A light-sensitive resin that responds to the process of “carbonizing,” or charring, could provide scientists with a new material that can be molded into complex, highly conductive 3-D structures with features just a few microns across. The findings hold promise for developing 3-D microelectrodes that could interface with the brain. Researchers have access to a variety of materials that can be used to make complex 3-D structures, but the commercially available resins that work best with modern 3-D shaping techniques do not respond to carbonization, a necessary part of the electrode preparation process. In this stage, a structure is baked at a temperature high enough to turn its surface to carbon. The process of carbonizing, or charring, increases the conductivity of the resin and increases its surface area, both of which make it a good electrode. However, this process also destroys the resin's shape; for example, a sphere becomes an unrecognizable charred blob. Two microstructures made with the new material, containing the lowest concentration of RDGE. (Left) Precarbonization. This circular table and bunny models, just slightly larger than a typical bacterium, were formed using two-photon polymerization, the preferred method of 3-D shaping. (Right) The same models after they have been carbonized, or charred. Notice that they shrink significantly but maintain their shape. Images courtesy of Optical Materials Express. Scientists at Yokohama National University, in collaboration with physicists and chemists from Tokyo Institute of Technology and C-MET Inc. of Kanagawa, Japan, set out to develop materials that could be crafted using 3-D shaping techniques but that also would survive the charring process. They designed a light-sensitive resin that included a material called resorcinol diglycidyl ether (RDGE), typically used to dilute other resins but never before used in 3-D sculpting. The new mixture has a unique advantage over other compounds: It is a liquid and therefore potentially suitable for manipulation using the preferred 3-D sculpting methods. The work could help researchers trying to create conductive materials in almost any complex shape at the microscopic or cellular level. “One of the most promising applications is 3-D microelectrodes that could interface with the brain,” said Yuya Daicho, a graduate student at Yokohama National University and lead author of the paper. Brain interfaces — rows of needle-shaped electrodes pointing in the same direction like teeth on combs — can send or receive electrical signals from neurons and can be used for deep-brain stimulation and other therapeutic interventions to treat disorders such as epilepsy, depression and Parkinson's disease. “Although current microelectrodes are simple 2-D needle arrays, our method can provide complex 3-D electrode arrays” in which the needles of a single device have different lengths and tip shapes, giving researchers more flexibility in designing electrodes for specialized purposes, Daicho said. The investigators also envision making microscopic 3-D coils for heating applications. Daicho and colleagues tested three different concentrations of RDGE in their new compounds. Though there was shrinkage, the materials held their shapes during the charring process. Controlled shrinkage can be good in cases where miniaturization of a structure is desired. The researchers observed that the resin with the lowest concentration of RDGE shrank 30 percent, while the material with the highest concentration shrank 20 percent. Two microstructures made with the new material, containing the highest concentration of RDGE. (Left) Pre-charring. These pyramid and bunny models did not respond to the preferred method of 3-D shaping, so they were created using an alternative process. (Right) Post-charring. Notice that the pyramid and bunny shrink significantly less than those made from the material with a lower concentration of RDGE. The resin’s ability to be manipulated using techniques suited specifically for 3-D shaping was also tested. In one technique, called microtransfer molding, the light-sensitive liquid was molded into a desired shape and then hardened by exposure to UV light. Another technique, preferred because of its versatility, made use of the liquid resin’s solidifying property when exposed to laser beams. In the two-photon polymerization process, the scientists used the laser to “draw” a shape onto a liquid resin and build it up layer by layer. Once the objects were shaped, they were carbonized and viewed with a scanning electron microscope. In addition to crafting pyramids and disks, the investigators reproduced the well-known “Stanford bunny,” a shape commonly used in 3-D modeling and computer graphics. The rabbit sculpture was the size of a typical bacterium. When he first saw a picture of the rabbit taken with the scanning electron microscope, Daicho’s adviser Shoji Maruo said he was delighted at how well it had held up during the charring process. “When we got the carbon bunny structure, we were very surprised,” Maruo said. It was exciting to see that “even with a very simple experimental structure, we could get this complicated 3-D carbon microstructure.” The rabbit's shape would be much more difficult, expensive and time-consuming to create using any of the existing processes compatible with carbonization, he added. Next, the team hopes to fabricate usable carbon microstructures as well as char the resins at temperatures above the 800 ºC temperature tested in this study. Moving to higher temperatures may destroy the microstructures, Maruo said, but there is a chance that they will turn the surfaces into graphite, a higher-quality conductor than the carbonized surfaces they have created so far. The study appeared in Optical Materials Express (doi: 10.1364/OME.3.000875). For more information, visit: www.ynu.ac.jp/english