Mechanical principles of engineering fibrous tissues with programmable shape

The form and structure of biological tissues define their function. The emergence of tissue morphology during development is one of the wonders of nature. Cells mechanically probe and manipulate their surroundings while constructing structures from the extracellular matrix that they produce. Self-assembly of tissues is essentially a mechanical process, touching various engineering disciplines including solid and fluid mechanics as well as thermodynamics. Recent advancements in materials science, molecular biology, and microtechnology enabled the engineering of living tissues in vitro as a means to discover the physical principles of morphogenesis. The quest is far from being completed, particularly for the cases where cells are interacting through a fibrous matrix. There is no algorithm or protocol that can prescribe long-lasting shapes to these constructs with arbitrary complexity. Considering the potential impact of this endeavor in regenerative medicine and drug screening, novel perspectives are urgently needed.

The objective of this thesis is to provide an experimental and computational framework that would together enable the researcher to perform system identification and sculpturing on small scale biological samples. System identification involves dissecting the contributions of forces and motion on the evolution of the tissue shape. Sculpturing living matter means guiding the morphogenesis process through pre-programmed mechanical perturbations. Computational modeling of tissue mechanics aid both the interrogation and engineering phases. As the biological sample, a very simple composition that is based on a cell-laden collagen gel was used. Two novel technologies are introduced: (1) a fully-motorized robotic micromanipulation system and (2) a spatiotemporally resolved optochemical stimulation protocol. The platforms are integrated with modern imaging systems for real-time recording and automation.

The thesis starts with a detailed investigation of constrained fibrous microtissues where the emphasis was placed on the controlled perturbation of the mechanical state. Experiments revealed an opportunity and an important challenge. The study showed that surface stresses were as important as bulk stresses in the equilibrium configuration of the tissues. A physics-based computational model was developed that accurately captured tissue morphology by only considering bulk and surface contraction. The challenge is that tissues under the influence of these contractile stresses are inherently unstable and they are always inclined to deviate from the prescribed shape. Epithelial tissues undergo a fluid to solid transition. In the second part, this transition was harnessed to stabilize the tissue shape and enable reshaping upon mechanically induced fluidization. Unconstrained fibrous microtissues with an epithelial shell were successfully morphed into prescribed shapes and re-shaped under the guidance of precise mechanical manipulations. The final part of the thesis suggests various future directions that would capitalize on the presented results and advance the concept towards a robust and versatile tissue engineering solution.