Stents have been in use since 1986, and have transformed the manner in which coronary heart disease and other arterial occlusions are treated. Currently they are widely used to treat several vascular and endovascular diseases caused by the contraction or blockage of blood vessels.
Most stents on the market are made of metal and can cause serious medical complications as they are implanted permanently into the blood vessels. One probable solution to the above issue is to use stents manufactured from bio-absorbable materials.
Bio-absorbable stents are designed to dissolve in the human body after a period of time when its purpose is served. However, there is one major issue with the use of bio-absorbable material - machining the material.
This article covers the influence of laser process parameters on the quality and effectiveness of machining of bio-absorbable materials with the aid of High Q Spirit™ femtosecond laser system.
Ultrafast Laser Systems
Ultrafast laser systems designed with a pulse duration which is in the femtosecond time range, are widely used in several material processing applications.
The typical benefits of the ultrafast lasers such as high accuracy and high ablation efficiency of ablated structures on metal and on dielectric targets have been revealed in many studies. It was noticed that processing quality met industrial standards but processing speed required further improvement so as to meet efficient industrial usage. To manage this problem, ultrafast laser units consisting of high average power and repetition rate can be used.
Ultrashort pulse lasers are also required to be industrially dependable with smaller foot print. Since the development of femtosecond laser systems such as the Spirit™ platform by High Q Lasers, theses systems have made micro-processing of solid targets a lot easier in industrial applications.
Laser fusion cutting is a major part of stent production and has been widely used from the beginning. The initially produced stents were made from stainless steel consisting of 2.5 - 4.0mm diameter. It was possible for feature tolerances to be ±25µm or more, and part geometries and features were quite simple.
Laser cutting could easily be implemented using nanosecond-duration pulsed infrared lasers, thus enabling accuracy requirements for machining to be achieved easily. Thermal interactions of the nanosecond laser pulses with the metal tend to have an effect on the non-optimal surface finish on metal parts: melting, burring, and re-cast are regular features of laser fusion cutting.
Also heat deposition in the material results in a heat affected zone (HAZ) in the nearby cut edges. In the HAZ, material properties or composition are changed. All these effects reveal that laser cutting technology can be used to increase level of stent production with the progress and modification of many post-processing steps to eliminate rough and modified material edges.
Cleaning, deburring, etching, and final polishing are regularly required to attain the necessary surface properties of the stent and reliability for implantable devices. However, some of these post-processing steps can be skipped by producing stents using ultra short pulse lasers.
A Nitinol micro-stent machined by the Spirit laser is seen in figure 1. The main features include the absence of a HAZ, tight machining tolerances, clean cut edges, and no heat-induced distortion of the fragile lattice structure.
Figure 1. Micro-stent precision machined with Spirit™ 1040-4fs laser. The material is Nitinol, with tube diameter and wall thickness of 4.25mm and 45µm, respectively. Strut width is 35µm.
Bio-Absorbable Materials for Stent Production
Bio-absorbable materials are widely studied for use in stent production due to their ability to be absorbed by the human body over time. Stents made from bio-absorbable materials help in the prevention of unnecessary medical problems which may occur when metal stents are used.
There has been an increase in the use of bio-absorbable polymers for manufacture of bio-degradable stents which has led to an increase of focus on micromachining of bio-absorbable polymer.
Lasers used for the production of stents depend on the material type and cutting details. Since bio-absorbable polymers have a low melting temperature, less than 100°C, care must be taken to ensure that the heat load to the adjacent areas during laser processing be reduced. Thus, ultrafast laser pulses would be an ideal choice for micromachining of bio-absorbable polymers due to the non-thermal characteristic of laser-material coupling and the option of structuring extremely small micron scale features.
The influence of the processing conditions on the effectiveness and quality of laser processing of frequently used bio-absorbable poly-L-lactic acid (PLLA) has been closely studied in this article. The focus was more on the how the pulse duration influenced the ablation process. According to the parameter study, stent structures’ dross- and melt-free cut machined by femtosecond Spirit™ laser in PLLA is described.
Experimental Setup and Materials
Figure 2. New Spirit™ 1040-8-SHG femtosecond laser developed by High Q Laser and Spectra-Physics.
Ytterbium-doped crystals have been discovered to be the ideal choice of laser materials for the femtosecond generation.
Another alternative for robust laser system is the semiconductor saturable absorber mirrors (SESAM) which is capable of passive mode-locking by intracavity pulse stretching plan.
High Q Laser GmbH’s near-IR laser systems – the Spirit™ and the picoREGEN™ have been utilized for this work.
The Spirit™ laser system (figure 2) is based on a Yb-doped chirped pulse regenerative amplifier. It has average power of 4 and 8W at 520 and 1040nm, respectively, and flexible repetition rates up to 1MHz. It is possible to achieve maximum pulse energy of 40µJ with pulse duration of <400fs at 200kHz.
The picoREGEN™ unit is designed with an average power of 30W at 1064nm and a repetition rate up to 1MHz. The maximum pulse energy is 200µJ with pulse duration of 10ps.
Both these lasers possess excellent beam quality factor M2<1.2. The laser beam’s intensity was modified with an integrated fast (300kHz) signal attenuator. The laser beam had the option for expansion up to 16x magnification by a variable beam expander. The lasers spot size was established from the relationship between the laser fluence F and the diameter D of a crater formed by a single laser pulse.
A natural PLLA ribbon of 80µm thickness was used for the experiment.
Results and Discussion
Optimization of ablation rate
Throughput is an essential factor for laser machining, to consider when comparing results attained using various laser systems or technologies.
Quality is also an essential factor and it impacts speed. Based on the application, the accurate balance between quality and speed has to be established. To decide the removal rate for PLLA, a method devised by Schille et al. was used.
For this parameter study, squares measuring 2x2mm were filled with parallel rectangular crossed lines (figure 3). The machining result was judged by the depth of the ablated structures and the heat load i.e. melt formations throughout the laser processing.
Figure 3. Schematic of experimental setup. Here dp is the lateral pulse distance, dh is hatch distance.
In order to understand the best processing conditions, the repetition rate, the influence of laser fluence, pulse duration, and lateral pulse distance were analyzed. The parameter study was conducted with the aid of 350fs and 10ps laser pulses at wavelengths of 520 and 532nm, respectively. The repetition rates of 100kHz and 500kHz for the femtosecond and picosecond lasers were used in the study.
Optimal ablation rate was noticed at dp =dh =8µm for spot radius of ca. w0 =10µm. Likewise, machining quality, i.e. melt formation, was based on the pulse duration and the process strategy.
Microscope images of bottom and edge of test structures ablated in PLLA polymer (Figure 4) was attained in thermal (upper row) and athermal (lower row) laser machining system. Thermal laser ablation causes strong melting, which is then followed by re-solidification of the polymer material in the machining area.
PLLA melting alters the polymer’s mechanical and chemical properties, thus reducing the accuracy of the laser machining. These characteristics are not preferred effects in stents production. However, athermal machining compared to thermal ablation provides melt-free characteristics with almost no heat affected zone (HAZ).
Figure 4. Optical microscope images of bottom surface and edge of test structures ablated with ultrashort pulse laser in PLLA polymer obtained upon thermal (upper row) and athermal (lower row) laser machining regime.
The reliance of the removal rate for the picosecond and the femtosecond laser pulses on the laser fluence at repetition rate of 100 and 500kHz can be seen in figure 5. To conduct the ablation study, femtosecond laser pulses with peak fluence between 1.5 and 3.5J/cm2 was used.
Due to the higher ablation threshold fluence of 5J/cm2 for 10ps laser pulses in contrast to the threshold fluence of 2J/cm2 for 350fs pulses, very high peak fluences were essential for micro-machining of PLLA with the picosecond laser i.e. between 6.0 and 16.0J/cm2.
The ablation rate increased when increasing applied laser fluence for both laser pulse widths. Highest ablation rate of up to ca.16mm3/min was attained using 350fs laser pulses, which matched an ablation efficiency of 4.2mm3/min/W.
Although a higher maximum ablation rate of up to 22mm3/min was noticed for ps pulses, the ablation efficiency was lower by almost a factor of 2 compared to fs laser pulses (2.1mm3/min/W). Nevertheless the focus is more on the maximum rate at which athermal machining occurs than the overall maximum ablation rate.
Figure 5 shows how when using 350fs laser pulses at 520nm, athermal ablation rates in PLLA of up to 12 mm3/min can be obtained. At the same time, the maximum ablation rate reduces by almost a factor of 6 to a value < 2mm3/min when the pulse duration is increased from 350fs to 10ps.
The results can be viewed in figure 5, and they focus on the significance of the right choice of laser repetition rate and pulse duration for micro machining of PLLA. To achieve high repetition rate, the time between successive pulses should not be too long for the heat to diffuse out of the focal volume.
Accordingly, the energy from successive pulses gets collected and the temperature of the material increases from pulse to pulse. Limited heating of the PLLA causes melt formation upon laser processing. However, the other reaction is that melting effects can be strongly decreased by reducing pulse duration from 10ps to 350fs. Thus the pulse duration is the main parameter while choosing an appropriate laser system for laser micro-machining of bio-absorbable polymers.
Figure 5. Ablation rate for a PLLA polymer as a function of the applied peak fluence for 350fs (top) and 10ps (bottom) laser pulses at wavelength of 520 and 532nm, respectively. The experimental conditions at which athermal/thermal machining is observed are marked in green and red zone. The transition zone between athermal/thermal machining is marked in yellow.
Micromachining of PLLA Bio-Absorbable Polymer
The Spirit™ at wavelength of 520nm was used to cut a stent structure in 80µm thick PLLA ribbon.
The optical microscope and SEM images of the laser processed PLLA ribbon can be viewed in Figure 6. High speed, multi-pass processing using a scanning galvanometer was used to create the stent structures.
The 80µm thick PLLA ribbon could be cut with the processing speed of several mm/s with high resolution with struts measuring less than 100µm in width and low thermal effects by using femtosecond laser pulses at 520nm. There were no melting zone, microcracks and no heat-induced distortion.
Figure 6. Stent structures machined by femtosecond laser pulses in PLLA. The material is 80µm thick and struts are 100µm wide.
Stents manufacturing has taken a new turn with the use of bio-absorbable materials. femtosecond and picosecond laser pulses were used to observe laser micro-machining of bio-absorbable PLLA. The study revealed that the maximum ablation rate for PLLA of upto 12mm3/min could be attained in the athermal machining system by using 350fs laser pulses at 520nm. This was also possible by a factor of six more than the highest ablation rate examined for 10ps laser pulses. With this in mind, shorter pulses with ca. 350fs are considered to be more useful for laser micro-machining of bio-absorbable polymers based on decreased melt production and heat affected zones.
The femtosecond laser (Spirit™) at wavelength of 520nm was used to display possibility of laser cut of PLLA materials. The first round of results revealed that a strong and reliable femtosecond laser, such as Spirit™ was indeed a good option for machining heat sensitive bio-absorbable materials like the PLLA.
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This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.
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