Rapid prototyping compliant arterial phantoms for in-vitro studies and device testing

Abstract

Background: Compliant vascular phantoms are desirable for in-vitro patient-specific experiments and device testing. TangoPlus FullCure 930 is a commercially available rubber-like material that can be used for PolyJet rapid prototyping. This work aims to gather preliminary data on the distensibility of this material, in order to assess the feasibility of its use in the context of experimental cardiovascular modelling.

Methods: The descending aorta anatomy of a volunteer was modelled in 3D from cardiovascular magnetic resonance (CMR) images and rapid prototyped using TangoPlus. The model was printed with a range of increasing wall thicknesses (0.6, 0.7, 0.8, 1.0 and 1.5 mm), keeping the lumen of the vessel constant. Models were also printed in both vertical and horizontal orientations, thus resulting in a total of ten specimens. Compliance tests were performed by monitoring pressure variations while gradually increasing and decreasing internal volume. Knowledge of distensibility was thus derived and then implemented with CMR data to test two applications. Firstly, a patient-specific compliant model of hypoplastic aorta suitable for connection in a mock circulatory loop for in-vitro tests was manufactured. Secondly, the right ventricular outflow tract (RVOT) of a patient necessitating pulmonary valve replacement was printed in order to physically test device insertion and assess patient's suitability for percutaneous pulmonary valve intervention.

Results: The distensibility of the material was identified in a range from 6.5 × 10(-3) mmHg(-1) for the 0.6 mm case, to 3.0 × 10(-3) mmHg(-1) for the 1.5 mm case. The models printed in the vertical orientation were always more compliant than their horizontal counterpart. Rapid prototyping of a compliant hypoplastic aorta and of a RVOT anatomical model were both feasible. Device insertion in the RVOT model was successful.

Conclusion: Values of distensibility, compared with literature data, show that TangoPlus is suitable for manufacturing arterial phantoms, with the added benefit of being compatible with PolyJet printing, thus guaranteeing representative anatomical finishing, and quick and inexpensive fabrication. The appealing possibility of printing models of non-uniform wall thickness, resembling more closely certain physiological scenarios, can also be explored. However, this material appears to be too stiff for modelling the more compliant systemic venous system.

Figure 1

Aortic models. Reconstructed tract of descending aorta, extruded five times with increasing wall thickness (0.6, 0.7, 0.8, 1.0 and 1.5 mm) keeping the internal diameter constant (15.5 mm).

Figure 2

Compliance testing of aortic samples. Sample of the pressure-volume relationship resulting from a compliance test. The loop exhibits an area of hysteresis due to the viscoelastic behaviour of the material. Five data points, collected for the 0.7 mm thick “vertical” model, are shown at each dV, demonstrating the repeatability of the method.

Figure 3

Compliance changes with increasing wall thickness. Change in pressure for corresponding changes in volume (dV) normalised for the initial volume of the sample (V₀) for the range of tested thicknesses (0.6 – 1.5 mm).

Figure 4

Physiological distensibility range. Relationship between increasing wall thickness and decreasing distensibility (D) for the compliant aortic models (black circles). The values of D implemented by the rapid prototyped models are within a physiological range, as shown by the comparison with a clinical range of D values (grey diamonds) for different arteries, including ascending and descending aorta [16,17], pulmonary artery [18] and carotid artery [19].

Figure 5

Difference between printing modes. The vertical mode for printing always resulted in stiffer models. Note: the 0.6 mm thickness case is not reported as the vertical specimen was damaged and the comparison could not be performed.

Figure 6

Models for hydrodynamic tests, example of hypoplastic aortic arch. Extruding a model of hypoplastic aorta with wall thickness appropriate for implementing patient-specific distensibility as derived from CMR data (A) and the finished model (B), including tapered connections for insertion in a mock circulatory system for hydrodynamic tests, and side ports for pressure measurements at different locations (the latter indicated by the red arrows).

Figure 7

Models for device testing, example of right ventricular outflow tract. A patient-specific model of right ventricular outflow tract (RVOT): 3D volume derived from CMR data (left), from which a physical phantom is rapid prototyped (centre) and then used for physical insertion of a stent-graft (right) for assessing patient’s suitability for the device.