Patterning self-assembled monolayers using microcontact printing: A new technology for biosensors?

Electrochemical desorption of self-assembled monolayers noninvasively releases patterned cells from geometrical confinements

This report describes a method to pattern mammalian cells using self-assembled monolayers (SAMs), and then to use electrochemical desorption of these monolayers to release cells from their patterns. This method uses an oligo(ethyleneglycol)-terminated SAM to prevent,-and a methyl-terminated SAM to allow-adsorption of proteins and attachment of bovine capillary endothelial cells. Electrochemical removal of the oligo(ethyleneglycol)-terminated SAM allowed proteins to adsorb onto areas that had been previously inert and enabled cells to migrate into these areas. This straightforward technique is useful in bioassays for drug screening and for fundamental studies in cell biology.

Biochips beyond DNA: technologies and applications

Technological advances in miniaturization have found a niche in biology and signal the beginning of a new revolution. Most of the attention and advances have been made with DNA chips yet a lot of progress is being made in the use of other biomolecules and cells. A variety of reviews have covered only different aspects and technologies but leading to the shared terminology of "biochips." This review provides a basic introduction and an in-depth survey of the different technologies and applications involving the use of non-DNA molecules such as proteins and cells. The review focuses on microarrays and microfluidics, but also describes some cellular systems (studies involving patterning and sensor chips) and nanotechnology. The principles of each technology including parameters involved in biochip design and operation are outlined. A discussion of the different biological and biomedical applications illustrates the significance of biochips in biotechnology.

Topographical and physicochemical modification of material surface to enable patterning of living cells

Precise control of the architecture of multiple cells in culture and in vivo via precise engineering of the material surface properties is described as cell patterning. Substrate patterning by control of the surface physicochemical and topographic features enables selective localization and phenotypic and genotypic control of living cells. In culture, control over spatial and temporal dynamics of cells and heterotypic interactions draws inspiration from in vivo embryogenesis and haptotaxis. Patterned arrays of single or multiple cell types in culture serve as model systems for exploration of cell-cell and cell-matrix interactions. More recently, the patterned arrays and assemblies of tissues have found practical applications in the fields of Biosensors and cell-based assays for Drug Discovery. Although the field of cell patterning has its origins early in this century, an improved understanding of cell-substrate interactions and the use of microfabrication techniques borrowed from the microelectronics industry have enabled significant recent progress. This review presents the important early discoveries and emphasizes results of recent state-of-the-art cell patterning methods. The review concludes by illustrating the growing impact of cell patterning in the areas of bioelectronic devices and cell-based assays for drug discovery.

Photolithographic generation of protein micropatterns for neuron culture applications

Standard positive photoresist techniques were adapted to generate micropatterns of proteins on glass and oxide surfaces. Both lift-off and plasma-etching techniques were used to transfer the photoresist pattern into a layer of covalently immobilised protein. The surface properties of the areas adjacent to the patterns were altered by chemical surface modification. Using a combination of the lift-off and the etching process complementary patterns of two different proteins were generated. The biochemical and biological functionality of the protein patterns were assessed by immunostaining and by investigating the outgrowth of neurites from neurons plated on the patterned substrates. The investigated patterning processes are compatible with microstructuring and thin film processes, and may be used to generate functional surfaces for sensor and neuron culture applications.

Nanotiterplates for combinatorial chemistry

Miniaturization has grown to be a driving force in modern chemical and biochemical laboratories. Combinatorial explosion demands for new pathways for the synthesis and screening of new substances which can act as leads in drug discovery. Highly parallelized automata that can handle the smallest amounts of substances are needed. However, the development is not always straightforward since new problems also arise in miniaturization, e.g. increasing importance of surface properties of utilized devices and evaporation of liquids. This paper reports on recent developments on the field of miniaturized reaction vessels called nanotiterplates. A survey on fabrication technologies as well as applications of nanotiterplates is given. Special emphasis is given to results of the development of an automaton for miniaturized synthesis and screening. Besides the mere fabrication of nanotiterplates with integrated microsieve bottom membranes, examples of applications in chemical synthesis and bio-assays are given. Further topics are the characterization and specific adaption of surface properties and investigations on the evaporation of solvents and measures for prevention.

Tissue engineering: the biophysical background

Tissue engineering is the construction, repair or replacement of damaged or missing tissue in humans and other animals. This engineering may take place within the animal body or as tissue constructs to be made in a bioreactor for later grafting into the animal. The minimal set of materials for this are the appropriate types of cell. Usually, however, non-living substrata are used as well. These substrata may be nothing more than materials that bulk up any voids in the damaged tissue and provide the mechanical strength that has been lost when the tissue is damaged or removed. They may serve a similar pair of functions in the bioreactor. They can do much more in terms of pattern formation. The orientations and morphology of the cells, the arrangement of intercellular material as it is laid down and the relationships between different cell types in the repairing or construct tissue are all of importance, for these should resemble the correct normal tissue as closely as possible. Most of these requirements are ones involving pattern formation. This review discusses the various ways in which tissue pattern can be engineered chiefly from a biophysical standpoint. Unpatterned cells are effectively not tissue. This engineering includes the use of topography on the substrata, chemical patterning of adhesive and other cues for the cells, mechanical force application to cause cell orientation and appropriate synthetic responses and electrical fields. The review also discusses the methods used to impart the appropriate cues to and through the materials which are often biodegradable polymers. The article gives particular attention to regions of research and practice where the involvement of the physicist or biophysicist is of importance.

Modification of Si(100) surface by the grafting of poly(ethylene glycol) for reduction in protein adsorption and platelet adhesion

The modification of argon plasma-pretreated single-crystal Si(100) wafer surfaces via the UV-induced graft polymerization of poly(ethylene glycol) methacrylate (PEGMA) macromonomer (molecular weight approximately 340) for biomaterials applications was explored. The modified Si(100) surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy. Surface peroxide concentrations resulting from the argon plasma treatment and subsequent atmospheric exposure were determined by a coupling reaction with diphenylpicrylhydrazyl. The results suggested that a short plasma treatment time of 10 s and brief air exposure were sufficient for generating an optimum amount of peroxides and hydroperoxides for the subsequent UV-induced graft polymerization. The graft concentration of the PEGMA polymer increased with increasing PEGMA macromonomer concentration for the graft polymerization and with increasing UV graft polymerization time. The PEGMA graft-polymerized silicon surface with a high poly(ethylene glycol) graft concentration was very effective in preventing protein adsorption and platelet adhesion. The grafted PEGMA polymer layer on the Si(100) surface exhibited fairly good stability during storage in a buffer solution.

Nanobiotechnology: the fabrication and applications of chemical and biological nanostructures

Biology can teach the physical world of electronics, computing, materials science and manufacturing how to assemble complex functional devices and systems that operate at the molecular level. Our present capability to fabricate simple molecular tools, devices, materials and machines is primitive compared with the sophistication of nature. Nevertheless, the nanomanufacturing of 'biomimetic' devices is moving ahead strongly. Recent developments have been made in the use of biological systems in molecular self-assembly, spatial positioning, microconstruction, biocomposite fabrication, nanomachines and biocomputing.

Protein structure in KBr pellets by infrared spectroscopy

In this work we analyzed the secondary structure of 13 globular proteins in KBr pellet through Fourier transform infrared spectroscopy (FTIR). The quantification was based in singular value decomposition (SVD) theory, a pattern recognition method. The results show better correlation for alpha helix (0.90) and beta sheet (0.84) in amide I band, similar to the results obtained for proteins in solution. These results show that the protein secondary structure is conserved in solid state, in opposition to the results observed by FTIR using resolution enhancement techniques. The SVD analysis also show that in KBr pellets the protein secondary structures have absorbances in different wavenumbers when compared to those in solution. In this way, the use of KBr pellet and the pattern recognition method can be an ideal method to analyze protein secondary structure by FTIR.

Diffraction-based cell detection using a microcontact printed antibody grating

An optical detector has been fabricated that is specific for targeted bacterial cells, by stamping an antibody grating pattern on a silicon surface. The antibody grating alone produces insignificant optical diffraction, but upon immunocapture of cells, the optical phase change produces a diffraction pattern. This technique eliminates much of the surface modifications and the secondary immunochemical or enzyme-linked steps that are common in immunoassays. Microcontact printing provides an alternative to previously reported photolithographic-mediated antibody patterning processes and uses a photolithographic process simply to produce the elastomeric stamp. We have stamped antibodies directly onto clean native oxide silicon substrates with no other chemical surface treatments. Direct binding of the antibodies to the silicon occurs in a way that still allows them to function and selectively bind antigen. The performance of the sensor was evaluated by capturing Escherichia coli O157:H7 cells on the antibody-stamped lines and measuring the intensity of the first-order diffraction beam resulting from the attachment of cells. The diffraction intensity increases in proportion to the cell density bound on the surface.

Surface modifications for the development of piezoimmunosensors

Four different techniques for the immobilisation of proteins onto the gold electrode of a piezoelectric quartz crystal were investigated. The examined techniques were adsorption, avidin-biotin binding and two different types of covalent binding on self-assembled monolayers (SAM), dithiobis(succinimidylpropionate) (DSP) and a dextran modified thiol monolayer. The reaction of the immobilised proteins (bovine serum albumin (BSA) and anti-human IgG) with their specific antibodies, anti-BSA and hIgG (50 and 200 micrograms/ml) were studied using a quartz crystal microbalance and then compared. Many cycles of measurements were performed on the same crystal regenerating the gold surface with a solution of glycine.HCl, 100 mM, pH 2.1. The interactions of the immobilised reagents with non-specific antibodies were also studied. The adsorption protocol was the quickest, but did not allow regeneration with glycine.HCl. Thiol-dextran coated surfaces did not show any detectable response to non-specific reagents, but needed a very long and complicated protocol. DSP and avidin-biotin coating procedures were easy and not too long. They seemed to have the best characteristics of reproducibility among different crystals and possibility of regeneration of the coated surface, but the percentage of non-specific binding was high.

Capacitance measurements of antibody-antigen interactions in a flow system

Capacitive immunosensors were made by coupling monoclonal antibodies to thioctic acid, which had self-assembled on a gold electrode. Surface areas that were not covered were plugged with 1-dodecanethiol to make the layer dense and insulating. Cyclic voltammetry showed that the hexacyanoferrate redox reactions were blocked by this procedure. The capacitance of the electrode was evaluated from the current transients obtained when a potentiostatic step was applied. The immunosensor was placed in a flow system, and a capacitance decrease could be observed after injection of an unlabeled antigen. It was linear over almost three decades when plotted vs the logarithm of the antigen concentration. Human chorionic gonadotropin hormone could be determined in the range 1 pg/mL-1 ng/mL, with a detection limit of 0.5 pg/mL (15 10(-15) M). A similar response was obtained with immobilized F(ab)2 fragments. No cross-reactivity was observed with the thyrotropic hormone, which has one chain in common with gonadotropin. Monoclonal antibodies toward interleukin-2 immobilized on the immunosensor gave also a response over 1 pg/mL-1 ng/mL, with a detection limit of 1 pg/mL. An immunosensor with monoclonal antibodies toward human albumin gave a calibration curve with lower slope than the other proteins but still with a detection limit of 1 pg/mL.

Geometric control of cell life and death

Human and bovine capillary endothelial cells were switched from growth to apoptosis by using micropatterned substrates that contained extracellular matrix-coated adhesive islands of decreasing size to progressively restrict cell extension. Cell spreading also was varied while maintaining the total cell-matrix contact area constant by changing the spacing between multiple focal adhesion-sized islands. Cell shape was found to govern whether individual cells grow or die, regardless of the type of matrix protein or antibody to integrin used to mediate adhesion. Local geometric control of cell growth and viability may therefore represent a fundamental mechanism for developmental regulation within the tissue microenvironment.

Biosensors for chemical and biological agents of defence interest

This review discusses current developments in biosensors for toxic materials of defence interest with particular emphasis on the biological element of such devices. A wide variety of synthetic chemicals, toxins of plant or animal origin and biological materials--including various disease micro-organisms as well as some bacterial exotoxins--have either been used as warfare agents or are perceived as having the potential to be used for that purpose. Although an enormous effort is being put into developing biosensors, relatively few analytes, especially toxic materials, can yet be measured by commercially available devices. The factors which currently mitigate against the use of enzyme, natural receptor or antibody based biosensors for unattended continuous environmental monitoring of toxic materials include the inherent instability and availability of suitable proteins and--for receptors and antibodies--the essentially irreversible nature of the binding event, which necessitates a continuous supply of reagents for sequential measurements. Assays involving antibody or DNA based biosensors are time consuming when working in a hazardous environment. Nevertheless, biosensors are capable of being used for extremely sensitive and specific on-site measurements of contamination by specific toxic materials. Methods for improving the stability, extending the range and altering the binding characteristics of sensing molecules are discussed.