Inspired by nature’s ability to shape a petal, a new tool for manufacturing three-dimensional shapes easily and cheaply was created using photolithography and printing techniques. The method could advance robotics, tunable micro-optics and biomedicine. The new technique, halftone gel lithography for photopatterning polymer gel sheets, was developed by Ryan Hayward, Christian Santangelo and colleagues at the University of Massachusetts Amherst. The method may someday help biomedical researchers to direct cells cultured in laboratories to grow into the correct shape to form a blood vessel or a particular organ, they said. “We wanted to develop a strategy that would allow us to pattern growth with some of the same flexibility that nature does,” Hayward said. Curves, tubes and other shapes are created in many plants by varying growth in adjacent areas. While some leaf or petal cells expand, others nearby do not, and this contrast causes buckling into a variety of shapes, including curly edges and cones. For example, a lily petal’s curve comes from patterned areas of elongation that define a specific 3-D shape. Controlling growth in a polymer system at the microscale with a technique akin to half-tone printing, the polymer swells like a sponge when exposed to water. Printing “resist dots” in the polymer substrate creates points that will not swell. When the dot size changes, buckling occurs from the mismatch in growth from one area to another. With a proper half-tone pattern of resist dots, almost any 3-D shape can be achieved. (Image: Zina Deretsky, NSF) Using this concept, the scientists created a method that exposes ultraviolet-sensitive thin polymer sheets to patterns of light. The amount of light absorbed at each position on the polymer sheet programs how much that region expands when put in water, thus mimicking nature’s ability to direct certain cells to grow while suppressing the growth of others. The technique involves spreading a 10-µm-thick layer of polymer onto a substrate before exposure. Areas of the gel exposed to light became crosslinked, and their ability to expand was restricted. Nearby unexposed areas, on the other hand, swelled like a sponge as they absorbed water. This patterned growth, as in nature, caused the gel to buckle into the desired shape. However, in contrast to nature, these materials can be repeatedly flattened and reshaped if they are dried and rehydrated. The researchers have made a variety of shapes such as cones, spheres and saddles, as well as more complex shapes such as minimal surfaces. To create the latter, fundamental challenges that demonstrated the basic principles of the method were represented, Hayward said. He explained that, as happens when photographic film is exposed to patterns of light, a chemical pattern is encoded within the film. Later, when various solvents etch the exposed and unexposed regions of the film, images are produced on the photographic negative. To pattern growth in gel sheets, a similar process must be completed. The team employed a photolithography technique to simplify the complicated patterns needed for the gels. As in printing, photolithography using different color shades can be costly because each shade requires different inks; most of high-volume printing relies on a technique called “halftoning.” This method uses only a few ink colors to print varied-sized dots; smaller dots take up less space and allow more white light to reflect from the paper, so the smaller dots appear lighter than larger dots. Importantly, this concept applies equally well to patterning the growth of the gel sheets, the team discovered. Instead trying to make smooth patterns with many different levels of growth, they simply printed dots of highly restricted growth and varied the dot sizes to program a patterned shape. “By directly transferring the image onto the soft gel with half-tones of light, we direct its growth,” Santangelo said. “We aren’t sure yet how many shapes we can make this way, but for now it’s exciting to explore, and we’re focused on understanding the process better. “For biomedicine or bioengineering, one of the questions has been how to create tissues that could help to grow you a new blood vessel or a new organ,” he said. “We now know a little more about how to go from a flat sheet of cells to a complex organism.” The method appeared in Science. For more information, visit: www.umass.edu