3D printing of micro-scale graded shape memory components for in-vivo actuated medical devices

微型机器人在医疗条件评估和治疗方面具有巨大的潜力。这种装置需要在微尺度上高度受控的驱动，以提供受控运动、组织顺应性测试、活组织检查等，这是功能梯度形状记忆合金(SMA)提供的前景。形状记忆合金具有“记住”其原始形状的能力，当变形时，加热时会返回到其变形前的形状。这类合金自问世以来就引起了人们的极大兴趣。SMA的功能分级(即局部修改材料的属性以适应设备不同部分的SMA效应)允许设计更复杂的、因此更可控的执行机构。由功能梯度SMA制造的器件和组件可以响应外部刺激(应力或温度变化，例如通过感应加热)提供驱动，在工作输出密度方面优于电磁铁或电动马达等传统驱动机制。这种性能非常适合用于微创医疗应用的微型设备，如精确的切割、组织识别、疾病的触觉传感和镊子，以及更雄心勃勃的形状转换，用于原位的“拆包”结构和“智能”支架和贴片。这里的制造挑战是通过结合局部定制材料成分和热历史来实现微观规模的功能分级。这将通过开发一种新的工艺--功能梯度激光诱导前向转移(FG-LIFT)来实现。这一过程将使用多轨道的“施主色带”(类似于彩色打字机色带)将不同金属(例如钛、镍和铜)的“亚体素”(典型尺寸为几微米宽、数百纳米高)沉积到目标衬底上，以构建每个体素，每个体素由多个不同金属的亚体素层组成。通过改变激光参数，随后的热处理将用于控制体素内部和体素之间的互扩散，从而提供对成分的非常严格的局部控制。因此，将通过继续添加额外的体素来构建3D微结构。这种FG-Lift工艺将用于制造亚毫米和毫米尺度的形状记忆合金部件，其功能分级为10‘S微米。这一极具挑战性的概念需要在微尺度上对材料成分和热处理进行3D控制。通过将功能分级的SMA材料沉积到具有适当材料特性的基板上(例如碳纤维垫或痕量加热器)，将实现对设备整体性能的额外定制。

Micro-robots have great potential for evaluation and treatment of medical conditions. Such devices require highly controlled actuation at a micro-scale to provide controlled motion, testing of tissue compliance, biopsy, etc, and this is a prospect offered by functionally-graded shape memory alloys (SMAs). An SMA has the ability to "remember" its original shape and that when deformed returns to its pre-deformed shape when heated. Such alloys have sparked great interest ever since their first development. Functional grading of SMAs (i.e. locally modifying the properties of the material to tailor the SMA effect in different parts of the device) allow the design of more complex and hence much more controllable actuation mechanisms. Devices and components manufactured from functionally graded SMAs can provide actuation in response to external stimulation (stress or temperature variation, e.g. via induction heating), outperforming conventional actuation mechanisms such as electromagnets or electrical motors in terms of work output density. Such performance is ideal for micro-devices for minimally invasive medical applications such as precise incision, tissue identification, tactile sensing for disease and tweezing, as well as more ambitious shape transformations for "unpacking" structures in situ and "intelligent" stents and patches.The manufacturing challenge here is to achieve that functional grading at a micro-scale, by a combination of locally tailoring the material composition and thermal history. This will be achieved via development of a novel process, functionally graded Laser Induced Forward Transfer (FG-LIFT). This process will use a multi-track 'donor ribbon' (rather like a multicoloured typewriter ribbon) to deposit "sub-voxels" (of typical dimensions a few microns across and hundreds of nm high) of different metals, e.g. Ti, Ni and Cu onto a target substrate, in order to construct voxels each consisting of a number of subvoxel layers of different metals. By altering the laser parameters, subsequent thermal treatment will be used to provide control of interdiffusion within and between voxels providing very tight localised control of composition. 3D microstructures will hence be constructed by continuing to add additional voxels. This FG-LIFT process will be used to manufacture sub-mm and mm-scale SMA components with functional grading at a scale of 10's of microns. This highly challenging concept requires 3D control - at the micro-scale - of both material composition and thermal treatment. By depositing the functionally graded SMA material onto substrates with appropriate material properties (e.g. carbon fibre mats or trace heaters), additional tailoring of the overall performance of the device will be achieved.