Manufacturing Bits: July 17
New materials and tweezers enable 3D devices
The University of Maryland has made advances in the development of polymerisation materials and “optical tweezers” for the lithography steps in building tiny, three-dimensional (3D) components for medical diagnostics, sensors and electronics applications.
The process appears to be different than directed self-assembly (DSA), which is generating interest in the possible production of semiconductors. For some time, the University of Maryland has been working on multi-photon absorption polymerization (MAP) materials to create 3D devices.
3D molecular assembly. Source: University of Maryland.
In the MAP process, a laser is focused inside a pre-polymer resin. Then, the focal point is scanned in a 3D pattern to create a part. And finally, the unpolymerized resin is washed away in solvent, leaving the desired part. MAP has been used to create waveguide-based optical devices, such as resonators, from both acrylic and epoxy polymers.
In its latest research, the university has created MAP materials that allow the simultaneous 3D manipulation of microscopic objects using optical tweezers as the point-by-point method for lithography. The combination of these techniques enables complex 3D structures. The findings will be published in the August issue of Chemical Science.
“The key element was finding a radical photoinitiator that was water soluble and had a high radical yield under 2-photon excitation. We used this in combination with commercially available, water-soluble acrylic monomers,” said University of Maryland chemistry professor John Fourkas, in an e-mail exchange. “There is nothing about our system that self assembles. Rather, we use forces generated by light to manipulate and assemble objects, and can lock them into place using photopolymerization.”
The optical tweezers themselves are “a transparent, micron-scale object that has a refractive index that is greater than that of the surrounding liquid can be held in place by a tightly focused laser beam. Moving the focal point of the beam will then move the object,” he said.
“One of the exciting aspects of this set of techniques is that it is compatible with a wide range of materials. For instance, we can weave together threads with completely different compositions to create functional microfabrics or build microscopic devices ‘brick by brick’ with building blocks that have different chemical or physical properties,” he added.
Silver nanowires to enable flexible electronics
North Carolina State University has developed conductive and elastic conductors made from silver nanowires, thereby paving the way for stretchable electronic devices. Potentially, the technology could be used for flexible displays, robotics and other applications.
The technique embeds conductive silver nanowires in a polymer with feature sizes as small as 50 microns. The nanowires can withstand stretching without adversely affecting the material’s conductivity.
Silver nano wires can be printed to fabricate patterned stretchable conductors. Source: North Carolina State University.
In the fabrication process, silver nanowires are placed on a silicon plate. A liquid polymer is poured over the plate. The polymer is then exposed to high heat, which turns the polymer from a liquid into an elastic solid. Then, the polymer can be peeled off the silicon plate.
Stable conductivity of nanowires is achieved in a large range of tensile strain (0 to 50%) after a few cycles of stretching and releasing the substrate, according to researchers. “In addition to having high conductivity and a large stable strain range, the new stretchable conductors show excellent robustness under repeated mechanical loading,” said Yong Zhu, an assistant professor of mechanical and aerospace engineering at NC State, in a statement.
The paper was published online in Advanced Materials. The research was supported by the National Science Foundation.
—Mark LaPedus
Tags: directed self assembly, DSA, lithography, MAP, multi-photon absorption polymerization, nanotechnology, North Carolina State University, silver nano wires, University of Maryland