Page 37 - IJB-9-2

International Journal of Bioprinting Scaffolds printed with light sheet stereolithography

object is realized in a layer-by-layer approach. One of the the fact that the performance of the system now depends
most widely used technologies in 3D scaffold fabrication on laser beam characteristics instead of used projection
is direct light processing (DLP) [37-40] . Its ability to expose a devices opens up the possibility for further improvement
photosensitive material to a full 2D image projection has in the aspects of scaffold fabrication of even larger sizes and
resulted in 3D structures with ultra-high resolution as well shorter fabrication times as we demonstrated in this work.
as size and printing speed not possible to fabricate using
counterpart methods, such as stereolithography (SLA) 2. Methods and printing protocol
and two-photon polymerization (TPP) . For example, 2.1. LS-SLA
[37]
investigations on the optimization of the projection optics
of DLP devices led to 3D constructions with a lateral The 3D printer developed in this work is based on a VP
[41]
resolution of 4.1 μm and an axial resolution of 2.5 μm in bottom-up fabrication scheme (Figure 1A) . The polymer
acrylate-based resins against 75 μm resolution in SLA . resin is in contact with the bottom surface (FEP film) of the
[41]
In addition, at the highest resolution, the projection area resin vat and an inverted build platform, which is used to
achieved with DLP was 6.4 × 4 mm², against the tens control the height of the resin layer. The pattern on each
of micrometers typically found in TPP processes [41,42] . layer is built by the selective light exposure of the polymer
Losing some tens of microns in resolution to gain larger resin to a blue laser light (405 nm) that is focused on the
area expositions have directed many DLP devices into the bottom surface of the resin vat. Thereby, the exposed regions
fabrication of 3D constructs with lateral resolution that are hardened through polymerization of the resin, which
ranges between 20 and 100 μm, achieving, for example, subsequently adheres to the build platform. The elemental
a maximum projection area of 19.35 × 12.1 mm² at the hardened structures are referred to as struts and voxels in
maximum lateral resolution . The results achieved by DLP this work. After polymerization of one layer sequence, the
[43]
set a benchmark in 3D bioprinting technologies and pave build platform moves stepwise in the z-direction, allowing
the way toward rapid and high-precision manufacturing. new liquid resin to flow into the bottom of the resin vat
before the process repeats.
However, the magnification or resolution dependence
of DLP due to the projection optics and digital micromirror In this work, we propose breaking the symmetry of
devices (DMDs) hinder the scaling-up capabilities of rapid the illumination source of conventional VP systems by
and high-resolution 3D printing (e.g., down to several producing large length-to-width aspect ratio voxels. The
millimeters in size) [38,39,44-46] , making its implementation elongated voxels are created using cylindrical lenses.
impractical in large surface-to-volume ratio constructs Due to its curved face in only one plane of symmetry, the
found in wound dressing applications [30,46-48] . Furthermore, laser beam propagates in the same path with two possible
although stitching methods have been proposed to fabricate configurations . In one configuration, as depicted in the
[50]
larger areas in state-of-the-art, the printed structures top diagram of Figure 1B, the light propagating from a
exhibit inaccuracies, surface defects, and mechanical beam shaper passes through the cylindrical lens without
deformations that may affect the performance of the any effect. Then, the laser beam is focused on one of the
scaffolds [38,47,49] . It is worth mentioning that more research axes of the film plane producing a LS with a width only
must be carried out on the stitching effects of biomaterials limited by diffraction at the scan lens aperture. The
on scaffold fabrication. achievable width of the LS at the film plane is estimated
The requirement of scaffolds exhibiting large surface- using Equation 1.
to-volume ratios can only be satisfied with novel d=κλ(F/#) (1)
manufacturing strategies that comprise different exposure, Where, F/# is the F-number of the scan lens, λ is the
fabrication, and structuring modes. In this paper, wavelength of the light source, and k is a truncation factor
we propose an alternative technique that extends the with a value κ = 2.44 for a uniform illuminated entrance
capabilities of VP devices toward practical large size scaffold pupil of the scan lens .
[51]
fabrication with microscale features. Instead of using the
common projection systems found in DLP and based on The second dimension controls the length of the LS. In
the laser scanning setup of SLA devices, we use laser beam the bottom configuration of Figure 1B, the curvature of the
shaping to modify the profile of the voxels in 3D printing, cylindrical lens in combination with the scan lens forms
resulting in a technology we call light sheet SLA (LS-SLA). a collimated beam expander and magnifies the height of
We showed that it is possible to produce elongated voxels to the rectangle beam delivered by the beam shaper. The
conserve an excellent lateral resolution for a large printing magnification of the Y-Z configuration is determined by
area, resulting in submicron resolution for centimeter the ratio between the effective focal lengths f of the scan
sl
length exposures without stitching structures. Furthermore, lens and the cylindrical lens f , as shown in Equation 2.
cl
Volume 9 Issue 2 (2023) 29 https://doi.org/10.18063/ijb.v9i2.650