32
X. Sun et al.
integrity, which is very important for the performance and therapeutic efficacy of the
stents. Shape memory alloy (SMA) is widely used in cardiovascular stents, and nitinol
SMA is the first choice because of its super elasticity and good biocompatibility [4].
At present, laser cutting is widely used in the manufacturing of nitinol cardiovascular stent. In the process of laser cutting, there is no mechanical contact, and the
deformation of stent is small. Moreover, the edge defects are few, and nitinol stent
with complex shape can be manufactured [5]. However, the research of Li et al. [6]
showed that the laser processing of metal was essentially a kind of hot processing
technology, and it would also produce thermal damage on the processed surface.
Hock et al. [7–12] studied the water guide laser cutting and dry laser cutting respectively. The results showed that the water guide laser cutting had narrow cutting seam,
with no dross and no obvious heat affected zone (HAZ). However, the processing
time and equipment price of water guide laser cutting are much higher than that
of dry laser cutting. Meng et al. [13] carried out research on cutting technology of
stainless steel cardiovascular stent by using millisecond optical fiber laser. Through
experiments and analysis, the best cutting technology parameters were obtained, and
high-quality cardiovascular stent was manufactured. Lv et al. [14] studied the heat
affected zone of cutting nitinol alloy sheet by laser. The results showed that the argon
assisted cutting could obtain better surface than the air or nitrogen assisted cutting.
Raval et al. [15] used electrochemical polishing to treat cardiovascular stents, and
obtained high-quality cardiovascular stents. But they used acid system which was
difficult to control, and the electrolyte was harmful to the environment. Therefore,
it is urgent to explore the process technology of improving the surface integrity of
cardiovascular stent based on laser cutting, to eliminate or reduce thermal damage,
to enhance the surface integrity, and to improve the performance and therapeutic
efficacy of the stents.
In order to improve the surface integrity and biocompatibility of the cardiovascular stent, the collaborative manufacturing process of “fiber laser- electropolishing”
is used in this paper. The laser processing system used microsecond optical fiber
laser cutting, and the electropolishing system used alcohol-salt non-toxic electrolyte.
Then through this process technology, we explore the effects of this technology on
the surface morphology, heat affected zone (HAZ), recasting layer, dross, surface
roughness and surface chemical composition of the stents.
3.2 Experimental Materials, Equipment and Methods
3.2.1 Materials
In this study, the cardiovascular stent was made of Nitinol (56.14 at. % Ni - 43.86 at.
% Ti) thin-walled tubes (hereinafter referred to as Nitinol tubes) produced by Jiangsu
Peier Tech company, with a diameter of 2.6 mm, a wall thickness of 0.2 mm, and a
tolerance of ± 0.02 mm. The original nitinol tube for fiber laser cutting is shown in
X. Sun et al.
integrity, which is very important for the performance and therapeutic efficacy of the
stents. Shape memory alloy (SMA) is widely used in cardiovascular stents, and nitinol
SMA is the first choice because of its super elasticity and good biocompatibility [4].
At present, laser cutting is widely used in the manufacturing of nitinol cardiovascular stent. In the process of laser cutting, there is no mechanical contact, and the
deformation of stent is small. Moreover, the edge defects are few, and nitinol stent
with complex shape can be manufactured [5]. However, the research of Li et al. [6]
showed that the laser processing of metal was essentially a kind of hot processing
technology, and it would also produce thermal damage on the processed surface.
Hock et al. [7–12] studied the water guide laser cutting and dry laser cutting respectively. The results showed that the water guide laser cutting had narrow cutting seam,
with no dross and no obvious heat affected zone (HAZ). However, the processing
time and equipment price of water guide laser cutting are much higher than that
of dry laser cutting. Meng et al. [13] carried out research on cutting technology of
stainless steel cardiovascular stent by using millisecond optical fiber laser. Through
experiments and analysis, the best cutting technology parameters were obtained, and
high-quality cardiovascular stent was manufactured. Lv et al. [14] studied the heat
affected zone of cutting nitinol alloy sheet by laser. The results showed that the argon
assisted cutting could obtain better surface than the air or nitrogen assisted cutting.
Raval et al. [15] used electrochemical polishing to treat cardiovascular stents, and
obtained high-quality cardiovascular stents. But they used acid system which was
difficult to control, and the electrolyte was harmful to the environment. Therefore,
it is urgent to explore the process technology of improving the surface integrity of
cardiovascular stent based on laser cutting, to eliminate or reduce thermal damage,
to enhance the surface integrity, and to improve the performance and therapeutic
efficacy of the stents.
In order to improve the surface integrity and biocompatibility of the cardiovascular stent, the collaborative manufacturing process of “fiber laser- electropolishing”
is used in this paper. The laser processing system used microsecond optical fiber
laser cutting, and the electropolishing system used alcohol-salt non-toxic electrolyte.
Then through this process technology, we explore the effects of this technology on
the surface morphology, heat affected zone (HAZ), recasting layer, dross, surface
roughness and surface chemical composition of the stents.
3.2 Experimental Materials, Equipment and Methods
3.2.1 Materials
In this study, the cardiovascular stent was made of Nitinol (56.14 at. % Ni - 43.86 at.
% Ti) thin-walled tubes (hereinafter referred to as Nitinol tubes) produced by Jiangsu
Peier Tech company, with a diameter of 2.6 mm, a wall thickness of 0.2 mm, and a
tolerance of ± 0.02 mm. The original nitinol tube for fiber laser cutting is shown in