The control of hydrodynamic focusing in a microchannel has inspired new approaches for microfluidic mixing, separations, sensors, cell analysis and microfabrication. Achieving a flat interface between the focusing and focused fluids is dependent on Reynolds number and device geometry, and many hydrodynamic focusing systems can benefit from this understanding. For applications where a specific cross-sectional shape is desired for the focused flow, advection generated by grooved structures in the channel walls can be used to define the shape of the focused flow. Relative flow rates of the focused flow and focusing streams can be manipulated to control the crosssectional area of the focused flows. This manuscript discusses the principles for defining the shape of the interface between the focused and focusing fluids and provides examples from our lab that use hydrodynamic focusing for impedance-based sensors, flow cytometry, and microfabrication to illustrate the breadth of opportunities for introducing new capabilities into microfluidic systems. We evaluate each example for the advantages and limitations integral to utilization of hydrodynamic focusing for that particular application.
Electroconductive hydrogels (ECH) prepared as blends of UV-cross-linked poly(hydroxyethylmethacrylate) [p(HEMA)]-based hydrogels and electropolymerized polypyrrole (PPy) were synthesized as coatings on microlithographically fabricated interdigitated microsensor electrodes (IMEs) and microdisc electrode arrays (MDEAs). Hydrogels were synthesized from tetraethyleneglycol diacrylate (TEGDA), hydroxyethylmethacrylate (HEMA), polyethyleneglycol monomethacrylate (PEGMA), N-[tris(hydroxymethyl)methyl]-acrylamide (HMMA), and 3-sulfopropyl methacrylate potassium salt (SPMA) to produce p(HEMA-co-PEGMA-co-HMMA-co-SPMA) hydrogels. The conductive polymer was synthesized from pyrrole and 4-(3'-pyrrolyl)butyric acid by electropolymerization within the electrode-supported hydrogel. ECH films produced with different electropolymerization charge densities were investigated using cyclic voltammetry, electrical impedance spectroscopy, differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). Polymer morphology was studied by SEM. The ECH demonstrated the desired characteristics of high electrical conductivity (low impedance), as well as high thermal stability compared to pure hydrogel. Signal enhancement was achieved by modifying the surface of an MDEA biotransducer with the ECH, with a 10-fold increase in the voltammetric current response associated with the ferrocene monocarboxylic acid (FcCO(2)H) redox reaction.
Our interest is in the development of engineered microdevices for continuous remote monitoring of intramuscular lactate, glucose, pH and temperature during post-traumatic hemorrhaging. Two important design considerations in the development of such devices for in vivo diagnostics are discussed; the utility of micro-disc electrode arrays (MDEAs) for electrochemical biosensing and the application of biomimetic, bioactive poly(HEMA)-based hydrogel composites for implant biocompatibility. A poly(HEMA)-based hydrogel membrane containing polyethylene glycol (PEG) was UV cross-linked with tetraethyleneglycol diacrylate following application to MDEAs (50 mum discs) and to 250 mum diameter gold electrodes within 8-well culture ware. Cyclic voltammetry (CV) of the MDEAs revealed a reduction in the apparent diffusion coefficient of ferrocenemonocarboxylic acid (FcCO(2)H), from 6.68 x 10(-5) to 6.74 x 10(-6) cm(2)/s for the uncoated and 6 mum thick hydrogel coated devices, respectively. Single frequency (4 kHz) temporal impedance measurements of the hydrogels in the 8-well culture ware showed a reversible 5% change in the absolute impedance of the hydrogels when exposed to a pH change between 6.1 to 7.2 and a 20% drop between pH 6.1 and 8.8.
Electroconductive hydrogels (ECHs) were prepared as blends of ultraviolet cross-linked poly(hydroxyethyl methacrylate) [poly(HEMA)]-based hydrogels and in situ electrochemically synthesized polypyrrole (PPy). ECH blends, with potential for neuronal prosthetic devices, implantable biosensors, and electro-stimulated release devices, were produced on surface functionalized microfabricated and planar gold electrodes. Hydrogels were synthesized from hydroxyethyl methacrylate (HEMA), poly(ethylene glycol) monomethacrylate (PEGMA), N-[tris(hydroxymethyl)methyl]-acrylamide (HMMA), and 3-sulfopropyl methacrylate potassium salt (SPMA) to produce p(HEMA-co-PEGMA-co-HMMA-co-SPMA). The electroconductive polymer component was electropolymerized from pyrrole and 4-(3 0 -pyrrolyl)butyric acid to form P(Py-co-PyBA) within the electrode-supported hydrogel. The dynamic electrochemical properties of Au*|Gel-P(Py-co-PyBA) were investigated using multiple scan rate cyclic voltammetry and electrical/electrochemical impedance spectroscopy (EIS) over the range 0.1-100 kHz and compared to Downloaded from there was a three-fold decrease in the magnitude of the absolute impedance, subsequent to electropolymerization. The in vitro biocompatibility and cytotoxicity of the polymer-modified gold surfaces were investigated using murine pheochromocytoma (PC12) cells and human muscle fibroblasts (RMS13). For Au*|Gel-P(Py-co-PyBA) polymer films prepared with different electropolymerization times of 5, 25, and 50 s, there was an increase in cell proliferation of 49%, 61%, and 6% compared to initial cell seeding. These ECH blends have the desired characteristics of low interfacial impedance and noncytotoxicity that makes them good candidates for in vivo intramuscular and neural studies.
A complete electrochemical cell-on-a-chip that uses the MicroDisc Electrode Array (MDEA) working electrode (WE) design was evaluated for eventual intramuscular implantation for the continuous amperometric monitoring of glucose and lactate in an animal trauma model. The microfabricated ECC MDEA5037 comprises two discrete electrochemical cells-on-a-chip (ECCs), each with a reference, counter, and MDEA working electrode. Each MDEA comprises 37 microdiscs (diameter = 50 microm) arranged in a Hexagonal Closed Packed (HCP) arrangement with a center to center distance (d) of 100 microm. Cyclic Voltammetry (CV) and Electrical Impendence Spectroscopy (EIS) reveals that this device scales in its interfacial properties with a corresponding MDEA 050 device that comprises 5,184 microdiscs. Parallel development of miniaturized mixed-signal integrated electronics for wireless reprogramming, data acquisition and communication addresses the key issues involved in developing measurement electronics, AD/DA processing, power management and telemetry for implantable amperometric biosensors. A generalized electronics platform based on the Texas Instruments TI NC01101 chip has been developed that may be readily applied to many types of biotransducers with minor modifications.
The ischemia-induced death of cardiomyocytes results in scar formation and reduced contractility of the ventricle. Several preclinical and clinical studies have supported the notion that cell therapy may be used for cardiac regeneration. Most attempts for cardiomyoplasty have considered the bone marrow as the source of the "repair stem cell(s)," assuming that the hematopoietic stem cell can do the work. However, bone marrow is also the residence of other progenitor cells, including mesenchymal stem cells (MSCs). Since 1995 it has been known that under in vitro conditions, MSCs differentiate into cells exhibiting features of cardiomyocytes. This pioneer work was followed by many preclinical studies that revealed that ex vivo expanded, bone marrow-derived MSCs may represent another option for cardiac regeneration. In this work, we review evidence and new prospects that support the use of MSCs in cardiomyoplasty.
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