SU-8 Cantilever Research Articles

Enhancing cardiomyocytes contraction force measuring in drug testing: Integration of a highly sensitive single-crystal silicon strain sensor into SU-8 cantilevers

The development of efficient tools for predicting drug-induced cardiotoxicity in the preclinical phase would greatly benefit the drug development process. This study presents an SU-8 cantilever integrated with a single-crystal silicon strain sensor to enhance force sensitivity in toxicity screening methods based on changes in the contraction force of cardiomyocytes. The proposed cantilever device enables real-time measurements of cardiomyocytes contraction force with high sensitivity, thereby facilitating the assessment of drug cardiotoxicity. The experimental results obtained herein demonstrate the responsiveness of the proposed platform in detecting forces smaller than 0.02 μN with a force sensitivity that is nearly 17 times higher than those of conventional metal-based strain sensors. Moreover, the integration of strain sensors demonstrates the potential for manufacturing cantilever arrays that can be used in high-throughput screening applications. The developed methodology successfully facilitates in vitro culturing of cardiomyocytes and allows for continuous monitoring of their contraction force. The practical applicability of the proposed platform is further validated through cardiotoxicity analysis. The cultured cardiomyocytes are treated with two cardiovascular drugs, namely verapamil (an L-type calcium channel blocker) and isoproterenol (a sympathomimetic drug targeting β1 and β2 adrenergic receptors), to analyze the drug induced effects in the cardiomyocytes.

MEA-integrated cantilever platform for comparison of real-time change in electrophysiology and contractility of cardiomyocytes to drugs

Drug-induced cardiotoxicity is a potentially severe side effect that can alter the contractility and electrophysiology of the cardiomyocytes. Cardiotoxicity is generally assessed through animal models using conventional drug screening platforms. Despite significant developments in drug screening platforms, the difficulty in measuring electrophysiology and contractile profile together affects the investigation of cardiotoxicity in potential drugs. Some drugs can prove to be more toxic to contractility than electrophysiology, which demands the need for a reliable, dual, and simultaneous drug screening platform. Herein, we propose the microelectrode array integrated SU-8 cantilever for dual and simultaneous measurement of electrophysiology and contractility of cardiomyocytes. The SU-8 cantilever is integrated with microelectrode array (C-MEA) using conventional photolithographic techniques. Drug tests are conducted to verify the feasibility of the C-MEA platform using three cardiovascular drugs. Clinically recognized drugs, quinidine and verapamil, are used to activate both the hERG channel and the contractile characteristics of cardiomyocytes. The effect of ion channel blockers on the field potential duration (FPD) of the cardiomyocytes is compared with several contractility-based parameters. The contraction-relaxation duration (CRD) profile is relatively close to that of FPD in tested drugs (half-maximal (IC50) toxicities are 1.093 μM (FPD) and 1.924 μM (CRD) for quinidine and 166.2 nM (FPD) and 459.4 nM (CRD) for verapamil). Blebbistatin, a known myosin II inhibitor, primarily affects the contractile profile of cardiomyocytes but not their field potential, with no evident correlation between contractility and field potential profiles. The proposed cantilever-based mechano-electrophysiology measurements platform provides a promising and accurate means to assess cardiotoxicity.

SU-8 cantilever with integrated pyrolyzed glass-like carbon piezoresistor

Glass-like carbon (GC) is a nongraphitizing material composed entirely of carbon atoms produced from selected organic polymer resins by controlled pyrolysis in an inert atmosphere. The GC properties are a combination of the properties of glass, ceramic, and graphite, including hardness, low density, low thermal conductivity, high chemical inertness, biocompatibility, high electrical conductivity, and microfabrication process compatibility. Despite these unique properties, the application of GC in mechanical sensors has not been explored thus far. Here, we investigate the electrical, structural, and chemical properties of GC thin films derived from epoxy-based negative photoresist SU-8 pyrolyzed from 700 to 900 °C. In addition, we fabricated microGC piezoresistors pyrolyzed at 700 and 900 °C and integrated them into nonpyrolyzed SU-8 cantilevers to create microelectromechanical systems (MEMS) mechanical sensors. The sensitivities of the GC sensor to strain, force, surface stress, and acceleration are characterized to demonstrate their potential and limits for electromechanical microdevices.

Enhancement of cardiac contractility using gold-coated SU-8 cantilevers and their application to drug-induced cardiac toxicity tests.

Herein, we propose an array of gold (Au)-coated SU-8 cantilevers with microgrooves for improved maturation of cardiomyocytes and describe its applications to drug-induced cardiac toxicity tests. Firstly, we evaluated the effect of cell culture substrates such as polydimethylsiloxane (PDMS), polyimide (PI), and SU-8 on the cardiomyocyte's maturation. Among these, the SU-8 with microgroove structures exhibits improved cardiomyocyte maturation. Further, thin layers of graphene and Au are coated on SU-8 substrates and the effects of these materials on cardiomyocyte maturation are evaluated by analyzing the expression of proteins such as alpha-actinin, Connexin 43 (Cx43), and Vinculin. While both conductive materials enhanced protein expression when compared to bare SU-8, the Au-coated SU-8 substrates demonstrated superior cardiomyocyte maturation. The cantilever structure is constructed using microgroove patterned SU-8 with and without an Au coating. The Au-coated SU-8 cantilever showed maximum displacement of 17.6 ± 0.3 μm on day 21 compared to bare SU-8 (14.2 ± 0.4 μm) owing to improved cardiomyocytes maturation. Verapamil and quinidine are used to characterize drug-induced changes in the contraction characteristics of cardiomyocytes on bare and Au-coated SU-8 cantilevers. The relative contraction forces and beat rates changed according to the calcium and sodium channel related drugs. Matured cardiomyocytes are less influenced by the drugs compared to immature cardiomyocytes and showed reliable IC50 values. These results indicate that the proposed Au-coated SU-8 cantilever array could help improve the accuracy of toxicity screening results by allowing for the use of cardiomyocytes that have been matured on the drug screening platform.

Hybrid Pattern Recognition for Rapid Explosive Sensing With Comprehensive Analysis

This paper presents a hybrid pattern recognition with temperature compensation (HPR-TC) used within an E-Nose system. HPR-TC with E-nose has the novelty, amongst MEMS sensor platforms, of having two modes of operation i.e., rapid mode of detection to be used in time-critical conditions and comprehensive analysis mode for improved detection accuracy. Two modes of operations in HPR-TC are possible because of the implementation of hybrid PR featuring a combination of two different data analysis techniques for explosive sensing. The first part of the hybrid PR is the binary PR based on threshold-based detection and the second one is the analog PR based on PCA and K-mean. The E-Nose system with proposed HPR-TC is validated with two different highly sensitive MEMS sensor types, i.e., SU8 and Si3Nx piezo-resistive cantilever. These MEMS sensors are coated with surface receptors, 4-MBA, 6-MNA and 4-ATP, to improve the selectivity. The E-Nose system can detect explosive compounds such as TNT, RDX, and PETN, in a controlled environment at a concentration as low as 16ppb of TNT, 56ppb of RDX and 134ppb of PETN. Furthermore, measurements show that E-Nose with temperature compensated binary PR can detect the explosives with a detection accuracy higher than 74% as true positives and higher than 79% as true negatives in a short time, within initial 17 seconds of the experiment. However, the temperature compensated analog PR gives a detailed classification of explosives with a higher detection accuracy of 80% as true positives and 86% as true negatives after approximately 95 seconds.

Integration of sharp silicon nitride tips into high-speed SU8 cantilevers in a batch fabrication process.

Employing polymer cantilevers has shown to outperform using their silicon or silicon nitride analogues concerning the imaging speed of atomic force microscopy (AFM) in tapping mode (intermittent contact mode with amplitude modulation) by up to one order of magnitude. However, tips of the cantilever made out of a polymer material do not meet the requirements for tip sharpness and durability. Combining the high imaging bandwidth of polymer cantilevers with making sharp and wear-resistant tips is essential for a future adoption of polymer cantilevers in routine AFM use. In this work, we have developed a batch fabrication process to integrate silicon nitride tips with an average tip radius of 9 ± 2 nm into high-speed SU8 cantilevers. Key aspects of the process are the mechanical anchoring of a moulded silicon nitride tip and a two-step release process. The fabrication recipe can be adjusted to any photo-processable polymer cantilever.

Micro-patterned SU-8 cantilever integrated with metal electrode for enhanced electromechanical stimulation of cardiac cells

Over the past few years, cardiac tissue engineering has undergone tremendous progress. Various in vitro methods have been developed to improve the accuracy in the result of drug-induced cardiac toxicity screening. Herein, we propose a novel SU-8 cantilever integrated with an electromechanical-stimulator to enhance the maturation of cultured cardiac cells. The simultaneous electromechanical stimulation significantly enhances the contraction force of the cardiomyocytes, thereby increasing cantilever displacement. Fluorescence microscopy analysis was performed to confirm the improved maturation of the cardiomyocytes. After the initial experiments, the contractile behaviors of the cultured cardiomyocytes were investigated by measuring the mechanical deformation of the SU-8 cantilever. Finally, the proposed electromechanical-stimulator-integrated SU-8 cantilever was used to evaluate the adverse effects of different cardiac vascular drugs, i.e., verapamil, lidocaine, and isoproterenol, on the cultured cardiomyocytes. The physiology of the cardiac-drug-treated cardiomyocytes was examined with and without electrical stimulation of the cardiomyocytes. The experimental results indicate that the proposed cantilever platform can be used as a predictive assay system for preliminary cardiac drug toxicity screening applications.

Alleviation of the adhesive protrusion problem at the bonding interface of free-standing microstructures

The vertically allocated free-standing SU-8 microstructures are typically bonded to a glass cover by the usage of uncrosslinked SU-8 adhesives. Such a phenomenon can easily cause SU-8 protrusion and eventually result in the SU-8 cantilever to be immovable. Traditional methods are sensitive to the bonding conditions and have a short bondable thickness of SU-8 adhesives. In this study, we propose an approach, that is, improved structural features, to alleviate the protrusion problem while extending the bondable thickness for the freestanding SU-8 microstructures in an enclosed channel. We used concave and moat microstructures as solutions of the improved structural features. We investigated the influence of both microstructures on the bonding quality and compared the bondable thickness with the previous one. THB-151N was used in another example to demonstrate the availability of our method. The bonding quality at the interfaces was evaluated by SEM images and direct inspection through a transparent glass cover. The bonding method is advantageous to other microfluidic systems, particularly those with long narrow channels.

Fully automated high-throughput cardiac toxicity screening platform using interlocking-structured 192 SU-8 cantilever arrays

This paper proposes a high-throughput drug-screening platform integrated with 48 well plates containing 192 SU-8 cantilevers that assess cardiac toxicity levels by measuring changes in the contractile force of cardiomyocytes in vitro environment. To improve the reliability and reproducibility of the measured data, four interlocking-structured cantilevers were employed in each well and the displacement changes were measured at the nanometer scale using a laser-based sensor. The use of the mean value of the cantilever displacement in each well greatly improved the accuracy of the drug toxicity results. In addition, microgrooves patterned onto the SU-8 cantilevers greatly enhanced the alignment of cardiomyocytes, resulting in an increase in the contraction force by approximately 2.4 times. After the preliminary experiment, the contractile behaviors of cardiomyocytes on the surface-patterned SU-8 cantilevers were measured under various drugs, namely, Bay K8644, Verapamil, Isoproterenol, Quinidine, Lidocaine, E-4031, and Astemizole. In the case of Bay K8644, Verapamil, and Isoproterenol, the contractile force of cardiomyocytes was increased or decreased by 30%, 50%, and 20%, respectively, at the value of IC50. However, Quinidine only increased the half-value duration (HaVD) by approximately 22%, even though it did not affect the contractile force at IC50. Potassium-channel inhibitors E-4031 and Astemizole also showed no change in contractile force at low concentrations. However, the HaVD increased by approximately 3.4 times and abnormal peaks were observed at higher drug concentrations. The fully automated high-throughput screening capability of the proposed platform has immense potential as a preliminary drug-screening system in the next generation.

A micro vertically-allocated SU-8 check valve and its characteristics

This paper proposes and develops a novel fabrication method to realize a micro vertically-allocated SU-8 check valve. The ordinary vertically-allocated SU-8 check valve formed by the photolithography has a limited overlap between an SU-8 cantilever and its valve seat, resulting in the low diodicity (i.e. the ratio of the forward flow to reverse flow rates). This overlap prevents the leakage through a bottom gap (a gap between a substrate and the SU-8 cantilever) and a top gap (a gap between a cover plate and the SU-8 cantilever) in the backward flow. These gaps, which are necessary for the movement of the SU-8 cantilever in the forward flow, must cause the leakage of check valves in the backward flow. To reduce the leakage and improve the diodicity, we propose to fabricate a novel fully-overlapped valve seat composed of channel walls, a bottom block, and a top block. To keep the advantages on the batch process of vertically-allocated SU-8 check valves, these blocks are desirable to be fabricated by the multi-layer process. We propose a multi-layer process for the bottom block and a novel beam forming process (a dose-controlled UV exposure) for the top block. In this paper, we design two types of check valves: (a) the ordinary partially-overlapped check valve; and (b) the proposed fully-overlapped check valve, to compare their diodicity. By investigating their flow rate characteristics experimentally, we conclude that the fully-overlapped check valve can reduce the leakage of 53.6% compared with the partially-overlapped check valve and increase the diodicity from 1.7 to 3.5 (about 2 times) at the supplied pressure of 18 kPa. We also demonstrate that the novel check valve exhibits higher performance and also show that our fully-overlapped check valve can be potentially applied to various microfluidics.

Micro-Lego of 3D SU-8 structures and its application to a re-entrant surface

Deterministic multi-layer micro-scale assembly of SU-8 polymer objects as well as their cohesive interfacial joining strength are investigated. The process of SU-8 photoresist into retrievable individual micro-scale objects is reviewed and the construction of three-dimensional (3D) SU-8 micro-structures via micro-Lego, which involves transfer printing and permanent joining of SU-8 objects on a target site, is demonstrated. Owing to the additive 3D nature of micro-Lego, the demonstrative micro-scale architectures are unattainable through other means such as conventional photolithography. The mechanical joining strength between SU-8 objects assembled by micro-Lego is inspected by probing SU-8 cantilever specimens using a nanoindenter and the results reveal that the joining strength is ~ 0.988 J/m2, which is comparable to wafer scale silicon fusion bonding strength. In order to demonstrate a practical application of SU-8 micro-Lego, a SU-8 re-entrant surface is assembled by one step process. It is shown that the SU-8 re-entrant surface can repel a water droplet while SU-8 is natively hydrophilic with a low intrinsic contact angle (73 ± 5°).

Study on the fabrication of a SU-8 cantilever vertically-allocated in a closed fluidic microchannel

This paper describes the fabrication of a vertically-allocated SU-8 cantilever in a closed fluidic channel. The difficulties to fabricate the vertically-allocated SU-8 cantilever inside the closed channel mainly lie in which kinds of sacrificial layers under the cantilever and sealing methods for enclosing the channel are utilized for the movability of the cantilever. To obtain a suitable sacrificial layer and high sealing quality, the selectivity of sacrificial layers and the sealing conditions based on the SU-8 adhesive are discussed in this paper. The experiments to test different photoresist lead to the conclusion that AZ 5214E is adequate for the sacrificial layer. Also, the bonding results indicate that the thickness of the uncrosslinked SU-8 solid layer is the most important factor which affects the formation of a proper gap between the top coverglass and the SU-8 cantilever. The photos of the movement of the cantilever also show that the free-standing SU-8 cantilever is successfully fabricated by releasing the suitable sacrificial layer and is allocated inside the closed channel by the method of SU-8 adhesive. The fabrication method in this paper is also useful for other microfluidic applications.

Spiral-shaped SU-8 cantilevers for monitoring mechanical response of cardiomyocytes treated with cardiac drugs

In this paper, a spiral-shaped SU-8 cantilever with biocompatibility was proposed to monitor the changes of cardiac contraction in response to injection of drug in real time. The cantilever comprises a thick quadrate body and a filmy spiral cantilever with micro-grooves and gold patterns on the top of its surface. The longitudinally patterned micro-grooves facilitate the aligned growth of cardiomyocytes in order to increase the contraction force. In addition, the spiral shape increases the cantilever’s effective length and decreases its spring constant, which means its bending displacement can be increased for the same force as compared to that observed in a conventional rectangular cantilever. The bending displacement of the spiral SU-8 cantilever induced by the contraction of cardiomyocytes can be precisely measured at the nanoscale using a homemade laser-based measurement system. The displacement in the spiral SU-8 cantilever without micro-grooves was much greater than that in the case of traditional rectangular SU-8 cantilevers. The displacement produced by the spiral cantilever with grooves is 240% greater than that without grooves. Following preliminary experiments, cardiac drugs including Verapamil, Quinidine, Lidocaine, E-4031, and Bay K 8644 were used to evaluate their side effects on cardiomyocytes. Quantitative analysis of measurements shows that not only Verapamil and Quinidine but also Lidocaine and E-4031 treatment can decrease the cell beating frequency, and both the Verapamil and Lidocaine treatment can decrease the cell contraction force. Furthermore, Bay K 8644 assay results indicate that it can decrease the contraction force and increase the beating frequency of cardiomyocytes.

Surface-patterned SU-8 cantilever arrays for preliminary screening of cardiac toxicity

Arrays of a μgrooved SU-8 cantilever were utilized to analyze changes in the contraction force and beating frequency of cardiomyocytes in vitro. The longitudinally patterned μgrooves facilitates alignment of cardiomyocytes on top of the SU-8 cantilever, which increases the contraction force of cardiomyocytes by a factor of about 2.5. The bending displacement of the SU-8 cantilever was precisely measured in nanoscale using a laser-based measurement system combined with a motorized xyz stage. The cantilever displacement due to contraction of the cardiomyocytes showed the maximum on day 8 after their cultivation. Following preliminary experiments, Isoproterenol, Verapamil, and Astemizole were used to investigate the effect of drug toxicity on the physiology of cardiomyocytes. The experimental results indicated that 1µM of Isoproterenol treatment increased contraction force and beating frequencies of cardiomyocytes by 30% and 200%, respectively, whereas 500nM of Verapamil treatment decreased contraction force and beating frequencies of cardiomyocytes by 56% and 42%, respectively. A concentration of less than 5nM of the hERG channel suppression drug Astemizole did not change the contraction forces in the displacement but slightly decreased the beating frequencies. However, irregular or abnormal heartbeats were observed at Astemizole concentrations of 5nM and higher. We experimentally conformed that the proposed SU-8 cantilever arrays combined with the laser-based measurement systems has the great potential for a high-throughput drug toxicity screening system in future.

MEMS Based Mechanical Resonant Push-Pull Scanning Endoscope

Here we present a microfabricated SU-8 cantilever scanner. Current SU-8 rib waveguide design improved the output beam diameter of our previous design from ~50μm to ~5.0μm, further improves our spatial resolution. The proposed rib waveguide design also allows a relatively large waveguide cross section (4μm in height and 55μm in width) and broader wavelength range in single-mode operation (λ= 0.7μm to 1.3μm) with a minimum transmission loss (85% output transmission efficiency with Gaussian beam profile input). Our design also provides a new means to create a 2-D raster scanning pattern using a microfabricated push-pull actuator, verified by a transient finite element analysis (FEA). The scanner's line resolution and field of view (FOV) have been optimized through a parametric study using modal and harmonic analyses. This optical scanner may find application in the area of endoscopy, where there is a need for a minimally invasive device that reduces patient discomfort.

Polymeric cantilever sensors functionalized with multiamine supramolecular hydrogel

This paper presents a multiamine supramolecular hydrogel (SH) modified SU-8 cantilever sensor. A chemical process to integrate the SH on the surface of SU-8 cantilever is developed to improve the stability and reusability of the functionalized cantilever sensor, instead of physical surface functionalization methods such as direct coating and self-assembly. The network structure of the SH is formed through a ring-opening and amine-substituted reaction in epoxy groups of the SU-8. The surface functionalization of the SU-8 cantilever is characterized by using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The modified SU-8 cantilever sensor has been successfully applied for real-time measurement of metal ions concentration in aqueous solutions. Responses from the cantilever due to surface stress are induced by the axial coordination of ferric ions to amino groups of the SH and the chemical energy is converted into the mechanical energy which can be measured by a piezoresistive sensor. The concentration-dependent performance of the sensor is demonstrated and discussed, indicating a potential use of SH functionalized cantilever sensors.

Development of 2D Microdisplay Using an Integrated Microresonating Waveguide Scanning System

Our research team has developed a 2D micro image display device that can potentially overcome the size reduction limits while maintaining the high-image resolution and field of view obtained by mirror-based display systems. The basic design of the optical scanner includes a microfabricated SU-8 cantilever waveguide that is electromechanically deflected by a piezoelectric actuator. From the distal tip of the cantilever waveguide, a light beam is emitted and the direction of propagation is displaced along two orthogonal directions. The waveforms for the actuator and the LED light modulation are generated and controlled using a field programmable gate array. Our recent study is an update to the previously-reported mechanical scanner, replacing the hand-built PZT scanner and fiber waveguide with a microfabricated system incorporating aerosol-deposited PZT thin film and a polymeric SU-8 wave guide. In this article, we report on the design and fabrication of a prototype miniaturized 2D scanner, discuss optical and mechanical the modeling of the system's properties and present the experimental results.

Effects of supercritical carbon dioxide treatment on bending properties of micro-sized SU-8 Specimens

The bending properties of micro-sized photoresist patterns are quantitatively evaluated using micro-sized SU-8 cantilever type test specimens to clarify the effects of supercritical carbon dioxide-treatment (ScCO2-treatment). The SU-8 specimens were treated with different types of treatments before the bending tests, which were ScCO2-treatment, heat-treatment, and non-treatment of specimens. In addition, FT-IR analysis of these specimens was performed to clarify the relationship between the degree of crosslinking and the bending properties. The values of Young’s modulus, yield stress, and bending strength in heat-treated SU-8 specimens are all greater than those of the non-treated ones because of a higher degree of crosslinking. On the other hand, the bending properties in SU-8 only change slightly by the ScCO2-treatment in spite of the fact that the degree of the epoxide rings in SU-8 is significantly lower. This result suggests that ScCO2 does not only affect the bending properties by influencing the degree of crosslinking, but ScCO2 also seems to work as a plasticizer.

集成铜金属压阻层的SU-8胶悬臂梁微力传感器的制作

集成铜金属压阻层的SU-8胶悬臂梁微力传感器的制作

A microvalve for lab-on-a-chip applications based on electrochemically actuated SU8 cantilevers

Increasing the complexity and functionality of Lab-on-a-Chip devices requires integrated valves for internal flow regulation. The device introduced in this work is an electrochemical SU8 microvalve, featuring a cantilevered structure that seals an adjacent channel under the action of a growing electrolysis bubble. The valve is based on previously reported movable microcantilevers embedded in microchannels. As opposed to membrane valves moving perpendicular to the substrate, the cantilever valve is patterned next to the channel and closes it by moving horizontally, dramatically reducing footprint. Its compact, monolithic and 2D construction reduces assembly problems and results in negligible increase in dead volume (14.5 nl). Besides, its location in the way of the channels avoids the need for auxiliary filling ports or separate working media for actuation. Effective sealing with negligible leakage at 20 kPa has been achieved. The valve also displays passive flow regulation induced by the pressure differential generated across the valve chamber under flow conditions (42–72% flow decrease).