Laser-based printing and patterning for biological applications

1. Introduction
Laser 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 patterning
Paper-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).

Sones, Collin

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He, Peijun

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Katis, Ioannis

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Eason, Robert

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5 June 2017