Multimaterial 3D laser microprinting using an integrated microfluidic system

Abstract

Three-dimensional (3D) laser micro- and nanoprinting has become a versatile, reliable, and commercially available technology for the preparation of complex 3D architectures for diverse applications. However, the vast majority of structures published so far have been composed of only a single constituent material. Here, we present a system based on a microfluidic chamber integrated into a state-of-the-art laser lithography apparatus. This system is scalable in terms of the number of materials and eliminates the need to go back and forth between the lithography instrument and the chemistry room numerous times, with tedious realignment steps in between. As an application, we present 3D deterministic microstructured security features requiring seven different liquids: a nonfluorescent photoresist as backbone, two photoresists containing different fluorescent quantum dots, two photoresists with different fluorescent dyes, and two developers. Our integrated microfluidic 3D printing system opens the door to truly multimaterial 3D additive manufacturing on the micro- and nanoscale.

Fig. 1. Scheme of the microfluidic chamber.

(A) A high-NA oil-immersion microscope objective lens focuses femtosecond laser pulses into a chamber, which is clad by two thin glass windows (light blue). One of them serves as the substrate for the samples. The selection valve shown in Fig. 3 allows for switching between different photoresists (here, one nonfluorescent and four fluorescent) and solvents (acetone and mr-Dev 600), which are injected into the microfluidic chamber. For clarity, the scheme is not to scale. A to-scale technical drawing is shown in Fig. 2B. (B) Structure formulae of the components of one of the fluorescent photoresists containing Atto dye molecules.

Fig. 2. Microfluidic sample holder for 3D laser lithography.

(A) Left-hand side: Scheme of the complete sample holder, which can be placed into a commercial 3D laser lithography machine. Right-hand side: Explosion drawing of the microfluidic chamber, which hosts a small coverslip (diameter, 10 mm) inside the chamber, onto which structures can be 3D-printed. The chamber is sealed using a solvent-resistant O-ring, and the top part features a circular glass window for the high-NA oil-immersion objective to focus inside the chamber. (B) Cross-sectional scale drawing of the sample holder. The sample holder features connectors for liquid tubing and channels for the liquids to be guided in and out of the microfluidic chamber. The liquid flow path is indicated using red arrows.

Fig. 3. Scheme of the system connected to the microfluidic chamber.

(A) It consists of an electronic pressure controller connected to a nitrogen bottle, up to 10 containers for the photoresists and solvents for development, and the star-shaped selection valve. Pumping individual liquids is possible by applying a pneumatic pressure to all liquid containers and opening the flow path for a single liquid using the selection valve. Following the selection valve, the liquid flow is guided through an overpressure valve and our homebuilt sample holder. Last, it is directed into a waste container. (B) Cross section through our homebuilt selection valve assembly. The assembly consists of commercial solenoid valves and a homebuilt 10-to-1 manifold that connects the 10 liquid containers to 10 solenoid valves, and the valve outputs to one manifold output port. An example flow path for one liquid is indicated with red arrows.

Fig. 4. Successive 3D printing of different photoresists.

Images taken using the camera integrated into our 3D laser lithography machine. Each image shows the uppermost layer of the 3D microstructure, but after different printing steps. For the first picture, the 3D support grid and blue fluorescent markers have been printed, whereas for the last picture, markers using all four fluorescent resists have been printed. For clarity, fluorescence emission colors are overlaid.

Fig. 5. Confocal laser scanning fluorescence microscopy of fabricated structures.

(A) On the left-hand side, a computer rendering of the design for the microstructure is shown. It consists of a nonfluorescent 3D support structure (gray) with fluorescent markers with different emission colors printed into it. On the right-hand side, a stack of images taken by using fluorescence microscopy is shown. (B) The designs of the test patterns were printed into the five different marker layers of the microstructure. (C) Measurement data from fabricated microstructures taken using fluorescence microscopy. Insets show the level of detail at which different photoresist structure elements can be printed.