Choosing one from the many: selection and sorting strategies for single adherent cells.
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Animals↗
Biomedical subjects
Mark Bachman
Publications and source records attributed to Mark Bachman.
Micropatterning of living cells on a heterogeneously wetted surface.
We report a simple approach to fabricate heterogeneously wetted surfaces which can be used to pattern living cells or biomolecules. An array of pedestals composed of SU-8 was fabricated on a glass surface which was then derivatized with a hydrophobic silane. Upon addition of aqueous solutions to the array, air was trapped within the hydrophobic cavities between the pedestals. The trapped air formed a "virtual wall" blocking access to these cavities. Cells cultured on the array were forced to grow only on the tops of the pedestals, i.e., the surfaces wetted by aqueous media. The virtual walls were stable during manipulation of the array and over long time periods (months).
Air↗
Simple photografting method to chemically modify and micropattern the surface of SU-8 photoresist.
SU-8 has gained widespread acceptance as a negative photoresist. It is also finding increasing use as a structural material in microanalytical devices. Consequently, methods to tailor the surface properties of SU-8 as well as to micropattern coatings on the surface of SU-8 are needed. The SU-8 photoresist consists of EPON SU-8 resin mixed with the photoacid generator triarylsulfonium hexafluoroantimonate. This photoacid generator can also serve as a photoinitiator generating free radicals when illuminated with UV light. Under the appropriate conditions, sufficient triarylsulfonium hexafluoroantimonate remains within cured SU-8 to act as a source of free radicals and initiate UV-mediated grafting of polymers onto the surface of the SU-8. UV-mediated grafting was used to coat SU-8 surfaces with poly(acrylic acid) and other water-soluble monomers. The SU-8 surface was chemically micropatterned by placing a mask between the UV light and SU-8. The X-Y spatial resolution of micropatterned poly(acrylic acid) on the SU-8 surface was 2 mum. Three applications of these chemically modified SU-8 surfaces were demonstrated. In the first, poly(ethylene glycol) was used to protect the SU-8 from interactions with proteins, yielding a surface resistant to biofouling. In the second demonstration, the SU-8 surface was micropatterned with a cell-resistant layer to guide cellular attachment and growth. In the final application, SU-8 micropallets were encoded with polymer lines. The bar codes were read by either absorbance or fluorescence measurements. Thus, UV-mediated graft polymerization is an efficient and effective method to micropattern coatings onto the surface of SU-8.
Animals↗
Micromechanical resonator array for an implantable bionic ear.
In this paper we report on a multi-resonant transducer that may be used to replace a traditional speech processor in cochlear implant applications. The transducer, made from an array of micro-machined polymer resonators, is capable of passively splitting sound into its frequency sub-bands without the need for analog-to-digital conversion and subsequent digital processing. Since all bands are mechanically filtered in parallel, there is low latency in the output signals. The simplicity of the device, high channel capability, low power requirements, and small form factor (less than 1 cm) make it a good candidate for a completely implantable bionic ear device.
Algorithms↗
Covalent micropatterning of poly(dimethylsiloxane) by photografting through a mask.
A new photografting method to micropattern a covalent surface modification on poly(dimethylsiloxane) (PDMS) provides advantages in simplicity and efficiency. To accomplish the entire process on the benchtop, the PDMS was initially treated with benzophenone dissolved in a water/acetone mixture. This process permitted limited diffusion of the photoinitiator into the PDMS surface. Polymerization of acrylic acid was initiated by exposure of the benzophenone-implanted PDMS to UV radiation through a photomask with a thin aqueous layer of acrylic acid sandwiched between the PDMS and photomask. This procedure resulted in patterned poly(acrylic acid) (PAA) on the PDMS surface. In the modified regions, PAA and PDMS formed an interpenetrating polymer network extending 50 microm into the PDMS with an X-Y spatial resolution of 5 microm. The carboxyl groups of the PAA graft could be derivatized to covalently bond other molecules to the patterned PAA. Two bioanalytical applications of this micropatterned surface were demonstrated: (1) a guide for cell attachment and growth and (2) a substrate for immunoassays. 3T3 cells were shown to selectively localize to modified surface regions where they could be cultured for up to 7 days. Additionally, the micropatterned surface was used to immobilize either protein A or antibody for heterogeneous immunoassays.
Acrylic Resins↗
Low-voltage polymer-based scanning cantilever for in vivo optical coherence tomography.
Novel hand-held optical coherence tomography (OCT) probes with polymer cantilevers have been developed for clinical oral and skin imaging. An electroactive ionic polymer-metal composite cantilever was used to generate 3-mm transverse scanning movement of an optical fiber with applied 2-V linear alternating voltage at 1 Hz. Low driving voltage ensures safety. Two different optical designs achieve both forward and sidewise scanning and make it possible to image everywhere within the human oral cavity. In vivo OCT imaging of the human tongue is demonstrated.
Equipment Design↗
Tailoring the surface properties of poly(dimethylsiloxane) microfluidic devices.
Poly(dimethylsiloxane) (PDMS) is an attractive material for microelectrophoretic applications because of its ease of fabrication, low cost, and optical transparency. However, its use remains limited compared to that of glass. A major reason is the difficulty of tailoring the surface properties of PDMS. We demonstrate UV grafting of co-mixed monomers to customize the surface properties of PDMS microfluidic channels in a simple one-step process. By co-mixing a neutral monomer with a charged monomer in different ratios, properties between those of the neutral monomer and those of the charged monomer could be selected. Mixtures of four different neutral monomers and two different charged monomers were grafted onto PDMS surfaces. Functional microchannels were fabricated from PDMS halves grafted with each of the different mixtures. By varying the concentration of the charged monomer, microchannels with electrophoretic mobilities between +4 x 10(-4) cm2/(V s) and -2 x 10(-4) cm2/(V s) were attainable. In addition, both the contact angle of the coated surfaces and the electrophoretic mobility of the coated microchannels were stable over time and upon exposure to air. By carefully selecting mixtures ofmonomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different applications.
Dimethylpolysiloxanes↗
Surface-directed, graft polymerization within microfluidic channels.
We demonstrate a simple procedure to coat the surfaces of enclosed PDMS microchannels by UV-mediated graft polymerization. In prior applications, only disassembled channels could be coated by this method. This limited the utility of the method to coatings that could easily and tightly seal with themselves. By preadsorbing a photoinitiator onto the surface of PDMS microchannels, the rate of polymer formation at the surface was greatly accelerated compared to that in solution. Thus, a gel did not form in the lumen of enclosed microchannels. We demonstrate that the photoinitiator benzophenone remained on the surface of PDMS even after extensive washing. After addition of a variety of monomer solutions (acrylic acid, poly(ethylene glycol) monomethoxyl acrylate, or poly(ethylene glycol) diacrylate) and illumination with UV light, a stable, covalently attached surface coating formed in the microchannels. The electroosmotic mobility was stable in response to air exposure and to repeated cycles of hydration-dehydration of the coating. These surfaces also supported the electrophoretic separation of two model analytes. Placement of an opaque mask over a portion of the channel permitted photopatterning of the microchannels with a resolution of approximately 100 microm. By using an appropriate mixture of monomers combined with masks, it should be possible to fabricate PDMS microfluidic devices with distinct surface properties in different regions or channels.
Journal Article↗
Fast electrical lysis of cells for capillary electrophoresis.
In the past decade, capillary electrophoresis has demonstrated increasing utility for the quantitative analysis of single cells. New applications for the analysis of dynamic cellular properties demand sampling methods with sufficient temporal resolution to accurately measure these processes. In particular, intracellular signaling pathways involving many enzymes can be modulated on subsecond time scales. We have developed a technique to rapidly lyse an adherent mammalian cell using a single electrical pulse followed by efficient loading of the cellular contents into a capillary. Microfabricated electrodes were designed to create a maximum voltage drop across the flattened cell's plasma membrane at a minimum interelectrode voltage. The influence of the interelectrode distance, pulse duration, and pulse strength on the rate of cell lysis was determined. The ability to rapidly lyse a cell and collect and separate the cellular contents was demonstrated by loading cells with Oregon Green and two isomers of carboxyfluorescein. All three fluorophores were detected with a separation efficiency comparable to that of standards. Parallel comparison of electrical lysis to that produced by a laser-based lysis system revealed that the sampling efficiencies of the two techniques were comparable. Rapid cell lysis by an electrical pulse may increase the application of capillary electrophoresis to the study of cellular dynamics requiring fast sampling times.
Animals↗
Cross-linked coatings for electrophoretic separations in poly(dimethylsiloxane) microchannels.
We have developed a strategy using ultraviolet light to polymerize mixed monomer solutions onto the surface of a poly(dimethylsiloxane) (PDMS) microdevice. By including monomers with different chemical properties, electrophoretic separations were optimized for a test set of analytes. The properties of surfaces grafted with a single neutral monomer, a neutral and a negative monomer, or a neutral, negative, and cross-linking monomer were assessed. The highest quality separations were achieved in channels with cross-linked coatings. The separation efficiency for biologically relevant peptides (kinase substrates) on these surfaces was as high as 18 600 theoretical plates in a 2.5 cm channel. The test peptides were fluorescein-AEEEIYGEFEAKKKK, fluorescein-GRPRAATFAEG, fluorescein-GRPRAA(T-PO(3))FAEG, fluorescein-DLDVPIP GRFDRRVSVAAE, and fluorescein-DLDVPIPGRFDRRV(S-PO(3))VAAE. Separations between two different peptides occurred in as little as 400 ms after injection into the separation channel. The simultaneous separation of five kinase and phosphatase substrates was also demonstrated. By carefully selecting mixtures of monomers with the appropriate properties, it may be possible to tailor the surface of PDMS for a large number of different electrophoretic separations.
Amino Acid Sequence↗
Surface modification of poly(dimethylsiloxane) microfluidic devices by ultraviolet polymer grafting.
Poly(dimethylsiloxane) (PDMS)-based microfluidic devices are increasing in popularity due to their ease of fabrication and low costs. Despite this, there is a tremendous need for strategies to rapidly and easily tailor the surface properties of these devices. We demonstrate a one-step procedure to covalently link polymers to the surface of PDMS microchannels by ultraviolet graft polymerization. Acrylic acid, acrylamide, dimethylacrylamide, 2-hydroxylethyl acrylate, and poly(ethylene glycol)monomethoxyl acrylate were grafted onto PDMS to yield hydrophilic surfaces. Water droplets possessed contact angles as low as 45 degrees on the grafted surfaces. Microchannels constructed from the grafted PDMS were readily filled with aqueous solutions in contrast to devices composed of native PDMS. The grafted surfaces also displayed a substantially reduced adsorption of two test peptides compared to that of oxidized PDMS. Microchannels with grafted surfaces exhibited electroosmotic mobilities intermediate to those displayed by native and oxidized PDMS. Unlike the electroosmotic mobility of oxidized PDMS, the electroosmotic mobility of the grafted surfaces remained stable upon exposure to air. The electrophoretic resolution of two test peptides in the grafted microchannels was considerably improved compared to that in microchannels composed of oxidized PDMS. By using the appropriate monomer, it should be possible to use UV grafting to impart a variety of surface properties to PDMS microfluidics devices.
Dimethylpolysiloxanes↗