From Problem to Solution: Bio-inspired 3D Printing for Bonding Soft and Rigid Material via Underextrusion

Abstract Vertebrate animals benefit from a combination of rigidity for structural support and softness for adaptation. Similarly, integratingrigidity and softness can enhance the versatility of soft robotics. However, the challenges associated with creating durablebonding interfaces between soft and rigid materials have limited the development of hybrid robots. Existing solutions requirespecialized machinery, such as polyjet 3D printers, which are not commonly available. In response to these challenges, wehave developed a 3D printing technique that can be used with almost all commercially available FDM printers. This techniqueleverages the common issue of underextrusion to create a strong bond between soft and rigid materials. Underextrusiongenerates a porous structure, similar to fibrous connective tissues, that provides a robust interface with the rigid part throughlayer fusion, while the porosity enables interlocking with the soft material. Our experiments demonstrated that this methodoutperforms conventional adhesives commonly used in soft robotics, achieving nearly 200% of the bonding strength in bothlap shear and peeling tests. Additionally, we investigated how different porosity levels affect bonding strength. We testedthe technique under pressure scenarios critical to soft and hybrid robots and achieved three times more pressure than thecurrent adhesion solution. Finally, we fabricated various hybrid robots using this technique to demonstrate the wide range ofcapabilities this approach and hybridity can bring to soft robotics.