Underwater femtosecond laser micromachining of thin nitinol tubes for medical coronary stent manufacture

Abstract

Microprofiling of medical coronary stents has been dominated by the use of Nd:YAG lasers with pulse lengths in the range of a few milliseconds, and material removal is based on the melt ejection with a high-pressure gas. As a result, recast and heat-affected zones are produced, and various post-processing procedures are required to remove these defects. This paper reports a new approach of machining stents in submerged conditions using a 100-fs pulsed laser. A comparison is given of dry and underwater femtosecond laser micromachining techniques of nickeltitanium alloy (nitinol) typically used as the material for coronary stents. The characteristics of laser interactions with the material have been studied. A femtosecond Ti:sapphire laser system (wavelength of 800 nm, pulse duration of 100 fs, repetition rate of 1 kHz) was used to perform the cutting process. It is observed that machining under a thin water film resulted in no presence of heat-affected zone, debris, spatter or recast with fine-cut surface quality. At the optimum parameters, the results obtained with dry cutting showed nearly the same cut surface quality as with cutting under water. However, debris and recast formation still appeared on the dry cut, which is based on material vaporization. Physical processes involved during the cutting process in a thin water film, i.e. bubble formation and shock waves, are discussed. N. Muhammad · L. Li ( ) Laser

Cavity formation and surface modeling of laser milling process under a thin-flowing water layer

“…Water is normally used to assist the process due to its high thermal conductivity, recyclability and cheap. The whole workpiece or only the surface of workpiece to be ablated by laser has to be submerged in water [1][2][3][4][5] or covered by a layer of water [4,6,7], where the laser ablation takes place in the confined volume of water. Since water can exponentially absorb the energy of laser with its travelling distance in water regarding the Beer-Lambert's law, the water layer has to be thin enough to cause less loss of laser intensity.…”

Experimental investigation of underwater laser beam micromachining (UW-LBμM) on 304 stainless steel

“…Commonly used liquid is water, as it is low cost, easily available, and transparent for laser transmission [17][18][19][20][21][22]. The mechanisms of UW-LBμM for various lasers are different due to change in absorption coefficient of water at different wavelengths.…”

“…Muhammad and Li [17] reported micromachining of nitinol tube for coronary stent application by using underwater femtosecond laser and found minimum surface roughness at low fluence as well as high speed during micromachining in air. HAZ is obtained at the highest fluence and it reduces with decrease in fluence.…”

Comparative study of CW, nanosecond- and femtosecond-pulsed laser microcutting of AZ31 magnesium alloy stents

“…In literature, limited amount of work regarding laser microcutting of stents is reported, where CW emission, 15 longpulsed, 14,16,17,21,22 short-pulsed, 23 and ultrashort pulsed 24,25 sources are used on different materials. A comparison regarding the different machining domains appears to be absent to authors' knowledge.…”