Cell-laden Microgels Research Articles

Characterization of the mechanical properties of microgels acting as cellular microenvironments

There is an increasing effort in the search and characterization of materials that can function as artificial three-dimensional cellular microenvironments. In particular, the encapsulation of cells in micrometer-size hydrogel particles (microgels) offers the ease of cell visualization, the transport of nutrients, and the ability to generate large combinatorial libraries of cellular microenvironments. In the present work, we report an atomic force microscopy (AFM) method for the characterization of an average Young's modulus and the stress relaxation time of micrometer-size polymer microgel particles. We studied the mechanical properties of agarose microgels with different polymer concentration at room temperature and 37 °C (the temperature of cell culture). We examined the temporal variation in the Young modulus of cell-free and cell-laden microgels under physiological conditions. Furthermore, we correlated the mechanical properties of the microgels laden with a single carcinoma associated fibroblast (CAF) cell with the spatial distribution of collagen around the cell. This work is potentially useful for studies of changes in cellular microenvironments, when cells locally degrade the matrix or secrete molecules that change the mechanical properties of the matrix.

Controlled Synthesis of Cell-Laden Microgels by Radical-Free Gelation in Droplet Microfluidics

Micrometer-sized hydrogel particles that contain living cells can be fabricated with exquisite control through the use of droplet-based microfluidics and bioinert polymers such as polyethyleneglycol (PEG) and hyperbranched polyglycerol (hPG). However, in existing techniques, the microgel gelation is often achieved through harmful reactions with free radicals. This is detrimental for the viability of the encapsulated cells. To overcome this limitation, we present a technique that combines droplet microfluidic templating with bio-orthogonal thiol-ene click reactions to fabricate monodisperse, cell-laden microgel particles. The gelation of these microgels is achieved via the nucleophilic Michael addition of dithiolated PEG macro-cross-linkers to acrylated hPG building blocks and does not require any initiator. We systematically vary the microgel properties through the use of PEG linkers with different molecular weights along with different concentrations of macromonomers to investigate the influence of these parameters on the viability and proliferation of encapsulated yeast cells. We also demonstrate the encapsulation of mammalian cells including fibroblasts and lymphoblasts.

Hydrogel microrobots actuated by optically generated vapour bubbles

A novel hydrogel microrobot made of poly(ethylene glycol) diacrylate (PEGDA) is reported. This disk-shaped microrobot is optothermally actuated by laser-induced bubbles. A pulsed laser is used to smoothly actuate an 80-μm-diameter bubble microrobot at up to 320 μm s(-1). A single microrobot or a pair of microrobots working in cooperation were used to assemble 20-μm-diameter polystyrene beads into different patterns. The microrobots were also used to assemble patterns made of single yeast cells and cell-laden agarose microgels. The patterned yeast cells and cell-laden microgels were cultured, and the cells successfully multiplied.

Directed assembly of cell‐laden microgels for building porous three‐dimensional tissue constructs

The organization of cells within a well-defined microenvironment is important in generating the resulting tissue function. However, the cellular organization within biodegradable scaffolds often does not resemble those of native tissues. In this study, we present directed assembly of microgels to organize cells for building porous 3D tissue constructs. Cell-laden microgels were generated by molding photocrosslinkable polyethylene glycol diacrylate within a poly(dimethyl siloxane) stencil. The resulting microgels were subsequently packed as individual layers (1 mm in height) on a glass substrate by removing the excess prepolymer solution around the microgels. These clusters were crosslinked and stacked on one another to fabricate thick 3D constructs that were greater than 1 cm in width and 3 mm in thickness. To generate pores within the engineered structures, sodium alginate microgels were integrated in the engineered constructs and used as a sacrificial template. These pores may be potentially useful for fabricating a vascular network to supply oxygen and nutrients to the engineered tissue constructs. This simple and versatile building approach may be a useful tool for various 3D tissue culture and engineering applications.

Interface-directed self-assembly of cell-laden microgels.

Cell-laden hydrogels show great promise for creating engineered tissues. However, a major shortcoming with these systems has been the inability to fabricate structures with controlled micrometer-scale features on a biologically relevant length scale. In this Full Paper, a rapid method is demonstrated for creating centimeter-scale, cell-laden hydrogels through the assembly of shape-controlled microgels or a liquid-air interface. Cell-laden microgels of specific shapes are randomly placed on the surface of a high-density, hydrophobic solution, induced to aggregate and then crosslinked into macroscale tissue-like structures. The resulting assemblies are cell-laden hydrogel sheets consisting of tightly packed, ordered microgel units. In addition, a hierarchical approach creates complex multigel building blocks, which are then assembled into tissues with precise spatial control over the cell distribution. The results demonstrate that forces at an air-liquid interface can be used to self-assemble spatially controllable, cocultured tissue-like structures.

A droplet-based building block approach for bladder smooth muscle cell (SMC) proliferation

Tissue engineering based on building blocks is an emerging method to fabricate 3D tissue constructs. This method requires depositing and assembling building blocks (cell-laden microgels) at high throughput. The current technologies (e.g., molding and photolithography) to fabricate microgels have throughput challenges and provide limited control over building block properties (e.g., cell density). The cell-encapsulating droplet generation technique has potential to address these challenges. In this study, we monitored individual building blocks for viability, proliferation and cell density. The results showed that (i) SMCs can be encapsulated in collagen droplets with high viability (>94.2 ± 3.2%) for four cases of initial number of cells per building block (i.e. 7 ± 2, 16 ± 2, 26 ± 3 and 37 ± 3 cells/building block). (ii) Encapsulated SMCs can proliferate in building blocks at rates that are consistent (1.49 ± 0.29) across all four cases, compared to that of the controls. (iii) By assembling these building blocks, we created an SMC patch (5 mm × 5 mm × 20 µm), which was cultured for 51 days forming a 3D tissue-like construct. The histology of the cultured patch was compared to that of a native rat bladder. These results indicate the potential of creating 3D tissue models at high throughput in vitro using building blocks.

Surface‐directed assembly of cell‐laden microgels

Cell-laden microscale hydrogels (microgels) can be used as tissue building blocks and assembled to create 3D tissue constructs with well-defined microarchitecture. In this article, we present a bottom-up approach to achieve microgel assembly on a patterned surface. Driven by surface tension, the hydrophilic microgels can be assembled into well-defined shapes on a glass surface patterned with hydrophobic and hydrophilic regions. We found that the cuboidic microgels ( approximately 100-200 microm in width) could self-assemble into defined shapes with high fidelity to the surface patterns. The microgel assembly process was improved by increasing the hydrophilicity of the microgels and reducing the surface tension of the surrounding solution. The assembled microgels were stabilized by a secondary crosslinking step. Assembled microgels containing cells stained with different dyes were fabricated to demonstrate the application of this approach for engineering microscale tissue constructs containing multiple cell types. This bottom-up approach enables rapid fabrication of cell-laden microgel assemblies with pre-defined geometrical and biological features, which is easily scalable and can be potentially used in microscale tissue engineering applications.

Method of Bottom-Up Directed Assembly of Cell-Laden Microgels

The paper describes a protocol to fabricate cell-laden microgel assemblies with pre-defined micro-architecture and complexity by a bottom-up approach, which can be used for tissue engineering applications. The assembly process was driven by the hydrophobic effect in the water/oil interface. By agitating hydrophilic microgels in hydrophobic medium, the shape-controlled microgel units assemble in an organized manner to locally minimize the interaction free energy (the surface area exposed to the oil). The assembly process was shown to be controlled by several parameters, such as external energy input, surface tension, and microgel dimensions. This assembly approach was used to build multi-component cell-laden constructs by assembling microgel building blocks and performing a secondary cross-linking reaction. This bottom-up approach for the directed assembly of cell-laden microgels offers a scalable method to fabricate 3D tissue constructs with biomimetic structure.

Directed assembly of cell-laden microgels for fabrication of 3D tissue constructs

We present a bottom-up approach to direct the assembly of cell-laden microgels to generate tissue constructs with tunable microarchitecture and complexity. This assembly process is driven by the tendency of multiphase liquid-liquid systems to minimize the surface area and the resulting surface free energy between the phases. We demonstrate that shape-controlled microgels spontaneously assemble within multiphase reactor systems into predetermined geometric configurations. Furthermore, we characterize the parameters that influence the assembly process, such as external energy input, surface tension, and microgel dimensions. Finally, we show that multicomponent cell-laden constructs could be generated by assembling microgel building blocks and performing a secondary cross-linking reaction. This bottom-up approach for the directed assembly of cell-laden microgels provides a powerful and highly scalable approach to form biomimetic 3D tissue constructs and opens a paradigm for directing the assembly of mesoscale materials.

Stop-flow lithography to generate cell-laden microgel particles

Encapsulating cells within hydrogels is important for generating three-dimensional (3D) tissue constructs for drug delivery and tissue engineering. This paper describes, for the first time, the fabrication of large numbers of cell-laden microgel particles using a continuous microfluidic process called stop-flow lithography (SFL). Prepolymer solution containing cells was flowed through a microfluidic device and arrays of individual particles were repeatedly defined using pulses of UV light through a transparency mask. Unlike photolithography, SFL can be used to synthesize microgel particles continuously while maintaining control over particle size, shape and anisotropy. Therefore, SFL may become a useful tool for generating cell-laden microgels for various biomedical applications.

Editorial: Tissue Engineering: Perspectives, Challenges, and Future Directions

Editorial: Tissue Engineering: Perspectives, Challenges, and Future Directions