Unleashing Continuous Precision: Microtube Synthesis with Two-Photon Direct Writing

Context: Continuous fabrication of high-aspect-ratio structures with nano/microscopic features is desirable for many applications in medicine and photonics, including custom-designed stents, micro-surgery equipment, and optical components.

Two-Photon Continuous Flow Lithography: This paper introduces a new adaptation of two-photon lithography known as two-photon vertical flow lithography.This technique is capable of fabricating high-resolution patterned particles and synthesizing 3D particles and fibers. It combines scanning two-photon lithography with continuous-flow lithography, overcoming the resolution limitations of traditional projection techniques.

By using a liquid negative tone photoresist that flows through a microfluidic chip attached to a two-photon polymerization printer, the method enables the continuous polymerization of 2D planar trajectories into interconnected 3D objects, as shown in Figure 1.

Figure 1. The fabrication setup constituting a continuous flow chip fixed on a glass substrate through which laser can be focused. The photoresist flow through the continuous flow chip. The laser is focused and maipulated around the resist inside the chip. The photoresist is flowing during the fabrication process. The liquid negative tone photoresist constantly passes through the inlet tubing and the fluid distributing branched channel network to the vertical outlet channel. The continuous flow of the photoresist allows for the polymerization of 2D planar trajectories into interconnected 3D objects that ascend with the liquid photoresist

This process allows for the fabrication of microtubes with high aspect ratios and small outer diameters, offering potential applications in various fields such as biohybrid cell cultivation scaffolds and microtube scaffolds. Designs of microtubules with or without pores can be seen in Figure 2. CCD camera images taken during fabrication of a tubule can be also be seen in Figure 3.

Figure 2. Design of nonporous and porous microtubes

3. CCD camera images taken during fabrication.

Fabrication of Particles: The stop-flow lithography technique employed in this method enables the high-resolution fabrication of patterned particles by precisely controlling the flow of the photoresist. By initiating the moving-beam fixed-sample fluid flow-coupled two-photon polymerization process, the focal spot of a near-infrared laser is focused into a fixed plane within the vertical outlet channel. This approach allows for the continuous polymerization of 2D planar trajectories, resulting in the formation of intricate 3D structures that ascend with the liquid photoresist. The ability to fabricate microtubes with high aspect ratios (>100:1) and small outer diameters (10 μm) showcases the versatility and precision of this technique.

Moreover, the combination of scanning two-photon lithography with continuous-flow lithography offers several advantages in synthesizing anisotropic particles in real time. By tuning process parameters such as laser power, scanning speed, and photoresist flow rate, researchers can achieve precise control over the fabrication process. This level of control allows for the creation of complex 3D structures with tailored properties, making the technique suitable for a wide range of applications in materials science, biotechnology, and microfluidics.

Application in Cell Cultivation : The ability to fabricate intricate 3D structures with high aspect ratios and small feature sizes makes this technique promising for creating scaffolds that mimic the natural extracellular matrix. These biohybrid scaffolds could provide a suitable environment for cell growth and differentiation, offering new possibilities in tissue engineering and regenerative medicine. While the proof of principle presented in the study is encouraging, further development is needed to optimize the cultivation conditions and assess the long-term viability of cells within the scaffolds. Cells grown on tubular matrices can be seen in Figure 4.

Figure 4. Cells growing on tubular scaffolds.

By combining the precision of two-photon lithography with the continuous flow of photoresist in a microfluidic system, researchers have demonstrated the ability to fabricate complex 3D structures with high resolution and control. The potential applications of this technology in materials science, biotechnology, and tissue engineering highlight its versatility and impact on various fields.

Reference: https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.201901356