Interview with an Expert: Dr Bastian Rapp of NeptunLab

Since earning his PhD from the University of Karlsruhe in 2008, Dr Bastian Rapp has risen to become the world’s leading authority on the application of 3D printing for microfluidics and related technologies. As the founder and head of NeptunLab, at the Institute of Microstructure Technology (IMT) of the Karlsruhe Institute of Technology, his work focuses on the development of microfluidics for biomedical applications and biotechnology. Bastian was kind enough to sit down with us to discuss the role 3D printing has played in his work and what he sees as the key areas where the technology needs to evolve.

So why 3D printing? How did you originally discover the technology?

My laboratory is focused on applications for microsystem engineering, material science, and analytics/diagnostics for biochemistry and biomedical applications. I was always interested in methods that would allow you to make components quickly — to get from a conceptual design to something that you could actually test in a very short amount of time. Microsystem engineering utilises technologies that make extremely fine, highly resolved structures, but these techniques are very time consuming.

I was intrigued to learn about the advancements in additive manufacturing. I started in this field almost 12 years ago. One thing that I was always particularly interested in was advancements in terms of resolution, because a lot of the things we do, the feature resolution is about the size of normal 3D printing’s roughness value. We’re talking about internal dimensions of 50 microns! We need extremely smooth surfaces, and we need extremely highly resolved features. So I was looking into methods that increase resolution, and ways to increase the choice of materials.

Most of the polymers that are used in 3D printing just do not work for the sort of applications that we were looking into. That’s why my laboratory has focused on technology and material development, in order to advance the field in this respect.

When you began exploring this technology, what was the implementation process like? Were you doing it in-house, or outsourcing it, for example?

The first 3D printed designs that I used in my research were actually manufactured by a Swiss company called ProForm, who were already moving into making very highly resolved features with micro-stereolithography. We worked with a lot of designs from ProForm, but eventually we found that most of the materials they could process were not really suitable, because their physical/chemical properties were just not what we needed. So about eight years back, we started developing our own instrumentation and also developed materials that you could process using these tools.

The basic problem with a lot of 3D printing technologies (although this is getting better) is that you can only use the specific materials that an instrument provider would supply you with. It’s very much the same problem as the old inkjet printers that only run on the manufacturer’s cartridges.

This is why we eventually said “Why do we need a conventional instrument when we could build our own, and make it an open platform for pretty much all materials?” That was the first working instrument that we set up in the laboratory to test new materials. Similar machines are now commercially available.

Our instrument was designed so the resolution would be significantly better than most stereolithography instruments, with an achievable resolution of 600 nanometers — significantly smaller than what you will typically find on the market. It also allows you to stitch parts together to achieve interesting lateral dimensions. For example, if you take a single DMD (digital micromirror device) chip and shrink it down to a 600 nanometer pixel size, your overall lateral field that you’re working with will be fractions of a millimeter, so you’ll need to stitch individual frames next to each other.

What were the early stages like? Were there specific challenges involved in applying this technology for the first time?

This is something I find quite interesting about the industry nowadays, as these were the days when you had to write custom software to print parts, and that sort of thing. Nowadays, you can download designs from the web, pass them through standard software and print them straight away. It’s advanced quite significantly.

How has it developed since then? What sort of applications are you finding for this technology?

We’ve done a lot of microfluidics using this technology, such as biosensors and analytical devices. We’ve also done a lot of optical devices, which do interesting things with light. For example, we’ve created projectors where you shine a laser pointer through a physical structure, which then generates a projection. Optical components like this will become more important in the upcoming years as we do more and more calculations with light, as opposed to electrons. We’ve also done a lot of chemistry-on-a-chip — reducing the large-scale chemistry that takes place in industry to a flow-through format.

What has the uptake been like among professionals?

In our community, we are very restricted in terms of dimensions. You can’t just buy any instrument from the market, because the resolution will just not be sufficient. As a result, my community has been picking up on these trends quite slowly, because to get started, you have to invest large amounts of money in order to buy the right instrument, and also a number of months to get it set up.

The other thing — and this is something that’s extremely important for our field — is that the choice of materials is still quite limited. A lot of materials that you can 3D print are not relevant for applications like bio-analytics, as the polymers are far too reactive. We recently published a paper on 3D printing with glass. This is an idea we are pushing: to make known materials accessible through novel instrumentation for additive manufacturing. It’s then not a question of how well I know a certain photopolymer, for example. I don’t care about that, because I can generate a structure in a known material, and the only new element is the process I use to make this component. In the end, it behaves identically to the material that we’ve been working with for decades, so this solves the problem of material acceptance. That’s why I often pitch these technologies as a material process innovation rather than a material innovation. We didn’t invent any new materials — it’s just a different way of making components with the materials we already have!

When I speak to people who are involved in additive manufacturing on an industrial scale, there are usually two points that are raised. The first is that the materials are just not there yet, and the second is that the resolution of the parts is just not there yet. For example, SLS is a good process, but needs extensive post-processing. If you compare this to processes like stereolithography or CLIP (continuous laser interface production), where you have a continuous build-up process and thus no steps, you can achieve very smooth surfaces, which are suitable for optical components. But stereolithography has its limitations, as it is a chemistry-based process. As a result, people who don’t see themselves as specialists in material chemistry won’t use stereolithography, and if they do, they only use the materials provided by the suppliers.

We’ve been trying to bridge these gaps, as stereolithography has a lot of advantages over other methods. The only disadvantage is that the materials need to be in a certain formulation, so they can be photo-cured. But this doesn’t need to be such a big problem. We’ve published a number of papers where we’ve successfully printed parts using a number of industrial thermoplastics, such as plexiglass, which you can now 3D print in very high resolutions.

Where do you see this going next? How do you envisage different industries applying this disruptive technology as it evolves?

One question that will need to be addressed is speed, as that’s still a problem in additive manufacturing. If you solve the material problems and have a known and established material that you can 3D print, but you can also use the same material in an industrial, scalable process, such as polymer replication, this will make additive manufacturing even more interesting. Companies can then prototype using 3D printing, using the same material that will then be used for manufacturing, so you’ve got a streamlined process with no material disruption between the concept phase and the manufacturing phase.

The second big problem is how you get the process to the point where industry can use it on a manufacturing scale. We’re seeing increases in build speed. CLIP for example, made stereolithography almost a hundred times faster, but it’s still too slow! With industrial replication, you don’t necessarily need to beat injection moulding, as that process is fully optimised and incredibly fast, but if you get to the point where you can make a component via a 3D printing process and the speed is only one order of magnitude slower, then suddenly you start doing your calculation differently. With additive manufacturing, you don’t need moulding tools, which are extremely expensive for most applications. If speed vs. material cost balances a bit better, then more people will be encouraged to explore additive manufacturing. This is where the technology will shine.

Speed, materials and resolution: these are the three things that will need to be addressed to really get the technology to kick off. The next big step will be in making other materials accessible for 3D printing that we’ve never seen before, including establish polymers and metals. There will definitely be more to come!

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(Images courtesy of NeptunLab)