Ionic liquid/carbon nanotube based actuators have been constantly improved in recent years owing to their suitability for applications related to human-machine interaction and robotics thanks to their light-weight and low voltage operation. However, while great attention has been paid to the development of better electrodes and electrolytes, no adequate efforts were made to develop actuators to be used in direct contact with the human skin. Herein, we present our approach, based on the use of parylene-C coating. Indeed, owning to its physicochemical properties such as high dielectric strength, resistance to solvents, biological and chemical inactivity/inertness, parylene fulfils the requirements for use in biocompatible actuator fabrication. In this paper, we study the influence of the parylene coating on the actuator performance. To do so, we analyzed its mechanical and electrochemical properties. We looked into the role of parylene as a protection layer that can prevent alteration of the actuator performance likely caused by external conditions. In order to complete our study, we designed a haptic device and investigated the generated force, displacement and energy usage.
Recently, materials that can convert electrical energy into mechanical work have drawn great attention. Applications in robotics, tactile or optical displays and microelectrochemical systems are currently investigated. Likewise, interest in actuators devices is increasing toward applications where low voltage and low weight properties are required. One way to achieve such prerequisites is to combine the mechanical and electronic properties of carbon nanotubes (CNTs) with the stability and conductivity of ionic liquids. Indeed, the CNTs can be dispersed in ionic liquids to form hybrid composites also named bucky gels, thanks to the non-covalent (π-π stacking and cation-π) interactions. In our previous studies, we demonstrated an improvement in actuator performance whilst using cross-linked CNTs. Indeed, our preliminary results showed an increase in the capacitance together with a faster response of the actuator. At the time, these results were explained by an actuation mechanism model.
Herein, we designed new experiments in order to allow us to get a deeper insight in the effect the crosslinking process on the carbon nanotubes properties. Thus, we present a set of electromechanical and electrochemical data that shed light on the chemical modification of the CNTs, the different cross-linking strategies and also on the uses of cross-linked CNTS polymer blends. Finally, corresponding bucky gels actuators performances will also be discussed.
An ideal plastic actuator for haptic applications should generate a relatively large displacement (minimum 0.2-0.6 mm,
force (~50 mN/cm2) and a fast actuation response to the applied voltage. Although many different types of flexible,
plastic actuators based on electroactive polymers (EAP) are currently under investigation, the ionic EAPs are the only
ones that can be operated at low voltage. This property makes them suitable for applications that require inherently safe
actuators. Among the ionic EAPs, bucky gel based actuators are very promising. Bucky gel is a physical gel made by
grounding imidazolium ionic liquids with carbon nanotubes, which can then be incorporated in a polymeric composite
matrix to prepare the active electrode layers of linear and bending actuators. Anyhow, many conflicting factors have to
be balanced to obtain required performance. In order to produce high force a large stiffness is preferable but this limits
the displacement. Moreover, the bigger the active electrode the larger the force. However the thicker an actuator is, the
slower the charging process becomes (it is diffusion limited). In order to increase the charging speed a thin electrolyte
would be desirable, but this increases the probability of pinholes and device failure. In this paper we will present how
different approaches in electrolyte and electrode preparation influence actuator performance and properties taking
particularly into account the device ionic conductivity (which influences the charging speed) and the electrode surface
resistance (which influences both the recruitment of the whole actuator length and its speed).