Micro electromechanical systems

Silicon carbide based MEMS

Microelectromechanical systems sensors have been intensively developed utilizing various physical concepts, such as piezoresistive, piezoelectric, and thermoresistive effects. Among these sensing concepts, the thermoresistive effect is of interest for a wide range of thermal sensors and devices, thanks to its simplicity in implementation and high sensitivity.1

Silicon carbide (SiC) is one of the most promising materials for applications in harsh environments due to its excellent electrical, mechanical, and chemical properties. Among several large band gap materials, SiC is one of the most promising semiconductors for MEMS transducers used in harsh environments due to its large energy band gap of 2.3 to 3.4 eV, excellent chemical inertness, and large Young’s modulus.2 Further, NTNLab research examines the piezoresistive effect of p-type single crystalline 3C-SiC characterized at high temperatures, using an in-situ measurement method. Experimental results show that the highly doped p-type 3C-SiC possesses a relatively stable gauge factor of approximately 25 to 28 at temperatures varying from 300 K to 573 K. The in-situ measurements also show that the combination of the piezoresistive and thermoresistive effects can increase the gauge factor of p-type 3C-SiC to approximately 20% at 573 K.3

The NTNLab has also found that the orientation dependence of the pseudo-Hall effect in p-type 3C–SiC four-terminal devices under mechanical stress. Research results indicate that the offset voltage of p-type 3C–SiC four-terminal devices significantly depends on the directions of the applied current and stress.4

The thermoresistive effect for advanced thermal sensors, including fundamental research, design and application development of high-temperature thermal sensors are being investigated in the NTNLab employing large band gap semiconductors such as silicon carbide. The development of thermal sensors could be driven towards miniaturization of a wide range of measurement without signal saturation, large working bandwidth and capability of detecting multi-directions of physical signals. An example from the NTNLab is the effect in pencil graphite, and a proof of concept of thermal flow sensors has been successfully demonstrated.5

The NTNLab has devised an innovative nano strain-amplifier employed to significantly enhance the sensitivity of piezoresistive strain sensors. Inspired from the dogbone structure, the nano strain-amplifier consists of a nano thin frame released from the substrate, where nanowires were formed at the centre of the frame. Analytical and numerical results indicated that a nano strain-amplifier significantly increases the strain induced into a free standing nanowire, resulting in a large change in their electrical conductance. The proposed structure was demonstrated in p-type cubic silicon carbide nanowires fabricated using a top down process. The experimental data showed that the nano strain-amplifier can enhance the sensitivity of SiC strain sensors at least 5.4 times larger than that of the conventional structures. This result indicates the potential of the proposed strain-amplifier for ultra-sensitive mechanical sensing applications.6

Silicon based MEMS

Over several decades, electronic sensing devices have been significantly developed, with an emphasis on miniaturization and high sensitivity with many successful applications. Nanowires (NWs), as an important achievement of the miniaturization process, are of interest, since these one-dimensional nanostructures offer extremely small sizes with great integration ability into micro/ nano systems. Recent research facilitating thermal-sensing nano-systems has focused on highly thermosensitive NWs, which are particularly useful for monitoring temperature variations within a narrow range. Research from the NTNLab shows highly thermosensitive silicon nanowires (SiNWs) for thermal-sensing applications. Crystalline Si was amorphized by Focused Ion Beam in the fabrication process of the SiNWs, and subsequently recrystallized by a thermal annealing process to improve their electrical conductivity. A temperature coefficient of resistance (TCR) from -8000 ppm/K to -12,000 ppm/K was measured for the SiNWs. This large negative TCR is attributed to the boundary potential barrier of 110 meV between silicon crystallites in the poly crystalline SiNWs.1

The piezoresistive effect in silicon nanowires (SiNWs) has attracted a great deal of interest for NEMS devices. The NTNLab is examining the piezoresistive effect of p-type silicon nanowires fabricated by a top-down process using focused ion beam (FIB) implantation and wet etching. In this work our Lab looks at characterization of the piezoresistive effect of SiNWs fabricated by localized amorphization through FIB implantation, selective wet etching, and re-crystallization by thermal annealing process. As the properties of SiNWs are changed due to amorphization and post-FIB thermal treatment, the piezoresistive effect of Si NWs fabricated using this process is expected to be different from that of bulk single crystalline Si. Our research examines the piezoresistive effect in released SiNWs fabricated by a top-down process using the FIB followed by wet etching of Si and thermal annealing. A relatively large gauge factor found in this top-down fabricated SiNWs indicates its high potential for nano electro mechanical systems (NEMS).2

Polymer based MEMS

Cells are well known to have the ability to sense and respond to mechanical stimuli, a process is referred to as mechanotransduction. Greater understanding of mechanotransduction is of interest to clinicians and scientists to improve clinical diagnosis and to understand medical pathology. However, the complexity involved in in vivo biological systems creates a need for better in vitro technologies, which can closely mimic the cells’ microenvironment using induced mechanical strain. This has motivated the NTNLab in the development of cell stretching devices for better understanding of the cell response to mechanical stimuli.1

Research from the NTNLab examines the design, fabrication and characterisation of a cell stretching device based on the side stretching approach. A unique PDMS-based micro fabrication process was developed that achieved parallelisation, controlled membrane thickness and an ultra-thin bottom layer. The platform was tested for cell growth under cyclic stretching, and preliminary results show that the device is compatible with all standard microscopes. Two types of cells were cultured and stretched in the device. The effective transferral of strain from the device membrane to the cells was demonstrated.2 Our research has found that the heterogeneity of the device provided an ideal platform for establishing strain requirement for the OEC culture. The cell stretching system developed may serve as a tool in exploring the mechanobiology of OECs for future SCI transplantation research.3

In further work, the NTNLab’s novel electromagnetic cell stretching platform can introduce a cyclic and static strain pattern on a cell culture. The platform was tested with fibroblasts, and the experimental results are consistent with cytoskeleton reorganisation and cell reorientation induced by strain. Our observations suggest that the cell orientation is highly influenced by external mechanical cues. Cells reorganise their cytoskeletons to avoid external strain and to maintain intact extracellular matrix arrangements.4

Droplet based microfluidics

Convective heat transfer in droplet-based microchannel heat sinks can be enhanced by the recirculating vortices due to the presence of interfaces. In rectangular microchannels, the three dimensional structures of the vortices and the ‘gutters’ (ie the space between the curved droplet interface and the corner of the microchannel) can significantly affect the heat transfer process. Numerical simulations of the heat transfer process are performed to study the three dimensional features in droplet-based microchannel heat sinks, with results showing that the ‘gutters’ can hinder the heat transfer process because of its parallel flow. On contrast, the recirculating flow in droplets and in slug regions between successive droplets can enhance the heat transfer by advecting hot fluid towards the center of the droplets/slugs and advecting fresh fluid towards the wall of the channel.1 Research from the NTNLab proposes to continuously generate droplets with programmable concentration using the ion concentration polarization (ICP) phenomenon. Via this method, our fabrication method has the advantages of simplicity, reliability, repeatability and low-cost. Sample droplets with up to 100-fold concentration were generated continuously with our device.2

Bioaffinity mass spectrometry screening is a novel approach using non-denaturing electrospray ionization (ESI) mass spectrometry (MS) in identifying drug leads. This technique can detect and preserve noncovalent protein-active drug ligand complexes under different physiological conditions. The screening challenge being the reduction of sample volume needed. The NTNLab demonstrates that analysis of samples can be performed using droplet-based microfluidics, with results showing that a MS instrument with a conventional ESI source can detect the samples and distinguish it with the separating oil phase.3

The NTNLab is committed to developing new micron-sized droplets. Delicate control of the droplet generation process is needed to address complex applications. At the liquid/liquid interface, an energy imbalance leads to instability and droplet breakup. Controlling the droplet generation process is a central topic of microfluidics.4 Work in the NTNLab presents state-of-the-art technologies to efficiently sort droplets. We classify the concepts according to the type of energy implemented into the system. Combining digital microfluidic devices allows for functional, flexible and powerful systems, covering specific technological areas and applications where electronics alone cannot compete or cannot offer an integrated chemical or biochemical functionality. Most powerful is the compatibility with digital electronics that allows for programmability and storage of droplets and droplet trains.5

Our work introduces an effective method to actively induce droplet generation using negative pressure. Droplets can be generated on demand using a series of periodic negative pressure pulses. Fluidic network models were developed using the analogy to electric networks to relate the pressure conditions for different flow regimes. Experimental results show that the droplet volume is correlated to the pressure ratio with a power law of 1.3.6

Automated droplet measurement (ADM) software has been developed based on OpenCV image processing library that has much higher throughput than currently available software. The process speed was found to be higher than the transfer speed even when visual feedback is enabled. Added to this, to shorten the total time taken on droplet measurement further, our Lab integrated newly developed ADM software and camera software development kit together in a new process flow. This was done by performing video transfer/streaming simultaneously with video processing. The total time for the droplet measurement using the integrated software was found significantly shorter than using the old process flow with DMV software.7

Micro optofluidics

For decades, microelectromechanical (MEMS) systems sensors have been intensively developed using various physical concepts, including piezoresistive, piezoelectric, and thermoresistive effects. The thermoresistive effect, which refers to the electrical resistance change with temperature variation, has many advantages in terms of simplicity in design and implementation. Based on this effect, various micro-sized thermal sensors, which can monitor temperature, flow and acceleration, have been successfully fabricated, thanks to the advancement in MEMS technologies.1

Research from the Nam-Trung Nguyen lab (NTNLab) examines the design and demonstration of an optofluidic in-plane bi-concave lens to perform both light focusing and diverging. With the concave lens hydrodynamically formed in a rectangular chamber with a liquid core cladding (L2) configuration. Our research has found that the focusing mode of the opto- fluidic bi-concave lens can be simply switched to diverging mode by stopping the pumping the liquid from core inlet. The previous cladding liquid (cinnamaldehyde) would join together in the chamber and become the core stream for the diverging mode. The auxiliary cladding liquid would change its function from avoiding light-scattering to becoming the cladding stream for the diverging mode. Combining the performance of focusing and diverging, the tunability of the focal length of the optofluidic lens and the width of the light beam can be greatly enhanced. This improvement of tunability will make this optofluidic bi-concave lens more adaptive to the lab-on-chip applications.2

NTNLab is also examining modelling and experimental results of a liquid-core liquid-cladding optofluidic lens under the combined effect of hydrodynamics and electro-osmosis. To allow the lens to be tuned by a voltage, the cladding fluids are electrically conducting, while the core fluid is non-conducting. Our results show that the interfaces between the cladding fluids and the core fluid have optically smooth arc shape. Under fixed cladding flow rates, the same voltage forms symmetric biconvex lens only. Different voltages can form biconvex lens, planoconvex lens, and meniscus lens.3

Integrating a flow cytometer into microfluidic networks helps to miniaturize the system and make it portable for field use with applications in environmental monitoring, industrial testing and biochemical studies. Integration of optical components, such as lenses, further improves the compactness. However, many current designs suffer from severe light scattering due to roughness of the solid-based lens interface. Our research examines a flow cytometer using an optofluidic lens to focus the light beam. Benefiting from the smooth liquid–liquid lens interface and the refractive-index matching liquid as cladding streams, a light beam can be well focused without scattering. The variations of the signal peak values are reduced, owing to the small beam width at the beam waist.4 We are also undertaking work on an optofluidic aperture stop formed by the liquid-core/liquid-cladding flow;5 and numerically and experimentally investigating and the design of an optofluidic in-plane bi-concave lens to perform both light focusing and diverging using the combined effect of pressure driven flow and electro-osmosis.6

Further work by the NTNLab illustrates the concept of liquid crystal based tunable optofluidic polarizer. Our research to date demonstrates the working principle, fabrication and characterization of a novel optofluidic tunable polarizer suitable for lab-on-a-chip applications. The concept and developed probe can find potential optics and lab-on-a chip applications.7

Lab-on-a-chip and point-of-care diagnostics

Rare cells are low-abundance cells in a much larger population of background cells. It is difficult to isolate and analyse rare cells because of their generally low selectivity and significant sample loss. Microfluidics provides robust solutions including apparent performance enhancements resulting in higher efficiencies and sensitivity levels. It also provides simpler handling of small sample volumes and multiplexing capabilities for high-throughput processing. The NTNLab continue to examine the design considerations of representative microfluidic devices for rare cell isolation and analysis.1

A system of multiple organs integrated on a single chip or human on a chip (HUC) has potential in drug discovery, and could overcome the limitations of animal models including cost and incompatibility with human physiology. The matching length scale of biological structures and micromachined components makes a microfluidic chip the ideal platform to investigate physiological events. The NTNLab continues to study the development of HUC, including the integration of cell culture on a chip to create an ethical human model and to provide insights into the sensitivity of different cell constructs on drugs.2

Understanding the process of fusion of olfactory ensheathing cell (OEC) spheroids will lead to improvement of cell transplantation therapies to repair spinal cord injuries. OECs are neuroglia cells that provide support and protection for olfactory sensory neurons in the peripheral nervous system (PNS). The NTNLab has developed a microfluidic device that enables injection and capture of spheroids of OECs.3

Piezoelectric thin films including zinc oxide and aluminium nitride are used in lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto substrates for making flexible devices, which have the potential to integrate disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. The NTNLab continues to engineer high performance piezoelectric thin films, with our research examining critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/designs and substrate selections.4

Current in vitro gut models lack physiological relevance, and various approaches have been taken to improve current cell culture models. For example, mimicking the 3D tissue structure or fluidic environment has shown improved physiological function of gut cells. The NTNLab’s research has incorporated a collagen scaffold that mimics the human intestinal villi into a microfluidic device, providing cells with both 3D tissue structure and fluidic shear. Our research suggests the combination of fluidic stimulus and 3D structure induces improvement in gut functions.5

Significant progress has been made in the fabrication and characterisation of MEMS thermal sensors that use the thermoresistive effect in metals such as platinum and semiconductors including silicon and polysilicon. Drawbacks include cost and the low capability to work in high temperature environments. Research in alternative thermal sensing materials with high thermoresistive sensitivity is being investigated by the NTNLab, as is proper packaging for thermal sensors as thermal expansion at high temperatures can cause cracking in devices. An alternative trend is low-cost materials for thermal sensors, which can be processed using cleanroom-free facilities and user-friendly techniques. For example, the thermoresistive effect in pencil graphite has been intensively investigated, and a proof of concept of thermal flow sensors has been successfully demonstrated.6