Page 240 - IJB-9-2

International Journal of Bioprinting Coronary and peripheral artery disease. State of the art.

4.1. Material extrusion 3DP as vascular stents top). The auxetic geometry maximizes the stent anchorage
manufacturing technique with vessel walls, thus minimizing the risk of malposition
Thermoplastics are a wide group of synthetic materials with and displacement. Different diameters, lengths and
versatile properties (flexibility, pliability, thermoresponsive) dimensions of the final stent were tested to obtain the
and acceptable mechanical strength. Some of them are best mechanical features (Figure 5, bottom). These studies
biodegradable and biocompatible and have high value revealed that for this particular geometry, the higher the
properties in the manufacturing of medical devices. Due to stent diameter, the lower the radial force provided by PLA.
their polymeric nature, thermoplastics have proven to be Additionally, the higher the wall thickness, the better the
useful drug carriers and delivery systems. In the particular compressive properties. Thus, PLA is a good material for
case of 3DP, thermoplastics are optimal feedstock for FDM the production of small-diameter stents with considerable
due to their ability to acquire semisolid, viscous consistency wall thickness and good recoverability, which are the
at high temperatures and recover solid state after cooling, features that guarantees the correct placement of the
whereas in the VS field, thermoplastics’ thermoresponsive medical device . The feasibility of PLA as VS raw material
[62]
behavior makes them valuable ingredients in the has also been confirmed by Jia et al. In this case, PLA
[63]
production of self-expandable VS. stent produced with FDM maintained its excellent shape at
room temperature during 1 week storage, and only recover
Polycaprolactone (PCL) and polylactic acid (PLA) have the original shape within 5 seconds after being exposed to
been used to formulate an ink for FDM printing [59,60] . The 70°C . The necessity of such a high temperature to trigger
[63]
stent composition and different BRS geometries in the VS expansion (70°C) implies the necessity of applying
final properties have been explored, including the effect heat into the implantation area. Given that, the term
on cellular proliferation. To understand the results and “self-expansion” is subjected to discussion, since external
optimize the 3DP process, PCL stents were printed onto stimuli is still needed for the implantation of these stents.
a computer-controlled rotatory platform under different Temperatures closer to the human body are needed for
conditions (nozzle temperature, fluid flow rate, printing these stents to be entirely “self-expandable” in order to
speed) [60,61] . The printing temperature and flow rate were obviate the need of external stimuli during deployment.
the factors reporting the strongest influence over PCL
printability. The increase in the printing speed reduced FDM was also used to print a stent made of PCL with
[36]
the PCL cooling rate, changing the final properties of a rotary mandrel, which is subjected to voltage . This
the material. This was ascribed to a more effective heat 3DP machine is somewhat based on electrospinning
dissipation . In another study dealing with PCL and technology. The raw PCL was shaped as particles that must
[61]
PLA, pure PCL showed 35% higher fibroblast proliferation be transformed into filaments to enable 3DP. To do so, PCL
than pure PLA stents, which was attributed to the different particles are melted in a heating chamber associated to
molecular weight of both ingredients (smaller molecular the printer, and subsequently transported to the extruder
weight usually lead to lower cellular proliferation). needle, which deposited them in a rotatory mandrel
Despite that, the mechanical properties revealed that the subjected to an electric voltage (4 kV). This 3DP technique
combination of PLA and PCL was the most appropriate made the fabrication of small stents with remarkable
[36]
for a stent due to the combination of PCL elasticity and resolution and reproducibility possible .
PLA rigidity: the former prevents PLA breakage during In another study, the acrylated, photocurable,
stent expansion, while the latter hindered PCL recoil after and thermoresponsive polymer PGDA (poly(glycerol
placement. The use of PCL in the external wall of the dodecanoate acrylate) was deposited at 45°C to produce
BRS VS has been proposed to increase endothelial cells an easily implantable stent (Figure 6B) . The intrinsic
[64]
proliferation, while an internal PLA wall will help to retain properties of its precursor (PGD) together with the
cellular proliferation and prevent restenosis . In another photocurable ability of PGDA allowed the printing of
[59]
study, PLA and polyvinyl alcohol (PVA) were combined 3D-tilted structures without collapsing (Figure 6A). Good
and 3D printed into a BRS VS with an arrow-like geometry, biocompatibility and cellular adhesion and proliferation
providing negative Poisson’s ratio or in other words, an were observed after the implantation of the 3D-printed VS
auxetic structure . Auxetics are structures with high in a mouse aorta (Figure 6C). Endothelial cells, adipocytes
[62]
energy absorption and fracture resistance. The complexity and connective tissue adhered and proliferated around the
of this geometry was accurately achieved through FDM stent within just 14 days .
[64]
by the combination of these two ingredients, in which
PVA is merely acting as a temporal support during the The intervention of a stenosis is rather complicated
printing process: after being printed, the water dissolution if it happens in a bifurcated vessel. In such a case, two
of PVA leaves a final, solid stent made of PLA (Figure 5, different stents are used (one for each branch) and then,

Volume 9 Issue 2 (2023) 232 https://doi.org/10.18063/ijb.v9i2.664