Chih-Chen Hsieh

Vapor-based tri-functional coatings†

The tri-functional coating synthesized via CVD copolymerization is comprised of distinguished anchoring sites of acetylene, maleimide, and ketone that can synergically undergo specific conjugation reactions to render surfaces with distinct biological functions, simultaneously. In addition, these tri-functional coatings can be fabricated in a micro-structured fashion on non-conventional surfaces.

Vapor-Deposited Parylene Photoresist: A Multipotent Approach toward Chemically and Topographically Defined Biointerfaces

Poly(4-benzoyl-p-xylylene-co-p-xylylene), a biologically compatible photoreactive polymer belonging to the parylene family, can be deposited using a chemical vapor deposition (CVD) polymerization process on a wide range of substrates. This study discovered that the solvent stability of poly(4-benzoyl-p-xylylene-co-p-xylylene) in acetone is significantly increased when exposed to approximately 365 nm of UV irradiation, because of the cross-linking of benzophenone side chains with adjacent molecules. This discovery makes the photodefinable polymer a powerful tool for use as a negative photoresist for surface microstructuring and biointerface engineering purposes. The polymer is extensively characterized using infrared reflection adsorption spectroscopy (IRRAS), scanning electron microscopy (SEM), and imaging ellipsometry. Furthermore, the vapor-based polymer coating process provides access to substrates with unconventional and complex three-dimensional (3D) geometries, as compared to conventional spin-coated resists that are limited to flat 2D assemblies. Moreover, this photoresist technology is seamlessly integrated with other functionalized parylenes including aldehyde-, acetylene-, and amine-functionalized parylenes to create unique surface microstructures that are chemically and topographically defined. The photopatterning and immobilization protocols described in this paper represent an approach that avoids contact between harmful substances (such as solvents and irradiations) and sensitive biomolecules. Finally, multiple biomolecules on planar substrates, as well as on unconventional 3D substrates (e.g., stents), are presented.

A facile approach toward protein-resistant biointerfaces based on photodefinable poly-p-xylylene coating.

A facile and versatile tool is reported that uses a photodefinable polymer, poly(4-benzoyl-p-xylylene-co-p-xylylene) to immobilize antifouling materials, such as poly(ethylene glycol), poly(ethylene glycol) methyl ether methacrylate, dextran, and ethanolamine. This immobilization process requires the polymers photoactivated carbonyl groups, which can facilitate light-induced molecular crosslinking and can rapidly react via insertion into CH or NH bonds upon photo-illumination at 365 nm. Importantly, the process does not require additional functional groups on the antifouling materials. The immobilized fouling materials were characterized using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS), and the resulting antifouling properties were examined through protein adsorption studies on fibrinogen and bovine serum albumin at surfaces that were spatially modified using a photomask during the photochemical process. In addition, the adsorbed fibrinogen was quantitatively analyzed using a quartz crystal microbalance (QCM), and the adsorption values were reduced to 32.8 ± 4.9 ng cm(-2), 5.5 ± 3.9 ng cm(-2), 21.4 ± 4.5 ng cm(-2), and 16.9 ± 3.4 ng cm(-2) for poly(ethylene glycol) (PEG), poly(ethylene glycol) methyl ether methacrylate (PEGMA), dextran, and ethanolamine, respectively. Finally, this antifouling modification technology was demonstrated on an unconventional substrate for a stent that was modified by PEGMA at selected areas using a microscopic patterning technique during photoimmobilization. Low levels of fibrinogen and BSA adsorption were also observed at the areas where PEGMA was attached.

Simulation guided design of a microfluidic device for electrophoretic stretching of DNA.

We have used Brownian dynamics-finite element method (BD-FEM) to guide the optimization of a microfluidic device designed to stretch DNA for gene mapping. The original design was proposed in our previous study [C. C. Hsieh and T. H. Lin, Biomicrofluidics 5(4), 044106 (2011)] for demonstrating a new pre-conditioning strategy to facilitate DNA stretching through a microcontraction using electrophoresis. In this study, we examine the efficiency of the original device for stretching DNA with different sizes ranging from 48.5 kbp (λ-DNA) to 166 kbp (T4-DNA). The efficiency of the device is found to deteriorate with increasing DNA molecular weight. The cause of the efficiency loss is determined by BD-FEM, and a modified design is proposed by drawing an analogy between an electric field and a potential flow. The modified device does not only regain the efficiency for stretching large DNA but also outperforms the original device for stretching small DNA.

Stretching DNA by electric field and flow field in microfluidic devices: An experimental validation to the devices designed with computer simulations.

We examined the performance of three microfluidic devices for stretching DNA. The first device is a microchannel with a contraction, and the remaining two are the modifications to the first. The modified designs were made with the help of computer simulations [C. C. Hsieh and T. H. Lin, Biomicrofluidics 5(4), 044106 (2011) and C. C. Hsieh, T. H. Lin, and C. D. Huang, Biomicrofluidics 6, 044105 (2012)] and they were optimized for operating with electric field. In our experiments, we first used DC electric field to stretch DNA. However, the experimental results were not even in qualitative agreement with our simulations. More detailed investigation revealed that DNA molecules adopt a globular conformation in high DC field and therefore become more difficult to stretch. Owing to the similarity between flow field and electric field, we turned to use flow field to stretch DNA with the same devices. The evolution patterns of DNA conformation in flow field were found qualitatively the same as our prediction based on electric field. We analyzed the maximum values, the evolution and the distributions of DNA extension at different Deborah number in each device. We found that the shear and the hydrodynamic interaction have significant influence on the performance of the devices.

Simulations of DNA stretching by flow field in microchannels with complex geometry

Recently, we have reported the experimental results of DNA stretching by flow field in three microchannels (C. H. Lee and C. C. Hsieh, Biomicrofluidics 7(1), 014109 (2013)) designed specifically for the purpose of preconditioning DNA conformation for easier stretching. The experimental results do not only demonstrate the superiority of the new devices but also provides detailed observation of DNA behavior in complex flow field that was not available before. In this study, we use Brownian dynamics-finite element method (BD-FEM) to simulate DNA behavior in these microchannels, and compare the results against the experiments. Although the hydrodynamic interaction (HI) between DNA segments and between DNA and the device boundaries was not included in the simulations, the simulation results are in fairly good agreement with the experimental data from either the aspect of the single molecule behavior or from the aspect of ensemble averaged properties. The discrepancy between the simulation and the experimental results can be explained by the neglect of HI effect in the simulations. Considering the huge savings on the computational cost from neglecting HI, we conclude that BD-FEM can be used as an efficient and economic designing tool for developing new microfluidic device for DNA manipulation.

Bihydrogel particles as free-standing mechanical pH microsensors

We propose a bihydrogel mechanical microsensor made from two hydrogels with different sensitivity to the environment. The idea was used to create pH microsensors consisting of two parallel phases of crosslinked hydrogel in which only one phase is pH-sensitive. Working similarly to a bimetallic strip, the bihydrogel particle bends depending on the environmental pH. These freestanding bihydrogel particles are low-cost and easy to manufacture. They are particularly suitable for use in confined environment where most microsensors are unable to fit in. The same principle can also be used to create sensors for temperature, ionic strength, and other environmental parameters.