Non-equilibrium cluster states in colloids with competing interactions
Entry by Max Darnell, AP 225, Fall 2011
Title: Non-equilibrium cluster states in colloids with competing interactions
Authors: Tian Hui Zhang, Jan Klok, R. Hans Tromp, Jan Groenewold and Willem K. Kegel
Journal: Soft Matter DOI: 10.1039/c1sm06570j (2011)
As lab-on-a-chip technology becomes more advanced, there is an ever-growing need for improved control of cells. Also, the link between electronic and microfluidic systems has not yet been fully exploited. This paper describes a system using microelectrodes and dielectrophoresis (DEP) in conjunction with microfluidics to trap cells in prescribed patterns. Finite-element models were used in the design of the device. The authors also show the range over which the electric fields used do not harm the cells. Increasing the control of cells within such setups has the potential to greatly expand the possibly designs of biosensors and actuators.
The image below shows the setup and construction for the microelectrodes. One can see that it is a standard electrode design on a silicon dioxide substrate. It should also be noted that the cell-side surface is coated with fibronectin to enhance cell adhesion in islands (standard microcontact printing) and that the Pluronic solution is a blocking agent to contain the "islands" of cells. On the right, one can see how the device works. When cells flow through the channel, many are attracted to the electrodes via DEP. This occurs because the cells are more polarizable than the surrounding media, meaning that there is a net electric force towards the electrode. The cells that migrate to the vicinity of the electrode can bind to the fibronectin, which stabilizes them in the flow. The unbound cells are then flushed out, leaving the user with attached cells until the voltage ceases and new flow can rinse away the cells. The authors used finite element models to optimize the parameters of this design.
The second figure shows the effects of DEP on cell health and morphology. In the first figure, it can be seen that above a threshold, the cells are not able to withstand the voltage, resulting in most of the cells dying. In the second plot, one can see that even after 48h, the cells with DEP proliferate a similar amount to those without. Also, one can see that with and without DEP, the morphology of the cells is similar, showing no large negative effect of the DEP on the cell viability.
Finally, the authors showed the improvement in making cells adhere to micropatterned islands when these electrodes are used. The figure below shows that when electrodes are used the cells find the islands with 70% accuracy but that this number drops to 17% accuracy without the electrodes.
Connection to Soft Matter
Electric fields and forces are extremely important in soft matter physics, especially as more biological systems are considered. This paper showed one application of electric fields in harnessing the properties of cells to control them, but there exist many more. For instance, considering cells such as cardiomyocytes or neurons, which use action potentials, the leveraging of the electrical considerations within soft matter physics could lead to novel interactions between artificial systems and these cell types. In addition, work such as that conducted in Bob Westervelt's lab shows the next steps in the work reported here, where the same phenomenon is leveraged, but where the location of the DEP field is also controllable.