CCFE - Center for Complex Fluid Engineering - Carnegie Mellon University

Gels are a common matrix for biosensors. Hindered transport through the polymeric matrix slows down the response rate in such sensors. As the number of diagnostic and analytical applications for gel-based sensor devices increases, so does the necessity of new pumping mechanisms for faster response. The network and mechanical properties of the gel make mechanical mixing schemes inappropriate. We are investigating novel internal pumping strategies based on electrically driven convection as a way to accelerate mass transfer in polyacrylamide gels. The gels are doped with charged colloids that drive local electroosmotic flow in response to externally applied electric fields. The advantages of modifying the flow process are twofold. Increased mass transport of reactants will improve the response-time and baseline restoration limitations of actual biosensors and will allow more accurate kinetic studies. Mass transfer limitations are one of the principal sources of inaccurate measurements in biochemical rate processes such as receptor-ligand binding rates.
Electroosmosis is the fluid motion induced by the movement of counterions in the electrical double layer adjacent to a charged surface when an electric field is applied. As the counterions are driven past the charged surface, they carry solvent molecules with them and induce a net flow close to the charged surface. This motion is transmitted to the bulk fluid. This basic principle can be exploited to induce flow where there are transport barriers in biosensors that otherwise suppress convection. Placing charged particles randomly in a polymer network can promote an “internal pumping” method to drive fluid convection. Considering that many of the biologically relevant analytes are charged species we would also have to take into account electrophoretic effects when analyzing data or modeling the transport. Electrophoresis is the direct motion of charged species as a result of the application of an electric field.	Figure 1: Schematic view of stationary particles in crosslinked gel.
Studies of silica particles with different sizes showed an improved enhancement of electroosmotic flows when using nanoscale silica particles. Figure 1 shows a schematic view of uniformly distributed charged particles inside a crosslinked gel in the presence of an applied electric field. Figure 2 demonstrates the principle of mass transport control by the addition of stationary charged silica particles with a 7nm diameter inside a gel and the application of a DC electric field. Amino-methylcoumarin, an intrinsically fluorescent dye was used to measure the flux across polyacrylamide gels. By the application of electric fields the transport of dye can be enhanced or hindered depending of the direction of the field applied. In the absence of charged silica particles no enhancement was observed and hindered transport when a reversed field was applied was not sufficient to reverse the flux. In the presence of particles enhancement was observed and when a field in opposite direction was applied the flux was reversed. Experimental data suggests that in the presence of charged silica particles there is control over the direction of the dye flux. Also as a result of the application of the DC electric fields pH changes are observed in the cathode and anode. These effects have to be considered in the data analysis. Figure 2: Flux measurements for amino-methylcoumarin through polyacrylamide gels varying the direction of the electric field applied in the presence and absence of particles.
We are currently performing experiments to understand the effects of particle volume fraction, electric field strength and buffer concentration on the mass transport. A model is also being built in order to understand the mass transport. In general, the transport will be affected by variations on conductivity and electric fields through out the gel, on top of the already coupled mass transfer, electric field and fluid dynamics equations.