In this paper, for the first time, we present an analysis of changes of physicochemical properties of poly(L-lactide) induced by the femtosecond laser. Introduced changes were characterized using Differential Scanning Calorimetry, Gel Permeation Chromatography, X-ray Photoelectron Spectroscopy and Fourier Transform Infrared spectroscopy. We have noted that even for these process parameters for which no thermal ablation effects occurred, we observed some changes in material properties. In GPC image we recorded an increase in polydispersity index and some reduction of the molecular weight. The FTIR spectra show a reduction in the number of both C=O and C−O−C bonds in the polymer as well as the appearance of new bands. By using the XPS it was determined that processing with femtosecond laser cause a small oxidation of the surface layer. Decay of the spectra indicates the possibility of carboxyl (−COOH) and hydroxyl (−OH) groups present in the modified polymer. Although the observed changes are relatively small compared to long pulse duration lasers or UV lasers, they cannot be neglected in biopolymer applications for tissue engineering.
This paper presents results of poly(L-lactide) and poly(L-lactide)/hydroxyapatite composite cutting optimization using 2<sup>nd</sup> harmonic of femtosecond fiber laser. There are several limitations regarding the use of femtosecond lasers for processing of heat-sensitive medical grade polymers such as poly(L-lactide) (PLLA). Improper use of ultrashort pulse laser may lead to heat load into surrounding material causing its melting and crack formation when excessive energy is deposited and pulse repetition frequency is too high. The optimization of laser parameters is necessary not only because of heat but also due to reflection and scattering of incident light in high aspect ratio V-shaped groove resulting in decrease of ablation rate with an increasing number of repetitions. This problem is especially important in case of polymer foils thicker than 200 μm and the beam spot size which is typically around 15 - 30 μm in commercial systems. In this work we present threshold fluence and ablation rates for three types of material: amorphous PLLA, crystalline PLLA and PLLA/hydroxyapatite composite. For those materials in form of thick foils (∼400 μm) we performed cutting optimization. A significant improvement of cutting efficiency in case of thick foils was made by applying a method of multiple, overlapped cuts.
Laser-induced periodic surface structures (LIPSS) can appear in different forms such as ripples, grooves or cones. Those highly periodic wavy surface features which are frequently smaller than incident light wavelength bring possibility of nanostructuring of many different materials. Furthermore, by changing laser parameters one can obtain wide spectrum of periodicities and geometries. The aim of this research was to determine possibility of nanostructuring pyrolytic carbon (PyC) heart valve leaflets using different irradiation conditions. The study was performed using two laser sources with different pulse duration (15 ps, 450 fs) as well as different wavelengths (1064, 532, 355 nm). Both low and high spatial frequency LIPSS were observed for each set of irradiation parameters. In case femtosecond laser pulses we obtained deep subwavelength ripple period which was even ten times smaller than applied wavelength. Obtained ripple period was ranging from 90 up to 860 nm. Raman spectra revealed the increase of disorder after laser irradiation which was comparable for both pico- and femtosecond laser.
This paper presents a method that enables fast and low-cost fabrication of microchannels with oval cross-section. The procedure is based on formation of a concave meniscus at the interface between an initially cured PDMS and a polymeric mould fabricated using excimer laser. The replica is formed by expanding gas trapped within the structures of the mould during thermal curing. A second shaping factor is connected with surface phenomena at the interface between the mould, gas and partially cured PDMS. The final shape of the meniscus is determined when the PDMS reaches the high cure extent.
The main advantage of laser processing is a non-contact character of material removal and high precision attainable thanks to low laser beam dimensions. This technique enables forming a complex, submillimeter geometrical shapes such as vascular stents which cannot be manufactured using traditional techniques e.g. injection moulding or mechanical treatment. In the domain of nanosecond laser sources, an ArF excimer laser appears as a good candidate for laser micromachining of bioresorbable polymers such as poly(L-lactide). Due to long pulse duration, however, there is a risk of heat diffusion and accumulation in the material. In addition, due to short wavelength (193 nm) photochemical process can modify the chemical composition of ablated surfaces. The motivation for this research was to evaluate the influence of laser micromachining on physicochemical properties of poly(L-lactide). We performed calorimetric analysis of laser machined samples by using differential scanning calorimetry (DSC). It allowed us to find the optimal process parameters for heat affected zone (HAZ) reduction. The chemical composition of the ablated surface was investigated by FTIR in attenuated total reflectance (ATR) mode.
In this paper, we present some examples of micromachining of poly(L-lactide) with a CO2 laser and an analysis of changes in material properties in the heat affected HAZ induced by the fluence well above the ablation threshold. The complexity of the processes of decomposition implies the need for simultaneous use of many selective analytical techniques which complement each other to give a full image of the changes. Introduced changes were characterized using Differential Scanning Calorimetry (DSC), Gel Permeation Chromatography (GPC), X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR). It turns out that CO2 laser processing of poly(L-lactide) mainly induces surface changes. However, oxidation of the surface was not observed. We recorded a bimodal distribution and some reduction in the molecular weight. Infrared spectroscopy in turn revealed the existence of absorption bands, characteristic for the vinyl groups (RCH=CH2). The appearance of these bands indicates that the decomposition of the polymer occurred, among others, by means of the cis-elimination reaction.
In this paper an alternative method of manufacturing SRR structures through the selective removal of a thin layer of silver-palladium deposited on the surface of the Al<sub>2</sub>O<sub>3</sub> ceramic by laser ablation process using nanosecond Nd:YAG laser (1064 nm) was presented. The SRR structures array were subject to transmittance measurements using the TDS (Time Domain Spectroscopy). Both electric and magnetic resonances were observed at frequencies determined by the structural parameters of the SRR. In case of the E field perpendicular to an SRR structure and one resonance area for 0.56 THz with the E field parallel to the structure, two characteristic resonant dips for 0.31 THz and 0.62 THz were obtained. Studies have confirmed that using selective laser removal process enables the preparation of the resonant structure in the range of THz.
Laser micromachining systems based on excimer lasers are usually oriented to work with mask projection regime because of the low pulse repetition rate as well as large beam aperture of the laser source. In case of fabricating of the complex 3D structures, this approach introduces a number of limitations. Alternative solution might be usage of direct writing laser mode. Some examples of the so called contour ablation approach for fabricating microlenses with an absolutely monotonically changing cross-sectional profile are presented in the literature. Based on this idea and introducing new variables like automatic mask selection as well as optimizing process algorithms led us to obtain more versatile method for shape approximation. Hence, there were fabricated structures with cross-sectional profiles described as functions that are monotonic on specified intervals such as Fresnel microlenses. In this paper we describe approximation of process parameters for obtaining desired cross-sectional profiles and finally fabrication of few exemplary microlenses. All structures were characterized by a digital optical microscopy and compared to the given profiles. The accuracy of reproduction of the desired structures at the level of single microns was achieved.
Laser-induced breakdown spectroscopy (LIBS) is a fast, fully optical method, that needs little or no sample preparation. In this technique qualitative and quantitative analysis is based on comparison. The determination of composition is generally based on the construction of a calibration curve namely the LIBS signal versus the concentration of the analyte. Typically, to calibrate the system, certified reference materials with known elemental composition are used. Nevertheless, such samples due to differences in the overall composition with respect to the used complex inorganic materials can influence significantly on the accuracy. There are also some intermediate factors which can cause imprecision in measurements, such as optical absorption, surface structure, thermal conductivity etc. This paper presents the calibration procedure performed with especially prepared pellets from the tested materials, which composition was previously defined. We also proposed methods of post-processing which allowed for mitigation of the matrix effects and for a reliable and accurate analysis. This technique was implemented for determination of trace elements in industrial copper concentrates standardized by conventional atomic absorption spectroscopy with a flame atomizer. A series of copper flotation concentrate samples was analyzed for contents of three elements, that is silver, cobalt and vanadium. It has been shown that the described technique can be used to qualitative and quantitative analyses of complex inorganic materials, such as copper flotation concentrates.
The irradiation of polylactide by KrF excimer laser with subthreshold fluence results in modification of its properties via photochemical reactions. A common approach is to modify chemical composition of polymer surface by UV irradiation, for example, in order to improve their wetting properties. In this paper, authors present a possibility of bulk modification of poly(L-lactide) which is related to photofragmentation and creation of new terminal groups. The irradiation results in decrease of molecular weight and increase of polydispersity. The appearance of new terminal groups is responsible for enhancement of absorption in UV-C range. The intensity of chemical composition changes introduced by UV irradiation can be precisely dosed thanks to a pulse character of laser source. Modifications can be controlled during the process by the analysis of energy transmitted through a polymer sheet. The distribution of absorption coefficient changes along with the depth of irradiated polymer and its correlation with polydispersity was discussed. Presented technique can be used for selective and controllable modification of hydrolytic degradation time of biodegradable polyesters utilized in biomedical applications.
In this paper a method for producing resonant structures using laser micromachining is presented. In the spot of laser
beam impact on AlN ceramics surface a conductive aluminum layer is formed. Compilation of process parameters allows
for the fabrication of structures with resistance at Rs ~ 0.01Ω/Rs. It has been also found out that the maximum value of resistance for which spiral resonator structures manifest their unique properties is at the level of Rs = 1.43 Ω.
Furthermore, the occurrence of mutual capacity which value is dependent on the arrangement of individual SR structures
with respect to each other was observed and examined. Based on satisfactory results for SR structures, it has been
attempted to produce a resonant structures dedicated to the THz range based on the process of direct metallization of
AlN ceramics surface. As a result, the Split Ring Resonator structure whose properties were verified by using the
THz -TDS method was manufactured. In case of the field E perpendicular to SRR structure and one resonance area for
0.50 THz with field E parallel to the structure, two characteristic resonant dips for 0.22 THz and 0.46 THz were
obtained. The studies confirmed that the method of direct metallization of AlN ceramics allows to produce resonant
structures in the THz range.
In this research the influence of laser micromachining on physicochemical properties of bioabsorbable polymer was
investigated. Poly(l-lactide) (PLLA), commonly used for manufacturing non-permanent biomedical devices, was
irradiated with varying fluences by CO<sub>2</sub> laser and by KrF excimer laser. To evaluate modification of the material, several
analytical techniques were used: ATR (attenuated total reflection), XPS (X-ray photoelectron spectroscopy) and DSC
(differential scanning calorimetry). We found that the laser-affected material has lower glass transition (T<sub>g</sub>) and melting
(T<sub>m</sub>) temperatures. CO<sub>2</sub> and KrF excimer lasers can be successfully used for cutting and drilling of polylactide.
Aluminum nitride (AlN) ceramics has a unique characteristic, namely the ability to form conductive structures on its
surface directly by laser-induced decomposition of the base material. Various research has been carried out on obtaining
low-ohmic structures depending on process parameters such as the laser power, overlap of subsequent pulses and the
type of shielding gas (air, nitrogen and argon). This paper focuses on explaining which factors have the greatest impact
on the resistance (resistivity) value of obtained structures. In order to explain the effect of the laser fluence (below and
above the ablation threshold of aluminum nitride) on the chemical structure of the conductive layers, qualitative EDX
analyses were performed. Optimization of the process allowed obtaining a resistivity of the conductive layers at a level
of ρ = 0.64·10<sup>-6</sup> Ω·m, with a thickness of aluminum up to 10 μm (sheet resistance R<sub>S</sub> = 10 mΩ/Sr). This technology can be
useful in making printed circuit boards (PCB), various types of sensors as well as radio-frequency identification (RFID)
and Lab-On-a-Chip (LOC) structures. This technology can also be useful for the production of metamaterials.