The extracellular matrix of native human tissues is complex, in terms of biochemical composition, multi-scale architecture, mechanical properties and cellular population. To artificially reproduce and to control the growth’s direction of such tissue, scientists try to insert adapted guiding cues for the cells to follow. If healthy cells – autologous or allogeneic – were to be inserted alone onto the injury site of a patient, cells could leak to the rest of the body and their survival won’t be insured as they arrive in a degraded environment. That’s why there is currently a consensus in the tissue engineering community to provide the transplanted cells a healthy 3D-environment, so the cell’s healing potential is optimized .
Hydrogel microparticles typically ranging from 1 to 350 µm are called microgels. In the last decade, microgels have drawn increasing attention in the scientific community, due to their tailorable size and viscoelasticity, their high water content, and their ability to encapsulate bioactive factors. Relative to a classic bulk hydrogel, they display a high specific surface area with space between each hydrogel microparticle. This feature allows (i) cells to more easily colonize the designed scaffold, as they have more space to grow into; (ii) a more efficient nutrient and waste transfer and thus (iii) improved cell-cell and cell-matrix interactions. Moreover, microgels are easily injectable in vivo. By avoiding invasive surgical procedures and associated complications, they are more pertinent for clinical uses .
Microgels can be implemented as an autonomous system, with microparticles linked together either through a chemical bond (permanent), a physical bond (reversible), via cell interaction, or by applying an external force, such as a magnetic field if the microgels contain a paramagnetic compound. “Click chemistry” is an example of recently developed techniques that support the precise tailoring of the designed biomaterial properties. Click chemistry supplies unique mild bioorthogonal reactions with a high reactivity and selectivity. Thus, fine-tuned biomaterials can achieve both spatial and temporal control over cellular behaviors with a large variety of microgels morphology: classic spherical, rod-shaped, cylindrical, etc. depending on the desired application .
Microgels can also be incorporated inside a “carrier” bulk hydrogel, in a “Hydrogel-Microgel” composite macroscopic system (see illustration below). Our last Article of the Month published last week presented e.g. magneto-responsive, rod-shaped, peptide-modified and PEG-based microgels, encapsulated into a fibrin hydrogel for various anisotropic tissue regeneration, depending on the concentration used and on the associated viscoelasticity of the produced scaffold .