User:Wanchentu/sandbox

Controlling cell microenvironment
Microfluidic systems expand their ability to control the local cell microenvironment beyond what is possible with conventional culture systems. Being able to provide different environments in a steady, sustainable and precise manner has a significant impact on cell culture research and study. Those environmental factors include physical (shear stress), biochemical (cell-cell interactions, cell-molecule interactions, cell-substrate interactions), and physicochemical (pH, CO2, temperature, O2) factors.

Oxygen concentration control
Oxygen plays an essential role in biological systems [2]. Oxygen concentration control is one of the key elements when designing the microfluidic systems, whether the aerobic species or when modulating cellular functions in vivo, such as baseline metabolism and function [2]. Multiple microfluidic systems have been designed to control the desired gas concentrations for cell culture. For example, generating oxygen gradients was achieved by single-thin-layer PDMS construction within channels (thicknesses less than 50 μm, diffusion coefficient of oxygen in native PDMS at 25 °C, D= 3.55x10-5 cm2 s-1) without utilizing gas cylinders or oxygen scavenging agents; thus the microfluidic cell culture device can be placed in incubators and be operated easily [3]. However, the PDMS may be problematic for the adsorption of small hydrophobic species [4]. Poly(methyl pentene) (PMP) may be an alternative material, because it has high oxygen permeability and biocompatibility like PDMS [5] [6]. In addition to the challenges of controlling gas concentration, monitoring oxygen in the microfluidic system is another challenge to address. There are numerous different dye indicators that can be used as optical, luminescence-based oxygen sensing, which is based on the phenomenon of luminescence quenching by oxygen, without consuming oxygen in the system [7]. This technique makes monitoring oxygen in microscale environments feasible and can be applied in biological laboratories [7].

Temperature control
Temperature can be sensed by cells and influences their behavior, such as biochemical reaction kinetics [8]. However, it is hard to control high-resolution temperature in traditional cell culture systems; whereas, microfluidic systems are proven to successfully reach the desired temperature under different temperature conditions through several techniques [8]. For example, the temperature gradient in the microfluidic system can be achieved by mixing two or more inputs at different temperatures and flow rates, and the temperature is measured in the outlet channels by embedding polymer-based aquarium thermocouples [9]. Also, by installing heaters and digital temperature sensors at the base of the microfluidic system, it has been demonstrated that a microfluidic cell culture system can continuously operate for at least 500 hours [10]. The circulating water channels in the microfluidic system are also used to precisely control temperatures of the cell culture channels and chambers [11]. Furthermore, putting the device inside a cell culture incubator can also easily control the cell culture temperature [12].

Applications of Cells in microfluidic systems
Microfluidic systems can be used to culture several cell types.

Culture of mammalian cells
Mammalian cell cultures can be seeded, grown for several weeks, detached, and passaged to a fresh culture medium ad nauseam by digital microfluidic (DMF) devices on a macro-scale [13].

Algae
Algae can be incubated, and their growth rate and lipid production can be monitored in a hanging-drop microfluidic system. For example, Mishra et al. developed a 25x75 mm, easily accessible microfluidic device. This design is used to optimize the conditions by changing well diameters, UV light exposure (causing mutagenesis), and light/no light tests for culturing Botryococcus braunii, which is one of the most common freshwater microalgae for biofuel production [14].

Bacteria and Yeast
Microfluidic systems can be used to incubate high volumes of C|bacteria and yeast colonies [11]. The two-layer microchemostat device is made to allow scientists to culture cells under chemostatic and thermostatic conditions without moving cells around and causing intercellular interaction [11]. Yeast cell suspension droplets can be placed on a plate with patterned hydrophilic areas and incubated for 24 hours; then the droplets are split the produced proteins from yeast are analyzed by MALDI-MS without killing the cells in the original droplets [15].