(63b) A Fully Automated Nanoliter Viscometer for Analyzing Newtonian and Non-Newtonian Body Fluids and Polymeric Liquids

In medical diagnostics, the viscosity of body fluids can uniquely predict or indicate disorders [1]. For instance, blood viscosity is an early predictor of cardiovascular diseases [2]. Blood and plasma viscosity is a risk factor for type 2 diabetes mellitus and correlates directly with the severity of retinopathy, a leading cause of blindness among adults in the United States. Other examples include amniotic fluid and synovial fluid viscosity. The wide range of maladies that can be predicted by simply measuring the viscosity of a body fluid indicates that a simple, quick and inexpensive test for viscosity that is widely available to patients and medical professionals will be very beneficial.

We have developed a nanoliter capillary viscometer that has been successfully tested with several Newtonian and non-Newtonian polymeric liquids and body fluids (Fig.1). The device is easy to use and calibrates itself during each run eliminating the need for trained personnel for its maintenance and operation. The device is a silicon-glass hybrid device that contains four microfluidic channels and measures 18mm on either side. The viscosity is calculated using capillary-pressure-driven Poiseuille flow. The capillary pressure is measured using an in-situ method of trapped air compression. As the liquid column imbibes further into the microfluidic channel, the velocity of the liquid column decreases with time resulting in a profile of decreasing shear rates that then allows us to analyze non-Newtonian viscosity is a single experimental run.

Further, the device has been successfully tested for robustness with a dilute as well as a semi-dilute solution of flexible elastic polymers including Polyethylene Oxide (PEO) and hydrolyzed Polyacrylamide (PAM) to an aqueous solution of a stiff rod-like polymer molecule of Xanthan Gum, a popular emulsifier and food thickener, with ink-jet printing inks and finally to a polymer suspension of whole bovine blood (Fig.2).

Finally we have used an electronic drop sensing technique to fully automate the operation of the viscometer. The location and velocity of the liquid column and subsequently the viscosity is electronically sensed and displayed in real time using a digital electrode method and an array of electrodes located at preset distances. The electrode methods utilize changes in electrical conductivity when the air-liquid interface of the droplet passes over a pair of electrode. Other sensing methods based on analog electronic output and thermal capacitance have also been demonstrated. The viscometer is completely controlled by a laptop computer, and the total time for operation including setup, calibration, sample addition and viscosity calculation is approximately 4 minutes. The present system of portable viscosity measurement may now be used as is in several applications including bedside patient monitoring for hyperviscosity and cardiovascular risk assessment [1,2].

References:1) “The Hyperviscosity Syndromes”, Seminars in thrombosis and hemostasis, 25(2):199-208 (1999). 2) “Blood Rheology, cardiovascular risk factors, and cardiovascular disease”, Throm Haemost, (84): 553-8(2000).