(H-306) The effects of microsphere seeding on the viscosity of blood analogue fluid used for Laser Doppler Velocimetry

Introduction:: The viscosity of blood is dependent on several factors, including blood composition (e.g., hematocrit), temperature, shear rate, vessel diameter (Fahraeus-Lindqvist effect), cell aggregation, shape, deformation, and orientation, as well as plasma viscosity[1]. In-vitro experiments with real blood leads to many challenges such as coagulation, storage, and handling. Therefore, the use of non-Newtonian fluid that mimics real blood is important for investigating hemodynamics in in-vitro experiments compromising devices and test sections. Blood is a non-Newtonian fluid that exhibits shear-thinning viscous properties. Hence, the researchers have developed blood analogue fluids (BAF) with rheological properties comparable to real blood using Xanthan gum, glycerin, and water[2]. Microspheres in BAF somewhat replicate the complex multiphase hemodynamics of circulatory systems, including the Fähraeus-Lindqvist effect. The significance of this research is that it assesses the effects of microspheres in BAF, which are widely used in laser Doppler velocimetry (LDV) experiments for testing implantable medical devices such as valves, stents, grafts, etc.

This study assesses the effect of polymer microsphere non-Newtonian viscosity of blood using aqueous Xanthan gum and glycerin solutions with water. It is an innovative study as the influence of polymer microspheres on the non-Newtonian viscosity of BAF have not been reported previously.


Materials and Methods:: Blood Analog Fluid. Blood is a non-Newtonian fluid. On a similar note, BAF must include the feature of non-Newtonian viscosity of blood. While glycerin in BAF controls the viscosity at infinite shear rates, the Xanthan gum controls the non-Newtonian behavior at low shear rates[3]. Therefore, the right mixture of water, glycerin, and Xanthan gum leads to a comparable BAF for conducting in vitro experiments [4, 5].

Blood Analog Fluid with Microspheres. The addition of microspheres in BAF is needed to generate a scattering signal for LDV measurements. The type and size of microspheres are important parameters of BAF that govern the quality of LDV measurements. Solid polymer microspheres of diameter 8 𝝁m are used for the current study. A 16 ml BAF sample solution was mixed with 1% (w/v) and 2% (w/v) of the polymer microsphere.

The viscosity of BAF with and without polymer microspheres was measured using a concentric cylinder viscometer (Brookfield Engineering Laboratories Inc., Middleborough, Massachusetts, United States) and compared with the Carreau model shown below [3].

μ = μ_inf + (μ₀ - μ_inf) × (1 + (λγ)²)^(n-1)/2 (1)

λ= time constant, n = power law index, μ₀ = zero shear viscosity (cP), μ_inf = infinite shear viscosity (cP), γ = shear rate (1/s)


Results, Conclusions, and Discussions:: RESULTS

The Carreau-model coefficients for BAFs, with and without polymer microspheres, were compared to blood[6] (Table 1 and Table 2). Figure 1 shows differences between blood viscosity (solid line) and different combinations of BAF components (BAF-1, BAF-2, and BAF-3). For example, BAF-3 (80% (v/v) distilled water, 20% (v/v) glycerol, and 0.027% (w/v) xanthan gum) was found to be comparable to the blood viscosity for shear rates ranging from 1 s^-1 to 100 s^-1. The Carreau-model coefficients for BAF-3 without polymer microspheres were reported as λ = 7.45, n = 0.11, μ0 = 221 cP, μinf = 5.1 cP. For BAF-1 and BAF-2, Table 1 may be referred to.

Figure 2 compares the viscosity of BAF with and without 1% (w/v) and 2 % (w/v) polymer microspheres. The 1% microspheres resulted in 111% increase in the infinite-shear rate viscosity (μinf) and 64% decrease in the zero-shear rate viscosity (μ0). The Carreau-model coefficients for BAF-3 with 1% microspheres were: λ = 1.5, n = 0.15, μ0 = 79.8 cP, μinf = 10.8 cP. Similarly, 2% microspheres resulted in a 135% increase in and a 55 % decrease in μ0. The Carreau-model coefficients for BAF-3 with 2% microspheres were: = 2.5, n = 0.05, μ0 = 99.03 cP, μinf = 11.99 cP (Table 2). Similar results were seen for other BAFs.

The viscosity of the BAF seeded with polymer microspheres was observed to be significantly higher for the higher shear rate values and lower for the low shear rate values than that of without the microspheres.

DISCUSSION

Microspheres allow velocity measurements during LDV measurements for testing medical devices such as valves, stents, and grafts. It is evident from Figure 2 that a small % of microsphere seeding results in a significant change in the shear-thinning viscosity characteristics of the BAF.

CONCLUSION

BAFs, infused with polymer microspheres show significant variations in the zero- and infinite-shear rate viscosity values of fluid when compared with real blood in humans. It is imperative that the influence of polymer microspheres needs to be included when LDV experiments are conducted, for accurate assessments of hemodynamics within test devices.

Acknowledgements (Optional): :

References (Optional): : [1] Sousa, P. C., Pinho, F. T., Oliveira, M. S. N., and Alves, M. A., 2011, "Extensional flow of blood analog solutions in microfluidic devices," Biomicrofluidics, 5(1).

[2] Brookshier, K. K., and Tarbell, J. M., 1991, "Effect of hematocrit on wall shear rate in oscillatory flow: do the elastic properties of blood play a role?," Biorheology, 28(6), pp. 569-587.

[3] Banerjee, R. K., Ashtekar, K. D., Helmy, T. A., Effat, M. A., Back, L. H., and Khoury, S. F., 2008, "Hemodynamic diagnostics of epicardial coronary stenoses: in-vitro experimental and computational study," Biomed Eng Online, 7, p. 24.

[4] Anastasiou, A. D., Spyrogianni, A. S., Koskinas, K. C., Giannoglou, G. D., and Paras, S. V., 2012, "Experimental investigation of the flow of a blood analogue fluid in a replica of a bifurcated small artery," Med Eng Phys, 34(2), pp. 211-218.

[5] D'Souza, G. A., Peelukhana, S. V., and Banerjee, R. K., 2014, "Diagnostic uncertainties during assessment of serial coronary stenoses: an in vitro study," J Biomech Eng, 136(2), p. 021026.

[6] Cho, Y. I., and Kensey, K. R., 1991, "Effects of the non-Newtonian viscosity of blood on flows in a diseased arterial vessel. Part 1: Steady flows," Biorheology, 28(3-4), pp. 241-262.