Nguyen Binh-Khiem

Polymer thin film deposited on liquid for varifocal encapsulated liquid lenses

We developed a process for making liquid lenses that have a tunable focal distance. In our process, polymer thin films are directly deposited on liquid droplets in vacuum. Conducting the deposition in vacuum helps preserve the delicate spherical droplet shape so that they can be used as lenses. The polymer film is mechanically stable, which protects the droplets from the ambient, but it is also extremely thin and flexible, which allows the droplets to deform. The droplets are deformed using electrostatic force to change their focal distance. Our lens is structurally simple and can be operated by an electrical input.

Tensile film stress of parylene deposited on liquid.

We found that liquid droplets encapsulated by Parylene deposited directly on a liquid surface deformed toward spherical shapes during Parylene deposition. This deformation suggested that the film stress was tensile. We calculated the film stress of such Parylene films by studying the surface mean curvature of the droplet shape and found the film stress measured about 0.7-0.9 MPa tensile. This film stress is of opposite type to that of as-deposited Parylene films deposited on solid substrates, which was compressive. This difference might indicate a profound change of the Parylene polymer due to the use of liquid surface as deposition substrate. The tensile film stress and its effect on the droplet shape also have implications in the fabrication and operation of Parylene microdevices that have encapsulated liquid structures such as microlens or micropumps.

Ratiometric optical temperature sensor using two fluorescent dyes dissolved in an ionic liquid encapsulated by Parylene film.

A temperature sensor that uses temperature-sensitive fluorescent dyes is developed. The droplet sensor has a diameter of 40 μm and uses 1 g/L of Rhodamine B (RhB) and 0.5 g/L of Rhodamine 110 (Rh110), which are fluorescent dyes that are dissolved in an ionic liquid (1-ethyl-3-methylimidazolium ethyl sulfate) to function as temperature indicators. This ionic liquid is encapsulated using vacuum Parylene film deposition (which is known as the Parylene-on-liquid-deposition (PoLD) method). The droplet is sealed by the chemically stable and impermeable Parylene film, which prevents the dye from interacting with the molecules in the solution and keeps the volume and concentration of the fluorescent material fixed. The two fluorescent dyes enable the temperature to be measured ratiometrically such that the droplet sensor can be used in various applications, such as the wireless temperature measurement of microregions. The sensor can measure the temperature of such microregions with an accuracy of 1.9 °C, a precision of 3.7 °C, and a fluorescence intensity change sensitivity of 1.0%/K. The sensor can measure temperatures at different sensor depths in water, ranging from 0 to 850 μm. The droplet sensor is fabricated using microelectromechanical system (MEMS) technology and is highly applicable to lab-on-a-chip devices.

Liquid-phase packaging of a glucose oxidase solution with parylene direct encapsulation and an ultraviolet curing adhesive cover for glucose sensors.

We have developed a package for disposable glucose sensor chips using Parylene encapsulation of a glucose oxidase solution in the liquid phase and a cover structure made of an ultraviolet (UV) curable adhesive. Parylene was directly deposited onto a small volume (1 μL) of glucose oxidase solution through chemical vapor deposition. The cover and reaction chamber were constructed on Parylene film using a UV-curable adhesive and photolithography. The package was processed at room temperature to avoid denaturation of the glucose oxidase. The glucose oxidase solution was encapsulated and unsealed. Glucose sensing was demonstrated using standard amperometric detection at glucose concentrations between 0.1 and 100 mM, which covers the glucose concentration range of diabetic patients. Our proposed Parylene encapsulation and UV-adhesive cover form a liquid phase glucose-oxidase package that has the advantages of room temperature processing and direct liquid encapsulation of a small volume solution without use of conventional solidifying chemicals.

Micro Liquid Prism

This paper presents a micro liquid prism. Two flat transparent plates float on a liquid droplet and these plates serve as prism faces. The two plates are positioned automatically by surface tension, just by putting the plates on the ellipsoidal droplet. Therefore the proposed prism can be fabricated accurately in micro scale without difficulty. As the liquid and the plates are encapsulated by Parylene, the prism can retain its shape. The prism which faces were 400¿m in diameter and whole size was smaller than 1mm3 was fabricated. The adequate function of the fabricated prism for Surface Plasmon Resonance (SPR) measurement was verified.

Liquid motor driven by electrowetting

We propose a liquid motor using surface tension. Our liquid motor is composed of a liquid droplet, a floating plate and a lower plate. The plate floating on the droplet is rotated continuously by electrowetting actuation. The floating plate is asymmetrically and has four cogs. By electrowetting actuation, the shape of the droplet is deformed. Then, the floating plate rotates to the position where the surface energy of the sandwiched liquid takes a minimum value. On the lower plate, electrodes are patterned in an annular shape. By choosing the voltage-applied electrodes, the position of the floating plate is controlled. In this research, a 2 mm floating plate with a 500 mum silicon cube was rotated at 180 rpm. This motor is useful for optical devices because the components of the liquid motor can be made of transparent materials.

Scanning micromirror using deformation of a Parylene-Encapsulated Liquid Structure

This paper presents a scanning micromirror using deformation of a parylene-encapsulated liquid structure (PELS). Silicone fluid is put between a silicon plate and an electrode-patterned plate. Both of the plates and liquid are encapsulated by a parylene membrane. Gold is deposited on the surface of the parylene membrane to fabricate an upper electrode. By applying voltage between the upper and lower electrodes, the encapsulated liquid is deformed and the silicon plate is tilted. The silicon plate is supported by the PELS, instead of usual torsion beams. When a set of four appropriately synchronized voltages is applied between the four lower electrodes and the upper electrode, two dimensional scanning motion is achieved.

Porous Parylene and effects of liquid on Parylene films deposited on liquid

We show that, in Parylene films fabricated by Parylene-on-liquid-deposition (POLD), the side in contact with the liquid during deposition develops a porous surface. Because of the liquid nature of such a deposition substrate, the molecules of Parylene monomer can diffuse into and polymerize inside the liquid, which is probably the main cause of this effect. We also found that the dimension of the characteristic features on such a surface as well as the thickness of the porous domain becomes larger when a liquid with larger molecular weight is used. We demonstrate three applications using this interaction between liquids and Parylene: POLD films as gas exchange membranes, POLD as a method to modify the hydrophobicity of Parylene surface, and in pattern-selective deposition of POLD films.

Multi-axis force sensor with dynamic range up to ultrasonic

This paper reports on the design, implementation and characterization of a multi-axis force sensor that has a dynamic frequency range up to more than 1.2 MHz. The multilayer structure of elastomer/thin polymer/viscous liquid is utilized to conduct forces as well as acoustic vibrations to sensing piezoresistive cantilevers. Experimental results showed that this sensor is capable of measuring normal and lateral forces with high linearity in the range of 40 kPa. Moreover, by evaluating the dynamic response, this sensor is proved to have a dynamic range covered ultrasonic frequencies with the first resonant frequency at 170 kHz.

High sensitive 3D tactile sensor with the structure of elastic pyramids on piezoresistive cantilevers

We propose a highly sensitive three-dimensional tactile sensor using the structure of elastic micro pyramids pressing on piezoresistive cantilevers. In the structure of the sensor we proposed, the forces acting on the surface of the elastomer were transmitted to the cantilevers through the pyramids. The key point of our sensor was that the cantilevers were not completely embedded inside the elastomer: a cavity under each cantilever enabled the larger deformation and thus the larger resistance change of the cantilever. Therefore the high sensitivity of the sensor could be obtained. Moreover, by using four cantilevers aligned with four pyramids, the three-dimensional force sensor was realized. The sensitivities of our sensor to forces in normal and lateral directions were about 50 times and 2.4 times higher, respectively, compared to those of a tactile sensor with the ultrathin cantilevers embedded inside an elastomer [1].