What image comes to mind when you think about robots? Perhaps something rigid and cold that cannot perceive its surrounding environment. While this is true for robotics in recent years, scientists around the world are now creating innovative new changes to make robots more “life-like”. To be more specific, recent attention in the field has been greatly focused on developing soft robots, namely, robots with soft bodies. Unlike conventional robots, most of which are made up of various rigid parts, soft robots comprise stretchable and flexible materials, like elastomers. This enables them to adaptively deform in response to environmental changes. For instance, a soft gripper made from electrostatic materials, can be delicate enough to pluck and hold a strawberry without damaging it, while also being strong enough to lift an apple.

With the advantages of high stretchability and flexibility, soft robotics has become an emerging field in areas related to human assistance and biomimetic robots for environmental monitoring.

To actuate or generate movement in a soft robot, fluidic elastomer actuators are widely used. In fluidic elastomer actuators, there are a series of hollow chambers with channels in between, which become inflated using gases or liquid, thereby generating deformation of the whole body. With asymmetry in actuator architecture or mechanical properties, such deformation can be made to result in bending or twisting.

In past decades, a great deal of effort has been made to develop materials and device geometries to improve the performance of fluidic elastomer actuators and their dexterity. A remaining issue, however, is enabling a soft robot to interact with and adapt to its surrounding environment. For this, diverse components like sensors must be integrated into them. Existing methods tend to focus on integrating commercially available, external sensors; however, these result in high process complexity, less compatibility with the actuators, and the integrated components tend to become separated after many uses; these issues impede the possibility of mass production.

To solve this issue, a group of researchers from Taiwan and Japan have developed a low-cost, intelligent soft robotic finger using paper electronics. Their findings were recently published in Advanced Intelligent Systems.

“Because paper is inexpensive and easy to obtain, it is usually used as a strain-limiting layer in fluidic elastomer actuators to trigger bending actuation. This made us wonder: Why don’t we print sensors on paper to provide sensing functions to actuators?” said Dr. Tilo Yang from National Taiwan University, the main contributor of the study. “In principle, arbitrary circuit layouts can be designed and printed on paper. The printing process is extremely easy and fast, which can be done even using an office printer. So, this method is very promising for large-area and mass production.”

The team fabricated two types of sensors on the same piece of paper: a resistive strain sensor and capacitive proximity sensor and incorporated them into fluidic elastomer actuators. Their results show that the resistive strain sensor was able to precisely measure the bending angle of the actuator finger to provide information about its exact position, and high consistency was obtained through 30 cycling tests. The strain sensor-embedded actuator was also used to measure the size of objects that it grasped, which would be useful in food handling. On the other hand, the capacitive sensor enabled the actuator to detect how close it was to a target object. The capacitive sensor was also capable of distinguishing between objects made of different materials based on their electrical properties.

Thanks to these multiple sensing functions, the team was able to realize a working, intelligent soft gripper. Such successful integration is promising for future prosthetic hands and human-robot interactive systems.

“While the concept is easy, it shows great potential for developing low-cost, mass-producible soft robotic systems with advanced sensing functionalities,” says Professor Jun Mizuno from Waseda University. The team expects to develop more paper electronic components, such as transistors, analog, wireless, and batteries when developing future autonomous soft robotic systems through a cheap, commercially viable process.

Reference: T.H. Yang, et al. ‘Low‐cost sensor‐rich fluidic elastomer actuators embedded with paper electronics,’ Advanced Intelligent Systems (2020). DOI: 10.1002/aisy.202000025