Sukru U. Senveli

Experimental study of PDMS bonding to various substrates for monolithic microfluidic applications

This paper presents a comprehensive experimental study and characterization of material and bonding of PDMS based structures to various substrates. A previously published method [1] of bonding is further improved with the inclusion of more substrate material and additional characteristics. Uncured PDMS is used as an adhesive to bond PDMS devices reversibly to various substrates including a number of commonly used substrate materials that are not supported by the widely used plasma treatment method. We have optimized parameters such as PDMS base to curing agent ratio, curing temperature, and PDMS device age to obtain better bond strengths and quality. Bond strengths are presented for semiconductor substrates (silicon, zinc oxide, and silicon dioxide), metals (gold, aluminum), photoresists (SU-8, AZxx) and glass. Silicon based substrates experienced minor amounts of surface residue, but the method is fully reversible for other tested substrates. Bond strengths were measured as maximum endurable pressure between PDMS and substrates. Maximum average bond strengths of more than 0.4 MPa were achieved for substrates with Si-O groups. Other substrates exhibited maximum average bond strengths in the range 0.2–0.3 MPa. Also presented is a method that avoids alignment step for PDMS microfluidic device bonding, named the non-aligned method. This method provides bond strengths of more than 0.1 MPa. Presented methods do not need special equipment or processes such as plasma generators or temperature increases. Biocompatibility tests are performed for materials used in fabrications to ensure applicability in bio-sensing related devices.

Finite element method analysis of surface acoustic wave devices with microcavities for detection of liquids

This paper introduces the use of finite element method analysis tools to investigate the use of a Rayleigh type surface acoustic wave (SAW) sensor to interrogate minute amounts of liquids trapped in microcavities placed on the delay line. Launched surface waves in the ST-X quartz substrate couple to the liquid and emit compressional waves. These waves form a resonant cavity condition and interfere with the surface waves in the substrate. Simulations show that the platform operates in a different mechanism than the conventional mass loading of SAW devices. Based on the proposed detection mechanism, it is able to distinguish between variations of 40% and 90% glycerin based on phase relations while using liquid volumes smaller than 10 pl. Results from shallow microcavities show high correlation with sound velocity parameter of the liquid whereas deeper microcavities display high sensitivities with respect to glycerin concentration. Simulated devices yield a maximum sensitivity of −0.77°/(% glycerin) for 16 μm wavelength operation with 8 μm deep, 24 μm wide, and 24 μm long microcavities.

A surface acoustic wave biosensor for interrogation of single tumour cells in microcavities

In this study, biological cells are sensed and characterized with surface acoustic wave (SAW) devices utilising microcavities. After tumour cells in media are transported to and trapped in microcavities, the proposed platform uses SAW interaction between the substrate and the cells to extract their mechanical stiffness based on the ultrasound velocity. Finite element method (FEM) analysis and experimental results show that output phase information is an indicator of the stiffness modulus of the trapped cells. Small populations of various types of cells such as MCF7, MDA-MB-231, SKBR3, and JJ012 were characterized and characteristic moduli were estimated for each cell population. Results show that high frequency stiffness modulus is a possible biomarker for aggressiveness of the tumour and that microcavity coupled SAW devices are a good candidate for non-invasive interrogation of single cells.

Rare Cell Immobilization on MEMS Scale Sensors Using Micro-Electromagnets

A novel method is presented for immobilizing rare single cells on MEMS scale sensor sites. The presented method does not require immobilization site to be chemically functionalized. Immobilizing single cells precisely on MEMS sensor sites is a requirement for many single cell analysis methods-reliability of cell analysis directly depends on the reliability of immobilization. The presented method uses magnetic forces to immobilize cells labeled with superparamagnetic beads on MEMS scale sites of interest. It can be coupled with many cell analysis techniques to form MEMS lab-on-a-chip devices due to the simple fabrication setup and principle of operation. The performance of the device was demonstrated with unbound superparamagnetic beads as well as with human breast tumor cells from the cell line MCF-7. Immobilization percentage and area were 95.2% and 1000 μm2, respectively, for superparamagnetic beads. MCF-7 cells labeled with superparamagnetic beads were immobilized at a percentage of 69.2% with the average distance from immobilized cells to micro-electromagnet being 50.8 μm. Thus, the presented method is a good candidate for integration with microfluidic sensing and analysis platforms.

A Novel Surface Acoustic Wave Sensor for Microparticle Sensing and Quantification

This paper presents a new method for sensing and the quantification of the number of solid microparticles using surface acoustic wave (SAW) devices. In contrast to the standard mass loaded delay line approach, microcavities with varying geometrical shapes and sizes are formed between SAW interdigitated transducer pairs. The system operation relies on the resonance condition occurring inside the microcavity through the coupling of Rayleigh waves to the sample, and the output phase angle is used for obtaining measurement results. It is shown through measurements that it is possible to interact with polystyrene solid microbeads trapped inside the microcavity and extract information about the size of the sample. Furthermore, the number of microbeads placed in a single file along the microcavity width can be quantified using this platform. Experimental results are compared and verified with finite-element method simulations. In essence, this novel approach resulted in a platform capable of analyzing sample volumes less than 10 pL in a non-invasive manner. For size differentiation, experimental phase shifts of 0.14° ± 0.05°, 0.81° ± 0.26°, and 3.54° ± 0.49° were obtained in rectangular microcavities for 10, 15, and 20

A novel surface acoustic wave sensor with embedded microcavities for size differentiation of solid microparticles

\mu \text{m}

Integrated surface acoustic wave based sensors for fluidic applications

microbeads, respectively. On the other hand, a distribution of phase shifts as 0.51° ± 0.19°, 0.98° ±0.12°, and 1.34° ± 0.15° are obtained for counting one, two, or three microbeads, respectively. The proposed system was designed, simulated, fabricated, and tested successfully.

Synthesis of ZnO nanowires using lower temperature vapor based methods

Here we present a novel method for sensing of solid microparticles using surface acoustic wave (SAW) devices. In contrast to the standard mass loaded delay line approach, microcavities with varying geometrical shapes and sizes are formed between SAW interdigitated transducer (IDT) pairs. The system operation relies on the resonance condition occurring inside the microcavity through coupling of Rayleigh waves to the sample, and the output phase angle is used for obtaining measurement results. We show through measurements that it is possible to interact with polystyrene solid microbeads trapped inside the microcavity and extract information about the size of the sample. Experimental results are compared and verified with finite element method (FEM) simulations. In essence, this novel approach resulted in a micro acoustic microscopy device with the capability of analyzing sample volumes less than 10 pL in a non-invasive manner. Experimental phase shifts of 0.14°±0.05°, 0.81°±0.26°, and 3.54°±0.49° were obtained in rectangular microcavities for 10 μm, 15 μm, and 20 μm microbeads, respectively The proposed system was designed, simulated, fabricated, and tested successfully.

Biosensors in the small scale: methods and technology trends

This paper illustrates the extension of Rayleigh wave based surface acoustic wave (SAW) device sensors previously developed by the authors to integration with microfluidics. The SAW device is first modeled with a microchannel and then analyzed using finite element method. Precise fabrication, alignment and bonding of polydimethylsiloxane (PDMS) microchannels on diced Y-Z lithium niobate substrates are accomplished. Low glycerin concentrations in deionized (DI) water are analyzed. Simulation results and vector network analyzer (VNA) measurements of the device with the microchannel integration are given. With new microchannel integrated SAW device, sensitivity of −0.53 (% glycerin) is achieved at peak frequency of 86.1 MHz. Minimum signal-to-noise ratio is found as 7.51 dB.

A novel approach for differentiation of liquid samples with surface acoustic wave transducers and embedded microcavities

In this paper, we elaborate on protocols for growing ZnO nanowires using different vapor deposition techniques to provide a comparative study for low temperature based deposition. Effects of various parameters ranging from process temperatures to material compositions were investigated. Growth from ZnO thin film seed layers and catalytic growth using Au nanoparticles were performed as well as growth on blank Si substrates for comparison. Detailed results of SEM and XRD studies are presented for the ZnO nanowires. The lowest temperature achieved was approximately 750 °C with nanowires having diameters of 30-50 nm and lengths of 200-300 nm using VS method with a ZnO thin film seed to obtain complete surface coverage. In order to make the vapor based methods compatible with biosensors with monolithic readout circuits, the conventional thermal budget of commonly employed CMOS technology (usually around 450 °C) needs to be considered. Thus, lower temperature growth is preferable. In this regard, we address the temperature aspect of the growth for CMOS compatibility. We identify the effects of important process parameters and present a comprehensive investigation and comparative study of various factors on ZnO nanowire growth.