Shrinking hydrogels: a big idea for nanodevices
Three years ago, when Professor Chen Shih-chi was visiting Carnegie Mellon University as an invited speaker from Hong Kong, he happened to strike up a conversation with Dr Leon Zhao Yongxin. As a result of their discussion, the two researchers became inspired to use their combined expertise to solve a long-standing challenge in the manufacture of nanodevices.
Chen is a professor in the Department of Mechanical and Automation Engineering at the Chinese University of Hong Kong. At Carnegie Mellon, Zhao’s Biophotonics Lab develops technique to study biological and pathological processes in cells and tissues. For example, expansion microscopy is a protocol that enlarges samples embedded in a hydrogel, enabling a view of fine details without any upgrade of existing microscopes. What Chen and Zhao wanted to do was the exact opposite: create the 3D pattern of a material in hydrogel and shrink it for nanoscale resolution.
“Shih-chi is known for inventing the ultrafast two-photon lithography system,” Zhao said. “We met during his visit to Carnegie Mellon and decided to combine our techniques and expertise to pursue this radical idea.”
Conventional 3D nanoscale printers focus a laser point to process materials serially, which is why it takes a long time to complete a design. Chen’s invention changes the width of the laser’s pulse to form patterned light sheets, allowing for a whole image containing hundreds of thousands of pixels to be printed at once. Called ‘femtosecond project two-photon lithography’, the method is up to 1,000 times faster than previous nanoprinting techniques.
For the process, researchers would direct the femtosecond two-photon laser to modify the network structure and pore size of the hydrogel, which then creates boundaries for water-dispersible materials. The hydrogel would then be immersed in water containing nanoparticles of metal, alloys, diamond, molecular crystals, polymers or fountain pen ink.
(A) Fluorescent image of two dragons of CdSe quantum dots without shrinking (B-F) scanning electron microscope (top) and energy dispersive X-rays (bottom) images of a monkey of silver; pig of gold-silver alloy; snake of titanium dioxide; dog of iron oxide; and rabbit of crystal formula, respectively. (G) Designed dragon patterns in A. (H) Optical microscopy image of an ox of diamond. (I-M) Fluorescent images of a tiger of graphene quantum dots; goat of fluorescent silver; horse of polystyrene; rooster of fluorescein; and mouse of fluorescent protein, respectively. (N-R) 3D models and fluorescent images of the fabricated structures in shapes of a C60 molecule, regular dodecahedron, regular octahedron, cube, and regular tetrahedron of different materials, respectively. (S) Top view of a five-layer split ring resonator structure; (T) trimetric view of the split ring resonator structure; (U) Scanning electron microscope image of the top layer of a split ring resonator structure after shrinking and dehydration. (V) 3D model of a woodpile structure containing 16 vertical rods along the z-axis. (W, X) Scanning electron microscope cross-sectional images of the fabricated woodpile at the two cut planes in (V), respectively. Credit: Chinese University of Hong Kong and Carnegie Mellon University
“Through fortuitous happenstance, the nanomaterials we tried were all attracted automatically to the printed pattern in hydrogel and assembled beautifully,” Zhao said. “As the gel shrinks and dehydrates, the materials become even more densely packed and connect to each other.”
This collaboration between Chen and Zhao may open doors for designing sophisticated nanodevices. The results have been published in the journal Science.
Zhao said: "In the end we would like to use the new technology to fabricate functional nanodevices, like nanocircuits, nanobiosensors, or even nanorobots for different applications. We are only limited by our imagination."