HAXELs: flexible zipping actuators for haptics

We have developed HAXELs: hydraulically amplified zipping electrostatic actuators, less than one millimeter in thickness, designed for seamless integration in textile-based haptic interfaces for Virtual Reality (VR) and Augmented Reality (AR) scenarios. 

For virtual reality (VR) to be truly immersive, the sense of touch must be stimulated. Our “HAXELs” can deliver a rich sense of touch on fingers or anywhere on our skin. Each HAXELs is less than 1 mm thick, changes in thickness by 60% in under 5 milliseconds, and generates 0.5 N of force. They are very lightweight, only 90 mg each, and can easily be incorporated in clothing. This work was published in Advanced Materials in 7-2020. First author: E. Leroy, work done at EPFL-LMTS.

HAXELs are capable of both of out-of-plane and in-plane motion, thus providing normal and shear forces to the user’s fingertip, hand or arm, with high spatial resolution. 

Combined with our textile brake, we develop thin and low-power  haptic gloves that make virtual objects feel real, yet allow for complete freedom motion of the hand.

Arrays of out-of-plane HAXELS

Row-column addressing can be used to address arrays, which also can be incorporated into thin comfortable sleeves.

How do HAXELs work?
Each HAXEL consists of a fluid-filled cavity whose shell is made of a non-stretchable polymer in the outer regions and of a stretchable elastomer for the central region. Electrodes are patterned on both the top and bottom polymer shells. When a voltage is applied to the annular electrodes, they are pulled together by Maxwell pressure, starting to zip at the outer regions where the electric field is highest. As the top and bottom polymer regions are pulled together, the fluid is rapidly forced into the central stretchable region, forming a raised bump. We obtain very high power density (>100 W/kg) and very high force density (3300 N/kg).

HAXELS are electrically controlled. No compressor or pressurized air source is needed, greatly adding to device portability, and enabling completely silent operation.

Shear force and normal force

We use our fingers to identify objects by sliding our fingertips over them. We can tell if if a glass is about to slip out of our hand by sensing shear forces. Simulate this in VR requires being able to generate normal and shear forces with high spatial resolution.

By segmenting the HAXEL electrodes, we can make the central bubble not only pop out of plane, but also move side to round, around, up and down… This enables us to use only one HAXEL to provide rich range of haptic feelings.schematic principle of HAXEL device

Testing on volunteers showed that both the normal and in-plane motion was easily felt and correctly identified. The device can generate static strain as well as vibro-tactile signals up to several hundred Hz.

HAXEL showing in-plane force generation

Other applications of HAXELS

Although HAXEL actuators were initially designed for haptics (either worn or to make surfaces haptically active), HAXELs can find applications in many other areas such as microfluidics (eg dense arrays of valves), soft robotic for locomotion or grasping. HAXELS can be scaled up to several square cm and down to sub-mm on a side.

For more info:
For more information on HAXEL design, fabrication and characterization, please see: E. Leroy, R. Hinchet and H. Shea, “Multimode Hydraulically Amplified Electrostatic Actuators for Wearable Haptics,” Advanced Materials, 2020. doi: 10.1002/adma.202002564

We gratefully acknowledge support from the Hasler Foundation Cyber-Human Systems program and from EPFL.