Nanyang Technological University: Thesis Validates Use of Bessel Beams in Laser-Based 3D Printing

Andy Wen Loong Liew has submitted a thesis, ‘Laser-based 3D printing using bessel beams for tissue engineering applications’ to Nanyang Technological University. Exploring a new technique for bioprinting, Liew studies the use of Bessel beams in connection with regenerative medicine techniques, and he also compares the benefits of traditional techniques.

Liew cites the benefits of Bessel beams (BB) in connection with 3D printing and customizing hydrogels, considering the following:

• Easy customization of construct architecture
• Reduced fabrication time
• High print resolution
• Ability to fabricate high aspect ratio tubular structures with anatomic relevance (and no supports)

This study also involves the use of collagen gel for improved success in sustaining cells and inducing endothelial sprouting.

Liew reminds us that the ultimate goal in tissue engineering is to begin 3D printing organs that are viable for transplant into humans; on the way, however, engineers have used a wide range of materials with customized hardware and software to begin engineering skin, cartilage, bone, and far more.

“The aforementioned tissues which have successfully made the transition are avascular which makes them easier to engineer compared to vascular tissues such as heart and kidney tissue which are more complex,” states Liew.

Liew asserts that so far there have been no studies of BB printing techniques published. Advantages and challenges are discussed regarding the new technique. Much of the work presented in the chapter 2 literary review is based on the following publication: Liew, Andy Wen Loong, and Yilei Zhang. ‘In vitro pre-vascularization strategies for tissue-engineered constructs – Bioprinting and others.’ International Journal of Bioprinting (2017) 3 3-17. There, Liew describes vascularization, the use of in vitro models, and current in vitro vascularization approaches like bioprinting, microfluidics (lithography), micropatterning, wire molding, and cell sheet engineering.

For the purposes of experimenting within this study, Liew makes it clear that while very little attention may have been paid to BBs in additive manufacturing processes, they are attractive due to a non-diffracting core that is not only highly localized but also offers high intensity.

“This high intensity BB core is the result of constructive interference of the beam after exiting the axicon. Unlike focused Gaussian beams, the high intensity BB core extends throughout the length of the non-diffracting zone while retaining its highly localized profile,” states Liew. “Thus by exposing a pre-polymer solution to the BB core, a long fiber-like structure can be produced through a single exposure as the high intensity BB core propagates through the entire height of the solution.”

Much of the discussion regarding the use of BBs also stems from Liew’s previous research, drawn from Liew, Wen Loong Andy, and Yilei Zhang. ‘Laser-based fabrication of 3D hydrogel constructs using Bessel beams.’ Bioprinting (2018) 9 44-51.

The researchers 3D printed at room temperature, relying on a vat filled with pre-polymer solution, ‘placed on the translation stage,’ and centered on the BB propagation axis. Print settings were optimized as follows for all samples fabricated in the study: Laser power = 120µW, Magnification (M) = 1 (refer to Section 5.2.2), translational speed of stage = 1mm/s.

The researchers noted that 3D printing time of samples was ‘significantly reduced’ as they compared results with conventional methods; in fact, with BB, the average printing time was decreased to an impressive 20 seconds—reflecting a savings of more than 50 percent.

“Encapsulation of fluorescent beads (simulating cells) within the tube walls was also successfully demonstrated with this technique as a proof-of-concept for subsequent chapters where the printing technique will be used for direct cell encapsulation,” stated Liew.

“Finally, 3D hydrogel scaffolds with controlled microscale features and in-built microchannels were fabricated with both naturally-derived and synthetic polymers using the BB technique, showcasing its superior print resolution compared to conventional printing techniques and flexibility. Overall, the technique displayed strong potential to be applied in the field of TE in future.”

In using BBs for tissue engineering, the following properties should continue to be evaluated:

• Limitations in design complexity
• Multi-material, multi-cellular construct printing
• Variances in printing time for construct designs
• Long term effects on cell phenotype/genotype from UV exposure
• Flexibility of systems in ‘tuning’ resolution
• Structural non-conformity to original design

“ … there are several drawbacks to using the BB technique for bioprinting applications including wall thinning and limited design complexity,” concluded Liew. “Future work should include a balanced evaluation of how the proposed 111 BB printing technique compares to established, commercially available bioprinting systems in order to establish it as a viable alternative to current technology.”

3D printing has had an enormous impact on tissue engineering in recent years, as researchers create new materials and structures like scaffolds, improve hydrogel microenvironments, refine bioprinting for bone regeneration, and much more.

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