Fabricating High-Performance Biomedical PLLA/PVDF Blend Micro Bone Screws through in situ Structuring of Oriented PVDF Submicron Fibers in Microinjection Molding

Poly(L-lactide) (PLLA) is regarded as a polymer with excellent biocompatibility, but its inherent brittleness property greatly restricts its application in the biomedical engineering field. Blending PLLA with other polymers is one of the sufficiently viable methods of property improvement. In this paper, the Poly(L-lactide) (PLLA)/Polyvinylidene fluoride (PVDF) blend was first prepared by melt-compounding, and then microinjection molded into the blend micro bone screw with high strength and toughness. The experimental results show that the PVDF dispersed phases could in situ form the highly oriented fibers with shish-kebab structures under the effect of strong shear stress field of microinjection molding along flow direction. These parallelly aligning PVDF fibers could simultaneously realize the significant enhancement in both toughness and strength of the micro bone screw. Specifically, in the bending test the blend micro bone screw with oriented PVDF fibers can withstand the compression displacement of nearly 2 mm, while the neat PLLA one would break within only 0.5 mm displacement. In addition, a bending recovery phenomenon was observed in the load-displacement curve of the blend, demonstrating the mechanical transition from brittle fracture to ductile fracture. Moreover, with addition of 30% PVDF, the elongation at break increases from 7.8% to 57.8%, while the tensile strength increases from 60.9 MPa to 74.3 MPa. The PLLA/PVDF micro screws possess better toughness and excellent biocompatibility compared to pure PLLA products. They also exhibit great potential for stimulating the proliferation of cells through piezoelectric output, opening up new possibilities for the development of next-generation fracture fixation materials.