Our lab has two research thrusts: mechanics of living cells (biomechanics), and nanoscale materials (nanomechanics). The underlying themes that link the two thrusts are the principles of mechanics and the processes that are relevant at small scale. We are interested at the fundamental mechanisms that determine the processes. We use both experiment and theory to address our questions. In order to carry out exploratory experiments, we often develop our own micro and nano-machines and apparatus (micro electro mechanical systems, or MEMS), and study their mechanics as well. A few contributions from our lab are summarized here.
Cells are basic units of life. There is increasing experimental evidence suggesting that extracellular and intracellular mechanical forces have a profound influence on a wide range of cell behavior such as growth, differentiation, apoptosis (programmed cell death), gene expression, adhesion and signal transduction. Study of cell mechanics has drawn considerable attention from diverse fields, including biology, physics, biochemistry, and bioengineering.
Our studies of cell mechanics are motivated by three primary reasons: (1) understanding mechanotransduction – how cells transduce mechanical stimuli into biochemical processes and vice versa, (2) disease detection and prognosis: is there a mechanical signature for a disease state in a cell, and (3) biological machines – can cell behavior be tuned by mechanical stimuli such that they self organize into functional units. A deeper understanding of these issues may have a revolutionary impact on biological and health sciences of the 21st century. Micro-nano technology may catalyze this revolution through the unique capabilities of probing biological phenomena at a cellular and sub-cellular scale. Our current projects involve neurons, cancer cells and interactions between clusters of cells to form biological machines. Our goal is to explore questions such as, do memory and learning in animals depend on mechanical forces in neurons? Can cardiac cells synchronize their beating through long-range mechanical forces transmitted through the elastic media? Does the mechanical stiffness of cancer tumors play a role in initiating metastasis, and if so, how? What mechanical cues may lead to the emergence of biological machines from clusters of living cells?
For a material sample, the smallness may appear in various forms, e.g., the physical size, the layer thickness of a multilayer system, or the grain size in a polycrystalline metal. Size brings interfaces. Smaller the size, higher is the interface to volume ratio. At nano scale, interfaces are abundant, and they play important roles in defining macroscopic properties of materials such as mechanical strength, energy dissipation and conductivity. Interfaces interfere with the mechanisms of deformation, and may generate new mechanisms.
Our goal is to explore and understand the role of small size scale of material samples (such as thin films or nano wires) and their microstructures in determining their thermo-mechanical properties, the interaction between the macroscopic mechanisms of deformation and the interfaces at nano scale, and the new mechanisms that interfaces may generate (structure-property relation, a fancier way of saying). For example, when large grains of a crystalline metal are sheared, dislocations (crystal defect) can move through the crystal. This dynamics results in the plastic deformation of the metal. When the grain size is small, grain boundaries impede the dynamics of dislocation, and change the deformation characteristics and the strength of the metal. On the other hand, single crystal silicon (most used material in micro electronics and micro machines (MEMS)), on the other hand, is brittle at room temperature, but becomes ductile at high temperatures when dislocation avalanche appears – a phenomenon known as brittle to ductile transition (BDT). At small scale, BDT temperature may decrease due to the dislocation avalanche originating from the free surface of the single crystal sample with large surface to volume ratio, and high flaw tolerance of small samples.
Effect of size on BDT (brittle to ductile transition) temperature in single crystal silicon