Femtosecond Technology for Technical and Medical Applications

Femtosecond laser processing of metal and plastics in the medical device industry
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The so far non-existent regulations and standards will be drafted and existing regulations and standards will be extended. Further experiments are dealing with the modelling of uncertainty associated with dose-response curves. These curves describe the probability of eye injury as a function of ocular energy and can be used to quantify the risk of ocular injury given a certain level of exposure to laser radiation. Keywords: safety, lasers, femtoseconds. Project ID:. Start date:. Project Duration:. Project costs:. Technological Area:. Market Area:. Laser Zentrum Hannover E.

Femtolasers Productions Gmbh. Landeskrankenanstalten Salzburg.

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Laservision Gmbh. Removal rate in this example was 0. The numbers definitely tell the story, as major medical device manufacturers are starting to understand the true ROI of more efficient machining technologies. The femtosecond laser is also the only current technology appropriate for machining medical products out of new, bioabsorbable polymers, which can be safely implanted in the body for controlled lengths of time before absorbing without causing harm or adverse interactions. Next-generation advanced bioabsorbables also called aspirants provide an alternative to traditional polymers or metal components, and are being designed to meet precise degradation rates and other specifications.

Introduction

The bioabsorbable material can be machined into any profile that can be used for stents. However, it must be machined correctly and without inducing heat. Failure to do so might lead to crystallization in the material, which would degrade its structure and affect its lifespan and ability to dispense medicine at the correct rate.

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Also, because bioabsorbables dissolve, they cannot be cleaned like most plastics, nor can they be touched with any liquid solutions—which is another reason that the femtosecond laser is a better choice for the material. Bioabsorbables are already being used for coronary stents in the EU, although they have not yet received FDA approval for use in the US. Mostly composed of polyesters, primarily homopolymers and copolymers of poly lactic acid and poly glycolic acid , bioabsorbables are showing promise for a variety of uses, including cardio stents for patients who may have been stented numerous times and can no longer tolerate a traditional fixed stent.

The material is also being used to deliver medicines into organs of the body—for example, a plastic material like a sponge is doped with medicine and inserted into the liver, dispensing medicine at a consistent rate and lasting from six months to three years before dissolving. After years of clinical trials, several firms are awaiting approval and already planning for the new innovation to hit the US market, and several have been qualifying use of femtosecond laser equipment to gear up for the precision micromachining required. The industrial robustness of the femtosecond disk laser needs to be matched to an equivalent system to deliver the day-in, day-out reliability that the medical device industry demands.

It is worth noting that the femtosecond laser cannot currently be fiber-delivered and therefore is directed and delivered to the focusing optics by fixed mirrors. So, designing a beam delivery system for a 4-axis tube cutter that can make off-axes cuts while maintaining alignment can be a challenge. The optical path design has to ensure that such key optical tools as the beam expander and fine tuning attenuator are easily accessible as needed for process development.

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The system design requires full mechanical isolation and in some cases ambient temperature stability to provide a system foundation for process repeatability. Miyachi is providing the first line of support, including sampling processing, quoting, and building of the work cell, as well as installation, training, service, and warranty.

The company uses its in-house femtosecond processing lab to provide a seamless transition from developing the application to the define-design-deliver approach used by the systems engineering group for both standard and custom system fulfillment. While there is no doubt that mechanical stiffness and isolation are required, this is simply the starting point.

Determining how delicate parts and materials will be repeatably positioned or clamped, implementing in-system part inspection and incorporating real-time optical beam diagnostics are several other key pieces of the puzzle. Optical alignment is important and ensuring that the beam profile and power levels are maintained requires the use of in-line, nonintrusive optical diagnostic tools to provide real-time information. Being in-line and nonintrusive enables data collection during processing that can be time- and date-stamped as part of the manufacturing data.

The femtosecond disk offers unique process capability with excellent beam quality and high peak powers. To maximize the process capability for production, the laser must be integrated into a system that enables high quality and repeatable processing.

New Applications of Femtosecond Lasers

The combination of a highly robust femtosecond product that has been on the market for more than eight years with an experienced micro systems provider with in-house parts processing capabilities results in a partnership that can develop ideal production system solutions for high-value medical products. The used setup was not optimized for filament generation, hence multifilament appearance is a result of beam self-organization inside the material. When the specific for the material critical power P cr is exceeded, the beam may break into several filaments each carrying the power comparable with critical power The effect of a breakup of cylindrical symmetry is connected with weak or random inhomogeneity in the input laser pulse 2.

Supramolecular structure of crystalline domain that cause strong scattering of light limits the filament formation. We observed terminals of filamentary channels at rear side of samples. The uplift of the material at the end of the channels Fig. The SEM images of sample cross-section showed multiple cavities with random distribution along single filament. Those observations suggest melting of the material. Recently, the investigation on filaments induced by tight focusing femtosecond laser beam inside PMMA led to conclusion that thermal effects that enhance fluidity of polymer play important role in formation of a void The propagation of shock wave as result of sudden thermalisation of energy carried by electrons along filament inside PMMA has been observed in pump-probe experiment where shock wave propagation was observed to propagate both cylindrically around the filament and toward the sample surface Microscopic observation revealed that at high overlapping of pulses, the ablated surface which is being created during the process causes distortion of wavefront and limits the filament formation.

When pulses are deposited close to each other the incubation of defects in material leads to stronger absorption. This effect is a result of lower overlapping at the boundary what allows for enhanced light penetration. Circular shape may also result from the refractive index change caused by previous pulses. Therefore the crater that would distort wavefront appears after the pulse.

At first stage of laser-material interaction, the free carrier population is not high enough to block light penetration. We suppose that first part of pulse penetrates the material deeply and propagates as filament while the second part of the pulse causes ablation of surface. We observed also an effect of perpendicular to filament ripple formation Fig. As these regular nanodefects correspond to laser wavelength they can be a result of periodic changes of electron density. Those instabilities resembles characteristic for ultrashort pulse high spatial frequency laser laser-induced periodic surface structures LIPSS which commonly occurs at the surface 7 , 39 but were also observed inside dielectrics Calorimetric curves confirm that at high pulse overlapping a kind of shield zone that protects the material inside is being formed.

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The material at the ablated surface does not influence the DSC curves in significant way. It confirms no filament formation and no other significant bulk modifications. For those samples the number and the length of filaments were the highest. It shows that the laser modification can selectively tune crystallization rate as well as crystalline structure in spatial sense. The samples with empty channels are brittle, however, after subsequent crystallization, their mechanical properties are recovered.

The mechanical properties should be however investigated in a framework of further research. The molecular weight is one of the key variables governing crystallization kinetics of PLLA. With decrease of molecular weight the cold crystallization peak shifts to lower temperature, owing to the increase of crystallization rate Moreover, in the second heating cycle the all DSC curves resemble unmodified material see Supplementary Fig.

Inside filament-induced channels there are possibly low molecular weight decomposition products however the amount of this fraction is too small to detect it using DSC. The change in cold crystallization behaviour is therefore not connected with molecular weight decrease within entire sample but presence of the filament-induced empty cavities.

Nucleating effect of filaments can be explained by three possible ways. Inside the filaments there is possibly a small fraction of monomers or oligomers from laser-induced degradation of PLLA and they cause increase of cold crystallization rate. Second possible explanation is related to presence of new surfaces introduced into bulk material.

Nucleation mechanism in bulk is different than surface nucleation from micro or nano pores Finally, the compaction of the material around filaments as a result of cylindrical shock wave propagation may also affect the crystallization dynamics. S4 revealed even weaker susceptibility to hydrolytic attack which is a surprising result. These changes suggest the possible decrease of molecular weight in water environment enhanced by filament — induced microchannels.

The results of hydrolytic degradation experiment suggest that microchannels formed by filament propagation accelerate hydrolysis stronger than multipulse surface ablation. Laser — induced cavities influence strongly the crystallization behaviour and the may be used to obtain certain crystal structure perfection in spatially selected regions. The ordering of crystalline phase in turn may influence even more the hydrolytic degradation profile.

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We believe that by using method presented here, adjusting filament density and applying proper conditioning, gradient properties of PLLA can be achieved in terms of both crystallinity and hydrolytic degradation profile. This aspect however requires further investigations. The femtosecond laser-induced bulk modifications inside PLLA were characterized. Calorimetric curves showed that the intensity of bulk modification in sense of cold crystallization behaviour can be adjusted by overlapping of pulses. The conclusions from microscopic cross-sectional analysis are in good agreement with DSC results.

The filamentation inside the material can be limited by appropriate pulse overlapping when the laser process is aimed at only surface modification. Using UV wavelength, only surface modification was obtained. We presented detailed analysis of crystalline phase developed during subsequent heating after laser modification.

Optics Letters

In consequence, subsequent conditioning can induce different crystalline phase morphology in different locations of specimen hence induce gradient mechanical and thermal properties. This method can be applied for controllable change of polymer crystallinity and hydrolytic degradation profile with additional thermal post processing after laser treatment.

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Editors: Dausinger, Friedrich, Lichtner, Friedemann, Lubatschowski, Holger (Eds.) Femtosecond technology, with its ultrashort light pulses, forms an innovative laser technology that can be used for numerous technical applications. This monograph gives a comprehensive overview of. Request PDF on ResearchGate | On Jan 1, , D. Breitling and others published Femtosecond Technology for Technical and Medical Applications: Interaction.

Furthermore, we observed that filamentary modifications accelerate hydrolysis stronger than multipulse surface ablation. The material was processed in hydraulic press in order to achieve amorphous polymer sheets. The values are expressed according to polystyrene standards. The femtosecond pulses generated by an oscillator based on all-PM-fibre laser mode-locked with a nonlinear loop mirror 43 were amplified in a two-stage amplifier using an ytterbium doped photonics crystal fibre rod in the final booster stage.

We used constant pulse energy of Samples were mounted in such a way to avoid influence of substrate ablation underneath or light reflection. Processing was performed in ambient air. The polarization was linear. The varying parameter was pulse overlapping which was changed in both scanning directions. Different overlapping of pulses we obtained by changing pulse spatial separation sp x and sp y Fig.

Overlapping factor was calculated as the percentage of the overlapped diameter in relation to total beam diameter in scanning direction. Second heating scan was also performed. The glass transition temperature T g from heating and cooling scans were taken as the inflection point in the heat flux curve. Experimental data was processed using the generic STAR e computer program. The background corrected WAXD patterns were resolved into Lorenzian shape diffraction peaks and diffusion maxima by using the Levenber-Marquardt non-linear fitting procedure implemented on OriginPro 9.

The degree of crystallinity X c was calculated according to the following relation:. The reference and laser treated samples were measured before and after crystallization in different conditions. Each sample was measured on both sides. A fitting procedure was performed using Omnic 8. The samples in form of laser scanned fields were cut out from polymer sheet and the mass of them was determined using high resolution laboratory balance accuracy 0.

Each sample was measured 5 times.

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Then the samples were placed in separate glass probes filled with distilled water. After drying, samples were analysed using laboratory balance in order to measure mass loss. Gattass, R. Femtosecond laser micromachining in transparent materials. Photonics 2 , — Mao, S. Dynamics of femtosecond laser interactions with dielectrics. A 79 Daskalova, A. FS laser processing of bio-polymer thin films for studying cell-to-substrate specific response. Paun, I. Laser micro-patterning of biodegradable polymer blends for tissue engineering.

Ortiz, R. Ultra-fast laser microprocessing of medical polymers for cell engineering applications. Szustakiewicz, K. Yada, S. Femtosecond laser induced periodic surface structure on poly-L-lactic acid. Express 23 , Jia, W. Effects of high-repetition-rate femtosecond laser micromachining on the physical and chemical properties of polylactide PLA. Shibata, A. Biodegradability of poly lactic-co-glycolic acid and poly l-lactic acid after deep-ultraviolet femtosecond and nanosecond laser irradiation.

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