The paper builds on previous work byresearchers in the laboratory of MIT Department of Materials Scienceand Engineering (DMSE) Prof. Geoffrey Beach, which involved thecreation of magnetic tracks on a microchip surface, and rapidlytransporting beads along those tracks—a technology the researchersliken to what is required to read and write magnetic data on acomputer's hard disk.
Microbeads are widely used in thebiomedical industry because they can be functionalized with surfacechemistries so that they can bind to molecules in fluid, explainsBeach.
"For cancer cells, for example, wecan find or produce magnetic microbeads that will bind to thatmaterial," he says. These particles are used for materials ofisolation. You can put a bar magnet in fluid, like blood, pullmicrobeads to the edge and dump out what you don't want, andisolate the material for further study."
But rather than do this in a test tube,"we wanted to be able to do it on a chip and have the functionalityof doing it with very small quantities of fluid, like a single dropof blood," says Beach.
"The real core of this research ismeant to develop a system where we have nanomagnets on a chip and canmove them around to capture magnetic particles, and capture them on achip as well," he says. "The impetus for this came from a lot ofinterest in using magnetic nanotracks in the data storage industry.We can use these domain walls to not just carry ones and zeros, butto also use their magnetic fields to grab onto magnetic nanoparticlesand move them."
"In particular, we were interested inthe drag force on a particle, which depends on the size of theparticle or if things are stuck to its surface. These domain wallsallow us to have the level of throughput you have with microfluidics,but at the same time, grab single particles and have digital controlof each object."
The new paper shows that the strongmagnetostatic interaction between a bead and a domain wall leads to adistinct magneto-mechanical resonance that reflects thesusceptibility and hydrodynamic size of the trapped bead. Numericaland analytical modeling is used to quantitatively explain thisresonance, and the magneto-mechanical resonant response undersinusoidal drive is experimentally characterized both optically andelectrically.
"What we have demonstrated in ourpaper is that using commercially available microbeads, we can tellthe difference between beads based on size," says Beach. "What wewant to do now is further develop the surface binding chemistry thatattaches molecules to that bead."
Graduate student Elizabeth Rapoport,who did her undergraduate work in chemistry and is now a student inthe DSME, says she became interested in Beach's work because "itleverages technology that enables researchers to do fastercomputing."
"We're taking something that isreally well studied in the field, but you wouldn't normally thinkto use, to create a system that is really powerful," she says. "Weare also developing junctions where we can do active sorting ofparticles. We can pause this process, shake it around, see what thebeads are attached to and then based on this information, sort themand send them on to different routes."
According to the researchers, theobserved bead resonance presents a new mechanism for microbeadsensing and metrology, and the dual functionality of domain walls asboth bead carriers and sensors is a promising platform for thedevelopment of lab-on-a-bead technologies.
"The application space we are lookingat presently is certainly diagnostics," says Beach. "Thefabrication of these devices is very simple, and one can make manydisposable chips."
The work, published in a Lab on aChip paper, was supported by MIT's Center for Materials Scienceand Engineering and the Deshpande Center for TechnologicalInnovation. Device fabrication was carried out at MIT'sNanoStructures Laboratory.
