Designing Fluid-Exuding Cartilage for Biomimetic Robots Mimicking Human Joint Lubrication Function
Akihiro Miki, Yuta Sahara, Kazuhiro Miyama, Shunnosuke Yoshimura, Yoshimoto Ribayashi, Shun Hasegawa, Kento Kawaharazuka, Kei Okada, Masayuki Inaba
TL;DR
The paper addresses high friction in open-type robotic joints and proposes a low-cost, bio-inspired solution: fluid-exuding cartilage created via 3D-printed rubber sheets with embedded absorbents that release synovial fluid under load. It validates the concept through a planar friction test and then demonstrates a curved cartilage integrated into an open-type ball joint with bones, ligaments, synovial fluid, and a joint capsule. The best observed friction reduction reaches $bc \,\approx\$ $0.053$ under optimized conditions, highlighting the approach's potential to approach bearing-like lubrication while remaining printable and adaptable. This work opens a path toward liquid-enabled, soft, open joints in biomimetic robots and motivates future refinements in ligaments, capsules, and active actuation for broader motion ranges.
Abstract
The human joint is an open-type joint composed of bones, cartilage, ligaments, synovial fluid, and joint capsule, having advantages of flexibility and impact resistance. However, replicating this structure in robots introduces friction challenges due to the absence of bearings. To address this, our study focuses on mimicking the fluid-exuding function of human cartilage. We employ a rubber-based 3D printing technique combined with absorbent materials to create a versatile and easily designed cartilage sheet for biomimetic robots. We evaluate both the fluid-exuding function and friction coefficient of the fabricated flat cartilage sheet. Furthermore, we practically create a piece of curved cartilage and an open-type biomimetic ball joint in combination with bones, ligaments, synovial fluid, and joint capsule to demonstrate the utility of the proposed cartilage sheet in the construction of such joints.