The present application deals with the development on an innovative soft hybrid actuator, combining two different technologies: (i) hydraulically amplified self-healing electrostatic (HASEL) and (ii) Electrorheological Fluids-based- (ERFs) actuators. Such novel actuators are predicted to be able to overcome the limitations of conventional Dielectric Elastomer Actuators (DEAs), such as the elevated activation voltage (in the kV range), which makes the latter still unsuitable for applications involving interfacing with humans, and the inability to sustain heavy loads, along with a limited maximum strain range. All the shortcoming will be addressed by combining the advantages of two cutting edge technologies, whose synergistic operation is still unexplored in the scientific community. Starting from the latest development of HASEL actuators, the proposed approach involves the employment of two different types of sealed pouches, filled with dielectric liquids and a Giant Electrorheological Fluid (GER), respectively. The first pouch type will be realized starting from inexpensive and commercially available heat sealable Biaxially Oriented Polypropilene (BOPP) sheets and suitable geometries will be created by using the ¿hot end¿ of a commercial 3D printer. The GER-filled pouch-type will be employed to sustain the "heavy loads" and provide a strain-enhancement mechanism, by properly designing "stiffening sleeves", taking advantage of the liquid-solid transition of the electrorheological fluid upon application of a suitable electric field.
Several types of soft actuators have emerged in recent years which, depending on the responsive material they are based on, can be classified in: (i) SMMs, which change shape in response to electric current or heat (ii) FFAs, actuators having a flexible inflatable structure and actuated by a fluid (1), (iii) EAPs, also known as ¿artificial muscles¿ for their ability to emulate muscle action in a soft mechanism when stimulated through electricity (2), (iv) DEAs, a subclass of EAPs, basically variable capacitance actuators, whose dynamics is based on the application of a suitable electrostatic field. DEAs offer several advantages over other soft actuators, in that they don¿t need a cumbersome power supply unit and their displacement and movements can be precisely controlled by simply applying the proper activation voltage.
However, traditional soft robots based on DEAs realized upon elastomeric membranes, while being able to reach considerable strains (>100%), are prone to dielectric breakdowns, due to the high electric fields required for actuation, making them, moreover, incompatible with applications where interfacing with humans is required. According to the IEC 60364 standard, for safe operation the supply voltage should not exceed 50 V in AC and 120 V in DC (the corresponding voltage range is termed Extra-low voltage or ELV). Unfortunately, DEAs requires high voltages for effective actuation, frequently exceeding several kV and even newly develop HASEL have onset voltages of 2/10 kV for the actuation to take place.
An important innovation of the present project deals with the drastic reduction of the actuation voltage, by employing dielectric liquids (either pure liquids, liquid mixtures or liquid suspensions) having considerably higher relative permittivity than those employed in HASEL actuators, thus drastically reducing the required actuation voltage and paving the way for possible applications for human-rehabilitation purposes.
The present project will involve the realization of two different soft pouch designs, one filled with a high permittivity liquid dielectric, enabling the actuation with a lower onset voltage and the second one filled with an ERF. Moreover, the latter pouch type will feature an extensible and stiffening sleeve, having a second pair of electrodes: upon actuation of the first pouch type the ERF will rise up the sleeves and once they¿re fully extended, a second voltage, applied to the sleeves¿ electrodes, will trigger the transition of the ERF from the liquid to the solid state, thus obtaining fully-extended stiffened sleeves.
Therefore, the second goal of the project will deal with the realization of a locally stiffening mechanism, allowing the actuated device to sustain loads unachievable by traditional soft actuators.
Moreover, by properly designing the sleeves¿ geometry, it will be possible to dramatically increase the maximum obtainable strain, as well.
Biblio
1. De Greef A, Lambert P, Delchambre A. Towards flexible medical instruments: Review of flexible fluidic actuators. Precis Eng. 2009;33(4):311¿21.
2. Bar-Cohen Y, Anderson IA. Electroactive polymer (EAP) actuators¿background review. Mech Soft Mater [Internet]. 2019 Dec 22;1(1):5. Available from: http://link.springer.com/10.1007/s42558-019-0005-1