(B-52) Degradation Characteristics of Digital Light Processing-Printed, Load-Bearing, Unit-Cell Tissue Engineering Scaffolds

Introduction:: Load-bearing scaffolds can emulate the strain environments of native tissues, e.g., for osteochondral tissue regeneration, and protect cells from high strains¹. Advanced 3D printing methods, such as digital light processing (DLP), enable the use of custom monomers and the fabrication of exquisite unit-cell micro-structures structures, including octet-truss and triply periodic minimal surface (TPMS)-sheet and -solid structures, to recapitulate the mechanical environment of native tissues² and facilitate tissue regeneration³.

Degradable chemistries are being increasingly used to allow synthetic materials to be replaced by a newly formed extracellular matrix. Theatrically, a load-bearing scaffold should have a controlled degradation rate to commensurately shift mechanical support over time to the newly formed and maturing matrix. Recent studies demonstrate a range of promising 3D printed polymers with tunable chemistry with enhanced scaffold degradability⁴ and the ability to design materials with controlled surface and bulk degradation⁵. However, we lack quantitative data that describe how degradation is influenced by design features of common, 3D printed, unit-cell micro-structures including pore size and morphology, feature size, porosity, and material properties. Here, we evaluate how the design of DLP-printed load-bearing unit-cell micro-structures influences degradation in a physiological environment. This work will guide future scaffold designs with controlled degradation characteristics for tissue engineering applications, e.g., bone and osteochondral tissue regeneration.


Materials and Methods::
A poly(β-amino ester) diacrylate (PBAE-dA) resin^5,6 was used. Briefly, macromers were synthesized by mixing butanediol diacrylate and benzhydrazide at a 3:2 molar ratio and heated at 90°C for 24 h. 0.85 wt% diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide was added as a photoinitiator, and 0.15 wt% Sodium 2-(1,3-dioxoindan-2-yl)quinolinedisulfonate was added as a photoabsorber to control the printed layer height.




Two truss-like unit-cell structures: octet-truss and TPMS diamond-solid, and two TPMS-sheet structures: diamond-sheet and gyroid-sheet, were evaluated (Fig. 1A). Surface models of each design (4.2 mm diameter x 2.2 mm height with 20% relative density) were sliced into 2D binary image stacks with 25 μm layer height and subsequently printed with a custom DLP system. Each layer was exposed for 8 sec at an irradiation intensity of I = 14.4 mW × cm² at λ = 405 nm. Printed scaffolds were washed in 100% ethanol to remove excess resin and stored in PBS. N = 24 scaffolds (n = 6 / design) underwent accelerated degradation tests in 0.25 M NaOH (72 h). Every 24 h, n = 2 scaffolds/group were cleaned with 100% ethanol, air dried, and weighed to evaluate material loss from degradation. At day 0, scaffolds were scanned with micro-CT (Zeiss XRM) to assess initial structural geometric properties. The remaining scaffolds in each group were subjected to unconfined compression (MTS Systems) to 15% strain to evaluate modulus at each degradation timepoint.




Results, Conclusions, and Discussions::
Micro-CT analysis showed that gyroid-sheet and diamond-sheet scaffolds had higher surface-volume ratio, at 25.9 mm^-1and 31.6 mm^-1, respectively. In comparison, octet-truss and diamond-solid structures had 18.1 mm^-1 and 19.4 mm^-1surface-volume ratios. All structures underwent gradual degradation over the three days. The two truss-like structures displayed ~7% weight loss / day, while the two TPMS-sheet structures lost 11-12% weight / day (Fig.1 B). All four designs showed that the percentages of weight loss converged when normalized to each structure’s surface-volume ratio. We previously demonstrated that the bulk PBAE material degrades predominantly through surface erosion due to hydrolysis^5,6, thus weight loss from degradation depends on the scaffold’s surface area.




Unconfined compression testing of undegraded (day 0) scaffolds revealed similar compression modulus (0.27-0.31 MPa) for the octet-truss and TPMS-sheet structures, whereas the diamond-solid structures were less stiff (0.15 MPa). This observation aligned with previous studies⁷ where the stretch-dominated octet-truss and TPMS-sheet structures showed superior relative compressive moduli. In contrast, the bending-dominated, diamond-solid structure had lower mechanical strength. After three days of degradation, the three TPMS structures lost ~16% compressive modulus / day, while Octet-truss lost only 9.4% / day (Fig.1 C).



In conclusion, this study revealed varied patterns of degradation among four different 3D printed load-bearing unit-cell scaffolds. Although the structure surface area was associated with the rate of weight loss, there was no direct correlation between weight loss and modulus. Additional testing will evaluate how surface degradation and material loss, along with parametric studies of the role of specific design features such as pore size and feature dimensions, affect each structure's mechanical stiffness and integrity. When completed, these results will guide the design of printable unit-cell design scaffolds that, when formed from tunable bio-degradable materials (PBAE), will effectively transfer load to the forming matrix over time.


Acknowledgements (Optional): :

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