Raquel Filipa Penacho Parreira
Tissues morphogenesis and homeostasis involve the spatiotemporal regulation of mechanics at multiple scales. Characterization of mechanical properties of biological systems as well as investigating the effects of mechanical forces on biological function are instrumental. However, existing biomanipulation systems are bulky and invasive, therefore, they do not allow application of forces within native tissues or biomimetic platforms. Investigation done with cells cultured on planar substrates provide limited information on multicellular organization and the interactions between cells and the surrounding extracellular matrix (ECM). Recent work has introduced biochemically and mechanically tunable synthetic matrices, with which the mechanics of the cellular microenvironment can be engineered. What has been missing is an actuated polymer system that can be freely shaped at microscale, and be seamlessly interfaced with living systems. Considering the small size of the actuators, the power must be transmitted wirelessly. Application of physiologically relevant forces using physiologically acceptable energy input requires an efficient transduction mechanism. This thesis is built upon two important nanoscale phenomena. Gold nanoparticles efficiently transduce visible light into localized heat due to plasmon resonance, and certain class of polymers display powerful contractions in the course of microseconds by going through a thermally-induced hydrophilic to hydrophobic transition. The thesis exploits these two phenomena on the same platform using nanotechnology and chemical synthesis, and introduces a series of microengineering techniques that would transform the active nanomaterial into microscale soft actuators and machines. To this end, the thesis explores a number of bottom-up engineering approaches including droplet microfluidics, magnetic and hydrodynamic interactions, and thermocapillary effects. The deformation generated by the actuators is transformed into a desired set of mechanical operations using rationally designed hydrogel mechanisms.The use of wirelessly-powered microactuators for mechanobiology research is demonstrated at two different levels. First, a high-throughput microscale compression device is built for bulk mechanical loading of three-dimensional (3D) culture models such as spheroids and organoids. Second, the chemical crosslinking of microactuators to collagen fibers is achieved to apply local forces to cells residing within reconstituted collagen I gels. The materials and methods are not restricted to the cell types and ECM components that are explored in this thesis, therefore, the technology can be applied to study almost any mechanobiology process in vitro. As a benchmark to estimate stress and strain that must be applied by wireless actuators in order to initiate a biological response, we performed a detailed study on the mechanical loading of mammary acini inside collagen matrices using a tethered robotic micromanipulator. The results of the study show that externally applied mechanical tension facilitate transition to an invasive phenotype, which involves long-distance force transmission, activation of mechanotransduction pathways, and plasticity of collagen. These conventional micromanipulation techniques are complementary to the presented novel wireless actuation scheme, together pushing the boundaries of our understanding of living systems.
EPFL2021
