Selective Laser Etching: creating three-dimensional microfluidic structures

Interview with Dr. Arnold Gillner and Sebastian Nippgen, Fraunhofer Institute for Laser Technology ILT

Stacked, in parallel lines and multi-branched - complex glass forms are beautiful to look at but also difficult to make because solid glass is hard and challenging to work with. When it is used in microfluidics, the size of the delicate shapes ranges in the micrometer range. Manufacturers subsequently need to use a laser to deliver a three-dimensional structural design into their workpieces.

02/01/2016

Sebastian Nippgen; ©Fraunhofer ILT, Aachen

Sebastian Nippgen; ©Fraunhofer ILT, Aachen

In this interview with COMPAMED.de, Dr. Arnold Gillner and Sebastian Nippgen explain how Selective Laser Etching (SLE) works, how you can utilize it to structure channels in the glass interior in an unlimited manner and what advantages the technique offers in the manufacturing of microfluidic components.

Dr. Gillner, Mr. Nippgen, we are talking about the selective laser etching technique and its application to produce microfluidic components. How does SLE work?

Sebastian Nippgen: The SLE technique focuses ultrashort-pulsed laser radiation inside of transparent dielectrics such as fused silica or sapphire. Due to the high intensity, temperatures of several thousand degrees develop inside this small focal point. The properties of the material are altered to where these modified areas exhibit a significantly increased etchability over those that are unmodified. Yet this does not cause any further material damage since the laser pulse is so ultrashort.

In a next step, we are able to selectively remove the modified areas in an ultrasonic bath with a wet etching technique, for instance by using a solution of potassium hydroxide or hydrofluoric acid. To do this, the modification inside the workpiece needs to have contact to the surface so that the etching solution is able to attack.

Very small areas are modified within a short time using this technique. How do you focus the laser beam in this case?

Nippgen:
Currently by using microscope objectives with 20x to 100x magnification. The focal length for exposure is less than 2 µm in diameter. The laser is diverted with a scanner through the objective. Layer by layer this creates three-dimensional structures in the glass which we previously generated using a CAD program.
Dr. Arnold Gillner; ©Fraunhofer ILT, Aachen

Dr. Arnold Gillner; ©Fraunhofer ILT, Aachen

How long do exposure and etching approximately take?

Nippgen: This also depends on the structural complexity, but it is essential to know how much space the laser needs to "color". When large spaces need to be exposed, it takes a very long time. If you need a lot of structure, it also takes longer.

The etching process for sapphire takes up to ten days. We are normally done after one day at the latest in the case of fused silica unless the channels are very long. Yet you can etch hundreds or thousands of parts at the same time in the solution.

How long does it take to produce microfluidics with other proven methods?

Dr. Arnold Gillner: This depends on the material. For components made of glass, the glass is structured using the reactive ion etching method for instance. Its rates of erosion are comparatively low and only one piece at a time is being processed.

Components made of plastic can be made much fast faster using injection molding of course. You only need to manufacture the respective injection molding tool once; then you can produce many parts in a very short amount of time.

Nippgen: Compared to glass, the drawback of plastic is that it exhibits a lower degree of robustness towards chemicals, a lesser degree of biocompatibility and is difficult or not possible to clean.

Gillner: Glass also does not exhibit fluorescence. Apart from cyclo olefin copolymers, it is substantial in plastics, which affects optical diagnostic and analysis procedures.
Photo: Microfluidics etched into glass

Delicate structures: using SLE, microfluidics like these can be made in glass parts. Firstly, a laser is used to alter the glass's structure; secondly, the altered structures are etched away; ©Fraunhofer ILT, Aachen/ Volker Lannert

What advantages does this technique provide for the manufacturing of microfluidic components?

Nippgen: One clear advantage is the unlimited 3D structurability of the channels. We can place them on top of each other without the need to adhere these layers. It is a monolithic component.
Photo: Microfluidics etched into glass

Microfluidics in glass have a higher biological and chemical compatibility, are easier to clean than plastic ones and can be re-used in most cases; ©Fraunhofer ILT, Aachen/ Volker Lannert

How long do exposure and etching approximately take?

Nippgen: This also depends on the structural complexity, but it is essential to know how much space the laser needs to "color". When large spaces need to be exposed, it takes a very long time. If you need a lot of structure, it also takes longer.

The etching process for sapphire takes up to ten days. We are normally done after one day at the latest in the case of fused silica unless the channels are very long. Yet you can etch hundreds or thousands of parts at the same time in the solution.

How long does it take to produce microfluidics with other proven methods?

Dr. Arnold Gillner: This depends on the material. For components made of glass, the glass is structured using the reactive ion etching method for instance. Its rates of erosion are comparatively low and only one piece at a time is being processed.

Components made of plastic can be made much fast faster using injection molding of course. You only need to manufacture the respective injection molding tool once; then you can produce many parts in a very short amount of time.

Nippgen: Compared to glass, the drawback of plastic is that it exhibits a lower degree of robustness towards chemicals, a lesser degree of biocompatibility and is difficult or not possible to clean.

Gillner: Glass also does not exhibit fluorescence. Apart from cyclo olefin copolymers, it is substantial in plastics, which affects optical diagnostic and analysis procedures.

What advantages does this technique provide for the manufacturing of microfluidic components?

Nippgen: One clear advantage is the unlimited 3D structurability of the channels. We can place them on top of each other without the need to adhere these layers. It is a monolithic component.
Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.
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