Page 246 - IJB-9-2

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

stent . The quality of stent drug-coating is crucial for two stent design and manufacturing orientation together with
[19]
main reasons: (i) it influences the drug release rate, and (ii) some basic rules during PBF printing process. With PBF
it affects the superficial texture of the VS as an irregular printing, layer plane supports are needed when angles
coating could lead to rough surfaces. Therefore, a smooth smaller than 45° are created; overhanging regions up to
stent surface would minimize the injury of blood vessels 1 mm can be built without supports; minimum gaps of
during the implantation of the medical device. They aimed 0.3 mm are recommended between separate features, etc.
[37]
to improve the traditional stent drug-coating procedures, According to these requirements, they designed a stent with
thereby reducing the time required and material waste. hexagonal, zig-zag pattern forming a closed cell without
Thus, they MJT-printed PLA/simvastatin and PLA/ flex-connectors (Figure 10A). This geometry avoided the
paclitaxel solutions over different already-existing requirements of support structures during PBF 3DP. Their
BMS (Presillion and Cypher stents) by means of a attention was clearly centered on the optimization of the
TM
TM
piezodriven dispenser. The authors reported that “tips with 3DP manufacturing, not in the final properties of the final
300 pL aqueous droplet volume are suitable for stent coating stent, so the tensile strength, recoil and fatigue resistance
(…) while smaller volumes (100 pL) resulted in clogging of were not characterized in this study. Depending on the laser
the nozzle” . Another important factor to bear in mind scanning pattern, the laser pulse duration and peak power
[19]
is the voltage used, since an inadequate voltage could lead must be optimized. They also concluded that increased
to bubble formation inside the ink tip, jeopardizing the peak power and pulse duration reduced surface roughness.
printability. The comparison between the final coating of For the particular geometrical design of this VS, the
two commercial stents with different geometries indicates parallel strategy gave rise to irregular geometry, thickness,
the need for MJT drug-coating optimization according to and surface roughness, thus highlighting the inadequacy
the shape of each medical device to avoid irregular drug- of this strategy for the production of micro-geometries
coating. The in vitro drug release profiles of simvastatin (Figure 10C). On the contrary, concentric scanning
and paclitaxel, separately, revealed burst release for the and higher printability were achieved (Figure 10C).
first 5 days, followed by a first-order release until day 30 Additionally, PBF 3DP produces items with very rough
in both cases. A successful implantation in male Wistar surfaces, implying that post-processing techniques such
rats was reported, without inflammatory and cytotoxicity electro-polishing are compulsory to reduce harm during
response within 7 days. Therefore, MJT-printing for stent stent implantation (Figure 10B) .
[37]
drug-coating can be used to optimize and maximize the Despite the fact that metallic substrates are the most
performance of DES. frequent in PBF, there are also some exceptions. For
instance, some years ago, Flege et al. adapted the PBF
4.4. Powder bed fusion 3DP as vascular stent 3DP technique to produce a BRS vascular stent made
manufacturing technique of PLLA and PCL . Since this 3DP technique requires
[78]
PBF 3DP is usually associated to strong, hard materials. In powdery raw materials for the printing process, PLLA
fact, metal powders are most frequently used in PBF 3DP and PCL polymers were subjected to solvent-evaporation
techniques to obtain metallic VS. Despite the long-term processes to obtain homogeneous, small particles so that
complications and disadvantages of permanent metallic they could then be used in PBF; both PCL and PLLA
stents, they are the most widely used (Table 1, Figure 2). particles possessed good flowability, regular spherical
Nevertheless, in terms of cutting-edge technologies and shape, narrow particle size distribution, and high density.
research, the efforts are centered on BRS, DES, which Moreover, the laser of the PBF printer was adapted by
means that the number of studies dealing with PBF 3DP adding a power attenuator. The attention of this study was
techniques is scarce. therefore centered on the ability to produce stents from
Laser-cutting and braiding are the most commonly these raw materials and to monitor the stability of the
used manufacturing techniques for the production of resultant ingredients after the PBF printing and gamma-
metallic VS. When it comes to 3DP, PBF is commonly irradiation sterilization. The biocompatibility of the stents
used to print metallic materials. Demir and Previtali and the materials was assessed using human arterial
demonstrated the feasibility and convenience of PBF and smooth muscle cells (haSMCs), human umbilical vein
the subsequent electrochemical polishing to produce a endothelial cells (HUVECs), and endothelial progenitor
CoCr stent with respect to the conventional manufacturing cells (EPCs). First, the manufacturing of PCL and PLLA
cycles (microtube production followed by laser-cutting) . powder particles did not jeopardize their biocompatibility,
[37]
In this technique, different scan strategies can be followed unlike the printing process, which reduced metabolic
(parallel or concentric scanning), giving rise to different activity of the EPCs. The authors ascribed this result
results. The authors highlighted the importance of a proper to the presence of polymerization initiators in stent

Volume 9 Issue 2 (2023) 238 https://doi.org/10.18063/ijb.v9i2.664