A team led by Prof. Ashis Kumar Sen, the corresponding author of the paper from the Department of Mechanical Engineering, IIT Madras, used a 2-cm-long microchannel device that employs capillary force to draw blood into the device to separate plasma from whole blood and test glucose level in diabetic patients.
The first part of the microchannel device has hydrophilic walls (top and two side walls) that help the blood sample to be drawn in through capillary force. But one centimetre away, all the four walls of the microchannel are hydrophobic. Like a drop of water on a Teflon surface, the blood comes together and forms a large contact angle (more than 90 degrees) when it enters the hydrophobic region. The forward movement of the blood is suddenly impeded and the blood cells tend to accumulate in the hydrophobic region of the microchannel.
Unlike blood cells, the plasma with its low viscosity continues to move forward due to the momentum gained while passing through the hydrophilic region. “The blood cells slow down and then stop moving at the hydrophobic region and form a self built-in filter, while the plasma continues to move past the cells,” says Prof. Sen. “By creating a differential wetting behaviour in the microchannel we were able to separate the plasma from the blood cells.” Separating the plasma from blood cells is essential as it improves sensitivity and reliability. Most blood analyses are based on optical detection techniques, and blood cells present tend to interfere with the optical path resulting in low sensitivity.
The device does not require any external or internal power as it relies on capillary force to draw blood and the separation of plasma from blood cells is achieved through differential wetting behaviour of the microchannel walls.
“Only 5 microlitre of blood is required and in 15 minutes we get 450 nanolitre of plasma which further increases with time. With suitable design modifications we have also achieved higher plasma volume up to 2 microlitre in 15 min, which is adequate for detection of most analytes,” says M. Sneha Maria, the first author of the paper from the Department of Mechanical Engineering and Department of Biotechnology, IIT Madras. It takes 15-20 minutes to test the samples and get the results.
The detection platform for different diseases and conditions can be integrated within the device inside the hydrophobic region. “This is a proof-of-concept study so we used commercially available glucose test strips to detect glucose level in the blood samples,” says Ms. Maria. The sensitivity of the disposable device is comparable to conventional blood tests, says Prof. Sen.
Unlike the microchannel device used by the IIT team, many commercial glucometers rely on whole blood for testing. Using whole blood can cause measurement errors due to various hematocrit levels (the ratio of the volume of red cells to the volume of whole blood). When the hematocrit levels are high the viscosity of blood is more and this leads to low glucose concentration and underestimation. Overestimation results when the hematocrit levels are low. “There is a likelihood of more than 10 per cent error in glucose detection when whole blood is used,” says Ms. Maria.
The team is now testing the device for diagnosis of dengue. Currently, rapid diagnostic test kits either use whole blood which affects the sensitivity or centrifuged plasma for dengue detection. This is where the device can score over others. Prof. Sen is hopeful that the device can be used for parallel detection of analytes for several diseases using just one blood sample. “We intend to separate the plasma to multiple detection sites for studying several diseases in one go,” he says.