Additively manufactured stretchable zipping electrostatic actuators

The use of soft and stretchable materials allows the development of adaptive robotic systems and human-machine interfaces that are more natural and comfortable to interact with. One of the application fields that benefits the most from these compliant materials is haptic feedback. The rise of Virtual and Augmented Reality applications for training, teleoperation, and entertainment purposes demands interfaces to stimulate the sense of touch with similar fidelity as visual and auditory feedback. Thanks to actuation principles exploiting flexible and stretchable polymeric materials, compliant, lightweight wearable devices providing mechanical stimuli to the user skin have been developed.Most of everyday activities would be impossible without being able to gain information about the physical properties of the surrounding environment through our fingertips, and this is reflected in the development of uncountable haptic feedback platforms for the finger. However, there is always a trade-off between intensity of mechanical output, actuation frequency bandwidth, and unobtrusiveness of the wearable hardware. One actuation principle that stands out for achieved energy density, frequency of produced stimuli, low weight, and silent operation and compactness of power supplies is zipping actuation, which exploit electrostatic forces and hydraulic coupling to produce mechanical work. An important limitation of these actuators is the lack of material solutions and fabrication processes suitable for stretchable, sub-millimetric zipping actuators for wearable haptic devices. The goal of this thesis is to fill this gap by developing additive manufacturing processes for stretchable zipping electro-static actuators. Additive fabrications enable unprecedented design flexibility, limited fabrication-related material waste, fast prototyping and adaptable fabrication processes for customized actuators. Stretchable zipping actuators based on the hydraulically amplified taxel (HAXEL) principle are fabricated by employing a technological platform based on inkjet printing, which allows thin depositions of stretchable materials. A new technical solution for the fabrication of stretchable, independently sealed fluidic pouches is presented, along with the design strategies suitable for on-skin wearable arrays for the fingertip.The presented actuators are first modelled analytically thanks to an energy-based approach capable of synthesizing electrical, mechanical and geometrical properties of the actuator into its electro-mechanical behaviour, which is validated experimentally. The fabricated stretchable HAXELs are less than 0.8 mm-thick, weight less than 25 mg, and are able to generate perceivable mechanical stimuli with a bandwidth spanning from quasi-static to 1 kHz. Actuation under 50% stretch is proven, as well as a lifetime higher than a whole week. Haptic studies on users confirmed detectable and localizable haptic stimuli, with 86% of provided haptic stimuli correctly identified by the users.In the last part of this thesis, an alternative fabrication process is developed in order to encapsulate the filling fluid directly in the printed structures, thus achieving further digital control and automation of the fabrication process. First actuators produced through the new approach are tested, thus opening the way to truly fully-printed liquid filled zipping electrostatic actuators based on thin and stretchable materials.