In this interview conducted by Nanoscribe, Prof. Bastian E. Rapp, co-founder and CTO, Glassomer, and Dr Alexander Quick, head of materials, Nanoscribe, share insights on developing a photoresin and process for the 3D printing of filigree glass parts.
Prof. Bastian E. Rapp, co-founder and CTO, Glassomer.
Dr Alexander Quick, head of materials, Nanoscribe.
When it comes to glass manufacturing, it is not only images of glowing molten glass and the subsequent mechanical forming or machining processes that dominate but also of the chemical etching process required for its structuring. Structuring glass is still a real challenge. This is especially true for microstructures, where classical glass processing reaches its limits. With Prof. Bastian E. Rapp and Dr Alexander Quick, we talk about a co-developed liquid polymer-silica nanocomposite and two-photon polymerisation (2PP)-based 3D printing process for creating glass microstructures and parts at room temperature.
How did you come up with the initial idea of developing liquid glass?
Bastian Rapp: For most of my career, I have been particularly interested in the combination of materials and processes and, quite naturally, 3D printing was a topic that fascinated me from the beginning. I have been working for some time on materials that can be 3D printed at high resolution if required, for example, in microsystem technology, optics or microfluidics. For many of these applications, we need materials that are very resilient mechanically, thermally and chemically. Of course, glass was one of the first materials we took a closer look at. However, glass is difficult to work with, especially on such a small scale. We researched and developed for quite a while until we found a suitable methodology for glass structuring. The polymer-silica nanocomposite route was the most promising but also one of the most challenging. Fortunately, we were still quite ignorant about this at that early stage.
What are the challenges and difficulties in glass manufacturing, and what is special about the processing of the nanocomposite?
Bastian Rapp: In short, with the nanocomposite approach, glass can be structured like a polymer. The term ‘Glassomer’ goes back to precisely this fact: you structure glass but pretend that it is actually just a polymer that you are processing. The approach is very similar to building a sandcastle. You take glass in its smallest form, like tiny grains of sand, and bind them together, in our case, with small amounts of a polymer. This creates a free-flowing liquid that essentially turns into glass after curing and post-processing. Until you have fixed the final shape, you work with a polymer during fabrication and thus with polymer processing techniques. Once you have defined the shape, the polymer-silica nanocomposite undergoes a post-treatment that removes the polymer and leaves only the silica particles. In the final step, you fuse them together to get a dense piece of glass. The name of this polymer-silica nanocomposite is GP-Silica, and it was jointly developed with Nanoscribe for the 2PP-based 3D printing process in the OptoGlass3D research project.
Can you explain in more detail what GP-Silica consists of, and what is advantageous about it in terms of processing?
Bastian Rapp: The photoresin’s formulation consists of an organic binder, which essentially is a polymer that can be cured to a solid, cross-linked piece of plastic when exposed to light. Within this organic binder, we suspend a large amount of silica nanoparticles. The nanoparticles are so small that they remain in suspension; the gravitational forces are too small to let them sink. So, essentially you have a clear liquid with a large amount of tiny sand particles in it.
Alexander Quick: And to make our new photoresin GP-Silica perfectly suited to the high-precision 2PP-based 3D printing process, it contains this special suspension and a few other ingredients. The liquid photoresin contains ‘initiating’ components that effectively trigger the curing reaction. Curing itself, similar to other Nanoscribe IP photoresins, is achieved by radical polymerisation, a well-established process in 2PP. The printed green part is a composite material consisting of the polymer and silica nanoparticles, which is processed into fused silica glass in a final, thermal post-treatment. In a nutshell, we use the processing advantages of polymer technologies to create glass structures.
What is the story behind the development of the 2PP-based 3D printing process, and why is it so well suited for the fabrication of glass microstructures?
Alexander Quick: We are both former members of the KIT (Karlsruhe Institute of Technology) community, have known each other for a long time and have a strong track record for working together successfully. After Glassomer was founded, we specifically wanted to explore the potential of combining our technologies. Through the OptoGlass3D project, which is funded by the ATTRACT project, we jointly developed the 2PP-based 3D printing process. An article detailing the success of the project was published in the journal Advanced Materials at the beginning of 2021, which showed us there is significant interest in this fundamentally new manufacturing opportunity.
Bastian Rapp: Glass is obviously one of the most important materials in optical applications and the nanocomposite approach is a versatile system to achieve previously inaccessible structures. However, our material requires a manufacturing platform that is suitable for high-resolution structuring. Most additive manufacturing approaches have intrinsic manufacturing artefacts such as layering artefacts or support structures. Additionally, in many cases, the materials and thus the object manufactured have rather bad optical properties. 2PP does not have these disadvantages, so the printing material and the 2PP-based microfabrication process are a perfect match for these applications.
What advantages does glass offer compared with the polymers that dominate 3D printing?
Bastian Rapp: The main advantage of glass over polymers is its outstanding optical properties. Low aberration, extremely high transmission, even in the lower UV (ultraviolet) range, and constant optical properties over a wide temperature range are just some of the advantages of glass. In addition, glass does not age or grey over time. It is mechanically durable and chemically inert, thus enabling applications that require constant material properties under harsh environmental conditions.
What are the differences between glass and polymer 3D printing?
Alexander Quick: As the first and still dominant material class for 2PP, polymers and processes developed around them have been continuously improved for over a decade. With GP-Silica, we benefit from such polymer processes to introduce glass printing. Of course, both the printing and post-processing of the composite material are unique. With the 10x objective lens, we focus on enabling fabrication of meso-scaled structures. An important reason for this decision is the significant simplification of thermal post-processing, for example, the possibility of removing the printed green part from the substrate. This is the easiest way to control the non-negligible shrinkage during thermal treatment as an inherent part of the sintering step.
Bastian Rapp: Besides low shrinkage, one of the key advantages of polymer over glass is the versatility of the material class. Polymers come in many variants covering a wide range of physical properties. As an intuitive example, the refractive of glass is not exceedingly high, somewhere in the range of 1.45. If you want to design more compact optics, you require higher refractive indices. There are polymers that significantly bypass this value. So, if you are looking for a material class that allows you to cover a wide range of refractive indices, polymers would be the right material class.
You have both mentioned some shrinkage in the glass 3D printing process, how is this dealt with?
Bastian Rapp: Shrinkage is an inherent property of the process. You cannot create a nanocomposite with 100 vol% solid loading as this would be dense glass. However, shrinkage is isotropic and can be factored into your model before printing. This is where additive manufacturing in general, and 2PP in particular, has a clear advantage over, for example, replication processes, as increasing the components in size is rather trivial when working with digital model data. In replication moulding, you would have to manufacture your mould slightly larger. But this is also a common prerequisite in reactive injection moulding.
Alexander Quick: As is common for polymer systems, the printing step of GP-Silica leads to a certain structure-dependent shrinkage of the part. So overall, shrinkage during printing and thermal post-processing must be taken into account. Admittedly, this is a bit more complex than everyday polymer printing. To get a glass structure with the desired dimension and shape, it is necessary to run a few iteration cycles of the whole process and compensate for the shrinkage of the structure by modifying the design. This, by the way, is how we made all our demonstrational structures. Fortunately, shrinkage during thermal treatment is isotropic and therefore predictable. It can be compensated quite easily. We also benefit from very short iteration cycles of less than 24 hours, which are common for the 2PP-based 3D printing process due to the small dimensions of the printed parts. You can prepare and print your design today and have the glass part ready for inspection the next day. Thermal treatment can even be performed at the touch of a button.
Short iteration cycles are beneficial in many fields, but for which applications is 3D printable glass attractive and what are the advantages?
Alexander Quick: For many experts who hear about the combination of Nanoscribe’s microfabrication technology and the glass printing material, the application field of microoptics is obvious. We share this view, especially for applications that tolerate modest shape fidelity, i.e. for many illumination optics. However, in principle, any application that is currently non-addressable with polymers but becomes feasible with the superior material properties and resilience of glass is worth considering. As an example, in microfluidics, structures benefit from withstanding harsh environments and being impermeable, especially when it comes to chemical miniaturisation, filtering or chromatography. In microrobotics and micromachines, structures may need to withstand high temperatures or high mechanical stresses during use or in subsequent processing steps. In life sciences, structures that can be reliably and repeatedly sterilised allow for the blueprinting of new ideas.
What do you expect will be the future markets for 3D printed glass microstructures?
Bastian Rapp: We envision this technology being applicable to a wide variety of applications ranging from precision and integrated optics to microfluidics, data communications and photonics. Basically, whenever you need materials with high precision, outstanding optical properties and unsurpassed resilience, this is a technology to be considered.
Alexander Quick: Our Glass Printing Explorer Set includes GP-Silica as well as silicon substrates, several print accessories and detailed processing instructions. It is therefore a starting point, aimed at Nanoscribe customers and system users trying glass printing for the first time. Many of our academic users are world-leading researchers in their respective scientific fields. We can therefore expect to see new cutting-edge scientific excellence with glass printing in various fields in the near future, followed by opportunities to address shortcomings in industrial applications.
Nanoscribe
Glassomer
[1]The ATTRACT project is funded by the European Union’s (EU’s) Horizon 2020 programme and brings together Europe’s fundamental research and industrial communities to develop the next generation of detection and imaging technologies.
[1]Kotz, F., Quick, A., Risch, P., Martin, T., Hoose, T., Thiel, M., Helmer, D. and Rapp, B. (2021). Two‐photon polymerization of nanocomposites for the fabrication of transparent fused silica glass microstructures. Advanced Materials, volume 33, issue 9, p.2170062.Available at: https://bit.ly/3MZRr4V