*Article* **The Mechanics of Bioinspired Stiff-to-Compliant Multi-Material 3D-Printed Interfaces**

**Dolev Frenkel † , Eran Ginsbury † and Mirit Sharabi \***

Department of Mechanical Engineering and Mechatronics, Ariel University, Ariel 407000, Israel

**\*** Correspondence: miritsh@ariel.ac.il; Tel.: +972-3-6453159

† These authors contributed equally to this work.

**Abstract:** Complex interfaces that involve a combination of stiff and compliant materials are widely prevalent in nature. This combination creates a superior assemblage with strength and toughness. When combining two different materials with large stiffness variations, an interfacial stress concentration is created, decreasing the structural integrity and making the structure more prone to failure. However, nature frequently combines two dissimilar materials with different properties. Additive manufacturing (AM) and 3D printing have revolutionized our engineering capabilities concerning the combination of stiff and compliant materials. The emergence of multi-material 3D-printing technologies has allowed the design of complex interfaces with combined strength and toughness, which is often challenging to achieve in homogeneous materials. Herein, we combined commercial 3D-printed stiff (PETG) and compliant (TPU) polymers using simple and bioinspired interfaces using a fused deposition modeling (FDM) printer and characterized the mechanical behaviors of the interfaces. Furthermore, we examined how the different structural parameters, such as the printing resolution (RES) and horizontal overlap distance (HOD), affect the mechanical properties. We found that the bioinspired interfaces significantly increased the strain, toughness, and tensile modulus compared with the simple interface. Furthermore, the more refined printing resolution elevated the yield stress, while the increased overlap distance mostly elevated the strain and toughness. Additionally, 3D printing allows the fabrication of other complex designs in the stiff and compliant material interface, allowing various tailor-designed and bioinspired interfaces. The importance of these bioinspired interfaces can be manifested in the biomedical and robotic fields and through interface combinations.

**Keywords:** interface; bioinspired materials; stiffness; strength; biomimetics; 3D printing; mechanical testing; stiff-to-compliant
