Exploring the biomedical significance of time-dependent design enabled by additive manufacturing

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Additive manufacture (AM) has the unique ability of creating the component material simultaneously with the geometry, something which is not possible with traditional subtractive manufacturing techniques. As such, this opens up the opportunity to vary how the material is produced, and therefore the intrinsic material properties, as the component is being built. This form of time-dependent design during manufacture, otherwise known as Temporal Design for Additive Manufacture (TDfAM) or sometimes called 4D printing, has the potential to afford a designer a new dimension of design freedom; being able to introduce heterogeneity of material or surface properties within a homogenous part.AM also enables the manufacture of geometries that would otherwise be impossible to manufacture, for example: lattices. This is another way in which AM has the ability to afford a designer increased control over material properties, through the use of different lattice topologies. Not only does a lattice unit cell topology influence characteristics such as relative density and overall stiffness, but specification of different lattice topologies gives increased control over surface area to volume ratio, an important characteristic if considering degradation. As such, combining different lattice designs with bioresorbable materials can result in different variations in stiffness over time during degradation, resulting in a design that is time dependent during service.This type of additional control and variation in properties over time could see many applications in areas where a time dependent response is required but manually actuating such a variation is difficult or impossible, such as in biomedical implants. In the case of fracture fixation, the optimum stiffness requirement varies over time: stability is required immediately after fracture but after approximately 6 weeks, interfragmentary motion governs the efficacy of healing. In this instance, changing the compliance of the fixation could offer enhanced healing, by enabling micromotion around the fracture site which changes over time. Using current methods, it is not possible to satisfy this changing and contradictory design requirement over time without the need for multiple surgeries, which would be more traumatic than its worth. Designing a fixation solution with tailored time-dependent stiffness that satisfies the varying stiffness requirements would solve these issues.In this project this is achieved through the design and modelling of lattice structures additively manufactured from bioresorbable materials, such as zinc, looking specifically at the application of fracture fixation in a high tibial osteotomy as a case study. This project experimentally investigates the potential additional control over material and surface characteristics that TDfAM (variation of process parameters during the build) can afford a designer in a biomedical context; and also investigates bioresorbable lattices as a vehicle to enable time dependent design of stiffness within fracture fixation, through the use of design, finite element modelling and computational validation.