Multimaterial 3-D laser microprinting usingan integrated microfluidic system13 February 2019, by Thamarasee Jeewandara

Left: Scheme of the microfluidic chamber. (A) A high-numerical aperture (NA) oil-immersion microscopeobjective lens focuses femtosecond laser pulses into achamber, which is clad by two thin glass windows (lightblue). One of them serves as the substrate for thesamples. The selection valve shown allows for switchingbetween different photoresists (here, one nonfluorescentand four fluorescent) and solvents (acetone and mr-Dev600), which are injected into the microfluidic chamber.(B) Structure formulae of the components of one of thefluorescent photoresists containing Atto dye molecules.Right: Microfluidic sample holder for 3D laserlithography. (A) Scheme of the complete sample holder,which can be placed into a commercial 3D laserlithography machine and explosion drawing of themicrofluidic chamber, which hosts a small coverslip(diameter, 10 mm) inside the chamber, onto whichstructures can be 3D-printed. The chamber is sealedusing a solvent-resistant O-ring, and the top partfeatures a circular glass window for the high-NA oil-immersion objective to focus inside the chamber. (B)Cross-sectional scale drawing of the sample holder. Thesample holder features connectors for liquid tubing andchannels for the liquids to be guided in and out of themicrofluidic chamber. The liquid flow path is indicatedusing red arrows. Credit: Science Advances, doi:10.1126/sciadv.aau9160

Complex, three-dimensional (3-D) structures areregularly constructed using a reliable commercialmethod of 3-D laser micro- and nanoprinting. In arecent study, Frederik Mayer and co-workers inGermany and Australia have presented a new

system in which a microfluidic chamber could beintegrated on a laser 3-D lithography device toconstruct multimaterial structures using more thanone constituent material. The new method caneliminate the existing need to transfer betweenlithography techniques and chemistry labs for astreamlined manufacturing process.

As a proof-of-principle, the scientists created 3-Ddeterministic microstructured security featuredevices using seven materials. These included (1)a nonfluorescent photoresist (light sensitivematerial) to build the device backbone, (2) twophotoresists containing different fluorescent quantum dots, (3) two more photoresists withdifferent fluorescent dyes and (4) two developers.3-D optical security features are typicallymanufactured through multi-step laser lithographyand chemistry techniques.

Microstructures for such security features usuallycontain a nonfluorescent 3-D cross-grid scaffoldand built-in fluorescent markers realized withsemiconductor quantum dots arranged onto thescaffold at will to encode a message. The resultingmicrostructure/security features can be read usingoptical sectioning methods such as 3-D confocalfluorescence scanning microscopy. The newsystem proposed by Mayer et al. therefore opens adoor to engineer multimaterials in 3-D additivemanufacture at the micro- and nanoscale on acombined microfluidic-lithography setup.

3-D laser printing technology or 3-D laser micro-and nano-printing emerged more than 20 years agoand is now widespread. Current applications areubiquitous from 3-D photonic crystals to photonicwire bonds, 3-D printed free-form surfaces, micro-optics for 3-D optical circuitry and micromirrors.Applications also include optical microlens systemsbased on 3-D mechanical metamaterials, 3-Dsecurity features, to 3-D microscaffolds for cellculture and 3-D printed micromachines. In amajority of published microstructures, however,

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scientists only used one main material to create the3-D architecture, with notable exceptions in recentliterature.

Scheme of the system connected to the microfluidicchamber. (A) It consists of an electronic pressurecontroller connected to a nitrogen bottle, up to 10containers for the photoresists and solvents fordevelopment, and the star-shaped selection valve.Pumping individual liquids is possible by applying apneumatic pressure to all liquid containers and openingthe flow path for a single liquid using the selection valve.Following the selection valve, the liquid flow is guidedthrough an overpressure valve and homebuilt sampleholder. Last, it is directed into a waste container. (B)Cross section through the homebuilt selection valveassembly. The assembly consists of commercial solenoidvalves and a homebuilt 10-to-1 manifold that connectsthe 10 liquid containers to 10 solenoid valves, and thevalve outputs to one manifold output port. An exampleflow path for one liquid is indicated with red arrows.Credit: Science Advances, doi: 10.1126/sciadv.aau9160.

During design, it is important to streamline thechemical process and 3-D laser printing techniquein the same compact tabletop machine tool toachieve multimaterial printing. At present, microfluidic devices are also commercially wellsuited to engineer interconnected systems since

mature components of the technology are readilyavailable. Much like cable components in anelectronic system, the connectors, flow switches,valves, flow controllers and switch flow matricescan be bought off the shelf. When constructing thecombined setup (microfluidics and laserlithography), Mayer et al. addressed two mainquestions:

1. Can all process steps be performed in theregime of laminar flow?

2. Can an attractive system with the defineddesign constraints be realized on a singledevice?

To address these questions in the new system,Mayer et al. constructed the device capabilities as adeterministic, multi-structured 3-D fluorescentsecurity feature with multiple emission colors. Thescientists used seven different liquids in themicrofluidic setup as detailed onset.

They constructed the microfluidic chamber andplaced the structure inside a commercial 3-D laserlithography machine. The microfluidic chambercontained a small coverslip onto which structurescould be 3-D printed. Structural alterations madeduring the experiment to the 3-D laser lithographysystem did not limit the possibilities of the device.Mayer et al. printed structures with a tunableprinting resolution, alongside large samplefootprints depending on sample size.

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Successive 3D printing of different photoresists. Imagestaken using the camera integrated into the 3D laserlithography machine. Each image shows the uppermostlayer of the 3D microstructure, but after different printingsteps. For the first picture, the 3D support grid and bluefluorescent markers have been printed, whereas for thelast picture, markers using all four fluorescent resistshave been printed. For clarity, fluorescence emissioncolors are overlaid. Credit: Science Advances, doi:10.1126/sciadv.aau9160.

The scientists designed the apparatus toreproducibly open and close the microfluidicchamber. To prevent pressure-induced glassrupture in the setup, they measured the criticalpressure via independently controlled combustiontests. To reduce overpressure inside themicrofluidic chamber, the scientists connected theoutput of the microfluidic chamber to the wastecontainer using a tube. They never set the pressurecontroller to an overpressure exceeding 2 bar andinstalled a pressure relief valve between thedistributor valve and the input to the chamber. Inthis way, Mayer et al. installed precautions toensure the glass window remained intact undercontrolled flow of photoresist and liquids in themicrofluidic system, throughout the experiment.

Animation of the scan through different z-positions of thefluorescent 3D microstructure. Images for the movie weretaken using a confocal laser scanning microscopy withoutinterpolation. Credit: Science Advances, doi:10.1126/sciadv.aau9160

The entire setup contained the microfluidicchamber, an electronic pressure controllerconnected to a nitrogen bottle, several reservoirswith different photoresist and developer liquids. Thesystem also contained a homebuilt distributor valveand tubes connecting the different compartments.The scientists maintained computer-aided control ofthe switch valves and included a simple amplifiercircuit with a microcontroller board. When deployingthe microfluidic system in a 3-D lithographic setup,Mayer et al. reduced unnecessary consumption ofphotoresist as far as possible and augmented thedevice setup for optimal function, addressing bothquestions of the study design.

The scientists demonstrated the capabilities of thesystem by fabricating 3-D fluorescent securityfeatures, similar to an established protocol. In theworkflow, they injected nonfluorescent photoresistinto the microfluidic chamber to create a 3-Dsupport grid. Then they 3-D printed fluorescentparts of the structure by repeatedly injectingfluorescent photoresists. The blue and green-emitting photoresists contained quantum dots, andthe orange and red-emitting resists containedorganic Atto dyes. The scientists imaged the writtensecurity structure using a camera built in to the 3-Dlaser lithography system.

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Confocal laser scanning fluorescence microscopy offabricated structures. (A) On the left-hand side, acomputer rendering of the design for the microstructure isshown. It consists of a nonfluorescent 3D supportstructure (gray) with fluorescent markers with differentemission colors printed into it. On the right-hand side, astack of images taken by using fluorescence microscopyis shown. (B) The designs of the test patterns wereprinted into the five different marker layers of themicrostructure. (C) Measurement data from fabricatedmicrostructures taken using fluorescence microscopy.Insets show the level of detail at which differentphotoresist structure elements can be printed. Credit:Science Advances, doi: 10.1126/sciadv.aau9160.

When they visualized the 3-D fluorescent securityfeature as a computer design, it contained a 3-Dcross-grid surrounded by walls for support andfluorescent markers arranged around every gridpoint. The entire microstructure could store around7.8 kbit of information. To characterize the 3-Dprinted structures, Mayer et al. used confocal laserscanning microscopy (LSM) and imaged thedifferent fluorescent parts. The scientists examinedthe level of detail at which the fluorescent parts ofthe structure were printed by scanning throughdifferent levels of the fluorescent 3-Dmicrostructure. In the work, they showed thatresults between the designed test patterns andmeasured data were in good agreement.

In this way, Mayer et al. introduced a microfluidic

system that could perform photoresist injection andsample development steps within a commerciallyavailable laser lithography machine. The systemfacilitated the fabrication of multimaterial 3-D laserlithography structures. As a proof-of-principle, theyprinted complex 3-D security features using thecombined system in the study.

The scientists envision that combined microfluidic-laser lithography systems will become widely usedin the future to manufacture complex 3-D micro-and nanostructures with multiple materials. Suchmaterials and systems will have applications indiverse fields such as 3-D scaffolds for cell culture,3-D metamaterials, 3-D micro-optical systems and3-D security features as shown in the study.

More information: Frederik Mayer et al.Multimaterial 3-D laser microprinting using anintegrated microfluidic system, Science Advances(2019). DOI: 10.1126/sciadv.aau9160

Satoshi Kawata et al. Finer features for functionalmicrodevices, Nature (2002). DOI:10.1038/35089130 V

Melissinaki et al. Direct laser writing of 3-Dscaffolds for neural tissue engineering applications,Biofabrication (2011). DOI:10.1088/1758-5082/3/4/045005

Judith K. Hohmann et al. Influence of Direct LaserWritten 3-D Topographies on Proliferation andDifferentiation of Osteoblast-Like Cells: TowardsImproved Implant Surfaces, Advanced FunctionalMaterials (2014). DOI: 10.1002/adfm.201401390

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APA citation: Multimaterial 3-D laser microprinting using an integrated microfluidic system (2019,February 13) retrieved 3 April 2022 from https://phys.org/news/2019-02-multimaterial-d-laser-microprinting-microfluidic.html

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