3D printing glass with micrometer precision

Nanoscribe: Thinking back, 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. And the 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 composite?

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 – 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 printing material’s name of this polymer-silica nanocomposite for Two-Photon Polymerization (2PP) is GP-Silica that we jointly developed with Nanoscribe in the research project OptoGlass3D.

What an achievement. Can you explain in more detail what GP-Silica as a nanocomposite 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 the printing material perfectly suited to the high-precision printing process based on 2PP, our new photoresin GP-Silica contains this special suspension and a few other ingredients. The liquid photoresin thus contains “initiating” components that effectively trigger the curing reaction. Curing itself, similar to other Nanoscribe IP Photoresins, is achieved by radical polymerization, 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.

That’s interesting. And why exactly is the high-precision 3D printing process based on Two-Photon Polymerization apparently so well suited for the microfabrication of glass microstructures?

Alexander Quick: As we were both based in the KIT community in the past (Editor’s note: Karlsruhe Institute of Technology, KIT in short form), we have known each other for a long time and we have been working together successfully. After Glassomer was founded, we wanted to specifically explore the potential of combining our technologies. In OptoGlass3D, a funded ATTRACT project, we jointly developed a direct fabrication process for glass microstructures based on 2PP. We were able to publish the project success in the journal Advanced Materials at the beginning of 2021, which showed us the strong interest in these fundamentally new manufacturing possibilities.

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 to 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 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.

And what are the differences in the 3D printing process of glass compared with polymers?

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, e.g. to the possibility to remove 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 now both mentioned some shrinkage in the glass microfabrication process. Is there no way to avoid this?

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 molding, you would have to manufacture your mold slightly larger. But this is also a common prerequisite in reactive injection molding.

But then the question arises: How can we deal with this cleverly?

Alexander Quick: As it is also 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 2PP glass printing due to the small dimension 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 the vast majority, the combination of Nanoscribe and the printing material glass suggests the application field of microoptics. We share this view for applications that tolerate modest shape fidelity, i.e. for many illumination optics. But in principle, any application that is currently non-addressable with polymers but becomes feasible with the superior materials 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 miniaturization, 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 sterilized allow for the blueprinting of new ideas.

And where do you expect the future markets for 3D-printed glass microstructures to be?

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: With GP-Silica and our Glass Printing Explorer Set as a starting point, Nanoscribe customers and system users can apply glass printing for scientific microfabrication 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 current shortcomings in industrial applications.