We present a method for multiple length scale structural optimisation. We first optimise isotropic microstructures for maximum bulk modulus at five solid fractions. Shape interpolation between these optimised microstructures produces a continuous set that smoothly varies in both geometry and mechanical properties. This smooth set is used for macroscopic optimisation via the material distribution method. The approach is computationally efficient and the geometric smoothness makes it clear how the microstructures can be transitioned between neighbouring elements. Performance comparisons are made to traditional structural optimisation for some example compliance optimisation problems. The interpolated microstructure designs are most advantageous for two dimensional problems involving multiple loading cases. In these cases, intermediate densities are utilised to more effectively distribute the load. In three dimensions, the method would be useful for a number of applications where specific microstructural requirements, such as a connected pore space, are needed within a multiple-scale design.

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We present a new approach to designing three-dimensional, physically realizable porous femoral implants with spatially varying microstructures and effective material properties. We optimize over a simplified design domain to reduce shear stress at the bone-prosthetic interface with a constraint on the bone resorption measured using strain energy. This combination of objective and constraint aims to reduce implant failure and allows a detailed study of the implant designs obtained with a range of microstructure sets and parameters. The microstructure sets are either specified directly or constructed using shape interpolation between a finite number of microstructures optimized for multifunctional characteristics. We demonstrate that designs using varying microstructures outperform designs with a homogeneous microstructure for this femoral implant problem. Further, the choice of microstructure set has an impact on the objective values achieved and on the optimized implant designs. A proof-of-concept metal prototype fabricated via selective laser melting (SLM) demonstrates the manufacturability of designs obtained with our approach.

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