Nano/micro particle-based functional composites and applications

In this thesis, we introduce particle-based conductive composites synthesized by mixing conductive nano/micrometer-sized particles with polydimethylsiloxane (PDMS) matrix. Experiments show that such composites exhibit excellent conductivity as well as good mechanical and thermal properties. By using micro molding and softlithographic techniques, these composites can be patterned into two-dimensional and three-dimensional conducting microstrucures that can be integrated into PDMS-based microchips for wide microfluidic functionalities. Based on such composites, a series of microfluidic devices have been designed and implemented, such as microheater, paperlike thermochromic display and micro thermoindicator. Furthermore, magnetically responsive elastic microspheres shaped into either core-shell or solid structure and consisting of PDMS and magnetic nano/micro-particles, are fabricated by a microfluidic flow-focusing device. Measured by a modified electronic balance, an appreciable magnetostrictive effect of the two types of spherical elastomers is exhibited. Owing to the smart controllability by an external magnetic field from magnetic particles as well as bio-compatibility and non-toxicity of PDMS, these microspherical elastomers may have potential application in drug delivery and release

Paperlike thermochromic display

The authors report the design and implementation of a paperlike, thermally activated display fabricated from thermochromic composite and embedded conductive wiring patterns, shaped from mixture of metallic nanoparticles in polydimethylsioxane using soft lithography. The display exhibits good image quality and ease of control. Use of electric heating pulses is shown to reduce energy consumption while improving image quality control. The display has excellent mechanical bending flexibility. (c) 2007 American Institute of Physics

Micro thermoindicators and optical-electronic temperature control for microfluidic applications

The authors report the design and implementation of sensing and control of local temperature in polydimethylsiloxane microfluidic reaction chip, based on the fabrication of a microtemperature sensor with thermochromic color bars and the associated optical and electronic feedback controls. The thermochromic color bar demonstrates easy and accurate local temperature monitoring. In combination with a microheater, this contactless microchip temperature control approach may have wide application potentials in microchemical and microbiological analyses. (c) 2007 American Institute of Physics

The synthesis and electrorheological properties of BaTiO3-coated PMMA microspheres

Coated-PMMA microspheres consisting of poly (methyl methacrylate)(PMMA) core and barium titanate (BaTiO3) shell were synthesized by the modified sol-gel processing and then adopted as an electrorheological (ER) materials. The structure and morphology of coated powders were characterized by SEM and FT-IR; the shear stress of the suspensions of coated-PMMA particles and pure PMMA particles in silicone oil with a 20 vol\% were investigated. The results show that the BaTiO3 coated PMMA microspheres based suspension in silicone oil exhibited typical ER behavior and stronger ER effects

Microheaters fabricated from a conducting composite

The authors report the fabrication of microheaters based on a conducting composite of silver microparticles embedded in polydimethylsioxane (PDMS). Experimental results show that the microheaters exhibit good performance in temperature rise and decay characteristics, with localized heating at targeted spatial domains. A unique feature of the microheater here is its excellent integration capability with biocompatible PDMS and other polymer materials, with potential microchip applications for bioprocessing and chemical reactions.<br/

Design and fabrication of magnetically functionalised core/shell microspheres for smart drug delivery

The fabrication of magnetically functionalized core/shell microspheres by using the microfluidic flow-focusing (MFF) approach is reported. The shell of each microsphere is embedded with magnetic nanoparticles, thereby enabling the microspheres to deform under an applied magnetic field. By encapsulating a drug, for example, aspirin, inside the microspheres, the drug release of the microspheres is enhanced under the compression–extension oscillations that are induced by an AC magnetic field. This active pumping mode of drug release can be controlled by varying the frequency and magnitude of the applied magnetic field as well as the time profile of the magnetic field. UV absorption measurements of cumulative aspirin release are carried out to determine the influence of these factors. The drug release behavior is found to be significantly different depending on whether the applied field varies sinusoidally or in a step-function manner with time

Wettability of urea-doped TiO2 nanoparticles and their high electrorheological effects

Aimed at the increasement of ER effects, a novel composite, urea doped-TiO2 particles (TU) were prepared by using a modified sol-gel method. The structure and morphology of the TU particles were observed and analyzed by scanning electron micrpscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry(FT-IR) and X-ray photoelectron spectrum (XPS). The dielectric properties of the TU particles and the ER effects based on the TU particles were investigated. The influence of wettability on the ER performance between the particles and silicone oil was examined

Real-time detection, control, and sorting of microfluidic droplets

We report the design and implementation of capacitive detection and control of microfluidic droplets in microfluidic devices. Integrated microfluidic chip(s) with detection/control circuit enables us to monitor in situ the individual volume of droplets, ranging from nanoliter to picoliter, velocity and even composition, with an operation frequency of several kilohertz. Through electronic feedback, we are able to easily count, sort, and direct the microfluidic droplets. Potential applications of this approach can be employed in the areas of biomicrofluidic processing, microchemical reactions as well as digital microfluidics.<br/

Magnetically responsive elastic microspheres

We report the design, fabrication, and characterization of magnetically responsive elastic microspheres consisting of polydimethylsioxane (PDMS) and magnetic nano-/microparticles. The microspheres can have either core-shell or solid structure, fabricated by using a microfluidic technique. The mechanical characteristics are determined with a modified electronic balance, and the results show that the microspheres exhibit magnetostrictive effect. Such microspheres can in addition behave as a smart material controllable through an external magnetic field. Owing to the transparency, biocompatibility and nontoxicity of PDMS, the magnetically responsive elastic microspheres may have potential applications in drug delivery, biosensing, bioseparation, and medical diagnosis