Fatigue and cyclic loading of 3D printed soft polymers for orthopedic applications

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Miller, Andrew Todd
Guldberg, Robert E.
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Processing-structure-property relationships were developed for several popular, soft biomedical polymers under fatigue loading to assist in the use and success of such polymers in load-bearing orthopedic applications. Results demonstrated that materials with sufficient energy dissipation at the testing temperature, such as polycarbonate urethane (PCU), resisted fatigue fracture under cyclic compressive loading however also demonstrated cyclic ratcheting. Bovine meniscal tissue demonstrated similar ratcheting behavior with a lack of fatigue fracture. Further studies on PCU demonstrated an increase in monotonic stiffness, shear failure stress, and improvements in tensile fatigue from a stress-based standpoint with increasing hard segment content. Effects of hard segment content on tensile failure strain, and strain-based fatigue performance, were more complex and seemingly influenced by interaction between phases. Results demonstrated that the fused deposition modeling (FDM) printing process is a very effective processing method for PCU as printed samples matched or exceeded injection molded samples in terms of monotonic tension, compression, shear, and tensile fatigue performance. Lastly, the effects of printed architecture on fatigue performance were examined for FDM PCU as well as an elastomeric polyurethane (EPU) printed via continuous liquid interface production. Results indicated that both materials were relatively tolerant of architectures and notches. Introduction of porosity led to a decrease in tensile failure stress based on gross area as expected, without a significant effect on tensile failure strain. Effects on fatigue performance were small and were found to be dependent on notch severity, with the largest effects in the high cycle regime.
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