Addition and Correction to "Thermo-Transitioning Core-Shell Microgels Combine Cohesive Reinforcement and Noncohesive Reconfigurability to Enable 3D Bioprinting and Stabilize Tissues During Incubation".

Hydrogel microparticles (microgels) have significant potential for use as building blocks in tissue engineering, as bioinks for 3D bioprinting, and as drug and cell carriers for cell-based therapies targeting damaged and diseased tissues. Various fabrication techniques have been developed for producing microgels with predefined shapes and sizes. However, for practical applications in biological laboratories and clinics, it is necessary to reduce time costs and simplify instrumentation and synthesis protocols, improving their reproducibility and reliability. Here we demonstrate a three-step experimental approach to develop microfluidic flow-focusing droplet generators that enable the introduction of all liquids by creating negative pressure in the outlet reservoir for the generation of spherical, core-shell, and Janus alginate microgels with living cells. This approach allows the use of a simple experimental setup that is easy to operate and robust and provides highly reproducible results, achieving a synthesis performance of up to 200 μL of microgels per hour. The size and the structure of the microgels were determined by the chip design and remained stable under pressure variations within the operating range of -7 to -15 kPa. This enabled the reliable and reproducible encapsulation of CT26 and HepG2 cells into core-shell and Janus alginate microgels with diameters ranging from 80 to 120 μm, maintaining over 80% cell viability during long-term incubation. Our findings offer a new perspective for the automation and scaling of multicomponent alginate microgel fabrication, paving the way for their implementation in tissue engineering and 3D bioprinting.

The pace of progress in tissue engineering and biomedical research could be accelerated by developing improved biofabrication methods that are capable of precisely assembling cells into complex structures. Embedded 3D bioprinting, which often uses packed microgel particles as a support environment, is a promising way to manufacture and culture tissue constructs. The facile reconfigurability of noncohesive microgel support materials enables precise printing but possesses limited mechanical stability. By contrast, cohesive microgels provide enhanced stability yet create kinetic or energetic constraints to reconfiguration during embedded 3D printing processes. Here, we introduce a microgel system that combines the benefits of both cohesive and noncohesive microgels by grafting a poly(N-isopropylacrylamide) (PNIPAM) shell onto polyethylene glycol (PEG) microgel core. These PNIPAM-coated PEG microgels exhibit temperature-dependent interparticle interactions. At room temperature, the microgels remain noncohesive, minimizing constraints on particle reconfiguration and enabling high-quality biofabrication. Upon incubation at 37 °C, the microgels transition to a cohesive state, providing additional structural integrity during tissue culture. We find a phase partitioning behavior between PNIPAM polymer chains and bare PEG microgels that underpins the surface grafting process and correlates with a unique transition in its yielding behavior that is not exhibited by bare PEG microgels. Additionally, the PEG/PNIPAM microgels exhibit only weakly varying linear material properties across temperature shifts, in contrast to pure PEG microgels, which soften dramatically at higher temperatures. Tests of 3D bioprinting structures made from MDCK and 3t3 cells demonstrate the PNIPAM-coated PEG microgel system's ability to maintain cell viability and structure during tissue culture. The work reported here highlights the potential of this thermally tunable microgel system for use in advanced tissue engineering applications, offering precision during fabrication and stability during tissue culture.

Construction of 3D printed constructs based on microfluidic microgel for bone regeneration