A game of fire and ice

New freeze-drying method preserves tiny electronic structures assembled using light

Imagine moving an object simply by shining a laser or projecting a light pattern. It sounds like a page ripped out of a science fiction novel, yet optical micromanipulation has led to the development of tools for handling objects with light.

Though this concept was first demonstrated over three decades ago, an international group involving researchers from Scotland, Belgium, China, and France recently reported an advancement that may facilitate bringing this technology to microassembly facilities to create nanowires, bio-lasers, and tiny electronics.

The team used optoelectronic tweezers (OET), an optical micromanipulation technique involving light patterns projected on a conductive surface, to trap and move particles. The conductive surface must be photosensitive, allowing the light to create a mismatch in conductivity between the area exposed to light, and the area left in shadow.

The team showed that OET could move solder beads — particles of tin, lead, and silver commonly used in electronics production — and place them in precise positions. They handled multiple beads simultaneously and could, in theory, move 10,000 beads in parallel.

These findings proved OET’s promise in manufacturing tiny electronic components. However, the technique required a liquid environment, and the researchers lacked an effective method for removing liquid from the beads after assembly.

The most obvious answer, evaporation, produced an undesirable side effect: as the liquid droplet shrank, it pulled the beads with it, disrupting the orderly line that had just been assembled.

This problem perplexed Shuailong Zhang, a postdoctoral fellow at U of T, formerly of the Micromanipulation Research Group at the University of Glasgow. The solution came to him on a snowy day. To avoid a liquid-to-gas transition, he could freeze the liquid to make it solid.

Unlike the snow he saw melting that day, Zhang didn’t need to return to the liquid phase to get rid of the solid. Instead, he used a technique called freeze-drying. After freezing, he would place the solid, with its embedded bead assemblies, in a low-pressure chamber. At low pressure — the exact pressure depending on the substance — solids can turn directly into gas.

The method worked. The beads stayed in place as the surrounding medium was removed.

In the last step of what Zhang called a “fire and ice game,” he heated the solder beads to fuse them together, and showed that the fused line of beads conducted electricity.

In other words, the research team used OET to form an electrical component, albeit simple, which could be reliably removed from its formative liquid medium.

Zhang envisions the technique being used to manufacture more complex devices, such as nanowires and nano-photonic devices. However, he thinks “the biggest potential application is in biology and medical science.”

Optical micromanipulation is not limited to moving metal particles; it has also been used to trap atoms, viruses, and cells. With its ability to precisely position individual cells, it presents a new way of probing biological questions. “It can be used for cell patterning, it can be used for cell sorting, and also it can be used to study cell competition and communication,” said Zhang.

The team is now developing a phone and tablet app to make light-based particle handling easier for future users.

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