When engaging in virtual reality (VR) and manipulating an object, users want to be able to feel what they’re touching in virtual space. A hard object needs to feel hard and a soft object needs to feel soft; mechanisms that can accurately deliver these touch sensations are called haptic systems.
Haptic systems achieve their goals through a number of approaches. One method involves the use of components called electroadhesive clutches. An electroadhesive clutch uses electrical voltage to create adhesion between two electrodes. The mechanism is typically made up of two overlapping, metallized electrodes separated by a non-metal, insulating material called a dielectric. The sheets can slide past one another when there is no electric field present, but when a voltage is applied, an electric field creates a strong attraction across the sheets, causing them to stick together in the same way your hair sticks to a balloon when you create an electric charge through rubbing, only much stronger. Gloves used in VR haptic systems can be embedded with these electroadhesive clutches to simulate touch sensations by changing from stiff to flexible and vice versa.
However, electroadhesion through the traditional dielectric setup requires anywhere from 100 to 10,000 volts of electricity to operate, levels that can be unsafe for direct human use. Additionally, their ability to switch back and forth from stiff to flexible is limited. Improving these clutches would not only improve a user’s experience in virtual reality, but could improve other applications such as robotic exoskeletons and prosthetics, finger-gripping systems, and shape-shifting robots.
A new design may now offer the best of all worlds: an electroadhesive material that is strong, operates at low voltages and quickly switches states. This system, which utilizes ionoelastomers instead of dielectrics, is explained in a paper published in Advanced Materials. This work is the result of a collaboration between Kevin Turner, Professor and Chair of Mechanical Engineering and Applied Mechanics (MEAM) in Penn Engineering, James Pikul, Associate Professor of Mechanical Engineering at the University of Wisconsin-Madison, and Ryan Hayward, Professor of Chemical and Biological Engineering at the University of Colorado Boulder.
“Ionoelastomers are flexible, elastic ion conductors that can stretch and bend while still allowing electricity to pass through them,” says Pikul. “The material itself feels like gummy rubber and was inspired by electroadhesive materials found in nature. Organisms like mussels and sandcastle worms use proteins with opposite charges to create a bioadhesive to secure their shells to the reef. The bond works due to the laws of electrostatics, where opposite charges attract and create a very strong adhesion on the molecular level. Biology activates this adhesion with chemicals, whereas we activate electrostatic adhesion in our materials by applying a voltage, i.e. electroadhesion.”
To leverage these capabilities in haptics, the team created a new ionoelastomer electroadhesive clutch that could successfully simulate realistic tactile experiences.
“We set out to create a material that would allow a system to simulate the feeling of holding and squeezing a soft rubber ball as well as the feeling of touching and pressing upon a hard, wooden table,” says Turner. “We wanted to be able to switch back and forth between those two very different sensations while also keeping the wearable device safe for human contact and repeated use.”
Fortunately, ionoelastomers allow charges to accumulate near the interface, decreasing the distance over which the voltage is applied and increasing the electric field. This results in a significant decrease, about 40-fold, in the required operating voltage of clutches. The ability to create such a strong bond in the soft ionoelastomers, however, gives rise to a different problem: a hard time letting go.
“Ionoelastomer clutches are soft and ‘sticky,’ and thus can have a hard time releasing when the voltage is removed,” continues Turner. “And that might cause interruptions in the sensations in a virtual environment.”
To tackle this issue, the team manipulated the surface roughness of the clutch. This roughness decreases the amount of surface area that can experience adhesion in the clutch, and therefore decreases the stickiness to a level that improves the switchability between stiff and flexible states. The team also integrated a metal mesh into the ionoelastomer to increase the system’s force capacity that allows the clutch to hold more weight and resist higher force, abilities essential to creating the sensation of pushing upon a hard surface in virtual reality.
The team put the clutch to the test in real-world scenarios and observed that it was successful in securing a cell phone to a mount in all orientations and in simulating tension and weight when attached to a finger in a wearable haptics device.
“We were able to create a clutch that outperforms the best existing clutch by 5-fold in force capacity with a 40-fold decrease in the required operating voltage,” says Pikul. “Our clutch can be applied to a wide range of technologies because it is strong, small and easily switches from stiff to flexible.”
But there is always room for improvement. Along with exploring the wide range of applications for this new electroadhesive clutch, the team wants to improve some of its properties to make it even better.
“Making the clutch tougher and more durable to last through many ‘on-off’ cycles would be an area to address in the future,” says Pikul. “We also want to find ways for the ionoelastomers to stick to a wider range of materials, including metals and polymers, so they can be integrated into a variety of soft and hard robots.”
Electroadhesive clutches are a relatively new technology being explored in many different types of robotics research. Improvements to these mechanisms not only move research forward, but also make future technology such as realistic VR gloves and even more ambitious products such as in-home robot assistants that much more possible.
This story was written by Poornima Apte and Melissa Pappas.