flow in the channels. They include changing the channel width and length, altering the wettability of the paper, diverting some fluid through a parallel channel, or changing the viscosity of the fluid. The flow in PADs can be turned off with dissolvable sugar bridges, Corona discharge treatment to alter a coating on the paper from a hydrophobic to hydrophilic state, or the use of an expandable polymer triggered by the flow to close the flow path. Electronic integration Integration of microfluidic platforms and electronic components have the potential to generate micro total analysis systems (µTAS), which are devices that include and automate all essential steps for sample preparation and analysis. Paper electronics rely on functional structures like conductors to be fabricated on the surface of paper, but paper-based microfluidics rely on channels and barriers to be fabricated inside the substrate. This incompatibility led to a majority of µTAS being developed using traditional microfluidic platforms with polymer-based channels. However, in 2009, screen-printed electrodes were integrated into a paper-based microfluidic device to create a biosensor for glucose, lactate, and uric acid. This first report of electronic integration for paper-based microfluidics illustrated how this material can improve the design of these µTAS due its flexibility and low-cost. Coupling electronic components into the hydrophobic channels created on the paper-based microfluidic devices are based upon physical and chemical integration techniques; these two strategies are discussed in the sections below. Physical Integration Physical integration methods adapt commonplace techniques (e.g., inkjet printing, pencil-on-paper, and screen printing) to create a network of conductive traces on paper. A promising physical technique is inkjet printing, which allows for conductive materials to be deposited in a precise and reproducible fashion onto paper. As a proof-of-concept, Ko et al. developed a paper-based electrical chip using a home office printer, an ink made of carbon nanotubes, and magazine paper. Similarly, silver nanoparticles were printed into microfluidic channels to sense changes in the permittivity of fluids, revealing information about concentration and mixing ratios. Research groups have found, however, that these nanoparticle containing inks can self-aggregate on the paper due to uneven drying, which leads to non-uniform coverage and non-linear responses. The pencil-on-paper technique is also a great example of electrical integration on paper-based microfluidics using inexpensive, common office supplies. Here, graphitic circuitry is created on the paper-based microfluidic device by the analyst repeatedly sketching with a pencil. For example, this electrical integration method was