Yen Peng Kong

Duo-mold imprinting of three-dimensional polymeric structures

We present a new method of imprinting three dimensional (3D) polymeric micro- and nanostructures using a duo-mold process. In this method, two patterned Si molds are surface treated to have different surface energies. A polymer solution is spin-coated onto one of the molds, forming a planarized thin film on the mold. The two molds are pressed together at an appropriate temperature and pressure and then released. The patterned thin film adhering to one of the molds is then pressed onto a substrate at an appropriate temperature and pressure to form supported 3D structures. Alternatively, the patterned thin film on the mold may be released to form freestanding 3D structures. A key success factor in this process is the silane-based surface treatments to enable the patterning of the thin polymer film and final release from the mold to form 3D structures. We demonstrate various polymethyl methacrylate 3D structures with combinations of grating, circular-, and square-patterned molds. The duo-mold process is a po...

Imprinting of polymer at low temperature and pressure

We have developed a method to pattern polymeric materials, including nonthermoplastic polymers and biomaterials, at low temperature and low pressure. In this method, plasticizers are added to increase the chain mobility of the polymers, resulting in lower imprinting temperature and/or pressure. Three established imprinting and transfer techniques were chosen to demonstrate this method: conventional nanoimprint lithography (NIL), microcontact printing (μCP), and soft ink-pad (SIP). These three techniques were used to pattern poly(3,4-ethylenedioxythiophene) (PEDOT) and chitosan. PEDOT and chitosan were chosen because both of them are nonthermoplastic polymer and therefore cannot be easily patterned using conventional NIL. Imprinting of PEDOT and chitosan films from the poly(dimethylsiloxane) mold was achieved at a low pressure of 10kPa and 25°C by controlled addition of glycerol as a plasticizer using conventional NIL; well-defined arrays of 2μm wide, 185nm high PEDOT dots have also been demonstrated by μC...

Polymer inking as a micro- and nanopatterning technique

A polymer inking technique was developed to form micro- and nanopatterns on a substrate. In this process, a polymer thin film is spin coated on a patterned mold. After contacting the substrate at a suitable temperature and pressure, the polymer on the protruded surfaces of the mold is transferred to the substrate and a positive image of the mold is obtained. A selective surface treatment method has been developed to improve the edge smoothness of the inked pattern. During selective surface treatment, the protruded surfaces of the mold are first treated with a flat poly(dimethylsiloxane) stamp impregnated with a silane that has medium surface energy. The mold is then immersed into the solution of another silane with very low surface energy to treat the trenches of the mold. Because the surface energy of the sidewalls is lower than that on the protrusions, polymer dewetting from the sidewalls is promoted, which makes the polymer film discontinuous along the edges of patterns. Therefore, inked polymer patter...

Stability of functional polymers after plasticizer-assisted imprint lithography

Poly(3,4-ethylenedioxythiophene) (PEDOT) and chitosan structures on Si, patterned by plasticizer-assisted imprint lithography (PAIL), are examined under a variety of imprinting conditions. The stabilities of pattern dimension and chemical functionality of these polymers are presented. Thermal annealing for 5min at 80°C is found to be an effective method to stabilize imprinted PEDOT patterns. Biofunctionality in chitosan as a function of imprint temperature and pressure is examined through fluorescence spectroscopy. The accessibility of the amine group of chitosan is observed to decrease for imprint temperatures above 80°C, whereas the chemical functionality is not affected by pressure up to 1MPa. Fluorescence spectra of the chitosan are observed to be strong functions of exposure time to O2 plasma.

Stacked polymer patterns imprinted using a soft inkpad

A soft inkpad imprinting technique to produce stacked micrometer and submicrometer polymer patterns on substrates is presented. A thin soft inkpad is used to coat a polymer film onto the protrusions of a surface treated hard mold. The polymer film on the protrusions of the hard mold is then transferred to a substrate. Simultaneously, a negative pattern of the hard mold is formed on the soft inkpad that may also be transferred to a substrate. Numerical simulations are used to study the mechanisms of pattern transfer by soft inkpad imprinting. With the use of polymer blends, both positive and negative polymeric gratings with 700 nm period were produced. The soft inkpad allows multiple transfers of polymers with similar solubilities to the hard mold since no chemical solution is used for coating. High aspect ratio polymer stacks can be formed without alignment. This capability is an important advantage when forming submicrometer and nanometer multiple-layered polymer structures because current nanoimprint sy...

Novel low cost fabrication of microneedle arrays for drug delivery applications

This paper reports a process used for the microfabrication of an array of hollow microneedles. The purpose of the array is for painless transdermal drug delivery. The fabrication process uses wet bulk silicon technology and copper electroplating technology. First, a microneedle array mold on <100>-oriented silicon was fabricated by wet anisotropic etching using KOH solution, then the silicon mold was electroplated with copper. After which, the hollow copper microneedle array was released by a lift-off process or by etching off the silicon mold in KOH solution. The hollow copper microneedle array has been mounted on a polycarbonate platform, which consist of laser ablated cavities and channel for external connection to drug source. In consideration of the contour of human’s skin and the geometry of the microneedle tip, which has walls of sloping gradient corresponding to the (111)-planes, the height of the microneedle array is 200 μm. Two arrays of hollow copper microneedle were fabricated. They have square base of dimensions 390 μm and 400 μm and square tips of size 100 μm and 120 μm with square holes of size 88 μm and 94 μm respectively. Both arrays have microneedle tips at 1900 μm apart from one another and consist of 10 × 10 microneedle tips.

Pulmonary drug delivery using droplets generated by Rayleigh instability-driven breakup of filaments

For effective pulmonary drug delivery of insulin for example, drug particles must be in the range of 1 to 5 microns. A piezoelectrically actuated MEMS atomizer based on Rayleigh instability-driven breakup of filaments has been designed to produce drug particles in this range. Although the formation of droplets from jets have been used extensively in ink-jet printing, the currently presented mode of droplet formation has not yet been demonstrated by any MEMS device. Testing of the vaporiser reveals that the droplets generated lie primarily in the range of 1.0 through 3.0 microns, a range that covers the designed droplet size of 2.5 microns. We thus show that it is possible to implement this mode of droplet generation that will achieve better device efficiency.

Plasticizer-assisted polymer imprint and transfer

We have developed a new method to pattern polymeric materials, including non-thermoplastic polymers, at low temperature and low pressure. In this method, plasticizers are added to increase the chain mobility of the polymers, resulting in lower imprinting temperature and/or pressure. Two established imprinting and transfer techniques were chosen to demonstrate this method, namely, conventional nanoimprint lithography (NIL) and microcontact printing (μCP). These two techniques were used to pattern poly(3,4-ethylenedioxythiophene) (PEDOT). PEDOT was chosen because it is a non-thermoplastic polymer and therefore cannot be easily patterned using conventional NIL. Successful imprint of PEDOT films from the PDMS mold was achieved at a low pressure of 10 kPa and 25°C by controlled addition of glycerol as a plasticizer using conventional NIL; well-defined arrays of 2μm wide, 185 nm high PEDOT dots have also been demonstrated by μCP. In contrast, patterning of PEDOT film without plasticizer requires higher temperature (80°C) and pressure (10 MPa), which could cause severe deformation of the transferred patterns. This method of plasticizer-assisted imprint lithography (PAIL) broadens the applicapability of NIL to a wide range of polymeric materials.

Three-dimensional simulation of micropumps

A 3D model of one type of micro pumps was supposed and analyzed using finite element method (FEM). The pump had square shape cavity and was driven by a square shape PZT component. The finite element analysis (FEA) took into consideration of the effects of PZT component dimensions, membrane thickness, pump chamber pressure and other geometric parameters. Modal analyses were also conducted. Compression ratio of the pump chamber was taken as the prime parameter for the analyses. It was found that the membrane thickness and the PZT plate thickness played major roles in determining the compression ratio. For each membrane thickness, there was always an optimum PZT plate thickness that gave the maximum compression ratio. Curves showing the relationship between the optimum PZT plate thickness and the membrane thickness at different chamber pressures were given, based on the FEA results. A set of optimum pump design parameters was proposed.

Novel self-oscillating anemometer with capacitance-based sensing

In this paper, the feasibility of a self-oscillating anemometer is examined. A 2D numerical study of a novel self-oscillating anemometer that can be fabricated using micromachining techniques is performed. The device is essentially a square cylinder suspended in the fluid flow by a fixed beam. The flow velocity can be easily measured by determining the frequency of oscillation obtained from capacitance sensing. Anemometer with different length scales can be fabricated to enable different ranges of velocities to be measured.