Flexible Membrane Tactile Sensor for Contact Traction Distribution Measurement on a Microscale Siddarth Kumar 1 , Gang Liu 2 , Mandayam A. Srinivasan 3 MIT Touch Lab, Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT A novel thin film polydimethylsiloxane (PDMS) based flexible tactile micro sensor has been developed for contact traction distribution measurement between an indenter and an object (e.g. finger pad). The tactile sensor is a five layer device consisting of 2 layers of textured patterns sandwiched between 3 layers of PDMS. The textured patterns are protected by a thin layer of PDMS (25 microns) on each side and are separated by a thicker PDMS layer (100 microns). The sensor thus has a total thickness of 150 microns, much less than the thickness of the human fingertip skin. The entire device is fabricated bottom up on a silicon wafer using soft lithography and microfabrication techniques. The sensor is designed to be used with an imaging apparatus (in our case we use an Optical Coherence Tomography (OCT) apparatus). The deflection of the patterns due to an applied load is imaged by the OCT and is used to estimate the stress at the sensor and object interface by solving the stress inverse problem analytically. Results are presented to illustrate the use of the microsensor to estimate the contact traction distribution due to a spherical indenter of radius 500 µm. KEYWORDS: Flexible pressure sensor, PDMS, micro fabrication, skin mechanics, tactile, OCT. 1 INTRODUCTION Form and texture perception at the fingertips is a critical component of the sensory and motor function of the hand [1,2]. When an object comes into contact with the human finger, tactile information is relayed to the brain via stresses/strains exerted on nerve endings embedded in the skin. The transmission of tactile information to these nerve endings from the skin surface is dependent on the mechanics of contact between the skin and the object. To understand how incident loads on the skin affect tactile perception, there is a need to develop a quantitative understanding of the types of spatiotemporal loads imposed on the surface of the skin. The human finger is able to sense textures of the order of microns [3]. Thus, there is a need for the development of high resolution tactile sensors capable of sensing load distributions exerted by non-planar geometries on the skin at a microscale. There is not much literature on thin pressure transducers developed on flexible substrates. Lim et at [4] developed a pressure sensor with amorphous silicon contacts fabricated onto a flexible 51 µm thick Kapton E ® (DuPont) substrate. The sensing elements were wired according to a full Wheatstone bridge layout. A flexible shear sensor based on cantilevers embedded in PDMS was developed by Noda et al [5]. Wettels et al [6] developed a biomimetic tactile sensor array consisting of a rigid core (with embedded electrodes) surrounded by a weakly conducting fluid encased in an elastomeric membrane. An applied load altered the impedance of the volumetric flow path from a given contact to the reference electrode and enabled the incident load to be inferred. These sensors however are not able to sense pressure distributions and give us only the average pressure exerted by an incident load. In recent literature, larger area tactile sensors to aid in sensing textures has been developed by Someya et al [7], Maheshwari et al [8], Kamiyama et al [9] and Johnson and Adelson [10]. However, there is not much literature on the development of thin sensors capable of high-resolution pressure/shear measurement over small areas (few hundred square microns) with the required spatial resolution on the order of tens of microns. The following paper presents a description of the design and fabrication of a high-resolution tactile microsensor device which will be used in conjunction with an imaging device (the Optical Coherence Tomography (OCT) imaging apparatus) to quantify load distributions due to arbitrary shaped micro indenters. 2 DEVICE DESIGN Figure 1. A schematic illustrating the sensor design. A section of the sensor is shown. There are 20 such SU8 features on each of the two layers. Each feature is 40 microns in width and 10 microns in height 1 siddarth@mit.edu 2 gangliu@mit.edu 3 srini@mit.edu 627 IEEE World Haptics Conference 2011 21-24 June, Istanbul, Turkey 978-1-4577-0298-3/11/$26.00 ©2011 IEEE