Utilizing Microfluidics for Optimizing Stem Cell Therapies
The attraction to utilizing microfluidics stems from its potential to improve efficiency and therefore significantly minimize costs. There are two main factors of microfluidics which yield the above advantages. The first is simply that microfluidics only uses microliters or nanoliters of solutions which significantly reduces material’s cost. The second factor is the advantages that can be exploited from the physical properties of fluids at these small volumes. There are two dimensionless numbers, Reynold’s number and Peclet number, which characterize fluid flow and how molecules within that fluid act, respectively. The Reynold’s number is able to classify the fluid dynamics into either laminar or turbulent, which are depicted in the figure below. Turbulent flow is what most people are familiar with; it is described as a fluid which chaotically flows with no distinct streamlines. Laminar flow is that of a fluid which has distinct streamlines which are all parallel to the direction the fluid is moving.
The equation to calculate the Reynold’s number is Re = ρvl/n where, ρ is the density of the fluid, ν is the velocity of the fluid, l is the characteristic length of the channel, and n is the viscosity of the fluid. For large Reynold’s numbers, typically greater than 1000, fluid flow is classified as turbulent. In the case of microfluidic devices, since the characteristic length is so small, Reynold’s numbers are mostly always less than 1 which I falls into the category of laminar flow. The advantage of laminar flow is that due to the steady streamlines, fluids can be manipulated very precisely, and molecules within the fluid can be controlled easily to form predictable gradients.
The calculated Peclet number determines whether diffusion or convection is the dominate means of mass transport by values larger than 1 and less than 1, respectively. The values of the Peclet number can be calculated by the equation Pe = vl/D where ν is the velocity of the fluid, l is the characteristic length of the fluid, and D is the diffusion coefficient. A typical example of these two processes is mixing of sugar in a cup of coffee. If one uses a spoon to mix the sugar then the means of transport of the sugar in the coffee is convection. This is because the spoon is causing a bulk flow of the fluid which in turn causes the sugar molecules to evenly disperse. However, if one simply puts the sugar in the coffee without stirring, the sugar molecules will slowly mix with the coffee by the process of diffusion. Diffusion is a process where molecules automatically have a net flux down a concentration gradient. Although convection occurs much faster than diffusion, for very small volumes, as such in microfluidic devices, the more efficient means of transport occurs by diffusion. This property allows for very exact and reproducible gradient profiles to be generated in a variety of ways. The exploitation of these properties has led to a myriad of devices ranging from very basic T-channels to sophisticated integrated circuits. The application of these devices has been for both cell-based assays and diagnostic testing.
Most microfluidic devices are made by a process called soft lithography. Soft lithography utilizes an elastomer called polydimethylsiloxane (PDMS) to make a negative mold of a master mold which is made by photolithography.