Programmable delay in paper-based devices using laser direct writing

Demand for low-cost alternatives to conventional medical diagnostic tools has been the driving force that has spurred significant developments in the diagnostics field. Paper-based fluidics, proposed by the Whitesides’ group in 2007 has been regarded as one such alternative, and consequently, this field has been progressing rapidly and a range of paper-based fluidic devices that implement different assays have since been demonstrated. Research into the development of methodologies that control, and in particular delay the flow of fluids in these devices is an urgently needed requirement that would enable greater functionalities in such paper-based devices.In this work, to control fluid-flow, we report the use of a new approach that is based on the laser-based photo-polymerisation technique that we have reported earlier for the creation of fluidic patterns (channels/wells) in paper. The delay or slowing down, of the fluid-flow in a fluidic channel is achieved via the introduction of barriers aligned across the direction of the fluid-flow – in a fashion similar to how speed-bumps enable traffic-calming control on a road. The schematic in Figure 1a shows how the delay can be introduced via the creation/insertion of barriers which are solid and impermeable and by controlling the ‘depth’ of the solid/impregnable barriers (Figure 1) to allow for controlled leakage of the fluids under the barriers. The control over the depth of the barriers is obtained by simply adjusting the laser-writing parameters such as the output power and writing/scanning speed. We observe that solid/impregnable barriers of various depths decrease the fluid flow by a rate that is proportional to their depth. Having patterned these barriers at pre-defined locations in the fluidic channel, using a pulsed laser operating at 266nm (20Hz, 10ns) we have achieved flow-delays with a time span ranging from few minutes to over an hour. We have also performed a study to understand the influence of the number of barriers and their position on the flow-delay, and this is shown in Figure 2.Since the channels and flow-delay barriers can be written via a common laser-writing procedure, this technique has a distinct advantage over certain other methods that require specialist operating environments, or custom-designed equipment to enable both these aspects. We believe this rapid and versatile technique is therefore suited for fabrication of ‘sample-in-read-out’ type automated paper-based microfluidic devices that can implement single/multistep analytical assays

Depth resolution of Piezoresponse force microscopy

Given that a ferroelectric domain is generally a three dimensional entity, the determination of its area as well as its depth is mandatory for full characterization. Piezoresponse force microscopy (PFM) is known for its ability to map the lateral dimensions of ferroelectric domains with high accuracy. However, no depth profile information has been readily available so far. Here, we have used ferroelectric domains of known depth profile to determine the dependence of the PFM response on the depth of the domain, and thus effectively the depth resolution of PFM detection

Laser-printed fluidic channels for the manufacture of multiplexed paper-based diagnostic sensors

Paper-based microfluidics is a rapidly progressing inter-disciplinary technology driven by the need for low-cost alternatives to conventional point-of-care diagnostic tools. For transport of reagents/analytes, such devices often consist of interconnected hydrophilic fluid-flow channels that are demarcated by hydrophobic barrier walls that extend through the thickness of the paper. Here, we present a laser-based fabrication procedure that uses laser-induced polymerisation of a photopolymer to produce the required fluidic channels in paper or other porous materials. Experimental results showed that the structures successfully guide the flow of fluids and also allow containment of fluids in wells, and hence the technique is suitable for fabrication of paper-based microfluidic devices.&amp; more...<br/

Domain engineering techniques and devices in lithium niobate

This thesis presents the results from investigations directed at novel approaches to domain engineering single-crystal congruent lithium niobate at the micron/sub-micron scale for practical device applications. Experimental etch-rate measurements from a parametric study of etch-rates and etch-quality of single-crystal lithium niobate z-faces, as a function of specific ratios for mixtures of HF/HNO3, to ascertain whether the widely-employed 1:2 mixture was in fact optimum for achieving the largest differential etch-rates between lithium niobate z-faces, revealed that pure HF produced an etch-rate that is a factor of two higher than that for the more frequently used 1:2 mixture. The observed etch-quality as compared to the 1:2 ratio was also improved for either pure HF or HF/HNO3 in a 1:4 ratio. A discussion of the etch-chemistry involved, and an explanation of the observed difference in etch-rates between the +z and -z faces has been proposed. The experimental results are also suggestive of a second differential etch-rate between virgin and newly poled z-faces. The observed variation in the differential etch-rate as a function of time-delay following poling, was suggestive of small atomic displacements following poling, and was quantified by the evidenced shifts in six major Raman spectral peaks. The noticeable modifications in the etch-behaviour of undoped congruent z-cut lithium niobate by pre-illumination with sub-picosecond UV-laser radiation of 248 nm wavelength at energy fluences below the ablation threshold, demonstrates the potential applicability of this technique for µm-scale surface structuring of lithium niobate. An innovative technique for surface domain-inversion, based on the conventional e-field poling, but involving an intentional over-poling step, was employed to fabricate 1D and 2D periodic structures with good domain uniformity. Domain periods as short as ~1µm have been achieved, and the technique shows full compatibility with standard waveguide fabrication techniques in lithium niobate. Quasi-phase matched harmonic generation at the fundamental wavelength of 1.064 µm, by means of the first-order (G10) reciprocal lattice vector, from a surface hexagonally poled planar annealed proton exchanged waveguide, with domain period of 6.7 µm, was demonstrated. First-order quasi-phase matched blue light generation with reasonable efficiencies at 413.17 nm, with domain periods of 2.47 µm from a surface poled Ti-indiffused channel waveguide was also demonstrated. Finally a novel route, sequentially employing techniques such as photolithographic patterning, e-field poling, direct-bonding and domain-sensitive differential wet etching for the fabrication of free-standing piezoelectric micro-cantilevers in single-crystal lithium niobate, with MEMS/MOEMS end-applications, was demonstrated

Latent ultrafast laser-assisted domain inversion in congruent lithium niobate

The combination of light with external electric fields has been successfully used for the domain engineering of ferroelectric lithium niobate crystals [1,2]. It has been shown that whereby the application of the electric field is delayed with respect to the illumination of the crystal. Furthermore, the local coercive field reduction becomes fixed after the first poling cycle. Hence, the initially illuminated and domain inverted regions will re-invert at lower voltages for subsequent poling cycles. The most significant implication of the latency is the decoupling of the laser illumination and E-field application steps which significantly simplifies the experimental setup and allows for high resolution light patterning, e.g. using a phase mask

UV laser induced surface microstructures in congruent lithium niobate single crystals

Ultra violet illumination of the -z face of lithium niobate single crystals, under specific conditions, results in an organized arrangement of submicron etch-resistant features that reflect the illuminating intensity distribution. Consequently, spatially resolved illumination can produce periodic structures with submicron periodicity. Furthermore, a size self-adjustment of the submicron etch resistant features was observed which is related to characteristic lengths (e.g. grating period) of the overall structure. The effect occurs for a narrow range of illuminating intensities and is attributed to a photo-induced electrostatic charge distribution which modifies the electrochemical interaction of the acid with the surface. The size and periodicity of the structures which can be achieved with this method are suitable for the fabrication of 2D photonic crystal structures in this electro-optically tunable material

Microstructuring lithium niobate: towards new hybrid devices

Lithium niobate is among the most important nonlinear optical materials used today in the photonics industry as it combines a variety of very important properties which, apart from the optical nonlinearity, includes electrooptic, pyroelectric, piezoelectric behaviour and an optical transparency which extends from the UV (350nm) to the infrared (5µm) spectral region. In order to benefit from both the optical and electro-mechanical properties of lithium niobate it is necessary to develop methods for the fabrication of suitable surface and/or bulk structures depending on the application involved. Such methods for surface and bulk microstructuring have been developed and are presented here aiming to show that there is significant scope for the broadening of the utility of this very useful material. &amp; more..

Laser-based printing and patterning for biological applications

1. IntroductionLaser direct-write methodologies are highly flexible, non-contact and serial pattern generation procedures that allow a user to create patterns either on the surface or in the volume of a material of choice through point-by-point scanning of the laser beam across the material work-piece. Pattern generation can be through either addition or subtraction of the material or through modifications to its physical properties, and the scale lengths typically range from nm-mm. Here we show the usefulness and versatility of such laser-based approaches for fabrication of paper-based sensors for medical diagnostics.2. Laser-based patterningPaper-based microfluidics has been a rapidly progressing inter-disciplinary technology driven by the need for low-cost alternatives to conventional point-of-care diagnostic tools since it was proposed by the Whitesides group in 2007 [1, 2]. For transport of reagents/analytes, such devices often consist of interconnected hydrophilic fluid-flow channels that are demarcated by hydrophobic barrier walls that extend through the thickness of the paper. Here, we present a laser-based fabrication procedure that uses laser-induced polymerisation of a photopolymer to produce the required fluidic channels in paper or other porous materials. Experimental results showed that the structures successfully guide the flow of fluids and also allow containment of fluids in wells, and hence the technique is suitable for fabrication of paper-based microfluidic devices.As shown in the schematic (Figure 1a), the process is conceptually simple. The minimum width for the hydrophobic barriers that successfully prevented fluid leakage was ~120 μm and the minimum width for the fluidic channels that can be formed was ~80 μm, the smallest reported so far for paper-based fluidic patterns. Some of the example devices are shown in Figure 1b-1e. The patterns can be produced rapidly using simple low power c.w. laser sources at a writing speed of order 1 m/s and we have also successfully demonstrated the use this technique for introduction of a range of additional functionalities such as controlled delay, three-dimensional flow and multiplexed flow of several different fluids.3. Laser-based printing With the end-goal of developing low-cost colorimetric point-of-care diagnostic sensors on paper, we also report our results on LIFT-printing of antibodies, both untagged and conjugated with the enzyme horseradish peroxidase (HRP). LIFT is an additive direct-write technique used commonly for depositing materials from a thin donor film onto a receiver substrate. The donor (a glycerol film containing the antibodies) is pre-deposited onto a carrier (a fused silica substrate) that is transparent to the incident laser light, and photons from the laser (KrF-excimer, 248nm, 1Hz, 10 ns, and maximum energy ~400mJ/pulse) provide the driving force that transfers a small volume of the donor onto the accepting receiver (paper). The viability of the untagged (target) antibodies post-transfer was validated by an indirect colorimetric Enzyme Linked Immunosorbent Assay. HRP-tagged antibody attaches specifically to the LIFT-printed target antibody and on addition of the corresponding chromogenic substrate, the printed pixels turn blue (Figure 2)

Local deposition assisted laser-based direct-write method for fabrication of paper-based microfluidic devices

Demand for low-cost alternatives to conventional medical diagnostic tools has been the driving force that has spurred significant developments in the diagnostics field. Paper-based fluidic devices, proposed by the Whitesides’ group in 2007 have been regarded as one such alternative, and consequently, this field has been progressing rapidly.In our previous works, we have demonstrated the usefulness and versatility of a laser direct-write (LDW) approach in the patterning of fluidic devices in porous materials such as cellulose for the fabrication of diagnostic devices. This lab-based non-lithographic approach with high flexibility has the potential to be up-scaled for mass-production of paper-based devices at affordable costs. A decrease in the total number of fabrication-steps would however not only make this LDW process more efficient as a consequence of reduced fabrication times, but would also make it more cost-effective because of the reduced usage of expensive reagent – translating it into a truly mature technique adoptable for commercial manufacture. To optimise our original technique, we propose the inclusion of a deposition tool that allows localised deposition of the photopolymer at only specific locations on the paper where the fluid containing wall/structures need to be formed within the substrates to create the microfluidic device. This selective photopolymer deposition eliminates the (global) soaking step required to impregnate the photopolymer within the paper, prior to the laser illumination step, and furthermore also makes redundant the subsequent solvent developing step inherent in our original technique.Overall, we believe this optimised LDW technique is suitable for roll-to-roll manufacture of paper-based microfluidic devices that can be used for a variety of applications

Surface domain inversion in ferroelectric lithium niobate

Periodic inversion is reported for ferroelectric domains near the surface of z-cut lithium niobate crystals by a modified electric field poling technique. The depth of the inverted domain region extends to values of order a few µm (5-10 µm) below the surface of the crystal, thereby removing the high aspect ratio instability problems associated with bulk poling, and therefore allowing the fabrication of fine period ferroelectric domain structures. Using this method periods as short as 1 µm have been achieved. Such periodic domain distributions have been fabricated in Ti-indiffused and proton exchanged lithium niobate waveguides and our first quasi-phase-matching results are shown