Poster M14 - Evaluation of Triply Periodic Minimal Surface (TPMS)-based Biomimetic Composite Systems for Cartilage Tissue Regeneration

Saturday, October 26, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Yashveer Soni (he/him/his)

Graduate Student Research Assistant
University of Colorado, Boulder
Arvada, Colorado, United States

Co-Author(s)

BL

Boyuan Liu, PhD

Post-Doctoral Researcher
University of Colorado, Boulder, United States
RM

Robert R. McLeod, PhD

Professor - Elect, Comp & Energy Engr Dept.
University of Colorado, Boulder, United States
VF

Virginia L. Ferguson, PhD

Professor - Paul M. Rady Mech Engineering
University of Colorado, Boulder
Boulder, Colorado, United States

Last Author(s)

SB

Stephanie J. Bryant, PhD

MSE Faculty Director, Professor - Chemical & Biological Engineering
University of Colorado, Boulder, United States

Introduction: For cartilage tissue engineering, load-bearing scaffolds serve as promising modalities to mimic the native mechanical environment and protect cells from excessively high strains[1]. Recently, Triply Periodic Minimal Surface (TPMS) structures have garnered interest in tissue engineering due to their improved load bearing capacity over traditional truss and pillar scaffolds, with choice of bending - versus stretching-dominated modes of failure[2], [3]. Gyroid TPMS for example, showed up to 6-fold increase in compressive strength and improved bone formation in critical defects in rabbit femur condyles over strut-based lattices, attributed to high permeability and curvature profile of the TPMS[4]. Additive manufacturing methods such as digital light projection (DLP) 3D printing are promising approaches to fabricate TPMS, affording both high resolution and expansive material libraries driven by photoinitiated radical polymerization. Since TPMS structures are mathematically designed using level-set approximations, they uniquely enable tissue specific scaffold designs[5]. The aims of this study are to evaluate the printability of TPMS microstructures using DLP-based printing technology, and to provide insight into the choice of TPMS as a load bearing structure with a focus on cartilage tissue engineering, factoring in geometry, morphology, pore size, and feature size. Towards this first aim, the appropriate DLP-printing parameters to achieve high resolution 25 µm layer printing were determined. Towards the second aim, five different TPMS geometries were 3D printed using DLP and assessed by scanning electron microscopy. Our long-term goal is to assess how smooth surface topologies and enhanced load bearing capacity afforded by the TPMS geometries impact chondrogenic cell function.

Materials and

Methods: A 3D printing resin consisting of poly(β-amino ester) diacrylate (PBAE-dA) macromer[6], [7], 0.85 wt% diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as the photoinitiator, and 0.15 wt% Quinoline Yellow (QY) as the UV absorber was used. The macromer was synthesized in a one-step Aza-Michael click reaction mixing 1,4-butanediol diacrylate and benzhydrazide (10 M in DMSO) at a 3:2 molar ratio at 90ºC for 18 h, and used without further purification[6], [8].

Five TPMS Structures (Gyroid-Sheet and -Solid, Diamond-Sheet and -Solid, and Primitive-Sheet) were designed using MSLattice Software[9] in a cylindrical shape (ɸ = 4.2 mm x h = 2 mm; 25% Relative Density; 1 mm unit cell), then sliced into 25 µm layer image projections. UV-vis spectroscopy and Near-IR RT-FTIR analysis was used to validate optical depth of penetration, acrylate (C=C) conversion (6100-6200 cm-1), and critical exposure times. A custom-built DLP system[10] with an irradiation intensity of I = 25 mW cm-2 at λ = 405 nm was used to print the desired structures with 25 µm layer heights. Prints were sonicated in pure ethanol for 1 hour to remove excess resin, then allowed to sit in 70% ethanol for 24 h before drying at 60ºC for 2 h. Structures were sputtered with platinum and scanning electron microscopy (SEM, Hitachi TM-4000PlusE-2) images were taken in SE mode with 5 kV accelerating voltage for surface analysis. Micro-CT (Zeiss XRM) scan with 60 kV accelerating voltage, 5 W power, 1 s exposure time, and 201 projections was employed to assess geometric properties of the printed structures.

Results, Conclusions, and Discussions: UV-vis measurements yielded a depth of penetration of Dp = 30 µm using the molar absorptivity of TPO (204 M-1cm-1) and QY (22,246 M-1cm-1). Monitoring acrylate (C=C) bond conversion in RT-FTIR, a maximum final conversion of 96% after 30s of irradiation was achieved (Figure 1). An effective working curve was constructed as per prior work[6] using conversion data (Figure 2), from which printing at a prescribed intensity of I = 25 mWcm2, a 10 second exposure time (E = 250 mJ cm-2) afforded 25 µm layers. Cross-section SEM images (Figure 3) of Diamond- and Gyroid- Sheet structures verify 25 µm layers and show a continuous and non-intersecting pore distribution throughout the structure. SEM images of all printed structures reveal consistent pore sizes and feature spacing. For example, visible pore openings in the Diamond Sheet, Primitive Sheet, and Gyroid-Solid structures are all on the 300 µm scale, whereas Gyroid-Sheet, Diamond-Solid, as well as the top surface features of Gyroid-Solid, show protruding features which are separated by a relatively larger distance on the 400-500 µm scale. MicroCT analysis (Figure 4) of the printed scaffolds revealed elevated solid volume fractions (Gyroid-Sheet: 27.03%, Gyroid-Solid: 26.55%, Diamond-Sheet: 29.99%, Diamond-Solid: 27.25%, Primitive-Sheet: 26.47%) compared to the initial designs (25% Relative Density). However, all of them except for the diamond-sheet are within a 10% difference from the desired relative density.

Future work will incorporate cell-laden hydrogels infilled into TPMS structures to evaluate the composite biomaterial for cartilage tissue regeneration, both in a free-swelling environment and under dynamic compressive loading within a custom-built bioreactor. Analysis of cell distribution, chondrogenic and osteogenic differentiation, and collagen production will provide insight into the application of DLP-printed TPMS structures for osteochondral tissue regeneration.

Acknowledgements (Optional): References
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