Researchers 3D Printed Soft Walking “Insect-like” Robots

Researchers at the University of California, San Diego have used 3D printing technology to produce soft and flexible walking “insect-like” robots.

The cost-effective additive manufacturing technology for manufacturing robots can reduce the entry cost of 3D printing soft robots and open new applications of the technology in places that are not safe for humans (for example, navigating in disasters or theaters).

“We hope these flexible skeletons will lead to the creation of a new class of biologically-inspired soft robots,” said Nick Graves, a professor of mechanical engineering at the Jacobs School of Engineering at the University of California, San Diego. “We want to make soft robots easier to build for researchers around the world.”

Make 3D printed soft robots more accessible

The researchers say that one of the main challenges in creating insect-inspired robots is to reshape the complexity of the mechanics of exoskeletons. The shell needs to serve a variety of functions, including structural support, joint flexibility, and body protection, while providing functional surface features for sensing, gripping, and adhesion.

What the San Diego research team observed is that the limb mobility of insects is determined by the arrangement of rigid, flexible and graded rigid elements, while the exoskeleton of insects is a hybrid structure of rigid and flexible mechanical parts. Therefore, future iterations will require a hybrid construction method to better reflect the insect models on which they are based.

Previous attempts to create insect-inspired robots required the use of multi-material 3D printers and multi-step casting processes. Scientists at the University of Rochester, for example, created jumping robot insects inspired by water riders in 2015. However, according to San Diego researchers, this bio-inspired robot looks more like a rigid industrial robot, including a stiff link and a rigid high-ratio motor. Recently, robot experts have started to intentionally use multi-material 3D printing, laser cutting, lamination, and die casting to incorporate the adaptability of the body and limbs into the robot design. These manufacturing techniques also have disadvantages because they often come at the cost of obtaining expensive and time-consuming manufacturing tools that provide limited material options.

To enable them to 3D print flexible and elastic exoskeletons in a more economical way, the research team designed a novel hybrid method called flexible bone printing. The use of fused deposition modeling (FDM) 3D printers and standard filament materials (such as acrylonitrile butadiene styrene (ABS)) makes this method cheaper and more readily available. In addition, by directly printing the 3D rigid wire directly onto the heated thermoplastic film, the new technology is different from the traditional method to soft robot manufacturing. This method provides a flexible and strong base layer for the deposited material, and can accurately control the stiffness and characteristics of the joints and pillars in the robot.

The flexoskeleton manufacturing technique is inspired by the outer shells of insects. Image via Soft Robotics.

3D printing “insect-like” soft robot

In standard FDM printing, plastic filaments (such as ABS or polylactic acid (PLA)) are extruded through the orifice of a heated nozzle and deposited on a flat printing surface. On the other hand, the flexible skeleton process uses a modified Prusa i3 MK3S or LulzBot Taz 6 FDM 3D printer to deposit filaments directly onto the heated thermoplastic base layer. This results in high bond strength between the deposited material and the non-extensible flexible substrate, thereby improving fatigue resistance. The bonding process of the flexible skeleton printing also does not require additional adhesives or curing agents, because the filaments are directly bonded to the base layer during the extrusion process.

To test the strength and fatigue resistance of the components produced, the team manufactured flexible beams with a uniform rectangular geometry. Each beam is bent to a constant stress state and held in this position for 10 seconds to simulate the situation where the robot legs are bent and fixed in place to support the load. The research team then measured the creep angle of the beam by taking an image of the unloaded beam deflection angle, which was measured from a neutral position before the test. By adding a polycarbonate (PC) layer, the San Diego researchers found that they were able to reduce the creep deformation of the 3D printing beam by 70% in 300 load cycles.

To demonstrate the walking capabilities of robots produced using flexible bone production methods, the team built a four-legged walking robot driven by tendons. The limbs produced with fully flexible bones are designed and assembled, and the robot’s chassis is driven by four miniature servos. The length of the two legs is 70 mm, each leg takes 30 minutes for 3D printing, and has two joints: one bend and one stretch. After the production of the entire robot is completed (it takes about three hours), insert each limb into the robot chassis (main body) and connect it to a micro-servo via a tendon and winch. The final product has interchangeable legs, which are designed for different terrains, and during the test, the completed robot can reach a speed of nearly 5 cm per second.

The external structure of the flexoskelton robot not only protects its internal components, but also mixes soft and rigid components so that it can be produced in a complex 3D layout. According to Gravish, the innovative production technology used to create robots may allow the production of new multifunctional robots for use in factory environments.

“The ultimate goal is to create an assembly line that can print the entire flexible skeleton robot without manual assembly. A small group of such small robots can complete the work of a large robot alone, or even more.” Gravish said.

The construction process of the flexoskeleton robot, which took around six hours to complete. Image via Soft Robotics.

Soft robotics in 3D printing

In recent years, research on the additive manufacturing of soft robots has taken various forms. For example, in January 2020, researchers at Cornell University developed 3D printed soft robot muscles that have the ability to “sweat”. Using a hydrogel-based composite resin and stereolithography (SLA), a soft finger-like actuator that can retain water and respond to temperature is produced.

In August 2019, researchers at the Delft University of Technology (TU Delft) in the Netherlands created multi-color 3D printed sensors to help soft robots improve their self-awareness and adaptability. By innovating a flexible embedded 3D printing sensing method, researchers have increased the interaction between robots and objects.

A pair of NASA researchers announced that they have used 3D printing technology in 3D printing to produce soft robot actuators in May 2019. The new components responsible for animating and controlling the moving parts of the robot represent an important step in bringing soft robot technology into space.

The research results of the researchers are detailed in the article “Flexible Skeleton Printing Makes Multifunctional Manufacturing of Multifunctional Soft and Rigid Robots” published in the “Soft Robot” magazine on April 7, 2020. The study was co-authored by Jiang Mingsong, Zhou Ziyi and Nicholas Gravish.

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