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: lucia.beccai@iit.it) 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-

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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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