Fishes, especially neoteleost species with anosteocytic bone such as pike (Esox lucius), tilapia (Oreochromus aureus), and mackerel (Scomber scombra), swim continuously without their bones breaking, as they exhibit remarkable resistance to fatigue 1. The fin rays are important for fish locomotion, transmitting movement forces critical for manoeuvring and stabilizing the body in the water 2. Therefore, it is important to understand the fatigue response of fin rays and design bioinspired engineered-materials to ensure that they are durable and able to withstand the dynamic mechanical forces for a long period of time 3. The aim of this study was to develop fatigue resistant nano- and micro-sized structures by combining melt electrowriting (MEW) and electrospinning technologies, bioinspired by the ray fin microstructure. The MEW technique is a solvent-free additive manufacturing method that uses electrohydrodynamics to produce micron-scale fiber struts with highly ordered pore size and interconnected porous structure 4,5. In the present study, we first fabricated highly ordered MEW scaffolds using polycaprolactone (PCL), which is gold standard thermoplastic polyester for MEW 5, with a 0-90° orientation. Subsequently, the surfaces of PCL-MEW scaffolds were modified through alkaline surface treatment and APTES surface modification to enhance the adhesion of PCL-gelatin (GEL) electrospun nanofibers layers on top of the PCL-MEW scaffolds. Finally, the hierarchical scaffolds were immersed at various time points into a hydroxyapatite (HAP) solution to design engineered scaffolds bioinspired by fishbone microstructure. The scanning electron microscopy (SEM) results revealed that MEW scaffolds with 12 layers were successfully fabricated, and all the fibers were deposited homogeneously layer by layer. Also, SEM images showed that the layer of electrospun nanofiber layers was bead-free and uniformly superimposed on the PCL-MEW scaffolds. Additionally, after the electrospinning process, no layer delamination was observed. Furthermore, Fourier transform infrared (FTIR) spectroscopy confirmed the presence of HAP, PCL and GEL. Additionally, preliminary quasistatic nanoindentation and uniaxial tensile tests were performed on strips cut from the neat PCL-MEW scaffolds. PCL-MEW scaffolds showed a reduced elastic modulus of 1.1 GPa and a hardness of 70 MPa. These findings suggest that integrating organic and inorganic components into MEW scaffolds offers a promising strategy for enhancing fatigue resistance under various loading conditions. This approach highlights the potential of MEW technology in developing advanced engineered-materials for fatigue response studies.

Keywords: melt electrowriting, electrospinning, hydroxyapatite, fishbone, fatigue properties

References
1. Ofer L, Dean MN, Zaslansky P, et al. A novel nonosteocytic regulatory mechanism of bone modeling. PLoS Biol. 2019. doi:10.1371/journal.pbio.3000140
2. Lauder G V., Madden PGA, Tangorra JL, Anderson E, Baker T V. Bioinspiration from fish for smart material design and function. Smart Mater Struct. 2011. doi:10.1088/0964-1726/20/9/094014
3. Teoh SH. Fatigue of biomaterials: A review. Int J Fatigue. 2000. doi:10.1016/S0142-1123(00)00052-9
4. Dalton PD. Melt electrowriting with additive manufacturing principles. Curr Opin Biomed Eng. 2017. doi:10.1016/j.cobme.2017.05.007
5. Unalan I, Occhipinti I, Miola M, Vernè E, Boccaccini AR. Development of Super‐Paramagnetic Iron Oxide Nanoparticle Coated Melt Electrowritten Scaffolds for Biomedical Applications. Macromol Biosci. 2023:2300397.