Periodic Reporting for period 1 - e-Gates (Precision diagnostics using electrogating of liquids in capillary-driven microfluidics)
Reporting period: 2016-08-15 to 2018-08-14
This project aimed to introduce a novel concept for controlling the flow of liquids filling microchannels in microfluidic chips. The concept is based on the stop-and-go actuation of gates, located at key locations along a flow path. When a liquid or sample is pipetted onto a loading pad (See. Fig. 1a), it spontaneously fills the flow path serviced by the loading pad due to capillary action and stops precisely at any gates that the liquid encounters. Stopping the flow is achieved by having a geometrical discontinuity along the flow path that pins the liquid due to a Laplace pressure barrier. However, the flow can be resumed by applying a voltage to the meniscus of liquid pinned at the gate, (Fig. 1b). The voltage increases the wetting properties of the geometrical discontinuity, which makes the liquid able to pass the gate. We termed such gates ""electrogates"" and bias them using a peripheral and specific protocols (selection of gate, voltage, voltage duration). The peripheral can be controlled using a smartphone application and standard wireless communication (e.g. Bluetooth), (Fig. 1c, d).
Electrogates can be integrated to microfluidic chips in silicon to select a flow path post-fabrication of the chips. This brings significant flexibility to such chips with flow paths and timing being programmed in a convenient manner using a simple user interface on a smartphone. One can envision deploying such a method to microfluidic chips used for diagnostics where duration of key biochemical steps can be optimized to match a desired sensitivity or where a liquid or sample can be passed inside specific microchannels for mixing, filtration, processing, etc. depending on what step would be optimal for a test. This offers remarkable opportunities considering that such a simple but efficient control over liquids can still be obtained using capillary forces and a simple peripheral for activating the desired electrogates. In addition, electrogates remain easy to fabricate and use low voltage and little power. They can also be multiplied on a microfluidic chip to allow multiplexed control of various liquids filling microfluidic chips in parallel.
Next, we went through the theoretical description and modeling of capillary-driven microfluidics and electrowetting phenomena, physics of microdevices and compatibility of microfabrication processes to define the optimal design features of the electrogating units in microfluidic chips. We consequently invented a novel but simple mechanism to control efficiently liquid filling in capillary-driven microchannels based on geometrical effects and electrowetting phenomena.
For the liquid pinning geometry, we have taken advantage of a suitable semicircular trench etched on an insulating hydrophilic surface at the bottom of the microchannel and a metal electrode patterned over the trench. The trench creates an efficient pinning of the liquid meniscus at the electrogate. The flow is resumed beyond the electrogate when a low DC voltage is applied between the electrode on the trench and an electrode in contact with the liquid sample in the loading pad. Using this simple but optimized electrogate geometry, we were able to stop and resume flow of numerous biological buffers (e.g. the phosphate buffered saline (PBS) solution) in a controlled manner.
Human serum can be more difficult to pin in microchannels because the adsorption of proteins from serum to surfaces can increase the wettability of microchannels. We found that the retention capability of human serum by electrogates can be strengthened using patterned Pd tiles before the trench. Electrogates were then successfully tested using biological samples such as human serum and artificial urine showing high reliability, short response time (< 1 sec), low voltage requirements (< 8 V) and exhibiting long-term stability (> 40 min). Moreover, they are easy to fabricate and compatible with multiple gates on one chip. The electrogates were implemented in Si microfluidic chips (1 x 2 cm2) comprising a loading pad to receive a liquid sample and metal electrodes to provide bias voltage for actuation of the electrogates.
The full functionality of electrogates was completed with the implementation of a small low-cost peripheral device having a battery, a control board and a BluetoothTM module for communication with a smartphone. This peripheral energizes the microfluidic chip (containing electrogates) according to a protocol sent by the smartphone so that assay protocols can be easily updated or modified almost “on the fly”. Communication is bidirectional and the chip/peripheral updates the smartphone application with the current state of filling of the microfluidic chip.