FORCE FEEDBACK-BASED MICROINSTRUMENT FOR MEASURING TISSUE

PROPERTIES AND PULSE IN MICROSURGERY

A. Menciassi, A. Eisinberg, G. Scalari, C. Anticoli, M.C. Carrozza, P. Dario

Scuola Superiore Sant’Anna - MiTech Lab

via Carducci, 40 - 56127 Pisa (Italy)

e-mail: arianna@sssup.it

AbstractMiniaturized and “smart” instruments capable of

characterizing the mechanical properties of tinybiological tissues are needed for research in biology,physiology and biomechanics, and can find veryimportant clinical applications for diagnostics andminimally invasive surgery (MIS). We are developing aset of robotic microinstruments designed to augment theperformance of the surgeon during MIS. Thesemicrotools are intended to restore (or even enhance) thefinger palpation capabilities that the surgeon exploits tocharacterize tissue hardness and to measure pulsatingvessels in traditional surgery, but that are substantiallyreduced in MIS.

This paper describes the main features and theperformance of a prototype miniature robotic instrumentconsisting of a microfabricated microgripper,instrumented with semiconductor strain-gauges as forcesensors. For the (in vitro) experiments reported in thispaper, the migrogripper is mounted on a workstation andteleoperated. An haptic interface provides force-feedbackto the operator. We have demonstrated that the systemcan discriminate tiny skin samples based on theirdifferent elastic properties, and feel microvessels basedon pulsating fluid flowing through them.

Keywords: tissue characterization, palpation,micromanipulation, microgrippers, force-feedback.

1. Introduction

Measuring the mechanical properties of cells and tinybiological tissues is important for research in biology,physiology and biomechanics. Cells of the body areexposed to mechanical stresses and strains throughoutlife, and this is critical to the health and functions ofvarious tissues and organs of the body [1-2]. It is clearthat microfabrication and microrobotics will provide verypowerful means for the development of a novelgeneration of research instruments. In fact, a variety oflaboratory apparatuses and microfabricated instrumentshave been developed recently for investigating cells and

tissues properties by mechanical stimulation [3-5].Miyazaki et al. [6] designed a micro tensile test systemfor cells and fine biological fibers and determined tensileproperties of collagen fibers isolated from the patellartendon. Sato et al. [7] measured the viscoelasticproperties of cultured porcine aortic endothelial cells andanalyzed them using a standard linear viscoelastic model.Wang and Coceani [8] set up an in vitro preparation offetal lambs pulmonary arteries and veins to examine localfactors responsible for hemodynamic control

In addition to basic research in biomechanics,miniaturized robotic instruments are fundamental toolsfor minimally invasive diagnostics and surgery. Perhapsthe most critical factor in MIS is the severe reduction ofsensory and dexterous manipulation capabilities of thesurgeon. Restoring (or even augmenting) thesecapabilities by developing new “smart” surgical toolswould have a major impact on the future of the wholefield of MIS.Palpation is a procedure that surgeons exploit regularlyand “naturally” in traditional surgery in order to estimatetissue hardness and to locate blood vessels hiddenbeneath opaque tissues. This is very important becausethe accidental puncturing of blood vessels during MIS isa rather frequent life threatening complication.

Research on robotics palpation has received someattention recently. A tactile array system aimed at findinghidden arteries has been proposed by Howe et al. [9], anda commercial tool for MIS has been instrumented inorder to enhance the surgeon’s haptic perception of themanipulated tissue by Bicchi et al. [10].

The authors are investigating a new class of roboticmicroinstruments whose ultimate goal is to augment theperformance of the surgeon during MIS. In previouspapers we have presented some preliminary work on thefabrication, characterization and control of microgrippersfor micromanipulation tasks [11-13]. We have alsodescribed some initial results on the use of microgrippersfor characterizing soft tissues [14].

This paper discusses the main features and theperformance of a prototype miniature robotic instrumentconsisting of a microfabricated microgripper,instrumented with semiconductor strain-gauges as force

Proceedings of the 2001 IEEE International Conference on Robotics & Automation

Seoul, Korea • May 21-26, 2001

0-7803-6475-9/01/$10.00© 2001 IEEE

sensors. In vitro experiments have been carried out on themigrogripper mounted on a workstation and teleoperated.An haptic interface provides force-feedback to theoperator. The system can discriminate tiny skin samplesbased on their different elastic properties, and feelmicrovessels based on pulsating fluid flowing throughthem.

2. The Modular Workstation

The apparatus we developed for testing themicrorobotic system is modular and teleoperated. Itcomprises: a microfabricated instrumented probe whichcan exert controllable force-displacement cycles on softtissues and measure force generated by pulsating flow inmicrovessels; a 3 d.o.f. motorized manipulator whichmoves the microgripper; a fiber optic microscope (50-200X) with monitor which allows the operator tovisualize the sample and the microgripper position; a PC-based control unit; and a haptic interface (Phantom 1.0,SensAble Technologies Inc.) which provides force-feedback to the operator. A scheme of the apparatus isillustrated in Figure 1.

Figure 1 Scheme of the apparatus

2.1 The instrumented microprobeThe microprobe selected as the end-effector of the testingapparatus is a LIGA-fabricated microgripper made out ofelectroplated nickel coated with a thin gold layer [11].The geometry of the microgripper (overall length 17 mm,overall width 7.5 mm, thickness 0.4 mm) is showed inFigure 2.The microgripper exploits flexure joints in order togenerate large displacement at the fingertips in a compactstructure, relatively easy to fabricate and assemble bymicrofabrication technologies. The microgripper isactuated by a low voltage multi-layer PZT stack (TOKINAE0203D16), whose maximum driving voltage is 150 V,and maximum displacement is about 17 µm). The voltageis supplied by a power amplifier which receives an inputsignal directly from a PC through an A/D interface (DAQAT-A06-10, National Instruments Inc.). When the

actuator pushes the rear part of the microgripper, theflexure joint-based deformable structure amplifies thedisplacement, thus producing much larger displacementat the tip.

Figure 2. Microgripper design

The microprobe is instrumented with semiconductorstrain gauges (ESU-025-1500 Entran Devices Inc.) [12].In order to obtain accurate and repeatable measurementsof force, the symmetry of the microgripper structureshould be matched by a symmetrical configuration of thestrain gauge sensors. A symmetrical configuration is alsouseful for thermal compensation and better signal tonoise ratio [15]. The optimal symmetrical configurationconsists of a full Wheatstone bridge sensor comprisingfour active strain gauges. The sensor can be used toimplement a PI closed loop force control, as described in[12-13]. The four strain gauges have been mounted intwo pairs on the microgripper, each pair located at aflexure joint: one strain gauge of the pair measurescompression and the other measures tension. Figure 3illustrates the approximate location of the strain gaugesin the microgripper structure.

Figure 3. Scheme of the microgripper showing thelocation of the strain gauge sensors

As the fingertips grasp a tissue sample, the strain gaugesmeasure the deformation of the microprobe structure.After proper calibration, the output signal of the straingauge bridge can be read as a force signal. To thispurpose, the strain gauge sensor system was calibrated byopening the microgripper fingertip against a load cell(Model GM2 3M, PTC Electronics Inc.; full scale 300mN, accuracy 0.01 mN). Calibration tests showed thegood linearity of the strain gauge sensors and indicatedthat the microstructural deformation, and hence the forceexerted by the micro gripper, can be monitored ratheraccurately using the Wheatstone bridge configuration.

OPERATOR

Microgripper

PC

Phantom 1.0

The output signal of the instrumented microgripper in theforce range 0-6 mN is illustrated in Figure 4. The forcesignal measured by the instrumented microprobe isdelivered to the Phantom haptic system, thus providingforce feedback to the human operator.

Figure 4. Calibration of the strain gauge sensor mountedon the microgripper

We have also developed a new version of microgripper,fabricated in superelastic alloy (Ni50.8Ti49.2) by wiremicro Electro Discharge Machining (µEDM). µEDMallows to fabricate high aspect-ratio structures made outof different conducting and semiconducting materialswith good surface finishing and without any thermalalterations even in the smallest features. Moreover, evenwhen the above characteristics are not strictly required(in our design the aspect ratio is ~5), µEDM is anelective choice to machine hard materials which couldnot be machined by other technologies.Superelastic alloy is the material of choice because of itsfavorable mechanical properties. This alloy allows toobtain a more robust and stiffer microprobe and tofabricate flexure joints which reach a large displacementamplification factor still within the elastic range of thematerial. A photograph of the new microgripper is shownin Figure 5.

Figure 5. The new superelastic alloy microgripperfabricated by micro Electro Discharge Machining

2.2 The haptic interface

The system control is implemented by means of aPC, which interfaces the microgripper actuating andsensing circuits with the Phantom haptic interface. Agraphic interface continously displays the state of thesystem to the operator. The system is activated throughthe graphic interface and various modes of functionalitycan be selected.For calibration purposes, the Phantom can be by-passedby retrieving the closure commands via a mouse or via anautomated sine wave generator. In normal operation, theoperator drives the actuators by the haptic interface and,at the same time, "feels” the grasping forces measured bythe instrumented microprobe. By using this apparatus,different micro-samples of soft tissue have been testedand pulse in microvessels has been “felt”. However, inorder to extract quantitative parameters from the testedsamples, signals must be processed. To this aim a methodhas been devised for the identification of the probe-sample system, as illustrated in the following paragraph.

3. Testing Methods and Experimental Results

While for research in biology and physiology accuratemeasurements of tissue mechanical properties arerequired, but on-line monitoring is not strictly necessary,most microsurgery tasks (e.g. the identification ofmicrovessels embedded in the organ or tissue to operate)require an instrument which is able to provide someinformation to the operator in real time, even if theinformation is less accurate.In order to evaluate the performance of the proposedmicrorobotic system in different measurement tasks, wehave selected two experiments: one directed to test theelastic properties of micro samples of biological tissue,and the second to detect pulsating flow in microvessels.These experiments are described in the next two sections.

3.1 Testing the elastic properties of microsamples ofbiological tissue

Although the main intended application of theapparatus is for in vivo experiments in physiology and formicrosurgery, in this phase we elected not to make testswith animals for ethical reasons. Instead, we used tinysamples (width about 100 µm) of human skin freshlyexcised from areas around the fingernails of threevolunteers. In order to obtain information on the elasticproperties of the microsamples, the following procedurewas used: the microgripper grasped the sample and,whilst grasping, a small signal step voltage was appliedto the piezo-actuator resulting in a sudden closure of thegripper. This results in small oscillations of the gripperfingertips at frequencies dependent on the resonantfrequencies of the mechanical system (“gripper plussample”). The step response of the system in idling

conditions and for the three different skin samples isshown in Figure 6.

Figure 6. Step response of the system in idling conditions(a) and when grasping three different skin samples (b, c,d) As clearly visible in Figure 6, the step response signalsare not sufficient per se to discriminate the different skinsamples. Additional processing is needed to provide theapparatus with the required discrimination capabilities.The method we elected to use for analyzing theexperimental data presented in Figure 6 is based onsystem identification [16]. The microgripper system wasidentified by means of a small step impulse excitation,filtering and Fast Fourier Transform (FFT) of the outputsignals. The experimental data were frequency analyzedusing MATLAB 5.3 (The MathWorks, Inc.). Results areillustrated in Figure 7.

Figure 7. Bode plots of the frequency response of thegripper microsystem in idling conditions (blue line) andwhen grasping three different skin samples

In idling conditions the gripper exhibits two mainresonant frequencies. When the gripper grasps a sample,

the fundamental mode of oscillation is blocked, thesecond resonant frequency is shifted depending on themechanical properties of the sample, and the dampingincreases due to internal friction. This behavior can beobserved in the Bode diagrams of Figure 7 which refer tothe idling condition and to the cases when three differentskin samples are grasped. In the system identificationprocedure, the resonant frequency and the fading constantof the poles relative to the secondary mode of oscillationare calculated for each system “gripper plus sample”starting from the acquired oscillating signals. Figure 8illustrates the Bode diagrams related to the identifiedsystems for the cases of idling condition and when thethree different skin samples are grasped.

Figure 8. Bode plots of the identified systems. Thesignals corresponding to the gripper grasping threedifferent tissue samples are much easier to discriminate

In idling conditions, the following experimental transferfunction was obtained:

�

�

�

====

=

�

��

�

�++⋅

���

��

�

�++

⋅=

−

025.090000005.0195

10

:

1212)(

11

00

13

1

1

12

20

02

2

2

pp

pp

p

p

ppo

p

p

HzfHzf

K

with

wws

wws

sKsG

ζζ

ζζ

When grasping skin samples, the following resonantfrequencies were derived from the diagrams in Figure 8:

Skin sample 1: f = 690 Hz, ς = 0.1, K = 0.2Skin sample 2: f = 800 Hz, ς = 0.25, K = 0.2Skin sample 3: f = 820 Hz, ς = 0.04, K = 0.2

In order to extract information on the mechanicalproperties of each sample, the resonant frequency and thefading constant of the poles relative to the secondarymode of oscillation should be interpreted using modelsfor the cases of the gripper alone (idling condition) and ofthe complete system “gripper plus sample”. The system“gripper and grasped sample” has been modeled as a 2d.o.f. system and this simplified model has been used tocalculate the first two resonating frequencies of thesystem [14]. The characteristic equations of this systemhave been obtained and the relative frequencies of themodes of oscillation have been calculated. Finally, thepoles of the system have been calculated by resolving thefollowing second order polynomial equation :

( ) ( ) ( )[ ]( )( )[ ]2

2221

221212

421

kkkkk

kkmkkmmmP

c

c

−+++

+++++= ωωω

where ω is the resonant frequency of the system; k1 andk2 are the elastic constants of the microgripper (accordingto our 2 d.o.f. model); m1 and m2 are the characteristicmasses of the system; kc is the elastic constant of thesample.The resonant frequencies ωi of the system “microgripperand grasped sample” obtained experimentally (see Figure6) corresponds to the roots of the equation above. Byknowing the value of k1, k2, m1 and m2 from mechanicalsimulations of the microgripper, the experimentalresonant frequencies can be related to the elastic constant(kc) of each sample.

3.2 Testing pulse in microvessels

The same system (connected to the haptic interface)was used to to measure and “feel” pulse in microvessels.For ethical reasons, microvessels were substituted bypolymeric microtubes which simulate microvessels (forexample those in the coronary tree) with goodapproximation. Pulsating blood flow was simulated by asimple microfluidic circuit comprising a Micro AnnularGear Pump (mzr-2903, Mikrosysteme GmbH) andpolymeric microtubes (Zeus Scientific Inc), with anexternal diameter of 800 µm and an internal diameter of500 µm. The micro pump is PC-controlled by a softwareinterface that allows dosing and continuos flow of fluidwith an accuracy of more than 99.5% on deliveredvolume. Experimental parameters such as accelerationtime, maximum speed and time interval among dosingpulses are variable in a wide range. Physiological bloodflow in different sites and conditions can be simulatedquite accurately in the microfluidic circuit, and thereforethe performance of the proposed microrobotic system canbe evaluated in realistic conditions. Water with a flowrate of 40 µl and a pulsating frequency of 8.3 Hz

(reproducing physiological parameters in coronarysystem of mice) was supplied through the microtubegrasped by the instrumented microgripper, as illustratedin Figure 9.

Figure 9. Microgripper grasping a microtube undermicroscope

The pulse signal was measured by the strain gauge sensorand also “felt” distinctly by the operator through thePhantom interface. Figure 10 shows the recorded straingauge signal and the operator hand who “feels” the pulsesignal through the haptic interface.

Figure 10. Pulsating microtube signal and hapticinterface sensing

4. Conclusion and Future Work

In this paper we have presented a technique suitablefor measuring in vivo the mechanical properties ofdifferent tissues, although in a rather qualitative way. Theproposed technique can be adopted for research andclinical purposes, as it does not require that the biologicalsample is excised and prepared by drying and fixation,like in traditional (and more accurate) techniques.The main features and components of the apparatusdeveloped in order to implement the proposed techniquehave been described. The key component of theapparatus is a LIGA microfabricated and instrumentedmicrogripper which is used to grasp and measure themechanical properties of the tissue sample. A method hasbeen presented for the identification of the microgripper-sample system. Different samples of human skin havebeen tested ex vivo and successfully distinguished basedon their mechanical characteristics. Furthermore the

system has been used successfully to sense pulse in amicrovessel.Further work is in progress along the two differentdirections described in this paper.The first direction aims to improve the performance ofthe microfabricated probe. In fact, a limitation of theLIGA microgripper used for the experiments presented inthis paper is its low stiffness, comparable to that of sometissue micro-samples. When the probe is much stifferthan the tested samples, the discrimination accuracy ofthe apparatus improves considerably [14]. The new probefabricated in superelastic alloy by using micro ElectroDischarge Machining has these characteristics and thus itwill be extensively used in the near future.

A second direction aims to further exploit the forcefeedback capabilities of the system. In fact, the hapticinterface provides the most intuitive approach to tissueand microvessel palpation and provides real time, highbandwidth feedback signal. This feature may make thePhantom interface very useful to provide a “feeling” oftissue “hardness” to the operator, and even an “alarmsignal” during delicate microsurgical operations. Weintend to exploit these characteristics in order to developa new a class of robotic microinstruments designed toaugment the performance of the surgeon during MIS.

ACKNOWLEDGEMENTS

The work described in this paper has been supportedin part by some Projects funded by the Commission ofEuropean Communities: “MEDEA” (MicroscanningEndoscope with Diagnostic and Enhanced ResolutionAttributes), BIOMED2 Contract No. BMH4-CT97-2399); “MINIMAN” (A Miniaturised Robot for MicroManipulation”), ESPRIT Contract No. 33915. TheScuola Superiore Sant’Anna has provided the financialsupport for basic research on micromanipulation. Theauthors wish to thank Mr. C. Filippeschi and Mr. P.Francabandiera for their technical contributions to thepaper.

REFERENCES

[1] F. Guilak, M. Sato, C.M. Stanford, R.A. Brand:“Editorial – Cell Mechanics”, Journal of Biomechanics33 (2000), pp. 1-2.

[2] T. Brown: “Techniques for MechanicalStimulation of Cells in vitro: a Review”, Journal ofBiomechanics 33 (2000), pp. 3-14.

[3] M.E. Fauver, D.L. Dunaway, D.H. Lilienfeld, H.G. Craighead, G.H. Pollack: “MicrofabricatedCantilevers for Measurement of Subcellular andMolecular Forces”, IEEE Transaction on BiomedicalEngineering, Vol. 45, No. 7, July 1998, pp. 891-898.

[4] G. Lin, K.S.J. Pister, K.P. Roos: “SurfaceMicromachined Polysilicon Heart Cell Force

Transducer”, Journal of MicroelectromechanicalSystems, Vol. 9, No. 1, March 2000, pp. 9-17.

[5] H. Miyazaki, Y. Hasegawa: “A Newly DesignedTensile Tester for Cells and Its Application toFibroblasts”, Journal of Biomechanics 33 (2000), pp. 97-104.

[6] H. Miyazaki, K. Hayashi: “Tensile Tests ofCollagen Fibers Obtained from the Rabbit PatellarTendon”, Biomedical Microdevices, No. 2, 1999, pp.151-157.

[7] M. Sato, N. Ohshima, R.M. Nerem: “ViscoelasticProperties of Cultured Porcine Aortic Endothelial CellsExposed to Shear Stress”, Journal of Biomechanics Vol.29, No. 4 (1996), pp. 461-467.

[8] Y. Wang, F. Coceani: “Isolated PulmonaryResistance Vessels From Fetal Lambs ContractileBehavior and Responses to Indomethacin andEndothelin-1”, Circulation Research Vol 71, No 2August 1992, pp. 320-330.

[9] R. Howe, W.J. Peine, D.A. Kontarinis, J.S. Son:“Remote Palpation Technology for SurgicalApplications”, IEEE Engineering in Medicine andBiology Magazine, 14 (3) (1995), pp 318-323.

[10] A. Bicchi, G. Canepa, D. De Rossi, P.Iacconi,E.P. Scilingo: “A sensorized Minimally Invasive SurgeryTool for Detecting Tissutal Elastic Properties”, Proc. of1996 IEEE ICRA, Minneapolis, Minnesota, April 1996.

[11] M.C. Carrozza, P. Dario, A. Menciassi, A. Fenu:“Manipulating Biological and Mechanical Micro-Objectswith a LIGA-Microfabricated End Effector”, Proc. of1998 IEEE ICRA, Leuven, Belgium, May 16-20, 1998,pp. 1811-1816.

[12] M.C. Carrozza, A. Eisinberg, A. Menciassi, D.Campolo, S. Micera, P. Dario: “Towards a Force-controlled Microgripper for Assembling BiomedicalMicrodevices”, Journal of Micromechanics andMicroengineering, Vol. 10 (2000) pp. 271-276.

[13] A. Eisinberg, A. Menciassi, S. Micera, D.Campolo, M.C. Carrozza, P. Dario: “PI Force Control ofa Microgripper for Assembling BiomedicalMicrodevices”, IEE Proceedings on Circuits, Devicesand Systems. (In press).

[14] A. Menciassi, G. Scalari, A. Eisinberg, C.Anticoli, P. Francabandiera, M.C. Carrozza, P. Dario:“An Instrumented Probe for in vivo Characterization ofSoft Tissues”. (Accepted for Publication on BiomedicalMicrodevices)

[15] A.L. Window and G.S. Holister, eds.: “StrainGauge Technology”, Elsevier Applied Science, London,UK (1989).

[16] A. Cohen: “Biomedical Signal Processing”, CRCPress Inc., Boca Raton, FL, U.S.A. (1986).