IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 60, NO. 8, AUGUST 2011 2967 Design and Characterization of a Nanocomposite Pressure Sensor Implemented in a Tactile Robotic System Alessandro Massaro, Fabrizio Spano, Aimé Lay-Ekuakille, Paolo Cazzato, Roberto Cingolani, and Athanassia Athanassiou Abstract—In this paper, we present the implementation of a new class of optical pressure sensors in a robotic tactile-sensing system based on polydimethylsiloxane (PDMS). The sensor con- sists of a tapered optical fiber, where an optical signal goes across, embedded into a PDMS–gold nanocomposite material (GNM). By applying different pressure forces onto the PDMS-based nanocom- posite, changes in the optical transmittivity of the fiber can be detected in real time due to the coupling between the GNM and the tapered fiber region. The intensity reduction of a transmitted light is correlated to the pressure force magnitude. Light intensity is converted into an electric signal by a system suitable for robotic implementation. High sensitivity using forces by applying weights of a few grams is proved. Sensitivity on the order of 5 g is checked. A detailed algorithm for the detection of roughness and shapes by means of a robotic finger is proposed. Index Terms—Light coupling, nanocomposite materials, optical tactile sensors, pressure sensing, robotic implementation. I. I NTRODUCTION S ENSORY information of the human skin for feeling mate- rials and determining their physical properties is provided by sensors on the skin. Currently, many researchers are attempt- ing to apply the five senses to intelligent robotic systems. In particular, many kinds of tactile sensors, combining small force sensors, have been introduced into intelligent robots. These tactile sensors, which are capable of detecting contact force, vibration, texture, and temperature, can be recognized as the next generation of an information collecting system. Future applications of implemented tactile sensors include robotics in Manuscript received November 3, 2010; revised January 14, 2011; accepted January 17, 2011. Date of publication April 5, 2011; date of current version July 13, 2011. The Associate Editor coordinating the review process for this paper was Dr. Zheng Liu. A. Massaro and F. Spano are with the Center for Biomolecular Nano- technologies, Italian Institute of Technology, 73010 Leece, Italy (e-mail: alessandro.massaro@iit.it). A. Lay-Ekuakille is with the Department of Innovation Engineering, Faculty of Engineering, University of Salento, 73100 Lecce, Italy. P. Cazzato is with the National Nanotechnology Laboratory, Institute of Nanoscience, National Research Council, 73100 Lecce, Italy. R. Cingolani is with the Italian Institute of Technology, 16163 Genova, Italy. A. Athanassiou is with the Center of Biomolecular Nanotechnologies, Italian Institute of Technology, 73010 Leece, Italy, and also with the National Nano- technology Laboratory, Institute of Nanoscience, National Research Council, 73100 Lecce, Italy. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2011.2121290 medicine for minimally invasive microsurgery, military uses for dangerous and delicate tasks, and automation in indus- tries. Some tactile and small force sensors using microelectro- mechanical system (MEMS) technology have been introduced. MEMS tactile-sensing work has been focused mainly on silicon-based sensors that use piezoresistive [1]–[3] or ca- pacitive sensing [4]–[6]. These sensors have been realized with bulk and surface micromachining methods. Polymer-based devices that use piezoelectric polymer films [7]–[9] such as polyvinylidene fluoride for sensing have also been constructed, but polymeric piezoelectric materials are not the only ones used for sensing applications. There are a lot of different polymers investigated for this kind of application and oriented on MEMS technology. Although these sensors offer good spatial resolu- tion due to the use of MEMS techniques, they still pick out problems in applications for practical systems. In particular, de- vices that incorporate brittle sensing elements such as silicone- based diaphragms or piezoresistors are not reliable for robotic manipulation [10], [11]. Previous efforts have been hindered by rigid substrates, fragile sensing elements, and complex wiring. Moreover, the polymeric solutions found in the literature for fabrication of pressure sensor systems [12]–[24] require com- plex fabrication processes and postprocessing analysis. All these drawbacks can be compensated by utilizing flexible op- tical fiber sensors and transducers. In addition, optical fiber sensors are immune from electromagnetic (EM) fields and can be easily multiplexed and integrated with small light- emitting diode sources, thus providing a good alternative for the implementation of robotic tactile sensors [25]. Moreover, the proposed optical fiber sensor is obtained by means of a simple fabrication process: A used nanocomposite material, which the fiber is embedded in, is achieved simply by chemical reduction that allows to obtain nano/micro gold particles in a polymeric material [i.e., a gold nanocomposite material (GNM)]. The use of elastomers such as polydimethylsiloxane (PDMS) has many advantages with respect to silicon or glass. PDMS is cheaper than silicon, is more flexible, and bonds easier to other materials than silicon or/and glass. PDMS conforms to the surface of a substrate over a large area and can adjust to surfaces that are nonplanar. PDMS is a homogenous and optically trans- parent material down to about 300 nm. PDMS is waterproof and permeable to gases. The surface properties of PDMS can easily be changed by exposure of its surface in oxygen plasma. In this way, PDMS can bond to other materials that have a wide 0018-9456/$26.00 © 2011 IEEE