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. Tools development for mechanobiology


Fig 1

While many biochemical signals have been shown to mediate cell functions, emerging evidence suggest that biomechanical stimuli, including substrate rigidity and external forces, also play pivotal roles in many physiological processes. Therefore, mechanobiology becomes an emerging research field that aim to clarify the biological foundation of cellular responses to these biomechanical stimulation.
Our group has pioneered the development of a novel technique to use polydimethylsiloxane (PDMS) micropost arrays (PMAs), geometrically regulated elastomeric microposts that present the same surface geometry but different post heights to control substrate rigidity (a). We also applied novel photosensitive polydimethylsiloxane (photoPDMS) chemistry (b) to create photosensitive, biocompatible photoPDMS andfabricated photoPDMS micropost arrays. This technique allows us to independently modulate micrometer scale “structure rigidity” and nanoscale bulk rigidity. We further integrated PMAs with microfluidic channels (c) to apply fluid shear stress to cells and simultaneously measure the dynamic intracellular contractile forces within the cells. Recently, we integrated PMAs with a novel acoustic tweezing cytometry technique (d) that utilizes ultrasound excitation of membranebound gaseous microbubbles to generate controllable subcellular mechanical stimulations to live single cells.
These examples show that the research in our lab seamlessly integrates the engineering principles with biomedical applications.  We can ask critical biological questions that biologists often try to avoid due to the lack of tools, because as engineers, we will have the capability to develop novel tools and assays. These tools will not only help dissect the mechanosensing principles of cells but also facilitate applications such as stem cell based therapy and cancer diagnostics. 
[1] Di Chen*, Yubing Sun*, Cheri X. Deng, and Jianping Fu. Improving survival of disassociated human embryonic stem cells by mechanical stimulation using acoustic tweezing cytometry. Biophysical Journal (Biophysical Letter), vol. 108, pp. 1315-1317, 2015. [web link]
[2] Zhenzhen Fan*, Yubing Sun*, Di Chen*, Weiqiang Chen, Cheri Deng, and Jianping Fu. Acoustic tweezing cytometry for live-cell subcellular control of intracellular cytoskeleton contractility. Scientific Reports, vol. 3, 2176, 2013.[web link]
[3] Raymond Hiu-Wai Lam, Yubing Sun, Weiqiang Chen, and Jianping Fu. Elastomeric microposts integrated into microfluidics for flow-mediated endothelial mechanotransduction analysis. Lab on a Chip, vol. 12, pp. 1865-1873, 2012.[web link]
[4] Yubing Sun, Liang-Ting Jiang, Ryoji Okada, and Jianping Fu. UV-modulated substrate rigidity for multiscale study of mechanoresponsive cellular behaviors. Langmuir, vol. 28, pp. 10789-10796, 2012. [web link]
[5] Jennifer M. Mann*, Raymond Hiu-Wai Lam*, Shinuo Weng, Yubing Sun, and Jianping Fu. A silicone-based stretchable micropost array membrane for monitoring live-cell subcellular cytoskeletal response. Lab on a Chip, vol. 12, pp. 731-740, 2012. [web link]

2. Mechanosensitive hPSCs differentiation


Stem cells have significant potential to regenerate human tissues and restore organ function in chronic illnesses such as diabetes and also to study disease pathology in vitro. The current inability to differentiate sufficient numbers of stem cells into physiologically functional derivatives is a significant bottleneck in the large-scale applications of stem cells. Engineering mechanical properties (e.g. rigidity) of stem cell microenvironment is a promising approach to overcome this hurdle. For example, using the PMA system, we demonstrate that hPSCs are intrinsically mechanosensitive, and that substrate rigidity is an extracellular switch that directs cell identity and differentiation potential of hPSCs. Indeed, neuroepithelial vs. neural crest lineage decisions and anterior vs. posterior patterning are dictated by substrate rigidity. By modulating substrate rigidity, the purity and yield of functional motor neurons (MNs) derived from hPSCs was improved four- and twelve-fold, respectively, over conventional protocols, which is considered a major breakthrough to both the regenerative medicine and mechanobiology fields. We also showed that the Hippo-YAP pathway regulates the mechanosensitive neuroepithelial conversion of hPSCs. This will facilitate the elucidation of the biophysical interactions between hPSCs and the cell microenvironment to improve the large-scale culture of hPSC in the future.
However, the restricted understanding of the mechanism(s) underlying cellular mechanosensing currently limits this approach. Our research in this direction will focus on two themes: 1) Elucidating the signaling pathways that are responsible for the mechanosensitivity of hPSCs self-renewal and differentiation, and 2) design materials system to modulate the mechanical properties of stem cell microenvironment.
[1] Yubing Sun and Jianping Fu. Harnessing mechanobiology of human pluripotent stem cells for regenerative medicine. ACS Chemical Neuroscience, vol. 5, pp. 621-623, 2014.[web link]
[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. [web link]
[3] Yubing Sun and Jianping Fu. Mechanobiology: A new frontier for human pluripotent stem cells. Integrative Biology, vol. 5, pp. 450-457, 2013. [web link]
[4] Weiqiang Chen, Yubing Sun, and Jianping Fu. Microfabricated nanotopological surfaces for study of adhesion-dependent cell mechanosensitivity. Small, vol. 9, pp. 81-89, 2013. [web link]
[5] 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. [web link]
[6] Yubing Sun, Luis G. Villa-Diaz, Raymond Hiu-Wai Lam, Weiqiang Chen, Paul H. Krebsbach, and Jianping Fu. Matrix mechanics regulates fate decisions of human embryonic stem cells. PLoS ONE, vol. 7, e37178, 2012. [web link]

3. 3D organotypic culture and developmental biomechanics

Current development biology studies using animal models often provide limited insights in the human development and are lack of cellular and molecular details. What if stem cells turn into embryos in a dish? In this way, it is possible to study human development directly bypassing the ethical and technical difficulty of using human embryo. 
To make that happen, it is important to first understand the fundamental principles of development process. The development processes, in fact, involve various mechanical cues such as folding of tissues and cell shape changes. More importantly, the formation of well-organized three-dimensional complex tissue/organ structures requires precise control of local mechanical forces and mechanical properties. 
Therefore, it is our goal to develop innovative approaches to first experimentally and theoretically explore the principles and dynamics of mechanical factors in development. Based on the insights we gain from those studies, we will be able to build tools to guide the differentiation/assembly process. For example, we demonstrate that geometrical constraint could mimic the neural plate border specification and using such in vitro model, we find that mechanical forces and corresponding cell shape changes might be required for neural plate specification. The long term goal of this research is to build highly complex tissues/organs models using human stem cells as innovative tools to understand human development.