Magnetic wire traps and programmable manipulation of biological cells

Abstract

We present a multiplex method, based on microscopic programmable magnetic traps in zigzag wires patterned on a platform, to simultaneously apply directed forces on multiple fluid-borne cells or biologically inert magnetic microparticles or nanoparticles. The gentle tunable forces do not produce damage and retain cell viability. The technique is demonstrated with T-lymphocyte cells remotely manipulated (by a joystick) along desired trajectories on a silicon surface with average speeds up to 20 microm/s.

Figure 1

(a) Schematic of a rectangular zigzag wire with a head-to-head (HH) domain wall (DW) at the vertex, associated field H_DW, and a trapped magnetic particle (gray circle). (b) Array of zigzag wires patterned on platform with perpendicular (H_z) and in-plane (H_//) magnetic fields. Sketch in (a) is an enlarged view of the dotted circle around a vertex. (c) Schematic of electromagnets and coil to create H_// and H_z. Cell movement observed by optical microscope (Reichert) with 20× objective lens and high speed camera. (d) Image of superparamagnetic spheres (2.8 μm diameter, dark circles) selectively attracted from solution and trapped only at HH and TT (tail-to-tail) domain walls under no external fields. The FeCo wires patterned on Si are 2 μm wide, 40 nm thick with 16 μm between adjacent vertices.

Figure 2

Calculated field gradient, force and energy profiles from a 1000 nm wide domain wall localized on a 40 nm thick, 1 μm wide FeCo wire. (a) Magnetic field gradients above the wire based on “point charge” (solid line) model and OOMMF simulations (dashed line) increase rapidly above 10⁴ T/m as distance z to the domain wall decreases. (b) Variation of axial force F_z determined from “point charge' model on a magnetic bead (χ = 0.85) lying 1.42 μm above domain wall with external field H_z. (c)–(f) potential energy profiles transform from attractive (c, d, e) to repulsive (f) by changing H_z further negative. Calculated maximum force F_x^max along length of wire shows its tunability with H_z. Note for distances z >1300 nm relevant to this study, the magnetic energies are not sensitive to the precise magnetization profiles within the domain wall.

Figure 3

(a) Sequential applications of planar (H_//) and perpendicular (H_Z) fields transport (indicated by dots) a microsphere on a Si platform along a zigzag wire. (b) Transport of several T-lymphocyte cells along the wires. The cells (dashed circles in top panel) are conjugated to 1 μm magnetic spheres. (c) Trajectory (white dots) of a single T-lymphocyte cell away from the wires and its controlled return to a neighboring vertex on the same wire. The arrows (and dashed circle in first panel) identify the cell. (d) Simultaneous back and forth transport of five fluid borne T-cells between zigzag wire (1) (2) and (3). Dots identify trajectory of five cells.

Figure 4

(a) Orientation of a zigzag wire, magnetization M (open arrows), and external H_//, H_Z fields. (b) – (f) Variation of magnetic potential energy within “point charge” model with distance d from the HH vertex along the wire. Orientations of H_// and H_z are indicated. Inset photographs show close correlation between locations of a microsphere with a diameter of 2.8 μm on the wire and the calculated local energy minimum position.