Laser printing constitutes an interesting alternative to more conventional printing techniques in the microfabrication of biomedical devices. The principle of operation of most laser printing techniques relies on the highly localized absorption of strongly focused laser pulses in the close proximity of the free surface of the liquid to be printed. This leads to the generation of a cavitation bubble which further expansion results in the ejection of a small fraction of the liquid, giving place to the deposition of a well-defined droplet onto a collector substrate. Laser printing has proved feasible for printing biological materials, from single-stranded DNA to proteins, and even living cells and microorganisms, with high degrees of resolution and reproducibility. In consequence, laser printing appears to be an excellent candidate for the fabrication of biological microdevices, such as DNA and protein microarrays, or miniaturized biosensors. The optimization of the performances of laser printing techniques requires a detailed knowledge of the dynamics of liquid transfer. Time-resolved microscopy techniques play a crucial role in this concern, since they allow tracking the evolution of the ejected material with excellent time and spatial resolution. Investigations carried out up to date have shown that liquid ejection proceeds through the formation of long, thin and stable liquid jets. In this work the different approaches used so far for monitoring liquid ejection during laser printing are considered, and it is shown how these techniques make possible to understand the complex dynamics involved in the process.
Liquids laser printing has been usually addressed through laser-induced forward transfer, a technique that allows the deposition of microdroplets with good resolution and control. However, it presents a significant drawback that can compromise its future industrial implementation: the need for the preparation of the liquid to be printed in thin film form. Such constraint results especially detrimental when very high degrees of resolution need to be achieved. In order to overcome the problem, we have recently shown that in the case of solutions transparent or weakly absorbing to the laser radiation, liquid printing is possible directly from the liquid contained in a reservoir, without the requirement of thin film preparation. The principle of operation of the film-free laser printing technique is the tight focusing of ultrashort laser pulses underneath the free surface of a liquid. Subsurface absorption leads to the formation of a cavitation bubble through optical breakdown, and the subsequent bubble expansion displaces some liquid towards the substrate, where the pattern is formed. Though the feasibility of the technique for microdroplets printing has already been proved, there is not much insight yet in the mechanisms of liquid ejection and transport. In this work we investigate the mechanisms of liquid printing during film-free laser forward printing. The study, essential for the optimization of the technique, reveals that the process is complex: the bubble expansion-collapse cycle results in the formation of two consecutive jets which display completely different dynamics, and which behavior is strongly dependent on the laser pulse energy density.
The development of organic electronic requires a non contact digital printing process. The European funded e-LIFT project investigated the possibility of using the Laser Induced Forward Transfer (LIFT) technique to address this field of applications. This process has been optimized for the deposition of functional organic and inorganic materials in liquid and solid phase, and a set of polymer dynamic release layer (DRL) has been developed to allow a safe transfer of a large range of thin films. Then, some specific applications related to the development of heterogeneous integration in organic electronics have been addressed. We demonstrated the ability of LIFT process to print thin film of organic semiconductor and to realize Organic Thin Film Transistors (OTFT) with mobilities as high as 4 10<sup>-2</sup> cm<sup>2</sup>.V<sup>-1</sup>.s<sup>-1</sup> and I<sub>on</sub>/I<sub>off</sub> ratio of 2.8 10<sup>5</sup>. Polymer Light Emitting Diodes (PLED) have been laser printed by transferring in a single step process a stack of thin films, leading to the fabrication of red, blue green PLEDs with luminance ranging from 145 cd.m<sup>-2</sup> to 540 cd.m<sup>-2</sup>. Then, chemical sensors and biosensors have been fabricated by printing polymers and proteins on Surface Acoustic Wave (SAW) devices. The ability of LIFT to transfer several sensing elements on a same device with high resolution allows improving the selectivity of these sensors and biosensors. Gas sensors based on the deposition of semiconducting oxide (SnO<sub>2</sub>) and biosensors for the detection of herbicides relying on the printing of proteins have also been realized and their performances overcome those of commercial devices. At last, we successfully laser-printed thermoelectric materials and realized microgenerators for energy harvesting applications.
Lasers are adequate tools for the production of patterns with high spatial resolution owing to the high focusing power of
their radiation. Laser induced forward transfer (LIFT) is a direct-writing technique allowing the deposition of tiny
amounts of material from a donor thin film through the action of a pulsed laser beam. A laser pulse is focused onto the
donor thin film through a transparent support, what results in the transference of a small area of the film onto a receptor
substrate that is placed parallel to the film-support system. Although LIFT was originally developed to operate with
solid films, it has been demonstrated that deposition is also viable from liquid films. In this case, a small amount of
liquid is directly ejected from the film onto the receptor substrate, where it rests deposited in the form of a microdroplet.
This makes LIFT adequate for biosensors preparation, since biological solutions can be transferred onto solid substrates
to produce micrometric patterns of biomolecules. In this case, the liquid solvent acts as transport vector of the
biomolecules. The viability of the technique has been demonstrated through the preparation of functional miniaturized
biosensors showing similar performances and higher scales of integration than those prepared through more
Multi-pulse Nd:YAG (λ = 1.064 μm, τ ~ 100-300 ns, ν = 1-30 kHz laser irradiation of titanium at low intensities, below or in some cases just above the single-pulse melting threshold of titanium led to the development of a large variety of surface structures. The morphology evolution was strongly influenced by the number of the subsequent laser pulses as well as the ambient gas. In air the formation of crown-, or dome-shaped micro-structures was evidenced. In vacuum the micro-relief is characterized by smooth polyhedral structures developing in the surface plane. In nitrogen the cumulative laser irradiation induced the growth of uniformly distributed micro-column arrays with a high aspect ratio, protruding above the non-irradiated target surface. Morphological, structural and chemical characterizations of the laser treated surface areas were performed by scanning electron microscopy, X-ray diffractometry, Raman spectroscopy, and wavelength dispersive X-ray spectroscopy. The growth mechanisms which lead to the formation of the specific structures are investigated. Moreover, the potential applications of the laser processed surfaces are discussed.
Biomolecule microarrays are a kind of biosensors that consist in patterns of different biological molecules immobilized on a solid substrate and capable to bind specifically to their complementary targets. In particular, DNA and protein microarrays have been revealed to be very efficient devices for genen and protein identification, what has converted them in powerful tools for many applications, like clinical diagnose, drug discovery analysis, genomics and proteomics. The production of these devices requires the manipulation of tiny amounts of a liquid solution containing biomolecules without damaging them. In this work laser induced forward transfer (LIFT) has been used for spotting a biomolecule in order to check the viability of this technique for the production of microarrays. A pulsed Nd:YAG laser beam (355 nm wavelength) has been used to transfer droplets of a biomolecule containing solution onto a solid slide. Optical microscopy of the transferred material has been carried out to investigate the morphological characteristics of the droplets obtained under different irradiation conditions. Afterwards, a DNA microarray has been spotted. The viability of the transference has been tested by checking the biological activity of the biomolecule in front of its specific complementary target. This has revealed that, indeed, the LIFT technique is adequate for the production of DNA microarrays.