No abstract

This paper describes the influence of the composition of poly(dimethylsiloxane) (PDMS) on the attachment and growth of several different types of mammalian cells: primary human umbilical artery endothelial cells (HUAECs), transformed 3T3 fibroblasts (3T3s), transformed osteoblast-like MC3T3-E1 cells, and HeLa (transformed epithelial) cells. Cells grew on PDMS having different ratios of base to curing agent: 10:1 (normal PDMS, PDMSN), 10:3 (PDMSCA), and 10:0.5 (PDMSB). They were also grown on "extracted PDMS" (normal PDMS that has reduced quantities of low molecular-weight oligomers, PDMSN,EX) and normal PDMS that had been extracted and then oxidized (PDMSN,EX,OX); all surfaces were exposed to a solution of fibronectin prior to cell attachment. Generally, fibronectin-coated PDMS is a suitable substrate for culturing mammalian cells. Compatibility of cells on some surfaces, however, was dependent on the cell type: PDMSN,EX,OX caused cell detachment of 3T3 fibroblasts and MC3T3-E1 cells, and PDMSCA caused detachment of HUAECs and HeLa cells. Growth of cells on PDMSN, PDMSN,EX, and PDMSB was comparable to growth on tissue culture-treated polystyrene for most of the cell types. All cells grew at similar rates on PDMS substrates regardless of the stiffness of the substrate, for substrates having Young's moduli ranging from E=0.60 +/- 0.04 to 2.6 +/- 0.2 MPa (for PDMSB and PDMSN,EX, respectively).

It is difficult to harness the power generated by biological motors to carry out mechanical work in systems outside the cell. Efforts to capture the mechanical energy of nanomotors ex vivo require in vitro reconstitution of motor proteins and, often, protein engineering. This study presents a method for harnessing the power produced by biological motors that uses intact cells. The unicellular, biflagellated algae Chlamydomonas reinhardtii serve as ''microoxen.'' This method uses surface chemistry to attach loads (1-to 6-m-diameter polystyrene beads) to cells, phototaxis to steer swimming cells, and photochemistry to release loads. These motile microorganisms can transport microscale loads (3-m-diameter beads) at velocities of Ϸ100 -200 m⅐sec ؊1 and over distances as large as 20 cm.biological motors ͉ Chlamydomonas ͉ phototaxis ͉ microfluidics ͉ microspheres T his study demonstrates the biological propulsion of microscale loads by the unicellular photosynthetic algae Chlamydomonas reinhardtii (CR). We exploit the chemistry of the algal cell wall to attach single 1-to 6-m polymer beads to CR. Cells with these ''loads'' attached swim at velocities as high as 100-200 m⅐sec Ϫ1 , approximately the velocity of unmodified cells. CR is phototactic and can be guided by using visible light ( Ϸ 500 nm); we have used this phototaxis to control the transport of microscale loads. A photocleavable linker between the surface of the bead and the cell wall allows us to release loads from the surface of the cell photochemically. We have combined these processes to pick up, transport, guide, and drop off beads by using motile cells.There are many examples of nanometer-scale motors in nature. Within the cell, linear motors, including DNA and RNA polymerase, dyneins, kinesins, and myosin, play a critical role in transcription, mitosis, meiosis, muscle contraction, and transporting organelles and synaptic vesicles (1-5). In eukaryotic mitochondria, a rotary motor, ATP synthase, produces ATP by harnessing the flow of protons down an electrochemical proton gradient (6, 7). Outside of the cell, ciliary dyneins drive the beating of eukaryotic flagella and cilia. In bacteria, a complex of Ϸ20 proteins makes up the remarkable rotary motor that powers the motion of flagella (8).Interest in biological motors is based on both their transduction of energy and their small size and hence their possible relevance to micro͞nanotechnology; the remarkable work of Walker, Vale, Kinosita, Hirokawa, Yanagida and others (9-17) has transformed our understanding of molecular motors. One outcome has been the design and fabrication of new synthetic motors composed entirely of biological molecules (18, 19); another outcome has been the integration of components onto recombinant biological motors (20)(21)(22).Here, we use biological motors intact in cells that use flagella (23). An advantage of this strategy over that using isolated and reconstituted motors is its simplicity. It (i) avoids purification and reconstitution of individual motor proteins, (ii) takes...

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This manuscript describes the use of water-soluble polymers for use as sacrificial layers in surface micromachining. Water-soluble polymers have two attractive characteristics for this application: 1) They can be deposited conveniently by spin-coating, and the solvent removed at a low temperature (95-150 8C), and 2) the resulting layer can be dissolved in water; no corrosive reagents or organic solvents are required. This technique is therefore compatible with a number of fragile materials, such as organic polymers, metal oxides and metals-materials that might be damaged during typical surface micromachining processes. The carboxylic acid groups of one polymer-poly(acrylic acid) (PAA)-can be transformed by reversible ion-exchange from water-soluble (Na + counterion) to water-insoluble (Ca 2 + counterion) forms. The use of PAA and dextran polymers as sacrificial materials is a useful technique for the fabrication of microstructures: Examples include metallic structures formed by the electrodeposition of nickel, and freestanding, polymeric structures formed by photolithography.

This manuscript describes the fabrication and use of a three-dimensional magnetic trap for diamagnetic objects in an aqueous solution of paramagnetic ions; this trap uses permanent magnets. It demonstrates trapping of polystyrene spheres, and of various types of living cells: mouse fibroblast ͑NIH-3T3͒, yeast (Saccharomyces cerevisiae), and algae (Chlamydomonas reinhardtii). For a 40 mM solution of gadolinium (III) diethylenetriaminepentaacetic acid ͑Gd· DTPA͒ in aqueous buffer, the smallest cell (particle) that could be trapped had a radius of ϳ2.5 m. The trapped particle and location of the magnetic trap can be translated in three dimensions by independent manipulation of the permanent magnets. This letter a1so characterizes the biocompatibility of the trapping solution.The ability to position cells, on surfaces and in suspension, is broadly useful in cellular biology. Microcontact printing of self-assembled monolayers 1-3 and other techniques of surface engineering 4,5 are used for confining and controlling the mobility of cells on surfaces. Optical traps can confine and manipulate cells and microspheres in suspension, and have been used to determine the elasticity of the cell membrane, 6-8 to observe cell division, 9 to measure inhibition of cell adhesion, 10 and to strain cells to induce activity in signaling pathways. 11 While optical traps have enabled many experiments in biophysics, they also have limitations. The laser power required to trap a micron-sized particle (for example, a cell) is proportional to the ratio of the refractive index of the particle to that of the medium. 12 Since the ratio of the refractive indices for most biological materials to biocompatible fluid media is near unity, trapping requires lasers having powers up to ϳ100 mW. This high laser power can raise the local temperature in the liquid by several degrees, 13 and this heating can damage or kill a trapped cell. In addition, traditional optical tweezers cannot trap objects with ratios of refractive indices of the object to the environment of less than unity (e.g., a gas-filled glass sphere in water 14 or a water drop in liquid parafilm 15 ) or greater than 1.5 (e.g., diamond particles in water). 16 Optical tweezers are restrictive in the size of the particle that can be trapped; particles must be ഛ10 m in diameter. The trapping force of optical tweezers is minimal outside the focus of the laser beam, requiring objects to enter the focus of the laser before they can be trapped. The small capture volume for many particles (only a few cubic microns) results in significant waiting periods before objects are trapped in dilute samples. The short working distance of the objective lens used to focus the light restricts the trap to regions near ͑Ͻ200 m͒ the surface. The high-powered lasers and infinity-corrected, high numerical aperture objectives used to construct an optical trap are expensive.Although the use of magnetism to manipulate objects was described by Thales ͑c.500 B.C.͒, the trapping of objects in a stable magnetic equilibrium...

This work describes a method for patterning a gold substrate with multiple, aligned self-assembled monolayers (SAMs) using light at different wavelengths. It describes the synthesis and characterization of an alkanethiolate SAM that is photosensitive to light at both 220 and 365 nm. A photomask acts as an area-selective filter for light at 220 and 365 nm, and a single set of exposures at these two wavelengths through one photomask, without steps of alignment between the exposures, can produce three aligned SAMs on one gold substrate. We demonstrate the versatility of this method of photopatterning by modifying individual aligned SAMs chemically to produce surfaces having different properties. We characterize the modified SAMs using immunolabeling, matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy, and surface plasmon resonance spectroscopy. We also describe the patterning of two aligned SAMs that resist the adsorption of proteins and a third region that does not resist the adsorption of proteins. The ability to produce multiple, aligned patterns of SAMs in a single step, without alignment of photomasks in separate steps, increases the versatility of SAMs for studying a range of physical phenomena.

Abstract-Challenges facing the scaling of microelectronics to sub-50 nm dimensions and the demanding material and structural requirements of integrated photonic and microelectromechanical systems suggest that alternative fabrication technologies are needed to produce nano-scale devices. Inspired by complex, functional, self-assembled structures and systems found in Nature we suggest that self-assembly can be employed as an effective tool for nanofabrication. We define a self-assembling system as one in which the elements of the system interact in pre-defined ways to spontaneously generate a higher order structure. Self-assembly is a parallel fabrication process that, at the molecular level, can generate three-dimensional structures with sub-nanometer precision. Guiding the process of self-assembly by external forces and geometrical constrains can reconfigure a system dynamically on demand. We survey some of the recent applications of self-assembly for nanofabrication of electronic and photonic devices. Five self-assembling systems are discussed: 1) self-assembled molecular monolayers; 2) self-assembly in supramolecular chemistry; 3) self-assembly of nanocrystals and nanowires; 4) self-assembly of phase-separated block copolymers; 5) colloidal self-assembly. These techniques can generate features ranging in size from a few angstroms to a few microns. We conclude with a discussion of the limitations and challenges facing self-assembly and some potential directions along which the development of self-assembly as a nanofabrication technology may proceed.