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

加法制造(AM)具有与几何形状同时创建零部件材料的独特能力，这是传统减法制造技术无法实现的。因此，这打开了机会，以改变材料的生产方式，从而改变固有的材料性能，因为组件正在建造。这种在制造过程中依赖时间的设计形式，也称为用于添加制造的时间设计(TDfAM)或有时称为4D打印，具有为设计者提供新的设计自由度的潜力；能够在同质部件中引入材料或表面属性的异质性。AM还使得能够制造否则不可能制造的几何图形，例如：晶格。这是AM通过使用不同的晶格拓扑为设计者提供对材料属性的更多控制的另一种方式。晶格单胞拓扑不仅影响相对密度和整体刚度等特性，而且不同晶格拓扑的规范增加了对表面积与体积比的控制，如果考虑到退化，这是一个重要的特征。因此，将不同的晶格设计与生物可吸收材料相结合可以在降解过程中导致不同的硬度随时间的变化，从而产生在使用期间依赖于时间的设计。这种类型的附加控制和随时间变化的特性可以在需要依赖时间的响应但手动启动这种变化是困难或不可能的领域中看到许多应用，例如在生物医学植入物中。在骨折固定的情况下，最佳的僵硬要求随着时间的推移而变化：骨折后立即需要稳定性，但大约6周后，骨折间的运动控制着愈合的效果。在这种情况下，改变固定的顺应性可以通过使骨折部位周围随时间变化的微动来提供增强的愈合。使用目前的方法，如果不需要多次手术，随着时间的推移，不可能满足这种不断变化和相互矛盾的设计要求，这将比它的价值更具伤害性。设计一种满足不同刚度要求的具有随时间变化的刚度的固定解决方案将解决这些问题。在本项目中，这是通过设计和建模由可生物吸收材料(如锌)附加制造的格子结构来实现的，并以骨折固定在胫骨高位截骨中的应用为例进行研究。该项目通过实验研究了TDfAM(在构建过程中改变工艺参数)可能对材料和表面特性进行额外控制的可能性，并通过使用设计、有限元建模和计算验证，将生物可吸收晶格作为一种媒介来研究骨折内固定中的刚度随时间变化的设计。

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.