Picosecond Laser Machining of Bioabsorbable Stents
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Solution Overview
Problem
Laser machining of stents often results in adverse energy transfer beyond the cutting edge, affecting the properties of the substrate and leading to reduced mechanical performance, such as radial strength and fracture resistance, particularly in biodegradable polymers like PLLA and PLGA, due to thermal and photochemical damage.
Innovation Solution
The use of a picosecond laser with a wavelength in the green range (e.g., 532 nm) and pulse width of 1-10 ps to minimize damage by controlling voids and cracks within a depth of 2 microns, thereby preserving the mechanical properties of the stent, including radial strength and fracture resistance, by optimizing laser parameters like pulse width, wavelength, and cooling gas flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional laser machining is used to cut through the substrate wall, then the stent structural elements are formed, but thermal and photochemical damage occurs beyond the cutting edge, reducing mechanical properties
Solution Approach 1:
The patent employs pulsed laser operation with specific pulse widths (1-1000 picoseconds) to deliver energy in periodic bursts rather than continuous exposure. This allows the material to cool between pulses, reducing thermal diffusion and photochemical damage beyond the cutting edge while maintaining precise cutting capability through repeated pulse application.
Solution Approach 2:
The patent systematically varies laser parameters including pulse width (1-1000 ps), wavelength (355-1064 nm), and fluence to optimize the balance between cutting efficiency and damage minimization. By changing these parameters, the laser energy is controlled to remove material precisely without excessive thermal or photochemical effects in the heat-affected zone.
2Productivity
If higher laser energy is used to increase cutting speed, then productivity improves, but the extent of damage to the substrate increases
Solution Approach 1:
High productivity is achieved through high repetition rate pulsing (e.g., 80-100 kHz) where many short-duration pulses are delivered in rapid succession. Each individual pulse maintains low energy to minimize damage, but the cumulative effect of many pulses achieves high cutting speeds and productivity without proportionally increasing the heat-affected zone or photochemical damage.
Solution Approach 2:
The continuous delivery of pulses at high repetition rates maintains steady cutting action throughout the process, ensuring consistent material removal rate. This continuous useful action keeps the cutting process efficient and productive while each individual pulse remains low-energy to limit damage accumulation.
3Power
If longer pulse width is used to increase energy delivery, then cutting through the wall is achieved, but thermal damage and void formation increase
Solution Approach 1:
The patent uses pulsed operation where the total energy delivery is distributed across multiple pulses rather than delivered in a single long pulse. This periodic delivery allows the material to respond to each pulse individually, preventing the cumulative thermal buildup and void formation that would occur with a single long-duration high-energy pulse, while still achieving complete wall penetration through accumulated effect.
Solution Approach 2:
The patent optimizes pulse width within the 1-1000 ps range to balance energy delivery efficiency with damage minimization. By selecting appropriate pulse widths and combining them with suitable repetition rates and wavelengths, sufficient total energy is delivered to cut through the wall while keeping individual pulse duration short enough to prevent excessive thermal diffusion and photochemical damage.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach minimizes thermal and photochemical damage, maintaining the mechanical properties of the stent, enhancing radial strength and fracture resistance, and reducing the extent of damage to the substrate, resulting in improved performance and durability of the stent.
Implementation Method 1
Laser machining refers to removal of material accomplished through laser and target material interactions. Material is removed by melting and blow away, or by direct vaporization/ablation.
Implementation Method 2
Laser machining processes transport photon energy into a target material in the form of thermal energy or photochemical energy.
Implementation Method 3
Laser machining processes transport photon energy into a target material in the form of thermal energy or photochemical energy. Material is removed by melting and blow away, or by direct vaporization/ablation.
Data Source
AI summary
The present invention involves laser machining polymer substrates to form a stent with laser parameters that minimize damage to the substrate in a surface region adjacent to the machined edge surface. The wavelength and pulse width are selected for this unique application and they can be controlled to minimize the surface modifications (such as voids, cracks which are induced by the laser-material interaction) which contribute to the variation in mechanical properties with distance from the edge surface, bulk mechanical properties, or a combination thereof.