Loïc Jacot-Descombes

Fabrication of epoxy spherical microstructures by controlled drop-on-demand inkjet printing

Well-controlled spherical microstructures open new possibilities for several MEMS devices, such as hemispherical microfluidic channels or micro-optical elements. However, machining of micro-spherical shapes has proven to be difficult with conventional planar micro-fabrication processes. This paper presents a fabrication method allowing the fabrication of controlled micro-spherical cap structures with defined edge angles. Drops of 30 pL of an epoxy solution were accurately inkjet printed on circular platforms. The deposited volume is confined by the rim of the platforms. This allows a fine tuning of the spherical cap edge angle as well as its height and radius of curvature. The presented method allowed fabricating large arrays of well-controlled micro-spherical shapes of different diameters, ranging from 50 to 930 μm, with a maximum controlled edge angle tuning of 85°. Theoretical investigations of the underlying phenomena are also presented. Good agreement between experimental results and theoretical expectations has been observed, with standard deviations below 3%. Using the proposed method, several 2D arrays up to 900 micro hemispheres with an edge angle of 90° ± 2° have been fabricated with a yield above 98%.

Microlenses with defined contour shapes

Ink-jet printing of optical ink over SU-8 pillars is here proposed as a technology for obtaining microlenses with shape control. To demonstrate the flexibility of this method, microlenses with five different contour shapes (ranging from circular and elliptical to toric or more advanced geometries) have been fabricated. Furthermore, the optical properties of the different fabricated lenses have been experimentally investigated. Focal distance, numerical aperture (NA) and full-width at half maximum (FWHM) of the microlenses have been determined. Arrays of microlenses showed an identical behavior with a standard deviation in the total intensity of only 7%. Additionally, the focal plane of the fabricated symmetric microlenses and the Sturm interval of the non-symmetric ones have been obtained. The experimental results demonstrate the validity and flexibility of the proposed technology.

Inkjet Printing of High Aspect Ratio Superparamagnetic SU-8 Microstructures with Preferential Magnetic Directions

Structuring SU-8 based superparamagnetic polymer composite (SPMPC) containing Fe3O4 nanoparticles by photolithography is limited in thickness due to light absorption by the nanoparticles. Hence, obtaining thicker structures requires alternative processing techniques. This paper presents a method based on inkjet printing and thermal curing for the fabrication of much thicker hemispherical microstructures of SPMPC. The microstructures are fabricated by inkjet printing the nanoparticle-doped SU-8 onto flat substrates functionalized to reduce the surface energy and thus the wetting. The thickness and the aspect ratio of the printed structures are further increased by printing the composite onto substrates with confinement pedestals. Fully crosslinked microstructures with a thickness up to 88.8 μm and edge angle of 112° ± 4° are obtained. Manipulation of the microstructures by an external field is enabled by creating lines of densely aggregated nanoparticles inside the composite. To this end, the printed microstructures are placed within an external magnetic field directly before crosslinking inducing the aggregation of dense Fe3O4 nanoparticle lines with in-plane and out-of-plane directions.

Acousto-fluidic system assisting in-liquid self-assembly of microcomponents

In this paper, we present the theoretical background, design, fabrication and characterization of a micromachined chamber assisting the fluidic self-assembly of micro-electro-mechanical systems in a bulk liquid. Exploiting bubble-induced acoustic microstreaming, several structurally-robust driving modes are excited inside the chamber. The modes promote the controlled aggregation and disaggregation of microcomponents relying on strong and reproducible fluid mixing effects achieved even at low Reynolds numbers. The functionality of the microfluidic chamber is demonstrated through the fast and repeatable geometrical pairing and subsequent unpairing of polymeric microcylinders. Relying only on drag and radiation forces and on the natural hydrophobicity of SU-8 in aqueous solutions, assembly yields of approximately 50% are achieved in no longer than ten seconds of agitation. The system can stochastically control the assembly process and significantly reduce the time-to-assembly of building blocks.

Fluid-mediated parallel self-assembly of polymeric micro-capsules for liquid encapsulation and release

Fluid-mediated self-assembly is one of the most promising routes for assembling and packaging smart microsystems in a scalable and cost-efficient way. In this work the pairwise fluidic self-assembly of 100 μm-sized SU-8 cylinders is studied with respect to two driving mechanisms: capillary forces at the liquid–air interface and the hydrophobic effect while fully immersed in liquid. The pairwise self-assembly is controlled by shape recognition and selective surface functionalization. Surface energy contrast is introduced through oxygen plasma treatment and local deposition of a hydrophobic self-assembled monolayer, respectively leading to face-selective hydrophilic and hydrophobic behavior. When in bulk liquid, after less than a day face-wise self-assembly of more than 650 components is achieved with a yield of up to 97% and with less than 1% of the cylinders assembled incorrectly. This technique is subsequently adopted for self-assembling half-capsules into closed micro-capsules, thereby entrapping a liquid during their self-assembly. The release of the liquid can subsequently be triggered in another medium, as intended for applications involving e.g. chemical reactors, environmental engineering and drug release.

Organic-inorganic-hybrid-polymer microlens arrays with tailored optical characteristics and multi-focal properties

Plano-convex microlens arrays of organic-inorganic polymers with tailored optical properties are presented. The fine-tuning of each microlens within an array is achieved by confining inkjet printed drops of the polymeric ink onto pre-patterned substrates. The lens optical properties are thus freely specified, and high numerical apertures from 0.45 to 0.9 and focal lengths between 10 μm and 100 μm are demonstrated, confirming theoretical predictions. Combining nanoimprint lithography approaches and inkjet printing enables using the same material for the microlenses and their substrates, improving the optical performances. Microlens arrays with desired specifications are printed reaching yields up to 100% and high lens reproducibility with standard deviations of the apparent contact angle under 1° and of the numerical apertures and focal lengths under 6%. Microlens arrays involving lenses with different characteristics, e.g. multi focal length, and thus focal planes separated by only few microns are printed with the same reproducibility.

High-aspect-ratio nanoimprint process chains

Different methods capable of developing complex structures and building elements with high-aspect-ratio nanostructures combined with microstructures, which are of interest in nanophotonics, are presented. As originals for subsequent replication steps, two families of masters were developed: (i) 3.2 μm deep, 180 nm wide trenches were fabricated by silicon cryo-etching and (ii) 9.8 μm high, 350 nm wide ridges were fabricated using 2-photon polymerization direct laser writing. Both emerging technologies enable the vertical smooth sidewalls needed for a successful imprint into thin layers of polymers with aspect ratios exceeding 15. Nanoridges with high aspect ratios of up to 28 and no residual layer were produced in Ormocers using the micromoulding into capillaries (MIMIC) process with subsequent ultraviolet-curing. This work presents and balances the different fabrication routes and the subsequent generation of working tools from masters with inverted tones and the combination of hard and soft materials. This provides these techniques with a proof of concept for their compatibility with high volume manufacturing of complex micro- and nanostructures.

Fabrication of HepG2 Cell Laden Collagen Microspheres using Inkjet Printing

In this study, drop-on-demand system using piezo-elecrtric inkjet printers was employed for preparation of collagen microspheres, and its application was made to the HepG2 cell-laden microsphere preparation. The collagen microspheres were injected into beaker filled with mineral oil and incubated in a water bath at 37℃ for 45 minutes to induce gelation of the collagen microsphere. The size of collagen microsphere was 100μm in diameter and 80μm in height showing spherical shape. HepG2 cells were encapsulated in the collagen microsphere. The cell-laden microspheres were inspected by the microscopic images. The encapsulation of cells may be beneficial for applications ranging from tissue engineering to cell-based diagnostic assays.

Printing non-circular microlenses

Microlenses are used in a myriad of applications, ranging from cellphones and cameras to micro-opto-electro-mechanical systems (MOEMS)1 and high-resolution imaging platforms for biological applications.2 Normally, circular microlenses result in circular light spots. However, certain advanced micro-optical systems demand non-circular lenses, for instance, to correct optical defects3 or enhance new applications such as lab-on-achip systems.4 Despite progress in microfabrication techniques, preparing such structures at the microscale remains a challenge. Direct printing has emerged as a promising option for scalable fabrication of non-circular microlenses, and ink-jet printing (IJP) allows us to dispense small drops of a lens material in liquid form onto a surface. A subsequent curing step yields solid transparent microlenses with a curvature radius that depends primarily on the contact angle between the substrate and the dispensed droplet. This technique can merge adjacent drops into microlenses with non-circular shapes,5 but it has two drawbacks. First, it relies on precise control of the surface wettability, which may vary over time. Second, the landing position of the dispensed droplets must be precisely controlled for accurate alignment. To overcome these limitations, we have combined IJP with photolithography methods to better tune and control the shape of the deposited lens material.6 To this end, we have fabricated ‘platforms’—prestructured substrates that define the contour shape and edge angle (Ea) of the deposited material,7 as shown in Figure 1—with the desired contours on the substrate. We subsequently printed liquid lens materials with micrometer precision. We tested this process using two different substrates. We first used a silicon (Si) substrate patterned by photolithography and reactive ion etching to make Si platforms. We deposited a Figure 1. Schematic (not to scale) of the ink-jet printing (IJP) process over a prepatterned substrate containing ‘platforms’ formed by photolithography. The deposited drops are confined on the platforms, fixing the contour shape and the edge angle (Ea) of the final microlenses.

Polymeric hemispherical pico-liter micro cups fabricated by inkjet printing

The fabrication of precise hemispherical shape is challenging with standard planar lithography techniques. A suitable alternative is the fabrication by inkjet printing. This paper presents a method based on drop-on-demand inkjet printing on pre-patterned silicon substrates allowing the controlled fabrication of SU-8 hemispherical cup-like structures with inner cavities of sub-nano-liter volumes. Examples are given for cups of 100μm in diameter with inner cavity volumes of 5pL, 20pL and 45pL. Arrays of 360 hemispherical SU-8 cups have been fabricated with a yield above 96%. The 4% of exceptions are also described and shown as a method for achieving almost complete SU-8 spheres.