Engineers have introduced a new magnetic shepherding approach for moving or positioning tiny floating objects found within organisms. The technique will advance potential applications in fields ranging from medicine to nanotechnology.
The authors of a new research article said their method avoids pitfalls of using tiny light beams, electric currents or even a competing magnetic approach to micromanipulate so-called “colloidal” objects.
“Biology is composed primarily of colloidal materials, things larger than a few billionths of a metre that are suspended in solution and don’t settle rapidly,” said Benjamin Yellen, who developed this “magnetic nanoparticle assembler” technique while obtaining his doctorate at
“They could be cells or large molecules; they are also being investigated for a variety of new devices, such as miniature lasers or semiconducting components,” added Yellen, who in September will become an assistant professor of mechanical engineering and materials science at Duke University’s Pratt School of Engineering.
Yellen is first author of a research paper on the method published today in the Proceedings of the National Academy of Sciences (PNAS). His co-authors are Gary Friedman, the Drexel professor of electrical and computer engineering who supervised his Ph.D. work, and Drexel graduate student Ondrej Hovorka.
According to the paper, other investigators are currently focusing either on using laser light beams or electric fields to “transport, sort or assemble microscopic objects.” But Yellen’s research group contends that “neither technique has demonstrated sufficient flexibility required for widespread adoption.”
Yellen, who is a postdoctoral researcher at Children’s
Meanwhile, using electricity as a micromanipulator requires space-consuming grids of electrical circuitry, he added. And electrical fields can also trigger disruptive chemical reactions.
“The big advantage to using magnetism is that very few things in nature are magnetically susceptible,” he said.
The PNAS authors’ paper described how they demonstrated their technique by first patterning permanent rectangular and circular “magnetic traps,” each with millionths of a metre dimensions, on silicon or glass wafers. Each trap was made of cobalt, an element that is magnetic.
Over those trap-patterned wafers the authors then added a fluid containing swarms of suspended magnetic iron oxide nanoparticles.
Into this ferrofluid they then floated non-magnetic microscopic beads of the colloid latex, each bead measuring between 90 and 5,000 nanometres.
Finally, the researchers set up an additional switchable external magnetic field that, when switched on, could alter the magnetic field surrounding the permanent magnetic traps.
This arrangement allowed the non-magnetic latex beads to be herded around, even arranged into a variety of complex patterns, by varying how the duelling magnetic fields influenced the shepherding swarms of magnetic iron oxide nanoparticles.
Under the direction of changeable magnetic fields, the particle swarms acted collectively to push and pull the comparatively large beads of colloids. The beads themselves were colour-labelled so their movements could be traced under microscopic observation.
“In a way, bead movement is analogous to the movement of a train along a railroad track,” wrote the authors in their PNAS paper.
While “trap magnetisation establishes the track,” fields from the switchable external magnet “provide locomotion,” they explained. Moreover, the track could be switched to new orientations by adjusting the interplay of fields between the permanent traps and the switchable magnetic source.
The authors suggested that the micromotions of this magnetic nanoparticle assembler might be made programmable by modifications of today’s magnetic recording technology.
They listed a number of potential applications, ranging from the speedier assembly of molecules for biosensors or hybridisation experiments, to precision arrangements of cells, bacteria and viruses in futuristic medical diagnostic devices, to the assembly of advanced microelectronic components, such as nanowire transistors.
Their paper also noted that a competing magnetic micromanipulation technique already exists that requires pre-bonding to “magnetic particle carriers.”
“You have to do a lot of chemical steps along the way, so it’s not so convenient,” Yellen said of that competing approach. “It would be much more convenient to just simply mix the nonmagnetic materials with a ferrofluid and have them moved around without having to attach them to a magnetic carrier.”
Once he arrives at Duke, Yellen said he plans to apply his magnetic nanoparticle assembler approach to designing advanced biosensors and cell membrane probes.