Hongkai Wu - Academia.edu (original) (raw)
Papers by Hongkai Wu
Analytical Chemistry, 2002
This paper describes a simple and convenient method for the rapid screening of potential inhibito... more This paper describes a simple and convenient method for the rapid screening of potential inhibitors of bacterial adhesion and for the quantitative evaluation of the efficacy of the inhibitors using arrays of self-assembled monolayers (SAMs) of alkanethiolates on gold that are presented on a 96-well microtiter plate. The SAMs present mixtures of r-D-mannopyranoside (a ligand that promotes the adhesion of uropathogenic Escherichia coli by binding to the FimH proteins on the tip of type 1 pili), and tri(ethylene glycol) moieties (organic groups that resist nonspecific adsorption of proteins and cells). The SAMs provide surfaces for studies of adhesion of uropathogenic E. coli to specific ligands; they also provide excellent resistance to nonspecific adhesion. Using arrays of mannoside-presenting SAMs, inhibitors of bacterial adhesion were easily screened by observing the number of bacteria that adhered to the surface of the SAMs in the presence of inhibitor. The potency of the inhibitor was quantified by measuring the percentage of inhibition as a function of the concentration of the inhibitor. The properties of SAMs, when combined with the convenience and standardization of a microtiter plate, make arrays of SAMs a versatile tool that can be applied to high-throughput screening of inhibitors of bacterial, viral, and mammalian cell adhesion and of strongly binding ligands for proteins.
Journal of Physical Chemistry B, 2001
Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in t... more Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in thin silver films. Electroplating silver onto these patterns increased the structural integrity of the metal layer. Separating the metal film from the substrate resulted in free-standing, 2D structures. Folding of these structures along predesigned "hinges" produced three-dimensional (3D) objects. Additional electrodeposition of nickel welded hinges into position, strengthened the structure, and joined separate pieces. By printing onto cylindrical surfaces, it was possible to generate complex shapes efficiently and to minimize joining steps.
Electrophoresis, 2000
Microfluidic devices are finding increasing application as analytical systems, biomedical devices... more Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.
Lab on A Chip, 2005
A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is tran... more A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 uC for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 mm, a y200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.
Analytical Chemistry, 2002
A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this meth... more A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this method, separation in the first dimension is carried out in a 1D capillary, with this system physically isolated from the capillaries that provide the separation in the second dimension. After completion of the first separation, the 1D channel is physically connected to the 2D capillaries, and a second separation is carried out in an orthogonal set of parallel capillaries. The ability of poly(dimethylsiloxane) (PDMS) to support the fabrication of 3D microfluidic systems makes it possible to produce membranes that both enclose the gel used in the first separation in a capillary and provide passages for the proteins to migrate into the array of orthogonal capillaries. The elastomeric nature of PDMS makes it possible to make reversible connections between pieces of PDMS. The feasibility of this system is demonstrated using a protein mixture containing fluorescein-conjugated carbonic anhydrase, fluorescein-conjugated BSA, and Texas Red-conjugated ovalbumin. This work suggests one type of design that might form the basis for a microfabricated device for 2D capillary electrophoresis.
Analytical Chemistry, 2000
Langmuir, 2002
This paper reports a simple and versatile technique for generating structures on the surfaces of ... more This paper reports a simple and versatile technique for generating structures on the surfaces of poly-(dimethylsiloxane) (PDMS), approximately sinusoidal waves with periods between 0.1 and 10 µm, and the use of these structures to study cell contact guidance. 1 The features are generated by stretching PDMS slabs mechanically, oxidizing them in an oxygen plasma, and allowing them to relax. These surface features are similar to photolithographically fabricated grooves that have traditionally been used to investigate cell contact guidance, although their edges are rounded rather than angular. Bovine capillary endothelial cells align and elongate on these features. The morphology and cytoskeletal structure of the aligned cells are similar to those of cells described in previous studies of contact guidance on surfaces with other types of topography. These observations and comparisons indicate that sharp edges in the features defining the grooves are not essential in eliciting contact guidance. This technique provides a method for fabricating microfeatures for the studies of the interactions between cells and their environment that does not require a cleanroom or access to photolithographic tools.
Electrophoresis, 2000
Microfluidic devices are finding increasing application as analytical systems, biomedical devices... more Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.
Analytical Chemistry, 2002
This paper describes the application of reduction photolithography, using arrays of microlenses a... more This paper describes the application of reduction photolithography, using arrays of microlenses and gray scale masks, to generate arrays of micropatterns having multilevel and curved features in photoresist. This technique can fabricate, in a single exposure, three-dimensional microstructures (e.g., nonspherical microlens arrays) over areas of ∼2 × 2 cm 2 . The simple optical configuration consisted of transparency film (having centimeter-sized features) as gray scale photomasks, an overhead projector as the illumination source, and arrays of microlenses as the size-reducing elements. Arrays of 40-and 100-µm lenses achieved a lateral size reduction of ∼10 3 and generated patterns of well-defined, multilevel structures; these structures may find use in applications such as diffractive optics.
Analytical Chemistry, 2002
This paper describes a simple and convenient method for the rapid screening of potential inhibito... more This paper describes a simple and convenient method for the rapid screening of potential inhibitors of bacterial adhesion and for the quantitative evaluation of the efficacy of the inhibitors using arrays of self-assembled monolayers (SAMs) of alkanethiolates on gold that are presented on a 96-well microtiter plate. The SAMs present mixtures of r-D-mannopyranoside (a ligand that promotes the adhesion of uropathogenic Escherichia coli by binding to the FimH proteins on the tip of type 1 pili), and tri(ethylene glycol) moieties (organic groups that resist nonspecific adsorption of proteins and cells). The SAMs provide surfaces for studies of adhesion of uropathogenic E. coli to specific ligands; they also provide excellent resistance to nonspecific adhesion. Using arrays of mannoside-presenting SAMs, inhibitors of bacterial adhesion were easily screened by observing the number of bacteria that adhered to the surface of the SAMs in the presence of inhibitor. The potency of the inhibitor was quantified by measuring the percentage of inhibition as a function of the concentration of the inhibitor. The properties of SAMs, when combined with the convenience and standardization of a microtiter plate, make arrays of SAMs a versatile tool that can be applied to high-throughput screening of inhibitors of bacterial, viral, and mammalian cell adhesion and of strongly binding ligands for proteins.
Analytical Chemistry, 2003
This paper extends rapid prototyping for several types of lithography to the 8-25-µm size range, ... more This paper extends rapid prototyping for several types of lithography to the 8-25-µm size range, using transparency photomasks prepared by photoplotting. It discusses the technical improvement in photomask quality achieved by photoplotting, compared to the currently used image setting, and demonstrates differences in the resolution that can be obtained with photomasks with features in the 8-100-µm size range. These high-resolution photomasks were used to microfabricate microelectrodes, microlenses, and stamps for microcontact printing, following methods described previously.
Journal of Physical Chemistry B, 2001
Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in t... more Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in thin silver films. Electroplating silver onto these patterns increased the structural integrity of the metal layer. Separating the metal film from the substrate resulted in free-standing, 2D structures. Folding of these structures along predesigned "hinges" produced three-dimensional (3D) objects. Additional electrodeposition of nickel welded hinges into position, strengthened the structure, and joined separate pieces. By printing onto cylindrical surfaces, it was possible to generate complex shapes efficiently and to minimize joining steps.
Analytical Chemistry, 2000
This paper describes a practical method for the fabrication of photomasks, masters, and stamps/mo... more This paper describes a practical method for the fabrication of photomasks, masters, and stamps/molds used in soft lithography that minimizes the need for specialized equipment. In this method, CAD files are first printed onto paper using an office printer with resolution of 600 dots/ in. Photographic reduction of these printed patterns transfers the images onto 35-mm film or microfiche. These photographic films can be used, after development, as photomasks in 1:1 contact photolithography. With the resulting photoresist masters, it is straightforward to fabricate poly(dimethylsiloxane) (PDMS) stamps/molds for soft lithography. This process can generate microstructures as small as 15 µm; the overall time to go from CAD file to PDMS stamp is 4-24 h. Although access to equipmentsspin coater and ultraviolet exposure tools normally found in the clean room is still required, the cost of the photomask itself is small, and the time required to go from concept to device is short. A comparison between this method and all other methods that generate film-type photomasks has been performed using test patterns of lines, squares, and circles. Three microstructures have also been fabricated to demonstrate the utility of this method in practical applications.
Journal of The American Chemical Society, 2002
This paper describes an approach to the fabrication of three-dimensional (3-D) structures of mill... more This paper describes an approach to the fabrication of three-dimensional (3-D) structures of millimeter-scale spherical beads having a range of latticesstetragonal, cubic, and hexagonalsusing hierarchical self-assembly. The process has five steps: (i) metal-coated beads are packed in a rod-shaped cavity in an elastomeric polymer (poly(dimethylsiloxane), PDMS); (ii) the beads are embedded in a second polymer (PDMS or polyurethane, PU) using a procedure that leaves the parts of the beads in contact with the PDMS exposed; (iii) the exposed areas of the beads are coated with a solder having a low melting point; (iv) the polymer rodsswith embedded beads and exposed solder dropssare suspended in an approximately isodense medium (an aqueous solution of KBr) and allowed to self-assemble by capillary interactions between the drops of molten solder; and (v) the assembly is finished by several procedures, including removing the beads from the polymer matrix by dissolution, filling the voids left with another material, and dissolving the matrix. The confinement of the beads in regular structures in polymer rods makes it possible to generate self-assembled structures with a variety of 3-D lattices; the type of the lattice formed can be controlled by varying the size of the beads, and the size and shape of the cross-section of the rods.
Lab on A Chip, 2005
A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is tran... more A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 uC for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 mm, a y200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.
Analytical Chemistry, 2002
A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this meth... more A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this method, separation in the first dimension is carried out in a 1D capillary, with this system physically isolated from the capillaries that provide the separation in the second dimension. After completion of the first separation, the 1D channel is physically connected to the 2D capillaries, and a second separation is carried out in an orthogonal set of parallel capillaries. The ability of poly(dimethylsiloxane) (PDMS) to support the fabrication of 3D microfluidic systems makes it possible to produce membranes that both enclose the gel used in the first separation in a capillary and provide passages for the proteins to migrate into the array of orthogonal capillaries. The elastomeric nature of PDMS makes it possible to make reversible connections between pieces of PDMS. The feasibility of this system is demonstrated using a protein mixture containing fluorescein-conjugated carbonic anhydrase, fluorescein-conjugated BSA, and Texas Red-conjugated ovalbumin. This work suggests one type of design that might form the basis for a microfabricated device for 2D capillary electrophoresis.
Analytical Chemistry, 2002
This paper describes a simple and convenient method for the rapid screening of potential inhibito... more This paper describes a simple and convenient method for the rapid screening of potential inhibitors of bacterial adhesion and for the quantitative evaluation of the efficacy of the inhibitors using arrays of self-assembled monolayers (SAMs) of alkanethiolates on gold that are presented on a 96-well microtiter plate. The SAMs present mixtures of r-D-mannopyranoside (a ligand that promotes the adhesion of uropathogenic Escherichia coli by binding to the FimH proteins on the tip of type 1 pili), and tri(ethylene glycol) moieties (organic groups that resist nonspecific adsorption of proteins and cells). The SAMs provide surfaces for studies of adhesion of uropathogenic E. coli to specific ligands; they also provide excellent resistance to nonspecific adhesion. Using arrays of mannoside-presenting SAMs, inhibitors of bacterial adhesion were easily screened by observing the number of bacteria that adhered to the surface of the SAMs in the presence of inhibitor. The potency of the inhibitor was quantified by measuring the percentage of inhibition as a function of the concentration of the inhibitor. The properties of SAMs, when combined with the convenience and standardization of a microtiter plate, make arrays of SAMs a versatile tool that can be applied to high-throughput screening of inhibitors of bacterial, viral, and mammalian cell adhesion and of strongly binding ligands for proteins.
Journal of Physical Chemistry B, 2001
Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in t... more Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in thin silver films. Electroplating silver onto these patterns increased the structural integrity of the metal layer. Separating the metal film from the substrate resulted in free-standing, 2D structures. Folding of these structures along predesigned "hinges" produced three-dimensional (3D) objects. Additional electrodeposition of nickel welded hinges into position, strengthened the structure, and joined separate pieces. By printing onto cylindrical surfaces, it was possible to generate complex shapes efficiently and to minimize joining steps.
Electrophoresis, 2000
Microfluidic devices are finding increasing application as analytical systems, biomedical devices... more Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.
Lab on A Chip, 2005
A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is tran... more A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 uC for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 mm, a y200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.
Analytical Chemistry, 2002
A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this meth... more A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this method, separation in the first dimension is carried out in a 1D capillary, with this system physically isolated from the capillaries that provide the separation in the second dimension. After completion of the first separation, the 1D channel is physically connected to the 2D capillaries, and a second separation is carried out in an orthogonal set of parallel capillaries. The ability of poly(dimethylsiloxane) (PDMS) to support the fabrication of 3D microfluidic systems makes it possible to produce membranes that both enclose the gel used in the first separation in a capillary and provide passages for the proteins to migrate into the array of orthogonal capillaries. The elastomeric nature of PDMS makes it possible to make reversible connections between pieces of PDMS. The feasibility of this system is demonstrated using a protein mixture containing fluorescein-conjugated carbonic anhydrase, fluorescein-conjugated BSA, and Texas Red-conjugated ovalbumin. This work suggests one type of design that might form the basis for a microfabricated device for 2D capillary electrophoresis.
Analytical Chemistry, 2000
Langmuir, 2002
This paper reports a simple and versatile technique for generating structures on the surfaces of ... more This paper reports a simple and versatile technique for generating structures on the surfaces of poly-(dimethylsiloxane) (PDMS), approximately sinusoidal waves with periods between 0.1 and 10 µm, and the use of these structures to study cell contact guidance. 1 The features are generated by stretching PDMS slabs mechanically, oxidizing them in an oxygen plasma, and allowing them to relax. These surface features are similar to photolithographically fabricated grooves that have traditionally been used to investigate cell contact guidance, although their edges are rounded rather than angular. Bovine capillary endothelial cells align and elongate on these features. The morphology and cytoskeletal structure of the aligned cells are similar to those of cells described in previous studies of contact guidance on surfaces with other types of topography. These observations and comparisons indicate that sharp edges in the features defining the grooves are not essential in eliciting contact guidance. This technique provides a method for fabricating microfeatures for the studies of the interactions between cells and their environment that does not require a cleanroom or access to photolithographic tools.
Electrophoresis, 2000
Microfluidic devices are finding increasing application as analytical systems, biomedical devices... more Microfluidic devices are finding increasing application as analytical systems, biomedical devices, tools for chemistry and biochemistry, and systems for fundamental research. Conventional methods of fabricating microfluidic devices have centered on etching in glass and silicon. Fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS) by soft lithography provides faster, less expensive routes than these conventional methods to devices that handle aqueous solutions. These soft-lithographic methods are based on rapid prototyping and replica molding and are more accessible to chemists and biologists working under benchtop conditions than are the microelectronics-derived methods because, in soft lithography, devices do not need to be fabricated in a cleanroom. This paper describes devices fabricated in PDMS for separations, patterning of biological and nonbiological material, and components for integrated systems.
Analytical Chemistry, 2002
This paper describes the application of reduction photolithography, using arrays of microlenses a... more This paper describes the application of reduction photolithography, using arrays of microlenses and gray scale masks, to generate arrays of micropatterns having multilevel and curved features in photoresist. This technique can fabricate, in a single exposure, three-dimensional microstructures (e.g., nonspherical microlens arrays) over areas of ∼2 × 2 cm 2 . The simple optical configuration consisted of transparency film (having centimeter-sized features) as gray scale photomasks, an overhead projector as the illumination source, and arrays of microlenses as the size-reducing elements. Arrays of 40-and 100-µm lenses achieved a lateral size reduction of ∼10 3 and generated patterns of well-defined, multilevel structures; these structures may find use in applications such as diffractive optics.
Analytical Chemistry, 2002
This paper describes a simple and convenient method for the rapid screening of potential inhibito... more This paper describes a simple and convenient method for the rapid screening of potential inhibitors of bacterial adhesion and for the quantitative evaluation of the efficacy of the inhibitors using arrays of self-assembled monolayers (SAMs) of alkanethiolates on gold that are presented on a 96-well microtiter plate. The SAMs present mixtures of r-D-mannopyranoside (a ligand that promotes the adhesion of uropathogenic Escherichia coli by binding to the FimH proteins on the tip of type 1 pili), and tri(ethylene glycol) moieties (organic groups that resist nonspecific adsorption of proteins and cells). The SAMs provide surfaces for studies of adhesion of uropathogenic E. coli to specific ligands; they also provide excellent resistance to nonspecific adhesion. Using arrays of mannoside-presenting SAMs, inhibitors of bacterial adhesion were easily screened by observing the number of bacteria that adhered to the surface of the SAMs in the presence of inhibitor. The potency of the inhibitor was quantified by measuring the percentage of inhibition as a function of the concentration of the inhibitor. The properties of SAMs, when combined with the convenience and standardization of a microtiter plate, make arrays of SAMs a versatile tool that can be applied to high-throughput screening of inhibitors of bacterial, viral, and mammalian cell adhesion and of strongly binding ligands for proteins.
Analytical Chemistry, 2003
This paper extends rapid prototyping for several types of lithography to the 8-25-µm size range, ... more This paper extends rapid prototyping for several types of lithography to the 8-25-µm size range, using transparency photomasks prepared by photoplotting. It discusses the technical improvement in photomask quality achieved by photoplotting, compared to the currently used image setting, and demonstrates differences in the resolution that can be obtained with photomasks with features in the 8-100-µm size range. These high-resolution photomasks were used to microfabricate microelectrodes, microlenses, and stamps for microcontact printing, following methods described previously.
Journal of Physical Chemistry B, 2001
Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in t... more Microcontact printing (µCP) and wet chemical etching generated two-dimensional (2D) patterns in thin silver films. Electroplating silver onto these patterns increased the structural integrity of the metal layer. Separating the metal film from the substrate resulted in free-standing, 2D structures. Folding of these structures along predesigned "hinges" produced three-dimensional (3D) objects. Additional electrodeposition of nickel welded hinges into position, strengthened the structure, and joined separate pieces. By printing onto cylindrical surfaces, it was possible to generate complex shapes efficiently and to minimize joining steps.
Analytical Chemistry, 2000
This paper describes a practical method for the fabrication of photomasks, masters, and stamps/mo... more This paper describes a practical method for the fabrication of photomasks, masters, and stamps/molds used in soft lithography that minimizes the need for specialized equipment. In this method, CAD files are first printed onto paper using an office printer with resolution of 600 dots/ in. Photographic reduction of these printed patterns transfers the images onto 35-mm film or microfiche. These photographic films can be used, after development, as photomasks in 1:1 contact photolithography. With the resulting photoresist masters, it is straightforward to fabricate poly(dimethylsiloxane) (PDMS) stamps/molds for soft lithography. This process can generate microstructures as small as 15 µm; the overall time to go from CAD file to PDMS stamp is 4-24 h. Although access to equipmentsspin coater and ultraviolet exposure tools normally found in the clean room is still required, the cost of the photomask itself is small, and the time required to go from concept to device is short. A comparison between this method and all other methods that generate film-type photomasks has been performed using test patterns of lines, squares, and circles. Three microstructures have also been fabricated to demonstrate the utility of this method in practical applications.
Journal of The American Chemical Society, 2002
This paper describes an approach to the fabrication of three-dimensional (3-D) structures of mill... more This paper describes an approach to the fabrication of three-dimensional (3-D) structures of millimeter-scale spherical beads having a range of latticesstetragonal, cubic, and hexagonalsusing hierarchical self-assembly. The process has five steps: (i) metal-coated beads are packed in a rod-shaped cavity in an elastomeric polymer (poly(dimethylsiloxane), PDMS); (ii) the beads are embedded in a second polymer (PDMS or polyurethane, PU) using a procedure that leaves the parts of the beads in contact with the PDMS exposed; (iii) the exposed areas of the beads are coated with a solder having a low melting point; (iv) the polymer rodsswith embedded beads and exposed solder dropssare suspended in an approximately isodense medium (an aqueous solution of KBr) and allowed to self-assemble by capillary interactions between the drops of molten solder; and (v) the assembly is finished by several procedures, including removing the beads from the polymer matrix by dissolution, filling the voids left with another material, and dissolving the matrix. The confinement of the beads in regular structures in polymer rods makes it possible to generate self-assembled structures with a variety of 3-D lattices; the type of the lattice formed can be controlled by varying the size of the beads, and the size and shape of the cross-section of the rods.
Lab on A Chip, 2005
A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is tran... more A thin layer of polydimethylsiloxane (PDMS) prepolymer, which is coated on a glass slide, is transferred onto the embossed area surfaces of a patterned substrate. This coated substrate is brought into contact with a flat plate, and the two structures are permanently bonded to form a sealed fluidic system by thermocuring (60 uC for 30 min) the prepolymer. The PDMS exists only at the contact area of the two surfaces with a negligible portion exposed to the microfluidic channel. This method is demonstrated by bonding microfluidic channels of two representative soft materials (PDMS substrate on a PDMS plate), and two representative hard materials (glass substrate on a glass plate). The effects of the adhesive layer on the electroosmotic flow (EOF) in glass channels are calculated and compared with the experimental results of a CE separation. For a channel with a size of approximately 10 to 500 mm, a y200-500 nm thick adhesive layer creates a bond without voids or excess material and has little effect on the EOF rate. The major advantages of this bonding method are its generality and its ease of use.
Analytical Chemistry, 2002
A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this meth... more A method for carrying out 2D gel electrophoresis in a capillary format is presented. In this method, separation in the first dimension is carried out in a 1D capillary, with this system physically isolated from the capillaries that provide the separation in the second dimension. After completion of the first separation, the 1D channel is physically connected to the 2D capillaries, and a second separation is carried out in an orthogonal set of parallel capillaries. The ability of poly(dimethylsiloxane) (PDMS) to support the fabrication of 3D microfluidic systems makes it possible to produce membranes that both enclose the gel used in the first separation in a capillary and provide passages for the proteins to migrate into the array of orthogonal capillaries. The elastomeric nature of PDMS makes it possible to make reversible connections between pieces of PDMS. The feasibility of this system is demonstrated using a protein mixture containing fluorescein-conjugated carbonic anhydrase, fluorescein-conjugated BSA, and Texas Red-conjugated ovalbumin. This work suggests one type of design that might form the basis for a microfabricated device for 2D capillary electrophoresis.