“Although nature does not necessarily provide the optimal form, it still outperforms many artificial systems and offers valuable insights for designing functional machines based on elegant principles,” said Josie Hughes, head of the Computational Robot Design and Fabrication Lab at EPFL, known as the CREATE Lab. Nature-inspired approaches are what lie at the heart of the exciting work in bio-inspired robotics that takes advantage of nothing but the exoskeleton of the carcass of langoustine’s abdomen.
The langoustine shell consists of a complex composite of chitinous mineralized plates and flexible joint membranes, which has been refined over millions of years for aquatic agility. Its anisotropic stiffness, which can be easily bent against resisting extension, lends the creature to high torque and fast aquatic motions. CREATE Lab engineers immediately recognized that this capability can be used for direct biological inspiration, robotics actuation, without the need for attempting its recreation in man-made materials.
They cleaned, processed, and modified the shells by suturing them together with thermoplastic polyurethane (TPU) elastomers through the dorsal surface to emulate spring restorations. Inextensible braided fishing lines were inserted into the ventral parts of these shells to model tendons, which were carefully planned to enable varied bending according to anchor points and pulleys. This tendon-controlled continuum-robot design enabled progressive bending for enveloping grips or bending the middle parts for precision grips.
The augmented exoskeletons stood mounted on motorized bases and had the strength to lift as much as 500 g objects, which was in excess of their own weight by more than 200 times and had the capacity to move at elbow bends up to 8 Hz. As exoskeleton grips, the shells modified in a passive manner for objects including tomatoes and pens. For water transportation, the exoskeleton “fins” propelled the underwater robot at speeds of up to 11 cm/s in the pool tests.
The need for durability, typically not seen in necrobots, has been met with silicone coatings. Without protection, the membranes became stiff after approximately five hours because of dehydration. Ecoflex coatings prolonged this to 25 hours, and Dragon Skin to 39 hours. These rubber coatings, commonly employed in soft robotics, provided protection against water and could withstand mechanical stress without affecting flexibility.
The sustainable fact of this project has to do with its cyclic approach. This system is biodegradable, while the synthetic parts of the system are easily removable and reusable due to their mechanical connectors in contrast to other robots that are either glued or welded. The raw material cost of exoskeletons, which are nothing but trash generated from deceased langoustines, amounts to nothing in areas where langoustines are consumed.
From the materials science viewpoint, the research is relevant to the development of biodegradable elastomers and hydrogels. Although the crustacean shells have slow biodegradation rates, the elastomeric component can be substituted with biologically sourced polymers like poly(glycerol sebacate), referred to as PGS, that can be composted faster. Adding these components would also be compliant with ISO 17088 norms of industrially compostable materials. Moreover, this would ensure prevailing norms against the disposal of these components at the end of their life.
Biological variability is a technical challenge. No two langoustine tails will ever have exactly the same geometry, meaning that there will be small asymmetries between the motion of the grippers. While this can possibly be dealt with via adaptive augmentation or dynamic control systems, existing software that models the kinematics of the segmented shell can probably also be applied towards automatching exoskeletons based on their actual values for described parameters.
What the prototypes of CREATE Lab have shown is that bio-hybrid robots can have high-quality manipulation and locomotion capabilities while incorporating sustainability principles into their design. In fact, by utilizing the evolved exoskeletons of crustaceans and collaborating on biodegradable synthetic actuators, they have moved necrobots not only beyond the bounds of gadgets but even beyond those of practical engineering. Moreover, they have revealed a whole design landscape comprising different arthropod shells that feature different bearing capabilities and joint structures and may thus be used in a variety of applications ranging from medical implants to environmental monitoring networks that will not leave any trace upon completion of their tasks.
