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Abstract² We present a novel and flexible system to be

employed for tactile transduction in the realization of artificial

³URERW� VNLQ´�� 7KH� PHFKDQLFDO� GHIRUPDWLRQ� GHWHFWLRQ�� ZKLFK�

functionally reproduces the sense of touch, is based on Organic

Thin Film Transistors (OTFTs) assembled on a flexible plastic

foil, where each device acts as a strain sensor. OTFT-based

mechanical sensors were fabricated employing a solution-

processable organic semiconductor, namely 6,13-

bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)

deposited by drop casting. It will be shown that the surface

deformation induced by an external mechanical stimulus gives

rise in both cases to a marked, reproducible, and reversible

(within a certain range of surface deformation) variation of the

device output current.

Starting from these results, more complex structures, such as

arrays and matrices of OTFT-based mechanical sensors, have

been fabricated by means of inkjet printing. Thanks to the

flexibility of the introduced structure, we will show that the

presented system can be transferred on different surfaces (hard

and soft) and employed for a wide range of applications. In

particular, it can be successfully employed for tactile

transduction in the realizDWLRQ�RI�DUWLILFLDO�³robot skin´�

INTRODUCTION

HE light weight, low-cost processing, and the

mechanical flexibility of conjugated polymers led them

to be considered as a valuable alternative to most

common inorganic materials for the realization of electronic

devices. Organic Thin Film Transistors (OTFTs) are

recognized as key tools/building blocks for the

implementation of electronic logic circuits [1-3] and have

been intensively studied for many applications, such as

displays, smart tags and sensors [4-6]. Despite the low

mobility of organic materials (compared to crystalline

semiconductors, it is about three orders of magnitude lower)

there are applications, as the recently suggested electronic

skin [3, 7, 8], in which the lower speed is tolerable and the

use of organic materials seems to be more beneficial than

Manuscript received January 31st 2012.

1Department of Electrical and Electronic Engineering, University of

&DJOLDUL��3LD]]D�G¶$UPL��������&DJOLDUL��,WDO\� 2CNR- Institute of Nanoscience S3 Via Campi 213A, I-41100 Modena,

Italy 3Dipartimento di Informatica, Sistemistica e Telematica, Università di

Genova, via Opera Pia 13, 16145 Genova

(*email address: piero.cosseddu@diee.unica.it)

detrimental. In fact, being able to obtain large sensing areas

is certainly a benefit for a wide set of applications and using

printing techniques for creating sensing devices on unusual

substrates could surely widen the set of possible applications

where sensing is required.

Reproducing the human sense of touch with an artificial

system is a very challenging task, primarily because the term

"touch" is actually the combined term for several senses. In

fact, when a person touches something or somebody this

gives rise to various feelings: the perception of pressure

(hence shape, softness, texture, vibration, etc.), relative

temperature and sometimes pain. The high degree of

dexterity which characterizes grasping and manipulative

functions in humans, and the sophisticated capability of

recognizing the features of an object are the result of a

powerful sensory-motor integration which fully exploits the

wealth of information provided by the cutaneous and

kinaesthetic neural afferent systems [9]. A very accurate

description of tactile units is available [9], where a

classification of these units according to receptive fields and

response time is given.

Several approaches have been introduced over the past

years for the fabrication of flexible tactile sensors for bio-

inspired applications. The tentative specifications for tactile

sensors have been defined [10] as: i) the sensor surface or its

covering should be both robust and durable; ii) the sensor

should provide stable and repeatable output signals. Loading

and unloading hysteresis should be minimal; iii) linearity is

important, while a monotonic response is absolutely

necessary. Some degree of non-linearity can be corrected

through signal processing; iv) the sensor transduction

bandwidth should not be less than 100 Hz, intended as tactile

image frame frequency. Individual sensing units should

accordingly possess faster responses to allow multiplexing;

v) spatial resolution should be at least of the order of 1-2

mm, as a reasonable compromise between gross grasping

and fine manipulation tasks.

The development of tactile sensors is one of the most

challenging aspects of robotics research. Many technologies

have been explored, including carbon-loaded elastomers,

piezoelectric materials, and micro-electromechanical

systems. Artificial skin examples, able to detect pressure,

already exist, but these are difficult to manufacture in large

enough quantities to cover a robot body, and, moreover, they

GR�QRW� VWUHWFK��7KH�PRVW�SURPLVLQJ�H[DPSOHV�RI�³HOHFWURQLF�

Inkjet printed Organic Thin Film Transistors based tactile

transducers for artificial robotic skin

Piero Cosseddu

1, 2*, Laura Basiricò

1, 2, Alberto Loi

1, Stefano Lai

1, P. Maiolino

3, E. Baglini

3, S. Denei

3,

F. Mastrogiovanni3, G. Cannata

3, Annalisa Bonfiglio

1, 2

T

The Fourth IEEE RAS/EMBS International Conferenceon Biomedical Robotics and BiomechatronicsRoma, Italy. June 24-27, 2012

978-1-4577-1200-5/12/$26.00 ©2012 IEEE 1907

skin-like" systems with large areas are based on organicsemiconductors and have been reported by Takao Someya'sgroup at the University of Tokyo [11]. They have developedconformable, flexible, wide-area networks of thermal andpressure sensors, in which measurements of temperature andpressure mapping were performed simultaneously.

In first examples of large area pressure network fabricatedon a plastic reported by Someya et al. [3, 11], organictransistors were not used as sensors in themselves but asaddressing elements of a flexible matrix which was used toread out pressure maps from pressure-sensitive rubberelements containing graphite. The device can detect a fewtens of kilopascals, which is comparable to the sensitivity ofdiscrete pressure sensors and the time response of thepressure-sensitive rubber is typically of the order ofhundreds of milliseconds.

After the pioneristic work of Someya, low-costmanufacturing processes have been further optimized for thedevelopment of flexible active matrices using ink-jet printedelectrodes and gate dielectric layers [12]. This workdemonstrates the feasibility of a printed organic FET activematrix as a read out circuit for sensor applications. Othergroups have also attempted to use organic semiconductorsas sensing elements. For instance, Rang et al. [13] haveinvestigated the hydrostatic-pressure dependence of I-Vcurves in organic transistors. The authors found a large andreversible dependence of drain current and hole mobility onhydrostatic pressure, suggesting that this kind of devicecould be suitable for sensor applications. However, theproposed device was not flexible, therefore not suitable forapplications like robot skin, e-textiles, etc.

Strain sensors using an organic semiconductor as thesensor (resistive) element of a strain gauge have been alsoreported by Jung and Jackson [14].

One of the main advantages of employing OTFTs asmechanical sensors is that transistors are multiparametricdevices, in which different electronic parameters, not onlyone as for piezoresistive sensors, can be extracted from theirelectrical characterization, offering the possibility of using acombination of variables in order to characterize theirresponse to the parameter to be sensed. Finally, activesensors combine in the same device both switching andsensing functions, and this allows to easily obtain a sensingmatrix of limited size and improved reliability.

Although mechanical flexibility is one of the mainadvantages of organic materials, organic semiconductorsstrain properties have not yet been fully exploited in order torealize devices for detecting physical parameters as forinstance pressure or bending. The effect of strain on themechanical and electronic properties of organicsemiconductors is an emerging research topic in fundamentalphysics and applications [15-17].

In this work, we report on the fabrication andelectromechanical characterization of OTFT-based strainsensors, and their employment as tactile transducers for therealization of artificial robotic skin.

I. EXPERIMENTAL

Organic Thin Film Transistors (OTFTs) have been fabricatedusing a bottom-gate/bottom-contact structure (Fig. 1) on aflexible plastic substrate, namely 175 urn thick polyethyleneterephthalate film (i.e., PET, Goodfellow). Beforeproceeding with device fabrication, the substrate wassonicated in acetone and isopropyl alcohol, washed in de­ionized water and dried under nitrogen flow. A 1.5 urn thickParylene C film was employed for the realization of the gatedielectric. Parylene was deposited from vapor phase, using aLabcoater 2 SCS PDS 2010.

Figure 1: Schematic representation of the flexible employedOTFT structure.

Gate, Source and Drain electrodes were fabricated by inkjetprinting using a Fujifilm Dimatix Materials Printer 2800, apiezoelectric drop-on-demand inkjet printer, widely used fororganic electronic applications, with a DMC-11610cartridge. This cartridge contains 16 nozzles with a diameterof21.5 um and each nozzle generates 10 pL drops of ink. Inthis case the electrodes have been made of silver-based ink(Cabot Conductive Ink CCI-300) which contains surfacemodified ultra-fine silver nano-particles dispersed in a liquidvehicle composed of ethanol and ethylene glycol. Beforefilling the cartridge the ink has been sonicated for 15 minand then filtered with a 0.2 urn nylon filter in order to avoidnano-particles agglomeration which can cause nozzleclogging. After deposition, the samples have been annealedin an oven at 60°C in order to allow the sintering of the ink.6,13-bis(triisopropylsilylethynyl) pentacene (i.e., TIPS­pentacene - provided from Sigma-Aldrich), was depositedby drop casting from a 0.5% in weight solution (usingtoluene as organic solvent) for the fabrication of the activelayer of the transistors. During the deposition of the TIP S­based solution, the substrate was kept at 90°C to promotefast solvent evaporation and subsequently to obtain goodcrystallization of the semiconductor.

Measurement of the drain-source current ID were carriedout at room temperature in air. An Agilent HP 4155Semiconductor Parameter Analyzer, provided with gold tipsfor contacting the electrodes, was used to control the gatevoltage VGS and the drain-source voltage VDS, as well as tomeasure IDS (the source being the common ground).

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Finally, mechanical tests on the matrices of OTFTs have

been performed using a dedicated readout electronics

developed ad hoc in our lab.

II. RESULTS

The first part of the activity was focused on the

electromechanical characterization of OTFTs. In Fig. 2 the

typical output and transfer characteristics of the fabricated

devices are shown. A good electrical performance has been

obtained, with charge carrier mobility up to 7×10-2

cm2/Vs,

threshold voltages usually ranging around +10V and

Ion/Ioff ranging around 105.

Figure 2: Output characteristic (a) and transfer characteristic

(b) of an OTFT with inkjet printed silver interdigitated

electrodes and TIPS-pentacene deposited by drop-casting.

In order to induce a tensile deformation on the active layer

all devices have been deformed by bending the substrate

with different bending radii using the experimental set up

shown in Fig. 3. Experiments have been performed using

bending radii varying from 4 cm down to 0.3 cm,

corresponding to a surface strain H�in the range of 0.2 to 3%.

The strain induced in the active layers was calculated

directly from the bending radius of the device [18], a model

that was already successfully applied in various bending

experiments.

Figure 3: Experimental setup employed for the

electromechanical characterization of a single OTFT.

Figure 4: Electrical response of OTFT mechanical sensors.

Normalized transfer characteristics recorded at different

bending radii (a) and sensitivity vs. surface strain (b).

The main results are shown in Fig. 4, where transfer

characteristics upon deformation and sensitivity vs. surface

strain are depicted. The reported transfer characteristics

(Fig. 4a) have been normalized by dividing ID values by ID0,

where ID0 is the maximum current recorded (at VGS = -40V)

1909

in the same device before bending its surface for the first

time: in this way it is much easier to evaluate the relative

current decrease induced by surface deformation. The

sensitivity of the devices to strain was calculated as variation

of the current (recorded at VGS=-40 V) of the bent device,

IDn, with respect to the unbent status, ID0n (normalized to the

pristine value), i.e. (IDn-ID0n)/ID0n.

As can be clearly observed from the plots reported in Fig.

4, the devices are characterized by a marked and

reproducible response to the induced surface deformation,

moreover, the electrical response of the fabricated OTFTs

was found to be linear within the range of deformation

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We assume that such a sensitivity to mechanical

deformation can be related to changes taking place in the

organic semiconductor thin films. In fact, a tensile stress has

a double effect on the organic semiconductor thin film. On

one hand the size of TIPS-pentacene crystals can change due

to stretching, on the other hand the tensile stress could also

cause an increase of the mutual distance between the TIPS-

pentacene crystals and reduce the charge carriers hopping

process from one crystal to another. In both cases the

reduced charge mobility could be explained using the

hopping transport model. Tensile stress leads to the

increasing of potential barrier for thermal activated

tunnelling which reduces the charge carrier mobility in

polycrystalline organic semiconductor films such as TIPS-

pentacene [19, 20].

Figure 5: Schematic (a) and photograph (b) of OTFTs in the

matrix configuration on PET substrate; magnification of a

single OTFT device, showing the inkjet printed interdigitated

source and drain electrodes (c).

These results are particularly important since they

demonstrate that also a solution processable material, such as

TIPS-pentacene, that can be deposited with easy and low

cost techniques over large areas, can be employed for the

fabrication of mechanical sensors.

Starting from these results, we have developed and tested

a system, based on a matrix of OTFTs sensors, for

reproducing the sense of touch in robotic applications. We

created a basic module by realizing a number of matrices of

8×8 OTFT-based sensors, covering an area of 16 cm2, with

lateral pitch (in both x and y directions) of 5 mm. These

sensors were fabricated by inkjet printed silver electrodes,

ZLWK� /� � ����P� DQG�:� � ���PP, using the configuration

reported in Fig. 5.

As the devices are arranged in a common source

configuration, with 8 independent gate electrodes, one per

row, and 8 independent drain electrodes, one per column,

every single element of the matrix can be addressed

independently. A schematic of this layout, together with a

picture of the fabricated matrix of sensors, is shown in Fig.

5.

Figure 6: Picture of an inkjet printed matrix during a

mechanical deformation test (a); electrical response of 8

different elements on the same row of the reported matrix

during mechanical deformation (b).

Since the fabricated OTFT-based sensors are able to

detect surface strain, rather than pressure, the fabricated

matrices of sensors have been embedded between two layers

of a polydimethylsiloxane (PDMS) elastomer. In this way

when pressure is applied by an indenter on the matrix

1910

surface, it induces a surface deformation that can be detectedby the OTFTs strain sensors.

As can be clearly observed from the plots reported in Fig.6, when a certain pressure is applied on the device structure avariation of the output current can be detected. In this case,an arbitrary pressure has been exerted with a finger on asingle element of the matrix (CH6 in the last row on the rightend). After that, the finger was moved forward and backwardacross the entire row, and the device response of eachelement of the same row has been recorded. The results arereported in Fig. 6b, from which two different points can behighlighted: i) each device responds to the appliedmechanical deformation and their electrical response isshifted in time according to the propagation of themechanical stimulus (finger moving across the surface); ii) asmall, but visible, cross talking between adjacent elementscan be observed. This behavior is induced by the fact thatthe contact area between the indenter (the finger in this case)and the active matrix is comparable to the lateral distancebetween two close OTFTs (5mm). As a result, themechanical deformation induced by the finger on oneelement is also partially applied to the adjacent elements.

Figure 7: Picture of the employed mechanical finger (a) andcalibration curve (Current vs. Force) of inkjet printed OTFTsmechanical sensors (b).

After that, the system was calibrated by using a differentexperimental set-up. In this second case a hemisphericindenter (4 mm radius), which exerted a vertical normalforce on the matrix elements, was employed. As both verticalposition I1z and force F of the indenter can be preciselycontrolled, in this way it was possible to proper calibrate thesystem, by measuring the electrical response of the testeddevices induced by the applied external mechanical stimuli.The results are shown in Fig. 7, where the I1ID/IDo vs. F plotsare represented. The results can be summarized in threemajor points: i) the reproducibility is good over the wholerange of applied forces; ii) the response is linear for appliedforces up to 1 N (in the "soft touch" range), while it saturatesfor higher forces; iii) the resolution (defined as the lowestdetectable force) is around 0.05 N.

Figure 8: Electrical response of an OTFT -sensor during theapplication of mechanical stimuli. Data are shown aspercentage relative variation with respect to the initial state,highlighted with the red line (a); comparison betweentransfer characteristics acquired before, and after acontinuous mechanical solicitation, showing no significantdegradation of the device behavior (b).

Also, in order to investigate the durability of the devices,mechanical stress tests have been performed, as reported inFig. 8. Up to 1000 rapid pressure events have been exertedwith a finger on an OTFT -based sensor, and the percentagevariation of the output current with respect to the initial statehas been recorded. Repeatability of the response over thetime and under the application of mechanical stimuli

1911

indicates a high durability of this system. From the reported

plots it can be clearly observed first of all that the device

response is highly reproducible over the entire stress test

duration. Moreover, as it can be also noticed from Fig. 8b,

even after several hours of continuous solicitations, the

device electrical behavior remains unchanged.

III. CONCLUSION

We have demonstrated that OTFTs can be successfully

employed for the fabrication of mechanical sensors, giving

rise to a pronounced, reproducible, and linear (within a

certain range) response to the applied mechanical stimulus.

Moreover, matrices of 8×8 inkjet printed OTFTs have

been fabricated and tested as tactile transducers, showing a

reproducible, linear response for pressures up to 1 N, with a

resolution of 0.05 N. These results represent a step forward

for the fabrication, at low costs and over large areas, of

flexible and compliant electronics for artificial skin for

robots.

ACKNOWLEDGMENT

The research leading to these results has received funding

IURP� WKH� (XURSHDQ� &RPPXQLW\¶V� 6HYHQWK� )UDPHZRUN�

Programme (FP7/2007-2013) under grant agreement no.

231500 (ROBOSKIN). P. Cosseddu acknowledges Regione

Autonoma della Sardegna (RAS) for funding his research

activity under the POR Sardegna FSE 2007-2013,

L.R.7/2007 CRP Prot. No. 1399/207. S. Lai and A. Loi

acknowledge Regione Autonoma della Sardegna (RAS) for

funding their Ph.D. activity under the POR Sardegna FSE

2007-2013.

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