Hydrogel-based artificial cell niche [electronic resource] : mimicking biophysical cues of extracellular matrix for tissue regeneration
Keywords: Materials Science and Engineering.

Human tissues have distinct biophysical properties such as stiffness and topography to serve their specific functions. Tissues show a broad range of stiffness spanning from 100 Pa (brain, lung) to MPa (bone). Also, some tissues including muscle and tendon, have unique anisotropic topography. Given cell fates are known to be modulated by tissue-specific stiffness and topography, recreating artificial cell niche mimicking such properties, is of great importance to construct functional tissues in vitro. My thesis work focuses on developing and engineering hydrogel-based artificial cell niche to explore how stiffness and topography of cell niche influence cell behaviors and tissue regeneration. While two-dimensional (2D) hydrogel substrates have been widely used to study effect of substrate stiffness on behaviors of human multipotent stem cells, very little is known on the effect on human pluripotent stem cells (hPSCs), a potentially promising cell source for tissue engineering and regenerative medicine. This is primarily due to lack of hydrogel platform that can support hPSC attachment and growth. Aim 1 of my thesis focuses on developing poly(acryl amide) hydrogel platform that enables robust hPSC attachment and growth to study mechanosensing of hPSCs. Using this platform, we showed that mechanosensing of hPSCs depends on not only substrate stiffness, but also biochemical cues. Moving from 2D to 3D, matrix stiffness has also been shown to play a significant role in regulating cell fate in 3D. To vary 3D matrix stiffness, hydrogel crosslinking density has often been controlled. While alteration in hydrogel crosslinking density can lead to simultaneous change in diffusion of various soluble factors, another potent regulator of cell fates, it is still unclear how altering hydrogel crosslinking density influences accumulation of soluble factors within 3D hydrogel. Second part of my thesis aims to systematically study effect of varying hydrogel crosslinking density and network structure on soluble factor accumulation and release. We showed that network structure (e.g. network mesh size and homogeneity) significantly influences accumulation and release of soluble factors. In addition to its role as a tool in deciphering cell-matrix interactions, hydrogels have also been used as temporary scaffold to encapsulate cells and be remodeled by the cells to construct 3D tissues in vitro. To date, it has been challenging to engineer 3D myocardial tissues using hPSC-derived cardiomyocytes (hPSC-CMs) due to poor cell viability and functionality within 3D hydrogel matrix, which may result from suboptimal stiffness and degradation rate of the hydrogel scaffold. Aim 3 of my thesis discusses engineering of stiffness and degradation rate of 3D gelatin hydrogel to enhance survival and functionality of hPSC-CMs to construct 3D myocardial tissues. We demonstrated that muscle-like stiffness and optimal degradation is crucial for constructing functional 3D myocardial tissues. Furthermore, to recapitulate anisotropic topography of 3D muscle tissues, hydrogels must provide topographical cues to guide cell alignment. While conventional tissue engineering strategies such as topographical patterning and electrospinning techniques, can help guide cell alignment and form anisotropic tissues, it often results in thin tissues and inhomogeneous cell distribution. In the last part, development of aligned microribbon-based hydrogel scaffold will be discussed for bioengineering 3D anisotropic smooth muscle tissues. We found that aligned microribbons effectively guided cell alignment and deposition of new matrix protein in an aligned manner, which is a hallmark of smooth muscle tissues. In summary, our developed hydrogel platforms help elucidate how cells respond to biophysical properties of their niche, which can be harnessed to accelerate functional tissue formation in vitro. Furthermore, such hydrogel platforms can be broadly applicable to other tissue engineering applications.