Sacrificial Template Replication: Fabrication of arbitrary three-dimensional suspended hollow microstructures in transparent fused silica glass

Glass is a versatile material, but it is very difficult to structure. Specifically, the generation of hollow channel structures as required, e.g., for synthesis-on-a-chip applications, has been very difficult to achieve. Here we demonstrate a novel approach to glass structuring enabling an entirely new real of complexity in terms of structure geometries and design of freedom.

Mar 29, 2019
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Glass is one of mankind’s oldest materials. Its first confirmed use dates back more than 5000 years B.C:. Today, glass fiber cables provide data transmission at the speed of light thus fueling the modern information society. In everyday life, glass is ubiquitous: From window panels, to drinking glasses, containers, appliances, decoration, art and architecture – our life would look different without glass. Its optical, mechanical and thermal resilience and material properties remain unmatched. However, its physical and chemical toughness come at a severe disadvantage: Glass is notoriously difficult to structure. High-temperature processes such as melting or glass blowing, grinding as well as chemical etching with hazardous chemicals such as hydrofluoric acid remain the only choices for glass structuring. This limits the degree of freedom as well as the structures which can be generated from glass.

One of the most prominent structuring capabilities missing is the generation of hollow structures in glass. Glass capillaries are a common tool in every laboratory, but a capillary is a very simple structure. The generation of complex three-dimensional hollow structures as required, e.g., for complex microfluidic layout, chemistry-on-a-chip or flow through synthesis applications are next to impossible to generate in glass.

We have recently developed a novel glass structuring methodology making use of a polymeric nanocomposite which can be cured at room temperature by exposure to light. This material consists of glass nanoparticles suspended in an organic binder. Once cured, the polymeric component of the material is thermally debound and the remaining particles are sintered to a dense piece of glass. We have shown that this process can be used to generate glass structures via UV replication, 3D-printing and subtractive machining. However, none of these methods is able to generate really three-dimensional, complex microchannels in glass.

In our most recent contribution, we demonstrate a method we refer to as Sacrificial Template Replication (STR). In this process, we generate the channel’s inverse shape, i.e., the volume which should later become a void volume. This structure can be generated by high-resolution additive manufacturing like two-photon polymerization. It is also possible, to use a polymeric filament as demonstrated with a complex structure network made by melt electrowriting. Once generated, this positive structure is then immersed in the nanocomposite which is then cured thus embedding the structure. During thermal debinding, the template as well as the binder are removed leaving the inverse hollow structure in glass. After sintering, the channel is obtained as the complementary form to the template.

We show that this process is able to generate structures in shapes and complexities which have been, until now, unachievable. This technique thus paves the way to very complex, three-dimensional channels and hollow structure layouts in high-purity optical-grade fused silica glass enabling applications such as flow-through synthesis and chemistry-on-a-chip as well as optical applications.

You can read more about our work in the Nature Communications Paper.

This post was co-authored by Frederik Kotz and Dorothea Helmer. 

Bastian Rapp

Prof. Dr.-Ing. habil., University of Freiburg

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