IEEE SENSORS JOURNAL 1 Piezoelectric Polymer Transducer...

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IEEE SENSORS JOURNAL 1

Piezoelectric Polymer Transducer Arraysfor Flexible Tactile Sensors

Lucia Seminara, Luigi Pinna, Maurizio Valle, Member, IEEE, Laura Basiricò, Alberto Loi, Piero Cosseddu,Annalisa Bonfiglio, Alberto Ascia, Maurizio Biso, Alberto Ansaldo, Davide Ricci, and Giorgio Metta

Abstract— In this paper, we propose a novel technological1

approach for the implementation of large-area flexible artificial2

skin based on arrays of piezoelectric polymer transducers.3

Polyvinylidene fluoride (PVDF) transducers are chosen for the4

high electromechanical transduction frequency bandwidth (up to5

1 kHz). A low-cost and scalable technique for extracting PVDF6

signals is used to directly provide the piezoelectric film with7

patterned electrodes. If the skin is meant to cover large areas8

of a robot body, specific requirements have to be fulfilled from9

the point of view of the overall system and of the technology.10

Experimental tests on the prototype skin modules demonstrate11

the feasibility of the proposed approach and reveal the potential-12

ity to build large area flexible skin.13

Index Terms— PVDF piezoelectric transducers, artificial skin,14

flexible sensors, ink jet printing, materials processing, robotic15

skin.16

I. INTRODUCTION17

AWIDE number of tactile sensors and systems have been18

proposed over the last two decades and more [1]–[5].19

To effectively use artificial skin on robots (and not only), it20

has been evidenced that the overall skin performance depends21

not only on the characteristics of individual elements but22

also on the system strategy to integrate the large number23

of components/arrays and to cover large areas of the robot24

body. Therefore, relevant technological issues must be faced25

based on a system perspective. One of the main issues in26

building skin systems is the deposition and patterning of27

metal electrodes on the sensing film. Inkjet printing has been28

chosen and it is proposed to deposit patterned metal layers29

on the sensing film. The appeal of this technology lies in30

Manuscript received April 27, 2013; accepted June 3, 2013. This work wassupported in part by the European project “ROBOSKIN” about Skin-BasedTechnologies and Capabilities for Safe, Autonomous and Interactive Robotsunder Grant 231500. The associate editor coordinating the review of this paperand approving it for publication was Dr. Ravinder S. Dahiya.

L. Seminara, L. Pinna, and M. Valle are with the Department ofTelecommunications, Electronic, Electric and Naval Engineering, Univer-sity of Genoa, Genoa 65000-65480, Italy (e-mail: [email protected];[email protected]; [email protected]).

L. Basiricò, A. Loi, P. Cosseddu, and A. Bonfiglio are with the Departmentof Electrical and Electronic Engineering, University of Cagliari, Cagliari09123, Italy (e-mail: [email protected]; [email protected];[email protected]; [email protected]).

A. Ascia, M. Biso, A. Ansaldo, D. Ricci, and G. Metta are with the Depart-ment of Robotics, Brain and Cognitive Sciences, Istituto Italiano di Tecnolo-gia, Genoa 16163, Italy (e-mail: [email protected]; [email protected];[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2268690

its scalability to large area manufacturing through a maskless 31

approach [6]. 32

If the aim is to mimic the behavior of the human skin, 33

different transducers have to be integrated, to cover the 34

0–1 kHz range of the stimulus frequencies required for 35

the application [7]. In this perspective, our research activity 36

focuses mainly on two different transducers, piezoelectric and 37

capacitive, studying methods and technologies enabling their 38

implementation on available robotic platforms. In this paper, 39

we address the study on piezoelectric polymer transducers 40

concentrating on the use of flexible films of polyvinylidene 41

fluoride (PVDF) to realize scalable, bendable and low-cost 42

sensing systems for large area skin [8]–[12]. 43

We choose PVDF as sensing material because in principle 44

it allows for the direct measurement of the mechanical stress, 45

decoupling tensile and shear stress components (according 46

to the way it is integrated in the artificial skin). In this 47

context, we focus on the T33 normal stress measurement 48

(through thickness direction) [13]. Other advantages of PVDF 49

are the large electro-mechanical transduction frequency band- 50

width (i.e. from few Hz up to 1kHz), the wide measurement 51

range, high elastic compliance/piezoelectricity/permittivity, 52

low weight and low cost [14], [15]. 53

The piezoelectric film has to be integrated in a flexi- 54

ble/conformable skin structure which requires tackling various 55

technological issues in order to make the process feasible, 56

controllable and scalable. In the perspective of system integra- 57

tion with capacitive transducers, triangular patches [16] have 58

been realized which can be organized in flexible structures 59

to adequately cover curved and large surfaces of a robot 60

body. However, the technology presented in this paper is not 61

constrained to the particular triangular substrate and patch 62

geometry. 63

To our knowledge, no paper addresses the exhaustive 64

electromechanical characterization of tactile sensing systems 65

based on piezoelectric PVDF polymers, also due to the diffi- 66

culty in making the whole manufacturing process controllable 67

and reproducible. Efforts have been made towards this direc- 68

tion and the results will be presented in this paper. 69

This paper is organized as follows. Section II introduces 70

the structure of the flexible piezoelectric transducer array, the 71

deposition technology of metal contacts on piezoelectric films 72

(which is one of the main contributions of this paper) and 73

the assembly technology. This section basically introduces 74

main technological solutions for developing reliable flexible 75

tactile sensor arrays based on PVDF piezoelectric polymers. 76

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2 IEEE SENSORS JOURNAL

Fig. 1. Artificial skin patch based on capacitive transducers: hexagonal patchmade up of six triangular modules.

Fig. 2. Right: the PCB substrate contains the lower PVDF electrodes (yellow)surrounded by a ground plane (pink), the PVDF film is provided with patternedelectrodes and the PDMS elastomer layer is polymerized on top. Left: sectionview of the robot skin triangular module.

Experimental results are introduced in Section III. Main goal77

of this section is to demonstrate that the proposed technology78

is feasible and measurements are repeatable. A mathematical79

model, which is used to evaluate the experimental results, is80

also introduced. Finally, section IV draws the conclusions and81

gives some hints on future developments.82

II. MATERIALS AND TECHNOLOGY83

A. Structure of the Flexible Piezoelectric Transducer Array84

The proposed artificial skin is a distributed system com-85

posed of spatially distributed tactile elements (i.e. taxels), orga-86

nized in surface compliant structures (patches) for covering87

large areas of a robot body [17]. Each patch is organized in88

a number of coupled triangular modules, comprising of a 2D89

tactile sensing array as well as of embedded and dedicated90

electronics (our reference system is reported in Fig. 1).91

In the current prototype, PVDF transducers have been92

integrated on the flexible PCB substrate and an elastomer93

coating has been directly polymerized on top of it to protect94

the sensors from being physically damaged by shock or95

chemically contaminated by oil and other materials (Fig. 2).96

The substrate is needed as a support and for the sensors signals97

extraction. It is provided with metal electrodes and tracks98

(back side) to extract the lower PVDF electrode signals to be99

connected to charge amplifiers for output charge reading [18].100

B. Technology of Deposition of Metal Contacts on101

Piezoelectric Films102

Different technologies for depositing and patterning con-103

ductive electrodes have been compared from the point of view104

(a)

(b)

(c)

Fig. 3. (a) Copper over Nickel metallization (MEAS_- Measurement Spe-cialties). (b) Silver ink screen printed metallization (MEAS_- MeasurementSpecialties). (c) Aluminum on Chrome metallization (Piezotech).

of conductive layer morphology, adhesion on the PVDF film, 105

robustness, low processing temperature, low cost and scala- 106

bility of the technology. We analysed commercially available 107

PVDF sampe foils. Samples from Measurement Specialties 108

Inc.1 (MEAS) and PiezoTech S.A.S2 were already provided 109

with continuous metal layers. The two MEAS samples con- 110

sisted in 110/122 μm thick PVDF poled foils provided with 111

either Cu/Ni (sputtered) or Ag ink (screen printed) electrodes. 112

PiezoTech solutions were based on 100 μm thick poled foils 113

provided with Cr/Al electrodes. 114

The morphology of the metal layers as provided by 115

the manufacturer has been characterized by Scanning Elec- 116

tron Microscopy (SEM). The Cu/Ni metallization by MEAS 117

presents cracks, defects and is very brittle due to the Ni- 118

Cu alloy that is formed at the bimetallic interface (Fig. 3a). 119

Results reported in Fig. 3b evidenced that the employed silver 120

1www.meas-spec.com/default.aspx

2www.piezotech.fr

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SEMINARA et al.: PIEZOELECTRIC POLYMER TRANSDUCER ARRAYS FOR FLEXIBLE TACTILE SENSORS 3

flakes present a very poor surface smoothness. Finally, in121

case of the Piezotech samples (Fig. 3c) the metal thin film122

is brittle and presents cracks. The same results have been123

confirmed in different tests, proving the reliability of the124

observations.125

Although metalized PVDF film foils are commercially avail-126

able, the brittleness of some of the available coatings, the high127

roughness of the silver ink solution and the very high cost128

of obtaining custom patterning of the conducting film, make129

these choices not suitable for scalable and low cost large area130

skin.131

Piezoelectric polymer foils without any metallization have132

been used and ad-hoc electrodes realized by employing133

dedicated deposition technologies.134

Different approaches have been considered and tested, con-135

sisting in the deposition and patterning of i) metal layers136

(gold/aluminium) by thermal vapour deposition; ii) gold layers137

by sputtering; iii) conductive polymers by inkjet printing.138

In the first two cases (deposition by thermal vapour deposi-139

tion and sputtering), the deposition takes place inside a High140

Vacuum Chamber (nominal pressure ranging from 5 × 10−5141

to 5 × 10−6 mbar). The evaporator system is provided with142

a Crystal Quartz Microbalance to control the deposition rate143

and the final film thickness. Gold and aluminium have been144

deposited. The final thickness of the deposited films ranges145

around 50–70 nm.146

Gold electrodes have also been deposited by employing a147

sputter coater, to achieve better adhesion of the deposited metal148

film on the PVDF surface.149

In both cases, the electrodes have been patterned by deposit-150

ing the metal through a shadow mask (1 cm2), the electrodes151

have been deposited on both sides of the PVDF film and152

aligned by an optical microscope.153

SEM images of the electrodes obtained with the above154

described deposition techniques are shown in Fig. 4.155

Among such deposition techniques, most uniform films are156

the sputtered ones. Evaporated gold films have a smaller grain157

size but the coverage is less uniform. However, all these films158

show a resistivity lower than 1 � per square and do appear159

adequate for the application.160

Among these three metal films, the most suitable material161

for the application would be the aluminium coating. However,162

the polymeric nature of the substrate excludes the use of stan-163

dard optical lithography and lift-off techniques for patterning,164

due to the need of compatible solvents such as acetone. A hard165

shadow masking should be needed with all the limitations166

that this implies especially regarding mask alignment during167

evaporation and metal coating shape design.168

We finally evaluated Ink Jet printing for electrode deposition169

and patterning . It allows patterning nearly any shape of metal170

contacts without the need of shadow masks or any other171

lithographic process.172

Commercial bare (no metallized) polarized 110 μm-thick173

PVDF sheets from Measurement Specialties Inc. have been174

purchased. An air plasma treatment (P = 75 W, p = 0.5 Torr,175

t = 60 s) has been used before the printing process to increase176

electrode adhesion to the polymer film, enhancing sensor177

robustness and reliability.178

(a)

(b)

(c)

Fig. 4. (a) Thermal evaporated gold thin film. (b) Thermal evaporated alu-minum thin film. (c) Sputtered gold thin film. Same scale as Fig. 3 (b) and (c).

Metal contacts have been patterned on both sides of PVDF 179

films by means of inkjet printing (Fujifilm Dimatix 2800). 180

For all the metal contacts we used Cabot Conductive Ink 300 181

(CCI-300), a metal ink (provided by Cabot Corporation) made 182

of silver nanoparticles in a liquid vehicle composed of ethanol 183

and ethylene glycol. All details about the printing process are 184

reported in [12]. 185

During printing, PVDF films were kept at 60 °C in order 186

to promote faster solvent evaporation. Two different patterns 187

were printed on the two sides of the substrate: a continuous 188

and homogeneous ink layer (common ground) is deposited 189

on top of the PVDF film. The second pattern was printed on 190

PVDF bottom side, once the first was dried, and consists in 191

12 circles with a diameter of 3 mm each (same geometry of 192

metal contacts on PCB), arranged in a triangular shape. Fig. 5 193

shows both layouts. 194

After deposition, samples have been annealed at 60 °C in 195

an oven for several hours. We observed that annealing at 196

temperatures above 60 °C leads to a huge deformation of the 197

polymer films. 198

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Fig. 5. Layout of the inkjet printed patterns: circular taxels (left) andcontinuous ground contact (right).

Fig. 6. Inkjet printed colloidal silver film.

SEM pictures after Ag inkjet printing the PVDF samples199

are reported in Fig. 6. Compared to thermal evaporated and200

sputtered films (Fig. 4), inkjet printed electrodes are the most201

uniform among all. The metal surface is shiny and SEM202

images demostrate the very small grain size of the silver203

particles. Combined with the material properties, the computer204

controlled patterning, the scalability and reproducibility of the205

deposition process made inkjet printed colloidal silver the best206

choice for the present application.207

Attention has been paid to test the possible influence of the208

electrode deposition process on the piezoelectric film electro-209

mechanical behaviour. Therefore, the frequency dependent d33210

piezoelectric coefficient [19] has been measured after plasma211

and annealing treatments, to check if some film degradation212

had occurred. Results are reported in Fig. 7 and show that the213

plasma treatment improved the PVDF piezoelectric behavior214

i.e. it increased the d33 value.215

C. Assembly Technology of the Tactile Sensing System216

The basic structure of skin prototypes based on PVDF arrays217

is reported in Fig. 2.218

As reported in Par. II.A, at this prototypal stage wires have219

to be soldered on the PCB back side, to connect PVDF output220

signals to interface electronics. Due to the central (back)221

position of the wire connector and to avoid thermal stress of222

the polymer at a higher temperature than 60 °C, the PCB223

substrate has first been wired. Glue has been used to reinforce224

the soldered wire structure.225

A preliminary procedure with 1 μm parylene deposition226

over PVDF contacts to enhance track robustness has been227

replaced by direct PDMS polymerization. Therefore, the228

PDMS layer is directly integrated on top of the PVDF film.229

A 2.5 mm elastomer thickness has been chosen as optimally230

meeting the application requirements. A Two-part silicone231

Sylgard 184 Silicone Elastomer (Dow Corning) that cures to232

a flexible elastomer (PDMS) has been employed. Sylgard 184233

Silicone Elastomer is supplied as two-part liquid component234

kits comprised of Part A/Part B to be mixed in a 10:1 ratio235

by weight or volume. A suitable quantity is prepared by236

(a)

(b)

Fig. 7. Frequency behavior of the d33 piezoelectric coefficient. (a) Beforeand (b) after plasma and annealing treatments for the PVDF film.

mixing the two components. Care should be taken to minimize 237

air entrapment. To reduce the liquid viscosity (enhancing its 238

capability to level off) a silicone fluid (47V5) has been added 239

at 5% to the liquid solution. The liquid is poured on the 240

PVDF substrate which was previously placed at the bottom 241

of a mold, kept horizontal by means of a level. An adhesion 242

promoter (Dow Corning 1200 OS Primer) has been previously 243

distributed on the PVDF surface to enhance Sylgard adhesion. 244

The mold is finally heat cured for 12 hours at 50 °C, not 245

to depass the PVDF Curie Temperature. Cure is evidenced 246

by a gradual increase in viscosity, followed by gelation and 247

conversion to a PDMS solid elastomer. 248

Patterned PVDF film triangles with elastomer on top were 249

finally glued on the PCB by the use of conductive Epoxy 250

(CW2400, all-spec industries), patterned contacts on the PVDF 251

bottom side corresponding to contacts on the PCB. The 252

optimal amount of conductive glue has been determined by 253

preliminary tests and the coverlay (Fig. 2) is used to avoid 254

short-circuits between taxels and to ensure that the piezoelec- 255

tric film is glued on a plane surface. In order to obtain a 256

constant pressure for the gluing process, 3 kg weight has been 257

placed on top of the PDMS layer. Soldered wires on the PCB 258

back side were kept in place passing through a hole on a 259

suitable support. 260

A completely assembled and ‘ready for testing’ proto- 261

type device is shown in Fig. 8 (a), (b). In Fig. 8 (c) the 262

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(a)

(b)

(c)

Fig. 8. (a), (b) Skin module prototype based on piezoelectric polymer arrays.(c) Triangular-shape skin module is adapted to a cylinder of diameter 3.3cmto show its bendability.

triangular-shape skin module is adapted to a cylinder (cur-263

vature radius = 1.65 cm) to show its bendability.264

III. EXPERIMENTAL RESULTS265

The goal of the experimental tests is to validate the proposed266

technology and to assess the performance of the tactile sensing267

system.268

A. Experimental Setup269

The mechanical chain of the experimental setup (Fig. 9) is270

basically constituted by a mechanical shaker (TV 50018, TIRA271

GmbH, Schwingtechnik Vibration Test Systems), the triangu-272

lar skin module (protective layer included) and a load cell273

(Model 208C01, PCB Piezotronics). The inductive mechanical274

actuator that moves the shaker is software controlled. The275

input parameters of the mechanical stimulus (range of fre-276

quencies, amplitude, DC offset) are set and the related signal277

is directly generated by the acquisition board and conveyed278

to the shaker via a conditioner. The charge developed by the279

PVDF film is measured by a charge amplifier [18].280

A 3 mm × 3 mm square indenter has been mounted on the281

shaker to stimulate one taxel at a time.282

Fig. 9. Mechanical setup to characterize the electromechanical behavior ofskin prototypes.

B. Modeling and Test of the Skin Structure 283

A mechanical model of the PVDF tactile sensor with a 284

protective layer on top has been used to test the feasibility and 285

reliability of the proposed system. As a matter of fact, a given 286

charge is expected as output of the transduction of the applied 287

mechanical stimulus, which is conveyed to the PVDF sensor 288

through the elastomer layer. If the measured charge value is in 289

accordance with the one predicted by the model, it means that 290

the mechanical chain of the measurement setup is working as 291

expected and the feasibility of the proposed technology for 292

skin manufacturing is demonstrated. 293

Approximately, the relation between a point load force 294

F applied on the outer surface and the stress at a given 295

point inside the cover layer is given by the Boussinesq’s 296

equation [20]: 297

T = 3

2π

F · er

r2 er ⊗ er (1) 298

where all bold faced symbols represent tensors (T is the stress 299

tensor) or vectors, er is the unit vector in the r -direction and 300

⊗ is the symbol of tensor product. Eq. (1) holds whenever the 301

Poisson ratio ν of the medium is close to 0.5 (this holds for 302

the present elastomer). A complete solution valid for all ν is 303

available [21] but it is considerably more complex. 304

Letting r̂ be the radial distance of the point where the force 305

is applied from the sensor center projected on the outer surface 306

(inset of Fig. 10), for a vertical force F3 (F1 = F2 = 0) 307

(1) allows to retrieve the normal T3 component on the PVDF 308

sensor3: 309

T3 = 3

2π

F3h3

(r̂2 + h2

)5/2. (2) 310

On the other hand, PVDF constitutive equations relate the 311

normal T3 component on the sensor to the output charge 312

density through the d33 piezoelectric modulus (sensor used in 313

thickness mode and electric field E3 across the PVDF sensor 314

assumed to be negligible4) [22] 315

D3 = d33T3. (3) 316

3Eq. (2) is derived from eq. (1) using r2 = r̂2+ h2 and er = sinθ(e1 cosφ+ e2 sinφ) – e3 cosθ , where cosθ = h/r, sinθ = r̂ / r.

4Because of the virtual ground at the operational amplifier inverting input.

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Fig. 10. Comparison between expected (model from Eq. (4)) and measuredcharge (two different taxels) at frequency of 16 Hz. Error bars related todeviation from the average value are too small to be visible in the plot (<5%).

TABLE I

COMMERCIALLY AVAILABLE METALIZED PVDF FILMS

Manufacturer Film Thickness Electrode Type

PiezoTech S.A.S 100µm Evaporated Cr/Al

Measurement Specialties Inc.

110µm Sputtered Cu/Ni

Screen printed Ag ink 122 µm

Using (2) and (3) and adding the hypotheses that the sensor317

size is sufficiently smaller than its distance from the point318

force, the total charge measured by the PVDF sensor can be319

directly related to the applied F3 load by:320

q = d333A

2π

F3h3

(r̂2 + h2

)5/2(4)321

where A is the area of the PVDF taxel.322

Concentrated sinusoidal force (at fixed frequency equal to323

16 Hz) has been applied on top of the elastomer layer, directly324

above taxels 9 and 12. Output charge has been measured and325

compared with theory. The data points reported in Fig. 10 are326

the average values of peak amplitudes (400 wave periods),

AQ:1

327

both for charge and force. The experimental results show very328

good accordance with what predicted by the model, confirming329

the feasibility of the employed skin manufacturing technology.330

C. Experimental Performance Tests331

Fig. 11a shows the response of a single taxel for four differ-332

ent stimulus frequencies belonging to the range of interest of333

the present application (f = 6, 62, 324, 824 Hz). As it can be334

seen, a linear behaviour is achieved over the whole explored335

range (1 Hz–1 kHz).336

On the other hand, by varying the amplitude of the applied337

stimulus at a fixed frequency (f = 6 Hz), the charge response338

of 5 different taxels belonging to three different triangles can339

be compared (Fig. 11b).340

(a)

(b)

Fig. 11. (a) Response linearity: same taxel, different stimulus frequencies.(b) System reproducibility: charge response of 5 different taxels (f = 6 Hz),placed on three different triangles (Samples 1, 2, 5). Taxel positions areindicated on the triangle on the right.

To better analyse the previous results, an identical triangular 341

prototype but based on a rigid substrate has been manu- 342

factured and tested. The scope is to verify the dependence 343

between the output charge and the thickness of the elastomer 344

layer. 345

In order to better compare the slopes of charge-to-force 346

curves corresponding to slightly different layer thicknesses, 347

thickness values on different taxels have been measured. An 348

average thickness value has been used to evaluate a d33 value 349

through (4), starting from measured charge q and force F3. 350

The dispersion of the ratio between this value and theoretical 351

d33 is directly compared to the dispersion in the values of the 352

point-by-point measured layer thickness. Results reported in 353

Fig. 12 show very good theoretical value reproducibility, as 354

a mean value. On the other hand, the taxel dispersion ouputs 355

results are due to the not uniform thickness of the elastomer 356

layer. Changes as small as 0.35 mm (corresponding to about 357

15% of the layer thickness, typical uncertainty at a prototype 358

stage) reflect in changes of the output charge as high as 50% 359

with respect to the mean value. Better precision in controlling 360

the elastomer layer thickness will be needed if better repro- 361

ducible systems will be targeted. This information becomes a 362

manufacturing process reference specs when moving from a 363

prototype stage to a production engineering step. 364

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Fig. 12. Taxel output charge compared with theory (black line) and withmeasured layer thickness variations (red crosses).

IV. CONCLUSION365

One of the main issues in building skin systems is the depo-366

sition and patterning of metal electrodes on the piezoelectric367

film. On the basis of a detailed experimental assessment, we368

realized that commercially available PVDF film foils are not369

suitable for scalable and low cost large area skin. On the other370

hand, the results that we provide in this paper demonstrate the371

feasibility of ink-jet printing technology. The evaluation has372

been made on account for metal layer morphology, adhesion373

on the PVDF film, robustness, low processing temperature,374

low cost and scalability of the deposition technology. Ink jet375

printing parameters and post-processing annealing temperature376

have been optimized. Moreover a plasma treatment has been377

used and tuned to increase electrode adhesion on the PVDF378

film.379

The tactile sensing system (i.e. robot skin) consists in the380

arrangement of one or more arrays of PVDF piezoelectric sen-381

sors fixed on a flexible substrate and coated with an elastomer382

layer. As evidenced in this paper, the proposed technology is383

not strictly constrained to a particular substrate (e.g. flexible384

PCB, etc.) and/or patch geometry (e.g. triangular shape) but it385

can be adapted to any shape and substrate material.386

The experimental methodology and setup have been pre-387

sented. We experimentally assessed the proposed technology388

and tested some integrated prototypes. A reference model389

based on continuum mechanics has been used to evaluate390

measurement results.391

Reported experimental results show that the proposed tech-392

nology is feasible and it is promising in terms of scalability,393

reproducibility, bendability and low cost. Manufacturing large394

area robotic skin becomes therefore a matter of following395

suitable engineering steps.396

Starting from the feasibility of the proposed technology,397

next step will be to develop a multimodal sensor system398

by integrating piezoelectric and capacitive transducers on the399

same substrate.400

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Proc. IEEE Sensors 2012, Oct., pp. 1–4. 437

[13] L. Seminara, M. Capurro, P. Cirillo, G. Cannata, and M. Valle, “Electro- 438

mechanical characterization of piezoelectric PVDF polymer films for 439

tactile sensors in robotics applications,” Sens. Actuators A, Phys., 440

vol. 169, pp. 49–58, 2011. 441

[14] H. S. Nalwa, Ferroelectric Polymers—Chemistry, Physics and Applica- 442

tions, New York, NY, USA: Marcel Dekker, 1995, pp. 203–214. 443

[15] T. T. Wang, J. M. Herbert, and A. M. Glass, The Applications of 444

Ferroelectric Polymers. Glasgow, U.K.: Blackie, 1988. 445

[16] M. Maggiali, G. Cannata, G. Metta, and G. Sandini, “Organization and 446

placement of tactile elements for a haptic sensory system,” U.S. Patent 447

0 128 764. AQ:3448

[17] A. Schmitz, P. Maiolino, M. Maggiali, L. Natale, G. Cannata, and 449

G. Metta, “Methods and technologies for the implementation of large- 450

scale robot tactile sensors,” IEEE Trans. Robot., vol. 27, no. 3, 451

pp. 389–400, Jun. 2011. 452

[18] L. Pinna, L. Seminara, and M. Valle, “Interface electronics for 453

robotic systems based on tactile sensor arrays,” in Proc. IEEE ICECS, 454

Dec. 2011, pp. 468–471. 455

[19] T. Ikeda, Fundamentals of Piezoelectricity. Oxford, U.K.: Oxford Sci- 456

ence, 1996. 457

[20] J. Boussinesq, Application des Potentielles à L’étude de L’équilibre et 458

du Mouvement des Solides Élastiques. Paris, France: Gauthier-Villars, 459

1885. 460

[21] A. P. S. Selvadurai, “On Boussinesq’s problem,” Int. J. Eng. Sci., vol. 39, 461

no. 3, pp. 317–322, 2001. 462

[22] R. S. Dahiya, M. Valle, G. Metta, L. Lorenzelli, and A. Adami, 463

“Piezoelectric oxide semiconductor field effect transistor touch sensing 464

devices,” Appl. Phys. Lett., vol. 95, p. 034105; Jul. 2009. 465

Lucia Seminara was born in London, U.K. in 1974. She received the M.S. 466

degree in Physics from the University of Genoa, Genoa, Italy, in 1999. In 467

2004, she received the Ph.D. degree in Physics from the École polytechnique 468

fédérale de Lausanne, Lausanne, Switzerland. From 2005 to 2009, she 469

worked in a telecommunications company, working on wireless networks 470

of nanostructured gas sensors. Since 2009, she has been a Post-Doctoral 471

Researcher with the Department of Electric, Electronic, Telecommunication 472

Engineering, and Naval Architecture (DITEN) of the University of Genoa, 473

Genoa, Italy. Her current research interests include the materials science 474

aspects related to the development of a robotic skin. 475

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Luigi Pinna was born in Cagliari, Italy, in 1978. He received the M.Sc. degree476

in electronic engineering from the University of Cagliari, Cagliari, Italy, in477

2005 and the Ph.D. degree in nanotechnologies from the University of Genoa,478

in 2010. Since 2010, he has been with the Department of Electric, Electronic,479

Telecommunication Engineering, and Naval Architecture (DITEN), University480

of Genoa. His current research interests include based-charge amplifiers circuit481

design for PVDF tactile sensor arrays, Organic Thin Film Transistors (OTFT)-482

based, and POSFET (Piezoelectric-Oxide-FET)-based tactile sensing arrays483

for humanoid robots.484

Maurizio Valle received the M.S. degree in Electronic Engineering in 1985485

and the Ph.D. degree in Electronic and Computer Science Engineering from486

the University of Genova, Italy, in 1990. From 1992 to 2006, he was an487

Assistant Professor. Since January 2007, he has been an Associate Professor488

in Electronic Engineering with the Department of Electrical, Electronic,489


of Genova. Dr. Valle is co-author of more than 180 articles on international491

scientific journals and conference proceedings. His current research interests492

include tactile sensors and systems, microelectronic embedded systems, and493

wireless sensor networks. He is member of IEEE.494

Laura Basiricò received the M.S. degree in Physics from the University495

of Bologna, Bologna, Italy, in 2008. In 2009, she joined the University496

of Cagliari with the Department of Electrical and Electronic Engineering497

as the Ph.D student. She received the Ph.D. degree in 2012. During the498

Ph.D. degree, she focused her research activity on the development of inkjet499

printing technique for the realization of Organic Field Effect Transistors500

and Organic ElectroChemical Transistors for the sensing application. She501

is currently a Researcher with ISMN-CNR, Bologna. Her current research502

interests include the field of organic electronic devices on the fabrication and503

physical characterization of Organic Field Effect Transistors for light emission504

and sensing applications.505

Alberto Loi received the Master Degree in Electronic Engineering from the506

University of Bologna, Italy, in 2010, discussing the thesis “Analysis of a507

30-V multifinger LDMOS power device.” In 2011, he joined the DEALAB508

group with the University of Cagliari, where he worked as the Ph.D. student.509

His current research interests include inkjet printing of organic materials,510

fabrication and characterization of Organic Field Effect Transistors, and511

fabrication and characterization of mechanical sensors and sensing systems512

based on organic devices.513

Piero Cosseddu graduated in Electronic Engineering, in 2003, and received514

the Ph.D. degree in Electronic and Computer Science Engineering in 2007515

from the University of Cagliari, Italy. Since May 2007, he has been a Post-516

Doctoral Fellow with the Department of Electrical and Electronic Engineering,517

University of Cagliari. His current research interests include the design,518

realization, and characterization of organic semiconductor-based devices for519

innovative applications such as artificial electronic skin, biomedical sensing,520

and wearable electronics.521

Annalisa Bonfiglio received the Laurea Degree in Physics from the University522

of Genoa, in 1991 and the Ph.D. degree in Bioengineering from the Politecnico523

di Milano, Milano, Italy, in 1995. She is currently an Associate Professor524

of Electronics and Electronic Bioengineering with the University of Cagliari,525

Italy. She is also a member of the Institute of Nanoscience- National Research526

Council. She is an author of more than 130 papers on international journals,527

conference proceedings, book chapters, and five patents. Her current research528

interests include innovative materials and electronic devices for wearable529

electronics and bioengineering and several international and national research530

Projects.531

Alberto Ascia received the master’s degree in Computer Science Engineering 532

and the Ph.D. degree in “Electronic Engineering, Automation and Control of 533

Complex Systems” from the University of Catania, Catania, Italy, in 2005 534

and 2009, respectively. During the Ph.D. degree, he focused on design and 535

characterization of transducers based on magnetic fluids. From January 2010 536

to July 2012, he has been a Post-Doctoral Researcher with the Istituto Italiano 537

di Tecnologia. Her current research interests include novel technologies for 538

the design and characterization of artificial skin for the iCub humanoid robot. 539

He is currently an Instrumentation Test Engineer with an aeronautic company. 540

Maurizio Biso received the M.Sc. degree in Industrial Chemistry and the 541

Ph.D. degree in Robotics, Neuroscience, and Nanotechnology. He is currently 542

a Postdoc with the Robotics, Brain, and Cognitive Sciences Department, 543

Istituto Italiano di Tecnologia, Genoa, Italy mainly working on composite 544

materials based on carbon nanotubes for robotics. His current research 545

interests include the development of “soft” materials for sensors and actu- 546

ators. He has a strong experience in electrochemistry and in synthesis and 547

characterization of polymeric materials. 548

Alberto Ansaldo received the M.Sc. degree in Electronic Engineering and 549

the Ph.D. degree in Communication Science and Technology, Genoa, Italy. 550

He is mainly working on Chemical Vapour Deposition of Carbon Nanotubes 551

for Electronic Applications. He is currently Research Technologist with the 552

Robotics Brain and Cognitive Science Department of the Italian Institute of 553

Technology, Genoa, Italy, in the Soft Materials Design Laboratory. His current 554

research interests include nanoscience, robotics, and bioengineering, with a 555

focus on the development of technologies based on novel materials such as 556

conductive polymers and carbon nanomaterials. 557

Davide Ricci received the M.Sc. degree in Physics and the Ph.D. degree 558

in Electronic Engineering and Computer Science from the University of 559

Genoa, Genoa, Italy, in 1989 and 1993, respectively. He is currently a Team 560

Leader with the Robotics Brain and Cognitive Science Department, Italian 561

Institute of Technology, Genoa, Italy. His current research interests include 562

the development of devices such as neural electrodes for Brain Machine 563

Interfaces, of flexible nano-actuators and sensors for robotics and of smart 564

interfaces for tissue engineering and prosthetics. He has co-authored more 565

than 100 papers on such topics. 566

Giorgio Metta is the Director of the iCub Facility department with the Istituto 567

Italiano di Tecnologia (IIT), Genoa, Italy, where he coordinates the iCub 568

project. He received the M.Sc. degree cum laude, in 1994, and the Ph.D. 569

degree, in 2000, in electronic engineering from the University of Genoa. 570

In 2001 and 2002, he has been a Post-Doctoral Associate with the MIT 571

AI-Lab. Since 2012, he has been the Professor of Cognitive Robotics with 572

the University of Plymouth, Plymouth, U.K. His current research interests 573

include the fields of biologically motivated and humanoid robotics and 574

developing robots that can adapt and learn from experience. He is an author 575

of approximately 200 scientific publications. 576

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AUTHOR QUERIES

AQ:1 = Table I is not cited in the body text. Please indicate where it should be cited.AQ:2 = Please provide the publisher name and location for ref. [7].AQ:3 = Please provide the date, month, and year for ref. [16].

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IEEE SENSORS JOURNAL 1

Piezoelectric Polymer Transducer Arraysfor Flexible Tactile Sensors

Lucia Seminara, Luigi Pinna, Maurizio Valle, Member, IEEE, Laura Basiricò, Alberto Loi, Piero Cosseddu,Annalisa Bonfiglio, Alberto Ascia, Maurizio Biso, Alberto Ansaldo, Davide Ricci, and Giorgio Metta

Abstract— In this paper, we propose a novel technological1

approach for the implementation of large-area flexible artificial2

skin based on arrays of piezoelectric polymer transducers.3

Polyvinylidene fluoride (PVDF) transducers are chosen for the4

high electromechanical transduction frequency bandwidth (up to5

1 kHz). A low-cost and scalable technique for extracting PVDF6

signals is used to directly provide the piezoelectric film with7

patterned electrodes. If the skin is meant to cover large areas8

of a robot body, specific requirements have to be fulfilled from9

the point of view of the overall system and of the technology.10

Experimental tests on the prototype skin modules demonstrate11

the feasibility of the proposed approach and reveal the potential-12

ity to build large area flexible skin.13

Index Terms— PVDF piezoelectric transducers, artificial skin,14

flexible sensors, ink jet printing, materials processing, robotic15

skin.16

I. INTRODUCTION17

AWIDE number of tactile sensors and systems have been18

proposed over the last two decades and more [1]–[5].19

To effectively use artificial skin on robots (and not only), it20

has been evidenced that the overall skin performance depends21

not only on the characteristics of individual elements but22

also on the system strategy to integrate the large number23

of components/arrays and to cover large areas of the robot24

body. Therefore, relevant technological issues must be faced25

based on a system perspective. One of the main issues in26

building skin systems is the deposition and patterning of27

metal electrodes on the sensing film. Inkjet printing has been28

chosen and it is proposed to deposit patterned metal layers29

on the sensing film. The appeal of this technology lies in30

Manuscript received April 27, 2013; accepted June 3, 2013. This work wassupported in part by the European project “ROBOSKIN” about Skin-BasedTechnologies and Capabilities for Safe, Autonomous and Interactive Robotsunder Grant 231500. The associate editor coordinating the review of this paperand approving it for publication was Dr. Ravinder S. Dahiya.

L. Seminara, L. Pinna, and M. Valle are with the Department ofTelecommunications, Electronic, Electric and Naval Engineering, Univer-sity of Genoa, Genoa 65000-65480, Italy (e-mail: [email protected];[email protected]; [email protected]).

L. Basiricò, A. Loi, P. Cosseddu, and A. Bonfiglio are with the Departmentof Electrical and Electronic Engineering, University of Cagliari, Cagliari09123, Italy (e-mail: [email protected]; [email protected];[email protected]; [email protected]).

A. Ascia, M. Biso, A. Ansaldo, D. Ricci, and G. Metta are with the Depart-ment of Robotics, Brain and Cognitive Sciences, Istituto Italiano di Tecnolo-gia, Genoa 16163, Italy (e-mail: [email protected]; [email protected];[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2268690

its scalability to large area manufacturing through a maskless 31

approach [6]. 32

If the aim is to mimic the behavior of the human skin, 33

different transducers have to be integrated, to cover the 34

0–1 kHz range of the stimulus frequencies required for 35

the application [7]. In this perspective, our research activity 36

focuses mainly on two different transducers, piezoelectric and 37

capacitive, studying methods and technologies enabling their 38

implementation on available robotic platforms. In this paper, 39

we address the study on piezoelectric polymer transducers 40

concentrating on the use of flexible films of polyvinylidene 41

fluoride (PVDF) to realize scalable, bendable and low-cost 42

sensing systems for large area skin [8]–[12]. 43

We choose PVDF as sensing material because in principle 44

it allows for the direct measurement of the mechanical stress, 45

decoupling tensile and shear stress components (according 46

to the way it is integrated in the artificial skin). In this 47

context, we focus on the T33 normal stress measurement 48

(through thickness direction) [13]. Other advantages of PVDF 49

are the large electro-mechanical transduction frequency band- 50

width (i.e. from few Hz up to 1kHz), the wide measurement 51

range, high elastic compliance/piezoelectricity/permittivity, 52

low weight and low cost [14], [15]. 53

The piezoelectric film has to be integrated in a flexi- 54

ble/conformable skin structure which requires tackling various 55

technological issues in order to make the process feasible, 56

controllable and scalable. In the perspective of system integra- 57

tion with capacitive transducers, triangular patches [16] have 58

been realized which can be organized in flexible structures 59

to adequately cover curved and large surfaces of a robot 60

body. However, the technology presented in this paper is not 61

constrained to the particular triangular substrate and patch 62

geometry. 63

To our knowledge, no paper addresses the exhaustive 64

electromechanical characterization of tactile sensing systems 65

based on piezoelectric PVDF polymers, also due to the diffi- 66

culty in making the whole manufacturing process controllable 67

and reproducible. Efforts have been made towards this direc- 68

tion and the results will be presented in this paper. 69

This paper is organized as follows. Section II introduces 70

the structure of the flexible piezoelectric transducer array, the 71

deposition technology of metal contacts on piezoelectric films 72

(which is one of the main contributions of this paper) and 73

the assembly technology. This section basically introduces 74

main technological solutions for developing reliable flexible 75

tactile sensor arrays based on PVDF piezoelectric polymers. 76

1530-437X/$31.00 © 2013 IEEE

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Fig. 1. Artificial skin patch based on capacitive transducers: hexagonal patchmade up of six triangular modules.

Fig. 2. Right: the PCB substrate contains the lower PVDF electrodes (yellow)surrounded by a ground plane (pink), the PVDF film is provided with patternedelectrodes and the PDMS elastomer layer is polymerized on top. Left: sectionview of the robot skin triangular module.

Experimental results are introduced in Section III. Main goal77

of this section is to demonstrate that the proposed technology78

is feasible and measurements are repeatable. A mathematical79

model, which is used to evaluate the experimental results, is80

also introduced. Finally, section IV draws the conclusions and81

gives some hints on future developments.82

II. MATERIALS AND TECHNOLOGY83

A. Structure of the Flexible Piezoelectric Transducer Array84

The proposed artificial skin is a distributed system com-85

posed of spatially distributed tactile elements (i.e. taxels), orga-86

nized in surface compliant structures (patches) for covering87

large areas of a robot body [17]. Each patch is organized in88

a number of coupled triangular modules, comprising of a 2D89

tactile sensing array as well as of embedded and dedicated90

electronics (our reference system is reported in Fig. 1).91

In the current prototype, PVDF transducers have been92

integrated on the flexible PCB substrate and an elastomer93

coating has been directly polymerized on top of it to protect94

the sensors from being physically damaged by shock or95

chemically contaminated by oil and other materials (Fig. 2).96

The substrate is needed as a support and for the sensors signals97

extraction. It is provided with metal electrodes and tracks98

(back side) to extract the lower PVDF electrode signals to be99

connected to charge amplifiers for output charge reading [18].100

B. Technology of Deposition of Metal Contacts on101

Piezoelectric Films102

Different technologies for depositing and patterning con-103

ductive electrodes have been compared from the point of view104

(a)

(b)

(c)

Fig. 3. (a) Copper over Nickel metallization (MEAS_- Measurement Spe-cialties). (b) Silver ink screen printed metallization (MEAS_- MeasurementSpecialties). (c) Aluminum on Chrome metallization (Piezotech).

of conductive layer morphology, adhesion on the PVDF film, 105

robustness, low processing temperature, low cost and scala- 106

bility of the technology. We analysed commercially available 107

PVDF sampe foils. Samples from Measurement Specialties 108

Inc.1 (MEAS) and PiezoTech S.A.S2 were already provided 109

with continuous metal layers. The two MEAS samples con- 110

sisted in 110/122 μm thick PVDF poled foils provided with 111

either Cu/Ni (sputtered) or Ag ink (screen printed) electrodes. 112

PiezoTech solutions were based on 100 μm thick poled foils 113

provided with Cr/Al electrodes. 114

The morphology of the metal layers as provided by 115

the manufacturer has been characterized by Scanning Elec- 116

tron Microscopy (SEM). The Cu/Ni metallization by MEAS 117

presents cracks, defects and is very brittle due to the Ni- 118

Cu alloy that is formed at the bimetallic interface (Fig. 3a). 119

Results reported in Fig. 3b evidenced that the employed silver 120

1www.meas-spec.com/default.aspx

2www.piezotech.fr

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flakes present a very poor surface smoothness. Finally, in121

case of the Piezotech samples (Fig. 3c) the metal thin film122

is brittle and presents cracks. The same results have been123

confirmed in different tests, proving the reliability of the124

observations.125

Although metalized PVDF film foils are commercially avail-126

able, the brittleness of some of the available coatings, the high127

roughness of the silver ink solution and the very high cost128

of obtaining custom patterning of the conducting film, make129

these choices not suitable for scalable and low cost large area130

skin.131

Piezoelectric polymer foils without any metallization have132

been used and ad-hoc electrodes realized by employing133

dedicated deposition technologies.134

Different approaches have been considered and tested, con-135

sisting in the deposition and patterning of i) metal layers136

(gold/aluminium) by thermal vapour deposition; ii) gold layers137

by sputtering; iii) conductive polymers by inkjet printing.138

In the first two cases (deposition by thermal vapour deposi-139

tion and sputtering), the deposition takes place inside a High140

Vacuum Chamber (nominal pressure ranging from 5 × 10−5141

to 5 × 10−6 mbar). The evaporator system is provided with142

a Crystal Quartz Microbalance to control the deposition rate143

and the final film thickness. Gold and aluminium have been144

deposited. The final thickness of the deposited films ranges145

around 50–70 nm.146

Gold electrodes have also been deposited by employing a147

sputter coater, to achieve better adhesion of the deposited metal148

film on the PVDF surface.149

In both cases, the electrodes have been patterned by deposit-150

ing the metal through a shadow mask (1 cm2), the electrodes151

have been deposited on both sides of the PVDF film and152

aligned by an optical microscope.153

SEM images of the electrodes obtained with the above154

described deposition techniques are shown in Fig. 4.155

Among such deposition techniques, most uniform films are156

the sputtered ones. Evaporated gold films have a smaller grain157

size but the coverage is less uniform. However, all these films158

show a resistivity lower than 1 � per square and do appear159

adequate for the application.160

Among these three metal films, the most suitable material161

for the application would be the aluminium coating. However,162

the polymeric nature of the substrate excludes the use of stan-163

dard optical lithography and lift-off techniques for patterning,164

due to the need of compatible solvents such as acetone. A hard165

shadow masking should be needed with all the limitations166

that this implies especially regarding mask alignment during167

evaporation and metal coating shape design.168

We finally evaluated Ink Jet printing for electrode deposition169

and patterning . It allows patterning nearly any shape of metal170

contacts without the need of shadow masks or any other171

lithographic process.172

Commercial bare (no metallized) polarized 110 μm-thick173

PVDF sheets from Measurement Specialties Inc. have been174

purchased. An air plasma treatment (P = 75 W, p = 0.5 Torr,175

t = 60 s) has been used before the printing process to increase176

electrode adhesion to the polymer film, enhancing sensor177

robustness and reliability.178

(a)

(b)

(c)

Fig. 4. (a) Thermal evaporated gold thin film. (b) Thermal evaporated alu-minum thin film. (c) Sputtered gold thin film. Same scale as Fig. 3 (b) and (c).

Metal contacts have been patterned on both sides of PVDF 179

films by means of inkjet printing (Fujifilm Dimatix 2800). 180

For all the metal contacts we used Cabot Conductive Ink 300 181

(CCI-300), a metal ink (provided by Cabot Corporation) made 182

of silver nanoparticles in a liquid vehicle composed of ethanol 183

and ethylene glycol. All details about the printing process are 184

reported in [12]. 185

During printing, PVDF films were kept at 60 °C in order 186

to promote faster solvent evaporation. Two different patterns 187

were printed on the two sides of the substrate: a continuous 188

and homogeneous ink layer (common ground) is deposited 189

on top of the PVDF film. The second pattern was printed on 190

PVDF bottom side, once the first was dried, and consists in 191

12 circles with a diameter of 3 mm each (same geometry of 192

metal contacts on PCB), arranged in a triangular shape. Fig. 5 193

shows both layouts. 194

After deposition, samples have been annealed at 60 °C in 195

an oven for several hours. We observed that annealing at 196

temperatures above 60 °C leads to a huge deformation of the 197

polymer films. 198

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Fig. 5. Layout of the inkjet printed patterns: circular taxels (left) andcontinuous ground contact (right).

Fig. 6. Inkjet printed colloidal silver film.

SEM pictures after Ag inkjet printing the PVDF samples199

are reported in Fig. 6. Compared to thermal evaporated and200

sputtered films (Fig. 4), inkjet printed electrodes are the most201

uniform among all. The metal surface is shiny and SEM202

images demostrate the very small grain size of the silver203

particles. Combined with the material properties, the computer204

controlled patterning, the scalability and reproducibility of the205

deposition process made inkjet printed colloidal silver the best206

choice for the present application.207

Attention has been paid to test the possible influence of the208

electrode deposition process on the piezoelectric film electro-209

mechanical behaviour. Therefore, the frequency dependent d33210

piezoelectric coefficient [19] has been measured after plasma211

and annealing treatments, to check if some film degradation212

had occurred. Results are reported in Fig. 7 and show that the213

plasma treatment improved the PVDF piezoelectric behavior214

i.e. it increased the d33 value.215

C. Assembly Technology of the Tactile Sensing System216

The basic structure of skin prototypes based on PVDF arrays217

is reported in Fig. 2.218

As reported in Par. II.A, at this prototypal stage wires have219

to be soldered on the PCB back side, to connect PVDF output220

signals to interface electronics. Due to the central (back)221

position of the wire connector and to avoid thermal stress of222

the polymer at a higher temperature than 60 °C, the PCB223

substrate has first been wired. Glue has been used to reinforce224

the soldered wire structure.225

A preliminary procedure with 1 μm parylene deposition226

over PVDF contacts to enhance track robustness has been227

replaced by direct PDMS polymerization. Therefore, the228

PDMS layer is directly integrated on top of the PVDF film.229

A 2.5 mm elastomer thickness has been chosen as optimally230

meeting the application requirements. A Two-part silicone231

Sylgard 184 Silicone Elastomer (Dow Corning) that cures to232

a flexible elastomer (PDMS) has been employed. Sylgard 184233

Silicone Elastomer is supplied as two-part liquid component234

kits comprised of Part A/Part B to be mixed in a 10:1 ratio235

by weight or volume. A suitable quantity is prepared by236

(a)

(b)

Fig. 7. Frequency behavior of the d33 piezoelectric coefficient. (a) Beforeand (b) after plasma and annealing treatments for the PVDF film.

mixing the two components. Care should be taken to minimize 237

air entrapment. To reduce the liquid viscosity (enhancing its 238

capability to level off) a silicone fluid (47V5) has been added 239

at 5% to the liquid solution. The liquid is poured on the 240

PVDF substrate which was previously placed at the bottom 241

of a mold, kept horizontal by means of a level. An adhesion 242

promoter (Dow Corning 1200 OS Primer) has been previously 243

distributed on the PVDF surface to enhance Sylgard adhesion. 244

The mold is finally heat cured for 12 hours at 50 °C, not 245

to depass the PVDF Curie Temperature. Cure is evidenced 246

by a gradual increase in viscosity, followed by gelation and 247

conversion to a PDMS solid elastomer. 248

Patterned PVDF film triangles with elastomer on top were 249

finally glued on the PCB by the use of conductive Epoxy 250

(CW2400, all-spec industries), patterned contacts on the PVDF 251

bottom side corresponding to contacts on the PCB. The 252

optimal amount of conductive glue has been determined by 253

preliminary tests and the coverlay (Fig. 2) is used to avoid 254

short-circuits between taxels and to ensure that the piezoelec- 255

tric film is glued on a plane surface. In order to obtain a 256

constant pressure for the gluing process, 3 kg weight has been 257

placed on top of the PDMS layer. Soldered wires on the PCB 258

back side were kept in place passing through a hole on a 259

suitable support. 260

A completely assembled and ‘ready for testing’ proto- 261

type device is shown in Fig. 8 (a), (b). In Fig. 8 (c) the 262

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(a)

(b)

(c)

Fig. 8. (a), (b) Skin module prototype based on piezoelectric polymer arrays.(c) Triangular-shape skin module is adapted to a cylinder of diameter 3.3cmto show its bendability.

triangular-shape skin module is adapted to a cylinder (cur-263

vature radius = 1.65 cm) to show its bendability.264

III. EXPERIMENTAL RESULTS265

The goal of the experimental tests is to validate the proposed266

technology and to assess the performance of the tactile sensing267

system.268

A. Experimental Setup269

The mechanical chain of the experimental setup (Fig. 9) is270

basically constituted by a mechanical shaker (TV 50018, TIRA271

GmbH, Schwingtechnik Vibration Test Systems), the triangu-272

lar skin module (protective layer included) and a load cell273

(Model 208C01, PCB Piezotronics). The inductive mechanical274

actuator that moves the shaker is software controlled. The275

input parameters of the mechanical stimulus (range of fre-276

quencies, amplitude, DC offset) are set and the related signal277

is directly generated by the acquisition board and conveyed278

to the shaker via a conditioner. The charge developed by the279

PVDF film is measured by a charge amplifier [18].280

A 3 mm × 3 mm square indenter has been mounted on the281

shaker to stimulate one taxel at a time.282

Fig. 9. Mechanical setup to characterize the electromechanical behavior ofskin prototypes.

B. Modeling and Test of the Skin Structure 283

A mechanical model of the PVDF tactile sensor with a 284

protective layer on top has been used to test the feasibility and 285

reliability of the proposed system. As a matter of fact, a given 286

charge is expected as output of the transduction of the applied 287

mechanical stimulus, which is conveyed to the PVDF sensor 288

through the elastomer layer. If the measured charge value is in 289

accordance with the one predicted by the model, it means that 290

the mechanical chain of the measurement setup is working as 291

expected and the feasibility of the proposed technology for 292

skin manufacturing is demonstrated. 293

Approximately, the relation between a point load force 294

F applied on the outer surface and the stress at a given 295

point inside the cover layer is given by the Boussinesq’s 296

equation [20]: 297

T = 3

2π

F · er

r2 er ⊗ er (1) 298

where all bold faced symbols represent tensors (T is the stress 299

tensor) or vectors, er is the unit vector in the r -direction and 300

⊗ is the symbol of tensor product. Eq. (1) holds whenever the 301

Poisson ratio ν of the medium is close to 0.5 (this holds for 302

the present elastomer). A complete solution valid for all ν is 303

available [21] but it is considerably more complex. 304

Letting r̂ be the radial distance of the point where the force 305

is applied from the sensor center projected on the outer surface 306

(inset of Fig. 10), for a vertical force F3 (F1 = F2 = 0) 307

(1) allows to retrieve the normal T3 component on the PVDF 308

sensor3: 309

T3 = 3

2π

F3h3

(r̂2 + h2

)5/2. (2) 310

On the other hand, PVDF constitutive equations relate the 311

normal T3 component on the sensor to the output charge 312

density through the d33 piezoelectric modulus (sensor used in 313

thickness mode and electric field E3 across the PVDF sensor 314

assumed to be negligible4) [22] 315

D3 = d33T3. (3) 316

3Eq. (2) is derived from eq. (1) using r2 = r̂2+ h2 and er = sinθ(e1 cosφ+ e2 sinφ) – e3 cosθ , where cosθ = h/r, sinθ = r̂ / r.

4Because of the virtual ground at the operational amplifier inverting input.

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Fig. 10. Comparison between expected (model from Eq. (4)) and measuredcharge (two different taxels) at frequency of 16 Hz. Error bars related todeviation from the average value are too small to be visible in the plot (<5%).

TABLE I

COMMERCIALLY AVAILABLE METALIZED PVDF FILMS

Manufacturer Film Thickness Electrode Type

PiezoTech S.A.S 100µm Evaporated Cr/Al

Measurement Specialties Inc.

110µm Sputtered Cu/Ni

Screen printed Ag ink 122 µm

Using (2) and (3) and adding the hypotheses that the sensor317

size is sufficiently smaller than its distance from the point318

force, the total charge measured by the PVDF sensor can be319

directly related to the applied F3 load by:320

q = d333A

2π

F3h3

(r̂2 + h2

)5/2(4)321

where A is the area of the PVDF taxel.322

Concentrated sinusoidal force (at fixed frequency equal to323

16 Hz) has been applied on top of the elastomer layer, directly324

above taxels 9 and 12. Output charge has been measured and325

compared with theory. The data points reported in Fig. 10 are326

the average values of peak amplitudes (400 wave periods),

AQ:1

327

both for charge and force. The experimental results show very328

good accordance with what predicted by the model, confirming329

the feasibility of the employed skin manufacturing technology.330

C. Experimental Performance Tests331

Fig. 11a shows the response of a single taxel for four differ-332

ent stimulus frequencies belonging to the range of interest of333

the present application (f = 6, 62, 324, 824 Hz). As it can be334

seen, a linear behaviour is achieved over the whole explored335

range (1 Hz–1 kHz).336

On the other hand, by varying the amplitude of the applied337

stimulus at a fixed frequency (f = 6 Hz), the charge response338

of 5 different taxels belonging to three different triangles can339

be compared (Fig. 11b).340

(a)

(b)

Fig. 11. (a) Response linearity: same taxel, different stimulus frequencies.(b) System reproducibility: charge response of 5 different taxels (f = 6 Hz),placed on three different triangles (Samples 1, 2, 5). Taxel positions areindicated on the triangle on the right.

To better analyse the previous results, an identical triangular 341

prototype but based on a rigid substrate has been manu- 342

factured and tested. The scope is to verify the dependence 343

between the output charge and the thickness of the elastomer 344

layer. 345

In order to better compare the slopes of charge-to-force 346

curves corresponding to slightly different layer thicknesses, 347

thickness values on different taxels have been measured. An 348

average thickness value has been used to evaluate a d33 value 349

through (4), starting from measured charge q and force F3. 350

The dispersion of the ratio between this value and theoretical 351

d33 is directly compared to the dispersion in the values of the 352

point-by-point measured layer thickness. Results reported in 353

Fig. 12 show very good theoretical value reproducibility, as 354

a mean value. On the other hand, the taxel dispersion ouputs 355

results are due to the not uniform thickness of the elastomer 356

layer. Changes as small as 0.35 mm (corresponding to about 357

15% of the layer thickness, typical uncertainty at a prototype 358

stage) reflect in changes of the output charge as high as 50% 359

with respect to the mean value. Better precision in controlling 360

the elastomer layer thickness will be needed if better repro- 361

ducible systems will be targeted. This information becomes a 362

manufacturing process reference specs when moving from a 363

prototype stage to a production engineering step. 364

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Fig. 12. Taxel output charge compared with theory (black line) and withmeasured layer thickness variations (red crosses).

IV. CONCLUSION365

One of the main issues in building skin systems is the depo-366

sition and patterning of metal electrodes on the piezoelectric367

film. On the basis of a detailed experimental assessment, we368

realized that commercially available PVDF film foils are not369

suitable for scalable and low cost large area skin. On the other370

hand, the results that we provide in this paper demonstrate the371

feasibility of ink-jet printing technology. The evaluation has372

been made on account for metal layer morphology, adhesion373

on the PVDF film, robustness, low processing temperature,374

low cost and scalability of the deposition technology. Ink jet375

printing parameters and post-processing annealing temperature376

have been optimized. Moreover a plasma treatment has been377

used and tuned to increase electrode adhesion on the PVDF378

film.379

The tactile sensing system (i.e. robot skin) consists in the380

arrangement of one or more arrays of PVDF piezoelectric sen-381

sors fixed on a flexible substrate and coated with an elastomer382

layer. As evidenced in this paper, the proposed technology is383

not strictly constrained to a particular substrate (e.g. flexible384

PCB, etc.) and/or patch geometry (e.g. triangular shape) but it385

can be adapted to any shape and substrate material.386

The experimental methodology and setup have been pre-387

sented. We experimentally assessed the proposed technology388

and tested some integrated prototypes. A reference model389

based on continuum mechanics has been used to evaluate390

measurement results.391

Reported experimental results show that the proposed tech-392

nology is feasible and it is promising in terms of scalability,393

reproducibility, bendability and low cost. Manufacturing large394

area robotic skin becomes therefore a matter of following395

suitable engineering steps.396

Starting from the feasibility of the proposed technology,397

next step will be to develop a multimodal sensor system398

by integrating piezoelectric and capacitive transducers on the399

same substrate.400

REFERENCES401

[1] M. H. Lee and H. R. Nicholls, “Tactile sensing for mechatronics—402

A state of the art survey,” Mechatronics, vol. 9, no. 1, pp. 1–31, 1999.403

[2] M. R. Cutkosky, R. D. Howe, and W. Provancher, “Force and tactile404

sensors,” in Handbook of Robotics, B. Siciliano and O. Khatib Eds.405

Berlin, Germany: Springer-Verlag, 2008, pp. 455–476.406

[3] R. S. Dahiya, G. Metta, M. Valle, and G. Sandini, “Tactile sensing 407

—From humans to humanoids,” IEEE Trans. Robot., vol. 26, no. 1, 408

pp. 1–20, Feb. 2010. 409

[4] Y. Ohmura, Y. Kuniyoshi, and A. Nagakubo, “Conformable and scalable 410

tactile sensor skin for curved surfaces,” in Proc. IEEE Int. Conf. Robot. 411

Autom., May 2006, pp. 1348–1353. 412

[5] T. Tajika, T. Miyashita, H. Ishiguro, and H. Norihiro, “Automatic catego- 413

rization of haptic interactions—What are the typical haptic interactions 414

between a human and a robot?” in Proc. IEEE-RAS Int. Conf. Humanoid 415

Robots 2006, Dec., pp. 490–496. 416

[6] M. Singh, H. M. Haverinen, P. Dhagat, and G. E. Jabbour, “Inkjet 417

printing—Process and its applications,” Adv. Mater., vol. 22, no. 6, 418

pp. 673–685, 2010. 419

[7] S. J. Lederman, Tactual Perception, W. Schiff, Ed. 1982, pp. 130–167. AQ:2420

[8] D. S. Tawil, D. Rye, and M. Velonaki, “Improved image reconstruction 421

for an EIT-based sensitive skin with multiple internal electrodes,” IEEE 422

Trans. Robot., vol. 27, no. 3, pp. 425–435, Jun. 2011. 423

[9] H.-K. Lee, S.-I. Chang, and E. Yoon, “A flexible polymer tactile sensor: 424

Fabrication and modular expandability for large area deployment,” 425

J. Microelectromech. Syst., vol. 15, no. 6, pp. 1681–1686, Dec. 2006. 426

[10] H. Chigusa, Y. Makino, and H. Shinoda, “Large area tactile sensor 427

based on proximity connection of tactile sensing elements,” in Proc. 428

23rd Sensor Symp., 2006, pp. 437–442. 429

[11] F. Vidal-Verdú, M. J. Barquero, J. Castellanos-Ramos, R. Navas- 430

González, J. A. Sánchez, J. Serón, and A. García-Cerezo, “A large area 431

tactile sensor patch based on commercial force sensors,” Sensors, vol. 11, 432

no. 5, pp. 5489–5507, 2011. 433

[12] L. Seminara, L. Pinna, M. Valle, L. Basiricò, A. Loi, P. Cosseddu, 434

A. Bonfiglio, A. Ascia, M. Bisio, A. Ansaldo, D. Ricci, and G. Metta, 435

“Piezoelectric polymer transducer arrays for flexible tactile sensors,” in 436

Proc. IEEE Sensors 2012, Oct., pp. 1–4. 437

[13] L. Seminara, M. Capurro, P. Cirillo, G. Cannata, and M. Valle, “Electro- 438

mechanical characterization of piezoelectric PVDF polymer films for 439

tactile sensors in robotics applications,” Sens. Actuators A, Phys., 440

vol. 169, pp. 49–58, 2011. 441

[14] H. S. Nalwa, Ferroelectric Polymers—Chemistry, Physics and Applica- 442

tions, New York, NY, USA: Marcel Dekker, 1995, pp. 203–214. 443

[15] T. T. Wang, J. M. Herbert, and A. M. Glass, The Applications of 444

Ferroelectric Polymers. Glasgow, U.K.: Blackie, 1988. 445

[16] M. Maggiali, G. Cannata, G. Metta, and G. Sandini, “Organization and 446

placement of tactile elements for a haptic sensory system,” U.S. Patent 447

0 128 764. AQ:3448

[17] A. Schmitz, P. Maiolino, M. Maggiali, L. Natale, G. Cannata, and 449

G. Metta, “Methods and technologies for the implementation of large- 450

scale robot tactile sensors,” IEEE Trans. Robot., vol. 27, no. 3, 451

pp. 389–400, Jun. 2011. 452

[18] L. Pinna, L. Seminara, and M. Valle, “Interface electronics for 453

robotic systems based on tactile sensor arrays,” in Proc. IEEE ICECS, 454

Dec. 2011, pp. 468–471. 455

[19] T. Ikeda, Fundamentals of Piezoelectricity. Oxford, U.K.: Oxford Sci- 456

ence, 1996. 457

[20] J. Boussinesq, Application des Potentielles à L’étude de L’équilibre et 458

du Mouvement des Solides Élastiques. Paris, France: Gauthier-Villars, 459

1885. 460

[21] A. P. S. Selvadurai, “On Boussinesq’s problem,” Int. J. Eng. Sci., vol. 39, 461

no. 3, pp. 317–322, 2001. 462

[22] R. S. Dahiya, M. Valle, G. Metta, L. Lorenzelli, and A. Adami, 463

“Piezoelectric oxide semiconductor field effect transistor touch sensing 464

devices,” Appl. Phys. Lett., vol. 95, p. 034105; Jul. 2009. 465

Lucia Seminara was born in London, U.K. in 1974. She received the M.S. 466

degree in Physics from the University of Genoa, Genoa, Italy, in 1999. In 467

2004, she received the Ph.D. degree in Physics from the École polytechnique 468

fédérale de Lausanne, Lausanne, Switzerland. From 2005 to 2009, she 469

worked in a telecommunications company, working on wireless networks 470

of nanostructured gas sensors. Since 2009, she has been a Post-Doctoral 471

Researcher with the Department of Electric, Electronic, Telecommunication 472

Engineering, and Naval Architecture (DITEN) of the University of Genoa, 473

Genoa, Italy. Her current research interests include the materials science 474

aspects related to the development of a robotic skin. 475

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Luigi Pinna was born in Cagliari, Italy, in 1978. He received the M.Sc. degree476

in electronic engineering from the University of Cagliari, Cagliari, Italy, in477

2005 and the Ph.D. degree in nanotechnologies from the University of Genoa,478

in 2010. Since 2010, he has been with the Department of Electric, Electronic,479


of Genoa. His current research interests include based-charge amplifiers circuit481

design for PVDF tactile sensor arrays, Organic Thin Film Transistors (OTFT)-482

based, and POSFET (Piezoelectric-Oxide-FET)-based tactile sensing arrays483

for humanoid robots.484

Maurizio Valle received the M.S. degree in Electronic Engineering in 1985485

and the Ph.D. degree in Electronic and Computer Science Engineering from486

the University of Genova, Italy, in 1990. From 1992 to 2006, he was an487

Assistant Professor. Since January 2007, he has been an Associate Professor488

in Electronic Engineering with the Department of Electrical, Electronic,489


of Genova. Dr. Valle is co-author of more than 180 articles on international491

scientific journals and conference proceedings. His current research interests492

include tactile sensors and systems, microelectronic embedded systems, and493

wireless sensor networks. He is member of IEEE.494

Laura Basiricò received the M.S. degree in Physics from the University495

of Bologna, Bologna, Italy, in 2008. In 2009, she joined the University496

of Cagliari with the Department of Electrical and Electronic Engineering497

as the Ph.D student. She received the Ph.D. degree in 2012. During the498

Ph.D. degree, she focused her research activity on the development of inkjet499

printing technique for the realization of Organic Field Effect Transistors500

and Organic ElectroChemical Transistors for the sensing application. She501

is currently a Researcher with ISMN-CNR, Bologna. Her current research502

interests include the field of organic electronic devices on the fabrication and503

physical characterization of Organic Field Effect Transistors for light emission504

and sensing applications.505

Alberto Loi received the Master Degree in Electronic Engineering from the506

University of Bologna, Italy, in 2010, discussing the thesis “Analysis of a507

30-V multifinger LDMOS power device.” In 2011, he joined the DEALAB508

group with the University of Cagliari, where he worked as the Ph.D. student.509

His current research interests include inkjet printing of organic materials,510

fabrication and characterization of Organic Field Effect Transistors, and511

fabrication and characterization of mechanical sensors and sensing systems512

based on organic devices.513

Piero Cosseddu graduated in Electronic Engineering, in 2003, and received514

the Ph.D. degree in Electronic and Computer Science Engineering in 2007515

from the University of Cagliari, Italy. Since May 2007, he has been a Post-516

Doctoral Fellow with the Department of Electrical and Electronic Engineering,517

University of Cagliari. His current research interests include the design,518

realization, and characterization of organic semiconductor-based devices for519

innovative applications such as artificial electronic skin, biomedical sensing,520

and wearable electronics.521

Annalisa Bonfiglio received the Laurea Degree in Physics from the University522

of Genoa, in 1991 and the Ph.D. degree in Bioengineering from the Politecnico523

di Milano, Milano, Italy, in 1995. She is currently an Associate Professor524

of Electronics and Electronic Bioengineering with the University of Cagliari,525

Italy. She is also a member of the Institute of Nanoscience- National Research526

Council. She is an author of more than 130 papers on international journals,527

conference proceedings, book chapters, and five patents. Her current research528

interests include innovative materials and electronic devices for wearable529

electronics and bioengineering and several international and national research530

Projects.531

Alberto Ascia received the master’s degree in Computer Science Engineering 532

and the Ph.D. degree in “Electronic Engineering, Automation and Control of 533

Complex Systems” from the University of Catania, Catania, Italy, in 2005 534

and 2009, respectively. During the Ph.D. degree, he focused on design and 535

characterization of transducers based on magnetic fluids. From January 2010 536

to July 2012, he has been a Post-Doctoral Researcher with the Istituto Italiano 537

di Tecnologia. Her current research interests include novel technologies for 538

the design and characterization of artificial skin for the iCub humanoid robot. 539

He is currently an Instrumentation Test Engineer with an aeronautic company. 540

Maurizio Biso received the M.Sc. degree in Industrial Chemistry and the 541

Ph.D. degree in Robotics, Neuroscience, and Nanotechnology. He is currently 542

a Postdoc with the Robotics, Brain, and Cognitive Sciences Department, 543

Istituto Italiano di Tecnologia, Genoa, Italy mainly working on composite 544

materials based on carbon nanotubes for robotics. His current research 545

interests include the development of “soft” materials for sensors and actu- 546

ators. He has a strong experience in electrochemistry and in synthesis and 547

characterization of polymeric materials. 548

Alberto Ansaldo received the M.Sc. degree in Electronic Engineering and 549

the Ph.D. degree in Communication Science and Technology, Genoa, Italy. 550

He is mainly working on Chemical Vapour Deposition of Carbon Nanotubes 551

for Electronic Applications. He is currently Research Technologist with the 552

Robotics Brain and Cognitive Science Department of the Italian Institute of 553

Technology, Genoa, Italy, in the Soft Materials Design Laboratory. His current 554

research interests include nanoscience, robotics, and bioengineering, with a 555

focus on the development of technologies based on novel materials such as 556

conductive polymers and carbon nanomaterials. 557

Davide Ricci received the M.Sc. degree in Physics and the Ph.D. degree 558

in Electronic Engineering and Computer Science from the University of 559

Genoa, Genoa, Italy, in 1989 and 1993, respectively. He is currently a Team 560

Leader with the Robotics Brain and Cognitive Science Department, Italian 561

Institute of Technology, Genoa, Italy. His current research interests include 562

the development of devices such as neural electrodes for Brain Machine 563

Interfaces, of flexible nano-actuators and sensors for robotics and of smart 564

interfaces for tissue engineering and prosthetics. He has co-authored more 565

than 100 papers on such topics. 566

Giorgio Metta is the Director of the iCub Facility department with the Istituto 567

Italiano di Tecnologia (IIT), Genoa, Italy, where he coordinates the iCub 568

project. He received the M.Sc. degree cum laude, in 1994, and the Ph.D. 569

degree, in 2000, in electronic engineering from the University of Genoa. 570

In 2001 and 2002, he has been a Post-Doctoral Associate with the MIT 571

AI-Lab. Since 2012, he has been the Professor of Cognitive Robotics with 572

the University of Plymouth, Plymouth, U.K. His current research interests 573

include the fields of biologically motivated and humanoid robotics and 574

developing robots that can adapt and learn from experience. He is an author 575

of approximately 200 scientific publications. 576

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AQ:1 = Table I is not cited in the body text. Please indicate where it should be cited.AQ:2 = Please provide the publisher name and location for ref. [7].AQ:3 = Please provide the date, month, and year for ref. [16].

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