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Abstract— A novel approach for the design of magnetically-
propelled microrobots is proposed as an effective solution for
swimming in a liquid medium. While intrinsic neutral
buoyancy of a microrobot per se simplifies propulsion in the
liquid environments, softness makes it compliant with delicate
environments, such as the human body, thus guaranteeing a
safe interaction with soft structures. With this aim, two groups
of soft microrobots with paramagnetic and ferromagnetic
behaviors were designed, fabricated and their features were
experimentally analyzed. In agreement with the theoretical
predictions, in the performed trials the ferromagnetic
microrobots showed orientation capabilities in response to the
magnetic field that could not be achieved by the paramagnetic
one. Moreover, it was observed that the ferromagnetic
microrobot could reach higher speed values (maximum value of
0.73 body length/s) than the paramagnetic prototype.
I. INTRODUCTION
ICROROBOTICS concerns the design and fabrication of
robots with characteristic size ranging from the
millimeter down to the micrometer. These microscale
robots are conceived to move in very narrow spaces and
micro-structured liquid environments, such as in inaccessible
districts of the human body for performing in vivo tasks for
biomedical diagnosis and therapy [1].
Microrobots could be able to navigate in different human
body environments, starting from the gastrointestinal tract
(GI) [2] until the circulatory system [3], the urinary system
[4], as well as the central nervous system (CNS) [5], when
the microrobot maximum dimensions are only a few
millimeters or less. The cerebrospinal fluid (CSF), a clear,
colorless fluid which baths the entire surface of the CNS,
represents a suitable environment for swimming
microrobots. Actually, CSF has properties similar to water
(relative viscosity: 1.020–1.027; density: 1.0032–1.0048 ·103
kg m−3
), therefore the prospective of using microrobots
within the CSF is challenging and promising for biomedical
applications [6].
The approach which is mostly pursued to propel
microrobots is based on external energy transfer. These
Manuscript received September 15, 2010.
S. Palagi, A. Menciassi and P. Dario are with the CRIM Lab, Scuola Superiore Sant'Anna and with the Center for MicroBioRobotics@SSSA,
Istituto Italiano di Tecnologia, viale Rinaldo Piaggio 34,56025 Pontedera
(Pisa), Italy. V. Pensabene, L. Beccai (corresponding author: [email protected]) and B. Mazzolai are with the Center for
MicroBioRobotics@SSSA, Istituto Italiano di Tecnologia, viale Rinaldo
Piaggio 34,56025 Pontedera (Pisa), Italy.
microrobots can harvest energy from an external power
source, such as a magnetic field generator, and use it to
implement movements and specific tasks. In this way, the
integration of actuators and power sources into microrobots
can be reduced, thus limiting their final complexity and,
consequently, their size [7].
Although a lot of research efforts have been spent in the
magnetic propulsion and control of microrobots, they mainly
focused on developing complex magnetic steering systems
[8-9], finally actuating and propelling rigid metallic micro-
structures [7, 10-11] or even small permanent magnets [9,
12]. Moreover, because of the high mass density of the
employed materials (mainly metals and magnetic alloys),
magnetic micro-swimmers [13] and, more generally,
microrobots moving in 3D workspaces [9] need complex
gravity compensation strategies, which normally limit their
degrees of freedom.
This paper presents a novel approach to design a new
generation of magnetically-propelled swimming
microrobots, which aims to obtain a “smart” behavior
provided by the intrinsic properties of the robot per se, rather
than that obtained using a multi-DoF external driving
system. In fact, the objective of this investigation is to
achieve neutral buoyancy and a high level of compliance,
which guarantees a safe interaction with soft tissues, thanks
to proper design choices and materials selection. In the
present investigation, two groups of microrobots,
implementing the aforementioned features, are proposed
enforcing two different magnetic behaviors, i.e.
paramagnetic and ferromagnetic. The different features of
such behaviors are analyzed since they could both represent
an attractive solution for implementing magnetically-
propelled swimming microrobots. Therefore, an
experimental analysis performed on the two groups of
prototypes is presented, in order to verify the
implementation of the desired magnetic characteristics and
to quantitatively measure and compare their performances.
II. DESIGN OF THE MICROROBOT
A. Swimming
In the field of microrobotics, the term “swimming”
usually refers to the capability of microrobots of generally
moving within a liquid medium.
Several swimming methods have been proposed for
magnetically-propelled microrobots, ranging from pulling by
Design and development of a soft magnetically-propelled swimming
microrobot
Stefano Palagi, Student Member, IEEE, Virginia Pensabene, Member, IEEE,
Lucia Beccai, Member, IEEE, Barbara Mazzolai, Member, IEEE, Arianna Menciassi, Member, IEEE,
Paolo Dario, Fellow, IEEE
M
2011 IEEE International Conference on Robotics and AutomationShanghai International Conference CenterMay 9-13, 2011, Shanghai, China
978-1-61284-380-3/11/$26.00 ©2011 IEEE 5109
means of static magnetic field gradients [7] to actuating bio-
inspired propellers through alternating fields [10]. Although
bio-inspired propulsion methods can be slightly more
efficient [14], magnetic pulling can be considered the
simplest method for implementing swimming capabilities in
microrobots that do not include any kind of propelling
appendage. Moreover, considering that microscale robotic
agents operate in a low-Re environment, in which inertia is
negligible over viscous forces [15], from a hydrodynamic
point of view the sphere represents a quite effective shape
(where the optimal one would be a prolate spheroid with
conical front and rear ends of angle 120° [16-17]) for the
microrobots body. For these reasons, the microrobots that
we propose in this work have near-spherical shape and
diameter of about 2 mm. The viscous drag force FD on such
a spherical microrobot is thus given by the Stokes’ law
(valid for Re < 1), namely
, (1)
where η is the fluid viscosity and d and v are, respectively,
the diameter and the velocity of the microrobot.
B. Magnetic Propulsion
Controlled magnetic fields are used to apply forces and
torques necessary to drive a microrobot. In particular, the
torque τM (N·m) generated by a magnetic field H (A/m) on a
microrobot with average magnetization M (A/m) can be
expressed as [18]
, (2)
where µ0 is the magnetic permeability of free space
( ) and V is the volume of the microrobot. τM
aims at aligning the magnetic dipole of the microrobot to the
external magnetic field. Therefore, this torque can be
exploited to orientate the microrobot in a desired direction in
the workspace. Moreover, the expression of the force FM
generated by the same magnetic field H on the microrobot,
which can be obtained by deriving the energy of the dipole
moment in presence of the field [18], results as follows
. (3)
This magnetic force can be used for wirelessly propelling
the microrobot within the operative medium through the
pulling propulsion method [14].
Hence, the magnetic torque and force depend respectively
on the magnetic field H and on its gradient H.
Depending on the nature and on the state of aggregation
of the magnetic components embedded inside its body, the
microrobot can implement different magnetic behaviors,
which strongly influence its propulsion performances and
the possibility to show additional degrees of freedom. Such
aspects need to be properly considered during both the
design and the fabrication stages. In particular, in order to be
magnetically propelled, the microrobot can have either
paramagnetic or ferromagnetic properties.
The average magnetization of a paramagnetic object, for
low values of the field (far from saturation), is linearly
related to the applied magnetic field through the magnetic
susceptibility χ. For paramagnetic materials, typical values
of χ range from 10-6
up to 10-3
[18]. The magnetic torque and
force acting on a paramagnetic or super-paramagnetic
microrobot can thus be written as
(4)
. (5)
Thus, the magnetic torque acting on a paramagnetic object
is always null, while the magnetic force varies not only with
the gradient of the field, but also with the field itself.
In the case of ferromagnetic objects, instead, the
magnetization is not linearly related to the applied field
because of the magnetic hysteresis exhibited by
ferromagnetic materials. In particular, this kind of objects,
once magnetized, retains a certain level of magnetization
(remanent magnetization, MR) when the magnetic field is
absent. For low values of the external magnetic field, the
magnetization of a ferromagnetic microrobot can be
considered as coinciding with its remanent magnetization;
the magnetic torque and force acting on the microrobot can
thus be expressed as
(6)
. (7)
The magnetic torque, which aligns the magnetization
vector of the microrobot to the external field, depends on
both the applied magnetic field and the angle between the
field and the magnetization vector. On the other hand, the
magnetic force is not depending on the magnetic field, but
only on its gradient. Hence, ferromagnetic microrobots could
be both orientated and pulled in the workspace, thus
presenting more degrees of freedom than paramagnetic
microrobots, and this could increase maneuverability in the
operative medium.
However, it is worth mentioning that nanometer-sized
particles of ferromagnetic materials exhibit no hysteresis in
the relationship between applied field and magnetization,
like paramagnetic materials, but considerably higher values
of magnetic susceptibility than them; this phenomenon is
known as super-paramagnetism. On the other hand,
aggregates of these nanoparticles can exhibit ferromagnetic
properties, as well. Therefore, the inclusion of a small
volume of super-paramagnetic nanoparticles in the robot
body can be considered a good solution for implementing,
by changing their state of aggregation, either a paramagnetic
or ferromagnetic behavior in swimming microrobots.
In this work, prototypes of both kinds of microrobot are
fabricated, by changing the state of aggregation of super-
paramagnetic nanoparticles embedded in the microrobot
body, thus showing paramagnetic and ferromagnetic
behavior. The developed prototypes are tested in order to
compare the magnetic actuation of the two types of
prototypes in terms of performances and maneuverability.
C. Buoyancy and Softness
All state-of-the-art magnetically-actuated microrobots,
independently of the implemented propulsion method, are
either microfabricated metallic structures [7, 10-11] or small
permanent magnets [9, 12]. These rigid and heavy micro-
5110
devices either have to move on a surface [11-12] or need
external gravity compensation for moving in a 3D space [9,
13], which can limit the degrees of freedom of the
microrobots, at least in some points of the workspace.
Hence, swimming magnetic microrobots usually require
complex control systems, implementing large magnetic field
gradient, for an effective steering in a 3D liquid
environment.
Regarding this aspect, a novel design approach is
followed in this work. In order to further simplify their
actuation and control in 3D liquid environments, the
microrobots are designed to be neutrally buoyant in their
operative fluid medium, having the same mass density of the
fluid they are moving within. In particular, in the design
process the mass density is imposed to be equal to that of
water (1·103 kg m
-3) which is very close to that of biological
fluids of interest, e.g. the CSF.
Moreover, a safe mechanical interaction between the
robots and the tissues has to be guaranteed. Moving in this
direction, soft-bodied microrobots, rather than the usual rigid
metallic micro-devices, are considered in this work.
Buoyancy and softness properties are implemented in the
developed prototypes by including a light organic liquid in
gel-based microrobot bodies.
III. FABRICATION OF THE MICROROBOT PROTOTYPES
A. Materials
Sodium alginate (alginic acid, sodium salt), calcium
chloride (CaCl2), sodium chloride (NaCl) and dodecane
(CH3(CH2)10CH3) were purchased from Sigma-Aldrich. not
functionalized super-paramagnetic Fe2O3 nanoparticles (NP,
iron(III) oxide, magnetic, NanoArc®) were purchased from
Alfa Aesar; anionic charged super-paramagnetic magnetite
NPs (fluidMAG-UC/A) were purchased from Chemicell
GmbH. The nanoparticles suppliers provide only data about
size and functionalization.
B. Fabrication process
Both the paramagnetic and the ferromagnetic prototypes
were fabricated by gelation of alginate solution droplets
embedding NPs with super-paramagnetic properties and
dodecane. The different final magnetic behavior
(paramagnetic or ferromagnetic) depends on two aspects: 1)
a different particle functionalization and 2) a different
gelling procedure under an external magnetic field or
without any external magnetic field. The preparation steps
are reported here in details:
1. preparation of a sodium alginate solution (10 mg/ml)
in de-ionized water;
2. addition of super-paramagnetic NPs (0.5 mg/ml):
anionic charged NPs for paramagnetic prototypes,
not-functionalized NPs for ferromagnetic prototypes;
3. addition of dodecane (0.04 ml/ml);
4. emulsion on the vortex (1 minute at 3000 rpm);
5. dispersion of NPs by sonication (90 minutes at room
temperature);
6. dripping in a calcium chloride aqueous solution (5
mg/ml) with a micropipette; in the case of
ferromagnetic prototypes a permanent magnet was
placed under the CaCl2 solution.
Hence, either paramagnetic or ferromagnetic prototypes
were fabricated through the same procedure by only
changing the functionalization of magnetic NPs and their
state of aggregation.
The employment of anionic charged NPs guarantees to
keep the super-paramagnetic properties of the particles
avoiding the formation of aggregates with different magnetic
behavior. On the contrary, the use of non functionalized
nanoparticles is needed for the production of ferromagnetic
prototypes. The formation of aggregates is enhanced by
employing a permanent magnet in the gelling solution and it
confers a residual magnetization to the aggregates, and thus
a ferromagnetic behavior to the gel structure.
Finally, the samples are stored in a sodium chloride
aqueous solution (5 mg/ml) to prevent swelling.
C. Characterization
Fig. 1 shows the two kinds of prototypes, in which the
final shape, dimension and structure are compared.
The volume of the magnetic structure can be defined and
controlled considering the volume of the drops of the initial
polymeric dispersions. In this case, aiming to obtain a final
spherical structure of 2 mm in diameter, and considering the
expression of the volume of the sphere, drops of 4.2 µl were
released in the calcium solution. The dropping mechanism
allows the obtaining of a structure which is not completely
spherical (as show in Fig. 1) and recalls the drop shape.
However, the final difference between the nominal and the
effective volume of the structure is below 9% in average,
and it can be neglected in the evaluation of the magnetic
propulsion. Moreover, the final results are highly
reproducible, with a standard deviation in terms of diameter
of about 3%. Beside this, considering that the polymeric
sphere represents a first simple prototype of microrobot
body, the asymmetric shape allows to observe different
response to the imposed magnetic field in the two cases, in
particular in terms of orientation and direction.
Without using the magnet in the gelling procedure, and
thanks to their charged surface, the NPs remain
homogeneously and finely dispersed within the gel droplets,
even if small aggregates can be found.
Fig. 1. On the left the paramagnetic prototype; on the right the ferromagnetic prototype (scale bars 2 mm). Images were collected
with Hirox optical microscope, maintaining the objects in a water
filled Petri dish.
5111
When employing non functionalized particles and placing
a permanent magnet in the gelling solution, the NPs are
attracted and move towards the magnet during completion of
alginate gelation. The level of aggregation is in this case
important and, being the NPs pure Fe2O3, they are all
attached and finally collected in a defined volume of the
polymeric matrix. The initial dispersion of NPs was not
stable and the concentration of NPs exceeded the saturation
level. This effect suggests a change in the final behavior of
the structure, since the NPs are aggregated in micrometric
volumes and should response to applied magnetic fields as a
single ferromagnetic material.
Dodecane, which was added in the polymeric solution
before the dropping phase, is maintained inside the structure
after the accomplishment of alginate polymerization. This
result is validated at the end of the process since the obtained
spheres show buoyancy in water (Fig. 2). The dispersion of
dodecane and the internal aspect of the polymeric matrix are
not analyzed in the present work, but can provide additional
details about the behavior of the dodecane in solution and
during the gelation process.
IV. MAGNETIC PROPULSION
A. Magnetic field generator
A dedicated system was set up for the generation of
magnetic fields [19]. It consists of two coils, nominally
identical, in the Maxwell configuration (distance L between
the coils equal to the square root of three times the radius R
of the coils) and independently current-supplied by two
custom amplification stages (Burr-Brown OPA549
operational amplifiers) driven by a software interface
(developed in NI LabView) through a USB multifunction
data acquisition device (NI USB-6259 BNC). Controlled
combinations of magnetic field and gradient can be
generated in the region around the centre of the workspace.
Moreover, defined x as the common axis of the coils, only
the x-components of both magnetic field and gradient are not
null in this region. Finally, the magnetic field is considered
positive when it is concordant with the x-axis.
For this reason equations (4) to (7) can be rewritten as
(8)
(9)
for paramagnetic or super-paramagnetic prototypes and as
(10)
(11)
for ferromagnetic prototypes, where θ is the angle between
the magnetization of the prototype and the external magnetic
field. Since in all the performed trials H is not null, the
consequent magnetic torque align nearly instantaneously the
magnetization vector to the magnetic field. For this reason
the angle θ is always equal to either 0 or 180°.
The magnetic field generator, which was developed for
performing preliminary tests on magnetically propelled
microrobots, represents a simplified steering system, made
up of only two coils and able to propel the prototypes of
microrobot only forward and backward along the x-direction
in a small workspace (of the same size of a 36 mm diameter
Petri dish). Moreover, no closed-loop control is implemented
in the system, although a micro-camera (IDS uEye USB UI-
2250-MM CMOS camera) was placed above the workspace
for giving visual feedback to the user and performing
automatic tracking of the prototypes. In spite of such
simplifications, the intensity of the generated magnetic fields
is close to those of devices currently developed for human-
body applications [9, 20]. It is noteworthy that, in the design
of a magnetic field generator for effective use for the human
body, dimensions and number of the coils should be
properly increased in order to achieve 3D navigation and
adequate workspace.
B. Magnetic response of microrobots prototypes
A first set of trials was performed with the two kinds of
prototypes in order to evaluate and compare their magnetic
response. Tests were repeated on 5 specimens of each group
of prototypes. Moreover, the paramagnetic or ferromagnetic
behavior of the devices could be observed and verified
through these trials.
Trials were performed maintaining the prototypes in
sodium chloride aqueous solution. The first experiment
consists in the following sequence of motion tasks:
A. starting from the centre of the workspace (x = 0) and
pulling forward along x-direction until x = 5 mm;
B. pulling backward along x-direction until x = 0;
Fig. 2. Buoyancy was observed in both kinds of prototypes in water: a) ferromagnetic and b) paramagnetic microrobots float underneath
the water surface.
Fig. 3. The magnetic field generator.
5112
C. pulling backward along x-direction until x = –5 mm;
D. pulling forward along x-direction until x = 0.
The motion range was not the maximum allowed by the
system; its choice represents, instead, a trade-off between
extent of displacement and duration of the task.
The combinations of magnetic field and gradient applied
during the four tracts of the trials, arbitrarily imposed by the
user, are reported in Table I. In particular, the amplitude of
the employed gradients, which is the most critical aspect in
magnetic propulsion, is in the typical range of those
employed for the magnetic navigation of microrobots in the
human body (100 – 500 mT/m ) [20].
As expected, the paramagnetic prototypes showed no
alignment with the field. In particular, no changes in the
orientation were observed during the four tracts of the trials,
while the direction of motion (forward or backward) was
directly imposed by the sign of the product between the
magnetic field and its gradient (see (8) and (9) and Fig. 4).
The ferromagnetic prototypes, instead, showed an
alignment with the field and a change of orientation
depending on the sign of the applied field. Moreover, the
direction of motion depended on the sign of the product
between the gradient of the magnetic field and (see
(10) and (11) and Fig. 5).
Hence, these experimental observations confirm the
effective implementation in the two different kinds of
prototypes of the two different magnetic behaviors
theoretically analyzed in the design stage.
C. Speed test and evaluation of magnetic properties
After having verified their magnetic behavior, a second
set of trials were performed on the two groups of microrobot
prototypes in order to quantitatively evaluate the magnetic
parameters χ and MR. Moreover, the performances in terms
of speed were evaluated and compared.
All the prototypes were placed along the x-axis at
and pulled until , by applying a
magnetic field and a magnetic gradient of 8 mT and 400
mT/m respectively, while their position was tracked during
all the trial. The speed of the prototypes in the centre of the
workspace was thus evaluated.
Hence, the average values of the measured speeds are
for paramagnetic prototypes and
for ferromagnetic prototypes.
These obtained speed values correspond to
and body length/s, for the paramagnetic and
ferromagnetic microrobots, respectively. This difference has
to be related to the different final magnetic behavior of the
nanoparticles in the two kinds of prototypes.
Finally, balancing the drag forces and the magnetic forces
and, in parallel, considering the experimentally evaluated
drag speeds, the magnetic parameters of the two groups of
prototypes were estimated; their average values are and .
An experiment relative to each trial is depicted in the
video file attached to the present work.
V. CONCLUSIONS
Magnetically-propelled swimming microrobots were
designed and developed with both paramagnetic and
ferromagnetic behaviors. According to the design, both types
of microrobots consisted of near-spherical millimeter-sized
neutrally buoyant soft-bodied prototypes.
Although the simple and not automated fabrication
process, good repeatability was observed in the dimension,
shape and buoyancy of the fabricated prototypes, especially
in the paramagnetic group, since the dispersion of the
functionalized super-paramagnetic NPs was very stable.
Moreover, since the values of speed are highly reproducible,
we can assume that this first simple fabrication step
guarantees a consistent final structure of microrobots.
The effective implementation of the two desired magnetic
behaviors was verified and their magnetic propulsion
capabilities were investigated. As expected, the
TABLE I
COMBINATIONS OF MAGNETIC FIELD AND GRADIENT
Tract B(x=0) = µ0H(x=0) [T] dB/dx (x=0) =
= µ0 dH/dx (x=0) [T/m]
A +8·10-3 +0.4
B +8·10-3 –0.4
C –8·10-3 +0.4
D –8·10-3 –0.4
Values of the magnetic induction field and of its gradient in the
centre of the workspace during the four tracts of the motion trials.
Fig. 5. Magnetic response of ferromagnetic prototypes showing re-orientation capabilities. In the images the center of the workspace
coincides with position x=0.
Fig. 4. Magnetic response of paramagnetic prototypes. In the images
the center of the workspace coincides with position x=0.
5113
ferromagnetic prototypes could be arbitrarily oriented by the
magnetic field, while the paramagnetic ones could not. This
property could be exploited for the design and fabrication of
a microrobot able to perform specific tasks, such as
assuming the best orientation for approaching a particular
environment, avoiding obstacles or picking nano-objects.
However, because of the much higher stability of charged
NPs in water dispersions with respect to not-functionalized
NPs, the amount of charged NPs within the microrobots can
be further increased, thus increasing the obtainable speeds.
For these reasons, the development of prototypes embedding
charged NPs that could be arbitrarily oriented is currently
under investigation. Moreover, future efforts will be devoted
to the fabrication of microrobots with different shapes and
sizes designing ad hoc microfluidic extrusion systems.
It is important to note that buoyancy, softness and
capability of being orientated, are intrinsically implemented
in the microrobots and not due to the external steering
system. For this reason, these robots could be proposed and
tested, without modifications, with more advanced steering
systems, such as those in [9] and [20], further on improving
their performances.
The achieved speeds are more than one order of
magnitude lower than typical values of the CSF peak
velocities [21]. Thus, in order to effectively employ these
microrobots in the CSF, their performances can be improved
selecting the most appropriate particles, increasing the
nanoparticles density in the polymeric structure and
accordingly designing an optimal external generator. The
design and development of microrobots characterized by
maximum dimensions of a few millimeters or less, and by
shape and materials highly compliant with the biological
medium, can open new scenarios for clinical applications.
Soft-bodied, self-buoyant microrobots developed with the
approach presented in this work, if properly equipped with
additional components implementing advanced specific
functionalities, could be used for achieving the most
appropriate interaction with the human body environment.
The use of the presented polymeric microrobots can be
foreseen in cardiovascular and lymphatic systems for
reaching tumor masses performing a controlled embolization
in specific microvascular vessels. Moreover, adding to the
polymeric structure thermal or chemical responsive
components, these controllable microrobots could work as
microsensing systems to be delivered and moved through the
central nervous system for monitoring pathological
conditions such as hydrocephalus, or for performing targeted
cancer treatments.
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