light they are exposed to.

To pattern the polymer gels, the group used two masks and UV photolithography in a technique similar to a common printing method called half-toning, which is used by printers to produce many shades of a color using dots of varying sizes. Smaller dots leave more of the white background showing, and from

600 μm A small square of polymer could be printed so that it swells into a sphere.

a distance the dots and unprinted areas appear to merge, producing a uniform light shade. Bigger dots produce areas of darker shades.

The researchers used the first mask with a low dose of UV light (0.2 J/cm2 at 365 nm) to define a potentially high swelling area, gelling the polymer and yielding a material that can swell to about eight times its dry size in area.

The second mask defines a pattern
of dots where the polymer is extensively
cross-linked; this restricts swelling to
only twice its dry size. When the sheet
swells, the difference between the two
areas creates stress. To relieve the stress,
the sheet bends. As long as the dots are
only a few times larger than the film
is thick, the sheet acts like a homog-
enous elastic material, yielding smooth
curves. Part of the group’s work involved
investigating how the film thickness, dot
diameter and spacing between the dots
is related to the curved surfaces. “With
a proper half-tone pattern of resist dots,”
the researchers write, “almost any 3-D
shape can be achieved.”
The method is simple, but the potential
for making 3-D shapes hasn’t been fully
explored. The group is still investigating
the kinetics of swelling and deswelling.

One-Step Rainbow Grating

University at Buffalo

Alow-cost, single-step method makes reflection gratings with tailored, variable periods. A group led by University at Buffalo (U.S.A.) engineering professors Qiaoqiang Gan and Alexander N. Cartwright reports making graded holographic photopolymer gratings that produce a rainbow-colored reflection image (Adv. Mater., doi: 10.1002/ adma.201104628). The gratings could be integrated with detectors or imagers to create compact spectroscopic analyzers.

To create the rainbow grating, they
sandwiched a syrup of monomer liquid
crystals and photo-initiators in solvent
between two glass slides. When a laser
beam is introduced at an angle through
the top slide, some of it reflects off the
bottom slide. The beam and its reflection
interfere. At high-intensity locations, the
monomers join into polymers, forming a
continuous pattern of parallel gratings.

A rainbow-colored grating, about 25 mm wide, lit by sunlight. Enlarged images show the grating surface with slowly varying period. Black bars = 10 µm.