SU-8 Micropillars Research Articles

Heterogeneous micro-architectonic integration of SU-8 and highly entangled polyacrylamide hydrogel to realize cut-resistant soft superhydrophobic surfaces

This paper presents a novel idea to create cut-resistant superhydrophobic (SHPo) surfaces by integrating an array of SU-8 micropillars on a highly entangled polyacrylamide (PAAm) hydrogel substrate. We begin by demonstrating that this highly entangled PAAm hydrogel exhibits superior resistance to cutting while being as transparent, flexible, and stretchable as other polymeric substrates like polydimethylsiloxane (PDMS). Currently, there are no well-known methods or chemicals to directly integrate SU-8 and PAAm with a covalent bond. To overcome this challenge, we introduce a thin layer of chemically modified PDMS between the SU-8 and PAAm so that covalent bonds can be formed between both the SU-8/PDMS interface and the PDMS/PAAm interface. After validating the reliability of the bonding in our experiments, we develop a heterogeneous integration process to fabricate the desired SHPo surface. To demonstrate the critical role of PAAm hydrogel in achieving the cut-resistant SHPo surface, we contrast this new SHPo surface with a reference version that uses a PDMS substrate instead. We conduct microscopic inspections using scanning electron microscopy (SEM) and a contact angle goniometer before and after cutting the two surfaces. These evaluations show significant differences in their structural integrity and behavior in water interaction.

Super-wetting enabled by an array of SU-8 micro-pillars etched with ion-beam

The wettability transition of an ion-beam-etched micro-pillar array surface is reported in the present study. The micro-pillar array, made of negative photoresist SU-8, was fabricated by ultraviolet lithography. The results indicate that the combination of the micro-pillar array and ion-beam etching technique can easily regulate surface wettability, which can be changed from hydrophobicity to super-hydrophilicity. Moreover, the super-hydrophilic property of the textured surface can be significantly affected by the etching time and etching energy, because the ion-beam etching could alter the shape and height of the micro-structure as well as the chemical composition of the surface. When a 2 µl water droplet spreads on the etched micro-pillar array surface, the contact angle (CA) decreases to almost 0° within 144 ms, indicating ultrafast spreading of the water droplet on the textured surface. It was found that the CA of the fabricated surface remained the same after two weeks’ exposure in the air, showing undependable wettability stability of the textured surface. The results of the present study can open a gate for potential applications of proposed super-wetting surfaces for micro-fluidics and anti-counterfeiting technologies.

Sensing film-coated QCM coupled resonator sensors: Approach, fabrication, and demonstration

We report the first sensing film-coated quartz crystal microbalance (QCM) as a coupled resonating sensor (CRS). QCM-CRSs are known for high sensitivity characteristics and, to date, they have been constructed by attaching high aspect ratio resonant micropillar structures on electrode surfaces. These pillars are composed of photoresist-based materials and do not accommodate exclusive sensing material in the form of films or other structures. This property restricts the sensing applications to pillar material properties. Here, we overcame this limitation by utilizing both sides of the QCM, i.e., one side is coated with a sensing film and the other side of the QCM is attached to resonating SU-8 micropillars. To show a potential application of the proposed sensor design, a zinc oxide (ZnO) thin film was chosen as an example sensing medium tosense the relative humidity (RH), and the enhanced mass sensitivity of a sensing film-coated QCM was demonstrated. Optimized micro-fabrication steps were used to construct the QCM-CRS, and the fabrication was finalized when the ZnO-coated QCM-CRS exhibited typical characteristics of a coupled resonator system Experiments were conducted to measure the resonance frequency shift (Δf0) for different values of RH. The QCM-CRS showed extraordinary sensitivity for changes in RH. It was observed that the Δf0 was 10 times higher than that of an identical QCM-based sensor without any coupled resonance features. The proposed QCM-CRS also showed the best time response and repeatability and had a tolerance in Δf0 of ±0.4%, indicating it had a high reliability and stability. The proposed technique can be applied to designing a variety of sensors by replacing the ZnO film with suitable sensing films.

Colon cancer cells adhesion on polymeric nanostructured surfaces

Abstract

Homogeneous transfer of graphene oxide into photoresist: Fabrication of high surface area three-dimensional micro-arrays by modified photolithography

Herein, we studied the homogeneous transfer of graphene oxide (GO) into photocurable SU-8 photoresist for fabricating GO/SU-8 three-dimensional (3D) composite micropillar arrays by modified photolithography. SU-8 is a negative toned epoxy based photoresist, which includes 8 epoxy groups in each SU-8 monomer. The effects of the concentration of GO flakes on the thermal, mechanical and specific surface area (SSA) properties of the resultant composites were investigated. The large amount of oxygen functional groups on the GO plane and at the edges allowed uniform distribution of GO sheets within the SU-8 resin. The GO/SU-8 micropillar arrays were fabricated by a versatile ultraviolet (UV) photolithography technique that allowed the synthesis of various 3D micro–nano integrated carbon microelectrode arrays. The integrated GO flakes were bonded to the surface or embedded within the primary structure of SU-8 micro-pillars. These SU-8 micropillars having surface-bonded or embedded GO flakes showed superior thermal stability, strength, and SSA characteristics than bare SU-8 micropillars. Thus, this new synthesis approach provides a novel route for developing high performance catalysts, sensors, and adsorbents, among other materials.

Deep proton writing of high aspect ratio SU-8 micro-pillars on glass

Deep proton writing (DPW) is a fabrication technology developed for the rapid prototyping of polymer micro-structures. We use SU-8, a negative resist, spincoated in a layer up to 720μm-thick in a single step on borosilicate glass, for irradiation with a collimated 12MeV energy proton beam. Micro-pillars with a slightly conical profile are irradiated in the SU-8 layer. We determine the optimal proton fluence to be 1.02×104μm−2, with which we are able to repeatably achieve micro-pillars with a top-diameter of 138±1μm and a bottom-diameter of 151±3μm. The smallest fabricated pillars have a top-diameter of 57±5μm. We achieved a root-mean-square sidewall surface roughness between 19nm and 35nm for the fabricated micro-pillars, measured over an area of 5×63.7μm. We briefly discuss initial testing of two potential applications of the fabricated micro-pillars. Using ∼100μm-diameter pillars as waveguides for gigascale integration optical interconnect applications, has shown a 4.7dB improvement in optical multimode fiber-to-fiber coupling as compared to the case where an air–gap is present between the fibers at the telecom wavelength of 1550nm. The ∼140μm-diameter pillars were used for mold fabrication with silicone casting. The resulting mold can be used for hydrogel casting, to obtain hydrogel replicas mimicking human tissue for in vitro bio-chemical applications.

Versatile multicharacterization platform involving tailored superhydrophobic SU-8 micropillars for the investigation of breast cancer estrogen receptor isoforms

Here, the authors report the fabrication of lotus-leaf-like tailored SU8 micropillars and their application in the context of a multitechnique characterization protocol for the investigation of the structural properties of the two estrogen receptors (ERα66/ERα46). ER (α) expression is undoubtedly the most important biomarker in breast cancer, as it provides the index for sensitivity to endocrine treatment. Beside the well-characterized ERα66 isoform, a shorter one (ERα46) is also expressed in ERα positive breast cancers and breast cancer cell lines. The superhydrophobic supports were developed by using a two-step approach including an optical lithography process and a plasma reactive ion roughening one. Upon drying on the micropillars, the biological samples resulted in stretched fibers of different diameters which were then characterized by synchrotron x-ray diffraction (XRD), Raman and Fourier-transform infrared spectroscopy. The evidence of both different spectroscopic vibrational responses and XRD signatures in the two estrogen receptors suggests the presence of conformational changes between the two biomarkers. The SU8 micropillar platform therefore represents a valid tool to enhance the discrimination sensitivity of structural features of this class of biomarkers by exploiting a multitechnique in situ characterization approach.

Characterization of the mechanical behavior of SU-8 at microscale by viscoelastic analysis

The mechanical properties of SU-8 at microscale were measured under both micropillar compression and nanoindentation on a film on a substrate. To the best of our knowledge, this paper reports the first set of results for microcompression of SU-8 micropillars for measurement of mechanical properties using viscoelastic analysis. The effects of loading rate and micropillar size are examined. It was determined that the SU-8 exhibits viscoelastic properties at room temperature, the time-average Young’s modulus increases in general with the loading rate. The average Young’s modulus determined by compression of a micropillar was 4.1 GPa at a strain rate near 10−3 s−1. For nanoindentation on a SU-8 film supported by a silicon substrate, the default output from the nanoindenter for the Young’s modulus was approximately 6.0 GPa with the consideration of elastic–plastic behavior of the SU-8. When the viscoelastic effects were considered, the time-average Young’s modulus at a given strain rate was determined to be near 3.6 GPa, which agrees with the reported values in the literature obtained from tension and bending, and also correlates reasonably well with data from microcompression. This work indicates that viscoelastic analysis is necessary to extract the valid mechanical properties at nano/microscales for SU-8.

Study of SU-8 reliability in wet thermal ambient for application to polymer micro-optics on VCSELs

We present experimental data on the reliability of SU-8 polymer when used as a core material for the integration of microlenses on vertical-cavity surface-emitting lasers (VCSELs). The respective effects of a hot and humid environment on structural, mechanical and optical properties of this epoxy resist are investigated. High aspect-ratio SU-8 micropillars are found to keep a good surface morphology and a stable optical transmission, as well as a good adherence on the wafer. Thermal cycling is also studied to check material stability under electro-thermal actuation in SU-8 micro-opto-electro-mechanical system (MOEMS). These results are of great importance for the collective integration of low-cost SU-8-based passive or active microlens arrays onto VCSELs wafers for optical interconnects and optical sensing applications.

Spatially Controlled Surface Energy Traps on Superhydrophobic Surfaces

Water wetting and adhesion control on polymeric patterns are achieved by tuning the configuration of their surface's structural characteristics from single to dual and triple length-scale. In particular, surfaces with combined micro-, submicrometer-,and nanoroughness are developed, using photolithographically structured SU-8 micro-pillars as substrates for the consecutive spray deposition of polytetrafluoroethylene (PTFE) submicrometer particles and hydrophobically capped iron oxide colloidal nanoparticles. The PTFE particles alone or in combination with the nanoparticles render the SU-8 micropillars superhydrophobic. The water adhesion behaviour of the sprayed pillars is more complex since they can be tuned gradually from totally adhesive to completely non adhesive. The influence of the hierarchical geometrical features of the functionalized surfaces on this behaviour is discussed within the frame of the theory. Specially designed surfaces using the described technique are presented for selective drop deposition and evaporation. This simple method for liquid adhesion control on superhydrophobic surfaces can find various applications in the field of microfluidics, sensors, biotechnology, antifouling materials, etc.

Flexible PDMS microtubes for examining local hydrophobicity

In this work, utilizing commercial glass micropipettes as molds, polydimethylsiloxane (PDMS) microtubes have been successfully fabricated. Such a microtube can be as long as 3 cm, its sidewall has a thickness in the range of 4---10 μm, and the outer and inner diameters of its tip can be as small as 26 and 17 μm, respectively. Compared with the rigid glass micropipette, the PDMS microtube is soft, making it flexible. The flexibility enables the PDMS microtube: (1) to be bendable for easily delivering liquid drops to confined locations, and (2) to have less damage to the contacted substrate. Its small tip makes it have access to narrow gaps, such as microgrooves. Three types of tests have been done to explore water-delivery properties of PDMS microtubes. Based on the understanding gained from these tests, a PDMS microtube is directly applied to examine local wetting properties of lotus and SU-8 micropillars.

Control of the water adhesion on hydrophobic micropillars by spray coating technique

We present an alternative approach for controlling the water adhesion on solid superhydrophobic surfaces by varying their coverage with a spray coating technique. In particular, micro-, submicro-, and nanorough surfaces were developed starting from photolithographically tailored SU-8 micropillars that were used as substrates for spraying first poly(tetrafluoroethylene) submicrometer particles and subsequently iron oxide nanoparticles. The sprayed particles serve to induce surface submicrometer and nanoscale roughness, rendering the SU-8 patterns superhydrophobic (apparent contact angle values of more than 150°), and also to tune the water adhesion between extreme states, turning the surfaces from “non-sticky” to “sticky” while preserving their superhydrophobicity. The influence of the chemical properties and of the geometrical characteristics of the functionalized surfaces on the wetting properties is discussed within the frame of the theory. This simple method can find various applications in the fabrication of microfluidic devices, smart surfaces, and biotechnological and antifouling materials.

Resonant Frequency Characteristics of a SAW Device Attached to Resonating Micropillars

Recently we reported experimental and simulation results on an increase in resonance frequency of a SAW resonator caused by mass loading of micropillars made of SU-8, attached normal to the surface of the resonator. We concluded that SAW resonator and the SU-8 micropillars in unison form a system of coupled resonators. We have now extended this work and performed a finite element method simulation to study the resonance frequency characteristics of the SAW-based coupled resonator. In this paper we report the effect of the resonance frequency of the micropillars on the resonance frequency of the system of coupled resonators, and observe the coupling of micropillar resonance and the propagating SAW as described in the well known Dybwad system of coupled resonators.

Fabrication of High Aspect Ratio SU-8 Microstructures on Piezoelectric Transducers

A methodology to fabricate high aspect ratio (HAR) SU-8 micro structures on piezoelectric and metallic substrates is presented. In this work, several fabrication trials were carried out to optimize the SU-8 fabrication process. The fabrication recipe mentioned in the SU-8 (Microchem, USA) datasheet is employed for the initial fabrication trials. The SU-8 structures (micropillars) fabricated during these trials resulted in poor bonding with the surface of piezoelectric substrate. In the later trials a thin film of OmniCoat (Microchem, USA) is coated over the substrate before coating the SU-8 film to improve the adhesion quality of the SU-8 micropillars to the substrate. The fabrication methodology used during the trails and results on the quality of the fabricated HAR SU-8 pillars are discussed in the paper.

Mass Loading in Coupled Resonators Consisting of SU-8 Micropillars Fabricated Over SAW Devices

Surface acoustic wave (SAW) sensors extensively employ mass loading effect of a sensing film coated over the surface of a SAW device. We report experimental and simulation results of mass loading by SU-8 micropillars (in place of sensing film) attached normal to the surface of SAW resonators. For certain size of micropillars, we have observed an increase in the resonance frequency of the resonator, in contrast to the decrease in the resonance frequency normally caused by mass loading. The SAW resonator and the pillars constitute a system of coupled resonators, and for high values of stiffness at the interface of the pillar and substrate the shift in the resonance frequency is positive.

Study on delamination mechanism of SU-8 micropillars on a Si-substrate under bend loading by Weibull analysis

Interfacial delamination mechanism between SU-8 micropillars with the widely change of aspect ratio and Si substrates was studied using Weibull distribution. Micropillars were fabricated by lithography on the substrate and delaminated from the substrate under bend loading. Delamination for aspect ratio of 1.4 and 2.2 with almost the same σmax indicates mode I fracture at interface and then shear stress increased as aspect ratio decreases. m value tends to become larger with distance from the complex stress area of σmax.