Ultra-Thin Optical Fibers for Tiny 3D-Print Micro­struc­tures

An optical fiber housed inside the needle to deliver light for 3-D printing microstructures. The light selectively hardens volumes inside the droplet of photopolymer on the glass slide. The new system could one day allow 3-D printing inside the body. (Source: D. Loterie & P. Delrot, EPFL)

For the first time, researchers have shown that an optical fiber as thin as a human hair can be used to create micro­scopic structures with laser-based 3D printing. The inno­vative approach might one day be used with an endo­scope to fabri­cate tiny biocom­patible structures directly into tissue inside the body. This capa­bility could enable new ways to repair tissue damage. “With further develop­ment our technique could enable endo­scopic micro­fabrication tools that would be valuable during surgery,” said research team leader Paul Delrot, from École Poly­technique Fédérale de Lausanne, Switzer­land. “These tools could be used to print micro- or nano-scale 3D structures that facilitate the adhesion and growth of cells to create engi­neered tissue that restores damaged tissues.”

With their new approach the researchers can create micro­structures with a 1.0-micron lateral and 21.5-micron axial printing reso­lution. Although these micro­structures were created on a micro­scope slide, the approach could be useful for studying how cells interact with various micro­structures in animal models, which would help pave the way for endo­scopic printing in people. To create the micro­structures, the researchers dipped the end of an optical fiber into a liquid known as photo­polymer that soli­difies, or cures, when illu­minated with a specific color of light. They used the optical fiber to deliver and digi­tally focus laser light point-by-point into the liquid to build a three-dimen­sional micro­structure.

By printing deli­cate details onto large parts, the new ultra-compact micro­fabrication tool could also be a useful add-on to today’s commer­cially available 3D printers that are used for everything from rapid proto­typing to making perso­nalized medical devices. “By using one printer head with a low resolution for the bulk parts and our device as a secon­dary printer head for the fine details, multi-reso­lution additive manu­facturing could be achieved,” said Delrot. Current laser-based micro­fabrication techniques rely on two-photon photo­polymeri­zation to selectively cure a volume deep inside a liquid photo­sensitive material. These techniques are diffi­cult to use for biomedical appli­cations because two-photon photo­polymeri­zation requires complex and expensive lasers that emit very short pulses as well as bulky optical systems to deliver the light.

“Our group has expertise in mani­pulating and shaping light through optical fibers, which led us to think that micro­structures could be printed with a compact system. In addition, to make the system more affor­dable, we took advantage of a photo­polymer with a nonlinear dose response. This can work with a simple conti­nuous-wave laser, so expensive pulsed lasers were not required,” said Delrot. To selec­tively cure a specific volume of material, the researchers took advantage of a chemical phenomenon in which solidi­fication only occurs above a certain threshold in light inten­sity. By performing a detailed study of the light scanning para­meters and the photo­polymer’s behavior, the researchers discovered the best para­meters for using this chemical pheno­menon to print micro­structures using a low-power, inex­pensive laser that emits conti­nuously.

To create hollow and solid micro­structures, the researchers used an organic polymer precursor doped with photo­initiator made of off-the-shelf chemical components. They focused a conti­nuous-wave laser emitting light at 488-nanometer wave­length through an optical fiber small enough to fit in a syringe. Using an approach known as wavefront shaping they were able to focus the light inside the photo­polymer so that only a small 3D point was cured. Performing a cali­bration step prior to micro­fabrication allowed them to digitally focus and scan laser light through the ultra-thin optical fiber without moving the fiber. “Compared to two-photon photo­polymeri­zation state-of-the-art systems, our device has a coarser printing reso­lution, however, it is poten­tially suffi­cient to study cellular inter­actions and does not require bulky optical systems nor expensive pulsed lasers,” said Delrot. “Since our approach doesn’t require complex optical components, it could be adapted to use with current endo­scopic systems.”

The researchers are working to develop biocom­patible photo­polymers and a compact photo­polymer delivery system, which are necessary before the technique could be used in people. A faster scanning speed is also needed, but in cases where the instru­ment size is not critical, this limi­tation could be overcome by using a commercial endo­scope instead of the ultra-thin fiber. Finally, a technique to finalize and post-process the printed structure inside the body is required to create micro­structures with biome­dical functions. “Our work shows that 3D micro­fabrication can be achieved with techniques other than focusing a high-power femto­second pulsed laser,” said Delrot. “Using less complex lasers or light sources will make additive manu­facturing more accessible and create new oppor­tunities of appli­cations such as the one we demonstrated.” (Source: OSA)

Reference: P. Delrot et al.: Single-photon three-dimensional microfabrication through a multimode optical fiber, Opt. Exp. 26, 1766 (2018); DOI: 10.1364/OE.26.001766

Link: Lab. of Applied Photonics Devices, School of Engineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland 

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