Abstract
Hydrogel microparticles (HMPs) are an emerging bioink that can allow three-dimensional (3D) printing of most soft biomaterials by improving physical support and maintaining biological functions. However, the mechanisms of HMP jamming within printing nozzles and yielding to flow remain underexplored. Here, we present an in-depth investigation via both experimental and computational methods on the HMP dissipation process during printing as a result of (i) external resistance from the printing apparatus and (ii) internal physicochemical properties of HMPs. In general, a small syringe opening, large or polydisperse size of HMPs, and less deformable HMPs induce high resistance and closer HMP packing, which improves printing fidelity and stability due to increased interparticle adhesion. However, smooth extrusion and preserving viability of encapsulated cells require low resistance during printing, which is associated with less shear stress. These findings can be used to improve printability of HMPs and facilitate their broader use in 3D bioprinting.
Highlights
Three-dimensional (3D) bioprinting fabricates scaffolds and extracellular matrices with living cells and has potentials to meet the needs for tissue engineering [1,2,3,4]
Similar to other reported Hydrogel microparticles (HMPs) bioinks [11, 12, 15], the poly(ethylene glycol) (PEG) HMP pellets that jammed in the printing nozzle demonstrated shear-thinning properties under all three different moduli, indicating that interparticle frictions are dissipated under pressure to allow the movement of HMPs (Fig. 1, B and C)
Recent advances suggest that the printability of these soft biomaterials can be largely improved after cross-linking them into solid granules or HMPs [23]
Summary
Three-dimensional (3D) bioprinting fabricates scaffolds and extracellular matrices with living cells and has potentials to meet the needs for tissue engineering [1,2,3,4]. Various strategies have been developed to modify existing 3D printing techniques and expand the bioink toolkit, including locking the printed shape in an uncross-linked state using suspension baths [5, 6], rapid improvement of the mechanical strength of bioinks after extrusion via double network formation [7], photopolymerization immediately before extrusion via transparent nozzles [8], and reinforcing the bioinks using rheological additives such as nanoparticles [9, 10] These advanced printing methods have enabled fabrication of intricate 3D structures from many soft biopolymers. Dedicated designs of cross-linking chemistries and suspension baths are required for these strategies, which limits their versatility to broad ranges of soft biomaterials of interest
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