Date Published: August 30, 2013
Publisher: Public Library of Science
Author(s): Katie L. Pitts, Marianne Fenech, Timothy W. Secomb.
It is desired to understand the effect of alginic acid sodium salt from brown algae (alginate) as a viscosity modifier on the behavior of blood in vitro using a micro-particle image velocimetry (µPIV) system. The effect of alginate on the shape of the velocity profile, the flow rate and the maximum velocity achieved in rectangular microchannels channels are measured. The channels were constructed of polydimethylsiloxane (PDMS), a biocompatible silicone. Porcine blood cells suspended in saline was used as the working fluid at twenty percent hematocrit (H = 20). While alginate was only found to have minimal effect on the maximum velocity and the flow rate achieved, it was found to significantly affect the shear rate at the wall by between eight to a hundred percent.
Viscosity modification of blood flows is of importance to biomedical research for many reasons. In a clinical setting, modifying the viscosity of blood can be desired in order to reverse non-standard physiology, or to incorporate a medical benefit such as drug delivery . While blood may be considered a homogeneous fluid for some macroscale studies, at the micro scale blood must be considered a suspension of flexible formed elements such as red blood cells (RBCs), white blood cells (WBCs) in a Newtonian matrix of plasma. Blood viscosity modification is frequently done by introducing other flexible elements in the form of polymers such as dextran, polyethylene glycol (PEG), polyethylene oxide (PEO), warfarin, heparin, and pluronics . Often the goal of these polymers is to thin the blood, avoid thrombosis or to decrease aggregation. Warfarin and heparin are common blood thinners used in modern medicine. Pluronics have been shown to decrease low shear blood viscosity and have been used in clinical trials as a viscosity modifier for sickle cell anemia . PEO has been shown to reduce the cell-free layer, the naturally occurring red blood cell depleted zone at the wall of a microchannel or vessel, in some concentrations . Dextran, PEO and PEG can induce the aggregation of RBC if the correct molecular weight or concentration. In general, long-chain polymers induce aggregation and short-chain polymers inhibit aggregation or have no effect . Alginic acid, alginate, is a straight-chain polyuronic acid with hydrophilic properties. Alginate is a polymeric viscosity modifier that has many uses in industry including biomedical applications such as tissue engineering and drug encapsulation . Alginate, a derivative of brown algae cells, is biocompatible and biomimetic . Additionally alginate has been suggested to have applications within in vivo or in vitro blood research .
Table 1 provides the calculated flow rate, shear rate and maximum velocity. There was almost no effect found on the maximum centerline velocity in the microchannel with the addition of alginate, at any of the programmed flow rates tested. In addition, there was little effect on the flow rate with the addition of alginate. The standard deviation of the calculated flow rates for both alginate and non-alginate samples are listed. There is a discrepancy between the calculated flow rate and the programmed flow rate of the pump. There is necessarily some compliance in the system due to the flexible nature of the PDMS and the tubing. This accounts for a portion of the discrepancy. Additionally, it is known that the PDMS deforms while flow is occurring . The discrepancy in flow rate is discussed more in depth later on. It can be seen that in most cases (except the highest flow rate) the presence of alginate increases the calculated flow rate.
Alginate has been shown to have minimal effect on the maximum velocity achieved in microflows of blood, as measured by µPIV. A significant reduction in the shear rate at the wall of the channel was found with the addition of alginate to the PBS. This could be due to the alginate-induced aggregation of the RBC as studied by Zhao et al.. The addition of alginate could be used in future research to adjust the flow rate and shear properties of blood solutions or blood substitutes. Alginate gives a blunted velocity profile at all flow rates, so much so the edge K value goes to zero when describing the velocity profile with a two-parameter shape equation. In conclusion, it has been demonstrated that alginate can be used as a viscosity modifier in blood flows of saline and RBCs to decrease the shear rate at the wall and flatten the overall velocity profile. This behavior could be used in applications of alginate as a viscosity modifier for blood in certain medical situations.