Single cell analysis is the study of individual cells isolated from cell populations. A typical workflow of single cell analysis starts from a physiologically heterogeneous cell culture, and involves a high-throughput and robust isolation method based on single or multiple parameters to prepare a subpopulation of cells for further downstream processing and analysis.
New technologies are demanded to conduct single cell analysis on a finer level. Microelectromechanical systems (MEMS) provide good interface with cells because of comparable feature size, and have the advantages of device miniaturization and process integration, thus are suitable for biological studies. In this dissertation, I focus on developing heterogeneously integrated optofluidic platforms for applications in cell manipulation and sensing.
We first demonstrate a novel dielectrophoresis-integrated pulsed laser activated cell sorter (DEP-PLACS). It consists of a microfluidic channel with 3D electrodes laid out to provide a tunnel-shaped electric field profile for sheathless focusing of microparticles/cells into a single stream in high-speed microfluidic flows. DEP-PLACS has achieved a sorting purity of 91% for polystyrene beads at a throughput of 1,500 particles/sec.
To achieve enhanced DEP forces in high conductivity media, which can potentially improve manipulation throughput in a biocompatible environment, hemispherically shaped, heavily doped (N++) silicon electrode is proposed to decouple the strong electric field region from the electrode interface and provides a large interface capacitance to prevent surface charging in high conductivity media, thereby effectively suppressing electrochemical reactions. Compared to conventional metal electrode, N++ electrode can provide 3 times higher threshold voltage and a corresponding 9-fold enhancement of maximum DEP force in 1� PBS buffer with an electrical conductivity of 1 S/m.
As one kind of downstream analysis, mechanobiology has been an emerging cross-disciplinary field that studies the mechanical properties of cells and tissues and how physical forces can contribute to the changes in biological events such as cell differentiation and disease development. Over the past years several tools and approaches have been developed for quantifying mechanical properties of biological samples. However, the throughput is usually low and obtaining statistically significant data can be difficult. To address this problem, we proposed a mechanobiological measurement platform consisting of two major steps: high-throughput cell patterning and parallel pressure sensing.
For the first step we developed a novel and simple technique called lift-off cell lithography (LCL). Our approach borrows the key concept of lift-off lithography from microfabrication and utilizes a fully biocompatible process to achieve high-throughput, high-efficiency cell patterning with nearly zero background defects across a large surface area. Using LCL, we reproducibly achieved > 70% patterning efficiency for both adherent and non-adherent cells with < 1% defects in undesired areas.
For the second step we developed a pressure-sensing substrate that can monitor pressure distribution across a large area with high spatial resolution by utilizing colorimetric interferometry and image-based measurements. Vertical optical micro-cavities are constructed on a silicon substrate and covered by a thin planar layer of polymer material that deforms under pressure. Local measurement of pressure is realized by monitoring the change of reflected color spectrum from each micro-cavity. Pressure distribution across 1 cm2 area with a spatial resolution of 50 μm has been achieved. We demonstrate a measurement range of 0-5psi with < 0.2 psi error. The measurement range can be customized by tuning the design and material properties depending on the specific application such as biological cell or tissue stiffness measurement.
