In Vitro Flow Study in an Intracranial Aneurysm Biomodel

Manufactured by Additive Manufacturing

Andrews Souza

1,2,3 a

, Diana Rodrigues

, Maria Sabrina Souza

, Conrado Ferrera

João Ribeiro

3,4 d

, Rui A. Lima

1,2 e

and Ana Moita

6,7 f

University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal

Mechanical Engineering and Resource Sustainability Center (MEtRICs), UMinho, Guimarães, Portugal

Centro de Investigação de Montanha (CIMO), Instituto Politécnico de Bragança (IPB),

Campus de Santa Apolónia, 5300-253 Bragança, Portugal

Instituto Politécnico de Bragança, 5300-253, Bragança, Portugal

Depto. de Ingeniería Mecánica, Energética y de los Materiales and Instituto de Computación Científica Avanzada,

Universidad de Extremadura, Badajoz, Spain

IN+, Center for Innovation, Technology and Policy Research, Instituto Superior Técnico,

Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

CINAMIL, Department of Exact Sciences and Engineering, Portuguese Military Academy,

R. Gomes Freire 203, 1169-203 Lisboa, Portugal

jribeiro@ipb.pt, rl@dem.uminho.pt, anamoita@tecnico.ulisboa.pt

Keywords: Aneurysm Intracranial Biomodels, in Vitro Tests, Additive Manufacturing, Polysmooth.

Abstract: The hemodynamics of Intracranial Aneurysm (IA) involves complex phenomena that influence its growth

and rupture. The progress of additive manufacturing techniques has allowed the development of biomodels

suitable to perform in vitro flow experiments. Hence, this work presents the manufacturing process to fabricate

flow biomodels by using the additive manufacturing technique known as Fused Deposition Modeling (FDM).

The biomodels obtained through the proposed technique has proved to be suitable for in vitro flow

experiments using imaging techniques and for validation of numerical studies.

1 INTRODUCTION

Intracranial Aneurysm (IA) is a disease associated

with weakening of the arterial wall, which causes

local dilation (Rodriguez-Régent et al., 2014). This

pathology has a high mortality rate of around 60%

after rupture (Amenta et al., 2012). Studies have

shown that changes in flow induce endothelial cell

responses, thus causing disease (Chiu & Chien,

2011), but the cause of development and disruption of

IAs are still not well understood (Tromp et al., 2014).

Therefore, to better understand IAs, it is important to

analyze the local hemodynamic, and how it affects the

vessel wall (Saqr et al., 2019).

Although there are non-invasive in vivo studies

capable of performing flow measurements using

https://orcid.org/0000-0003-2414-073X

https://orcid.org/0000-0002-4415-7267

https://orcid.org/0000-0002-2274-1374

imaging techniques such as Phase Contrast Magnetic

Resonance Imaging (PC-MRI), Magnetic Resonance

Angiography (MRA), these have difficulty in

visualizing the flow due to lack of resolution in small

vessels only underestimate wall shear values, have

low reproducibility and are expensive (Szajer & Ho-

Shon, 2018)(Roloff et al., 2018).

As an alternative to in vivo studies, in vitro tests

with transparent flow phantoms (biomodels) make it

possible to visualize the flow through the monitoring

of suspended tracer particles. It is possible to employ

different techniques for measurement, such as

Particle Image Velocimetry (PIV) (Yamaguchi et al.,

2022), Particle Tracking Velocimetry (PTV) and

image microscopy (Souza et al., 2020). Another

advantage of in vitro studies is the possibility of

https://orcid.org/0000-0001-6300-148X

https://orcid.org/0000-0003-3428-637X

https://orcid.org/0000-0001-9801-7617

Souza, A., Rodrigues, D., Souza, M., Ferrera, C., Ribeiro, J., Lima, R. and Moita, A.

In Vitro Flow Study in an Intracranial Aneurysm Biomodel Manufactured by Additive Manufacturing.

DOI: 10.5220/0011652100003414

In Proceedings of the 16th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2023) - Volume 1: BIODEVICES, pages 121-125

ISBN: 978-989-758-631-6; ISSN: 2184-4305

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

121

validating numerical simulations. The disadvantage

of this type of study is the difficulty of manufacturing

biomodels suitable for tests, as these flow phantoms

must be transparent, they must reproduce the

anatomical geometry, and the lost core material must

not interact with the biomodel material. In recent

years, the aid of Additive Manufacturing (AM)

techniques has enabled the proper fabrication of IA

biomodels. Souza et al (Souza et al., 2020) combined

AM processes and lost core casting with glycerin-

based soap, and Doutel et al (Doutel et al., 2015) used

caramel (melted sugar) as the lost core material.

Although these processes have been shown to be

adequate, they have many steps in their manufacture.

In this work, a biomodel manufacturing process is

presented, which uses AM techniques to directly print

the model of the lost core (vessel lumen), which is

coated with a transparent, biocompatible silicone,

which is Polydimethylsiloxane (PDMS). One of the

advantages of this silicone is its ease of manufacture,

when compared to models made of glass (Yu &

Durgesh, 2022). The biomodels were tested for their

transparency and high-speed video microscopy tests

were performed for qualitative analysis of flow

behavior and quantitative analysis of flow velocity.

2 BIOMODEL

MANUFACTURING PROCESS

The geometry used was previously created in the

SolidWorks 3D CAD Software, using the average

dimensions of real intracranial aneurysms (Parlea et

al., 1999). The geometry was saved in STL format

and then converted to G-code, a format used in 3D

printers. The spherical geometry was chosen, because

according to the study by (Philip et al., 2022) that

compares through numerical studies the idealized and

patient-specific geometries that the physics of the

flow inside the aneurysm sac is better predicted in this

type of geometry.

The Biomodels were manufactured using an

additive manufacturing technique, Fused Deposition

Modeling (FDM). The geometry was printed in

PolySmooth (chosen material) in the Ultimarker 3 3D

printer. After obtaining the model, it was positioned

in the Polysher machine for a surface treatment with

isopropyl alcohol, this treatment took place for a

period of 20 minutes, way that the lines of the

outermost layers of geometry are smoothed. With the

surface treatment completed, the mold was placed in

an acetate box and then the PDMS was poured by

gravity. PDMS was prepared in a ratio of 10:1 and its

curing process took place in 48 hours. After the

PDMS had completely cured, the lost core material

was removed with isopropyl alcohol. The process

steps are illustrated in Figure 1.

Figure 1: (a) PolySmooth geometry, (b) biomodel with lost

core material and (c) final biomodel.

3 EVALUATION OF THE

OPTICAL TRANSPARENCY OF

THE BIOMODEL

To evaluate the issue of optical distortion caused by

solid-liquid interaction, two fluids with different

physical properties were tested. At first, a fluid with

a refractive index similar to that of PDMS and was

used, a mixture of 61% glycerol and 39% distilled

water (w/w) and 0.06% suspended particles of

Polymethylmethacrylate (PMMA) with 60 μm (in

diameter). The second fluid considered was just

water. It is important to mention that the application

of this technique, to evaluate the optical transparency

of the biomodel under study, was based on the work

carried out by Hopkins et al (Hopkins et al., 2000).

The physical properties of the materials used in the

present study are shown in Table 1 (Souza et al.,

2020).

Table 1: Physical properties of materials used in the

evaluation of the optical transparency of biomodels.

Material Refractive

index

Viscosity

(

Pa.s

)

Density

(

)

Wate

1.333 0.920 × 10

-3

997

Glycerin

mixture

1.412 1.290 × 10

-2

1153

PDMS 1.412 - -

In the tests, a sheet with a rectangular structure

was used in which each rectangle has dimensions of

2.4 ×3.9 mm and under which the biomodel was

placed with the different fluids. Figure 2 shows the

images of the transparency tests, with image (a)

referring to the biomodel in which the injected fluid

was water and image (b) corresponding to the

situation in which the glycerin-based solution was

injected.

BIODEVICES 2023 - 16th International Conference on Biomedical Electronics and Devices

122

Figure 2: Evaluation of the optical transparency of the

biomodel with the fluid: (a) water and (b) glycerin-based

solution.

4 EXPERIMENTAL TEST OF

FLOW VISUALIZATION

The main objectives of this test were: to evaluate the

appearance of the fluid recirculation phenomenon as

a function of the flow rates used, through the

observation of the particle trajectories and; evaluate

the velocities in different zones of the biomodel.

For this, an experimental setup was used,

consisting of a set of equipment, namely: an ultra-

high-speed camera (Photron FASTCAM SA3),

coupled to an inverted microscope (IX71, Olympus,

Japan) and an objective (N-Achroplan 2.5x/0.07). At

first, the PDMS biomodel was fixed to the

microscope and a syringe pump was used to pump the

working fluid at a constant flow rate. Two different

flow rates were used: 5 ml/min and 20 ml/min.

4.1 Qualitative Analysis of Flow

Behavior

The recorded images, using the Photron FASTCAM

visualization software, were later processed in the

ImageJ software, where the particle trajectories and

velocities were obtained using the Z Project plugin

and the MTrackJ plugin, respectively. The image

processing of the two flows studied are shown in

Figure 3.

Figure 3: Trajectories of the PMMA particles for a flow rate

of: (a) 5 ml/min and (b) 20 ml/min.

Observing the previous figure, it can be concluded

that, for a flow rate of 5 mL/min, the phenomenon of

fluid recirculation still does not occur. However, it is

possible to verify that for the flow of 20 mL/min, the

phenomenon of fluid recirculation occurs. Although

the flow rates used are lower than those found in

cerebral arteries, the technique demonstrates the

potential of observing the different phenomena that

occur within IAs with an increase in the flow rate.

Thus, characterizing and visualizing the vortex zones,

which is a characteristic related to IAs growth and

rupture (Saqr et al., 2019).

4.2 Quantitative Analysis of Flow

Velocity

To study the velocities, the images were processed

with the ImageJ software with the MTrackJ plugin.

Velocity was evaluated in 5 zones: at the inlet, at the

outlet and at three locations in the center of the

biomodel, for both flows. Figure 4 shows the

trajectories where the velocities for the different flow

rates were obtained.

Figure 4: Trajectory of the marked particles, for the flow

rate of (a) 5ml/min and (b) 20 ml/min.

With the trajectories traced, it was then possible

to calculate the velocities in each of the marked areas.

Therefore, the results obtained for the velocity in the

study with flow rate of 5 ml/min are found in Table 2

and Table 3 shows the results for the flow rate of 20

ml/min.

Table 2: Velocities obtained considering a flow rate of 5

ml/min.

Track

Velocit

m/s

1Re

Inlet 0.0120

2 Yellow Outlet 0.0156

3Li

reen Cente

0.0061

4 Dark blue Cente

0.0040

5 Pink Center 0.0008

In Vitro Flow Study in an Intracranial Aneurysm Biomodel Manufactured by Additive Manufacturing

123

Table 3: Velocities obtained considering a flow rate of 20

ml/min.

Track

Velocit

m/s

1 Re

Inlet 0.0595

2 Yellow Outlet 0.0659

3 Li

reen Cente

0.0327

4 Li

ht blue Cente

0.0137

5 Dark blue Center 0.0039

With the velocity profiles traced, we observed that

the velocities of the fluid inside the aneurysm sac for

both studied flow rates are lower than the inlet and

outlet flows. At the flow rate of 20 ml/min where

recirculation occurs, the flow velocity decreases even

more as it approaches the upper part of the aneurysm

head. Although the flow rates used in our tests are

lower than the real values, the behavior of the

velocities found corresponds to previous studies

(Philip et al., 2022)(Cebral et al., 2011), where the

velocity decreases in the vortex zones.

5 CONCLUSIONS

Intracranial aneurysms are severe diseases that

require deeper understanding for a better diagnosis

and treatment of this kind of pathology. In vitro

hemodynamic studies are a promising way to improve

our understanding about the beginning, development,

and rupture of intracranial aneurysms. The obtained

experimental flow results have shown that the

polysmooth material that was used by FDM printing

technique was proved to be suitable for the

manufacture of biomodels, with good dimensional

accuracy, high quality flow visualizations and ease to

remove the material from the lumen. Through the

visualization tests, it was possible to identify the

recirculation regions at the highest flow. In addition,

it was possible to observe that at the central region of

the aneurysm, where the recirculation occurs, the

velocities are much lower when compared to the inlet

and outlet velocities.

For future work, it is intended to use fluids with

rheological properties closer to blood (blood

analogues), but with the same refractive index as

PDMS. In addition to using flow rates obtained from

medical examinations for a more realistic approach.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support

provided by Fundação para a Ciência e a Tecnologia

(FCT), through the projects EXPL/EME-

EME/0732/2021, PTDC/EEI-EEE/2846/2021,

funded by NORTE 2020, PORTUGAL2020, and

FEDER. This work was also supported by Fundação

para a Ciência e a Tecnologia (FCT) under the

strategic grants UIDB/04077/2020, UIDB/00690/

2020, UIDB/04436/2020 and UIDB/00532/2020.

Andrews Souza acknowledges the PhD scholarship

2021.07961.BD attributed by FCT. Partial support

from the Junta de Extremadura through Grants No.

GR21091 and IB20105 (partially financed by FEDER

funds) is gratefully acknowledged.

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