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

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.

The proposed FG-LIFT manufacturing solution will be transformative as a highly flexible micro-additive 3D printing process for micro-actuators. Our driver is the micro-scale actuation requirements in medical devices, including micro-robotics, which have great potential for evaluation and treatment of medical conditions.

The proposal builds on strengths of the UK in industrial lasers and their application to laser-driven manufacturing processes. Aligning with both the Productivity and Health priorities of EPSRC, the FG-LIFT process that will be developed provides a route to functional grading of materials at the microscale, with key initial applications being in medical devices. It will have significant impact across a broad range of stakeholders from manufacturing researchers right through to patients who will benefit from the micro-robotic measurement and treatment that will be enabled by the technology.

Researchers working with laser-based manufacturing will be provided with a novel 3D manufacturing approach that can be developed for other application areas, including in combination with other manufacturing processes. The degree of control that will be offered by this new process will far exceed anything that is currently available at this micro-scale.

The medical device industry will be provided with a new manufacturing technique suitable for a whole family of devices based on the principle of micro-actuation. These devices will be used by clinicians e.g. for detection and analysis of cancerous tissue, and "unpacking" structures in-situ; with consequent benefit to patients.