Synthetic and bio-artificial tactile sensing: a review

This paper reviews the state of the art of artificial tactile sensing, with a particular focus on bio-hybrid and fully-biological approaches. To this aim, the study of physiology of the human sense of touch and of the coding mechanisms of tactile information is a significant starting point, which is briefly explored in this review. Then, the progress towards the development of an artificial sense of touch are investigated. Artificial tactile sensing is analysed with respect to the possible approaches to fabricate the outer interface layer: synthetic skin versus bio-artificial skin. With particular respect to the synthetic skin approach, a brief overview is provided on various technologies and transduction principles that can be integrated beneath the skin layer. Then, the main focus moves to approaches characterized by the use of bio-artificial skin as an outer layer of the artificial sensory system. Within this design solution for the skin, bio-hybrid and fully-biological tactile sensing systems are thoroughly presented: while significant results have been reported for the development of tissue engineered skins, the development of mechanotransduction units and their integration is a recent trend that is still lagging behind, therefore requiring research efforts and investments. In the last part of the paper, application domains and perspectives of the reviewed tactile sensing technologies are discussed

Design, development, integration and experimental evaluation of a tactile sensing system for applications in neuro-robotics

The objective of this Master Thesis is the development a Micro Electro Mechanical System (MEMS)-based capacitive tactile sensing system, intended to be integrated in the distal phalanx of a robotic finger. The used sensor-array consists of sixteen capacitive force sensors, two resistive temperature sensors and two reference capacitors. The prototype is capable of measuring the application of local forces and temperature changes. The sensors are silicon-based and are fabricated by means of the Bonded and Etched-Back Silicon-On-Insulator (BESOI) technology, and are the result of a collaboration between The BioRobotics Institute of Scuola Superiore Sant’Anna and the School of Mechanical Engineering of University of Birmingham, in the framework of the EU-funded (FP7-NMP) Nanobiotouch project. The sensing principle is capacitance-based because of the advantages in terms of higher spatial resolution, long term drift stability, increased sensitivity, lower temperature sensitivity and power consumption as compared to piezoresistive, strain gauge and piezoelectric sensing principles. The readout electronics is implemented by means of four capacitance-to-analog converters (Irvine Sensors, MS3110), with time-division multiplexing via low-capacitance CMOS quad-channel switches (Analog Devices, ADG1212) along each row of sensors of the array. The MS3110 output voltage is a function of the differential capacitance provided as an input to the converter, and of configuration parameters having effect on the conversion gain and offset. Such configuration parameters of the MS3110 capacitance-to-analog converter are programmed by means of the MS3110BDPC Evaluation/Programming Board. During the Master Thesis, several experimental sessions were carried out in order to assess the subsystems of the tactile sensing system under study: 1. interfacing of readout electronics for the single-sensor; 2. interfacing of multiplexed electronics for the sensor-array; 3. probing of individual capacitive sensor unit. Within the first set of experimental activities, a single-sensor readout electronics was tested in order to evaluate the functionality of the MS3110 capacitance-to-analog converter: discrete capacitances, in the range between 0.25pF and 10pF, were tested as a function of the values of the converter parameters setting the gain and the offset. The values of the output voltage were recorded and elaborated as a function of the difference between the sensing capacitances and of the used configuration parameters, to verify the nominal characteristics of the capacitance-to-analog converter. Within the second set of experimental sessions, the interfacing of the multiplexed readout electronics for the sensor-array was tested in order to evaluate its functionality in measuring the capacitance of the 16 channels of the array, under a time-division policy. The signal to the capacitance-to-analog converter is switched between discrete capacitances using the quad-channel switches (ADG1212). Within this experimental protocol, the performance of one of the four measurement circuits was evaluated while operating a time-division multiplexing via the switch along each row of the measurement circuit. Discrete capacitances, in the range between 0.25pF and 10pF, were tested as a function of the values of the converter parameters. The experimental results show a general consistency between the nominal characteristics of the capacitance-to-analog converter and the experimental data. Within the third set of experimental activities, the mechanotransduction properties of the sensor were evaluated by direct probing of individual capacitive sensor units, i.e. by characterizing the response to indentation cycles. A loading system allowed precise positioning of a loading probe endowed with a spherical head: the contact between the probe and the sensor was first adjusted with three manual orthogonal micrometric translation stages (M-105.10, PI), and then precisely controlled along the indentation direction by means of a servo-controlled positioning system (M-111.1, PI); two camera permitted the proper positioning of the probe over the sensor. To measure the applied force, a six-component load cell was positioned at the interface between the probe and the translational stage. The values of output voltages were recorded and elaborated as a function of the applied load, to evaluate the characteristics of the capacitive tactile sensor. The results of this experimental protocol show, coherently with the expectations, that the sensor and the readout electronics responded to an increase in probe indentation depth with an increase in measured voltage change

Soft Smart Garments for Lower Limb Joint Position Analysis

Revealing human movement requires lightweight, flexible systems capable of detecting mechanical parameters (like strain and pressure) while being worn comfortably by the user, and not interfering with his/her activity. In this work we address such multifaceted challenge with the development of smart garments for lower limb motion detection, like a textile kneepad and anklet in which soft sensors and readout electronics are embedded for retrieving movement of the specific joint. Stretchable capacitive sensors with a three-electrode configuration are built combining conductive textiles and elastomeric layers, and distributed around knee and ankle. Results show an excellent behavior in the ~30% strain range, hence the correlation between sensors’ responses and the optically tracked Euler angles is allowed for basic lower limb movements. Bending during knee flexion/extension is detected, and it is discriminated from any external contact by implementing in real time a low computational algorithm. The smart anklet is designed to address joint motion detection in and off the sagittal plane. Ankle dorsi/plantar flexion, adduction/abduction, and rotation are retrieved. Both knee and ankle smart garments show a high accuracy in movement detection, with a RMSE less than 4° in the worst case