Search

Current advances in single cell sequencing, gene expression and proteomics require the isolation of single cells, frequently from a very small source population. In this work we describe the design and characterization of a manually operated microfluidic cell sorter that 1) can accurately sort single or small groups of cells from very small cell populations with minimal losses, 2) that is easy to operate and that can be used in any laboratory that has a basic fluorescent microscope and syringe pump, 3) that can be assembled within minutes, 4) that can sort cells in very short time (minutes) with minimum cell stress, 5) that is cheap and reusable. This microfluidic sorter is made from hard plastic material (PMMA) into which microchannels are directly milled with hydraulic diameter of 70 μm. Inlet and outlet reservoirs are drilled through the chip. Sorting occurs through hydrodynamic switching ensuring low hydrodynamic shear stresses, which were modeled or experimentally confirmed to be below the cell damage threshold. Manually operated, the maximum sorting frequencies were approximately 10 cells per minute. Experiments verified that cell sorting operations could be achieved in as little as 15 minutes, including the assembly and testing of the sorter. In only one out of 10 sorting experiments the sorted cells were contaminated with another cell type. This microfluidic cell sorter represents an important capability for protocols requiring fast isolation of single cells from small number of rare cell populations.

This paper describes the fabrication and integration of light-induced dielectrophoresis for cellular manipulation in biological microbeams. An optoelectronic tweezers (OET) cellular manipulation platform was designed, fabricated, and tested at Columbia University's Radiological Research Accelerator Facility (RARAF). The platform involves a light induced dielectrophoretic surface and a microfluidic chamber with channels for easy input and output of cells. The electrical conductivity of the particle-laden medium was optimized to maximize the dielectrophoretic force. To experimentally validate the operation of the OET device, we demonstrate UV-microspot irradiation of cells containing green fluorescent protein (GFP) tagged DNA single-strand break repair protein, targeted in suspension. We demonstrate the optofluidic control of single cells and groups of cells before, during, and after irradiation. The integration of optofluidic cellular manipulation into a biological microbeam enhances the facility's ability to handle non-adherent cells such as lymphocytes. To the best of our knowledge, this is the first time that OET cell handling is successfully implemented in a biological microbeam.

We describe the imbibition process from a point source into a homogeneous semi-infinite porous material. When body forces are negligible, the advance of the wetting front is driven by capillary pressure and resisted by viscous forces. With the assumption that the wetting front assumes a hemispherical shape, our analytical results show that the absorbed volume flow rate is approximately constant with respect to time, and that the radius of the wetting evolves in time as r ≈ t(1/3). This cube-root law for the long-time dynamics is confirmed by experiments using a packed cell of glass microspheres with average diameter of 42 μm. This result complements the classical one-dimensional imbibition result where the imbibition length l ≈ t(1/2), and studies in axisymmetric porous cones with small opening angles where l ≈ t(1/4) at long times.

The objective of this work is to develop and validate a numerical model that can conduct a transient analysis of heat transfer and the corresponding damage in skin burns. Once this model is developed, an examination of the effect of cooling on reducing damage from skin burns is carried out. A finite element numerical model is used to simulate the conduction of heat and the transient progress of irreversible injury in the skin. The damage function of Henriques and Moritz is used to model the damage that occurs in the skin during the burn and cooling periods. Numerical results are presented that describe the heat transfer during a skin burn. Comparison is made between different burns: a high-temperature, short-duration burn (99°C for 1 second) and a medium-temperature, long-duration burn (80°C for 15 seconds). Cooling parameters such as the nature of the cooling fluid, the duration of the cooling period, the temperature of the coolant fluid, and the delay between the termination of the burn and the initiation of the cooling therapy are examined. The authors find that the most influential way to significantly reduce the damage from a burn is to immediately cool the burn. In addition, it was found that cooling a burn for a prolonged period of time or with very cold water cannot be justified from purely a heat transfer point of view.

The shape of deposits obtained from drying drops containing colloidal particles matters for technologies such as inkjet printing, microelectronics, and bioassay manufacturing. In this work, the formation of deposits during the drying of nanoliter drops containing colloidal particles is investigated experimentally with microscopy and profilometry, and theoretically with an in-house finite-element code. The system studied involves aqueous drops containing titania nanoparticles evaporating on a glass substrate. Deposit shapes from spotted drops at different pH values are measured using a laser profilometer. Our results show that the pH of the solution influences the dried deposit pattern, which can be ring-like or more uniform. The transition between these patterns is explained by considering how DLVO interactions such as the electrostatic and van der Waals forces modify the particle deposition process. Also, a phase diagram is proposed to describe how the shape of a colloidal deposit results from the competition among three flow patterns: a radial flow driven by evaporation at the wetting line, a Marangoni recirculating flow driven by surface tension gradients, and the transport of particles toward the substrate driven by DLVO interactions. This phase diagram explains three types of deposits commonly observed experimentally, such as a peripheral ring, a small central bump, or a uniform layer. Simulations and experiments are found in very good agreement.