3D printing with sub-micron resolution. Dr Cameron Chai from AXT takes us on this fantastic journey
In the October/November issue of AMT last year, AXT described Projection Micro Stereolithography (PµSL), allowing 3D printing resolution to go beyond the limits of ~300µm of FDM techniques, using resin-based printing technology. Using PµSL, it is possible to achieve resolutions of just 2µm. But what if you want to print features in the nanometer range? The solution here lies in two-photon polymerisation (2PP), another resin-based printing technology that uses light curing or polymerisation.
In microfabrication, systems like the NanoOne from UpNano set the standard, and printing features down to 170nm. The NanoOne is ideal for fabricating parts with complex geometries for academic research and industrial applications as engineers continue to push the bounds of miniaturisation. Contrary to popular belief, 2PP is a fast printing process, and producing parts using 2PP can also be done at a reasonable price.
How 2PP printing works
2PP printing technology takes advantage of the spatial selectivity of 2-photon absorption (2PA), i.e. inducing polymerisation at the point where the two photons converge, enabling 3D structures to be directly created in a predetermined volume with a spatial resolution better than 1µm. The higher spatial resolution is possible because 2PA significantly reduces beyond the focus point. This minimises the fluorescence volume, culminating in higher spatial resolution.
The benefit becomes obvious when single and multi-photon beam paths are compared using fluorescent microscopy. As can be seen in the figure, 2PA only occurs at the focal point of the beam, meaning that monomer crosslinking takes place in this plane. Conversely, in the case of single-photon absorption (per other techniques such as stereolithography), the emitted light is absorbed along the whole beam, leading to a reduction in resolution.
This method allows printing directly within the resin volume. As such it avoids the need for repeated material coating that is required for SLA (stereolithography) processes. These processes can induce stresses on fine structures during the printing process.
Balancing speed and resolution: Adaptive resolution
The UpNano NanoOne takes things further using a patented process that adapts the printing resolution to cater for the required resolution. The software compensates and adapts the voxel size accordingly by classifying the required resolution as high or low. The system automatically enlarges the laser focal spot for low-resolution regions, thus increasing the print speed. Conversely, it reduces the focal spot size for high-resolution areas, allowing it to achieve resolutions in the nanometer size range. In either case, there is no effect on mechanical properties.
Materials
There are now many resins compatible with 2PP 3D printers that are optimised for properties such as resolution, optical transparency (for optical and microfluidic applications), biocompatible formulations for biomedical research, high-performance formulations for ultimate strength or temperature capabilities, and even silica ceramic formulations for producing fused quartz micro components.
Applications
2PP is ideally suited to the microfabrication of high-precision parts that incorporate features in the sub-micron realm. Systems like the NanoOne can print at >450mm3/hr, producing components up to 42x42x42mm with a surface finish that rivals injection moulding.
While the output is limited only by the operator’s imagination, examples of what can be produced include:
- Extracellular matrices
- Lab-on-chip devices
- Optoelectronics
- Photonic devices
- Micro needles
- Filter elements
- Micromechanical devices
- Microfluidic devices
- Microlenses
