Film-free laser forward printing of transparent and weakly absorbing liquids.

A laser-based technique for printing transparent and weakly absorbing liquids is developed. Its principle of operation relies in the tight focusing of short laser pulses inside the liquid and close to its free surface, in such a way that the laser radiation is absorbed in a tiny volume around the beam waist, with practically no absorption in any other location along the beam path. If the absorbed energy overcomes the optical breakdown threshold, a cavitation bubble is generated, and its expansion results in the propulsion of a small fraction of liquid which can be collected on a substrate, leading to the printing of a microdroplet for each laser pulse. The technique does not require the preparation of the liquid in thin film form, and its forward mode of operation imposes no restriction concerning the optical properties of the substrate. These characteristics make it well suited for printing a wide variety of materials of interest in diverse applications. We demonstrate that the film-free laser forward printing technique is capable of printing microdroplets with good resolution, reproducibility and control, and analyze the influence of the main process parameter, laser pulse energy. The mechanisms of liquid printing are also investigated: time-resolved imaging provides a clear picture of the dynamics of liquid transfer which allows understanding the main features observed in the printed droplets.

DNA deposition through laser induced forward transfer.

Laser induced forward transfer (LIFT) is a laser direct write technique that appears to be specially adequate for the production of biosensors, since it permits to deposit patterns of biomolecules with high spatial resolution. In the LIFT technique, a laser pulse is focused on a thin film of the material to be transferred through a transparent support, and under the action of the laser pulse, a small fraction of the film is transferred to a receptor substrate that is placed parallel to the film-support system. In the case of biomolecules transfer, the thin film consists in a liquid solution containing the biomolecules. In this work, microarrays of two different cDNAs have been both spotted by LIFT and pin microspotting onto a poly-L-lysine treated glass slide. Once transferred, all the microarrays have been submitted to hybridization with the complementary strands of the spotted cDNAs, each one tagged with a different fluorochrome. Comparative fluorescence scanner analyses have revealed that the microarrays transferred through LIFT are equivalent to those transferred through pin microspotting in terms of signal intensity and gene discrimination capacity, and that the action of the laser pulse does not result in significant damage of the transferred DNA.

Characterization of calcium phosphate coatings deposited by Nd:YAG laser ablation at 355 nm: influence of thickness.

Calcium phosphate coatings were deposited by pulsed laser ablation with a radiation of 355 nm from a Nd:YAG laser. All the coatings were obtained at the same conditions, but deposition was stopped after different number of pulses to get coatings with different thickness. The influence of thickness in the structural and mechanical properties of the coatings was investigated. Coatings structure was characterised by scanning electron microscopy, grazing incidence X-ray diffractometry and Raman spectroscopy. The mechanical properties were evaluated by scratch test. The morphology of the coatings is dominated by the presence of droplets. The coatings are composed mainly of hydroxyapatite, alpha tricalcium phosphate and amorphous calcium phosphate. Thinner coatings withstand higher loads of failure in the scratch test.

Pulsed laser deposition of pseudowollastonite coatings.

Pseudowollastonite (alpha-CaSiO3) is a bioactive ceramic material that induces direct bone growth. A process to obtain pseudowollastonite coatings that may be applied to implants is described and evaluated in this work. The coatings were first deposited on titanium alloy by laser ablation with a pulsed Nd:YAG laser tripled in frequency. After deposition, they were submitted to a soft laser treatment with a continuous wave Nd:YAG infrared laser. Coatings were characterised by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and energy dispersive spectroscopy before and after the laser treatment. As-deposited coatings are composed of pseudowollastonite and amorphous material. They have a porous structure of gathered grains and poor cohesion. After the laser treatment the coatings crystallinity and cohesion are improved. The laser treatment also makes the coatings dense and well adhered to the substrate. Therefore, this two-step process has been demonstrated as a valuable method to coat titanium implants with pseudowollastonite.

Influence of thickness on the properties of hydroxyapatite coatings deposited by KrF laser ablation.

The growth of hydroxyapatite coatings obtained by KrF excimer laser ablation and their adhesion to a titanium alloy substrate were studied by producing coatings with thicknesses ranging from 170 nm up to 1.5 microm, as a result of different deposition times. The morphology of the coatings consists of grain-like particles and also droplets. During growth the grain-like particles grow in size, partially masking the droplets, and a columnar structure is developed. The thinnest film is mainly composed of amorphous calcium phosphate. The coating 350nm thick already contains hydroxyapatite, whereas thicker coatings present some alpha tricalcium phosphate in addition to hydroxyapatite. The resulting coating to substrate adhesion was evaluated through the scratch test technique. Coatings fail under the scratch test by spallating laterally from the diamond tip and the failure load increases as thickness decreases, until not adhesive but cohesive failure for the thinnest coating is observed.

BS-SEM evaluation of the tissular interactions between cortical bone and calcium-phosphate covered titanium implants.

The improvement of the reliability of the contact between the osseous tissues and the implant materials has been tested by recovering the metallic implants with ceramic materials, usually calcium phosphates. In our study, the calcium phosphate recovering layers were deposited by means of a pulsed-laser deposition technique. Our aim was to to evaluate the tissue interactions established between cortical bone and titanium implants covered by five different layers, ranging from amorphous calcium phosphate to crystalline hydroxyapatite, obtained by altering the parameters of the laser ablation process. The surgical protocol of the study consisted in the simultaneous implantation of the five types of implants in both the tibial dyaphisis of three Beagle dogs, sacrificed respectively one, two and three months after the last surgical procedures. After the sacrifice, the samples were submitted to a scheduled procedure of embedding in plastic polymers without prior decalcification, in order to perform the ultrastructural studies: scanning microscopy with secondary and backscattered electrons (BS-SEM). Our observations show that both in terms of the calcified tissues appearing as a response to the presence of the different coatings and of time of recovering, the implants coated with crystalline calcium phosphate layers by laser ablation present a better result than the amorphous-calcium-phosphate-coated implants. Moreover, the constant presence of chondroid tissue, related with the mechanical induction by forces applied on the recovering area, strongly suggests that the mechanisms implied in osteointegration are related to endomembranous, rather than endochondral ossification processes.

Behavior in simulated body fluid of calcium phosphate coatings obtained by laser ablation.

Three types of calcium phosphate coatings onto titanium alloy substrates, deposited by the laser ablation technique, were immersed in a simulated body fluid in order to determine their behavior in conditions similar to the human blood plasma. Neither the hydroxyapatite coating nor the amorphous calcium phosphate coating do dissolve and the alpha-tricalcium phosphate phase of the coating of beta-tricalcium phosphate with minor alpha phase slightly dissolves. Precipitation of an apatitic phase is favored onto the hydroxyapatite coating and onto the coating of beta-tricalcium phosphate with minor alpha phase. Onto the titanium alloy substrate reference there is also precipitation but at larger induction times. However, onto the amorphous calcium phosphate coating no precipitate is formed.

Mechanical properties of calcium phosphate coatings deposited by laser ablation.

Amorphous calcium phosphate and crystalline hydroxyapatite coatings with different morphologies were deposited onto Ti-6Al-4V substrates by means of the laser ablation technique. The strength of adhesion of the coatings to the substrate and their mode of fracture were evaluated through the scratch test technique and scanning electron microscopy. The effect of wet immersion on the adhesion was also assessed. The mechanisms of failure and the critical load of delamination differ significantly depending on the phase and structure of the coatings. The HA coatings with granular morphology have higher resistance to delamination as compared to HA coatings with columnar morphology. This fact has been related to the absence of stresses for the granular morphology.

Bone growth on and resorption of calcium phosphate coatings obtained by pulsed laser deposition.

Three different calcium phosphate coatings of crystalline hydroxyapatite (HA), alpha- and beta-tricalcium phosphate (alpha+beta-TCP), or amorphous calcium phosphate (ACP) obtained by pulsed laser deposition on Ti-6Al-4V were incubated in a potentially osteogenic primary cell culture (rat bone marrow) in order to evaluate the amount and mode of mineralized bone matrix formation after 2 weeks with special emphasis on the type of interfacial structure that was created. Evaluation techniques included fluorescence labeling and scanning electron microscopy. The resistance to cellular resorption by osteoclasts was also studied. Bone matrix delaminated from the ACP coatings, while it remained on the HA and the alpha+beta-TCP coatings even after fracturing. A cementlike line was seen as the immediate contiguous interface with the nondegrading dense HA surface and with the surface of the remaining porous beta-TCP coating. Highly dense and crystalline HA coatings do not dissolve but are capable of establishing a strong bond with the bone matrix grown on top. Chemical and mechanical bonding were considered in this case. Cellular resorption was practically not observed on the HA coatings, but it was observed on the alpha+beta-TCP coatings. Resorption took place as dissolution that was due to the acidic microenvironment.

Application of dissolution experiments to characterise the structure of pulsed laser-deposited calcium phosphate coatings.

A dissolution test was performed with pulsed laser (Nd: YAG, 355 nm)-deposited calcium phosphate coatings composed of hydroxyapatite (HA) and alpha-tricalcium phosphate (alpha-TCP) in different proportions, as a result of the use of different deposition rates. During immersion in a Ca2+-free Hank's solution, the dissolution kinetics were determined while other structural and compositional properties of the coatings were derived. It was possible to infer that the alpha-TCP is distributed uniformly and that the coating is of a non-columnar compact grain structure. The mass ratio of the phases for each coating was also determined and was related to the X-ray diffraction intensities. When incomplete, the hydroxylation level of the HA in the coatings is completed after immersion.

Dissolution behaviour of calcium phosphate coatings obtained by laser ablation.

Pulsed laser deposited calcium phosphate coatings on titanium alloy have been tested under simulated physiological conditions in order to evaluate the changes in morphology, composition and structure. The coatings were deposited under different conditions to obtain different crystalline structures, ranging from amorphous and mixed crystalline phases to pure crystalline hydroxyapatite (HA). The coated samples were immersed in a Ca-free Hank's balanced salt solution for up to 5 days. Characterization of the coatings was performed by X-ray diffraction, scanning electron microscopy and Fourier-transform Raman spectroscopy before and after immersion. Their dissolution behaviour was also monitored through their mass loss and calcium release. Coatings of pure HA preserve their morphology and structure during the exposure time in solution. In multiphasic coatings, consisting of HA with tetracalcium phosphate (TetraCP) or beta-tricalcium phosphate (beta-TCP) with a-tricalcium phosphate (alpha-TCP), microporosity is induced by the complete dissolution of TetraCP or gamma-TCP. Amorphous calcium phosphate coatings totally dissolve.

ArF excimer laser irradiation of human dentin.

BACKGROUND AND OBJECTIVE: The use of excimer lasers for treatment of dental hard tissues has considerable potential because the combined characteristics of low wavelength and short pulse result in limited heat diffusion and, therefore, tissue ablation without the problems of collateral damage. To date, there are relatively few published studies concerning the effects of excimer laser irradiation on dental hard tissues. Thus the present study was conducted to examine the morphological changes in tooth dentin subsequent to ArF excimer laser irradiation. STUDY DESIGN/MATERIALS AND METHODS: The morphologic changes induced in normal, nondiseased human dentin following irradiation by an ArF excimer laser at fluences ranging from 1 to 4 J/cm2 and the number of laser pulses ranging from 50 to 1,000 were evaluated by scanning electron microscopy. RESULTS: Two modes of ablation, photochemical at low fluences and thermal at high fluences, were observed. A fluence of 1 J/ cm2 when combined with 50 or 100 pulses produced a uniform ablation of the dentin surface without signs of tissue melting. At fluences > 1.5 J/cm2, the thermal mode of ablation was more efficient at removing intertubular dentin than peritubular dentin. Further, when compared to the lower fluences, the higher settings produced a rougher ablation crater surface. Additionally, the higher fluences produced surface melting with each pulse and sealing of exposed dentinal tubules after irradiation with 100-300 laser pulses. CONCLUSIONS: The photochemical and thermal mechanisms of tooth dentin ablation were identified based on significant differences in tissue morphology following laser irradiation. The rates of tissue ablation and the observed morphologic changes indicate that the ArF excimer laser could be useful for caries removal and sealing of exposed dentinal tubules.