New Resin Suitable for Multiphoton Polymerization

Daniel S. Burgess

Scientists at Boston College in Chestnut Hill, Mass., and at Boston University have developed an acrylate resin for multiphoton absorption polymerization. To demonstrate the mechanical and optical properties of the material, they have used it to fabricate various three-dimensional microstructures, including on the surface of a human hair -- suggesting its compatibility with other biological materials.

Multiphoton absorption polymerization is the subject of investigation worldwide as a means of fabricating microstructures -- such as waveguides, interconnects and photonic crystals for applications in optical communications -- and of creating components for sensor arrays and microscopy, said John T. Fourkas, a professor of chemistry at Boston College. An indirect, two-photon absorption process initiates the polymerization of the resin. Because this occurs only where the two low-energy photons meet, it offers the user a high degree of spatial control over the process -- on the scale of the focal volume of the excitation source -- enabling the creation of arbitrarily complex 3-D objects by scanning the focal point's position through the resin. After excitation, a solvent washes away the unexposed resin, leaving behind the polymer structure.

The team is investigating the ability of the technique to fabricate microscale devices with multiple functionalities, Fourkas said. It thus is hoping to make available a range of materials for researchers' chemical toolboxes. "No single resin is suitable for all applications, so it is desirable to be able to develop a wide range of resins for different applications."

In this instance, the scientists set out to design a resin formulation with desirable mechanical properties using inexpensive, off-the-shelf components, including two acrylate monomers. Crucially, they selected as a photoinitiator BASF AG's Lucirin TPO-L, which typically is used in printing and coating applications. The substance normally is excited using ultraviolet radiation, but its absorbance spectrum has a secondary peak just beyond the visible wavelengths, making it suitable for two-photon absorption at 700 to 800 nm. Moreover, unlike most photoinitiators, it is a liquid, and it is compatible with most resin formulations.

"It really opens the door to the creation of designer resins with whatever properties are desired," Fourkas said.

To evaluate the acrylate resin, the scientists fabricated various complex 3-D microstructures, including hollow towers, interlocking rings, a transparent cantilever and pairs of pyramids connected by overlapping cables. The setup incorporated a Coherent Mira Optima 900-F mode-locked Ti:sapphire laser oscillator, a Carl Zeiss Axioplan 2 upright microscope and Ludl Electronic Products Ltd.'s BioPrecision computerized motorized stage. After polymerization, the unexposed resin was removed with ethanol.

Scientists have developed an acrylate resin for multiphoton absorption polymerization. Using the material, they fabricated microscale polymeric structures, including these three-dimensional letters on a 110-µm-diameter human hair. Courtesy of John T. Fourkas.

Another structure fabricated with the resin was a series of 3-D block letters on a human hair (see figure). The researchers noted a lack of sagging or distortion, even in the 5-µm features, indicating low shrinkage and a bulk modulus that they since have estimated to be on the order of a gigapascal.

This indicates potential applications that are straight out of science fiction, with microscopic polymeric devices constructed on the components of the human body.

"The fact that we were able to fabricate structures nondestructively on a hair suggests that the same thing may be possible on far more interesting biological tissues," Fourkas said.

Multiphoton absorption polymerization is unsuitable for directly mass-producing such assemblies, however. The process, he noted, is an inherently serial technique in which structures are fabricated point by point.

Consequently, the scientists are investigating the possibility of combining it with microtransfer molding, so that polymerization would be used to build an original from which a master mold would be produced. From that master would be created high-quality replicas of the original as well as other molds. They expect to be able to create wafer-scale devices with this approach in the near future, he said.