Development of an electronic skin with ultra-high pressure sensitivity is now of critical importance due its broad range of applications including prosthetic skins and biomimetic robotics. Microstructured conductive composite elastomers can acquire mechanical and electrical properties analogous to those of natural skin. One of the most prominent features of human skin is its tactile sensing property which can be mimicked in an electronic skin. Herein, an electrically conductive composite comprising polydimethylsiloxane and conductive fillers is used as a flexible and stretchable piezoresistive sensor. The electrical conductivity is induced within the elastomer matrix via carbon nanotubes whereas the piezoresistivity is obtained by means of microstructuring the surface of the substrate. An interlocked array of pyramids in micro-scale allows the change in the contact resistance between two thin layers of the composite upon application of an external load. Deformation of the interlocked arrays endows the sensor with an ultra-high sensitivity to the external pressures within the range of human skin perception. Moreover, using finite element analysis, the change in the contact are between the two layers was captured for different geometries. The structure of the sensor can be optimized through an optimization model in order to acquire maximum sensitivity.
KEYWORDS: Sensors, Piezoresistive sensors, Skin, Resistance, Monte Carlo methods, Systems modeling, Optimization (mathematics), Polymeric sensors, Polymers, Composites, Electrodes, 3D modeling
Human intervention can be replaced through development of tools resulted from utilizing sensing devices possessing a wide range of applications including humanoid robots or remote and minimally invasive surgeries. Similar to the five human senses, sensors interface with their surroundings to stimulate a suitable response or action. The sense of touch which arises in human skin is among the most challenging senses to emulate due to its ultra high sensitivity. This has brought forth novel challenging issues to consider in the field of biomimetic robotics. In this work, using a multiphase reaction, a polypyrrole (PPy) based hydrogel is developed as a resistive type pressure sensor with an intrinsically elastic microstructure stemming from three dimensional hollow spheres. Furthermore, a semi-analytical constriction resistance model accounting for the real contact area between the PPy hydrogel sensors and the electrode along with the dependency of the contact resistance change on the applied load is developed. The model is then solved using a Monte Carlo technique and the sensitivity of the sensor is obtained. The experimental results showed the good tracking ability of the proposed model.
Many efforts have been devoted to modeling the diffusive impedance of conjugated polymer (CP) based actuators using
their equivalent electrical circuits. Employing the same methodology, CP based mechanical sensors can also be treated
by an equivalent transmission line circuit and their overall impedance can be modeled, correspondingly. Due to the large
number of resources to study the electrical circuits, this technique is a practical tool. Therefore, in this study, an
equivalent RC-circuit model including electrochemical parameters is determined to obtain a better perception of the
sensing mechanism of these mechanical sensors. Conjugated polymers are capable of generating an output current or
voltage upon an induced mechanical deformation or force. This observed behavior in polymer based mechanical sensors
is considered as the reverse actuation process. Many outstanding properties of the conjugated polymer actuators
including their light weight and biocompatibility are still retained by these sensors. Sensors with a trilayer configuration
are capable of operating in air in response to a mechanically induced bending deformation. However, due to their
nonlinear behavior and multivariable characteristics, it is required to propose a systematic approach in order to optimize
their performance and gain the optimal values of their constituent decision variable. Therefore, the proposed
mathematical model is used to define the output voltage of the PPy based mechanical sensor along with the sensitivity of
the model to the applied frequency of the induced deformation. Applying a multiobjective optimization algorithm, the
optimization problem was solved and the tracking ability of the proposed model was then verified.
Polypyrrole (PPy) conducting polymers as one of the most well-known actuation materials have shown numerous
applications in a variety of fields such as biomedical devices as well as biomimetic robotics. This study investigates the
multiobjective optimization of a PPy/MWCNTs actuator through an electrochemomechanical model. The multilayer
actuator is composed of a PVDF layer, as the core membrane and an electrolyte reservoir, as well as two one layer of a
conjugated polymer and one layer of multiwalled carbon nanotubes deposited on each side of the PVDF layer. In order to
obtain the optimum values for each decision variable (i.e., geometrical and electrochemical), the two main outputs of the
bending actuator, the tip displacement and blocking force, have been mathematically modeled and formulated as the
objective functions. A multiobjective optimization algorithm is applied to simultaneously maximize the blocking force
and tip displacement generated by the actuator. Furthermore, a range for each design variable is defined within which
none of the objective functions of the film-type actuator dominates the other one while they are both kept within an
acceptable range. The results obtained from the mathematical model are experimentally verified. Moreover, in order to
determine the performance of the fabricated actuator, its outputs are compared with their counterparts of a neat PPy
actuator.
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