Microfluidic devices are able to process small volumes of liquid and are comprised of microscale components, but the devices themselves are not often small themselves. These labs-on-chips are often limited to lives in labs instead of the remote areas that could really benefit from their use. The limitation comes in the form of support equipment used to process or analyze assays that are expensive, bulky, energy consuming and/or require trained professional operators. Syringe pumps are often used in labs to drive liquids used in assays at specific flow rates and to ensure that the right volume is used. The need for complicated, external flow equipment was recently addressed by a group from Peking University. The group’s paper, “Squeeze-chip: a finger-controlled microfluidic flow network device and its application to biochemical assays” was recently featured on the cover of Lab on a Chip.
Squeeze-chip Design
The ‘squeeze-chip’ is based on a system of check valves and finger-operated pumps. Check valves allow fluid to flow in only one direction and, in this case, are fabricated from PDMS and integrated into a microfluidic card. The pump is a fluid reservoir that can be depressed by a finger. Squeezing the reservoir evacuates fluid through one of the check valves oriented to pass fluid away from the reservoir. After releasing the reservoir, it draws fluid in through a second check valve that’s oriented in a different direction so that it can only feed liquid into the reservoir. Alternatively, specially designed squeeze-chips can handle two immiscible fluids so that with each pump, a small plug of one fluid can be inserted into the system and is sandwiched by the other fluid. The displaced volume is not always equal, but the reservoirs feed into metering channels which only accept a specific volume, adding some control to the squeeze-chip. The authors have had success in delivering volumes ranging from nanoliters to microliters. This is the basic setup for a squeeze-chip, which can be combined with other units to create a more complex, sophisticated system.
Squeeze-chip Validation
The researchers demonstrated the squeeze-chip’s ability by running colorimetric assays to measure glucose and uric acid at clinically relevant concentrations of 0-10 mM and 0-15 mM respectively. These assays comprise a system of squeeze-chips that mix solutions, resulting in a 4mm thick readout chamber, allowing the user to see the solution’s color with the naked eye. The researchers were able to detect glucose as low as 1 mM and uric acid as low as 100 µM with initial sample consumption less than 5 µL per test. Limits of detection can be lowered by increasing the readout chamber thickness, which would make the color darker.
Discussion
I think that the squeeze-chip is a great component to make devices more viable outside of the lab, though it may not be suitable for every card. The metering chambers add some volume control, but, again, this may not be enough. More importantly, volumetric flow rate isn’t controlled, which eliminates the squeeze-chip as a viable option for applications requiring more stringent regulation. There are several considerations that need to be kept in mind when designing any lab-on-a-chip for use outside the lab. Despite any microscale magic taking place, the end-user and intended environment need elevated priority, meaning that these devices need to be relatively cheap, free of any tethers to an advanced lab and operable by people with limited education. The squeeze-chip certainly addresses cost and eliminates a connection to an external syringe pump. It can be operated by hand, or even operated by an actuated piston if the chip is predestined to function in some housing. Usability testing results would be interesting to see as well, including performance variation among users, but it looks like devices using the squeeze-chip can be readily used in areas of need.
Reference:
Li, W., Chen, T., Chen, Z., Fei, P., Yu, Z., Pang, Y., & Huang, Y. (2012). Squeeze-chip: a finger-controlled microfluidic flow network device and its application to biochemical assays Lab on a Chip, 12 (9) DOI: 10.1039/C2LC40125H