Inspired by nature's ability to shape a petal¹（花瓣） , and building on simple techniques used in photolithography（照相平印术） and printing, researchers at the University of Massachusetts Amherst have developed a new tool for manufacturing three-dimensional shapes easily and cheaply, to aid advances in biomedicine, robotics and tunable² micro-optics. Ryan Hayward, Christian³ Santangelo and colleagues describe their new method of halftone gel lithography（平版印刷术） for photo-patterning polymer gel sheets in the current issue of Science. They say the technique, among other applications, may someday help biomedical researchers to direct cells cultured in a laboratory to grow into the correct shape to form a blood vessel⁴ or a particular organ.

"We wanted to develop a strategy that would allow us to pattern growth with some of the same flexibility⁵ that nature does," Hayward explains. Many plants create curves, tubes and other shapes by varying growth in adjacent areas. While some leaf or petal cells expand, other nearby cells do not, and this contrast causes buckling⁶ into a variety of shapes, including cones⁷ or curly edges. A lily petal's curve, for example, arises from patterned areas of elongation that define a specific three-dimensional shape.

Building on this concept, Hayward and colleagues developed a method for exposing ultraviolet-sensitive thin polymer（聚合物） sheets to patterns of light. The amount of light absorbed at each position on the sheet programs the amount that this region will expand when placed in contact with water, thus mimicking⁸ nature's ability to direct certain cells to grow while suppressing the growth of others. The technique involves spreading a 10-micrometer-thick layer (about 5 times thinner than a human hair) of polymer onto a substrate before exposure.

Areas of the gel exposed to light become crosslinked, restricting their ability to expand, while nearby unexposed areas will swell⁹ like a sponge as they absorb water. As in nature, this patterned growth causes the gel to buckle¹⁰ into the desired shape. Unlike in nature, however, these materials can be repeatedly flattened¹¹ and re-shaped by drying out and rehydrating the sheet.

To date, the UMass Amherst researchers have made a variety of simple shapes including spheres, saddles and cones（锥形体） , as well as more complex shapes such as minimal¹² surfaces. Creating the latter represents a fundamental challenge that demonstrates basic principles of the method, Hayward says.

He adds, "Analogies to photography and printing are helpful here." When photographic film is exposed to patterns of light, a chemical pattern is encoded within the film. Later, the film is developed using several solvents¹³（溶剂） that etch the exposed and unexposed regions differently to provide the image we see on the photographic negative. A very similar process is used by UMass Amherst researchers to pattern growth in gel sheets.