Physical Vein Models to Quantify the Flow Performance of Sclerosing Foams

Abstract

Foam sclerotherapy is clinically employed to treat varicose veins. It involves intravenous injection of foamed surfactant agents causing endothelial wall damage and vessel shrinkage, leading to subsequent neovascularization. Foam production methods used clinically include manual techniques, such as the Double Syringe System (DSS) and Tessari (TSS) methods. Pre-clinical in-vitro studies are conducted to characterize the performance of sclerosing agents; however, the experimental models used often do not replicate physiologically relevant physical and biological conditions. In this study, physical vein models (PVMs) were developed and employed for the first time to characterize the flow behavior of sclerosing foams. PVMs were fabricated in polydimethylsiloxane (PDMS) by replica molding, and were designed to mimic qualitative geometrical characteristics of veins. Foam behavior was investigated as a function of different physical variables, namely (i) geometry of the vein model (i.e., physiological vs. varicose vein), (ii) foam production technique, and (iii) flow rate of a blood surrogate. The experimental set-up consisted of a PVM positioned on an inclined platform, a syringe pump to control the flow rate of a blood substitute, and a pressure transducer. The static pressure of the blood surrogate at the PVM inlet was measured upon foam administration. The recorded pressure-time curves were analyzed to quantify metrics of foam behavior, with a particular focus on foam expansion and degradation dynamics. Results showed that DSS and TSS foams had similar expansion rate in the physiological PVM, whilst DSS foam had lower expansion rate in the varicose PVM compared to TSS foam. The degradation rate of DSS foam was lower than TSS foam, in both model architectures. Moreover, the background flow rate had a significant effect on foam behavior, enhancing foam displacement rate in both types of PVM.

Keywords: foam; foam sclerotherapy; microfluidic; physical vein model; physician compounded foam; varicose vein.

Figure 1

Assembled PVMs representing simplified physiological (A) and varicose (B) vein models, and demonstration of perfusion using a red dye. These models were employed to test the flow behavior of foams. The main channel was punctured with a needle (16G), in order to mimic the clinical process of injection more closely.

Figure 2

Schematic of PVMs production via replica molding. Firstly, (A) a CAD design of positive molds was generated and 3D printed. Secondly, (B) two PDMS layers were produced from the molds, aligned together, and permanently bonded via oxygen plasma treatment to obtain a fully circular channel cross-section (C). Finally, the main channel was punctured with a 16G needle (D).

Figure 3

Schematic illustrating the set-up for evaluating the flow behavior of foams in the PVM model. A steady flow is imposed using a syringe pump (A), and a pressure transducer (B) is positioned in line with the inlet tubing and prior to the PVM (C). The foam is injected through the 16G needle into the main channel (D). The pressure transducer is connected to a National Instruments I/O module. The NI-DAQ system supports analog and digital inputs, and communicates with the NI-DAQ software. A Matlab code is employed to store pressure data in an automated fashion (E).

Figure 4

Representative pressure vs. time curve obtained by the CfAS. The plots were divided into three phases: (i) an increase of the pressure due to the expansion of the foam inside the channel, (ii) a decrease of the pressure caused by the degradation of the foam, and (iii) an initial peak due to the injection.

Figure 5

Pressure plots obtained from the CfAS while injecting DSS (A,C) and TSS (B,D) foams. The upper panels show the recordings obtained using the straight channel geometry, whereas the lower panels show the recording obtained using the serpentine-like channel geometry. As a control test (E), the pressure was measured without injecting foam, at a background constant flow rate (72 mL/h). The pressure inside the vein models was also measured while injecting the foam in the absence of a blood-surrogate flow (F).

Figure 6

Expansion rate values at the different flow rates investigated, and different foam production methods (TSS blue bars, DSS red bars). Measurements were obtained from the straight (A) and curved (B) channel geometry. Data represent the average of 6 measurements ± SD. An asterisk (^*) indicates that differences between mean values are statistically significant (p < 0.05). The experiment was repeated six times, and results are reported as mean value ± standard deviation.

Figure 7

Expansion time values at the different flow rates investigated, and different foam production methods (TSS blue bars, DSS red bars). Measurements were obtained from the straight (A) and curved (B) channel geometry. Two asterisks (^**) indicate that differences between mean values are very statistically significant (p < 0.01). The experiment was repeated six times, and results are reported as mean value ± standard deviation.

Figure 8

Degradation rate values at the different flow rates investigated, and different foam production methods (TSS blue bars, DSS red bars). Measurements were obtained from the straight (A) and curved (B) channel geometry. An asterisk (^*) indicates that differences between mean values are statistically significant (p < 0.05). The experiment was repeated six times, and results are reported as mean value ± standard deviation.

Figure 9

Degradation time values at the different flow rates investigated, and different foam production methods (TSS blue bars, DSS red bars). Measurements were obtained from the straight (A) and curved (B) channel geometry. An asterisk (^*) indicates that differences between mean values are statistically significant (p < 0.05) and two asterisks (^**) indicate that differences between mean values are very statistically significant (p < 0.01). The experiment was repeated six times, and results are reported as mean value ± standard deviation.

Figure 10

Bright field microscope images of HUVECs cultured within the fully circular PVM channels. Images on the left show the lower channel wall coated with collagen (A) and fibronectin (C), respectively. Images on the right show the upper channel wall coated with collagen (B) and fibronectin (D), respectively. Images (4x magnification) were taken after 48 h from cell seeding. Scale bars are 200 μm long.