Cellulose-based bioink formulations for 3D bioprinting

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Li, Na ORCID: https://orcid.org/0009-0001-5127-5712 (2024). Cellulose-based bioink formulations for 3D bioprinting. University of Birmingham. Ph.D.

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Abstract

In tissue engineering, although three-dimensional (3D) bioprinting has emerged as a revolutionary technology for precise manufacture of living biomimetic scaffolds, challenges persist in fulfilling the technical requirements concerning the processability of bioink formulations (scaffold precursors), the desired mechanical properties in scaffolds, and the cell-guided functionality of the biomaterials. To surmount the technical constraints of 3D bioprinting, cellulose-composited, including microfibrillated cellulose (MFC) (width of 0.3-2 µm) and nanofibrillated cellulose (NFC) (width of 30-100 nm), gelatin methacryloyl (G)/sodium alginate (A) bioink formulations were developed and constructed to cellulose-composited ionic-covalent entanglement (ICE) G/A hydrogel scaffolds.

Cellulose is proven to enhance the printability of G/A bioink formulation. Hydrogen bonds between cellulose and G/A impart a yield stress of the formulation, improving the stability of formulations before printing. The shear-induced orientation behaviours of MFC and NFC by hydrogen bond breaking can enhance the shear-thinning of G/A formulation, enabling it to smoothly pass through nozzles during printing. Also, due to the reformation of hydrogen bonds within cellulose network and between cellulose and G/A, cellulose introduces excellent thixotropy to G/A bioink and thus high recoverability after shear deformation, allowing it to maintain filament shape post-printing. MFC-composited G/A formulations with varying contents of MFC produce scaffolds with distinct printability indices under identical printing conditions, which can be explained by disparities in the yield stress and thixotropy among these formulations. The linear relationship between the layer numbers and the height of the 3D structure prepared by cellulose-composited G/A formulations can guide the building of complex structures with great shape fidelity.

Cellulose not only improves the macroscopic Young’s modulus of ICE hydrogel but also diversifies its microscopic Young’s modulus distributions, attributed to three different reinforcement types in hydrogel: cellulose network distributes energy evenly across the hydrogel; ICE network dissipates energy by ionic bonds breaking; synergy of fibril network and ICE G/A dissipates energy by hydrogen bonds. The increases in MFC concentration and fibril size from NFC to MFC are all beneficial to the strength of cellulose network and synergy of cellulose and ICE. From the perspective of micromorphology, the mechanical enhancement of ICE hydrogel by cellulose is because cellulose fibril networks fill large pores in ICE hydrogel and concentrate pores in a small range of 25 to 50 micros, thus avoiding stress concentration.

While the mechanical properties of cellulose-composited ICE hydrogels are enhanced, the high water-holding capabilities (87.83 to 89.71%) are also maintained, which is vital for hydrogels to balance between mechanical and biological functions. Also, cellulose extends the half-life of ICE hydrogels in enzymatic biodegradation from less than 7 to 12 days, which contributes to the biostability of hydrogels for performing mechanical support functions in the initial stage. Moreover, cells elongate in bioprinted cellulose-composited ICE scaffolds after 14 days and proliferate significantly after 21 days. Meanwhile, porosities of scaffolds rise after 21 days due to hydrogel degradation. Thus, degradation process of cellulose-composited ICE hydrogel scaffolds aligns exceptionally well with tissue remodelling process. Furthermore, cellulose prolongs the half-life of ampicillin release process from hydrogels and changes drug release kinetics from diffusion-controlled to anomalous transport, which facilitates the controlled, gradual release of drugs within hydrogel systems.

Overall, cellulose, particularly MFC, demonstrates great potential in simultaneously improving the printability of bioink formulations, the mechanical properties, and the biological functionality of hydrogel scaffolds. This thesis provides meaningful guidance for designing bioprinted biomimetic scaffolds that more closely align with the extracellular matrix of human tissues.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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