Designing Fluid-Exuding Cartilage for Biomimetic Robots Mimicking Human Joint Lubrication Function

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.

• The paper introduces a novel 3D printing technique with embedded absorbent materials to mimic fluid-exuding cartilage for improved joint lubrication.
• The methodology effectively reduces friction by exuding synthetic synovial fluid under load, validated by custom friction coefficient experiments.
• The assembled biomimetic ball joint demonstrates smooth operation and offers promising potential for advanced robotic and prosthetic applications.

Exploring Biomimetic Ball Joint Fabrication with Fluid-Exuding Cartilage for Robotic Applications

Introduction

The emulation of human joint flexibility and low friction in robotics has been a long-standing challenge, primarily due to the intricate nature of human joints which possess soft constraints, shock resistance, and adaptability. In this context, this paper introduces a novel approach by mimicking the fluid-exuding function of human cartilage using a rubber-based 3D printing technique combined with absorbent materials. The study evaluates the friction coefficient and fluid exudation function of a fabricated flat cartilage sheet and further demonstrates the practical assembly of a piece of curved cartilage and an open-type biomimetic ball joint.

Cartilage Mimicry

Human articular cartilage, mainly composed of extracellular matrix, collagen fibers, and proteoglycans, absorbs impacts and ensures smooth bone movement by exuding synovial fluid under load. The research leverages these biological principles by employing absorbent materials such as PVA (polyvinyl alcohol) sponges inserted into a rubber matrix to replicate the fluid exudation behavior. The fluid, mimicking synovial fluid, is effectively exuded under mechanical load, enhancing lubrication and reducing friction between contact surfaces.

Fabrication Techniques and Preliminary Experiments

The cartilage replication process involved 3D printing a planar sheet composed of rubber material with embedded PVA sponges. Preliminary experiments assessing the fluid-exudation under load conditions confirmed the effective release of synthetic synovial fluid from the sponge inserts upon applying pressure, with notable differences in exudation efficiency between various absorbent material types and configurations.

Friction Coefficient Measurement Experiments

Friction testing, conducted using a custom-built friction testing apparatus, aimed to quantify the effectiveness of the synthesized cartilage sheet in mimicking the low friction characteristics of natural human cartilage. The experiments demonstrated a significant reduction in the friction coefficient with the introduction of the fluid-exudation mechanism, especially when combined with a low-friction PTFE film and when the contact surface was submerged in synthetic synovial fluid.

Open-Type Ball Joint Construction

Beyond the flat cartilage sheet, the study extended to fabricating a curved cartilage piece and integrating it into a composite open-type ball joint structure comprising artificial bones, ligaments, synovial fluid, and a joint capsule. This construction aimed to validate the application of the developed cartilage fabric in a functional joint assembly. Despite the limitations in achieving the full range of motion observed in natural human joints, the assembled ball joint exhibited smooth operation, highlighting the potential of the fluid-exudation cartilage in robotic applications.

Conclusions and Future Directions

The findings from this research underscore the feasibility of developing biomimetic cartilage with a fluid-exudation mechanism for use in robotics, achieving lower friction coefficients and emulating the lubrication function intrinsic to human joints. However, challenges such as optimizing the range of motion in the assembled joints and refining the fabrication methods for better mimicry of natural cartilage's microstructure remain. Future efforts will focus on further experimentation with different materials, exploring 3D printing of spherical cartilage, and enhancing the biomimetic properties of the developed joints for broader robotic applications.

This innovative approach opens new avenues in the design and fabrication of robotic joints that better emulate the mechanical and tribological sophistication of human joints, with potential implications for both industrial robotics and prosthetics.