  A tunable stiffness bone rod was designed, optimized, and 3D printed to
address the common shortcomings of existing bone rods in the healing of long
fractured bones. The common deficiencies of existing bone fixations are high
stiffness, thereby negligible flexibility in deformation for best bone growth
results, and stress-shielding effect. Our novel design framework provides the
surgeons with ready-for-3D-printing patient-specific designs, optimized to have
desired force-displacement response with a stopping mechanism for preventing
further deformation under higher than usual loads such as falling. The
framework is a design optimization based on the multi-objective genetic
algorithm (GA) optimization to quantify the objectives, tunning the varied
stiffness while minimizing the maximum Mises stress of the model to avoid
plastic and permanent deformation of the bone rod. The optimum design
computational framework of tunable stiffness material presented in this paper
is not specific for a tibia bone rod. It can be used for any application where
bilinear stiffness is desirable.
