OctoMag: An Electromagnetic System
for 5-DOF Wireless Micromanipulation
B. E. Kratochvil, M. P. Kummer, J. J. Abbott, R. Borer, O. Ergeneman, and Bradley J. Nelson
Abstract—We demonstrate five-degree-of-freedom (5-DOF)
wireless magnetic control of a fully untethered microrobot
with a magnetic steering system we call OctoMag. Although
only occupying a single hemisphere, this system is capable of
isotropically applying forces on the order of 1–40 µN with
unrestricted control of the 2 orienting DOF. These capabilities
are enabled through the use of soft-magnetic-cores which
provide an increase of approximately 20× that of air cores
in magnetic-field strength, but comes at the cost of more
complicated interactions between coils. We propose a modeling
mechanism that assumes the field contributions of the individual
currents superimpose linearly when using cores with large
linear regions and negligible hysteresis. When designing the
system, the locations and quantity of electromagnets were
optimized with regards to the force generation in the worst-
case direction predicted by the model. The resultant system
is capable of both open and closed-loop operation over a
workspace of 4 cm3. OctoMag was primarily designed for the
control of intraocular microrobots for delicate retinal proce-
dures, but also has potential uses in other medical applications
or micromanipulation under an optical microscope.
One approach to the wireless control of microrobots is
through externally applied magnetic fields. These untethered
devices can navigate in bodily fluids to enable a number of
new minimally invasive therapeutic and diagnostic medical
procedures. We are particularly interested in intraocular mi-
crorobots, which have the potential to be used in ophthalmic
procedures such as drug delivery and remote sensing . One
particularly difficult procedure for vitreoretinal surgeons to
perform is retinal-vein cannulation—the injection of a clot-
busting enzyme into a tiny vein—which is at the limits of
human capabilities . A few groups have proposed robot-
assisted solutions for vitreoretinal surgery to attenuate the
surgeon’s hand tremor , . In these proposed robotic
solutions, the invasiveness of the procedure is not necessarily
reduced, and the delicate retina is still at risk from a tool that
is capable of inflicting irreparable damage.
In an effort to enable less invasive and safer retinal
surgery, as well as providing an increased level of dexterity
desired by clinicians, we embarked on the design of a
system for magnetic manipulation of a fully untethered
dexterous microrobotic device inside the eye. As opposed
to manual surgery and existing robots, which are fundamen-
tally position controlled, a magnetic device is fundamentally
force controlled, with localization required for closed-loop
This work is supported by the NCCR Co-Me of the Swiss National
Science Foundation. The authors are with the Institute of Robotics and
Intelligent Systems, ETH Zurich, 8092 Zurich, Switzerland. e-mail: bnel-
firstname.lastname@example.org. J. J. Abbott is now with the Department of Mechanical
Engineering, University of Utah, Salt Lake City, UT, 84112 USA.
position control. This makes a magnetic tool a safer device
for interacting with the retina: we can impose limits on
the system to make irreparable retinal damage impossible,
even in the event of patient movement or system failure.
Ophthalmic procedures are also unique among minimally
invasive medical procedures in that they provide a direct
line of sight for visual feedback, making closed-loop position
control of intraocular microrobots possible.
II. EXPERIMENTAL RESULTS
The prototype system is shown in Fig. 1, and consists
of eight stationary electromagnets with ferromagnetic cores.
Whereas magnetic manipulation has typically relied on or-
thogonal electromagnetic arrangements generating uniform
fields, which are simple in terms of modeling and control,
OctoMag’s unprecedented level of wireless control is due
to its utilization of complex nonuniform magnetic fields. In
its current configuration the OctoMag is capable of creating
field gradients up to 2 T/m and orienting fields up to
50 mT while steering the robot. A variety of devices is the
millimeter and submillimeter range can be controlled as end
effectors. We have thus far experimented with devices made
from microassembled CoNi pieces, stainless steel tubing, and
NdFeB permanent magnets.
Earnshaw’s theorem tells us that there can be no stable
static equilibria using ferromagnetism. That is, to maintain
a stable position of the microrobot, we must use feedback
control. Once we have implemented gravity compensation,
we empirically find that the drift of the microrobot in a
static field is quite slow. During open-loop control, a 500 µm
microrobot drifts with a velocity of roughly 50 µm/s at the
diameter electromagnets. The gap between two opposing electromagnets on
the lower set is 130 mm. The insets are the top and side-camera views, and
show a 2-mm-long microrobot of the type described in .
The OctoMag prototype contains eight 210-mm-long by 62-mm-
2010 IEEE International Conference on Robotics and Automation
Anchorage Convention District
May 3-8, 2010, Anchorage, Alaska, USA
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Fig. 2. Demonstration of rotation control. Both time-lapse image sequences Download full-text
show a single 2-mm-long microrobot in the z = 0 plane (viewed from
above). The left image sequence demonstrates rotation of the microrobot in
place at an arbitrary location in space by relying on pure open-loop control.
The right image sequence demonstrates rotation of the microrobot about a
remote center at an arbitrary location using closed-loop control.
center of the workspace and 140 µm/s at the extremity of
the workspace. We find that the human operator can regulate
the position quite well using only visual feedback (Fig. 2),
although not with the level of precision as when using the
computer-vision tracker for closed-loop control (Fig. 3).
A chick choriallantoic membrane (CAM) has been demon-
strated to be a viable model tissue for surgical retinal research
and simulation . Figure 4 shows a permanent magnetic
device equipped with a needle tip being used to puncture a
blood vessel. This demonstrates that an untethered magnetic
agent is able to be manipulated with enough dexterity and
force to potentially deliver enzymes or other clot-busting
agents to the delicate structures of the eye.
Although OctoMag was designed for the control of in-
traocular microrobots for minimally invasive retinal therapy
and diagnosis, it additionally has great potential for use as a
wireless micromanipulation system under a light microscope.
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long microrobot aligned with the Z axis (vertical). The left composite image
demonstrates movement in a plane offset by -1.5 mmoff the X-Z plane
(viewed from the side). The right image sequence demonstrates movement
in a plane offset 1.5 mmoff the X-Y plane (viewed from above). The
microrobot was moved along the edges of a cube as displayed in the
isometric graph (b). ◦ indicate waypoints. + indicate tracker data.
Demonstration of automated position control. (a) show a 2-mm-
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Fig. 4. The OctoMag controlling a 2.5-mm-long NdFeB agent with a needle tip to puncture vasculature on a CAM.