A valved responsive hydrogel microdispensing device with integrated pressure source

Stimulus responsive hydrogels provide the actuation pressure required for both valving and dispensing functions in the device presented. The microdispensing device uses an array of responsive hydrogels to deform a flexible membrane above a fluid reservoir chamber. When the microvalve is open, the deformation of the membrane reduces the volume of the reservoir chamber and pushes fluid through the microvalve. When the microvalve is closed, the expanding hydrogel array generates a storable pressure source that will result in fluid dispensing once the microvalve is opened. Experiments determined the pressure generated by this device to be 35 kPa. The device has a stroke volume of 45 /spl mu/L, and is able to hold the pressure for over 24 h.     

Ultra thin ICs and MEMS elements: techniques for wafer thinning,stress-free separation,assembly and interconnection

 Ultra thin chips with a thickness below 30 μm offer low system height, low topography and show enhanced mechanical flexibility. These properties enable diverse use possibilities and new applications. However, advanced wafer thinning, adapted assembly and interconnection methods are required for this technology. A new process scheme is proposed that allows manufacturing of ultra thin fully processed wafers. Secure handling is achieved by means of carrier substrates using reversible adhesive tapes for connection of support and device wafers. Well established backgrinding and etching techniques are used for wafer thinning. To avoid mechanical damage of thin ICs the “Dicing-by-Thinning” (DbyT) concept is introduced to process flow. Best results are obtained when preparing dry etched chip grooves at front side of device wafer and opening these trenches during backside thinning. The new process scheme was also applied to wafers with highly topographic surfaces. Results of 40 μm thin wafers with 15 μm high Nickel bumps are presented. Three different assembly methods are described, interconnection through the thin chip, face down assembly and isoplanar contacting. Received: 6 July 2001/Accepted: 26 February 2002 The authors would like to thank M. Küchler (IZM Chemnitz) for preparing and performing trench etching process and A. Ostmann (IZM Berlin) for performance of nickel bumping process. This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.     

Vertical liquid transportation through capillary bundle structure using centrifugal force

The use of three-dimensional (3D) microstructures is becoming essential attempt to develop next generations’ microdevices, to integrate many modules and various functions, and enhance the performance of device. In this paper, we present a new concept for lab on a chip using 3D structure and centrifugal pumping for integrated functional fluid systems such as high-throughput screening, and point of care testing systems which has stacked multiple structures with 3D-interconnection. The use of 3D structure brings many benefits for above high throughput systems, such as possibility to integrate various modules enabling to perform total assay operation, from sample preparation for biochemical reaction and their detection on one platform. For this concept, the most important key technology is control of a vertical valving and transportation of liquid between different 2D micro channel networks with different height levels. We demonstrated such vertical liquid transportation in 3D micro channel networks through the high aspect ratio capillary bundle filter by controlling spinning speed of device and centrifugal force as a pumping force, and confirmed capillary bundle could be employed as vertical microvalve for 3D fluidic systems using centrifugal force as a pumping method.     

Leveraging stimuli responsive hydrogels for on/off control of mixing

Microfluidic mixers are an important component in microfluidic devices. This paper presents a micromixer which can control mixing with responsive hydrogel actuators to modulate mixing between two adjacent fluids dependant on the chemistries of the fluid. This is achieved by the responsive hydrogels swelling or contracting under different stimuli, which alters the mixing between the two fluids. We present this device using standard pH responsive hydrogels for two different device designs and demonstrate altered mixing profiles based on the pH of the fluid streams. For the T-shaped device an increase in mixing efficiency from 18.3% to 34.5% is observed between the contracted and expanded hydrogel states. For the modified T-shaped device mixing efficiency in the contracted state is 25.0% while in the expanded state efficiency increases to 72.9%. This can be used as a mixer that has on/off functionality of an active mixer, based on the pH of the mixing streams, with the advantages a passive mixer offers. Other responsive hydrogel chemistries could be substituted into the device to achieve many different triggers for mixing.     

A hydrogel-actuated environmentally sensitive microvalve for active flow control

This paper reports on the fabrication and test of a hydrogel-actuated microvalve that responds to changes in the concentration of specific chemical species in an external liquid environment. The microvalve consists of a thin hydrogel, sandwiched between a stiff porous membrane and a flexible silicone rubber diaphragm. Swelling and deswelling of the hydrogel, which results from the diffusion of chemical species through the porous membrane is accompanied by the deflection of the diaphragm and hence closure and opening of the valve intake orifice. A phenylboronic-acid-based hydrogel was used to construct a smart microvalve that responds to the changes in the glucose and pH concentrations. The fastest response time (for a pH concentration cycle) achieved was 7 min using a 30-/spl mu/m-thick hydrogel and a 60-/spl mu/m-thick porous membrane with 0.1 /spl mu/m pore size and 40% porosity.     

An integrated liquid mixer/valve

We present an integrated liquid mixer/valve to be used for sample preparation for bioscience analysis systems. The mixer/valve is a glass-silicon bonded structure with a wafer-bonded cantilever-plate flapper valve and deep reactive-ion etched ports. It is passively pressure actuated and is distinguished by the fact that it can perform both a mixing and valving function simultaneously to mix two liquids noncontinuously. We present the design and fabrication of the mixer/valve and show that it successfully performs both its valving and mixing functions, including the discontinuous mixing of two liquids. We propose a method for characterizing mixing in this device using fluorescence microscopy and the pH dependence of fluorescein fluorescence. This method aims to allow one to extract the mixing length from a quantifiable observable. We present modeling and results of mixing length measurements using this method     

Design and simulation of a thermo transfer type MEMS based micro flow sensor for arterial blood flow measurement

Thermo transfer type MEMS (Micro Electro Mechanical System) based micro flow sensing device have promising potential to solve the limitation of implantable arterial blood flow rate monitoring. The present paper emphasizes on modeling and simulation of MEMS based micro flow sensing device, which will be capable of implantable arterial blood flow rate measurement. It describes the basic design and model architecture of thermal type micro flow sensor. A pair of thin film micro heaters is designed through MEMS micro machining process and simulated using CoventorWare; a finite element based numerical code. A rectangular cross section micro channel has been modeled where in micro heater and thermal sensors are embedded using the same CoventorWare tools. Some promising and interesting results of thermal dissipation depending upon very small amount of flow rate through the micro channel are investigated. It is observed that measuring the variation of temperature difference between downstream and upstream, the variation of fluid flow rate in the micro channel can be measured. The numerical simulation results also shows that the temperature distribution profile of the heated surface depends upon microfluidic flow rate i.e. convective heat transfer is directly proportional to the microfluidic flow rate on the surface of the insulating membrane. The simplified analytical model of the thermo transfer type flow sensor is presented and verified by simulation results, which are very promising for application in arterial blood flow rate measuring in implantable micro devices for continuous monitoring of cardiac output.     

Equilibrium swelling and kinetics of pH-responsive hydrogels: models, experiments, and simulations

The widespread application of ionic hydrogels in a number of applications like control of microfluidic flow, development of muscle-like actuators, filtration/separation and drug delivery makes it important to properly understand these materials. Understanding hydrogel properties is also important from the standpoint of their similarity to many biological tissues. Typically, gel size is sensitive to outer solution pH and salt concentration. In this paper, we develop models to predict the swelling/deswelling of hydrogels in buffered pH solutions. An equilibrium model has been developed to predict the degree of swelling of the hydrogel at a given pH and salt concentration in the solution. A kinetic model has been developed to predict the rate of swelling of the hydrogel when the solution pH is changed. Experiments are performed to characterize the mechanical properties of the hydrogel in different pH solutions. The degree of swelling as well as the rate of swelling of the hydrogel are also studied through experiments. The simulations are compared with experimental results and the models are found to predict the swelling/deswelling processes accurately.     

Piezoresistive biochemical sensors based on hydrogels

Already 8 years ago, the usage of piezoresistive sensors for chemical measurands was proposed at the Solid State Electronics Laboratory of the Dresden University of Technology. Adding functionalised polymer coating which shows swelling due to chemical or biological values leads to a similar deflection of the thin silicon bending plate like for pressure sensors. The application of “stimuli-responsive” or “smart” cross-linked gels in chemical sensors is based on their ability to a phase transition under the influence of external excitations (pH, concentration of additives in water, temperature). Combining a “smart” hydrogel and a micro fabricated pressure sensor chip allows to continuously monitor the analyte-dependent swelling of a hydrogel and hence the analyte concentration in ambient aqueous solutions. The sensitivity of hydrogels with regard to the concentration of such additives as H⁺-ions (pH sensor), transition-metal ions, salts, organic solvents and proteins in water was investigated.     

A three-dimensional dielectrophoretic particle focusing channel for microcytometry applications

In this paper, we have designed and fabricated a microfluidic channel to focus biological cells using dielectrophoresis for cytometry applications. The device consists of an elliptic-like channel fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Microelectrodes are patterned on the circumference of the channel to generate ac fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An analysis using a thin shell model and experiments with microbeads and human leukemia HL60 cells indicate that biological cells can be focused using an ac voltage of an amplitude up to 15 V/sub p-p/ and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.     

Phase change in microchannel heat sinks with integrated temperaturesensors

A unique technique of mask-less and self-aligned silicon etch between bonded wafers was developed and applied to fabricate a microchannel heat sink integrated with a heater and an array of temperature sensors. The technique allowed the formation of self-aligned and self-stopped etching of grooves between the bonded wafers. The device, consisting of distributed temperature microsensors, allowed direct temperature measurements for different levels of power dissipation under forced convection using either nitrogen or water as working fluids. The measured temperature distributions are used to characterize the micro heat sink performance under forced convection boiling conditions. The onset of critical heat flux (CHF) condition was investigated for different channel sizes and liquid flow-rates. The results suggest that the bubble dynamic mechanism in the microchannel might be different compared with conventional channels     

Wafer‐scale monolithic hybrid integration of Si‐based IC and III–V epi‐layers—A mass manufacturable approach for active matrix micro‐LED micro‐displays

Hybridization of silicon integrated circuits (ICs) with compound semiconductor device arrays are crucial for making functional hybrid chips, which are found to have enormous applications in many areas. Although widely used in manufacturing hybrid chips, the flip‐chip technology suffers from several limitations that are difficult to overcome, especially when the demand is raised to make functional hybrid chips with higher device array density without sacrificing the chip footprint. To address those issues, Beida Jade Bird Display Limited has developed its unique wafer‐level monolithic hybrid integration technology and demonstrated its advantages in making large‐scale hybrid integration of functional device arrays on Si IC wafers. Active matrix micro‐light‐emitting diode micro‐displays with a resolution of 5000+ pixel per inch were successfully fabricated using Beida Jade Bird Display Limited's monolithic hybrid integration technology. The general fabrication method is described, and the result is presented in this paper. The fabricated monochromatic micro‐light‐emitting diode micro‐displays exhibit improved device performance than do other micro‐display technologies and have great potentials in applications such as portable projectors and near‐to‐eye projection for augmented reality. More importantly, the wafer‐scale monolithic hybrid integration technology offers a clear path for low‐cost mass production of hybrid optoelectronic IC chips.     

Temperature-Regulated Nonlinear Microvalves for Self-Adaptive MEMS Cooling

In this paper, thermal buckling of doubly clamped microfabricated nickel beams is implemented as a passive actuation mechanism to drive temperature-regulated nonlinear microvalves for adaptive microcooling applications. The nonlinear buckling phenomenon is combined with the nonlinear change in flow rate through parallel plates with a variable spacing. The thermal buckling mechanism and parallel plate flow are modeled analytically, and nondimensional characteristic design curves have been generated. Passive flow-control microvalves were fabricated using deep reactive ion etching and a through-mold nickel electroplating process over a thin sacrificial layer. The model is validated with experimental results from the microfabricated temperature-regulated microvalves. Experimental characterization using an integrated micromachined heat exchanger with air as the working fluid shows the desired nonlinear valving behavior with mass flow rates of up to 5 mg/s for a temperature increase of 50 $^{circ}hbox{C}$, corresponding to 0.25 W of heat removal. It is shown that temperature-induced elastic instabilities in microfabricated structures can be modeled and manipulated to create a nonlinear adaptive valving mechanism. The modeling approach, microfabrication process, and full characterization of the microvalves are presented.$hfill$ [2008-0008]      

Switchable Polymer Based Thin Film Coils as a Power Module for Wireless Neural Interfaces

Reliable chronic operation of implantable medical devices such as the Utah Electrode Array (UEA) for neural interface requires elimination of transcutaneous wire connections for signal processing, powering and communication of the device. A wireless power source that allows integration with the UEA is therefore necessary. While (rechargeable) micro batteries as well as biological micro fuel cells are yet far from meeting the power density and lifetime requirements of an implantable neural interface device, inductive coupling between two coils is a promising approach to power such a device with highly restricted dimensions. The power receiving coils presented in this paper were designed to maximize the inductance and quality factor of the coils and microfabricated using polymer based thin film technologies. A flexible configuration of stacked thin film coils allows parallel and serial switching, thereby allowing to tune the coil's resonance frequency. The electrical properties of the fabricated coils were characterized and their power transmission performance was investigated in laboratory condition.     

Continuous generation of hydrogel beads and encapsulation of biological materials using a microfluidic droplet-merging channel

In this paper, we describe a method for encapsulation of biomaterials in hydrogel beads using a microfluidic droplet-merging channel. We devised a double T-junction in a microfluidic channel for alternate injection of aqueous fluids inside a droplet unit carried within immiscible oil. With this device, hydrogel beads with diameter <100 μm are produced, and various solutions containing cells, proteins and reagents for gelation could merge with the gel droplets with high efficiency in the broad range of flow rates. Mixing of reagents and reactions inside the hydrogel beads are continuously observed in a microchannel through a microscope. By enabling serial injection of each liquid with the dispersed gel droplets after they are produced from the oil-focusing channel, the device simplifies the sample preparation process, and gel-bead fabrication can be coupled with further assay continuously in a single channel. Instantaneous reactions of enzyme inside hydrogel and in-situ formation of cell-containing beads with high viability are demonstrated in this report.     

Fabricating a hollow optical waveguide for optical communication applications

This work demonstrates a direct amorphous Si low-temperature wafer bonding technique to fabricate a semiconductor hollow waveguide with omni-directional reflectors for use in near infrared applications. The 2% dilute KOH solution was used to bond two ODR Si wafers with an amorphous Si thin film on the top of Si wafers. The resultant bonding interface is very thin, with a thickness that is close to that of the SiO/sub 2/ layer in the ODR substrate. Hence, the far-field image shows that light is strongly confined in the waveguides. The propagation loss was reduced to 1.0/spl plusmn/0.5 db/cm in the TE and TM modes, broadening the development of the semiconductor hollow waveguide with omni-directional reflectors for use in optical communication applications.     

A synchronous microsystem process using a dual-cylinder tool for the surface microstructures from solar cell silicon wafers

The low yield of active epoxy and Si₃N₄ microstructure layers is an important factor in semiconductor production. This study investigates a synchronous removal process using a newly designed dual-cylinder micro electromachining (μ-ECM) tool. This method of removing defective layers from solar cell silicon wafers can replace the current ones that employ strong acid and mechanical grinding. Grinding can damage the physical structure of silicon wafers and acid treatment is a source of environmental pollution. A high rate of epoxy film removal can be achieved with a short dual-cylinder anode used with a small gap between the tool and the silicon wafer surface. A thin anode also corresponds to a higher rate of epoxy film removal. Small diameter cathode cylinders also provide more discharge space and with a higher current give a better removal effect. The precise engineering technology used in this approach provides a clean and efficient recycling process that removes surface microstructure from defective solar cell silicon wafers to put them back into production with a resulting reduction of cost and environmental pollution.     

Development of microfluidic devices incorporating non-spherical hydrogel microparticles for protein-based bioassay

This paper describes the fabrication of a microfluidic device for use in protein-based bioassays that effectively incorporates poly(ethylene glycol) (PEG) hydrogel microparticles within a defined region. The microfluidic device is composed of a polymerization chamber and reaction chamber that are serially connected through the microchannel. Various shapes and sizes of hydrogel microparticles were fabricated in the polymerization chamber by photopatterning and moved to the reaction chamber by pressure-driven flow. All of the hydrogel microparticles were retained within the reaction chamber due to an in-chamber integrated microfilter with smaller mesh size than hydrogel microparticles. Hydrogel microparticles were able to encapsulate enzymes without losing their activity, and different concentrations of glucose were detected by sequential bienzymatic reaction of hydrogel-entrapped glucose oxidase (GOX) and peroxidase (POD) inside the microfluidic device using fluorescence method. Importantly, there was a linear correspondence between fluorescence intensity and the glucose concentration over the physiologically important range of 1.00–10.00 mM. D. Choi and E. Jang contributed equally to this work.     

A dielectric affinity glucose microsensor using hydrogel-functionalized coplanar electrodes

This paper presents a dielectric affinity microsensor that consists of an in situ prepared hydrogel attached to a pair of coplanar electrodes for dielectrically based affinity detection of glucose in subcutaneous tissue in continuous glucose monitoring applications. The hydrogel, incorporating N-3-acrylamidophenylboronic acid that recognizes glucose via affinity binding, is synthetically prepared on the electrodes via in situ gelation. When implanted in subcutaneous tissue, glucose molecules in interstitial fluid diffuse rapidly through the hydrogel and bind to the phenylboronic acid moieties. This induces a change in the hydrogel’s permittivity and hence in the impedance between the electrodes, which can be measured to determine the glucose concentration. The in situ hydrogel preparation allows for a reduced hydrogel thickness (~10 µm) to enable the device to respond rapidly to glucose concentration changes in tissue, as well as covalent electrode attachment of the hydrogel to eliminate the need for semipermeable membranes that would otherwise be required to restrain the sensing material within the device. Meanwhile, the use of coplanar electrodes is amenable to the in situ preparation and facilitates glucose accessibility of the hydrogel, and combined with dielectrically based transduction, also eliminates mechanical moving parts often found in existing affinity glucose microsensors that can be fragile and complicated to fabricate. Testing of the device in phosphate-buffered saline at pH 7.4 and 37 °C has shown that at glucose concentrations ranging from 0 to 500 mg/dL, the hydrogel-based microsensor exhibits a rapid, repeatable, and reversible response. In particular, in the glucose concentration range of 40–100 mg/dL, which is of great clinical interest to monitoring normal and low blood sugar levels, the device response is approximately linear with a resolution of 0.32 mg/dL based on effective capacitance and 0.27 mg/dL based on effective resistance, respectively. Thus, the device holds the potential to enable reliable and accurate continuous monitoring of glucose in subcutaneous tissue.     

Glass substrate for micro display devices

Glass substrate suitable for wearable micro display devices for augmented/virtual reality was developed. This glass has highly matched thermal expansion coefficient with that of silicon, and it enables to suppress the amount of warpage caused by the boding between Si and glass wafers. The glass also shows lower thermal shrinkage than that of conventional non‐alkali glass for thin‐film transistor substrate. In this paper, the effect of the temperature dependence of the thermal expansion coefficient on warpage generation after bonding process was investigated using both numerical simulation and experiment. The newly developed glass showed remarkably low warpage after the bonding process.