Our research applies and integrates fundamental engineering principles, such as manufacturing, biomechanics, materials science, and micro/nanoengineering, to understand and harness the mechanobiology of stem cells for modeling currently incurable human diseases and for applications in regenerative medicine.

1. Biomimetic In Vitro Models for Studying Human Development

Direct investigation of human development is limited technically and ethically. For example, the neural development in humans initiates between embryonic day 17 – 19, while culturing human embryos to the neurulation stage is still technically challenging. Several groups, including ours, pioneered the concept of using in vitro cultured hPSCs to model gastrulation and neurulation, leading to a completely new field of “synthetic embryology”. We pioneered a neuroectoderm microtissue model with neuroepithelial cells in the center of the microtissue and neural plate border cells on the periphery. In this work, we found that mechanical forces-mediated BMP signaling activation was responsible for the formation of the two cell layers.
Supported by NSF, our group recently developed a new protocol to derive complete ectoderm with three layers (neuroepithelium, neural crest, and epidermis). In collaboration with Dr. Hongyan Yuan, we demonstrated that the time-dependent reaction-diffusion of BMP and WNT signaling can lead to spatial patterning of different cell types by modulating the wavelength and phase of the reaction-diffusion signaling patterns. This work for the first time provides a chemically defined and controllable platform to investigate the functional role of individual morphogen and signal transduction in human ectoderm development.
[1] Xue X, Sun Y, Resto-Irizarry AM, Yuan Y, Aw Yong KM, Zheng Y, Weng S, Shao Y, Chai Y, Studer L, Fu J. Mechanics-guided embryonic patterning of neuroectoderm tissue from human pluripotent stem cells. Nat Mater. 2018;17:633-41.
[2] Xie T, Kang J, Pak C, Yuan H, Sun Y. Temporal Modulations of NODAL, BMP, and WNT Signals Guide the Spatial Patterning in Self-Organized Human Ectoderm Tissues. Matter. 2020;2(6):1621-38.
[3] Tiankai Zhao, Yubing Sun, Xin Li, Mehdi Baghaee, Yuenan Wang, and Hongyan Yuan. A contraction-reaction-diffusion model for circular pattern formation in embryogenesis, Journal of the Mechanics and Physics of Solids, vol. 157, 104630, 2021.

2. Collective cell rearrangement and migration

Spindle-like cells such as 3T3 fibroblasts and neural stem cells are conventionally described as active nematics that form topological defects with half-integral winding numbers when geometrically confined. Elucidating the biophysics of the cell rearrangement and migration is important for understanding tissue homeostasis and wound repairs.
In collaboration with Dr. Min Wu at WPI, we recently discovered a completely different collective cell behavior of contractile, mesenchymal cells with isotropic actin meshwork. We described a new class of collective cell arrangement where Rat Embryonic Fibroblasts (REFs) develop radial-alignment boundaries when confined in circular and ring mesoscale patterns. Through combined experimental and computational studies, we found that the formation of such radial alignment is not a result of directed cell migration and requires a “condensation tendency”, a tendency of tissue contraction and condensation, and an emergent cell stiffness differential.
[1] Yubing Sun, Christopher S. Chen, and Jianping Fu. Forcing stem cells to behave: A biophysical perspective of cellular microenvironment. Annual Review of Biophysics, vol. 41, pp. 519-542, 2012.
[2] Yubing Sun, Koh Meng Aw Yong, Luis G. Villa-Diaz, Xiaoli Zhang, Weiqiang Chen, Renee Philson, Shinuo Weng, Haoxing Xu, Paul H. Krebsbach and Jianping Fu. Hippo/YAP-mediated rigidity-dependent motor neuron differentiation of human pluripotent stem cells. Nature Materials, vol. 13, pp. 599-604, 2014.
[3] Xie T, St Pierre SR, Olaranont N, Brown LE, Wu M, Sun Y. Condensation tendency and planar isotropic actin gradient induce radial alignment in confined monolayers. eLife. 2021;10:e60381.

3. Morphogen gradient induced patterning of brain organoids

Brain organoids derived from hPSCs provide a tractable in vitro system to study brain development and diseases. Specifically, human Forebrain Organoids (hFOs) are especially important for modeling neuropsychiatric diseases such as schizophrenia and autism spectrum disorders. Current brain organoids are mostly derived by exposing embryoid bodies in the media with uniform growth factors and small molecules, relying on the spontaneous differentiation capability of hPSCs. As a result, the obtained organoids often lack precise arealization within individual organoids and have undesired batch-to-batch variation, which hinders us from unlocking their full potential in standardized applications such as drug and toxin screening.
In collaboration with Dr. ChangHui Pak in the Department of Biochemistry and Molecular Biology, our group has developed a novel localized passive-diffusion (LPaD) based morphogen concentration generation device12 to pattern brain organoids. We recently demonstrated that this system can stably maintain morphogen gradients for about a week, and it can effectively induce dorsoventral patterning within single hFOs. We further developed a second system that utilizes surgical sutures as supporting structures and micro-holes to restrict diffusion to induce morphogen gradient. We have demonstrated the feasibility of using these systems to generate properly patterned hFOs by introducing a sonic hedgehog signaling gradient, which is critical for understanding interneuron migration and excitatory-inhibitory neuronal interactions.
[1] Li N, Yang F, Parthasarathy S, Pierre SS, Hong K, Pavon N, Pak C, Sun Y. Patterning Neuroepithelial Cell Sheet via a Sustained Chemical Gradient Generated by Localized Passive Diffusion Devices. ACS Biomaterials Science & Engineering. 2021;7(4):1713-21.